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I
UNDERSTANDING
CLIMATIC
CHANGE
^■m
NATIONAL ACADEMY OF SCIENCES
UNITED STATES COMMITTEE FOR THE
GLOBAL ATMOSPHERIC RESEARCH PROGRAM
National Research Council
UNDERSTANDING
CLIMATIC
CHANGE
A Program for Action
NATIONAL ACADEMY OF SCIENCES
WASHINGTON, D.C.
1975
notice: The project which is the subject of this report was approved by the Gov-
erning Board of the National Research Council, acting in behalf of the National
Academy of Sciences. Such approval reflects the Board's judgment that the project
is of national importance and appropriate with respect to both the purposes and
resources of the National Research Council.
The members of the committee selected to undertake this project and prepare
this report were chosen for recognized scholarly competence and with due con-
sideration for the balance of disciplines appropriate to the project. Responsibili v
for the detailed aspects of this report rests with that committee.
Each report issuing from a study committee of the National Research Council is
reviewed by an independent group of qualified individuals according to procedures
established and monitored by the Report Review Committee of the National
Academy of Sciences. Distribution of the report is approved, by the President of
the Academy, upon satisfactory completion of the review process.
The activities of the United States Committee for the Global Atmospheric Re-
search Program leading to this report have been supported by the National Oceanic
and Atmospheric Administration and the National Science Foundation under Con-
tract NSF-C310, Task Order No. 197.
Library of Congress Cataloging in Publication Data
United States Committee for the Global Atmospheric Research Program.
Understanding climatic change.
Includes bibliographies.
1. Climatic changes — Research. I. Title.
QC981.8.C5U54 1975 551.6 75-827
ISBN 0-309-02323-8
Available from
Printing and Publishing Office, National Academy of Sciences
2101 Constitution Avenue, Washington, D.C. 20418
Printed in the United States of America
U.S. COMMITTEE FOR THE GLOBAL ATMOSPHERIC
RESEARCH PROGRAM
Scientific Members
Verner E. Suomi, University of Wisconsin, Chairman
Richard J. Reed, University of Washington, V ice-Chairman
Francis P. Bretherton, National Center for Atmospheric Research
T. N. Krishnamurti, Florida State University
Cecil E. Leith, National Center for Atmospheric Research
Richard S. Lindzen, Harvard University
Syukuro Manabe, Geophysical Fluid Dynamics Laboratory (noaa)
Yale Mintz, University of California at Los Angeles
Allan R. Robinson, Harvard University
Joseph Smagorinsky, Geophysical Fluid Dynamics Laboratory (noaa)
William L. Smith, National Environmental Satellite Service (noaa)
Ferris Webster, Woods Hole Oceanographic Institution
Michio Yanai, University of California
John A. Young, University of Wisconsin
John S. Perry, National Research Council, Executive Scientist
John R. Sievers, National Research Council, Executive Secretary
Ex-officio Members
John W. Firor, National Center for Atmospheric Research
Robert G. Fleagle, University of Washington
Thomas F. Malone, Holcomb Research Institute
Invited Participants
Charles W. Mathews, National Aeronautics and Space Administration
Gordon H. Smith, Department of Defense
Edward P. Todd, National Science Foundation
John W. Townsend, Jr., National Oceanic and Atmospheric Administra-
tion
Robert M. White, Department of Commerce
Liaison Representatives
Eugene W. Bierly, National Science Foundation
William Chapin, Department of State
Rudolf J. Engelmann, Atomic Energy Commission
Albert Kaehn, Jr., Department of Defense
Douglas H. Sargeant, National Oceanic and Atmospheric Administration
Joseph F. Sowar, Federal Aviation Administration
Morris Tepper, National Aeronautics and Space Administration
iii
PANEL ON CLIMATIC VARIATION
Members
W. Lawrence Gates, The Rand Corporation, Co-Chairman
Yale Mintz, University of California at Los Angeles, Co-Chairman
Wallace S. Broecker, Lamont-Doherty Geological Observatory
Kirk Bryan, Geophysical Fluid Dynamics Laboratory
Jule G. Charney, Massachusetts Institute of Technology
George H. Denton, University of Maine
Harold C. Fritts, University of Arizona
John Imbrie, Brown University
Robert Jastrow, Goddard Institute for Space Studies
Edward N. Lorenz, Massachusetts Institute of Technology
Syukuro Manabe, Geophysical Fluid Dynamics Laboratory
J. Murray Mitchell, Jr., Environmental Data Service
Jerome Namias, Scripps Institution of Oceanography
Henry Stommel, Massachusetts Institute of Technology
Warren M. Washington, National Center for Atmospheric Research
Consultants
John E. Kutzbach, University of Wisconsin
Cecil E. Leith, National Center for Atmospheric Research
Abraham H. Oort, Geophysical Fluid Dynamics Laboratory
Richard C. J. Somerville, National Center for Atmospheric Research
IV
FOREWORD
The Preface of the U.S. Committee for the Global Atmospheric Research
Program document Plan for U.S. Participation in the Global Atmos-
pheric Research Program begins :
In late 1967 the International Council of Scientific Unions, acting jointly with
the World Meteorological Organization, proposed a Global Atmosphere Research
Program (garp) to accomplish the objectives stated in U.N. Resolutions 1721
(XVI) and 1802 (XVII), namely, "to advance the state of atmospheric sciences
and technology so as to provide greater knowledge of basic physical forces affect-
ing climate . . . ; to develop existing weather forecasting capabilities . . . ," and
"to develop an expanded program of atmospheric science research which will com-
plement the program fostered by the World Meteorological Organization."
Now, in 1974, a program to reach the "weather forecasting" objective of
garp is well under way. In this report, the U.S. Committee for the
Global Atmospheric Research Program (usogarp) outlines a program
to "understand the basic physical forces affecting climate." There is
ample evidence, summarized in Appendix A of this report, that climate
does change, and there is more than ample evidence from past history
and even recent events that changes in climate can profoundly affect
human activities and even life itself. Indeed, as a growing population
places ever greater demands on food and fiber resources, man's sensitiv-
ity to variations in climate will increase. We have an urgent need for
better information on global climate. Unfortunately, we do not have a
good quantitative understanding of our climate machine and what deter-
mines its course. Without this fundamental understanding, it does not
VI UNDERSTANDING CLIMATIC CHANGE
seem possible to predict climate — neither in its short-term variations nor
in its larger long-term changes. There are some who believe that impor-
tant variations in climate can occur with changes in the controlling fac-
tors that are so small they are difficult to measure. With such barriers
to be overcome, is there any assurance of success? We believe so.
First, the two garp objectives, dealing with weather and climate, are
strongly related to each other. A better understanding of the physical
processes that affect one means a better understanding of the processes
that affect the other. The difference lies mainly in how the processes
should be taken into account. Mathematical models fashioned to take
into account long-term changes will have to have some characteristics
that are different from those fashioned mainly for short-term (weather)
changes. There has been significant progress in garp's weather objec-
tive; therefore, there already has been important progress in garp's
climate objective.
Second, there has been a tremendous improvement in our ability to
observe the global weather, thanks to weather satellites. By the end of
this decade, we will have the ability to observe the entire earth with
needed meteorological observations. An ability to obtain better weather
observations is an ability to obtain better climate observations also.
Important as this is, meteorological satellites also allow us to monitor
those parameters that we now believe control the climate machine: the
sun's output, the earth's albedo, the distribution of clouds, the fields of
ice and snow, and the temperatures of the upper layers of the ocean.
These parameters control the average state of the weather and thus
climate. Meteorological satellites are observing some of these param-
eters and could measure all of them. In some instances, the data are
already being collected. These now need to be assembled to serve the
needs of climate research. The feasibility of collecting data from ocean
platforms has been established. A program to do it is needed. This
report outlines the key requirements.
Third, research into past climates has made significant advances. We
now not only know what happened in the past far better than we did a
decade or two ago, but these data will provide an important information
base against which theories and numerical models of climate can be
tested.
Last, but far from least, there is a new generation of atmospheric sci-
entists. Their tools are the computer, numerical models, and satellites,
and they know how to use them well. The usc-garp believes that this is
an adequate manpower base. We do not expect any breakthroughs, and
progress could be slower than desired, but the program outlined in this
FOREWORD VII
document is a rational approach toward obtaining progress as rapidly as
possible on this vital subject.
The usc-garp further believes that neither the scientific community
nor the nation can afford to be complacent with its present level of
understanding on this important aspect of the earth's physical environ-
ment. The natural forces determining the world's weather and climate
are beyond our control, but having better insight into what nature might
do should help the nation to plan for what it must do.
This report was prepared by the Committee's Panel on Climatic
Variation. On behalf of the usc-garp, I express full appreciation to its
Co-Chairmen, Yale Mintz and W. Lawrence Gates, and all of its mem-
bers for this important report.
verner e. suomi, Chairman
U.S. Committee for the
Global Atmospheric Research Program
PREFACE
The increasing realization that man's activities may be changing the
climate, and mounting evidence that the earth's climates have undergone
a long series of complex natural changes in the past, have brought new
interest and concern to the problem of climatic variation. The impor-
tance of the problem has also been underscored by new recognition of
the continuing vulnerability of man's economic and social structure to
climatic variations. Our response to these concerns is the proposal of a
major new program of research designed to increase our understanding
of climatic change and to lay the foundation for its prediction.
The need for increased understanding of the physical basis of climate
was recognized by the Panel on International Meteorological Coopera-
tion of the Committee on Atmospheric Sciences in its report of 1966,
which led to the development of the Global Atmospheric Research Pro-
gram (garp). This objective was embodied in the garp plan as a "sec-
ond objective" devoted to the study of the physical basis of climate, to
be undertaken along with the program's primary concern of improving
and extending weather forecasts with the aid of numerical models.
In March 1972, the United States Committee for garp appointed
the Panel on Climatic Variation to study the problem and to submit
recommendations appropriate for climatic objectives of garp observa-
tional programs, particularly the First garp Global Experiment (fgge)
planned for 1978. The Panel's charge was subsequently enlarged to
include recommendations for the design and implementation of a national
climatic research program.
ix
X UNDERSTANDING CLIMATIC CHANGE
In its initial deliberations, the work of the Panel seemed logically to
fall into three categories, depending on the time scale of climatic varia-
tion. First, the shorter-period variations, of the order 101 to 10 years,
which are documented by modern instrumental observations; second, the
variations of intermediate length, of the order 10 to 103 years, which are
largely documented by historical and proxy data sources; and third, the
longer-period variations, of the order 103 years and beyond, for which
documentation comes from paleoclimatic and geological records. Three
subpanels were therefore formed, and a report was issued in February
1973 by the subpanel concerned with monthly to decadal time scales
(W. L. Gates, Chairman), which is the basis of the main body of the
present report. The deliberations of the subpanels concerned with
decadal to millenial changes (J. M. Mitchell, Chairman) and with
millenial changes and beyond (W. S. Broecker, Chairman) were the
basis of Appendix A of this report.
From the beginning of the Panel's work it was realized that it would
be necessary to address a wide range of questions involving the use of
climatic data from instrumental and proxy sources, the use of numerical
simulation models, and the conduct of research on the physical mecha-
nisms of climatic change. It was also obvious in undertaking an assign-
ment of this magnitude that the Panel would not be able to refer to the
large number of studies that have an important bearing on the problem
of climatic variation. We have, therefore, generally cited only those
works that were useful in framing our recommendations and in making a
brief overview of present research (see Chapter 5). Some of our
recommendations have been made previously by other groups [see, for
example, C. L. Wilson (Chairman), 1971: Study of Man's Impact on
Climate (smic) Report, Inadvertent Climate Modification, W. H.
Matthews, W. W. Kellogg, and G. D. Robinson, eds. Massachusetts
Institute of Technology, Cambridge, Mass.], and we are also aware that
the problem of climatic change has been considered by several other
groups and is of concern to other committees of garp.
In addition to the contributions of the Panel's members, a number of
consultants to the Panel also made valuable contributions: J. E. Kutz-
bach and A. H. Oort on the observational and statistical aspects of
climatic change; C. E. Leith on the question of climatic predictability;
and R. C. J. Somerville on the evaluation of numerical model perfor-
mance. A. R. Robinson of Harvard University also contributed material
on the role of the oceans in climatic change. Useful comments on various
aspects of the Panel's work were also made by S. H. Schneider of the
National Center for Atmospheric Research; by E. W. Bierly, J. O.
Fletcher, and U. Radok of the National Science Foundation; by R. S.
PREFACE XI
Lindzen of Harvard University; by R. J. Reed and R. G. Fleagle of the
University of Washington; and by J. Smagorinsky of the Geophysical
Fluid Dynamics Laboratory. The organization and preparation of the
report as a whole was undertaken by W. L. Gates.
Appendix A, which is a survey of past climatic variations, was pre-
pared principally by J. Imbrie, W. S. Broecker, J. M. Mitchell, Jr., and
J. E. Kutzbach. The portions of this Appendix concerned with den-
drochronology were prepared by H. C. Fritts, and those concerned with
glaciology by G. H. Denton. Unpublished data and figures used in this
Appendix were also kindly supplied by A. H. Oort; C. Sancetta of Oregon
State University; A. Mclntyre, J. D. Hays, and G. Kukla of the Lamont-
Doherty Geological Observatory; V. C. LaMarche of the University of
Arizona; J. Kennett of the University of Rhode Island; and T. Kellogg,
N. G. Kipp, R. K. Matthews, and T. Webb of Brown University.
Appendix B, which presents a comparative review of selected climate
simulation capabilities of global general circulation models, was prepared
principally by W. L. Gates, K. Bryan, and W. M. Washington. Valuable
comments and contributions of unpublished material were also made by
S. Manabe, R. C. J. Somerville, Y. Mintz, and R. C. Alexander of The
Rand Corporation.
This report makes no claim to completeness, and many important mat-
ters are not touched upon. For example, we have not considered the
questions of instrumental design and logistical support necessary to carry
out the observational programs that we have recommended, nor have
we dealt with the training and educational activities necessary to supply
the additional scientific manpower. Although we have presented some
thoughts on possible organizational arrangements for the conduct of the
necessary research, and have made some preliminary cost estimates, such
questions were regarded as being outside the scope of the Panel's im-
mediate objectives and responsibility.
The principal purpose of this report is to recommend a comprehen-
sive research program, which we feel is necessary to increase significantly
our understanding of climatic variation, and the Panel will consider its
efforts to have been successful if the report serves as a useful planning
document to this end. In making its recommendations, the Panel is
aware of what has been called the problem of "(don't know)," 2 i.e.,
those who are called on to implement the program may not know that
we don't know the answers to the central questions. The presentation of
this report at least makes it clear that we don't know, and thereby re-
duces the exponent to unity. The successful execution of the program
should remove at least part of the remaining "don't know." In short, we
have attempted to describe here what should be done, and recognize
XII UNDERSTANDING CLIMATIC CHANGE
that what can be done and then what actually will be done remain to
be determined.
We wish to acknowledge the valuable advice and assistance of John
R. Sievers of the National Research Council and of Verner E. Suomi of
the University of Wisconsin throughout the preparation of this report.
We are also indebted to Viv Pickelsimer of The Rand Corporation for
her efficient handling of many of the details of the Panel's work and the
preparation of the typescript.
W. LAWRENCE GATES
YALE MINTZ
Co-Chairmen, Panel on Climatic Variation
CONTENTS
INTRODUCTION
Limits of Our Present Knowledge / 2
Need for Data; Need for Understanding; Need for Assessment
Future Efforts and Resources / 5
Research Approaches; The Question of Priorities
Purposes and Contents of This Report / 7
2 SUMMARY OF PRINCIPAL CONCLUSIONS AND
RECOMMENDATIONS
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 13
Climatic System / 13
Components of the System; Physical Processes of Climate; Defi-
nitions
Causes of Climatic Change / 20
Climatic Boundary Conditions; Climatic Change Processes and
Feedback Mechanisms; Climatic Noise
Role of the Oceans in Climatic Change / 25
Physical Processes in the Ocean; Modeling the Oceanic Circulation
Simulation and Predictability of Climatic Variation / 28
Climate Modeling Problem; Predictability and the Question of
Transitivity; Long-Range or Climatic Forecasting
xiii
XIV UNDERSTANDING CLIMATIC CHANGE
4 PAST CLIMATIC VARIATIONS AND THE PROJECTION
OF FUTURE CLIMATES 35
Importance of Studies of Past Climates / 35
Record of Instrumentally Observed Climatic Changes / 36
Historical and Paleoclimatic Record / 37
Nature of the Evidence; Summary of Paleoclimatic History
Inference of Future Climates from Past Behavior / 40
Natural Climatic Variations; Man's Impact on Climate
SCOPE OF PRESENT RESEARCH ON CLIMATIC
VARIATION 46
Climatic Data Collection and Analysis / 46
Atmospheric Observations; Oceanic and Other Observations; Ob-
servational Field Programs
Studies of Climate from Historical Sources / 49
Studies of Climate from Proxy Sources / 50
General Syntheses; Chronology; Monitoring Techniques; Proxy
Data Records and Their Climatic Inferences; Institutional Pro-
grams
Physical Mechanisms of Climatic Change / 54
Physical Theories and Feedback Mechanisms; Diagnostic and Em-
pirical Studies; Predictability and Related Theoretical Studies
Numerical Modeling of Climate and Climatic Variation / 56
Atmospheric General Circulation Models and Related Studies;
Statistical-Dynamical Models and Parameterization Studies;
Oceanic General Circulation Models; Coupled General Circulation
Models
Applications of Climate Models / 59
Simulation of Past Climates; Climate Change Experiments and
Sensitivity Studies; Studies of the Mutual Impacts of Climate and
Man
A NATIONAL CLIMATIC RESEARCH PROGRAM 62
The Approach / 63
What Climatic Events and Processes Can We Now Identify?; Why
Is a Program Necessary?
The Research Program (ncrp) / 66
Data Needed for Climatic Research; Research Needed on Climatic
Variation; Needed Applications of Climatic Studies
The Plan / 94
Subprogram Identification; Facilities and Support; Timetable and
Priorities within the Program; Administration and Coordination
CONTENTS XV
A Coordinated International Climatic Research Program
(icrp) / 105
Program Motivation and Structure; Program Elements; Program
Support
REFERENCES 111
APPENDIX A: SURVEY OF PAST CLIMATES 127
Introduction / 127
Nature of Paleoclimatic Evidence; Instrumental and Historical
Methods of Climate Reconstruction; Biological and Geological
Methods of Climate Reconstruction; Regularities in Climatic Series
Chronology of Global Climate / 148
Period of Instrumental Observations; The Last 1000 Years; The
Last 5000 Years; The Last 25,000 Years; The Last 150,000 Years;
The Last 1,000,000 Years; The Last 100,000,000 Years; The Last
1,000,000,000 Years
Geographic Patterns of Climatic Change / 1 63
Structure Revealed by Observational Data; Structure Revealed by
Paleoclimatography
Summary of the Climatic Record / 1 79
Future Climate : Some Inferences from Past Behavior / 1 82
Potential Contribution of Sinusoidal Fluctuations of Various Time
Scales to the Rate of Change of Present-Day Climate; Likelihood
of a Major Deterioration of Global Climate in the Years Ahead
References 190
APPENDIX B: SURVEY OF THE CLIMATE SIMULATION
CAPABILITY OF GLOBAL CIRCULATION MODELS 196
Introduction / 196
Development and Uses of Numerical Modeling / 198
Atmospheric General Circulation Models / 201
Formulation; Solution Methods; Selected Climatic Simulations
Oceanic and Coupled Atmosphere-Ocean General Circulation
Models / 218
Formulation; Solution Methods; Selected Climatic Simulations;
Coupled Ocean-Atmosphere Models
References 236
1
INTRODUCTION
Climatic change has been a subject of intellectual interest for many
years. However, there are now more compelling reasons for its study:
the growing awareness that our economic and social stability is pro-
foundly influenced by climate and that man's activities themselves may
be capable of influencing the climate in possibly undesirable ways. The
climates of the earth have always been changing, and they will doubtless
continue to do so in the future. How large these future changes will be,
and where and how rapidly they will occur, we do not know.
A major climatic change would force economic and social adjustments
on a worldwide scale, because the global patterns of food production
and population that have evolved are implicitly dependent on the climate
of the present century. It is not primarily the advance of a major ice
sheet over our farms and cities that we must fear, devastating as this
would be, for such changes take thousands of years to evolve. Rather,
it is persistent changes of the temperature and rainfall in areas com-
mitted to agricultural use, changes in the frost content of Canadian and
Siberian soils, and changes of ocean temperature in areas of high nutri-
ent production, for example, that are of more immediate concern. We
know from experience that the world's food production is highly de-
pendent on the occurrence of favorable weather conditions in the
"breadbasket" areas during the growing seasons. Because world grain
reserves are but a few percent of annual consumption, an unfavorable
crop year, such as occurred in the Ukraine in 1972, has immediate inter-
national consequences. The current drought in parts of Asia and in
£ UNDERSTANDING CLIMATIC CHANGE
central Africa is producing severe hardship and has already caused the
migration of millions of people.
As the world's population grows and as the economic development
of newer nations rises, the demand for food, water, and energy will
steadily increase, while our ability to meet these needs will remain sub-
ject to the vagaries of climate. Most of the world's land suitable for
agriculture or grazing has already been put to use, and many of the
world's fisheries are being exploited at rates near those of natural re-
plenishment. As we approach full utilization of the water, land, and air,
which supply our food and receive our wastes, we are becoming in-
creasingly dependent on the stability of the present seemingly "normal"
climate. Our vulnerability to climatic change is seen to be all the more
serious when we recognize that our present climate is in fact highly
abnormal, and that we may already be producing climatic changes as a
result of our own activities. This dependence of the nation's welfare, as
well as that of the international community as a whole, should serve
as a warning signal that we simply cannot afford to be unprepared for
either a natural or man-made climatic catastrophe.
Reducing this climatic dependency will require coordinated man-
agement of the nation's resources on the one hand and a thorough
knowledge of the climate's behavior on the other. It is therefore essential
that we acquire a far greater understanding of climate and climatic
change than we now possess. This knowledge will permit a rational
response to climatic variations, including the systematic assessment
beforehand of man-made influences upon the climate and will make pos-
sible an orderly economic and social adjustment to changes in climate.
LIMITS OF OUR PRESENT KNOWLEDGE
Although we have considerable knowledge of the broad characteristics
of climate, we have relatively little knowledge of the major processes
of climatic change. To acquire this knowledge it will be necessary to
use all the research tools at our disposal. We must also study each com-
ponent of the climatic system, which includes not only the atmosphere
but the world's oceans, the ice masses, and the exposed land surface
itself. Only in this way can we expect to make significant advances in
our understanding of the elusive and complex processes of climatic
change.
Need for Data
Observations are essential to the development of an understanding of
climatic change; without them, our theories will remain theories and
INTRODUCTION O
the potential uses of our models will remain untapped. Our observa-
tional records must be extended in both space and time, so that we
can adequately document the climatic events that have occurred in the
past, and so that we can monitor the climatically important physical
processes that are now going on around us. Much of the present climatic
data are of limited availability and need to put into forms that permit
the systematic determination of appropriate climatic statistics and the
assessment of the practical consequences of climatic variation. It is
especially important that climatic data be organized and assembled
to permit their use in conjunction with dynamical climate models.
The oceans in particular exert a powerful influence on the earth's
climates, yet we have inadequate oceanographic observations on the
space and time scales needed for climatic studies. The important heat,
moisture, and momentum exchanges that occur at the sea surface, and
the corresponding transports that occur within the ocean, are not at
all well known. Recent observations from the Mid-ocean Dynamics Ex-
periment (mode) reveal energetic oceanic mesoscale motions at sub-
surface levels, and our ignorance becomes even greater than we thought
it was.
The present international network of conventional meteorological
observations has grown largely in response to the need for weather fore-
casts, while most oceanographic data have been collected from ships
widely separated in space and time. For the proposed research program,
these data must be supplemented by truly global observations of the
large-scale geophysical boundary conditions and of the physical pro-
cesses that are important in climatic change. It is here that satellite
observations are expected to play a key role, as they offer an unparalleled
opportunity to monitor a growing list of variables, such as cloudiness,
temperature, and the extent of ice and snow. Other climatically im-
portant variables will require special monitoring programs, on either
a global or regional basis. It is essential, moreover, that the relevant
data be collected on a long-term basis in order to acquire the necessary
statistics of climate.
Need for Understanding
Our knowledge of the mechanisms of climatic change is at least as
fragmentary as our data. Not only are the basic scientific questions
largely unanswered, but in many cases we do not yet know enough to
pose the key questions. What are the most important causes of climatic
variation, and which are the most important or most sensitive of the
many processes involved in the interaction of the air, sea, ice, and land
components of the climatic system? Although there is evidence of a
4 UNDERSTANDING CLIMATIC CHANGE
strong coupling between the atmosphere and the ocean, for example, we
cannot yet say that we understand much about its consequences for
climatic change. There are also indications in paleoclimatic data that
the earth's climates may be significantly influenced by the long-term
astronomical variations of the sun's radiation received at the top of
the atmosphere. But here again we do not yet understand the processes
that may be involved.
There is no doubt that the earth's climates have changed greatly in
the past and will likely change in the future. But will we be able to
recognize the first phases of a truly significant climatic change when
it does occur? Like the familiar events of daily weather, from which the
climate is derived, climatic changes occur on a variety of space scales.
These range from the change of local climate resulting from the removal
of a forest, for example, to regional or global anomalies resulting from
shifts of the pattern of the large-scale circulation. But unlike the
weather, variations of climate take place relatively slowly, and we may
think in terms of yearly, decadal, and millenial climatic changes. But
the system is complex, and the search for order in the climatic record
has only begun.
Even the barest outline of a theory of climate must address the key
question of the predictability of climatic change. This question is
closely tied to the limited predictability of the weather itself and to
the predictability of the various external boundary conditions and inter-
nal transfer processes that characterize the climatic system. Although
there is evidence of regularity on some time scales, the climatic record
includes many seemingly irregular variations of large amplitude. How
do we separate the genuine climatic signal from what may be un-
predictable "noise," and to what extent are the noise and signal coupled?
These are important questions, and ones to which there are no ready
answers. The determination of the climate's predictability will require the
further development and application of both theory and dynamical
models, along with a greatly expanded data base. The answers, when
they are found, will determine the limit to which we can hope to predict
future climatic variations.
Special attention must be paid to the fundamental role of the world's
oceans in controlling the climate. The oceans not only are the primary
source of the water in the atmosphere and on the land, but they consti-
tute a vast reservoir of thermal energy. The timing and location of the
exchange of this energy with the overlying air has a profound effect on
the more rapidly varying atmospheric circulation. When the dynamics
of this ocean-atmosphere interaction are better known, we may find
that the ocean plays a more important role than the atmosphere in
climatic changes.
INTRODUCTION
Need for Assessment
We should add to these limits of our present knowledge the lack of
comprehensive assessment of the impacts of climatic variation on human
affairs. No one doubts that there are such impacts, for the specter of
drought and the consequences of persistently severe winter weather are
all too familiar in many parts of the world. Even so, we must admit that
we cannot now adequately answer the question: What is a change of
climate worth? A farmer may know what knowledge of the climatic
conditions of the next growing season would be worth to him, but the
answer in terms of national and international resource planning is more
elusive. This lack of assessment is brought into sharper focus when we
attempt to discern the economic and social consequences of possible
alternative future climates.
FUTURE EFFORTS AND RESOURCES
Research Approaches
Our future efforts must be guided by the realization that climatic
changes in any one part of the world are manifestations of changes in
the global climatic system. Since our fundamental goal is to increase
our understanding of climatic variations to the point where we may
predict (and possibly even control) them, we must subject our ideas
to quantitative test wherever possible.
The recent development of satellite-based observing systems, the
coming of a new generation of high-speed computers, and the emergence
of models suitable for climatic simulation combine to make such an
undertaking feasible at this time. The importance of climatic variations
requires, moreover, that we use all methods of inquiry that are likely to
yield useful information, and that we do so at the earliest possible time.
The principal approaches to the problem that are available to us
are shown in Figure 1.1, and we recognize the importance of maintaining
a balance of effort among them. These same approaches form the ele-
ments of the climatic research program recommended in this report and
broadly cover what we believe to be the needed efforts for observation,
analysis, modeling, and theory. The successful execution of the program
will require contributions from the physical sciences of meteorology,
oceanography, glaciology, hydrology, astronomy, geology, and paleontol-
ogy and from the biological and social sciences of ecology, geography,
archeology, history, economics, and sociology. A program of this sort
calls for a long-term commitment from the scientific research com-
munity, from the sponsoring government agencies, and from the public.
UNDERSTANDING CLIMATIC CHANGE
Monitoring
What is now .
\ going on? /
Numerical \
Models \
/ Empirical
/ Studies
What is shown
How does the
by climatic .
simulations? /
/Climatic \
/ Data Analysis\
^ What has V
V system work?
\ happened in /
/ Future
Theoretical \
\the past? /
/ Climates
Studies \
How and when
How much do
v is the climate
we really /
\ going to
understand? /
/ Climatic \
/ Impacts \
What does it
all mean to
man?
\ change?
FIGURE 1.1 The interdependence of the major components of a
climatic research program and a number of key questions.
The Question of Priorities
The various components of the recommended climatic research program
(fully described in Chapter 6) are to a great extent interdependent:
data are needed to check the coupled general circulation models and to
calibrate the simpler models; the models are needed to test hypotheses
and to project future climates; monitoring is needed to check the pro-
jections; and all are needed to assess the consequences. The question of
priorities then becomes a matter of the priority of questions (see
Figure 1.1), and there appear to be no a priori easy guidelines to relative
importance.
Our priorities are reflected in those actions and activities that we
recommend be implemented at once and in those subsequent activities
INTRODUCTION /
for which planning should begin as soon as possible. While anticipating
that much further planning will be necessary to implement the complete
program, we urge that the essential interdependence of the various
efforts be recognized and that all aspects of the problem be given support
as parts of a coherent research program.
PURPOSES AND CONTENTS OF THIS REPORT
Broadly speaking, the purposes of this report are twofold: first, to
advise the United States Government through the National Research
Council's United States Committee for garp on the urgent need for a
coherent national research program on the problem of climatic variation;
and, second, to advise on the steps necessary to address the same prob-
lem in the international scene.
As noted previously, our response to the Government is the recom-
mendation of a broadly based National Climatic Research Program
(ncrp), whose goal is the resolution of the problem of climatic varia-
tion. This program is presented in detail in Chapter 6, and its adoption
is the first of our major national recommendations summarized in
Chapter 2. In view of the possibly great impacts of future climatic varia-
tions on the nation's welfare, we believe that it is our responsibility
to call for a national commitment to this effort. We accordingly urge
strongly that resources to carry out such a program be made available
at the earliest possible time, including provision for the necessary ob-
servations, computers, and research facilities.
Our further response to the appropriate international bodies is the
proposal of a coordinated International Climatic Research Program
(icrp), which we believe to be a suitable mechanism for the pursuit
of the climatic aspects of garp. As discussed in Chapter 6, we view this
as a new program of considerably greater breadth than the present garp
activities, but one for which the garp is a necessary prelude. The U.S.
national program (ncrp) would form an integral part of the icrp, as
would the national programs of other countries. In addition, we recom-
mend a number of supporting programs whose observational require-
ments may impact on the First garp Global Experiment scheduled for
1978-1979.
The remainder of this report consists of ( 1 ) a summary of our princi-
pal conclusions and recommendations (Chapter 2); (2) a discussion
of the physical basis of climate and climatic change (Chapter 3); (3) a
summary of past climatic variations as drawn from the instrumental and
paleoclimatic record (Chapter 4); (4) a brief review of the scope of
present research on climatic variation (Chapter 5); and (5) the pro-
8 UNDERSTANDING CLIMATIC CHANGE
posed climatic research program (Chapter 6). Two technical appendixes
prepared specially for this report present further details of the record
and interpretation of past climates (Appendix A) and a brief com-
parative review of the ability of present atmospheric and oceanic gen-
eral circulation models to simulate selected climatic variables (Ap-
pendix B).
2
SUMMARY OF PRINCIPAL CONCLUSIONS
AND RECOMMENDATIONS
The principal conclusions and recommendations that have resulted
from the deliberations of this Panel, which are expanded upon else-
where in this report, may be summarized as follows:
1. To meet present and future national needs and to further the
national contribution to garp, we strongly recommend the immediate
adoption and development of a coherent National Climatic Research
Program (ncrp) with appropriate international coordination. The
major subprograms of the ncrp are summarized in Recommendations
2, 3, and 4.
2. To perform the needed analysis of selected climatic data, includ-
ing that from conventional instruments and satellites, historical records,
and paleoclimatic data sources, we recommend the establishment of
a Climatic Data Analysis Program (cdap) as a subprogram of the ncrp.
This program's functions would be to facilitate and coordinate the
preparation and maintenance of a comprehensive climatic data inven-
tory, the development of selected climatic data banks, and the prepara-
tion of suitable data analyses, based on both current and paleoclimatic
data.
To carry out these functions we recommend the development of new
climatic data-analysis facilities with access to suitable computing and
data processing and display equipment, as components of a national
network for climatic data analysis. We envisage these facilities as work-
ing closely with the various specialized climatic data depositories and
10 UNDERSTANDING CLIMATIC CHANGE
as an essential mechanism for the successful execution of the cdap and
of related components of the overall national program.
In response to immediate practical needs, we recommend the initia-
tion and continued support of empirical and statistical studies of the
impacts of climatic change on man's food, water, and energy supplies.
Support should also be given to studies of the broader social and eco-
nomic consequences of climatic variations.
3. To acquire the needed data on the important boundary conditions
and physical processes of climate, we recommend the development of a
global Climatic Index Monitoring Program (cimp) as a second subpro-
gram of the ncrp. This program's functions would include the monitor-
ing and collection, on appropriate climatic time and space scales, of data
on the components of the global heat balance (including the solar
constant), the ocean-surface temperature and the thermal structure of
the surface mixed layer, the extent of ice and snow cover and other land-
surface characteristics, the atmospheric composition and turbidity,
anthropogenic processes, and, if possible, ocean-current transports and
components of the hydrological cycle. This program will require a num-
ber of new observational schemes in the atmosphere, in the ocean, and
on land and will rely heavily on environmental satellites. We anticipate
that such data will also have important uses on a real-time basis and
that the cimp could serve as a national watchdog for climatic change.
4. To accelerate research on climatic variation, and to support the
needed development of climatic modeling on a broad front, we recom-
mend the establishment of a Climatic Modeling and Applications Pro-
gram (cmap) as a third subprogram of the ncrp. In this program,
emphasis should be given to the development of coupled global climate
models (cgcm's) of the combined atmospheric and oceanic general
circulation and to the improvement of the models' treatment of clouds,
mesoscale processes, and boundary-layer phenomena. Attention should
also be given to the processes of air-sea interaction and to treatment
of the ocean's surface layer, sea ice, and the oceanic mesoscale phe-
nomena. We note the importance of extended model integrations to
determine the annual and interannual variability of simulated climates
and urge that appropriate studies be made of the sensitivity of simulated
climates to physical and numerical uncertainties in the models'
formulation.
To provide the basis for the needed further modeling of climatic
variation, we recommend the development and support of a wide variety
of statistical-dynamical and other parameterized climate models. We
note the importance of calibration in such models and urge that ap-
propriate schemes be developed to permit extended climatic simula-
tions which include oceanic and cryospheric variables.
SUMMARY OF PRINCIPAL CONCLUSIONS AND RECOMMENDATIONS 11
To provide the needed further insight into the mechanisms of climatic
variation, we recommend the application of climatic models in support
of empirical and diagnostic studies, with particular attention to the roles
of climatic feedback processes in the coupled ocean-atmosphere sys-
tem, to the questions of climatic predictability and transitivity, and to
the climatic effects of changes in the geophysical boundary conditions.
To provide the needed reconstruction of past climates and to develop
a broader calibration of climate models, we recommend the initiation
and support of systematic efforts to reconstruct selected events and
periods in the climatic history of the earth. This should include the ap-
plication of the cgcm's to simulate selected equilibrium paleoclimates
and the use of statistical-dynamical or other parameterized climate
models to infer the time-dependent evolution of the coupled atmosphere-
hydrosphere-cryosphere climatic system.
To further the needed application of climatic models, we recommend
the systematic exploration with suitable climate models of a variety of
possible future climates, due either to natural or man-made causes.
These should include determination of the likely effects of changes in
solar radiation, land-surface character, cloudiness, pollution, and ice
extent. We urge that efforts be made to extract consistent physical
hypotheses from such experiments and that the necessary statistical
controls be developed.
To lay the basis for the needed assessment of the possibilities of
long-range or climatic forecasting, we further recommend the applica-
tion of climate models of all types in experimental integrations using
observed initial and boundary conditions. Appropriate climatic statistics
should be drawn from such integrations and compared with observation
insofar as possible, in order to establish the models' usefulness as long-
range forecast tools. Initial emphasis should be given to the time periods
of seasons to decades, for which there is presently the greatest practical
need for scientifically based information.
To assist in the performance of the needed research or climatic
modeling and applications, we recommend that efforts be made to
identify or form a number of cooperative research associations or cli-
matic research consortia, which we view as natural and useful co-
ordinating mechanisms for the effective performance and long-range
stability of the ncrp. We further recommend that the period prior to
1980 be used to develop additional scientific and technical manpower
through the establishment and support of fellowships in appropriate
areas of climatic research.
5. In order to further the aims of the international garp efforts
directed to the problem of climate and climatic variation, we recommend
the adoption and development of an International Climatic Research
12 UNDERSTANDING CLIMATIC CHANGE
Program (icrp). By the very nature of climate, the U.S. national pro-
gram is considered an integral part of the icrp, along with the climatic
research programs of other nations. In view of the differences of the
observational time scales and of the variables involved in weather fore-
casting and climatic studies, and in view of the latter's broadly inter-
disciplinary character, we visualize such a program being the logical
successor to garp in matters relating to climate. Recognizing that the
elements of the ncrp recommended above could equally well apply
to an international program, we suggest that they be considered by
the appropriate international organizations.
To help provide the observational framework needed for climatic
research, we recommend the designation of the period 1980-2000 as
International Climatic Decades (icd). During this period, efforts should
be made to secure broad international cooperation in the collection,
analysis, and exchange of all available climatic data, including con-
ventional observations and special data sets of particular climatic in-
terest (such as during droughts and following volcanic eruptions). Dur-
ing the icd we also recommend the initiation and support of regional
climatic studies in order to describe and model local climatic anomalies
of special interest.
We further recommend development of appropriate national and in-
ternational training programs and educational activities in order to
promote the participation of all nations in climatic research.
6. To provide the global paleoclimatic data needed for the recon-
struction of past climates, we recommend the development of an Inter-
national Paleoclimatic Data Network (ipdn) as a subprogram of the
icrp. This program should aim to assist each nation in the cooperative
identification, extraction, analysis, monitoring, and exchange of its
unique paleoclimatic records, such as those from tree rings, soil types,
fossil pollen, and data on sea and lake levels.
3
PHYSICAL BASIS OF CLIMATE
AND CLIMATIC CHANGE
CLIMATIC SYSTEM
The term climate usually brings to mind an average regime of weather.
The climatic system, however, consists of those properties and processes
that are responsible for the climate and its variations and are illustrated
in Figure 3.1. The properties of the climatic system may be broadly
classified as thermal properties, which include the temperature of the
air, water, ice, and land; kinetic properties, which include the wind and
ocean currents, together with the associated vertical motions, and the
motion of ice masses; aqueous properties, which include the air's
moisture or humidity, the cloudiness and cloud water content, ground-
water, lake levels, and the water content of snow and of land and sea
ice; and static properties, which include the pressure and density of the
atmosphere and ocean, the composition of the (dry) air, the oceanic
salinity, and the geometric boundaries and physical constants of the
system. These variables are interconnected by the various physical
processes occurring in the system, such as precipitation and evaporation,
radiation, and the transfer of heat and momentum by advection, con-
vection, and turbulence.
Components of the System
In general terms the complete climatic system consists of five physical
components — the atmosphere, hydrosphere, cryosphere, lithosphere,
and biosphere, as follows :
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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 15
The atmosphere, which comprises the earth's gaseous envelope, is
the most variable part of the system and has a characteristic response
or thermal adjustment time of the order of a month. By this we mean
that the atmosphere, by transferring heat vertically and horizontally,
will adjust itself to an imposed temperature change in about a month's
time. This is also approximately the time it would take for the atmo-
sphere's kinetic energy to be dissipated by friction, if there were no
processes acting to replenish this energy.
The hydrosphere, which comprises the liquid water distributed
over the surface of the earth, includes the oceans, lakes, rivers, and
the water beneath the earth's surface, such as groundwater and sub-
terranean water. Of these, the world's oceans are the most important for
climatic variations. The ocean absorbs most of the solar radiation that
reaches the earth's surface, and the oceanic temperature structure repre-
sents an enormous reservoir of energy due to the relatively large mass
and specific heat of the ocean's water. The upper layers of the ocean
interact with the overlying atmosphere on time scales of months to years,
while the deeper ocean waters have thermal adjustment times of the
order of centuries.
The cryosphere, w '^h comprises the world's ice masses and snow
deposits, includes the continental ice sheets, mountain glaciers, sea
ice, surface snow cover, and lake and river ice. The changes of snow
cover on the land are mainly seasonal and are closely tied to the
atmospheric circulation. The glaciers and ice sheets (which represent
the bulk of the world's freshwater storage) respond much more slowly.
Because of their great mass, these systems develop a dynamics of their
own, and they show significant changes in volume and extent over
periods ranging from hundreds to millions of years. Such variations are,
of course, closely related to the global hydrologic balance and to varia-
tions of sea level (see Appendix A) .
The lithosphere, which consists of the land masses over the surface
of the earth, includes the mountains and ocean basins, together with
the surface rock, sediments, and soil. These features change over the
longest time scales of all the components of the climatic system, ranging
up to the age of the earth itself. The processes of continental drift
and sea-floor spreading, which have resulted in mountain building and
in changes in the shapes and depths of the oceans, occur over tens and
hundreds of millions of years. These events are not generally regarded
as representing the same kind of interaction with other components of
the system as the variations described above. We note, however, that
there may be a significant relationship between the occurrence of major
glacial periods and the times when continental land masses occupied
16 UNDERSTANDING CLIMATIC CHANGE
positions near the rotational poles of the earth (see Appendix A).
The processes of isostatic adjustment and the accumulation of deep-
ocean sediments also represent significant changes of the lithosphere,
and as such may be viewed as earth-ice-ocean interactions. The intro-
duction of volcanic debris into the atmosphere and its subsequent dis-
persal may also be cited as an example of earth-air interaction.
The biosphere includes the plant cover on land and in the ocean and
the animals of the air, sea, and land, including man himself. Although
their response characteristics differ widely, these biological elements
are sensitive to climate and, in turn, may influence climatic changes.
It is from the biosphere that we obtain most of the data on paleoclimates
(see Appendix A). Natural changes in surface vegetation occur over
periods ranging from decades to thousands of years in response to
changes in temperature and precipitation and, in turn, alter the surface
albedo and roughness, evaporation, and ground hydrology. Changes in
animal populations also reflect climatic variations through the avail-
ability of suitable food and habitat. The anthropogenic changes due
to agriculture and animal husbandry are not known but may well be
appreciable in altering at least regional climates.
Physical Processes of Climate
The climate at any particular time represents in some sense the average
of the various elements of weather, along with the state of the other
components of the system. The physical processes responsible for
climate (as distinct from climatic change) are therefore basically the
same as those responsible for weather. These processes are expressed
in quantitative fashion by the dynamical equation of motion, the thermo-
dynamic energy equation, and the equations of mass and water substance
continuity, as applied to the atmosphere and ocean (see Appendix B).
A process of primary importance for the circulation of the atmosphere
and ocean is the rate at which heat is added to the system, the ultimate
source of which is the sun's radiation. The atmosphere and ocean re-
spond to this heating by developing winds and currents, which serve to
transport heat from regions where it is received in abundance, such as
in the equatorial and tropical areas, to regions where relatively little
radiation is received, such as the polar regions of the earth. In this
way, the atmosphere and ocean maintain the overall global balance
of heat. A great deal of this heat is transported by the disturbances re-
sponsible for much of our weather in middle and high latitudes, and
similar disturbances may occur in the ocean. These eddies of the general
circulation also participate in the transports necessary to maintain the
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 17
global balances of momentum, mass, and the total quantity of water
substance.
While this simple view is a fair summary of our basic understanding
of the general circulation, it is not without shortcomings. For example,
it does not consider the basically different circulation regime in the
low latitudes or the role of convective phenomena, and it does not
consider the important variations of the circulation with height. It
might also be noted that for other combinations of the planetary size
and rotation rate, atmospheric composition, and meridional heating
gradient, such as occur on other planets, an altogether different circula-
tion regime — and hence climate — could result.
Although the equations referred to above are fundamental in that
they form the basis of our ability to simulate numerically the climate
with dynamical models, they are not in themselves particularly reveal-
ing as far as the more subtle physical processes of climate are con-
cerned, to say nothing of the processes of climatic change. The heating
rate is itself highly dependent on the distribution of the temperature and
moisture in the atmosphere and owes much to the release of the latent
heat of condensation during the formation of clouds and to the subse-
quent influence of the clouds on the solar and terrestrial radiation.
These processes, together with others that contribute to the overall heat
balance of the atmosphere, are shown in Figure 3.2, in which data
derived from recent satellite observations have been incorporated (see,
for example, Vonder Haar and Suomi, 1971). Here the presence of
clouds, water vapor, and C02 is seen to account for over 90 percent of
the long-wave radiation leaving the earth-ocean-atmosphere system.
This effective blocking of the radiation emitted by the earth's surface,
commonly referred to as the greenhouse effect, permits a somewhat
higher surface temperature than would otherwise be the case. It is
interesting that this important effect is achieved by gases in the at-
mosphere that exist in near trace amounts.
We see from Figure 3.2 that the role played by clouds is an important
one: the reflection and emission from clouds accounts for about 46 per-
cent of the total radiation leaving the atmosphere; and in terms of the
shortwave radiation alone, clouds account for two thirds of the
planetary albedo. The largest single heat source for the atmosphere is
that supplied by the release of the latent heat of condensation, and this
is particularly important in the lower latitudes. There is also an ap-
preciable supply of sensible heat from the oceans, especially in the
middle and higher latitudes. It is therefore clear that water substance, in
either vapor or droplet form, plays a dominant role in the atmospheric
heat balance. And when we recall that the oceans themselves absorb
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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 19
most of the solar radiation reaching the surface, and that the presence
of ice and snow also affect the heat balance, the climatic dominance
of global water substance becomes overwhelming, even if ice is not
taken into account.
Definitions
It is useful at this point to introduce a number of definitions related
to climate and climatic change. In what may be called the "common"
definition, climate is the average of the various weather elements, usually
taken over a particular 30-year period. A more useful definition is
what we shall call the "practical" definition, which introduces the con-
cept of a climatic state. This and related definitions are as follows:
Climatic state. This is defined as the average (together with the
variability and other statistics) of the complete set of atmospheric,
hydrospheric, and cryospheric variables over a specified period of time
in a specified domain of the earth-atmosphere system. The time interval
is understood to be considerably longer than the life span of individual
synoptic weather systems (of the order of several days) and longer than
the theoretical time limit over which the behavior of the atmosphere can
be locally predicted (of the order of several weeks) . We may thus speak,
for example, of monthly, seasonal, yearly, or decadal climatic states.
Climatic variation. This is defined as the difference between climatic
states of the same kind, as between two Januaries or between two
decades. We may thus speak, for example, of monthly, seasonal, yearly,
or decadal climatic variations in a precise way. The phrase "climatic
change" is used in a more general fashion but is generally synonymous
with this definition.
Climatic anomaly. This we define as the deviation of a particular
climatic state from the average of a (relatively) large number of
climatic states of the same kind. We may thus speak, for example, of
the climatic anomaly represented by a particular January or by a par-
ticular year.
Climatic variability. This we define as the variance among a number
of climatic states of the same kind. We may thus speak, for example,
of monthly, seasonal, yearly, or decadal climatic variability. Although
it may be confusing, this definition of climatic variability includes the
variance of the variability of the individual climatic states.
The foregoing definitions are useful for two reasons. First, the con-
cept of climatic state preserves the essence of what is usually connoted
by climate, while circumventing troublesome problems of statistical
20 UNDERSTANDING CLIMATIC CHANGE
stability. Second, climatic states represent definite realizations or
samples of climate (rather than the climate per se) and are comparable
with the climates simulated by numerical general circulation experi-
ments. There are many other definitions in existence to distinguish
particular statistical characteristics of climate and climatic change
(such as climatic fluctuations, oscillations, periods, cycles, trends, and
rhythms). The above definitions are generally adequate for our pur-
poses, although we shall later consider another definition of climate
related to the climatic system. We shall also subsequently introduce
the concepts of climatic noise and climatic predictability. Except when
otherwise indicated, the use of the word "climate" in this report is to
be considered an abbreviation for climatic state.
It should be noted that we have included the oceans in the definition
of a climatic state, as well as information on other aspects of the physical
environment. The ensemble of statistics required to completely describe
a climatic state is presently available for only a few regions and for
limited periods of time. The climatic data-analysis and monitoring
programs recommended in Chapter 6 are intended to fill in as much of
the gap as possible with available data and to ensure that at least certain
critical data are systematically gathered for an extended period of time
in the future.
CAUSES OF CLIMATIC CHANGE
While the above discussion may describe the processes responsible for
the maintenance of climate, it is an inadequate description of the
processes involved in climatic change. Here we are on less secure
ground and must consider a wide range of possible interactions among
the elements of the climatic system. It is these interactions that are
responsible for the complexity of climatic variation.
Climatic Boundary Conditions
If we view the gaseous, liquid, and ice envelopes surrounding the
earth as the internal climatic system, we may regard the underlying
ground and the space surrounding the earth as the external system. The
boundary conditions then consist of the configuration of the earth's
crust and the state of the sun itself. Changes in these conditions can
obviously alter the state of the climatic system, i.e., they can be causes
of climatic variation.
Each of the external processes illustrated in Figure 3.1 may be used
to develop a climatic theory, on which basis one may attempt to explain
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 21
certain features of the observed climatic changes. For example, changes
of the distribution of solar radiation have been used since the time of
Milankovitch (1930) to explain the major glacial-interglacial cycles of
the order of 10 l to 10s years. Aside from the question of variations of
the sun's radiative output, variations of the earth's orbital parameters
produce changes in the intensity and geographical pattern of the seasonal
and annual radiation received at the top of the atmosphere and in the
length of the radiational seasons in each hemisphere. These effects,
which are known with considerable accuracy, have resulted in occasional
variations of the seasonal insolation regime several times larger than
those now experienced. These orbital elements (eccentricity, obliquity,
and precession) vary with periods averaging about 96,000 years, 41,000
years, and 21,000 years, respectively. Because the seasons themselves
represent substantial* climatic variations, such astronomical theories of
climatic change must be given careful consideration.
The separate question of the climatic effects of possible changes in
the sun's radiation (i.e., changes of the so-called solar constant) has a
much less firm physical basis. Not only are the measured short-period
variations of solar output quite small, but the repeated search for
climatic periodicities linked with the 11 -year and 80-year sunspot cycles
has not yielded statistically conclusive results. The question of still
longer-period solar variations cannot be adequately examined with
present data, although over periods of the order of hundreds of millions
of years the sun's radiation seems likely to have changed. The time
range of this and other possible causative factors of climatic change is
shown in Figure 3.3.
On time scales of tens of millions of years there are changes in the
shapes of the ocean basins and the distribution of continents as a
result of sea-floor spreading and continental drift (see Figure 3.3).
Over geological time, these processes must have resulted in substantial
changes of global climate. Just how much of the recorded paleoclimatic
variations may eventually be accounted for by such effects, however, is
not known, and applying climatic models to the systematic reconstruc-
tion of the earth's climatic history prior to about 10 million years ago
is an important component of the research program recommended in this
report (see Chapter 6). In such climatic reconstructions, the oceans
must be simulated along with the atmosphere, and eventually the ice
masses must also be reproduced. Accompanying the migration of the
land masses are the processes of mountain building, epeirogeny, iso-
static adjustment, and sea-level changes, all of which must also be taken
into account.
Yet another external cause of climatic variation is the changes in
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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 23
the composition of the atmosphere resulting from the natural chemical
evolution of the nitrogen, oxygen, and carbon dioxide content in re-
sponse to geological and biological processes, as well as from the efflu-
ents of volcanic eruptions. On shorter time scales, however, it is prob-
ably the injection of dust particles into the atmosphere by volcanoes that
has produced a more significant climatic effect by modifying the at-
mospheric radiation balance (see Figure 3.2). The progressive enrich-
ment of the atmospheric C02 content, which has occurred during this
century as a result of man's combustion of fossil fuels (amounting to an
increase of order 10 percent since the 1880's), must also be con-
sidered an external cause of climatic variation.
These considerations lead to the "physical" definition of climate
as the equilibrium statistical state reached by the elements of the at-
mosphere, hydrosphere, and cryosphere under a set of given and fixed
external boundary conditions. There is, of course, the possibility that a
true equilibrium may not be reached in a finite time due to the disparity
of the response times of the system's components, but this is neverthe-
less a useful definition. By progressively reducing the internal climatic
system to include only the atmosphere and ocean (in equilibrium with
the land and ice distribution ) , and then to include only the atmosphere
itself (in equilibrium with the ocean, ice, and land), a hierarchy of
climates may be defined which is useful for the analysis of questions of
climatic determinism.
Climatic Change Processes and Feedback Mechanisms
Important as the above processes may be for the longer-period varia-
tions of climate, there are other factors that may also produce climatic
change. These involve changes in the large-scale distribution of the
effective internal driving mechanisms for the atmosphere and ocean.
Variations of the global ice distribution, for example, have a sig-
nificant effect on the net heating of the atmosphere (by virtue of the
ice's effective control of the surface heat budget), and thereby may
change the meridional heating gradient that drives the atmospheric
(and oceanic) circulation. An equally significant change (for the
oceans, at least) may be introduced by widespread salinity variations,
as caused, for example, by the melting of ice. The salinity of the ocean
surface water is in turn closely related to the formation of relatively
dense bottom water, which by sinking and spreading fills the bulk of
the world's ocean basins.
Such processes may act as internal controls of the climatic system,
24 UNDERSTANDING CLIMATIC CHANGE
with time scales extending from fractions of a year to hundreds and
even thousands of years (see Figure 3.3). Some of these processes dis-
play a coupling or mutual compensation among two (or more) elements
of the internal climatic system. Such interactions or feedback mecha-
nisms may act either to amplify the value or anomaly of one of the
interacting elements (positive feedback) or to damp it (negative feed-
back). Because of the large number of degrees of freedom of the
ocean-atmosphere system (for the moment considering the ice distribu-
tion to be fixed), there are a large number of possible feedback
mechanisms within the ocean, within the atmosphere, and between the
ocean and the atmosphere. These same degrees of freedom, however,
invite a high risk of error in any qualitative analysis, and in some cases
equally plausible arguments of this sort lead to opposite conclusions.
Some of the more prominent feedback effects operate among the
shorter-period processes of climatic change, especially those concerning
the radiation balance over land and the energy balance over the ocean.
For example, a perturbation of the ocean-surface temperature may
modify the transfer of sensible heat to the overlying atmosphere, and
thereby affect the atmospheric circulation and cloudiness. These changes
may in turn affect the ocean-surface temperatures through changes in
radiation, wind-induced mixing, advection, and convergence (and may
subsequently affect the deep-ocean temperatures through geostrophic
adjustment to the convergence in the boundary layer). These processes
may result either in the enhancement or reduction of the initial anomaly
of sea-surface temperature. A number of studies have shown positive
feedback of this sort for several years' time in the North Pacific Ocean.
The greenhouse effect (in which the absorption of long- wave radiation
by water vapor produces a higher surface temperature), is probably the
best known example of a semipermanent positive feedback process, al-
though other positive feedbacks of climatic importance may be noted.
One of these is the snow cover-albedo-temperature feedback, in which
an increase of snow extent increases the surface albedo and thereby
lowers the surface temperature. This in turn (all else being equal)
further increases the extent of the snow cover. An example of negative
feedback is the coupling between cloudiness and surface temperature
noted earlier. In this scheme, an initial increase of surface temperature
serves to increase the evaporation, which is followed by an increase
of cloudiness. This in turn reduces the solar radiation reaching the
surface and thereby lowers the initial temperature anomaly. Here we
have ignored the effects of long-wave radiation and of advective pro-
cesses in both ocean and atmosphere, but these examples serve to illus-
trate the uncertainty that must be attached to such arguments.
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 25
While there is much evidence to support the existence of feedback
processes, the key phrase in their qualitative use is "all else being equal."
In a system as complex as climate, this is usually not the case, and an
anomaly in one part of the system may be expected to set off a whole
series of adjustments, depending on the type, location, and magnitude
of the disturbance. Any positive feedback must, in any event, be
checked at some level by the intervention of other internal adjustment
processes, or the climate would exhibit a runaway behavior. We do not
adequately understand these adjustment mechanisms, and their system-
atic quantitative exploration by numerical climate models is an
important task for the future (see Chapter 6). In that research it will
be essential to use coupled models of atmosphere and ocean, and these
must be calibrated with great care so as not to distort the feedback
mechanisms themselves.
Climatic Noise
Climatic states have been defined in terms of finite time averages and
as such are subject to fluctuations of statistical origin in addition to the
changes of a physical nature already discussed. Since these statistical
fluctuations arise from the day-to-day fluctuations in weather (the
autovariation of the atmosphere identified in Figure 3.3), they are un-
predictable over time scales of climatological interest and are therefore
appropriately defined as "climatic noise." The amplitude of this noise
decreases approximately as the square root of the length of the time-
averaging interval, but some remains at any finite time scale (Leith,
1973; Chervin et at, 1974). A key problem of climatic variation on any
time scale is therefore the determination of the "climatic predictability,"
which we may define as the ratio of the magnitude of the potentially
predictable climatic change of physical origin to the magnitude of this
unpredictable climatic noise.
ROLE OF THE OCEANS IN CLIMATIC CHANGE
It has been noted that the oceans play a prominent role in the determina-
tion of climate through the processes at the air-sea interface that govern
the exchanges of heat, moisture, and momentum. While these condi-
tions are actually determined mutually by the atmosphere and the
ocean, they are likely dominated by the ocean on at least the longer
climatic time scales. It is the high thermal and mechanical oceanic
inertia that requires that special consideration be given to the role of
the ocean in climatic change.
26 UNDERSTANDING CLIMATIC CHANGE
Physical Processes in the Ocean
Over half of the solar radiation reaching the earth's surface is absorbed
by the sea. This solar radiation, along with the surface wind stress, is the
ultimate energy source for a variety of physical processes in the ocean
whose climatic importance is essentially a function of their time scales.
The absorption of solar radiation is primarily responsible for the exist-
ence of a warm surface mixed layer of order 102 m deep found over
most of the world's oceans. This warm surface layer represents a large
reservoir of heat and acts as a significant thermodynamic constraint on
the atmospheric circulation.
The exchange of the ocean's heat with the atmosphere occurs over a
wide range of time scales and largely determines the relative importance
of other physical processes in the ocean for climatic change. Some of this
heat is used for surface evaporation, some is stored in the surface layer,
and some is moved downward into deeper water by various dynamical
and thermodynamical processes. The fluxes of latent and sensible heat
into the atmosphere are commonly parameterized in atmospheric models
as functions of the large-scale surface wind speed and the vertical
gradients of humidity and temperature in the air over the ocean surface.
These fluxes are actually accomplished by small-scale turbulent proc-
esses in the surface boundary layer whose behavior is not adequately
understood. Physical processes in the ocean such as vertical convective
motions (depending on the local vertical stratification of temperature
and salinity) and wind-induced stirring also affect the depth and struc-
ture of the mixed layer, as shown, for example, by the simulations of
daily variations of local mixed layer depth by Denman and Miyake
(1973). Other small-scale processes such as salt fingering and internal
waves also produce transports that may contribute significantly to the
overall vertical mixing in the ocean. Therefore, the dynamics of the
ocean's surface layer must be taken into account in even the simplest
of climate models.
It is becoming apparent that the most energetic motion scale in the
oceans is that of the mesoscale eddy, whose period is of the order of a
few months and whose horizontal wavelength is of the order of several
hundred kilometers. The kinetic energy of these motions, which is pre-
dominantly in the barotropic and first baroclinic vertical mode, may be
one or two orders of magnitude greater than that of the time-averaged
motions themselves. In a general sense, these slowly evolving eddies are
the counterpart of the larger-scale transient cyclones and anticyclones in
the atmosphere. An understanding of the physical processes responsible
for the origin and behavior of these eddies and their role in the oceanic
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 27
general circulation is essential for further insight into the dynamics of
the vast open ocean regions.
In addition to the surface interactions, vertical mixing processes, and
mesoscale motions, the study of the longer-period variations of climate
clearly requires consideration of the large-scale dynamics of the com-
plete oceanic circulation. This includes the large-scale pattern of wind-
driven and thermohaline currents and their associated horizontal and
vertical transports of heat, momentum, and salt. Of particular im-
portance here is the study of the local dynamics of the intense bound-
ary and equatorial currents and the relative roles of inertial and topo-
graphic influences. The characteristic variations of these large-scale
processes are on time scales of the order of seasons and years in the
near-surface waters but may occur in progressively longer time scales
in deeper water. The longest oceanic adjustment time associated with
the "permanent" ocean circulation is of the order 103 years (see Figure
3.3). For climatic variations on these time scales, therefore, the entire
water mass of the global ocean must be taken into account.
Modeling the Oceanic Circulation
The systematic examination of the various mechanisms and feedbacks
by which the oceanic thermal structure and circulation are maintained
on various time scales is largely a task for the future. In this research,
it will be necessary to conduct intensive observational programs in order
to gain greater understanding of the various oceanic physical processes
themselves and to construct numerical models of the oceanic circulation
in which these processes are correctly represented.
For climatic studies, it is important that the heat and energy balances
of the ocean be modeled correctly over the time and space scales of
interest, and this cannot now be said to have been achieved. The classical
ocean circulation models, which were initiated in the late 1940's and
further developed in the following decades, do account for the gross
features of the ocean circulation, such as the major current systems and
the large-scale oceanic thermal structure (see Appendix B). But even
these features are physically and geometrically distorted by the con-
sideration of only the larger-scale, relatively viscous motions. The
commonly used vertical thermal eddy diffusivity in such models is
also questionable and may be an order of magnitude too high, as in-
dicated, for example, by recent studies on oceanic tritium concentra-
tions. This alone will produce a distortion of the processes responsible
for deep-water formation in such models.
But perhaps more important is the fact that numerical ocean models
28 UNDERSTANDING CLIMATIC CHANGE
have not had a sufficiently fine horizontal resolution to portray the
mesoscale eddies, either in the open ocean or in the restricted regions
of concentrated currents. The accuracy with which the meandering and
vortex shedding of boundary currents such as the Gulf Stream or
Kuroshio must be modeled, or the resolution required for the transient
behavior of the equatorial and Antarctic current systems, depends on
the extent to which these features are coupled to the semipermanent or
primary current systems themselves and on the time scales under con-
sideration. It is unlikely, however, that these features, or the mesoscale
eddies, can be successfully modeled with constant eddy diffusion
coefficients.
To study the role of the oceans in climatic change, it is necessary
to construct dynamically and energetically correct oceanic general cir-
culation models and to couple them, in appropriate versions, to
similarly accurate and compatible atmospheric models. Some experi-
ence with simplified coupled models of coarse resolution has already
been gained, as discussed in Appendix B. Further tests of coupled
models are necessary in which the oceanic mesoscale eddies are re-
solved, in order that we may understand their role in the oceanic heat
balance and their relationship to the climatically important changes of
sea-surface temperature. Since computational limitations will likely
preclude the resolution of these eddies throughout the world ocean,
their successful parameterization will become an important problem for
future research.
Of particular importance for climate studies is the construction of an
accurate model of the oceanic-surface mixed layer, since all the physical
processes in the ocean ultimately exert their influence on the atmosphere
through the surface of the sea. Until the dynamics of this oceanic
boundary layer are better understood, our ability to model climatic
variations on any time scale will remain seriously limited.
SIMULATION AND PREDICTABILITY OF CLIMATIC VARIATION
Climate Modeling Problem
From the above remarks it is clear that the problem of modeling climatic
variation is fundamentally one of constructing a hierarchy of coupled
atmosphere-ocean models, each suited to the physical processes domi-
nant on a particular time scale. The attack on this problem is now in its
infancy. Whether we consider changes of the external boundary con-
ditions or changes of the internally controlled physical processes and
feedback mechanisms, we note from Figure 3.3 the wide range of time
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 29
intervals over which characteristic climatic events occur and that many
of these involve interactions among the atmosphere, oceans, ice, and
land. Because of the system's nonlinearity, we may expect a broad
range of response in both space and time in the individual climatic
variables. This is just what the climatic record shows.
To study the relative contribution of individual physical processes to
the overall "equilibrium" climatic state, one approach is to test the
sensitivity of the statistics generated by a climate model to perturbations
in the parameters that influence that particular physical process. In such
a modeling program, the effects of changes can first be tested in isolation
from other interacting components of the system and then in concert
with all known processes in a complete climatic model. In this research,
we should not rely exclusively on the general circulation models (gcm's)
but should employ a. variety of modeling approaches. We note, however,
that not only are the gcm's (and the coupled gcm's in particular)
useful in the calibration of the simpler models, but they are essential to
the detailed diagnosis of the shorter-period climatic states that are in
approximate statistical equilibrium with slowly changing boundary
conditions.
A fundamental approach to the problem of modeling climate and
climatic variation must proceed through the consideration of dynamical
models of the coupled components of the climatic system. In minimum
practical terms, this means the joint atmosphere-ocean system, although
for some purposes (such as the behavior of ice sheets and glaciers) the
cryosphere must be included as well. Efforts to assemble such models
are just getting under way, and their further development is given high
priority in the research program recommended in Chapter 6.
Predictability and the Question of Transitivity
It is possible to regard climatic change as a conventional initial/
boundary-value problem in fluid dynamics, if we define the climatic sys-
tem as consisting of the atmosphere, hydrosphere, and cryosphere. In
this deterministic view the behavior of the system is governed by the
changes of the external boundary conditions (see Figure 3.1). Over
relatively short periods, it is even possible to regard the land ice masses
as part of the external conditions as well. It is probably not possible,
however, to remove the hydrosphere from the internal system and still
talk meaningfully about climatic variation, as the surface layers of the
ocean interact with the atmosphere on the shortest time scales associated
with climate (see Figure 3.3). Decoupling of the ocean, however, is
exactly what has so far been done in conventional atmospheric and
30 UNDERSTANDING CLIMATIC CHANGE
oceanic general circulation models, although preliminary efforts to
consider the coupled system have been made (see Appendix B).
Even with the atmosphere (together with certain surface effects)
regarded as the sole component of the climatic system, and with all ex-
ternal boundary conditions held fixed, there is, in spite of our physical
expectations, no assurance that there will be a climate in the sense that
time series generated by the atmospheric changes will settle into a
statistically steady state; and no assurance that the climate, if it exists, is
unique in the sense that the statistics are independent of the initial state.
It is therefore useful to define a random time series (or the system
generating such a series) as transitive if its statistics (and hence its
climatic states) are stable and independent of the initial conditions and
as intransitive if not. As shown by Lorenz (1968), nonlinear systems,
which are far simpler than the atmosphere, sometimes display a ten-
dency to fluctuate in an irregular manner between two (or more) in-
ternal states, while the external boundary conditions remain completely
unchanged. This behavior is related to the system's transitivity and is
illustrated in Figure 3.4.
Let us assume that two different states of a climatic system are
possible at a time f=0, such as A and B in Figure 3.4, and let us con-
sider that A is the climatic state that would normally be "expected"
under the given constant boundary condition. In a completely transitive
system, the climatic state B would approach the state A with the
passage of time and eventually become indistinguishable from it. This
would correspond to a unique solution for the climate under fixed
boundary conditions. In a completely intransitive system, on the other
hand, the climatic state B would remain unchanged, and two possible
solutions would exist. There would in this case, moreover, be no way
in which we could continue to identify the state A as the "normal" or
correct solution, as state B would presumably furnish an equally
acceptable set of climatic statistics.
A third behavior, however, is perhaps the most interesting of all, and
is displayed by an almost-intransitive system. In this case, the system in
state B may behave for a while as though it were intransitive, and then
at time tx shift toward an alternate climatic state A, where it might re-
main for a further period of time. At time t2 the system might then return
to the original climatic state B, where it could remain or enter into
further excursions. The climate exhibited by such a system would thus
consist of two (or more) quasi-stable states, together with periods
of transition between them. For longer periods of time the system might
have stable statistics, but for shorter periods of time it would appear to
be intransitive.
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32 UNDERSTANDING CLIMATIC CHANGE
Because the atmosphere is constantly subject to disturbances, such as
those arising from flow over rough terrain or from the occurrence of
baroclinic instability, one might think that it could not be an almost-
intransitive system and fail to show greater excursions of annual and
decadal climatic states than it does. This depends, of course, on the level
of variability associated with individual climatic states and hence on the
time interval we select to define the climatic state itself (and on how
close neighboring quasi-stable states might be). What may appear to be
a climatic transition on one time scale may become the natural noise
level of a climatic state defined over a longer interval. This is consistent
with many of the climatic records presented in Appendix A.
Even so, it might still be possible for the coupled ocean-atmosphere
system or for the coupled ocean-atmosphere-ice system to be almost
intransitive. One cannot help but be struck by the appearance of those
proxy records that display repeated transitions between two states (see
Figures A. 13 and A. 14 in particular). This evidence suggests that the
glacial-interglacial oscillations that have characterized the past million
years of the earth's climatic history may be the climatic transitions of
an almost-intransitive system. Another possible example of this
phenomenon is the irregular and relatively sudden reversal of the earth's
magnetic poles. The search for further evidence of this sort in both the
paleoclimatic record and in the climatic history generated by numerical
models is an important task for future research.
As though the specter of almost-intransitivity were not enough, on
the longer-time scales of climatic variation it is equally important to
recognize another, potentially serious complication. If it turns out that
climatic evolution is influenced to a significant degree by environmental
impacts originating outside the atmosphere-ocean-cryosphere system,
then the predictability of climate will be additionally constrained by the
predictability of the environment in a larger sense. This, in turn, could
turn out to be the greatest stumbling block of all, as illustrated, for
example, by the difficulty of predicting the timing and intensity of vol-
canic eruptions (which inject radiation-attenuating layers of dust into
the upper atmosphere) and by the difficulty of predicting the behavior
of the sun itself, which is the ultimate source of the energy driving the
climatic system.
As noted earlier, the predictability of climatic variation is con-
strained by an inherent limitation in the detailed predicability of the
atmosphere and ocean. Climatic noise as previously defined thus arises
from the unavoidable uncertainty in the determination of the initial
state and from the nonlinear nature of the relevant dynamics, as shown,
for example, by Lorenz (1969). Fluctuations in the weather for periods
PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 33
beyond a few weeks may therefore be treated in large part as though
they were generated by an unpredictable random process. The observed
time series of many meteorological variables may be reasonably well
modeled by a first-order Markov process with a time (r) -lagged cor-
relation given by R(t) =exp(-v|r|), with a constant decay rate v of the
order of 0.3 day1. The corresponding power spectrum as a function of
frequency cu is given by P(o>) =/4/(v2+<»2), where A is a constant.
As <a— »0 for such a spectrum, we have P(o>)->A/v2, which is a constant,
and the very-low-frequency end of the spectrum therefore appears
"white." There is thus some contribution to climatic variations on all
time scales, no matter how long, arising from the fluctuations of the
weather.
While these considerations do not directly address the physical basis
of climatic change,, they are nevertheless basic to our view of the pre-
dictability of climatic change. What parts of climatic variations on
various time scales are potentially predictable, and what parts are just
climatic noise? In the power spectrum is there potentially predictable
variability above the "white," low-frequency end of the daily weather
fluctuations, or is it possible that some of the long-term compensation
processes, such as those shown in Figure 3.3, might depress the spec-
trum below its white extension to o> = 0?
The 250-year record of monthly mean temperatures in central
England compiled by Manley (1959) shows small lagged correlations
significantly above that of weather out to about 6 months, small but
perhaps significant lagged correlations at 2 and 4 years, and a generally
white spectrum with some evidence of extra variability for periods of a
few decades and longer (C. E. Leith, ncar, Boulder, Colorado, un-
published results). The 6-month lagged correlation may well be a re-
flection of the role of North Atlantic sea-surface temperature anomalies
on the English climate and illustrates the somewhat longer periods of
the autovariation of the coupled ocean-atmosphere system over those
of the atmosphere alone, as indicated in Figure 3.3. Additional evidence
of even longer-period variability is found in the historical and paleo-
climatic records (Kutzbach and Bryson, 1974; see Figure A. 5). Further
studies of this kind should be made with statistical tests not only of the
pessimistic null hypothesis that nothing is predictable but also of
hypotheses that are framed more optimistically.
Long-Range or Climatic Forecasting
As our understanding of the physical basis of climatic variation grows,
we hope to be able to discern the predictable climatic change signal
34 UNDERSTANDING CLIMATIC CHANGE
from the unpredictable climatic noise and to describe with some con-
fidence the character of both past and likely future climates. In view
of the questions posed by limited predictability, however, this discern-
ment may be limited to those circumstances in which there is a relatively
large change in the processes or boundary conditions of the climatic
system. The related problem of forecasting specific seasonal and annual
climatic variations rests upon the same physical basis and may prove
more difficult to solve. To reach these goals will require the coordinated
use of all our research tools, whether they be observational, numerical,
or theoretical. The capstone of these efforts will be the emergence of an
increasingly well-defined and tested theory of climatic variation.
Whether the predictability of climatic change turns out to be lower
than many would like to believe or to be limited to a finite range as in
the problem of weather forecasting, the quest for understanding must be
made. Our recommendations for the research that we believe to be
a necessary part of this effort are presented in detail in Chapter 6.
4
PAST CLIMATIC VARIATIONS AND THE
PROJECTION OF FUTURE CLIMATES
It is universally accepted that global climate has undergone significant
variations on a wide variety of time scales, and we have every reason to
expect that such variations will continue in the future. The development
of an ability to forecast these future variations, even on time scales as
short as one or two decades, is an important and challenging task. Study
of the instrumental, historical, and paleoclimatic records not only offers
a basis for projection into the future but furnishes insight into the
regional effects of global climatic changes. This chapter attempts to
summarize our knowledge of past climatic variations and to give some
indication of the further research that must be carried out on critical
aspects of this subject. Further details of the record of past climates are
given in Appendix A.
IMPORTANCE OF STUDIES OF PAST CLIMATES
In order to understand fully the physical basis of climate and climatic
variation, we must examine the earth's atmosphere-ocean-ice system
under as wide a range of conditions as possible. Most of our notions of
how the climatic system works, and the tuning of our empirical and
dynamical models, are based on observations of today's climate. In order
that these ideas and models may be useful in the projection of future
climates, it is necessary that they be calibrated under as wide a range
of conditions as possible. The only documented evidence we have of
climates under boundary conditions significantly different from today's
35
36 UNDERSTANDING CLIMATIC CHANGE
comes from the paleoclimatic record. It is here that paleoclimatology, in
conjunction with climatic modeling, can make an especially valuable
contribution to the resolution of the problem of climatic variation.
Modern instrumental data suggest that the atmosphere, at least, may
be capable of assuming quite different circulation patterns even with
relatively constant boundary conditions and that the resulting variability
of climate is strongly dependent on geographical location. Although the
data base is much less complete for the oceans, persistent anomalies of
sea-surface temperature appear to be related to atmospheric circulation
regimes over time scales of months and seasons, and the oceans may
show other longer-period variations of which we are now unaware.
In general, the record of past climate indicates that the longer the
available record, the more extreme are the apparent climatic variations.
An immediate consequence of this "red-noise" characteristic is that the
largest climatic changes are not revealed by the relatively short record
of instrumental observation but must instead be sought through paleo-
climatic studies. The record of past climates also contains important
information on the range of climatic variability, the mean recurrence
interval of rare climatic events, and the tendency for systematic time-
wise behavior or periodicity. Such climatic characteristics are in general
shown poorly, if at all, by the available instrumental records.
RECORD OF INSTRUMENTALLY OBSERVED CLIMATIC CHANGES
Our knowledge of instrumentally recorded climatic variations is largely
confined to the record of the past two centuries or so, and it is only in
the last 100 years that synoptic coverage has permitted the analysis of
the geographical patterns of climatic change over large portions of the
globe. It is only during the past 25 years or so that systematic observa-
tions of the free atmosphere (mainly in the northern hemisphere) have
been made and that regular measurements of the ocean surface waters
have been available in even limited regions. Enough data have been
gathered, however, to permit the following summary.
A striking feature of the instrumental record is the behavior of tem-
perature worldwide. As shown by Mitchell (1970), the average surface
air temperature in the northern hemisphere increased from the 1880's
until about 1940 and has been decreasing thereafter (see Figure A. 6,
Appendix A). Starr and Oort (1973) have reported that, during the
period 1958-1963, the hemisphere's (mass-weighted) mean tempera-
ture decreased by about 0.6 °C. In that period the polar and subtropical
arid regions experienced the greatest cooling. The cause of this variation
is not known, although clearly this trend cannot continue indefinitely.
PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 37
It may represent a portion of a longer-period climatic oscillation, al-
though statistical analysis of available records has failed to establish any
significant periodic variation between the quasi-biennial cycle and
periods of the order of 100 years. The corresponding patterns of pre-
cipitation, cloudiness, and snow cover have not been adequately deter-
mined, and it would be of great interest to examine the simultaneous
variations of oceanic heat storage and imbalances of the planetary radia-
tion budget, once the necessary satellite observations become available.
For the earlier instrumental period, there are scattered records of
temperature, rainfall, and ice extent, which clearly show individual
years and decades of anomalous character. The only apparent trend is
a gradual warming in the European area since the so-called Little Ice
Age of the sixteenth to nineteenth centuries.
HISTORICAL AND PALEOCLIMATIC RECORD
Two sources of data are available to extend the record of climate into
the pre-instrumental era: historical sources, such as written records,
and qualitative observations, which give rise to what may be called
"historical" climatic data; and various natural paleoclimatic recorders,
which give rise to what may be called "proxy" climatic data.
Nature of the Evidence
The historical record contains much information relating to climate
and climatic variation over the past several hundred to several thousand
years, and this information should be located, cataloged, and evaluated.
Historical data on crop yields, droughts, and winter severity from
manuscripts, explorations, and other sources sometimes provide the
only available information on the general character of the climate of
the historical past. Such information is especially useful in conjunction
with selected tree-ring, ice core, and lake sediment data in diagnostic
studies of the higher-frequency climatic variability on the time scales
of years, decades, and centuries.
For earlier periods, the paleoclimatic record becomes increasingly
fragmentary and ultimately nil for the oldest geological periods. But for
the past million years, and especially for the past 100,000 years, the
paleoclimatic record is relatively continuous and can be made to yield
quantitative estimates of the values of a number of significant climatic
parameters. Each record, however, must first be calibrated or processed
to provide an estimate of the climate. The elevation of an ancient coral
reef, for example, is a record of a previous sea level, but before it can
38 UNDERSTANDING CLIMATIC CHANGE
be used for paleoclimatic purposes the effect of local crustal movements
must be removed. The taxonomic composition of fossil assemblages in
marine sediments and the width of tree rings, for example, are known
to reflect the joint influence of several ecological factors; here multi-
variate statistical techniques can be used to obtain estimates of selected
paleoclimatic parameters such as temperature and precipitation.
In order to be useful, a proxy data source must also have a strati-
graphic character; that is, the ambient values of a climatically sensitive
parameter must be preserved within the layers of a slowly accumulating
natural deposit or material. Such sources include the sediments left by
melting glaciers on land; sediments in peat bogs, lakes, and on the
ocean bottom; the layers in soil and polar ice caps; and the annual
layers of wood formed in growing trees. Since no proxy source yields as
long and continuous a record as would be desired, and the quality of
data varies considerably from site to site, a coherent picture of past
climate requires the assembly of data from different periods and with
different sampling intervals. Such characteristics of the principal proxy
data sources are summarized in Table A.l of Appendix A.
After proxy data have been processed and stratigraphically screened,
an absolute chronology must be established in order to date specific
features in the climatic record. The most accurate dating technique is
that used in tree-ring analysis, where dates accurate to within a single
year may be determined over the past several thousand years under
favorable conditions. Annually layered lake sediments and the younger
ice cores also have a potential dating accuracy to within several years
over the past few millenia. For suitable materials, 14C-dating methods
extend the absolute time scale to about 40,000 years, with an accuracy
of about 5 percent of the material's true age. Beyond the range of 14C
dating, the analysis of the daughter products of uranium decay make
possible the reconstruction of the climatic chronology of the past million
years. For even older records, our chronology is based primarily on
potassium-argon radiometric dating as applied to terrestrial lava and
ash beds. Stratigraphic levels dated by this method are then correlated
with undated sedimentary sequences by the use of paleomagnetic
reversals and characteristic floral and faunal boundaries.
Summary of Paleoclimatic History
From the overview of the geological time scale, we live in an unusual
epoch: today the polar regions have large ice caps, whereas during most
of the earth's history the poles have been ice-free. As shown in Figure
A. 15 of Appendix A, only two other epochs of extensive continental
PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 39
glaciation have been recorded, one during late Precambrian time (ap-
proximately 600 million years ago) and one during Permo-Carbonif-
erous time (approximately 300 million years ago). During the era that
followed the Permo-Carboniferous ice age, the earth's climates returned
to a generally warmer, nonglacial regime.
Before the end of the Mesozoic era (approximately 65 million years
ago) climates were substantially warmer than today. At that time, the
configuration of the continents and shallow ocean ridges served to
block a circumpolar ocean current in the southern hemisphere. This
barrier was formed by South America and Antarctica, which lay in
approximately their present latitudinal positions, and by Australia,
then a northeastward extension of Antarctica. About 50 million years
ago, the Antarctic-Australian passage opened as Australia moved
northeastward and- as the Indian Ocean widened and deepened. By
about 30 million years ago, the Antarctic circumpolar current system
was established, an event that may have decisively influenced the sub-
sequent climatic history of the earth.
About 55 million years ago global climate began a long cooling trend
known as the Cenozoic climate decline (see Figure A.15). Approxi-
mately 35 million years ago there is evidence from the marine record
that the waters around the Antarctic continent underwent substantial
cooling, and there is further evidence that about 25 million years ago
glacial ice occurred along the edge of the Antarctic continent in some
locations. During early Miocene time (approximately 20 million years
ago) there is evidence that the low and middle latitudes were somewhat
warmer. There is widespread evidence of further cooling about 10
million years ago, including the growth of mountain glaciers in the
northern hemisphere and substantial growth of the Antarctic ice sheet;
this time may be taken as the beginning of the present glacial age. Evi-
dence from marine sediments and from continental glacial features
indicates that about 5 million years ago the already substantial ice sheets
on Antarctica underwent rapid growth and even temporarily exceeded
their present volume. Three million years ago continental ice sheets
appeared for the first time in the northern hemisphere, occupying lands
adjacent to the North Atlantic Ocean, and during at least the last 1 mil-
lion years the ice cover on the Arctic Ocean was never significantly less
than it is today.
Once the polar ice caps formed, they began a long and complex series
of fluctuations in size. Although the earlier record is still not clear, the
last million years has witnessed fluctuations in the northern hemisphere
ice sheets with a dominant period on the order of 100,000 years (see
Figure A. 2). These fluctuations may have occurred in parallel with
40
UNDERSTANDING CLIMATIC CHANGE
substantial changes in the volume of the West Antarctic ice sheet. By
comparison, however, changes in the volume of the ice sheet in East
Antarctica were quite small and were probably not synchronous with
glaciations in the northern hemisphere.
The major climatic events during the past 150,000 years were the
occurrence of two glacial maxima of roughly equal intensity, one about
135,000 years ago and the other between 14,000 and 22,000 years
ago. Both were characterized by widespread glaciation and generally
colder climates and were abruptly terminated by warm interglacial
intervals that lasted on the order of 10,000 years. The penultimate
interglacial reached its peak about 124,000 years ago, while the pres-
ent interglacial (known as the Holocene) evidently had its thermal
maximum about 6000 years ago.
Between 22,000 and 14,000 years ago the northern hemisphere ice
sheets attained their maximum extent (see Figure A. 24). The eastern
part of the Laurentide ice sheet (which covered portions of eastern
North America) and the Scandinavian ice sheet (which covered parts
of northern Europe) both attained their maxima between 22,000 and
18,000 years ago, several thousand years before the maximum of the
Cordilleran ice sheet. About 14,000 years ago deglaciation began rather
abruptly, and the Cordilleran sheet melted rapidly and was gone by
10,000 years ago. The interval of deglaciation (14,000 to 7000 years
ago) was marked in many places by significant secondary fluctuations
about every 2000 to 3000 years.
In general, the period about 7000 to 5000 years ago was warmer
than today, although the records of mountain glaciers, tree lines, and
tree rings reveal that the past 7000 years was punctuated in many parts
of the world by colder intervals about every 2500 years, with the most
recent occurring about 300 years ago.
For the last 1000 years, the proxy records generally confirm the
scattered observations in historical records. The cold period identified
above is seen to have consisted of two periods of maximum cold, one
in the fifteenth century and another in the late seventeenth century. The
entire interval, from about 1430 to 1850, has long been referred to as
the Little Ice Age and was characterized in Europe and North America
by markedly colder climates than today's.
INFERENCE OF FUTURE CLIMATES FROM PAST BEHAVIOR
Notwithstanding the limitations of our present insight into the physical
basis of climate, we are not altogether powerless to make certain in-
PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 41
ferences about future climate. Beginning with the most conservative
approach, we may use the climatic "normal" as a reference for future
planning. In this approach, it is tacitly assumed that the future climate
will mirror the recently observed past climate in terms of its statistical
properties. Depending on the sensitivity of the climate-related applica-
tion (and on the degree to which the climate is subject to change over
a period of years following that for which the "normal" is defined),
this kind of inference can be anything from highly useful to downright
misleading.
Of the various other approaches to the inference of future climate
in which the attempt is made to capture more predictive information
than is embodied in the "normal," the most popular have been those
based on the supposition that climate varies in cycles. Since the develop-
ment of modern techniques of time-series analysis, in particular those
involving the determination of the variance (or power) spectrum, it
has become clear that almost all alleged climatic cycles are either (1)
artifacts of statistical sampling, (2) associated with such small fractions
of the total variance that they are virtually useless for prediction pur-
poses, or (3) a combination of both. Other approaches, developed to a
high degree of sophistication in recent years, include several kinds of
nonlinear regression analysis (in which no assumption need be made
about the periodic behavior of the climatic time series), which appro-
priately degenerate to a prediction of the "normal" in cases where the
series possess no systematic temporal behavior. The full potential of such
approaches is not yet clear but appears promising, at least in certain
situations.
Natural Climatic Variations
Regardless of the approach taken to infer future climates, the view that
climatic variation is a strictly random process in time can no longer be
supported. It has been well established, for example, that many atmo-
spheric variables are serially correlated on time scales of weeks, months,
and even years. For the most part such correlations derive from "per-
sistence" and resemble the behavior of a low-order Markov process.
Unfortunately, nonrandomness of this kind does not lend itself to long-
range statistical prediction. In addition to persistence, long-term trends
have a tendency to show up in great number and variety in climato-
logical time series (see Appendix A). Many such trends are now
understood to originate from what are called inhomogeneities in the
series, as, for example, effects of station relocations, changes in observ-
42 UNDERSTANDING CLIMATIC CHANGE
ing procedures, or local microclimatic disturbances irrelevant to large-
scale climate. Even after statistical removal of such effects, many "real"
trends nonetheless remain and may be recognized as part of a longer-
term oscillation of climate. We must, moreover, recognize that the
climatic record may also reflect various natural environmental disturb-
ances, such as volcanic eruptions and perhaps changes of the sun's
energy output, which are themselves only poorly predictable, if at all.
Clearly, a climatic prediction based on the linear extrapolation into the
future of a record containing such effects would be highly unrealistic.
The behavior of longer climatic series is seemingly periodic, or quasi-
periodic, especially those series that extend into the geological past as
reconstructed from various proxy data sources. It is a fundamental
problem of paleoclimatology to determine whether this behavior is
really what it seems or whether it is an illusion created by the character-
istic loss of high-frequency information due to the limited resolving
power of most proxy climatic indicators. Illumination of this question
would be of great importance to the determination of the basic causes of
the glacial-interglacial climatic succession and to the assessment of
where the earth stands today in relation to this sequence.
Spectrum analyses of the time series of a wide variety of climatic
indices have consistently displayed a "red-noise" character (see, for
example, Gilman et al, 1963). That is, the spectra show a gradual in-
crease of variance per unit frequency as one proceeds from high fre-
quencies toward low frequencies. The lack of spectral "gaps" provides
empirical confirmation of the lack of any obvious optimal averaging in-
terval for defining climatic statistics. Most spectra of climatic indices
are also consistent in displaying some form of quasi-biennial oscillation
(see, for example, Brier, 1968; Angell et al., 1969; or Wagner, 1971).
This fluctuation is most obvious in the wind data of the tropical strato-
sphere but also has been shown to be a real if minor feature of the
climate at the earth's surface as well.
Time series of some of the longer instrumental records show some
suggestion of very-low-frequency fluctuations (periods of about 80 years
and longer), but the data sets are not long enough to establish the
physical nature and historical continuity of such oscillations. While
numerous investigators have reported spectral peaks corresponding to
almost all intermediate periods, the lack of consistency between the
various studies suggests that no example of quasi-cyclic climatic be-
havior with wavelengths between those on the order of 100 years and
the quasi-biennial oscillation have been unequivocally demonstrated on
a global scale. Further discussion of these questions is given in Appendix
A (p. 127 0).
PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 43
Man's Impact on Climate
While the natural variations of climate have been larger than those
that may have been induced by human activities during the past century,
the rapidity with which human impacts threaten to grow in the future,
and increasingly to disturb the natural course of events, is a matter of
concern. These impacts include man's changes of the atmospheric com-
position and his direct interference with factors controlling the all-
important heat balance.
Carbon Dioxide and Aerosols
The relative roles of changing carbon dioxide and particle loading as
factors in climatic 'change have been assessed by Mitchell (1973a,
1973b), who noted that these variable atmospheric constituents are not
necessarily external parameters of the climatic system but may also be
internal variables; for example, the changing capacity of the surface
layers of the oceans to absorb C02, the variable atmospheric loading of
wind-blown dust, and the interaction of C02 with the biosphere.
The atmospheric C02 concentrations recorded at Mauna Loa, Hawaii
(and other locations) show a steady increase in the annual average,
amounting to about a 4 percent rise in total C02 between 1958 and 1972
(Keeling et ai, 1974). The present-day C02 excess (relative to the year
1850) is estimated at 13 percent. A comparison with estimates of the
fossil C02 input to the atmosphere from human activities indicates that
between 50 and 75 percent of the latter has stayed in the atmosphere,
with the remainder entering the ocean and the biosphere. The C02
excess is conservatively projected to increase to 15 percent by 1980, to
22 percent by 1990, and to 32 percent by 2000 a.d. The corresponding
changes of mean atmospheric temperature due to C02 [as calculated
by Manabe (1971) on the assumption of constant relative humidity
and fixed cloudiness] are about 0.3 °C per 10 percent change of C02
and appear capable of accounting for only a fraction of the observed
warming of the earth between 1880 and 1940. They could, however,
conceivably aggregate to a further warming of about 0.5 °C between
now and the end of the century.
The total global atmospheric loading by small particles (those less
than 5 ^m in diameter) is less well monitored than is C02 content but
is estimated to be at present about 4xl07 tons, of which perhaps as
much as 1 x 107 tons is derived both directly and indirectly from human
activities. If the anthropogenic fraction should grow in the future at the
not unrealistic rate of 4 percent per year, the total particulate loading
44 UNDERSTANDING CLIMATIC CHANGE
of the atmosphere could increase about 60 percent above its present-
day level by the end of this century. The present-day anthropogenic
particulate loading is estimated to exceed the average stratospheric
loading by volcanic dust during the past 120 years but to equal only
perhaps one fifth of the stratospheric loading that followed the 1883
eruption of Krakatoa.
The impact of such particle loading on the mean atmospheric tem-
perature cannot be reliably determined from present information.
Recent studies indicate that the role of atmospheric aerosols in the heat
budget depends critically on the aerosols' absorptivity, as well as on
their scattering properties and vertical distribution. The net thermal
impact of aerosols on the lower atmosphere (below cloud level) prob-
ably depends on the evaporable water content of the surface in addition
to the surface albedo. Aerosols may also affect the structure and
distribution of clouds and thereby produce effects that are more im-
portant than their direct radiative interaction (Hobbs et al., 1914;
Mitchell, 1974).
Of the two forms of pollution, the carbon dioxide increase is probably
the more influential at the present time in changing temperatures near
the earth's surface (Mitchell, 1973a). If both the C02 and particulate
inputs to the atmosphere grow at equal rates in the future, the widely
differing atmospheric residence times of the two pollutants means that
the particulate effect will grow in importance relative to that of C02.
Thermal Pollution, Clouds, and Surface Changes
There are other possible impacts of human activities that should be
considered in projecting future climates. One of these is the thermal
pollution resulting from man's increasing use of energy and the inevitable
discharge of waste heat into either the atmosphere or the ocean. Al-
though it is not yet significant on the global scale, the projections of
Budyko (1969) and others indicate that this heat source may become
an appreciable fraction (1 percent or more) of the effective solar
radiation absorbed at the earth's surface by the middle of the next
century. And if future energy generation is concentrated into large
nuclear power parks, the natural heat balance over considerable areas
may be upset long before that time. Recent estimates by Haefele (1974)
indicate that by early in the next century, the total energy use over the
continents will approach 1 0 percent of the natural heat density of about
50 W/m2 and that in local industrial areas the man-made energy
density may become several hundred times larger.
There is also the possibility that widespread artificial creation of
PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 45
clouds by aircraft exhaust and by other means may induce significant
climatic variations, although there is no firm evidence that this has yet
occurred. Such effects could serve to increase the already prominent role
played by (natural) clouds in the earth's heat balance (see Figure 3.2).
Widespread changes of surface land character resulting from agri-
cultural use and urbanization, and the introduction of man-made sources
of evaporable water, may also have significant impacts on future
climates. When the surface albedo and surface roughness are changed
by the removal of vegetation, for example, the regional climatic
anomalies introduced may have large-scale effects, depending on the
location and scale of the changes. The creation of large lakes and
reservoirs by the diversion of natural watercourses may also have
widespread climatic consequences. The list of man's possible future
alterations of the earth's surface can be considerably lengthened by the
inclusion of more ambitious schemes, such as the removal of ice cover
in the polar regions and the diversion of ocean currents. Again, how-
ever, it is only through the use of adequately calibrated numerical
models that we can hope to acquire the information necessary for a
quantitative assessment of the climatic impacts.
5
SCOPE OF PRESENT RESEARCH
ON CLIMATIC VARIATION
The overview of the problem of climatic variation presented in the
preceding chapters and in the technical appendixes contains only those
references to the literature that were helpful in the illustration of a
particular viewpoint or necessary to document a specific source of in-
formation. In the course of its deliberations, however, the Panel found
it necessary to survey present research on climatic variation, as rep-
resented by the more recently published literature and by selected on-
going activities. Inasmuch as this information may serve as a useful
background to the Panel's recommendations for a national and inter-
national program of climatic research, it is summarized here. Even this
survey, in which emphasis is given to material published since 1970,
must be considered incomplete and necessarily gives precedence to
sources of information most readily available to the Panel. Further use-
ful references on various aspects of the problem of climatic variation
are to be found in other recent publications (Committee on Polar
Research, 1970; National Science Board, 1972; Wilson, 1970, 1971).
CLIMATIC DATA COLLECTION AND ANALYSIS
Here the current status of efforts to assemble climatic data for both the
atmosphere and ocean is summarized, and the various observational
field programs directed to the collection of specific data of climatic
interest are described.
46
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION
Atmospheric Observations
47
Climatological data banks are maintained by noaa's National Climatic
Center (ncc) and National Meteorological Center (nmc) and by the
military operational weather services, particularly the Air Force's En-
vironmental Technical Applications Center (etac) and the Navy's
Fleet Numerical Weather Central (fnwc). Using data from these
sources, atmospheric data sets specifically for climatic studies have been
assembled by the National Center for Atmospheric Research, the Geo-
physical Fluid Dynamics Laboratory, MIT, and other institutions.
Efforts to assemble the rapidly accumulating data from meteorological
satellites have also been made by noaa's National Environmental Satel-
lite Service (ness) and by the University of Wisconsin. Sustained efforts
to assemble and systematically analyze such data for the use of the
climatic research community are important tasks for the future.
In addition to the standard compilations of climatological statistics
prepared on a routine basis by governmental agencies, new summaries
of upper-air data have been prepared (Crutcher and Meserve, 1970;
Taljaard et al., 1969); these have permitted the initial construction of
the average monthly global distributions of the basic meteorological
variables of pressure, temperature, and dew points at selected levels.
The analysis of such data in terms of the various statistics of the global
circulation is less advanced, although intensive studies of a five-year
period in the northern hemisphere have recently been completed (Oort,
1972; Oort and Rasmusson, 1971; Starr and Oort, 1973). These studies
provide the most quantitative analyses of the annual climatic variations
of the atmosphere yet made, and plans are under way for their extension
to additional five-year periods.
Studies of the spatial patterns of observed variability over longer time
periods are almost entirely confined to surface variables in the northern
hemisphere (Hellerman, 1967; Kutzbach, 1970; Wagner, 1971). Such
studies should be extended to other portions of the atmosphere and
broadened to include other, less comprehensively observed climatic
elements.
An observational analysis of the tropical and equatorial circulation
has been completed (Newell et al., 1972), and statistics for the strato-
spheric climate are becoming increasingly available (Newell, 1972).
Comprehensive data on the components of the global atmospheric
energy balance are only beginning to be available (Newell et al., 1969),
although many rely on older and indirect data for the unobserved ele-
ments of the heat balance at the earth's surface (Budyko, 1956, 1963;
Lvovitch and Ovtchinnikov, 1964; Moller, 1951; Posey and Clapp,
48 UNDERSTANDING CLIMATIC CHANGE
1964). More recent direct observations from satellites, however, are
providing valuable new insight into both the time and space variations
of the overall radiation budget of the earth (Vonder Haar and Suomi,
1971) and promise to provide further data of climatic importance as
newer and more versatile satellite observational capabilities develop
(Chahine, 1974; cospar Working Group 6, 1972; Raschke et al, 1973;
Smithed/., 1973).
Oceanic and Other Observations
The observational data base for the oceans is much less developed than
that for the atmosphere, and oceanic climatic summaries are based
largely on observations that are more widely scattered in both space and
time. Even for the more traveled parts of the oceans, these data are
sufficient only to indicate the average large-scale features of the ocean's
structure and circulation (Fuglister, 1960; Hellerman, 1967; Sverdrup
et al, 1942; U.S. Navy Hydrographic Office, 1944). Updated com-
pilations of surface stress (Hellerman, 1967)) and sea-surface tem-
peratures (Alexander and Mobley, 1973; Washington and Thiel,
1970) have been made, and summaries of the observed subsurface
temperature structure have recently become available for selected
oceans (Born et al, 1973; Robinson and Bauer, 1971).
Significant repositories of oceanic data useful for climatic purposes
exist at a number of institutions, although a comprehensive oceanic
data inventory has not yet been prepared. The Navy's Fleet Numerical
Weather Central, the Scripps Institution of Oceanography, the Woods
Hole Oceanographic Institution, and the National Marine Fisheries
Service, for example, all have specialized oceanographic data banks, as
well as data from individual cruises and expeditions. Guides to the
oceanic data services of the Environmental Data Service (1973) of
noaa are also available.
An increasing amount of data on oceanic surface conditions is becom-
ing available from satellite observations and other remote-sensing
techniques (Munk and Woods, 1973; Shenk and Salomonson, 1972)
and offer the promise of routine global monitoring of sea-surface tem-
perature and sea-ice distribution. Satellite data collected by ness also
permit the determination of the snow and ice extent over land; this and
other glaciological data are being accumulated by the U.S. Geological
Survey. The further extension of oceanographic, sea-ice, and glaci-
ological observations by satellites, buoys, and ships is under active
consideration in connection with the fgge (garp, 1972; Stommel,
1973) and is part of other large-scale programs as well (International
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 49
Decade of Ocean Exploration, 1973; International Glaciological Pro-
gramme for the Antarctic Peninsula, 1973; Kasser, 1973; Mid-ocean
Dynamics Experiment-one, Scientific Council, 1973; Joint U.S. polex
Panel, 1974).
Observational Field Programs
Many observational data of importance to climatic research have been
acquired in special field programs. Some of these are directly related to
garp itself (amtex Study Group, 1973; garp Joint Organizing Com-
mittee, 1972, 1973; Houghton, 1974; Kondratyev, 1973), such as the
Complete Atmospheric Energetics Experiment (caenex), the Air-Mass
Transformation Experiment (amtex), the garp Atlantic Tropical Ex-
periment (gate), and the Arctic Ice Dynamics Joint Experiment
(aidjex). Others are part of the nsf's International Decade of Ocean
Exploration (idoe) (1973) program (Mid-ocean Dynamics Experi-
ment-one, Scientific Council, 1973), such as the Geochemical Ocean
Sections Study (geosecs), the Mid-ocean Dynamics Experiment
(mode), the North Pacific Experiment (norpax), and the Climate,
Long-range Investigation, Mapping, and Prediction (climap) project.
Other field programs are aimed at the monitoring of atmospheric com-
position and aerosols, such as those of ncar, the Environmental Pro-
tection Agency, and noaa's Environmental Research Laboratories.
Each of these programs is focused on physical processes of im-
portance in particular geographical regions and is a valuable source of
experience and information. There are also international programs of
this sort in various stages of planning, such as the Polar Experiment
(polex) (Joint U.S. polex Panel, 1974), the International Glaci-
ological Program for the Antarctic Peninsula (igpap) (1973), and the
International Southern Ocean Studies (isos) programs (isos Planning
Committee, 1973). Cooperative programs such as these will be neces-
sary for the comprehensive future monitoring, analysis, and modeling of
climate and climatic variation.
STUDIES OF CLIMATE FROM HISTORICAL SOURCES
The record of past climates as contained in various historical documents,
writings, and archeological material has been increasingly recognized as
an important source of information (Bryson and Julian, 1963; Butzer,
1971; Carpenter, 1965; LeRoy Ladurie, 1971; Lamb, 1968, 1972;
Ludlam, 1966, 1968). These sources permit the study of historical
climates over the past several thousand years. A systematic compilation
50 UNDERSTANDING CLIMATIC CHANGE
of material of this sort is being undertaken by the Climatic Research
Unit of the University of East Anglia (Lamb, 1973b).
STUDIES OF CLIMATE FROM PROXY SOURCES
The assembly of paleoclimatic information from proxy data sources has
attained new importance in recent years with the development of new
methods of dating and of new techniques of quantitative climatic in-
ference. In the following, the various efforts in this aspect of climatic
research are briefly summarized.
General Syntheses
Two broad surveys of paleoclimatology have appeared in recent years
(Funnell and Riedel, 1971; Schwartzbach, 1961), along with textbooks
(Flint, 1971; Washburn, 1973) and symposia (Black et al, 1973;
Turekian, 1971), which emphasize the glacial processes during the late
Cenozoic period. Other recent paleoclimatological syntheses have been
concerned with the broad range of Quaternary studies (Wright and
Frey, 1965), with the relationships between Pleistocene geology and
biology (Butzer, 1971; West, 1968), and with more recent paleoclimatic
fluctuations from a meteorological viewpoint (Lamb, 1969). The
review of the full range of paleoclimatic events on all time scales given
in Appendix A of this report has been made possible by the recent
application of improved dating methods to the stratigraphic record, of
ocean sediments and uplifted reefs. This synthesis illustrates the essential
need for an accurate time scale in the interpretation of proxy climatic
data.
Chronology
The methods of dendrochronology (Ferguson, 1970; LaMarche and
Harlan, 1973), the radiocarbon method (Olsson, 1970; Wendland and
Bryson, 1974), and other isotopic dating methods have recently been
used to infer the chronology of climate over the past several hundred
thousand years (Broecker and van Donk, 1970; Matthews, 1973;
Mesolella et al, 1969). Biostratigraphic and paleomagnetic correlations
between the marine and continental records have provided a reasonably
accurate chronology of the past 60 million years by the use of potassium-
argon and other isotopes (Berggren, 1971, 1972; Hays et al, 1969;
Kukla, 1970; Ruddiman, 1971; Sancetta et al, 1973; Shackleton and
Kennett, 1974a, 1974b; Shackleton and Opdyke, 1973).
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 51
Monitoring Techniques
Following the initial efforts to estimate paleotemperatures from isotopic
time series (Emiliani, 1955, 1968), recent work has made it possible
to separate the effects of temperature from those of ice-volume change
(Shackleton and Opdyke, 1973). Multivariate statistical techniques
have recently been developed that permit the quantitative estimation of
climatic parameters from the concentration of fossil plankton in deep-
sea sediments (Imbrie and Kipp, 1971; Imbrie et al, 1973; Kipp,
1974), the growth record of tree rings (Fritts et al, 1971), and the
continental distribution of fossil pollen (Webb and Bryson, 1972).
These methods have since been applied to the reconstruction of paleo-
ocean temperatures (Luz, 1973; Mclntyre et ah, 1972a; Pisias et al.,
1973; Sachs, 1973), as well as to pressure and precipitation anomalies
(Fritts, 1972). Isotopic studies of cores taken in the polar ice caps
provide measures of the air temperature at the time of ice formation
(Dansgaard et al., 1971). Further refinements of such monitoring tech-
niques will help to fill in the paleoclimatic record, especially when
several independent methods are available for the same period.
Proxy Data Records and Their Climatic Inferences
Proxy data come from a wide variety of sources; potentially, any bio-
logical, chemical, or physical characteristic that responds to climate
may provide proxy data useful in the reconstruction of past climates.
One of the more prolific sources of long-term climatic information has
been the extensive collection of deep-sea cores, obtained routinely over
the years on various oceanographic expeditions and more recently
from the Deep-Sea Drilling Project (Douglas and Savin, 1973; Shackle-
ton and Kennett: 1974a, 1974b). Analysis of the fossil flora and fauna
in such cores, with chronology provided from their isotopic content
and paleomagnetic stratigraphy, has been performed for all the princi-
pal oceans of the world (Emiliani, 1968; Gardner and Hays, 1974;
Hunkins et al, 1971; Imbrie, et al, 1973; Kellogg, 1974; Kennett and
Huddlestun, 1972; Moore, 1973) and provides a preliminary docu-
mentation of the temperature and large-scale displacements of the
surface waters during the last few hundred thousand years (Mclntyre
et al, 1972b; Shackleton and Opdyke, 1973). Other characteristics of
the sediment cores, such as the presence of volcanic ash (Ruddiman
and Glover, 1972), also indicate climatically important events, as well
as providing valuable core dating horizons. For periods of particular
interest, such as the glacial maximum of about 18,000 years ago, de-
52 UNDERSTANDING CLIMATIC CHANGE
tailed reconstructions of seasonal sea-surface temperature and salinity
have been made for the North Atlantic (Mclntyre et al, 1974) and
more recently have been extended to the world ocean under the
climap program.
The concentration of fossil pollen and the record of soil types in
relatively undisturbed continental sites is another source of proxy
data on terrestrial paleoclimates. In recent years, pollen data have
been analyzed from a number of continental areas (Bernabo et al, 1974;
Davis, 1969; Heusser, 1966; Heusser and Florer, 1974; Livingstone,
1971; Swain, 1973; Tsukada, 1968; van der Hammen et al, 1971)
and provide a preliminary documentation of the surface vegetational
changes during the late Cenozoic and Quaternary periods (Leopold,
1969; Wright, 1971). Soil records have been studied less extensively
but provide corroborative evidence of surface climatic conditions (Frye
and Willman, 1973; Kukla, 1970; Sorenson and Knox, 1973).
In many ways analogous to the records from deep-sea cores, proxy
climatic data from ice cores have recently been obtained from sites in
both Antarctica and Greenland (Dansgaard et al, 1969, 1971). Such
ice-core records provide a detailed history of atmospheric conditions
over the ice during the last hundred thousand years (Dansgaard et ah,
1973; Johnsen et al, 1972; Langway, 1970). The drilling of deeper
cores are planned, and their analysis and correlation with other proxy
data will contribute significantly to the reconstruction of global climatic
history.
Further climatic inferences are obtained from proxy data on marine
shorelines. By assembling data on dated terraces at selected continental
and island sites, and with the necessary adjustments for eustatic changes
in the earth's crust, the record of sea-level variations over the last
150,000 years is becoming established (Bloom, 1971; Currey, 1965;
Matthews, 1973; Mesolella et al, 1969; Milliman and Emery, 1968;
Steinen et al, 1973; Walcott, 1972), particularly as regards the timing
of high stands.
Closely related to the questions of ice, soil, and sea-level changes
are the proxy data from glacial fluctuations themselves. Considerable
attention has been given in recent years to the reconstruction of the
glacial history of the most recent major ice age in North America
(Andrews et al, 1972; Black et al, 1973; Dreimanis and Karrow, 1972;
Frye and Willman, 1973; Paterson, 1972; Porter, 1971; Richmond,
1972), as well as the relatively small but significant fluctuations in
mountain glaciers over the past 10,000 years (Denton and Karlen,
1973). Although local glacial margins fluctuate primarily in response
to the glacier's net mass accumulation, their overall pattern provides
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 53
evidence of larger-scale and longer-period climatic responses. When
these changes are combined with the more limited data on the glacial
history of the Antarctic ice sheet, a number of worldwide relationships
in the major fluctuations of glacial extent begin to emerge (Denton et al.,
1971; Hughes, 1973).
In the postglacial period, important proxy data on climatic variations
over the continents also come from the records of tree rings and closed-
basin lakes. Both of these features respond directly to the hydrologic
and thermal balances at the surface and when properly calibrated for
local effects can provide a record of climate over thousands of years.
With the introduction of new dating and analysis methods, the records
of tree-ring width variations from both living and fossil trees provide
an annually integrated record of climatic changes, especially in arid
regions (Ferguson, 1970; Fritts, 1971, 1972; LaMarche, 1974; La-
Marche and Harlan, 1973). The radiocarbon dating of debris in selected
arid lakes provides further evidence of climatic variations, particularly
as they affect the local water balance (Broecker and Kaufman, 1965;
ButzeretaL, 1972; Farrand, 1971).
Institutional Programs
Much of the present research on paleoclimates is performed in con-
junction with other glaciological and geological programs, such as those
of the U.S. Geological Survey, the Illinois Geological Survey, the
Lamont-Doherty Geological Observatory of Columbia University, and
the Army's Cold Regions Research and Engineering Laboratory. Other
efforts are conducted within the larger oceanographic research labora-
tories, such as the Scripps Institution of Oceanography of the University
of California, the Woods Hole Oceanographic Institution, the U.S.
Naval Oceanographic Laboratory, and the marine research laboratories
of the University of Miami, the University of Rhode Island, and Oregon
State University. In recent years, more specialized paleoclimatic re-
search efforts have been developed at a number of other universities,
joining the long-established Laboratory of Tree-ring Research of the
University of Arizona and the Institute for Polar Research at The Ohio
State University. These include the Quaternary Research Centers at
the University of Washington and the University of Maine, the Center
for Climatic Research at the University of Wisconsin, the Institute of
Arctic and Alpine Research at the University of Colorado, and the
paleoclimatic research- programs in the geology and geophysics depart-
ments of Brown University and Yale University.
Notable among the many cooperative activities of these and other
54 UNDERSTANDING CLIMATIC CHANGE
institutions are the nsf's idoe programs, including the climap and
norpax projects. Such cooperative programs have been instrumental
in developing an effective collaboration among the paleoclimatic,
oceanographic, and meteorological research communities and should be
broadened in the future.
PHYSICAL MECHANISMS OF CLIMATIC CHANGE
Although the problem of climatic change has been the subject of
speculation for over a century, recent research has concentrated on
the study of specific physical processes and on the interactions among
various components of the climatic physical system. Here the more
recent of such efforts are briefly surveyed, together with a review of
associated empirical, diagnostic, and theoretical studies.
Physical Theories and Feedback Mechanisms
Of particular interest in the problem of climatic change is the question
of the cause of the ice ages. Among the recent attempts to answer
this question are hypotheses that focus upon the roles of sea ice (Donn
and Ewing, 1968) and ice shelves (Wilson, 1964), the carbon dioxide
balance (Plass, 1956), and the ocean's salinity (Weye, 1968). Other
hypotheses emphasize the roles of variations of external boundary con-
ditions, particularly the incoming solar radiation (Alexander, 1974;
Budyko, 1969; Clapp, 1970; Manabe and Wetherald, 1967) and the
volcanic dust loading of the atmosphere (Lamb, 1970) .
It is generally believed that the astronomical variations of seasonal
solar radiation play a role in longer-period climatic changes (Milanko-
vitch, 1930; Mitchell, 1971b; Vernekar, 1972), although there is no
agreement on the physical mechanisms involved. Recent studies have
also been made of the long-standing question of possible short-term
relationships between the climate and solar activity itself (Roberts,
1973; Roberts and Olson, 1973). Other hypotheses of climatic change
reckon with the possibility that much of the observed variations of
climate are essentially the result of the natural, self-excited variability
of the internal climatic system (Bryson, 1974; Mitchell, 1966, 1971b;
Sawyer, 1966).
Of the many feedback processes involved in climate (Schneider and
Dickinson, 1974) the role of aerosols has recently received particular
attention (Chylek and Coakley, 1974; Joseph et al, 1973; Mitchell,
1971a, 1974; Rasool and Schneider, 1971; Schneider, 1971). Although
our knowledge of the physical properties and global distribution of
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 55
aerosols is limited, these studies indicate that the climatic effects may
be substantial (Rasool and Schneider, 1971; Yamamoto and Tanaka,
1972). Several research programs on aerosols are under way, including
the Global Atmospheric Aerosol Research Study (gaars) of ncar and
the Soviet caenex program (Kondratyev, 1973) previously noted.
Attention has also been focused on the regulatory roles of cloudiness
(Cox, 1971; Mitchell, 1974; Schneider, 1972) and air-sea interaction
(Namias, 1973; White and Barnett, 1972) in the global climatic sys-
tem. In both cases, however, an adequate quantitative understanding
has not yet been achieved.
Diagnostic and Empirical Studies
Related to the search for physical climatic theories and mechanisms
are many empirical and diagnostic studies of various aspects of climatic
change. Particular attention has been given to the analysis of the large-
scale variations of the atmospheric circulation that have been observed
during the past few decades (Angell et al., 1969; Bjerknes, 1969; Davis,
1972; Namias, 1970; Wahl, 1972; Wahl and Lawson, 1970; White and
Walker, 1973) and to their relationship to regional anomalies of
temperature and rainfall (Landsberg, 1973; Namias, 1972b; Winstanley,
1973a, 1973b). Satellite observations of the large-scale variations of
surface albedo and seasonal snow cover have brought new attention
to these features of the climatic system (Kukla and Kukla, 1974;
Wagner, 1973), as well as necessitating a significant revision of the
atmospheric radiative energy budget (London and Sasamori, 1971)
and the estimated oceanic energy transport (Vonder Haar and Oort,
1973).
Several recent diagnostic and empirical studies have also focused
on aspects of the atmosphere-ocean interaction on seasonal, annual, and
decadal time scales (Lamb and Ratcliffe, 1972; Namias, 1969, 1971b,
1972a) and have prompted new attention to their relevance to long-
range forecasting (Ratcliffe, 1973; Ratcliffe and Murray, 1970). The
larger-scale variations of ocean-surface temperature and sea level have
also been studied and have led to the identification of apparent tele-
connections with the atmospheric circulation (Namias, 1971a; Wyrtki,
1973,1974).
New studies of mesoscale oceanic features have been made (Bern-
stein, 1974) and provide further evidence of the dominance of this scale
in the oceanic energy spectrum (in agreement with the preliminary
results of the mode program). Other oceanic studies have concentrated
on the empirical evaluation of the turbulent fluxes of momentum, heat,
56 UNDERSTANDING CLIMATIC CHANGE
and water vapor across the air-sea interface (Holland, 1972; Paulson
et ah, 1971, 1972). The difficulties of estimating the transport of even
the strongest ocean currents or the heat balance over ice-covered seas
with the present data base have also received renewed emphasis
(Fletcher, 1972; Niiler and Richardson, 1973; Reid and Nowlin, 1971).
Predictability and Related Theoretical Studies
An important problem in climatic variation is the determination of the
degree of predictability that is inherent in the natural system, as well
as that which is achievable by simulation. A number of recent studies
of simplified models have shown that multiple climatic solutions may
exist under the same external conditions (Budyko, 1972b; Faegre,
1972; Lorenz, 1968, 1970) in a manner suggestive of certain features
of the observed climatic record. There is also evidence from simplified
models that the completely accurate specification of a climatic state
is not achievable in any case, because of the same kind of nonlinear
error growth that limits the accuracy of weather prediction (Fleming,
1972; Houghton, 1972; Leith, 1971; Lorenz, 1969; Robinson, 1971a).
Analyses of selected climatic time series indicate only limited pre-
dictability on yearly and perhaps decadal time scales (Kutzbach and
Bryson, 1974; Leith, 1973; Lorenz, 1973; Vulis and Monin, 1971),
while the general white-noise character of higher-frequency fluctuation
has been confirmed in model simulations (Chervin et al., 1974). Further
studies of climatic predictability are needed in order to identify both
the intrinsic and practical limits of climatic prediction.
NUMERICAL MODELING OF CLIMATE AND CLIMATIC VARIATION
The accurate portrayal of global climate is the scientific goal of much
of the atmospheric and oceanic numerical modeling effort now under
way (Smagorinsky, 1974). When such models are coupled, the direct
numerical simulation of at least the shorter-period climatic variations
becomes a realistic possibility. The study of longer-period climatic
variations, however, may require the construction of increasingly
parameterized models. Here the more recent modeling research in both
of these approaches is briefly reviewed.
Atmospheric General Circulation Models and Related Studies
Studies with global atmospheric general circulation models (gcm's)
have focused on the simulation of seasonal climate, with emphasis on
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 57
the analysis of the surface heat and hydrologic balances (Gates, 1972;
Holloway and Manabe, 1971 ; Kasahara and Washington, 1971 ; Manabe,
1969a, 1969b; Manabe et al, 1972; Somerville et al, 1974). As de-
scribed more fully in Appendix B, simulations of average January
climate have now been achieved by several gcm's. Although additional
global gcm's are under development (Corby et al, 1972), only two at
this writing have been integrated over time longer than one year
(Manabe et al, 1972, 1974b; Mintz et al., 1972).
Global atmospheric models have also recently been applied to the
simulation of specific regional circulations, such as those in the tropics
(Manabe et al., 1974). In such applications the model's parameteriza-
tion of processes in the surface boundary layer is of particular
importance (DeardorfT, 1972; Delsol et al., 1971; Sasamori, 1970). Con-
siderable recent interest has also been shown in the simulation of strato-
spheric climate with global gcm's (Kasahara and Sasamori, 1974;
Kasahara et al., 1973; Mahlman and Manabe, 1972). An overview
of global atmospheric (and oceanic) gcm's is given in Appendix B;
more detailed reviews of these and other models have recently been
prepared (Robinson, 1971b; Schneider and Kellogg, 1973), while
others are in preparation (garp Joint Organizing Committee, 1974;
Schneider and Dickinson, 1974).
Statistical-Dynamical Models and Parameterization Studies
Research on the development of dynamical climate models (in which
the transfer properties of the large-scale eddies are statistically
parameterized rather than resolved as in the gcm's) has accelerated in
recent years (Willson, 1973). These models include those that address
only the surface heat balance (Budyko, 1969; Faegre, 1972; Sellers,
1969, 1973), those that consider the time-dependent zonally averaged
motion (MacCracken, 1972; MacCracken and Luther, 1973; Saltzman
and Vernekar, 1971, 1972; Wiin-Nielsen, 1972; Williams and Davies,
1965), and those in which the statistical eddy fluxes are represented in
terms of the large-scale motions themselves (Dwyer and Petersen,
1973; Kurihara, 1970, 1973). A key problem in such models is the
correct parameterization of the heat and momentum transports by the
large-scale eddies. While a completely adequate formulation has not
yet been achieved, research is continuing by a variety of methods
(Clapp, 1970; Gavrilin and Monin, 1970; Green, 1970; Saltzman, 1973;
Smith, 1973; Stone, 1973). Because of the generally longer time scales
involved in the oceanic general circulation, relatively less attention has
been given to the corresponding formulation of statistical-dynamical
58 UNDERSTANDING CLIMATIC CHANGE
ocean models (Adem, 1970; Petukhov and Feygel'son, 1973; Pike,
1972). This problem, however, will assume greater importance with
the increased development of coupled ocean-atmosphere systems re-
viewed below.
Oceanic General Circulation Models
Although generally less advanced than their atmospheric counterparts
oceanic gcm's have recently been developed to the point where suc-
cessful simulations of the seasonal variations of ocean temperature
and currents have been achieved in both idealized basins (Bryan, 1969;
Bryan and Cox, 1968; Haney, 1974) and in selected ocean basins with
realistic lateral geometry (Cox, 1970; Gait, 1973; Holland and Hirsch-
man, 1972; Huang, 1973). The numerical simulation of the complete
world ocean circulation has only recently been achieved with baroclinic
models (Alexander, 1974; Cox, 1974; Takano et al, 1973); this shows
significant improvement over earlier global simulations with homoge-
neous wind-driven models (Bryan and Cox, 1972; Crowley, 1968). As
noted earlier, such models have not yet been able to resolve the energetic
oceanic mesoscale eddies, although a number of experimental calcula-
tions to that end are under way.
Recent studies have also shown the importance of improving the
models' treatment of the oceanic surface mixed layer (Bathen, 1972;
Denman, 1973; Denman and Miyake, 1973) and sea ice (Maykut and
Untersteiner, 1971) and of incorporating bottom topography (Holland,
1973; Rooth, 1972) and the abyssal water circulation (Kuo and
Veronis, 1973).
Coupled General Circulation Models
Although preliminary numerical calculations with a model of the
coupled atmosphere-ocean circulation were performed several years ago
(Manabe and Bryan, 1969; Wetherald and Manabe, 1972), it is only
recently that a truly globally coupled model has been achieved (Bryan
et al., 1974; Manabe et al, 1974a). These calculations underscore the
importance of the ocean's participation in the processes of air-sea
interaction and in the maintenance of large-scale climate. These and
other such coupled models now under construction will lay the basis
for the systematic exploration of the dynamics of the atmosphere-ocean
system and its role in climatic variation. The necessary calibration and
testing of coupled gcm's will require a broad data base and access to
the fastest computers available.
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 59
APPLICATIONS OF CLIMATE MODELS
The uses of climate models extend across a wide range of applications,
including the reconstruction of past climates and the projection of
future climates. Here the more recent use of models for such studies
is briefly reviewed, as distinguished from the research on model design
and calibration reviewed above.
Simulation of Past Climates
By assembling the boundary conditions appropriate to selected periods
in the past, numerical models may be applied to the simulation of
paleoclimates. The climate of the last ice age has recently received
increased attention, , both through the application of parameterized
and empirical models (Alyea, 1972; Lamb and Woodroffe, 1970; Mac-
Cracken, 1968) and through the use of atmospheric gcm's (Kraus,
1973; Williams et al, 1973). In the latter case, the specification of the
distribution of glacial ice and sea-surface temperature represents a
strong thermal control over the simulated climate. In order to provide
realistic information on the near-equilibrium ice-age climatic state,
these conditions should be constructed on the basis of the appropriate
proxy climatic records, while other portions of this same paleoclimatic
data base serve as verification. An initial effort of this sort is now
under way as part of the climap program.
At the present time, the simulation of the time-dependent evolution
of past climates over thousands of years can only be achieved with the
more highly parameterized models; the design and calibration of such
models of the air-sea-ice system are largely tasks for the future.
Climate Change Experiments and Sensitivity Studies
Numerical climate models also permit the examination of the climatic
consequences of a wide variety of possible changes in the physical sys-
tem and its boundary conditions; such studies, in fact, are a primary
motivation for the development of the climatic models themselves. As
previously noted, a number of experiments on the effect of solar radia-
tion changes have been performed with simplified models (Budyko,
1969; Manabe and Wetherald, 1967; Schneider and Gal-Chen, 1973;
Sellers, 1969, 1973), and further studies of this kind with global models
are under way. A number of recent experiments have been made with
atmospheric gcm's on the effects of prescribed sea-surface temperature
anomalies on the large-scale atmospheric circulation (Houghton et al,
60 UNDERSTANDING CLIMATIC CHANGE
1973; Rowntree, 1972; Spar, 1973a, 1973b), while others have been
concerned with the climatic effects of thermal pollution (Washington,
1972) and of sea ice (Fletcher 1972).
Although these experiments indicate that the models display a re-
sponse over several months' time to small changes in the components
of the surface heat balance, their longer-term climatic response is not
known. Such experiments serve to emphasize the need for extended
model integrations, preferably with coupled models, and underscore
the importance of determining the models' sensitivity and the conse-
quent noise levels in model-generated climatic statistics. The reduction
of this climatic noise has an important bearing on the determination of
the significance of climatic variations (Chervin et al, 1974; Gates,
1974; Gilman et al, 1963; Leith, 1973). This question is also closely
related to the problem of long-range or climatic prediction (Brier, 1968;
Kukla^a/., 1972; Lamb, 1973a).
Studies of the Mutual Impacts of Climate and Man
Although the influence of man's activities on the local climate has
long been recognized, renewed attention has been given in recent years
to the possibility that man's increasingly extensive alteration of the
environment may have an impact on the large-scale climate as well
(Sawyer, 1971). Here the more recent of such studies are briefly re-
viewed, along with studies of the parallel problem of climate's impact on
man's activities themselves.
Aside from the numerical simulations of anthropogenic climatic
effects noted earlier, there have been a number of recent studies of
the climatic consequences of atmospheric pollution (Bryson and Wend-
land, 1970; Mitchell, 1970, 1973a, 1973b; Newell, 1971; Yamamoto
and Tanaka, 1972) and of the possible effects of man's interference
with the surface heat balance, primarily through changes of the surface
land character (Atwater, 1972; Budyko, 1972a; Landsberg, 1970;
Sawyer, 1971). Aside from local climatic effects, such as those due to
urbanization, these studies have not yet established the existence of a
large-scale anthropogenic climatic impact (Machata, 1973). Like their
numerical simulation counterparts, such studies are made more diffi-
cult by the high levels of natural climatic variability and by the lack of
adequate observational data.
A longer-range question receiving increased attention is the problem
of disposing of the waste heat that accompanies man's production and
consumption of energy. When projected into the next century, this
effect poses potentially serious climatic consequences and may prove
SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 61
to be a limiting factor in the determination of acceptable levels of
energy use (Haefele, 1973; Lovins, 1974). These and other aspects of
man's impact on the climate have been considered extensively in the
scep and smic reports (Wilson, 1970, 1971).
Recent attention has also focused on the effects of climatic varia-
tions on man's economic and social welfare. From a general awareness
of these effects (Budyko, 1971; Johnson and Smith, 1965; Maunder,
1970) research has turned to the representation of climatic anomalies
in terms of the associated agricultural and commercial impacts (Pittock,
1972) and to the development of climatic impact indices (Baier, 1973).
Further studies are necessary in order to develop comprehensive climatic
impact simulation models, with both diagnostic and predictive capability.
6
A NATIONAL CLIMATIC RESEARCH PROGRAM
While there is ample evidence that past climatic changes have had
profound effects on man's activities, future changes of climate promise
to have even greater impacts. The present level of use of land for
agriculture, the use of water supplies for irrigation and drinking, and
the use of both airsheds and watersheds for waste disposal is approach-
ing the limit. A change of climate, even if sustained only for a few
years' time, could seriously disrupt this use pattern and have far-reach-
ing consequences to the national economy and well-being. To this vul-
nerability to natural climatic changes we must add the increasing
possibility that man's own activities may have significant climatic reper-
cussions.
If we are to react rationally to the inevitable climatic changes of
the future, and if we are ever to predict their future course, whether
they are natural or man-induced, a far greater understanding of these
changes is required than we now possess. It is, moreover, important
that this knowledge be acquired as soon as possible. Although much has
been accomplished, and further research is under way on many prob-
lems (as summarized in Chapter 5), the mechanics of the climatic
system is so complex, and our observations of its behavior so incom-
plete, that at present we do not know what causes any particular climatic
change to occur.
Our response to this state of affairs is the recommendation of an
integrated research program to contain the observational, analytical,
and research components necessary to acquire this understanding.
62
A NATIONAL CLIMATIC RESEARCH PROGRAM 63
Heretofore the many pieces of the climatic puzzle have been considered
in relative isolation from each other, a subdivision that is natural to
the traditional scientific method. We believe, however, that the time has
now come to initiate a broad and coordinated attack on the problem of
climate and climatic change. Such a program should not stifle the de-
velopment of new and independent lines of attack nor seek to assemble
all efforts under a single authority. On the contrary, its purpose should
be to provide a coordinating framework for the necessary research on all
aspects of this important problem, including the strengthening of those
efforts already under way as well as the initiation of new efforts. Only
in this manner can our limited resources be used to maximum benefit
and a balanced and coherent approach maintained.
THE APPROACH
From the summary of recent and current research on climate and
climatic variation presented in Chapter 5, it is clear that considerable
effort has been devoted to this problem. It is also clear that much
remains to be done. As an approach to the research program itself, we
here attempt to summarize what is now known and to identify those
elements that now make a greatly expanded effort both feasible and
desirable.
What Climatic Events and Processes Can We Now Identify?
From the analysis of accumulated instrumental climatic data, we can
identify some of the major characteristics of the climatic changes of
the past few decades. These include the presence of seasonal and
annual circulation anomalies over large regions of the earth, together
with some longer-term trends. More recent satellite observations have
documented changes in worldwide cloudiness, snow cover, and the
global radiation balance and have served to emphasize the climatic
role of the oceans. Although the necessary oceanic measurements have
not yet been made, satellite observations (together with atmospheric
data) indicate that the oceans accomplish between one third and one
half of the total annual meridional heat transport.
From the analysis of selected paleoclimatic data, it appears that
ancient climates have been somewhat similar in behavior to the present-
day climate, although the resolution is poorer. These data also suggest
the presence of seemingly quasi-periodic climatic fluctuations on time
scales of order 100,000 years, associated with the earth's major
glaciations.
64 UNDERSTANDING CLIMATIC CHANGE
From the solutions of numerical general circulation models, we
can identify a number of important physical elements in the mainte-
nance of global climate. Primary among these is the role played by
convective motions in the vertical heat flux and by the transfers of heat
at the ocean surface. Climate models also show that the climate is
sensitive to the extent of cloudiness and to the surface albedo. Recent
solutions of global atmospheric models have shown that the accuracy
of the simulations of cloudiness and precipitation is more difficult to
establish than the average seasonal distribution of the large-scale
patterns of pressure, temperature, and wind, which are simulated with
reasonable accuracy (see Appendix B). This may be due to the pre-
scription of the sea-surface temperature in the atmospheric models,
serving to mask errors in the models' heat balance.
Less experience has been gained with oceanic general circulation
models, although they are capable of portraying the large-scale thermal
structure of the oceans and the distribution of the major current sys-
tems when subject to realistic (atmospheric) surface boundary condi-
tions. These and other models are just beginning to identify the energetic
mesoscale eddy, which in some ways appears to be the oceanic counter-
part of the transient cyclones and anticyclones in the atmosphere.
From the analysis of a variety of climate models, as well as from
the analysis of climatic data, we can identify a number of links or
processes in the phenomenon of climatic change. On at least the shorter
climatic time scales, the climatic system is regulated by a number of
feedback mechanisms, especially those involving cloudiness, surface
temperature, and surface albedo. Underlying these effects is the increas-
ing evidence that large-scale thermal interactions between the ocean
and atmosphere are the primary factor in climatic variations on time
scales from months to millenia. These interactions must be examined
in coupled ocean-atmosphere models, whose development has just
begun. The role of the oceans in the climatic system raises the pos-
sibility of some degree of useful predictability on, say, seasonal or
annual time scales and is an obviously important matter for further
research.
From the analysis of the limited data available, we can identify a
number of areas in which man's actions may be capable of altering
the course of climatic change. Chief among these is interference with
the atmospheric heat balance by increasing the aerosol and particulate
loading and increasing the C02 content of the atmosphere by industrial
and commercial activity. While present evidence indicates that these
are not now dominant factors, they may become so in the future. To
these we must also add the possibility of man's direct thermal inter-
A NATIONAL CLIMATIC RESEARCH PROGRAM 65
ference with climate by the disposal of large amounts of waste heat
into the atmosphere and ocean. Although important large-scale thermal
pollution effects of this sort do not appear likely before the middle
of the next century, they may eventually be the factor that limits the
climatically acceptable level of energy production.
Why Is a Program Necessary?
Although the conclusions identified above represent important research
achievements, they are nevertheless concerned with separate pieces of
the problem. What we cannot identify at the present time is how the
complete climatic system operates, which are its most critical and sensi-
tive parts, which processes are responsible for its changes, and what are
the most likely future climates. In short, while we know something about
climate itself, we know very little about climatic change.
From among the present activities we can identify important prob-
lems requiring further research. In general, these concern new observa-
tions and the further analysis of older ones, the design of improved
climatic models of the atmosphere and ocean, and the simulation of
climatic variations under a variety of conditions for the past, present,
and future. As we attempt this research on a global scale, it becomes
increasingly important that we ensure the smooth flow of data and
ideas, as well as of resources, among all parts of the problem. The
attention devoted in each country (and internationally through garp)
to the improvement of weather forecasting (a problem whose physical
basis is reasonably well understood) must be matched by a program
devoted to climate and climatic variation, a problem whose global
aspects are even more prominent and whose physical basis is not at all
well understood. The need for a broad, sustained, and coordinated
attack is therefore a fundamental reason for a climatic research program.
Other circumstances also indicate that a major research program
on climatic change is both timely and necessary. First, for the past
few years we have had available to us the unprecedented observational
capability of meteorological satellites. This capability has steadily in-
creased from the initial observations of the cloudiness, radiation budget,
and albedo to include the vertical distribution of temperature and
moisture, the extent of snow and ice, the sea-surface temperature, the
presence of particulates, and the character of the land surface. The
regular global coverage provided by such satellites clearly constitutes
an observational breakthrough of great importance for climatic studies.
Second, the steady increase in the speed and capacity of computers,
which has been taking place since their introduction in the 1950's, has
66 UNDERSTANDING CLIMATIC CHANGE
reached the point where numerical integration of global circulation
models over many months or even years is now practical. Such calcula-
tions, along with the associated data processing, will form the quantita-
tive backbone of climatic research for many years to come, and their
feasibility clearly constitutes a computational breakthrough. This com-
puting capability, as represented, for example, by machines of the
ti-asc or illiac-4 class, will permit extensive experimentation for the
first time with the coupled global climatic system.
Finally, the recent development of unified physical models of the
coupled ocean-atmosphere may itself be viewed as a modeling break-
through of great importance. Up to now either the atmosphere or the
ocean has been considered as a separate entity in global modeling, and
their solutions have consequently described a sort of quasi-equilibrium
climate. The simulation of climatic variation with these models, on the
other hand, is just now beginning. A future modeling breakthrough
of equally great importance will be the successful parameterization of
the eddy transports of baroclinic disturbances in the atmosphere and
in the ocean.
Aside from the practical importance (or even urgency) of the climatic
problem, the breakthroughs noted above indicate that a time is at
hand during which progress will be in proportion to our efforts. By co-
ordinating these efforts into a coherent research program, we may
therefore expect to achieve significantly greater understanding of
climatic variation.
THE RESEARCH PROGRAM (NCRP)
We have here assembled our specific recommendations for the data,
the research, and the applications that we believe constitute the needed
elements of a comprehensive national research program on climatic
change. We recognize that some of the elements of this program re-
quire considerable further development and coordination. We also
recognize that some of the recommended efforts are already under way
or are planned by various groups, but we believe that their identification
as parts of a coherent program is both valuable and necessary. Our
recommendations for the planning and execution of this program are
given later in this chapter, including those items on which we urge im-
mediate action.
Data Needed for Climatic Research
The availability of suitable climatic data is essential to the success of
climatic analysis and research, and such data are an integral part of
A NATIONAL CLIMATIC RESEARCH PROGRAM 67
the overall program. The needed data are discussed below in terms of a
subprogram for climatic data assembly and analysis and a subprogram
for climatic index monitoring. These are the efforts that we believe to
be necessary to make the store of climatic data more useful to the
climatic research community and to ensure the systematic collection of
the needed climatic data in the future.
Climatic Data Analysis
Instrumental Data Instrumental observations of the atmosphere ade-
quate to depict even a decadal climatic variation are available only
for about the last half century for selected regions of the northern
hemisphere, and the observational coverage of the oceans is even poorer
in both space and time (see Appendix A). In order to assess more ac-
curately the present data base of conventional observations and the
needed extensions of such data, a number of efforts should be under-
taken:
A worldwide inventory of climatic data should be taken to determine
the amount, nature, and location of past and present instrumental
observations of the following variables: surface pressure, temperature,
humidity, wind, rainfall, snowfall, and cloudiness; upper-air tempera-
ture, pressure-altitude, wind, and humidity; ocean temperature, salinity,
and current; the location and depth of land ice, sea ice, and snow; the
surface insolation, ground temperature, ground moisture, and runoff.
This inventory should identify the length of the observational record,
the data quality, and the state of its availability. In addition to the
usual data sources, efforts should be made to locate data from private
sources, older records, and unpublished climatological summaries. Al-
though some of these data have been summarized, no overall inventory
of this type exists.
Selected portions of these data should be systematically transferred
to suitable computer storage, in a format permitting easy access and
screening by variable, time period, and location. These data should
then be used to compute in a systematic fashion a basic set of climatic
statistics for as many time periods and for as many regions of the world
as possible. These should include the means, the variances, and the ex-
tremes for monthly, seasonal, annual, and decadal periods, for both
individual stations and for various ensembles of stations up to and in-
cluding the entire globe. Research should also be devoted to the effects
of instrumental errors, observational coverage, and analysis procedures
on climatic statistics.
Recognizing that these data have large differences in quality, cover-
68 UNDERSTANDING CLIMATIC CHANGE
age, and length-of-record and were often collected as by-products of
other studies, new four-dimensional climatological data-analysis schemes
should be developed, based on suitable analysis methods or models,
to synthesize as much of the missing information as possible while
making maximum use of the available data. Efforts should also be
devoted to the design of suitable computerized graphical display and
output.
Once such syntheses are available, we recommend that suitable
climatological diagnostic studies be made using dynamical climate
models to generate systematically the various auxiliary and unobserved
climatic variables, such as evaporation, sensible heat flux, surface
wind stress, and the balances of surface heat, moisture, and momentum.
Such data, of course, would be artificial but may nevertheless be of
diagnostic use. Insofar as possible, the pertinent statistics of the
atmospheric and oceanic general circulations and their energy, mo-
mentum, and heat balances should be determined.
The results of such analyses should be made available in the form
of new climatological atlases, supplementing and extending those now
available for scattered portions of the record and for selected regions
of the world. The widely used climatological summaries of Sverdrup
(1942), Moller (1951), and Budyko (1963), for example, are largely
based on the subjective analyses of older data of uncertain quality.
Other analyses are more authoritative (Oort and Rasmusson, 1971;
Newell et al., 1972) but are in need of extension.
We wish to emphasize the great importance of the potentially un-
matched coverage of observations from satellites. Those that are of
climatic value should be systematically cataloged and summarized and
made available on as timely a basis as possible. These should include
observations of cloud cover, snow, and ice extent; planetary albedo; and
the net radiation balance. As remote techniques for measuring the at-
mosphere's composition, motion, and temperature structure (and the
surface temperature of land and ocean) are developed, these data
should be systematically added to the climatological inventory. They
should also be used in the analysis and model-based diagnostic efforts
described above and in the climatic index monitoring program out-
lined below. The presently available summaries of such data (e.g.,
Vonder Haar and Suomi, 1971) have yielded important new results and
should be continued on an expanded basis.
Historical Data As noted in Appendix A, a wealth of information
has been recorded on past variations of weather and climate in historical
sources such as books, manuscripts, logs, and journals during the past
A NATIONAL CLIMATIC RESEARCH PROGRAM 69
several centuries. While much of these data are fragmentary and not
of a quality comparable with that of instrumental observations, it is
nevertheless of value. We therefore recommend that
An organized effort be made to locate, classify, and summarize
historical climatic information and to identify and exploit new sources.
From the studies of this sort that have already been made (e.g., Bryson
and Julian, 1963; LeRoy Ladurie, 1971; Lamb, 1968, 1972), it is clear
that these efforts should involve historians, archeologists, and geog-
raphers on an international scale.
Efforts be made to relate this material to data from other proxy
sources whenever possible, and efforts made to interpret and focus
the material in a climatologically meaningful way.
Proxy Data We recognize the unique value of proxy data for studies
of climatic change. Such data are obtained from the analysis of tree-ring
growth patterns, glacier movements, lake and deep-sea sediments, ice
cores, and studies of soil and periglacial stratigraphy. Data from tree
rings, annually layered lake sediments, and some ice cores are capable
of providing information for individual years, while those from other
sources provide more generalized climatic information on time scales
of decades, centuries, and millenia. Such data constitute the only source
of records for the study of the structure and characteristics fluctuations of
ancient climates. As discussed in Appendix A of this report, some
of these past climates were quite different from the present regime and
provide our only documentation of the extreme states of which the
earth's climatic system is capable.
Because all proxy climatic data may contain both bias and random
error components, it is essential that a variety of independent proxy
records be studied. It is important that coverage be as nearly global
as possible, since most of the information on climatic variations is
contained in the spatial patterns of the data fields. While noting that
some such activity is already in progress, we urge that the assembly
and analysis of paleoclimatic data be initially focused on four time
spans (see below). This represents a strategic decision in order to
make the best use of the available resources. In each area it is im-
portant that steps be taken to increase greatly the degree of coordina-
tion and cooperation within the paleoclimatological community, and
that the cross-checking of overlapping data sets, the development of
complementary and independent proxy data sources, and the calibration
against instrumentally observed data be undertaken whenever possible.
The last 10,000 years. This is the interval within which we may hope
70 UNDERSTANDING CLIMATIC CHANGE
to gain insight into the current interglacial period by the systematic
assembly of a wide variety of proxy climatic data. This is also the
interval of greatest practical importance for the immediate future. For
this period particular attention should be given to six techniques:
Studies of the structural, isotopic, and chemical properties of tree
rings should be intensified and extended to a global coverage. Since
forests cover large areas of the globe, it is possible in principle to de-
velop climatic records over extensive continental areas and to recon-
struct the spatial patterns of past climate for the past several centuries
or millenia. The amount of effort depends on the availability of suitable
trees and on the resolution required in the climatic reconstruction. Data
on variations of the density of wood from x-ray techniques and on the
concentrations of trace elements and of stable isotopes of carbon, hy-
drogen, and oxygen in well-dated rings should also be developed.
Special efforts should be made to calibrate the few millenia-long tree-
ring records with information from other suitable proxy data sources,
such as pollen, varves, and ice cores.
Studies of pollen records in lakes and bogs should be extended. Most
pollen analyses to date have concerned bogs created by the retreat of
the last continental ice sheet. In order to permit synoptic reconstruc-
tion of the global vegetational record for the past 10,000 years or so,
pollen analyses with extensive 14C dating should be extended to the
nonglaciated areas of the world, particularly to low-latitude regions
and to the southern hemisphere.
Studies of the polar ice caps should be expanded. This should include
additional short ice cores in widely distributed locations, in both
Greenland and Antarctica, and more detailed isotopic analyses.
Studies of the major mountain glaciers should be expanded, to ob-
tain additional information on the various glaciers' advances and re-
treats, using chronological control where possible.
Studies of ocean sediments in the few basins of known high deposi-
tion rates should be greatly expanded. Particularly near the continental
margins, the synoptic reconstruction of even the decadal variations
of sea-surface temperatures (and possibly of currents as well) would
be of great paleoclimatic interest. This effort will involve lithologic,
faunal, and isotopic analyses of long cores collected specifically for
this purpose.
The records from varved sediments in closed basin lakes or land-
locked seas should be extended. Such data are particularly sensitive to
the climatic fluctuations in arid regions and would further our knowledge
of the long-term behavior of deserts and drought.
The last 30,000 years. This interval is dominated by the waxing and
A NATIONAL CLIMATIC RESEARCH PROGRAM 71
waning of continental ice sheets. In this interval the radiocarbon dating
method provides a good chronology, and the possibilities for studying
the relative phases of different proxy climatic records on a global basis
are a maximum. In this period particular efforts should be made in the
following areas :
Pollen records for the interval 10,000 to 30,000 years ago should
be obtained in a wide variety of sites in both hemispheres.
Ice-margin data should continue to be collected for northern hem-
isphere glaciers and should be extended into southern hemisphere
mountain areas.
Additional deep-sea cores should be obtained, especially in the
Pacific and Southern Oceans, in order to reveal further the geographic
pattern of marine paleoclimates. These data would be particularly useful
from high-deposition-rate basins.
Additional data should be obtained on the fluctuations in the extent
and volume of the polar ice sheets during this time interval. Particular
attention should be given to the smaller ice sheets, such as the West
Antarctic and Greenland ice sheets, which react more rapidly to climatic
variations.
More extensive analyses of sea-level records should be made, empha-
sizing the removal of tectonic and isostatic effects. Present studies on
raised coral reefs should be extended, and estuarine borings should be
carefully dated and given thorough lithologic analysis.
The last 150,000 years. Here we should seek to increase our knowl-
edge of the last 100,000-year glacial-interglacial cycle. This interval
includes the last period in the climatic history of the earth that was evi-
dently most like that of today. The data of this period also provide the
best example of how the last interglacial period ended. Efforts should be
made to further develop a number of proxy data sources, including:
Extensive collection and analyses of marine sediment cores to pro-
vide adequate global coverage of the world ocean.
Further studies of the fluctuations of the Antarctic and Greenland
ice caps, with emphasis on records extending beyond the beginning of
the last interglacial. This should include a geographic network of ice
cores of sufficient length to penetrate this time range, of which those
at Camp Century, Byrd, and Vostok are now the only examples.
Further systematic study of the loess-soil sequences in suitable regions
around the world, including Argentina, Australia, China, and the Great
Plains of North America.
Systematic studies of desert regions and arid intermountain basin
72 UNDERSTANDING CLIMATIC CHANGE
areas in order to examine the patterns of long-term changes in aridity.
Present records are limited to about the last 40,000 years, and their
extension will require long borings in selected lakes and playas.
Extended studies of sea-level variations from coral reef and island
shorelines features.
Further studies of long pollen records covering previous interglacial
periods. This should include data from previously unsampled regions
of the world, particularly in the southern hemisphere.
The last 1,000,000 years and beyond. Fluctuations in this time range
should not be ignored simply because of their antiquity. Here we have
the opportunity to compare the circulation patterns that have char-
acterized the last several full-glacial and interglacial periods, and
thereby to contribute evidence on the question of the degree of de-
terminism of the earth's climatic system. Efforts should therefore be
made to extend suitable proxy records into this time range, including:
Additional marine sediment cores of sufficient length (say, up to
100 m long) to cover several glacial cycles should be obtained. This
will require new innovations in drilling technology, as piston cores
do not penetrate deeply enough for this purpose, and rotary drills
presently in use greatly disturb the sedimentary record.
The record of the Antarctic ice sheet (and the associated sea-level
variations) should be extended as far back as possible and in as much
detail as possible. This ice mass is a living climatic fossil and may
contain information about the global climate for the past several million
years.
Climatic Index Identification and Monitoring In addition to the data
provided by conventional surface and upper-air observations, climatic
studies require other contemporary data that are not now readily
available. The one hope for obtaining truly global coverage of many
current climatic variables rests with satellite observations. We expect
that climatic studies in the foreseeable future will have to rely on a
combination of conventional observations, satellite observations, and
special observations designed to monitor selected climatic variables
as discussed below. We should therefore make full use of the temporary
expansion of the observational network planned for the fgge in 1978
in order to design a longer-lived climatic observing program. In addition,
efforts should be made to process the monitored data from both satel-
lites and other systems into forms that are useful for climatic studies.
Support should be given to the development of new satellite-based ob-
A NATIONAL CLIMATIC RESEARCH PROGRAM 73
servational techniques, including those designed to monitor the oceans
and the earth's surface.
There remain, however, a number of processes that are important to
climate that are now beyond the reach of satellite observations. Primary
among these is the pattern of the planetary thermal forcing, which
drives the atmospheric and oceanic circulation, and the related balance
of energy at the earth's surface. Even a measurement of the average
pole-to-equator temperature difference tells us something about the
circulation; and, in a similar way, the discharge of a river gives us
some information on the hydrologic balance in the river's basin.
Such measurements, which represent time and space integrals of
climatically important procesess, we term "climatic indices." While
efforts to monitor indices of this sort are already under way, we recom-
mend that further efforts be made to identify and monitor a variety of
such indices in a coordinated and sustained fashion, as part of a compre-
hensive global Climatic Index Monitoring Program (cimp) whose
elements are outlined below.
Atmospheric Indices The heat balance of the atmosphere is basic
to the character of the general circulation and hence is a principal de-
terminant of climate. It is therefore important that the primary ele-
ments of this balance be monitored with as much accuracy and with
as nearly global coverage as possible. In particular, we recommend that
further efforts be made to
Monitor the solar constant and the spectral distribution of solar
radiation with appropriate satelliteborne instrumentation.
Monitor the net outgoing shortwave and long-wave radiation by
satellite-based measurements, from which determinations of the ab-
sorbed radiation and planetary albedo may be made.
Monitor the latent heat released in large-scale tropical convection,
possibly with the aid of satellite cloud observations.
Develop methods to monitor remotely the surface latent heat flux
into the atmosphere, possibly with the aid of satellite measurements
of the vertical distribution and total amount of water vapor. These
methods (and those for the sensible heat flux discussed below) will
require calibration against field appropriate measurements, especially
over the oceans.
Develop methods to monitor remotely the surface sensible heat flux
into the atmosphere, especially that from the oceans, such as occurs in
winter off the east coasts of the continents and in the higher latitudes.
Efforts should also be made to monitor remotely the vertical sensible
74 UNDERSTANDING CLIMATIC CHANGE
heat flux that occurs as a result of convective motions both over the
oceans and over land.
Expand the satellite monitoring of global cloud cover to include
information on the clouds' height, thickness, and liquid water content,
so that their role in the heat balance may be determined.
Monitor the distribution of surface wind over the oceans, possibly
by radar measurements of the scattering by surface waves or from
the microwave emissivity changes created by foam.
Oceanic Indices In view of the fundamental role the oceans play
in the processes of climatic change, special efforts should be made to
monitor those oceanic variables associated with large-scale thermal
interaction with the atmosphere. In addition to the low-level air tem-
perature, moisture, cloudiness, surface wind, and surface radiation, the
surface heat exchange depends critically on the sea-surface temperature
and heat storage in the oceanic surface layer itself. We therefore recom-
mend that further efforts be made to
Monitor the worldwide distribution of sea-surface temperature by
a combination of all available ship, buoy, coastal, and satellite-based
measurements. Sea-surface temperature analyses, such as now per-
formed operationally by the Navy's Fleet Numerical Weather Central
in Monterey, should be extended and supplemented for climatic pur-
poses on a global basis by improved satellite observations capable of
penetrating cloud layers. The drifting buoy observations of sea-surface
temperature planned for the fgge should be expanded and maintained
on a routine basis.
Monitor the heat storage in the surface layer of the ocean by a
program of observations from satellite-interrogated expandable drifting
buoys and by expendable bathythermograph (xbt) observations from
ships-of-opportunity in those areas of the world ocean traveled by
commercial ships. It is estimated that there are several hundred such
transits each year across most major oceans of the world. An expan-
sion of xbt observations from merchant ships-of-opportunity is being
undertaken by the North Pacific project (norpax), in cooperation with
the Navy's Fleet Numerical Weather Central and noaa's National
Marine Fisheries Service. Similar programs should be undertaken in
the other oceans, and especially in the oceans of the southern hemisphere,
with special efforts made to place instruments aboard ships on uncon-
ventional routes and on selected government vessels. This xbt program
should be supplemented by buoy measurements in selected locations
A NATIONAL CLIMATIC RESEARCH PROGRAM 75
and by xbt's launched from aircraft on meridional flight paths in the
more inaccessible ocean areas.
Expand the present data buoy programs now under way by noaa and
others, so that the volume and heat transport of the major ocean cur-
rents can be monitored. Suitably deployed bottom-mounted sensors,
moored buoys, or both should be used to monitor the transport of the
Gulf Stream, Kuroshio, and Antarctic circumpolar currents in selected
locations, such as is planned for the Drake Passage as part of the Inter-
national Southern Ocean Studies (isos). The water mass balance of
individual basins such as the Arctic should also be monitored.
Monitor the complete temperature structure in selected regions of
the ocean, such as meridional cross sections through the major gyral
circulations. The several long-term local observational series (such
as the Panulirus, Plymouth, and Murmansk sections) should be main-
tained and new efforts started in regions of special interest.
Monitor the vertical salinity structure of the oceans in those high-
latitude regions where salinity plays an important role in determining
the density field of the upper ocean layers. Near-surface salinity is
also important in regions where ocean bottom water is formed, such
as in the Weddell Sea. This might best be done by a combination of
moored buoys and ship observations.
Monitor the large-scale distribution of sea level by the use of an
expanded network of tide gauges. Such a measurement program at
island sites in the equatorial Pacific is being undertaken in connection
with norpax, and other measurements are planned in the Indian Ocean
as part of the Indian Ocean Experiment (index). Radar altimeters
such as those proposed for the seasat-a satellite should also be useful
for this purpose.
Monitor the oceanic chemical composition at selected sites and in
selected sections, including the concentrations of dissolved gases and
trace substances. Such measurements now being performed as part of
the geosecs program should be expanded and continued.
Cryospheric Indices In view of the great influence of snow and ice
cover on the surface energy balance, further efforts should be made to
Monitor the distribution of sea ice in the polar oceans and the ice
in major lakes and estuaries. Efforts should also be made to measure
as many as possible of the ice's physical properties by remote sensing.
Devote further study to the current mass budgets of the Antarctic
and Greenland ice caps, from both glaciological field observations and
76 UNDERSTANDING CLIMATIC CHANGE
from airborne and satellite measurements. Such observations should
include changes in ice-edge locations, in the numbers and sizes of ice-
bergs, and in the ice caps' firnline height. Methods for the remote aerial
sensing of surface temperature and possibly ice accumulation rate should
also be further developed.
Extend the monitoring of the movement and mass budget of se-
lected mountain glaciers.
Monitor the extent, depth, and characteristics of worldwide snow
cover.
Surface and Hydrologic Indices In association with the monitoring
of the elements of the surface heat balance, and of the various oceanic
and cryospheric climatic indices, initially lower priority but neverthe-
less important efforts should be made to
Monitor the natural changes of surface vegetative cover, possibly
by observations from earth resources satellites.
Monitor the variations of soil moisture and groundwater, possibly
by satellite-based techniques.
Monitor the flow and discharge of the major river systems of the
world.
Monitor the level and water balance of the major lakes of the world.
Monitor the total precipitation (especially rainfall over the oceans),
possibly by satelliteborne radar observations and surface gauges.
Composition and Turbidity Indices In view of the role that at-
mospheric constituents and aerosols play in the heat balance of the
atmosphere, further efforts should be made to
Monitor the chemical composition of the atmosphere at a number
of sites throughout the world, with particular reference to the content
of C02. Measurements such as those at Mauna Loa should be con-
tinued and extended to additional selected sites. The composition of
the higher atmosphere should also be periodically determined, especially
the water vapor in the stratosphere and the ozone concentration in the
stratosphere and mesosphere.
Monitor the total aerosol and dust loading of the atmosphere, to-
gether with determinations of the vertical and horizontal aerosol distri-
bution, by an extension of such programs as ncar's Global Atmospheric
Aerosol Study (gaars). In addition to turbidity measurements, the
aerosol particle-size distribution and optical properties should be de-
termined when possible. Efforts should also be made to monitor the
A NATIONAL CLIMATIC RESEARCH PROGRAM 11
occurrence of large-scale forest fires and volcanic eruptions, together
with estimates of their particulate loading of the atmosphere.
Anthropogenic Indices In view of man's increasing interference with
the environment, further efforts should be made to
Monitor the addition of waste heat into the atmosphere and ocean.
Although the present levels of thermal pollution are relatively small on
a global basis, steadily increasing levels of energy generation pose a
threat to the stability of at least the local climate and possibly the
larger-scale climate as well. Therefore both the local thermal discharges
of power generating and industrial facilities should be monitored, along
with the thermal pollution from urbanized areas.
Monitor the climate-sensitive chemical pollution of the atmosphere
and ocean. Measurement programs such as those of the Environmental
Protection Agency and the Atomic Energy Commission should be ex-
panded on a global basis and extended to the oceans.
Monitor the changes of large-scale land use, including forest clear-
ing, irrigation, and urbanization, possibly by the use of earth resources
satellites.
Summary of Climatic Index Monitoring A summary of the elements
of the recommended program is given in Table 6.1. Here we have not
made an assessment of the required accuracy of the various monitored
indices, nor has the capability of presently available instrumentation
been thoroughly reviewed. Further analysis is also needed to determine
the characteristic variability of each climatic index. In general, the
surface heat and hydrologic balances should be monitored with an
accuracy of a few percent, so that space- and time-averaged climatic
statistics will have at least a 5 percent accuracy. It is important that this
monitoring activity be undertaken on a continuing and long-term basis
for at least two decades in order to assemble a meaningful body of
data for climatic analyses. As noted below, these efforts should be
coordinated on an international scale and be a part of an international
climatic program.
Research Needed on Climatic Variation
We here outline the research that we believe needs to be performed,
in terms of model development, theoretical research, and empirical
and diagnostic studies. While research in some of these areas is already
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80 UNDERSTANDING CLIMATIC CHANGE
efforts are the necessary ingredients of the much broader climatic
research program that we recommend be carried out in the years ahead.
Theoretical Studies of Climatic Change Mechanisms We recognize
the importance of theoretical studies in a problem as complex as climatic
variation and the essential interaction that must take place between
theory and the complementary observational and numerical modeling
studies. Our present knowledge of the mechanisms of climatic variation
is so meager, however, and progress in this area so difficult to anticipate,
that any recommendations are subject to modification as new avenues
of attack open up or as old ones prove fruitless. There are, however,
certain fundamental problems to which further study must be directed:
The question of the degree of predictability of (natural) climatic
change must be given further theoretical attention. While the local
details of weather do not appear to be predictable beyond a few weeks'
time, the consequences of this fact for climatic variations are not clear.
In such studies a clear definition of the internal climatic system needs
to be made, and particular attention must be given to the roles of the
ocean and ice. This question has an obvious and important bearing on
our eventual capability to predict climatic variation.
The related question of the possible intransitivity of climatic states
needs further study, again with particular attention to the oceans and
ice. The whole question of climatic variation may be viewed as a
stability problem for a system containing elements with very different
time constants, and support should be given to such theoretical ap-
proaches.
Theoretical research should be directed to the nature and stability
of the various climatic feedback mechanisms identified earlier, par-
ticularly those involving the sea-surface temperature, cloudiness, albedo,
and land-surface character.
Further theoretical research should be directed to the general prob-
lem of the development of statistical-hydrodynamical representations
of climate and to the parameterization of transient phenomena on a
variety of time and space scales.
Additional theoretical studies should also be made of specific climatic
phenomena, such as drought and the growth of arid regions, ice ages and
the stability of polar ice cover, and the effects of global pollution from
natural and artificial sources.
Atmospheric General Circulation Models The global dynamical
models of the atmospheric general circulation (or gcm's) that have been
A NATIONAL CLIMATIC RESEARCH PROGRAM 81
developed in recent years represent the most sophisticated mathe-
matical tools ever available for the study of this system and are the
testing grounds for many of our theoretical ideas. The latest versions
of these models (see Appendix B) embody much of the physics that
governs the larger scales of atmospheric behavior, along with physical
parameterization s of smaller-scale processes. In addition to the simula-
tion of the free-air temperature, pressure, wind, and humidity distribu-
tions over the globe with a resolution of several hundred kilometers,
such atmospheric models provide solutions for the various components
of the heat and moisture balances, such as the fluxes of shortwave and
long-wave radiation, sensible heat flux, evaporation, precipitation, sur-
face runoff, and ground temperature. The surface boundary conditions
usually assumed are the distributions of sea-surface temperature and
sea ice, and the assumption of a heat balance over land surfaces. After
a spin-up period of a month or so during which the temperature comes
into statistical equilibrium (with the sun's heating and the ocean surface
temperature), the average global climate simulated by such models
shows a reasonable resemblance to observation; several examples of
such simulations are shown in Appendix B.
In order to improve the fidelity of such global atmospheric models
for the simulation of the various processes of climatic change, and to
ensure their increased availability for the conduct of climatic experi-
ments, efforts should be made to
Improve the models' treatment of clouds, especially those of the
nonprecipitating high-altitude cirrus and the low-level stratus type.
Account should be taken of the liquid-water content of clouds and the
full interaction of clouds with atmospheric radiative transfer. Attention
should also be given to the modeling of cloud evaporation and advection.
Improve the parameterization of turbulent, convective, and mesoscale
processes by comparing the performance of alternative schemes against
appropriate observations of the fluxes of heat, moisture, and momentum.
Particular attention should be given to improved parameterizations of
the fluxes within the surface boundary layer, to the parameterization of
cumulus convection, and to the treatment of energy flux by gravity
waves.
Improve the treatment of ground cover and land usage in the calcula-
tion of the surface heat and moisture balances. Particular attention
should be given to the improvement of the prognostic schemes for
snow cover, as this may prove of importance in seasonal climatic
variations.
Parameterize the role of aerosols in such models, so that the effects
82 UNDERSTANDING CLIMATIC CHANGE
of both natural and anthropogenic particulates on the heating rate of
the atmosphere may be determined.
Improve the numerical resolution of the solutions by the use of finer
grids (or the use of graded meshes in regions of special interest) and
increase the computational efficiency by the development of more
accurate numerical algorithms and improved solution methods.
Simulate the annual cycle of atmospheric circulation with models
using observed forcing functions to obtain the surface fluxes of heat,
momentum, and water vapor. Such numerical integrations are necessary
in order to ensure adequate model calibration and to simulate climatic
statistics for the atmosphere.
Determine the noise level or sensitivity of the model-simulated climate
to changes in the initial conditions (including random errors) and to
changes in the parameterizations of the model. Such studies are neces-
sary in order to determine the physical significance of numerical climate-
change experiments made with atmospheric models.
Oceanic General Circulation Models The oceanic general circulation
models (or gcm's) are generally at a less advanced stage of develop-
ment than their atmospheric counterparts and have only recently been
extended to the global ocean (see Appendix B). With the surface bound-
ary conditions of specified thermal forcing and wind stress (plus the
kinematic and insulated wall boundary conditions at the bottom and
lateral sides of the ocean basin), such models simulate with fair ac-
curacy the large-scale distributions of ocean temperature and current
with a resolution of several hundred kilometers. If the density structure
is specified from observations, a model will spin up from rest in a few
months' time and show a reasonable correspondence with observed drift
current patterns at the surface. The simulated transport of the major
western boundary currents in the models is generally less than that
indicated by available observations but nevertheless quantitatively
more accurate than the predictions of previous theories. Areas of
coastal and equatorial upwelling show the same strong relationship to
the surface wind-stress pattern in the model as is observed in the real
ocean.
The more relevant calculation with respect to climate modeling is
one in which the density field as well as the velocity field is predicted
from boundary conditions that determine the vertical flux of momentum,
heat, and water at the ocean surface. However, this problem involves
much longer time scales — the spin-up time of a prestratified ocean is
of the order of two or three decades; but if changes in the abyssal
thermal structure are to be predicted, then the "turnover" time of the
A NATIONAL CLIMATIC RESEARCH PROGRAM 83
ocean is the order of several centuries. Preliminary results (see, for ex-
ample, Bryan and Cox, 1968) show that such models can successfully
simulate the gross features of the density structure of the world ocean,
although more detailed calculations must be made to provide a critical
test.
In order to improve the accuracy of ocean models and to lay the
foundation for their successful coupling with atmospheric models, efforts
should be made to
Improve our knowledge of the structure, behavior, and role of
mesoscale eddies in the ocean. In the atmosphere there is a peak in
the kinetic energy spectrum observed at wavelengths of a few thousand
kilometers, whereas in the ocean the peak kinetic energy is in eddies
that have a radius (or quarter-wavelength) of the order of 102 km.
Thus an ocean circulation model requires about an order of magnitude
greater horizontal resolution to resolve its most energetic eddies than
does an atmospheric gcm. Further field studies such as those conducted
under the Mid-ocean Dynamics Experiment (mode), the North Pacific
Experiment (norpax), and those planned under the joint Soviet-
American (polymode) experiment, are needed to determine the transfer
of heat and momentum by such eddies. Such observational experiments
should provide the basis for the interpretation of high-resolution nu-
merical experiments, which are necessary to resolve the details of the
eddy motions and to establish their role in the oceanic general
circulation.
Intensify research on the parameterization of turbulent and meso-
scale motions both in the surface mixed layer and the deeper ocean
layers, including thermohaline convection, so that the results of field
measurements may be usefully incorporated into global ocean circulation
models.
Improve the prediction of sea-surface temperature and heat transport
by the inclusion of the depth and structure of the surface mixed layer
as a predicted variable in oceanic general circulation models. This
should include experiments on the numerical forecasting of the oceanic
surface layer, as driven by observed surface conditions, and the forma-
tion and behavior of pools of anomalously warm or cold water.
Simulate the annual cycle of sea-surface temperature and currents
with models using observed forcing functions to obtain the surface
fluxes of momentum, heat, and water (precipitation minus evaporation).
Such numerical integrations must be carried out over several annual
cycles, in order to ensure adequate model calibration and to simulate
climatic statistics for the ocean.
84 UNDERSTANDING CLIMATIC CHANGE
Subject the ocean models to the same kind of diagnostic testing and
sensitivity analysis as performed for atmospheric models, in order to
determine the roles of possible oceanic feedback processes and the
levels of predictability associated with various oceanic variables.
Apply high-resolution versions of global oceanic circulation models
(or regional versions thereof) to the study of the behavior of local in-
tense currents, such as the eddying motion of western boundary cur-
rents and the structure of equatorial currents.
Develop more accurate models of sea ice, which include the effects
of salinity and the dynamic and thermodynamic factors governing the
distribution of the polar ice packs. The data base being assembled by
the Arctic Ice Dynamics Joint Experiment (aidjex) in the Beaufort
Sea should be useful in the design of models that can predict those
properties of the polar ice pack that are important in the surface heat
balance, such as the ice thickness and the occurrence of open-water
leads.
Search for new computational algorithms for predicting oceanic
circulation that will provide the greatest accuracy for the least pos-
sible cost. At present, the methods used in modeling the ocean are
similar to those used in the atmospheric gcm's. The presence of lateral
boundaries and the need to resolve mesoscale motions may make
alternative numerical methods of particular use in numerical ocean
models.
Coupled Global Atmosphere-Ocean Models Tests of climatic change
extending over one or more years are not adequate unless they are made
with a model of the coupled ocean-atmosphere system. While the un-
coupled atmospheric and oceanic gcm's are useful for many purposes,
the thermal and mechanical coupling between the ocean and atmosphere
is fundamental to climatic variation. We note that a global ocean model
may require only a fraction of the computational effort needed by an
atmospheric gcm of the same resolution but emphasize that care must
be taken to avoid erroneous drift in the simulated climate due to sys-
tematic biases in the model or in the oceanic initial state.
Assuming that coupled models (cgcm's) will incorporate the develop-
ments and improvements recommended above for the separate at-
mospheric and oceanic models, emphasis should be given to the follow-
ing research with cgcm's:
Investigation of the simulated climatic variability, on seasonal and
annual time scales, of all climatic variables of the coupled system,
including the simulated exchange processes at the air-sea interface.
A NATIONAL CLIMATIC RESEARCH PROGRAM 85
Of particular importance in the coupled models is the simulation of
the sea-surface temperature, as this has a key role in the evolution
of the system. This will require integration over many years of simu-
lated time in order to generate adequate climatic statistics and to
examine the models' stability. Particular attention should be given
to evidence of climatic trends and intransitivity in the numerical solu-
tions. The statistics of such simulations with cgcm's will also prove
valuable in the calibration of statistical climate models.
The sensitivity of the climate simulated by coupled models should be
systematically examined in experiments extending at least through an
annual cycle. These studies should include the climatic consequences
of uncertainties in the simulations' initial state (including random
errors), in the parameterization of the various physical processes (such
as convection, cloudiness, boundary-layer fluxes, and mesoscale oceanic
eddies), and in the computational procedures. Such studies are neces-
sary in order to establish the characteristic noise levels of the models
and are of great importance in the use of the models for climate ex-
periments.
A program of climate change hypothesis testing should be under-
taken with coupled models, as soon as their stability and calibration
are reasonably assured. This should include examination of the various
feedback mechanisms among components of the climatic system, such
as ice and snow, cloudiness, sea-surface temperature, albedo, radiation,
and convection.
The coupled models should be used in a program of long-range
integrations with observed initial and boundary conditions, in order
to assess both their overall fidelity and their usefulness as long-range
or climatic forecasting tools.
Although not a research task in itself, special efforts should be
made to appropriately store, analyze, and display the rather staggering
amounts of data generated during the integration of cgcm's, so that
subsequent diagnosis can be performed efficiently.
Statistical-Dynamical Climate Models Although the coupled numeri-
cal models of the global circulation offer the most comprehensive and
detailed solutions available, even with the fastest computers envisaged
relatively few century-long climatic simulations will be possible, and
it is likely that none will be performed for periods as long as a millenium.
Such models will therefore find their greatest use in climatic research
in the exploration of the character of relatively short-period (say annual
to decadal) climatic variations and in the calibration of other, less-
detailed models. We therefore emphasize that statistical-dynamical
86 UNDERSTANDING CLIMATIC CHANGE
climate models (defined as those in which the structure and motion
of the individual large-scale transient disturbances are not resolved in
detail) will have to be used to simulate the longer-period climatic
variations. While such models provide less resolution of the details of
climatic change, they may display less climatic noise than do the global
circulation models.
In order to ensure the availability of the hierarchy of models needed
in a comprehensive research program on climatic change, the following
research should be carried out:
Statistical-dynamical models of the coupled time-dependent at-
mospheric and oceanic circulation should be constructed and calibrated
that embody suitable time- and space-averaged representations of the
climatic elements. In their extreme form, such models address the
steady-state globally averaged quantities, while others, for example,
consider time-dependent zonally averaged variables. Further efforts
should be made to represent the climatically important land-sea distri-
bution in such models and to calibrate them systematically against
observations as well as against other climatic models.
Simulation of climatic variation over extended time periods should
be made by the integration of suitably calibrated time-dependent
statistical-dynamical models. Depending on the time range, appropriate
components of the climatic system's atmosphere, hydrosphere, cryo-
sphere, lithosphere, and biosphere should be introduced, along with
appropriate variations of the external boundary conditions (see Figures
3.1 and 3.2).
Coupled time-dependent models in which the global circulation is
represented by low-order spatial resolution should also be further de-
veloped, such as those using a limited number of orthogonal components
or spectral modes.
Coupled models should be constructed and calibrated that embody
new forms of time-averaged representations of the climatic system. We
recognize that the parameterization of the effects of the transient eddies
poses a difficult problem in statistical hydrodynamics and urge that
full use be made of both model-generated and observed statistics, as
well as of theory, to develop a variety of such models for different types
and ranges of time averaging.
In each type of statistical-dynamical model, particular attention
should be given to the inclusion of the ocean and ice. In such models,
attention should also be given to the possibility of treating the at-
mosphere statistically while simulating the ocean in detail and perhaps
of treating both the atmosphere and ocean statistically while simulating
A NATIONAL CLIMATIC RESEARCH PROGRAM 87
the growth of ice sheets in detail. It is particularly important that such
models be calibrated with respect to both the mean and variance of the
climatic elements and that their stability and sensitivity be systematically
determined.
Empirical and Diagnostic Studies of Climatic Variation Although we
have recommended some diagnostic and empirical studies in connection
with the analysis of instrumental and proxy climatic data, such studies
should also be made on a phenomenological basis as part of the climatic
analysis and research program. As the record of past climates is made
more complete, there will be increased opportunity to carry out such
investigations with both instrumental and proxy data. In particular:
Studies should be made of the temporal and spatial correlations
among various data, including regional and global estimates of the
trends of key climatic elements such as temperature and precipitation.
Further empirical studies should be made of the surface oceanic
variables of temperature, salinity, sea level, and sea ice and of the
planetary heat balance, albedo, and cloudiness from satellite-based ob-
servations. The studies of Bjerknes (1969), Kukla and Kukla (1974),
Namias (1972a), and Wyrtki (1973) are examples of the sort of
empirical synthesis that can be achieved and should be systematically
extended to other regions of the world and to other climatic variables.
In these efforts, particular attention should be given to the various pos-
sible climatic feedback processes and to the forcing functions of the
general circulation. Here the diagnostic use of climatic models should
prove valuable.
Further studies should be made of the statistical characteristics of
climatic data, both observed and simulated. Power spectrum analyses
should be made for as many variables and locations as possible, and
with the longest records available, as the spectrum's "redness" has an
important bearing on questions of climatic cycles and climate prediction.
Needed Applications of Climatic Studies
Although closely related to the climatic data analysis and climatic
research recommended above, the needed applications of climatic
studies (and of climate models in particular) are so important that
they warrant identification as a separate component of the program.
It is in these applications that the program reaches its fruition, and
if attention to them is delayed until our understanding is complete or
our models perfect, they may never be undertaken. With due regard
88 UNDERSTANDING CLIMATIC CHANGE
for scientific caution, we believe that the time has come for a vigorous
attack on the areas of climate model application described below.
Simulation of the Earth's Climatic History
The evidence presented in Appendix A (and summarized in Chapter 4)
shows that the climatic history of the earth has been remarkably variable
and that this history provides information that is of value in the study
of present and possible future climates. The data assembled by paleo-
climatologists show conclusively that the flora, fauna, and surface
characteristics of many regions of the world have often been markedly
different in past times than they are today. Compared with this long-
period panorama, instrumental observations provide a frustratingly
short record.
It is at this juncture that the intersection of paleoclimatic and numeri-
cal modeling studies offers the most promise: the global climatic models
have the potential ability to simulate at least a near-equilibrium ap-
proximation to past climates subject to the appropriate geological
boundary conditions, while the paleoclimatic records can be used as
verification data. Initial efforts in this direction have already begun
(see Chapter 5), and we may expect increasing insight into the nature
of past climates as both the models and proxy data base improve.
In order to explore the nature of past climates systematically and
to lay the foundation for the study of possible future climates, the fol-
lowing studies should be made :
The geophysical boundary conditions at a number of selected times
in the history of the earth should be systematically assembled with a
view toward their use in climate models. This should include global
data on the continental land-mass positions and elevations, sea-level
ice-sheet elevations and margins, sea-ice extent, soil type and vegeta-
tive cover, and surface albedo. Estimates should also be made of the
earth's rotation rate and of the solar insolation (due to orbital
parameter changes). The selection of the time period might be based on
criteria such as the occurrence of an ice age, the distribution of the
continents and mountains, the opening or closing of a major oceanic
passage, or the large-scale flooding or draining of lowlands. Periods of
particular climatic stress such as indicated by the disappearance of
species might also be considered.
The various proxy records of temperature, salinity, and precipita-
tion should also be systematically assembled for the same selected
A NATIONAL CLIMATIC RESEARCH PROGRAM
89
times, to serve as verification data for the coupled climate models' simu-
lations and as possible input or boundary conditions for uncoupled
models.
Dynamical global models should be used to simulate the quasi-
equilibrium paleoclimate at selected times in the past when the boundary
conditions external to the ocean-atmosphere system can be reasonably
well specified. Such experiments should be focused on times when the
global climate might be expected to be in a particularly interesting
state (as judged from the available geological and proxy evidence) or
when the climate might be expected to be in the process of changing
most rapidly from one characteristic regime to another. The simulations
should extend long enough to accumulate realistic climatic statistics
and should use the assembled paleoclimatic data for vertification. By
using part of the paleoclimatic evidence (namely, the sea-surface
temperature) as a boundary condition, atmospheric gcm's may also be
used for this purpose.
Coupled statistical-dynamical models, or other coupled climate
models, should be used to simulate the time-dependent climatic evolu-
tion between the various "equilibrium" states identified above. For this
application the dynamics of ice sheets should be incorporated into the
coupled ocean-atmosphere models and note taken of the possible
time dependence of the remaining boundary conditions, such as solar
radiation and continental drift. In particular, the astronomical changes
of seasonal radiation resulting from the variation of the earth's orbital
parameters should be incorporated in a climate model, and the resulting
simulated climatic changes compared with the paleoclimatic evidence.
This recommendation parallels one made earlier in connection with the
development of the statistical-dynamical models themselves.
Studies should be made of possible methods to accelerate the simu-
lation of quasi-equilibrium climatic states in the global circulation
models, so that realistic statistics can be obtained without integration
over long time periods.
Exploration of Possible Future Climates
One of the most important applications of climate models is the sys-
tematic conduct and evaluation of climatic experiments designed to
explore the effects of either natural or anthropogenic changes in the
system. It is from such model-based experiments, calibrated with respect
to observed behavior, that we must draw our conclusions as to how
the climatic system operates and on which we should base our projec-
90 UNDERSTANDING CLIMATIC CHANGE
tions of likely future climates. The program in this area should include
the determination of the global climatic effects of the following (with
both coupled global circulation models and parameterized models):
The changes of incoming solar radiation. These experiments should
be performed with coupled models, in view of the dominance of the
oceans in the planetary heat storage, and should include changes in both
the amount and spectral distribution of solar radiation.
The changes of land surface character and albedo, as introduced by
deforestation, urbanization, irrigation, and changes of agricultural
practices.
The changes of cloudiness. These experiments should consider the
effects of the introduction or removal of both condensation and freezing
nuclei and the production of artificial clouds by aircraft.
The changes of evaporation, as introduced by reservoirs, irrigation,
and transpiration.
The disposal of waste heat. These experiments should be made with
coupled models and should include a broad range of rates and locations
of heat release in both atmosphere and ocean.
The introduction of dust and particulates into the troposphere, the
stratosphere, or both. These experiments should consider the effects of
scattering, absorption, fallout, and scavenging by precipitation and
should be designed to simulate the effects of both man-made pollution
and volcanic dust.
The partial or complete removal of the Arctic sea ice or the Antarctic
ice sheet. These experiments should be performed with a coupled model
that includes the mass and heat budget of pack ice.
The diversion of ocean currents. These experiments should be per-
formed with coupled models.
In climatic simulations of this kind the physical basis of each experi-
ment should be carefully examined in order to ensure the adequacy of
the particular model or models to be employed. The experiments sug-
gested above are those that we believe should be performed as part of
the climatic research program, as they involve processes or areas of
likely maximum climatic sensitivity or changes to which the climate's
response is relatively uncertain, and/or they represent conceivable (or
in some cases likely) future alterations by nature or by man.
It is important in such climatic experiments that the synoptic and
statistical significance of the results be carefully examined. This should
include the repetition of the experiment under slightly different (but
admissible) conditions to determine its stability and noise level and the
A NATIONAL CLIMATIC RESEARCH PROGRAM 91
analysis of independent simulations with other models. Only in this way
can we hope to accumulate the necessary experimental knowledge on
which to base our expectations of future climatic states. This, together
with the knowledge gained from the observational and research por-
tions of the program outlined above, will lay the scientific foundation
for what might be called climatic engineering.
Development of Long-Range or Climatic Forecasting
A third important area of application of climatic studies is the problem
of long-range or climatic forecasting on time scales of months, seasons,
and years. There have been numerous studies of this question almost
since the beginning of recorded observations. This research has not
solved the problem but has at least identified some of its ingredients.
We believe that further efforts should be made to systematically acquire
the data and perform the research necessary to attack this problem
anew, especially with the aid of climatic models.
Clearly the demand for climatic or long-range forecasts greatly
exceeds present capability. An accurate prediction of the temperature
or rainfall anomaly over, say, the central plains of North America or
over the Ukraine a decade, a year, or even a season in advance would be
of great value. And even a somewhat less accurate (but reliable)
prediction of the likelihood of such anomalies would be of great use to
those involved in agriculture, energy supply allocation, and commerce.
At present, the skill of the experimental long-range outlooks prepared
by the National Weather Service for the 30-day temperature anomaly
at some 100 U.S. cities is only 11 percent greater than chance and only
2 percent greater than chance for the 30-day precipitation anomaly.
These forecasts are principally prepared by a mixture of empirical and
statistical methods and have also been applied to the seasonal predic-
tion of temperature (Namias, 1968).
The ability of numerical models to perform useful long-range or
climatic forecasting (i.e., forecasts over monthly, seasonal, or annual
periods) has not been systematically examined because of the large
amounts of computation involved and the unavailability of suitable
models. Such efforts must also contend with the crucial questions of
climatic predictability, noted in Chapter 3, and the long-range stability
of the models themselves. We believe that further attention should be
given to these problems, using the expanded data base, the coupled
dynamical models, and the new computer resources called for in the
climatic program. We therefore recommend that
92 UNDERSTANDING CLIMATIC CHANGE
The coupled global circulation models should be systematically ap-
plied to the preparation of a series of long-range forecasts using ob-
served initial conditions wherever possible. These integrations should
extend over at least several seasons, well beyond the limit of local
predictability. Appropriate climatic statistics should be drawn from these
integrations and systematically compared with the observed variations
of all the climatic elements available and statistically analyzed for pos-
sibly significant trends of regional climatic anomalies.
The statistical-dynamical models and other appropriate members
of the parameterized climate model hierarchy should be used in the
preparation of similar long-range forecasts.
Systematic empirical and diagnostic studies of longer-period varia-
tions in the climatic system should be undertaken with the aid of models
and the expanding data base of monitored variables.
Assessment of Climate's Impact on Man
While the above efforts are concerned with the physical aspects of the
problem of climatic variation, a climatic research program should also
include studies of the impact of climate and climatic change on man
himself; this is best done with the guidance and insight provided by
climate models. While many studies have been made in this important
area, such as those of the Department of Transportation's Climatic
Impact Assessment Program (ciap), more comprehensive research
should be undertaken on a long-term basis. These studies may be
characterized as seeking answers to such questions as "What is a
1 -degree change of mean winter temperature worth, after all?" or even
"Climatic variation: so what?" The study of the impacts of climatic
variations on man is also a way of establishing priorities for research.
Climate and Food, Water, and Energy
That climate has a dominant influence on agricultural food production,
water supply, and the generation and use of energy is generally recog-
nized. The kinds and amounts of crops that may be grown in various
regions, the water available for domestic, agricultural, and industrial
use, and the consumption of electrical energy and fossil fuels all depend
in large measure on the distribution of temperature, rainfall, and sun-
shine. During the global warming of the first part of this century, for
example, the average length of the growing season in England (as
measured by the duration of temperatures above 42 °F) increased by
A NATIONAL CLIMATIC RESEARCH PROGRAM 93
two to three weeks and during the more recent cooling trend since the
1940's has undergone a comparable shortening (Davis, 1972). Al-
though Maunder (1970), Johnson and Smith (1965), and others have
surveyed the vast literature on the effects of climatic change on man,
further quantification of these effects is needed, particularly as a func-
tion of the time and space scales of atmospheric variability. Accordingly,
we recommend that research be devoted to the following:
The systematic assembly from both national and international sources
of data on worldwide food production and the analysis of their re-
sponse and sensitivity to variations of climate on monthly and seasonal
time scales. Such analyses should then be used to model or simulate
the total agricultural response to hypothetical climatic variations. We
note that in some cases it may be the variance or extremes of climate,
rather than the averages themselves, that will prove to be the more
important factor. An applied systems study of this problem has been
recently initiated by R. A. Bryson and colleagues at the University of
Wisconsin, with the aim of developing predictive relationships between
climate and food supply, which will be useful for policy decisions.
The systematic assembly of worldwide data on available water supply,
both from rainfall and snowpack, and its patterns of use and loss.
Analysis should then be undertaken of the water supply system's re-
sponse and sensitivity to variations of climate and simulation models
constructed.
The systematic assembly of worldwide data on the production and
use of energy and the determination of its response and sensitivity to
climatic variations. As in the cases of food and water, simulation models
should be constructed, so that the consequences of various patterns
of hypothetical climatic change can be estimated.
Social and Economic Impacts
Although it is difficult to obtain useful measures of the social and eco-
nomic impacts of climatic change, increased attention should be given
to this aspect of the problem. This is a problem in which the "noise
level" of nonclimatic factors is very high and for which the physical
scientist's knowledge must be supplemented by the skills and methods
of social and political scientists. The goal of this research should be the
development of an overall model of societal response to climatic change.
This is an area in which international cooperation should be sought,
and efforts such as those now being proposed by the International
94 UNDERSTANDING CLIMATIC CHANGE
Federation of Institutes of Advanced Study should be supported and
expanded.
THE PUN
Our recommendations for the planning and execution of the climatic
research program outlined above are given here in terms of what we
believe to be the appropriate subprograms, the necessary facilities and
support, and the desirable timetable for both the short-range and long-
range phases. We also offer some observations on the program's ad-
ministration and coordination, although we recognize that a program
of this scope will require much further planning and that the support
and cooperation of many persons and agencies will be necessary for its
successful execution.
Subprogram Identification
In a program as broad as that envisaged here, it is convenient to think
in terms of a number of components or subprograms, each concerned
with a specific portion of the overall effort. Such subprograms also
represent the necessary division of effort for the practical execution of
the program. The ncrp itself should ensure the coordination of the
various subprograms and maintain an appropriate balance of effort
among them.
Climatic Data- Analysis Program (CDAP)
In order to promote the extensive assembly and analysis of climatic
data outlined above, we recommend that a Climatic Data-Analysis Pro-
gram (cdap) be established as a subprogram of the ncrp. The purposes
of this subprogram are to facilitate the exchange of data and informa-
tion among the various climatic data depositories and research projects
and to support the coordinated preparation, analysis, and dissemination
of appropriate climatic statistics.
Climatic Index Monitoring Program (CI MP)
In order to promote the monitoring of the various climatic indices out-
lined above, we recommend that a Climatic Index Monitoring Program
(cimp) be established as a second subprogram of the ncrp. The pur-
poses of this subprogram are to support and coordinate the collection of
A NATIONAL CLIMATIC RESEARCH PROGRAM 95
data on selected climatic indices and to ensure their systematic dis-
semination on a timely and sustained basis.
Climatic Modeling and Applications Program {CM A?)
In order to promote the construction and application of the climatic
models outlined above, we recommend that a Climatic Modeling and
Applications Program (cmap) be established as a third subprogram of
the ncrp. The purposes of this subprogram are to support and co-
ordinate the development of a broad range of climatic models, to sup-
port necessary background scientific research, and to ensure the sys-
tematic application of appropriate models to the problems of climatic
reconstruction, climatic prediction, and climatic impacts.
Facilities and Support
The availability of adequate facilities and support and the design of
coordinating mechanisms are necessary to carry out the various sub-
programs recommended for the ncrp and should be given careful con-
sideration. Of primary importance are the roles of climatic data-analysis
facilities and research consortia, the needed high-speed computers, and
the required levels of funding.
Climatic Data- Analysis Facilities
To assist in the implementation of both the Climatic Data Analysis Pro-
gram (cdap) and Climatic Index Monitoring Program (cimp), we
recommend the development of new climatic data-analysis facilities at
appropriate locations, including linkage to the various specialized data
centers and climatic monitoring agencies by a high-speed data-trans-
mission network. Such facilities should have access to machines of the
highest speed and capacity available and be staffed by specialists in
data analysis, transmission, and display. Collection of certain climatic
data by a group of specialized facilities appears more desirable than
does collection of all data by a single centralized facility.
We envisage these facilities as performing the bulk of the recom-
mended cdap. This would include the inventory, compilation, processing,
analysis, and documentation of both conventional and proxy climatic
data. Close working cooperation is envisaged with specialized data
depositories; for conventional atmospheric and oceanic data these
include noaa's National Climatic Center and National Oceanographic
96 UNDERSTANDING CLIMATIC CHANGE
Data Center, for satellite data the National Environmental Satellite
Service, for glaciological data the Geological Survey's Data Center A in
Tacoma, for ice-core data the Army's Cold Regions Research and Engi-
neering Laboratory, for marine cores Columbia University's Lamont-
Doherty Geological Observatory, and for pollen and tree-ring data the
universities of Wisconsin and Arizona.
We also envisage the data-analysis facilities as playing a prominent
role in the cimp and in the processing, analysis, and dissemination of the
results on as nearly a real-time basis as possible. Certain of the facilities
could serve as global climatic "watchdogs" and might have a resident
scientific staif to perform diagnostic research as appropriate.
Climatic Research Consortia and Manpower Needs
We envisage the broad range of research and analysis recommended here
as being best performed by a number of institutions and groups. This
is desirable in order to ensure the breadth of viewpoint and diversity
of approach necessary in a problem as close to the unknown as is cli-
matic variation. An attempt to carry out all the recommended activities
and research by a single institution would in any case be a practical
impossibility.
Research on climate and climatic variation at the present time is
principally performed in governmental laboratories and in a variety of
research projects in universities and other institutions, usually with the
support of the federal government. Chief among the laboratories con-
cerned with elements of the climatic problem are noaa's Geophysical
Fluid Dynamics Laboratory, noaa's National Environmental Satellite
Service and Environmental Data Service, nsf's National Center for
Atmospheric Research, and nasa's Goddard Institute for Space Studies.
More specialized research on problems related to climate is also per-
formed by the U.S. Geological Survey and by the operational services
and laboratories of the U.S. Army, Navy, and Air Force. Many of
the climate-related research projects in universities and other institu-
tions are supported by the National Science Foundation through its
programs for atmospheric, oceanic, and polar research; by dot's Cli-
matic Impact Assessment Program; and by arpa's Climate Dynamics
Program. These include the various Quaternary research groups, geo-
logical and oceanographic laboratories, numerical modeling groups, and
polar studies and environmental institutes.
Each of these efforts makes a contribution to the national climatic
research picture, and they represent a valuable reservoir of experience
A NATIONAL CLIMATIC RESEARCH PROGRAM 97
and talent. In order to promote greater cooperation and exchange, to
ensure an appropriate balance of effort, and to give such research the
needed stability and coherence, we recommend that efforts be made to
coordinate present research more effectively as parts of a national
climatic research program. We believe that this can be achieved best by
the formation of cooperative associations of existing climatic research
groups and the initiation of whatever new research efforts may be re-
quired as parts of such associations. We accordingly recommend the
formation of a number of climatic research consortia among various
research groups as appropriate to their interests, with each such con-
sortium having links to computing facilities of the highest speed and
capacity available. Such research consortia would serve as valuable
coordinating mechanisms for the broad range of climatic research en-
visaged in the Climatic Modeling and Applications Program (cmap),
as well as giving both coherence and flexibility to the ncrp as a whole.
The present mode, norpax, and climap programs may serve as useful
examples for such consortia. As the national program develops, the
possible need for new institutional structures or facilities should be-
come clear. Our recommendations reflect the consensus that maximum
use should be made of existing institutions while further consideration
is given to the possible need for their expansion.
Aside from institutional arrangements, however, we believe that the
proposed research program unquestionably calls for the initiation and
support of new mechanisms to provide an expanded base of appro-
priately trained scientific and technical manpower. We accordingly
recommend that programs for technical training be developed and that
both predoctoral and postdoctoral fellowships in the broad area of
climatic research be established as soon as possible.
Computer Requirements
The required access to high-speed computers has been alluded to several
times in the discussion of the recommended program. Although it is
difficult to make precise projections, the volume of data processing
involved in the analysis and monitoring portions of the program alone
indicate that a dedicated machine of at least the cdc 7600 class is
required for the implementation of the cdap and cimp. The computer
needs of the research consortia and of the other research groups involved
in the modeling portions of the program are even more demanding, in
view of the variety. of the needed climatic models and tests and the
number and the length of the necessary climatic simulation experiments
98 UNDERSTANDING CLIMATIC CHANGE
and applications. Our estimates of the ncrp's overall computer require-
ments are given in Table 6.2 and call for a very significant increase over
present levels of computer usage.
If anything, these estimates may be too low. In its computer planning,
ncar has estimated a climate-related usage of several cdc 7600 units
by 1980 for the needs of ncar and the university community it serves
(W. M. Washington, personal communication), while the installation of
the ti-asc system at gfdl in 1974 will likely significantly raise their
machine usage for climatic studies. As shown in Table 6.2, it is estimated
that climatic data analysis and monitoring will require the full-time use
of at least one fourth-generation machine, and that climatic modeling
and applications will require the full-time use of at least one fifth-genera-
tion machine. We therefore recommend that machines of the cdc-7600
class be secured as soon as possible for the use of the data-analysis
facilities and the associated elements of the cdap and cimp and that
planning begin for the acquisition of computers of the ti-asc or illiac-4
class for the use of the climatic research consortia and the associated
elements of the cmap. It will also be necessary to provide broadband
communication links among the various facilities and cooperating groups
and with the climatic research community as a whole.
TABLE 6.2 Estimated Computing Needs for the National Climatic
Research Program °
Present Use * Projected Use
Climatic data analysis and monitoring
Atmospheric gcm development
Oceanic gcm development
Coupled gcm's (climate models)
Development and tests
Climatic reconstructions
Climatic experiments and projections
Other models and studies
total 1.5 16.5
" In units of cdc 7600 years.
6 Estimated 1973 national total, exclusive of operational agencies.
c For the program year circa 1980.
d Envisaged as use by climatic data-analysis facilities.
e Estimating 0.2 usage at ncar, 0.5 usage at gfdl, and 0.1 total usage elsewhere.
f Envisaged as use by cooperative climatic research consortia.
0.2
1.5*
0.8 c
3.0
0.2
2.0
0.1
3.0'
-0
2.0'
0.1
3.0 f
0.1
2.0
A NATIONAL CLIMATIC RESEARCH PROGRAM 99
Estimated Costs
The cost of the recommended national climatic research program is
difficult to determine accurately without a great deal of information on
observational, computing, and support costs from the various agencies
and institutions presently engaged in the many aspects of climatic re-
search. Rather than seeking such detailed data, we have restricted our-
selves to gross projections on the basis of estimates of the costs of
present efforts. Our estimates of the expenditures for climatic research
{not including the costs of instruments, observing platforms, or opera-
tional and service-related activities) are given in Table 6.3. Our pro-
jections of the growth of these (direct) costs during the early phases
of the program (i.e., to the year 1980) are shown in Figure 6.1, along
with the percentage increases over the preceding year; these estimates,
of course, depend directly on the base figures that are used and are sub-
ject to further refinement. These figures are intended for order-of-
magnitude guidance only and will require revision as the program
develops.
We recognize that the ultimate distribution of resources among the
various subprograms of the ncrp will be determined by the sense
of priorities of the government and by the capabilities of the research
community. The estimates shown in Figure 6.1 for the year 1980 are
based on our preception of the needed increases over present efforts in
the areas of data analysis and monitoring (cdap and cimp), especially
those concerning satellite data and the monitoring of oceanic climatic
indices. In the area of climatic modeling and applications (cmap),
the largest increases over present efforts are envisaged for the develop-
TABLE 6.3 Estimated Expenditures for Climatic Research" (in $106/yr)
Present Projected
(1974) (c.1980)
Climatic data assembly and analysis
Climatic index monitoring h
Climatic modeling and applications
~18 67
° Based on estimates of the climate-related research sponsored by the nsf, dot, and dod and that
conducted by gfdl, ncar, nasa, and noaa but not including essentially operational or service-
related activities.
b Not including costs of instruments or observing platforms. {
5
18
4
12
9
37
100
70
UNDERSTANDING CLIMATIC CHANGE
60 -
50 -
o 40
o on -
30
20 -
10
-
67
61
51
CMAP
-
39
-
28
_
CIMP
CDAP
22
18
(20%)
(30%)
| (40%)
(30%)
(20%)
(10%)
1975
1976
1977
1978
1979
1980
1974
(present)
FIGURE 6.1 Projected costs of the National Climatic Research Program (NCRP). The
numbers in parentheses are the percent increase over the preceding year's expenditures.
ment and application of coupled global climate models and climatic
impact studies. The relatively rapid growth rate during the program's
third and fourth years are projected to include the acquisition of the
necessary computers and networks. Overall, the recommended pro-
gram calls for an approximate fourfold expansion of the support of
research on climatic variation by the year 1980; the program's costs
beyond this time are more difficult to estimate and will depend on the
progress and opportunities developed prior to that time.
It is useful to compare these cost projections with the direct and
indirect costs of present garp efforts and those of closely related pro-
grams. In fiscal year 1973 the direct garp expenditures totaled $13.2
million, about 54 percent of which represented expenditures by the De-
A NATIONAL CLIMATIC RESEARCH PROGRAM
101
partment of Commerce and nasa directed toward the improvement of
weather forecasting, with the remainder expended by nsf for research
on both forecasting and general circulation studies. Some of these costs
are included in the estimates in Table 6.3, insofar as they can be identi-
fied as directed toward climatic research. The indirect costs associated
with garp amounted to $29.0 million in fiscal year 1973 and are not
reflected in the present climatic research estimates.
In addition to these efforts, there are other current programs that
contribute to garp and whose costs should not be overlooked. The
implementation of the World Weather Watch (www) and its satellite
system represented $1.5 million direct costs and $54.5 million indirect
costs in fiscal year 1973, while systems design and technological de-
velopment represented $2.4 million direct costs and $50.1 million in-
direct costs in the same period. The extent to which elements of the
recommended cdap and cimp subprograms of the ncrp may be con-
sidered as add-ons to such existing programs needs further considera-
tion, as does the extent to which the future costs of garp itself may be
merged with those envisaged for the ncrp.
Also in need of further study are the United States' contributions
to the costs of the various subprograms recommended as part of the In-
ternational Climatic Research Program (icrp) described below, as well
as the impacts of inflation. We also note that funds will be required for
the training of additional scientific manpower in all aspects of the
research program.
Timetable and Priorities within the Program
We recognize the need for flexibility in a research program of this
kind, and that future technological and research discoveries may have
important impacts on the direction of climatic research. In spite of
these unknown factors, however, some consideration of goals and
priorities is useful. Here we present our recommendations for the ob-
jectives of the initial phase of the program (1974-1980) and the
necessary sequence of planning activities for both these goals and those
of the long-term phase (1980-2000). Our recommendations for a
coordinated international program are considered subsequently.
The Initial Phase (1974-1980)
Once the decision is made to develop a national climatic research pro-
gram, we recommend that planning begin immediately for the implemen-
tation of its component activities and subprograms. Our specific recom-
102
UNDERSTANDING CLIMATIC CHANGE
mendations for both the immediate and subsequent objectives during
this phase of the program are shown in Table 6.4 in terms of the data-
analysis, index-monitoring, and modeling subprograms identified earlier.
Here our sense of relative priorities is given implicitly by the ranking
into immediate and subsequent objectives; these time scales refer to
the expected times of the achievement of first useful results, with the
recognition that initial development must in some cases begin earlier
TABLE 6.4 Goals for the Initial Phase of the NCRP (1974-1980)
Immediate Objectives
Subsequent Objectives
Subprogram
(1974-1976)
(1
976-1980)
Climatic data
1.
Development of climatic data-
1.
Development of global
analysis
analysis facilities
climatic data-analysis
(cdap)
2.
Statistical analysis of climatic
system (fgge)
variability, predictability,
2.
Assembly and process-
feedback processes
ing of global climatic
3.
Statistical climatic-impact
studies (crops, human affairs)
3.
data (conventional,
satellite, historical,
proxy data)
Development of cli-
matic impact models
Climatic index
1.
Monitoring of oceanic mixed-
1.
Satellite monitoring of
monitoring
layer
global heat-balance
(cimp)
2.
Monitoring of ice, snow, and
components
cloud cover
2.
Monitoring selected
3.
Expansion of proxy data
sources
physical processes
(fgge)
4.
Monitoring system simulation
studies
3.
Development of global
climatic index monitor-
ing system
Climatic model-
1.
Development of oceanic
1.
Development of fully
ing and appli-
mixed-layer models
coupled atmosphere-
cations
2.
Development and analysis of
ocean-ice gcm's
(cmap)
provisionally coupled gcm's
(sensitivity, predictability
studies)
2.
Development of statis-
tical-dynamical cli-
mate models
3.
Development of simplified cli-
matic models and related
theoretical studies
3.
Parameterization of
mesoscale processes,
simulation of climatic
4.
Selected paleoclimatic recon-
structions
4.
feedback mechanisms
(fgge)
Experimental seasonal
climatic forecasts by
dynamical models
A NATIONAL CLIMATIC RESEARCH PROGRAM 103
and that further development and application will continue later. This
ranking also reflects a balance between the relative ease of accomplish-
ment and the relative potential for initial practical usefulness. We
believe that progress toward the subsequent objectives will require
the support of all immediate objectives of the program, with new
priorities evolving as a function of achievement and opportunity.
Relationship to the FGGE (1978-1979)
The First garp Global Experiment (fgge), now planned for 1978-
1979, is primarily an attempt to collect a definitive global data set for
use in the improvement of weather prediction by numerical atmospheric
models. The potential value' of these data for climatic research lies not
so much in their display of seasonal and interhemispheric variations,
valuable as that will be, but in the fact that many of the short-period
physical processes to be intensely measured or parameterized in fgge
are also important for the understanding of climate. Among these are
the processes of convection, boundary-layer dynamics, and the at-
mosphere's interaction with the surface of the ocean.
The observational requirements during the fgge call for measurement
of the atmospheric temperature, water vapor, cloud cover and eleva-
tion, wind, and surface pressure, together with the surface boundary
variables of sea-surface temperature, soil moisture, precipitation, snow
depth, and sea-ice distribution. To enhance their value for climatic
studies, we recommend that these data be supplemented during fgge
insofar as possible by observations of the global distributions of ozone,
particulates, surface and planetary albedo, incoming solar and outgoing
terrestrial radiation, vegetal cover, and the continental freshwater
runoff. We recommend that special observations also be made in con-
junction with regional programs, such as norpax and polex, which
are expected to be in operation during the fgge.
The Long-Term Phase (1980-2000)
The long-range goals and full-scale operation of the ncrp in the period
beyond 1980 are portrayed in the upper part of Figure 6.2. During this
period, the full interaction among the observational, analysis, modeling,
and theoretical components of the program will occur, leading to the
development of an operational global climatic data system and, it is
hoped, to the acquisition of an increasingly accurate theory of climatic
variation. Although priorities cannot be set at such long range, the
eventual practical payoffs of this program will be the determination of
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A NATIONAL CLIMATIC RESEARCH PROGRAM 105
the degree to which climatic variations on seasonal, annual, decadal,
and longer times scales may be predicted and the degree to which they
may be controlled by man.
Administration and Coordination
The administrative structure and coordination of the recommended
program are the responsibility of the federal government and were not
given extensive consideration. However, noting the concern with the
problem of climatic variation in many parts of the government, and the
widespread participation of many governmental and nongovernmental
groups in climatic research, we believe that the program should be ad-
ministered in such a way that the interests of all are effectively repre-
sented and coordinated. It is particularly important that the advice of
the scientific community be used in the design and development of
the major elements of the research program.
Both the short-term and long-term goals of the ncrp are also shared
by the International Climatic Research Program (icrp) recommended
below. The development of this international program should proceed
in parallel with the ncrp and should be closely coordinated with garp.
The principal activities within garp up to the present time have focused
on the problem of improving the accuracy and extending the range of
weather forecasts, and the United States' contributions to garp in par-
ticular have emphasized the development and use of numerical models
for this purpose. These efforts are necessary steps in the development
of an adequate modeling capability for both weather prediction and
climate, and were they not already under way as part of garp they
would have had to be undertaken through some other means as a
prelude to the climatic research program.
A COORDINATED INTERNATIONAL CLIMATIC RESEARCH
PROGRAM (ICRP)
Many of the efforts envisaged within the ncrp are of an obvious inter-
national character, and the degree to which these should be regarded
as national as opposed to international activities is not of critical im-
portance for our purposes. The important point is that there are inter-
national efforts now under way within garp of direct relevance to the
climatic problem, of which we note especially the International Study
Conference on the Physical Basis of Climate and Climate Modeling
held in Sweden in July and August 1974 under the auspices of the
icsu/wmo garp Joint Organizing Committee. The recommendations
106 UNDERSTANDING CLIMATIC CHANGE
and programs resulting from this and subsequent planning conferences
should be closely coordinated with the U.S. national program. We offer
here our recommendations for an appropriate international climatic re-
search program and some observations on how such a program might
best be coordinated with garp itself.
Program Motivation and Structure
The observational programs planned in support of garp offer an un-
paralleled opportunity to observe the global atmosphere, and every
effort should be made to use these data for climatic purposes as well
as for the purposes of weather prediction. The climatic system, how-
ever, consists of important nonatmospheric components, including the
world's oceans, ice masses, and land surfaces, together with elements
of the biosphere. While it is not necessary to measure all of these com-
ponents in the same detail with which we observe the atmosphere, their
roles in climatic variation must not be overlooked.
In addition to the fundamental physical differences discussed in
Chapter 3, the problem of climatic variation also differs from that
of weather forecasting by the nature of the data sets required. The
primary data needs of weather prediction are accurate and dense
synoptic observations of the atmosphere's present (and future) states,
while the data needed for studies of climatic variation are longer-term
statistics of a much wider variety of variables. When climatic variations
over long time scales are considered, these variables must be supplied
from fields outside of observational meteorology. Thus, an essential
characteristic of climate studies is its involvement of a wide range of
nonatmospheric scientific disciplines.
The types of numerical models needed for climatic research also
differ from those of weather prediction. The atmospheric gcm's (which
represent the ultimate in weather models ) do not need a time-dependent
ocean for weather-forecasting purposes over periods of a week or two.
For climatic change purposes, on the other hand, such numerical models
must include the changes of the oceanic heat storage. Such a slowly
varying feature may be regarded as a boundary or external condition
for weather prediction but becomes an internal part of the system
for climatic variation.
International Climatic Research and GARP
In view of these characteristics, we suggest that while the garp concern
with climate is a natural one, as indicated above the problem of climate
A NATIONAL CLIMATIC RESEARCH PROGRAM 107
goes much beyond the present basis and emphasis of garp. Accordingly,
we recommend that the global climate studies that are under way within
garp be viewed as leading to the organization of a new and long-term
international program devoted specifically to the study of climate and
climatic variation, which we suggest be called the International Climatic
Research Program (icrp).
International Climatic Decades (1980-2000)
We suggest that the observational programs of garp, and especially
those of the fgge, be viewed as preliminary efforts, later to be expanded
and maintained on a long-term basis. In particular, we recommend
that the special data needs of climatic studies be supported on an inter-
national scale through the designation of the period 1980-2000 as the
International Climatic Decades (icd), during which intensive efforts
would be made to secure as complete a global climatic data base as
possible.
The general outline of the envisaged international program (icrp)
is sketched in the lower part of Figure 6.2, and the program's scientific
elements are discussed in more detail below.
Program Elements
Climatic Data Analysis
The main thrust of the international climatic program should be the
collection and analysis of climatic data during the icd's, 1980-2000.
During this period, the participation of all nations should be sought in
order to develop global climatic statistics for a broad set of climatic
variables. We urge that these efforts include international cooperation
in the systematic summary of all available meteorological observations
of climatic value, including oceanographic observations in the waters
of coastal nations.
International Paleoclimatic Data Network (IPDN)
We urge the development of an international cooperative program for
the monitoring of selected climatic indices and the extraction of histori-
cal and proxy climatic data unique to each nation, such as indices of
glaciers, rain forest precipitation, lake levels, local desert history, tree
rings, and soil records. Specifically, we recommend that this take the
form of an International Paleoclimatic Data Network (ipdn), as a
108 UNDERSTANDING CLIMATIC CHANGE
subprogram of the icrp. The cooperation of such organizations as
scar, scor, and the International Union for Quaternary Research
(inqua) should be sought in this program.
The contents of these international observational efforts might
possibly broadly follow those recommended for the U.S. national effort,
with modifications as appropriate to each nation's needs and capabilities.
In addition, we recommend that the icrp undertake the following:
The international collection of special climatic data sets on such
events as widespread drought and floods and following major environ-
mental disturbances such as volcanic eruptions;
Programs to encourage international exchange of climatic data and
analyses.
Climatic Research
Although cooperative research studies are desirable, we recognize that
the large-scale numerical simulation of climate with cgcm's can now
be carried out in only a relatively few countries. To promote wider
international participation in climatic research, we therefore recommend
that the icrp include the following:
Programs and activities to encourage international cooperation in
climatic research and to facilitate the participation of developing na-
tions that do not yet have adequate training or research facilities.
Internationally supported regional climatic studies in order to describe
and model local climatic anomalies of special interest.
The contents of these and other research activities of the icrp might
also broadly follow those recommended for the U.S. national effort,
with appropriate modifications for each nation's interests and capabilities.
Global Climatic Impacts
While all nations are tied in some fashion to the world pattern of
climate, some are more vulnerable to climatic variations than others by
virtue of their locations and the delicacy of their climatic balance. We
therefore recommend that the icrp include the following:
International cooperative programs to assess the impacts of observed
climatic changes on the economies of the world's nations, including
A NATIONAL CLIMATIC RESEARCH PROGRAM 109
the effects on the water supply, food production, and energy utilization.
This should include the impacts of variations of oceanic climate for
those nations whose economies are dependent on the sea. The coopera-
tion of appropriate international agencies of the United Nations and of
other groups such as the International Federation of Institutes of Ad-
vanced Study should be sought.
Cooperative analyses of the regional impacts of possible future cli-
mates. Such studies could be of great importance to many countries,
particularly emerging nations making long-range policy decisions con-
cerning the development of their resources.
Program Support
The question of the details of support of the icrp was not dealt with.
It seems clear, however, that an appropriate balance of effort should be
maintained among icrp, the various national climatic research pro-
grams, and other international programs such as the World Weather
Watch (www) and the United Nations Environment Program (unep).
The services of groups performing the function of the present garp
Joint Organizing Committee and its Joint Planning Staff will also be
necessary for the success of the international program.
In order to assist in the coordination of the icrp, we urge that sup-
port be made available by the appropriate agencies of the United Na-
tions on a scale commensurate with the breadth and importance of the
problem. This should include a budget adequate for the effective inter-
national coordination of the icrp on a scale significantly greater than
that of garp and on a continuing long-term basis. We also urge that
scientific assistance be sought from the International Council of Scientific
Unions in support of selected icrp subprograms.
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APPENDIX A
SURVEY OF PAST CLIMATES
INTRODUCTION
The earth's climates have always been changing, and the magnitude
of these changes has varied from place to place and from time to time.
In some places the yearly changes are so small as to be of minor inter-
est, while in others the changes can be catastrophic, as when the
monsoon fails or unseasonable rain delays the planting and harvesting of
basic crops. On a longer time scale, certain decades have striking
and anomalous characteristics, such as the severe droughts that affected
the American Midwest during the 1870's, 1890's, and 1930's and the
high temperatures recorded globally during the 1940's. And on still
longer time scales, the climatic regimes that dominated certain centuries
brought significant changes in the global patterns of temperature, rain-
fall, and snow accumulation. For example, northern hemisphere winter
temperatures from the midfifteenth to the midnineteenth centuries
were significantly lower than they are today. The late nineteenth century
represented a period of transition between this cold interval — sometimes
known as the Little Ice Age — and the thermal maximum of the 1940's.
Some idea of the magnitude of the climatic changes that characterized
the Little Ice Age can be gained from a study of proxy or natural
records of climate, such as those of alpine glaciers. As shown in Figure
A.l, as late as the midnineteenth century the termini of these glaciers
were still advanced well beyond their present limits.
The practical as well as the purely scientific value of understanding
127
128
UNDERSTANDING CLIMATIC CHANGE
FIGURE A.l The Argentiere glacier in the French Alps, (a) An etching made about 1850,
showing the extent of the glacier during the waning phase of the Little Ice Age. (b) Photo-
graph of the same view taken in 1966. [From LeRoy Ladurie (1971).]
APPENDIX A 129
the processes that bring about climatic change is self-evident. Only by
understanding the system can we hope to comprehend its past and to
predict its future course. This objective can be achieved only by study-
ing the workings of the global climate machine over a time span ade-
quate to record a representative range of conditions in nature's own
laboratory, and for this the record of past climates is indispensable.
From the evidence discussed below and summarized in Figure A. 2
we conclude that a satisfactory perspective of the history of climate
can be achieved only by the analysis of observations spanning the entire
time range of climatic variation, say, from 10"1 to 10° years. Near the
short end of this range there is a rich instrumental record to collate and
analyze, although as discussed elsewhere in this report, awkward gaps
exist in our knowledge of many parts of the air-sea-ice system during
even the past hundred years. As the time scale of observations is
lengthened to include earlier centuries, the direct instrumental record
becomes less and less adequate. A continuous time series of observa-
tions as far back as the seventeenth century is available for only one
area. For earlier times the instrumental record is blank, and indirect
means must be found to reconstruct the history of climate.
The science of paleoclimatology is concerned with the earth's past
climates, and that branch which seeks to map the reconstructed climates
may be referred to as paleoclimatography. So defined, the science of
paleoclimatology does far more than satisfy man's natural curiosity
about the past; it provides the only source of direct evidence on pro-
cesses that change global climate on time scales longer than a century.
When calibrated and assembled into global arrays, these data will be
essential in the reconstruction of paleoclimates with numerical models.
Nature of Paleoclimatic Evidence
The subject of ancient climates may conveniently be approached in
terms of the nature of the climatic record, whether from human
(historical) recordings or from proxy or natural climatic indicators. It
is therefore convenient to identify historical climatic data and proxy
climatic data as sources of paleoclimatic evidence.
Prior to the period of instrumental record, historical climatic data
are found in books, manuscripts, logs, and other documentary sources
and provide valuable (although fragmentary) climatic evidence before
the advent of routine meteorological observations. Lamb (1969) has
pioneered the collection of such data and has charted the main course
of climate over Western Europe during the past 1000 years [Figure
A.2(b)]. Where the historical or manuscript record overlaps the instru-
130
UNDERSTANDING CLIMATIC CHANGE
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the five-year average surface temperatures over the region 0-80 °N during the last 100 years
(Mitchell, 1963). (b) Winter severity index for eastern Europe during the last 1000 years
(Lamb, 1969). (c) Generalized midlatitude northern hemisphere air-temperature trends
during the last 15,000 years, based on changes in tree lines (LaMarche, 1974), marginal
fluctuations in alpine and continental glaciers (Denton and Karlen, 1973), and shifts in
vegetation patterns recorded in pollen spectra (van der Hammen et al., 1971). (d) Gen-
eralized northern hemisphere air-temperature trends during the last 100,000 years, based
on midlatitude sea-surface temperature and pollen records and on worldwide sea-level
records (see Figure A.13). (e) Fluctuations in global ice-volume during the last 1,000,000
years as recorded by changes in isotopic composition of fossil plankton in deep-sea core
V28-238 (Shackleton and Opdyke, 1973). See legend for identification of symbols (1)
through (6).
APPENDIX A 131
mental record, the climatic reconstructions may be confirmed and
calibrated by the latter.
In contrast, the proxy record of climate makes use of various natural
recording systems to carry the record of climate back into the past.
Records from well-dated tree rings, annually layered (or varved) lake
sediments, and ice cores resemble the historical data in that values
can be associated with individual years and may be calibrated with
modern data to extend the climatic record for many centuries, and in
certain favored sites for as long as 8000 to 10,000 years. Other record-
ing systems, such as the pollen concentration in lake sediments and
fossil organisms and oxygen isotopes in ocean sediments, have less
resolution but may provide continuous records extending over many
tens of thousands of years. These and other characteristics of proxy
climatic data sources are summarized in Table A.l.
In general, the older geological records provide only fragmentary
and generally qualitative information but constitute our only records
extending back many millions of years. For the past one million years,
however, and especially for the past 100,000 years, the record is rela-
tively continuous and can be made to yield quantitative estimates of
the values of a number of significant climatic parameters. These in-
clude the total volume of glacial ice (and its inverse, the sea-level),
the air temperature and precipitation over land, the sea-surface
temperature and salinity for much of the world ocean, and the general
trend of air temperature over the polar ice caps.
Like sensing systems made by man, each natural paleoclimatic
indicator must be calibrated, and each has distinctive performance
characteristics that must be understood if the data are to be interpreted
correctly. In discussing these sources it is useful to distinguish between
those paleoclimatic indicators that are more or less continuous re-
corders of climate, such as tree rings and varves, and those whose
records are episodic, such as mountain glaciers. We should also con-
sider the minimum attainable sampling interval that is characteristic of
a particular paleoclimatic indicator (see Table A.l). Thus, tree rings,
varves, and some ice cores can be sampled at intervals of one year,
pollen or other sedimentary fossil samples only rarely represent less
than about 100 years, and many geological series are sampled over
intervals representing a thousand years or more. These figures reflect
differences in the resolving power of each proxy indicator. Climate-
induced changes in a plant community as reflected in pollen concentra-
tions, for example, are relatively slow; the high-frequency information is
lost, but low-frequency changes are preserved. In contrast, tree-ring
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134 UNDERSTANDING CLIMATIC CHANGE
records and isotopic records in ice cores respond yearly and even
seasonally in favored sites.
Each proxy record also has a characteristic chronologic and geo-
graphic range over which it can be used effectively. Tree-ring records
go back several thousand years at a number of widely distributed con-
tinental sites, pollen records have the potential of providing synoptic
coverage over the continents for the past 12,000 years or so, and a
nearly complete record of the fluctuating margins of the continental
ice sheets is available for about the last 40,000 years. Planktonic and
benthic fossils from deep-sea cores can in principle provide nearly
global coverage of the ocean going back tens of millions of years, al-
though sampling difficulties have thus far limited our access to sediments
deposited during the last several hundred thousand years.
Although instrumental records best provide the framework neces-
sary for the quantitative understanding of the physical mechanisms of
climate and climatic variation with the aid of dynamical models, the
increasingly quantitative and synoptic nature of paleoclimatic data
will add a much-needed perspective. As discussed elsewhere in this
report, it is therefore important that the historical and proxy records
of past climate be systematically assembled and analyzed, in order to
provide the data necessary for a satisfactory description of the earth's
climates.
Instrumental and Historical Methods of Climate Reconstruction
Over the past three centuries, the development of meteorological
instruments and appropriate "platforms" for sensing the state of the
atmosphere-hydrosphere-cryosphere system has produced an important
storehouse of quantitative information pertaining to the earth's climates.
Time series of these records show that climate undergoes considerable
variation from year to year, decade to decade, and century to century.
From a practical viewpoint, much of this information mirrors the
economically more important climatic variations, as found, for example,
in the changes of crop and animal production, the patterns of natural
flora and fauna, and the variations of the levels of lakes and streams
and the extent of ice. Generally the term "climate" is understood to
describe in some fashion the "average" of such variations. As discussed
in Chapter 3, a complete description of a climatic state would also
include the variance and extremes of atmospheric behavior, as well as
the values of all parameters and boundary conditions regarded as ex-
ternal to the climatic system.
In discussing the reconstruction of climates from instrumental data,
several characteristics of past and present observational systems should
APPENDIX A 135
be considered. First, instrumental observations have been obtained for
the most part for the purpose of describing and forecasting the weather.
Hence, although extensive records of such weather elements as tempera-
ture, precipitation, cloudiness, wind, and observations are available,
they are inadequate for many climatic purposes. There exist few direct
measurements related to the thermal forcing functions of the at-
mosphere-hydrosphere-cryosphere system, such as the solar constant;
the radiation, heat, and moisture budgets over land and ocean surfaces;
the vegetative cover; the distribution of snow and ice; the thermal
structure of the oceanic surface layer; and atmospheric composition
and turbidity.
Second, observational records may be expected to contain errors due
to changes in instrument design and calibration and to changes in
instrument exposure and location. There is therefore a need to establish
and maintain conventional observations at reference climatological
stations and a need to identify, insofar as possible, "benchmark" records
of past climate. Such observations are needed to supplement the climatic
monitoring program described elsewhere (see Chapter 6).
Third, the time interval over which portions of the climatic system
need to be described are very different. If, for example, the fluctuations
in the volume and extent of the polar ice caps are to be studied, a
time interval of order 100,000 years (the maximum residence time of
water in the ice caps) is required. Or if atmospheric interaction with
the deep oceans is to be considered, then a time interval of the order
500 years (the residence time of bottom water) is required. It is there-
fore apparent that the period of instrumental records covering the past
century or two is long enough only to have sampled a portion of such
climatic responses, and that our information on older climates must
come from historical sources and from the various natural (proxy)
indicators of climate described earlier. Although such records will al-
ways be fragmentary, we should recognize their unique value in describ-
ing the past behavior of the earth's climatic system.
For practical reasons, it has been convenient to compute climatic
statistics over relatively short intervals of time, such as 10, 20, or
30 years, and to designate the 30-year statistics as climatic "normals."
It is important to note, however, that the most widely accepted climatic
"normals" (for the period 1931-1960) represent one of the most ab-
normal 30-year periods in the last thousand years (Bryson and Hare,
1973). As noted elsewhere in this report, the entire last 10,000 years are
themselves also abnormal in the sense that such (interglacial) climates
are typical of only about one tenth of the climatic record of the last
million years.
While continuous observations of atmospheric pressure, temperature,
136 UNDERSTANDING CLIMATIC CHANGE
and precipitation are available at a few locations from the late seven-
teenth century, such as the record of temperature in Central England
assembled by Manley (1959), it is only since the early part of the
eighteenth century that the spatial coverage of observing stations has
permitted the mapping of climatic variables on even a limited regional
scale. These and other scattered early observations of rainfall, wind
direction, and sea-surface temperature have been summarized by Lamb
(1969). Only since about 1850 are reliable decadal averages of surface
pressure available for most of Europe, and only since about 1900 are
there reliable analyses for the midlatitudes of the northern hemisphere,
as shown in Figure A. 3. And only since about 1950 does the surface
observational network begin to approach adequate coverage over the
continents; large portions of the oceans, particularly in the southern
hemisphere, remain inadequately observed.
For the climate of the free atmosphere, the international radiosonde
network permits reliable analyses for the midlatitudes of the northern
hemisphere only since the 1950's, and less than adequate coverage
exists over the rest of the globe. Beginning in the 1960's, routine ob-
servations from satellite platforms have begun to make possible global
observations of a number of climatic variables, such as cloudiness, the
planetary albedo, and the planetary heat budget. Yet many important
quantities, such as the heat and moisture budgets at the earth's surface
and the thermal structure and motions of the oceanic surface layer,
remain largely unobserved on even a local scale.
Biological and Geological Methods of Climate Reconstruction
During the first three decades of the nineteenth century, Venetz in
Switzerland and Esmark in Norway inferred the existence of a pre-
historic ice age from the study of vegetation-covered moraines and other
glacial features in the lower reaches of mountain valleys. After a century
of effort, the literature of paleoclimatology has become so diverse, and
so burdened by stratigraphic terminology, that it is useful to provide a
summary of paleoclimatic techniques.
The quantitative description of past climates as determined by bio-
logical and geological records requires the development of paleoclimatic
monitoring techniques and the construction of time scales by suitable
chronometric or dating methods. In general, the second of these prob-
lems is the more difficult.
Beyond the range of 14C dating (the past 40,000 years), it is only
since about 1970 that the main chronology of the climate of the past
100,000 years has become clear; and only since 1973 that the main
features of the chronology of the past million years have been estab-
APPENDIX A
137
• a
mm ! i
80" 0' 20*
100* 140*
S>. ! !■
FIGURE A.3 Growth of the network of surface pressure observa-
tions and of the area that can be covered by reliable 10-year
average isobars (Lamb, 1969). (a) 1750-1759, (b) 1850-1859, (c)
1950-1959.
138 UNDERSTANDING CLIMATIC CHANGE
lished. Key discoveries in these time ranges have been in the sea-level
records of oceanic islands and in the sedimentary records of deep-sea
cores. In preparing this survey, the chronology of these records has been
used as a framework into which the data from more fragmentary or
poorly dated records have been fitted.
Monitoring Techniques
The problem of developing a paleoclimatic monitoring technique —
or finding something meaningful to plot — may be broken down into
three subproblems. A natural climatic record must be (a) identified,
(b) calibrated, and (c) obtained from a stable recording medium.
Identification of Natural Climatic Records A number of different
monitoring techniques that can provide data for paleoclimatic inference
are summarized in Table A.l and are based on observations of fossil
pollen, ancient soil types, lake deposits, marine shore lines, deep-sea
sediments, tree rings, and ice sheets and mountain glaciers. The tech-
niques that are emphasized here are those that in general yield more or
less continuous time series. Other types of proxy data are also useful
in the reconstruction of climatic history (see, for example, Flint, 1971,
or Washburn, 1973).
Calibration of Paleoclimatic Records Many proxy records must be
calibrated to provide an estimate of the climatic parameter of interest.
The elevation of an ancient coral reef, for example, is a record of a
previous sea level; but before it can be used for paleoclimatic purposes
the effect of local crustal uplift or subsidence must be removed (Bloom,
1971; Matthews, 1973; Walcott, 1972).
Another example may be cited from paleontology, where the taxo-
nomic composition of fossil assemblages and the width of tree rings
are known to reflect the joint influence of several ecological and en-
vironmental factors of climatic interest. Here appropriate statistical
techniques are used to define indices that give estimates of the individual
paleoclimatic parameters, such as air temperature, rainfall, or sea-
surface temperature and salinity. In the case of tree rings, although
each tree responds only to the local temperature, moisture, and sun-
light, for example, by averaging over many sites, the trees' response may
be related to the large-scale distribution of rainfall and surface tempera-
ture. In this way a statistical relationship may be established with a
variety of parameters, even though they may not be direct causes of
tree growth. When such tree-ring data are carefully dated they can
APPENDIX A 139
thus provide estimates of the past regional variations of climatic elements
such as precipitation, temperature, pressure, drought, and stream flow
(Fritts et al, 1971). These methods yield what are called transfer func-
tions, which serve to transform one set of time-varying signals to another
set that represents the desired paleoclimatic estimates. In addition to
their application to tree-ring data, multivariate statistical-analysis
techniques have been successfully applied to marine fossil data (Imbrie
and Kipp, 1971; Imbrie, 1972; Imbrie et al., 1973) and to fossil pollen
data (Bryson et al., 1970; Webb and Bryson, 1972). Typical results
indicate, for example, that average winter sea-surface temperatures
18,000 years ago in the Caribbean were about 3°C lower than today,
while those in midlatitudes of the North Atlantic were about 10°C
below present levels.
The oxygen isotope ratio 180/160 as it is preserved in different ma-
terials is used in three separate paleoclimatic monitoring techniques.
Although the results are interpreted differently, in each technique the
ratio is measured as the departure 8180 from a standard, with positive
values indicating an excess of the heavy isotope. One technique ex-
amines the ratio in polar ice caps, where the values of 8180 are generally
on the order of 30 parts per thousand lower than in the oceanic reservoir,
because of the precipitation and isotopic enrichment that accompanies
the transport of water vapor into high latitudes. As shown by Dans-
gaard (1954) and by Dansgaard et al. (1971) the value of 8lsO in each
accumulating layer of ice is closely related to the temperature at which
precipitation occurs over the ice. Although complicating effects make it
impossible to convert the 8lsO curve into an absolute measure of air
temperature, the isotopic time series are extraordinarily detailed.
Another isotopic technique records 8lsO in the carbonate skeletons
of planktonic marine fossils (Emiliani, 1955, 1968). Here the ratio is
determined by the isotopic ratio and temperature in the near-surface
water in which the organisms live. Work by Shackleton and Opdyke
(1973) demonstrates that the observed ratio is predominantly influ-
enced by the isotopic ratio in the seawater. Hence the isotopic curve
reflects primarily the changing volume of polar ice, which, upon melting,
releases isotopically light water into the ocean.
A third technique measures the isotopic ratio in benthic fossils whose
skeletons reflect conditions prevailing in bottom waters. By making
the assumption that the temperature of bottom water underwent little
change over the past million years, the difference between the isotopic
ratio observed in benthic and planktonic fossils can be used to estimate
changes in surface-water temperatures. Initial application of this tech-
nique (Shackleton and Opdyke, 1973) provides an independent con-
140 UNDERSTANDING CLIMATIC CHANGE
firmation of the previously cited estimate of glacial-age Caribbean
temperatures obtained by paleontological techniques. Over time spans
on the order of tens of millions of years, measurements of 8lsO in
benthic fossils offer a means of tracing changes in bottom water in
which the effects of changing polar temperatures and ice volumes are
combined (Douglas and Savin, 1973).
Evaluation of the Recording Medium All paleoclimatic techniques
require that ambient values of a climatic parameter be preserved within
individual layers of a slowly accumulating natural deposit. Such de-
posits include sediments left by melting glaciers on land; sediments
accumulating in peat bogs, lakes, and on the ocean bottom; soil layers;
layers accumulating in polar ice caps; and the annual layers of wood
formed in growing trees. Ideally, a recording site selected for paleo-
climatic work should yield long, continuous, and evenly spaced time
series. The degree to which these qualities are realized varies from
site to site, so that distortions and nonuniformities in each record must
be identified and removed. The stratigraphic techniques by which this
screening is accomplished will not be discussed here, although the reader
should be aware that (with the exception of tree rings) some degree of
chronological distortion will occur in all paleoclimatic curves where
chronometric control is lacking.
To enable the reader to form his own judgments as to the chronology
of past climatic changes, most of the paleoclimatic curves given in this
report show explicitly the time control points between which the data
are spaced in proportion to their relative position in the original sedi-
mentary record. This procedure assumes that accumulation was con-
stant between the time controls, which is a reasonable assumption in
favorable environments. In other cases this assumption introduces a
distortion in the signal and a consequent uncertainty in the timing of
the inferred climatic variations.
Each of the recording media used in paleoclimatography has char-
acteristic limitations and advantages. As summarized in Table A.l,
the reconstruction of past climates requires evidence from a variety of
techniques, each yielding time series of different lengths and sampling
intervals and reflecting variations in different regions. The tree-ring
record, for example, provides evenly spaced and continuous annual
records, but only for the past few thousand years. The ice-margin record
of both valley and continental glaciers is discontinuous, especially
prior to about 20,000 years ago, because each major glacial advance
tends to obliterate (or at least to conceal) the earlier evidence. Records
of lake levels and sea levels are also discontinuous. The former rarely
APPENDIX A 141
extend back more than 50,000 years, although the latter extend back
several hundred thousand years. Soil sequences display great variability
in sedimentation rate but provide continuous climatic information for
sites on the continents where other records are not available (or are
discontinuous); in favored sites, the soil record extends back about a
million years. Pollen records are usually continuous but are rarely
longer than 12,000 years. Deep-sea cores provide material for the
study of fossils, oxygen isotopes, and sedimentary chemistry. These
records are relatively continuous over the past several hundred thousand
years and are distributed over large parts of the world ocean. Their
relatively uniform but low deposition rates, however, generally limit the
chronological detail obtainable. Cores taken in the continental ice
sheets provide a detailed and generally continuous record for many
thousands of years, although their interpretation is handicapped by the
lack of fully adequate models of the ice flow with its characteristic
velocity-temperature feedback.
Chronometric Techniques
The problem of constructing a paleoclimatic chronology has been ap-
proached by four direct methods and one indirect method.
Dendrochronology The most accurate direct dating is achieved in
tree-ring analysis, in which many records with overlapping sets of rings
are matched. With sufficient samples, virtual certainty in the dates
of each annual layer may be obtained, and a year-by-year chronology
can be established for periods covered by the growth records of both
living and fossil trees. Such records are especially valuable for studying
variations of climate during the last few hundred years and can be
extended to many of the land regions of the world.
Analysis of Annually Layered Sediments In favored locations, lakes
with annually layered bottom sediments provide nearly the same time
control as do tree rings. Some ice cores and certain marine sediment
cores from regions of high deposition rates also contain distinct annual
layers. These data, along with tree rings and historical records, are the
only source of information on the high-frequency portion of the
spectrum of climatic variation.
Radiocarbon Dating The advent of the 14C method in the early 1950's
was a major breakthrough in paleoclimatography, for it made possible
the development of a reasonably accurate absolute chronology of the
;
142 UNDERSTANDING CLIMATIC CHANGE
past 40,000 years in widely distributed regions. Prior knowledge was
essentially limited to dated tree-ring sequences (for the past several
thousand years) and to varve-counted sequences in Scandinavia (ex-
tending back to about 12,000 years). The 14C method has an accuracy
of about ±5 percent of the age being determined; that is, material
10,000 years old could be dated within the range 9500-10,500 years.
The calibration of llC ages against those determined from dendro-
chronology gives insight into the variations of atmospheric 14C produc-
tion rates over the past 7000 years (Suess, 1970).
Decay of Long-Lived Radioactivities These methods employ daughter
products of uranium decay or the production of 10Ar through potassium
decay. Used under favorable circumstances, one of the uranium methods
(the decay of 230Th) can provide approximate average sedimentation
rates in deep-sea cores. The other method (the growth of 230Th) can
be used successfully on fossil corals to provide discrete dates for shore-
line features recording ancient sea levels. Together, these techniques
have provided a reasonably satisfactory chronology of the past 200,000
years with a dating accuracy of about ±10 percent. Our chronology
for older climatic records is based on the well-known K/Ar technique,
applied to terrestrial lava flows and ash beds. This technique has pro-
vided, for example, the important dates for paleomagnetic reversal
boundaries.
Stratigraphic Correlation with Dated Sequences Much of the absolute
chronology of climatic sequences is supplied by an indirect method,
namely, the stratigraphic correlation of specific levels in an undated
sequence with dated sequences from another location. For example, a
particular glacial moraine that lacks material for 14C dating may be
identified with another formed at the same time that has datable ma-
terial. Such correlation by direct physical means is limited to relatively
small regions, however, and stratigraphic correlation techniques must
be used. Three such methods form the backbone of the chronology
of paleoclimate : biostratigraphy, isotope stratigraphy, and paleomag-
netic stratigraphy.
The techniques of biostratigraphy use the levels of extinction or
origin of selected species as the basis for correlation. This method has
enabled Berggren (1972), for example, to devise a time scale of the
past 65,000,000 years that is widely used as a basis for historical inter-
pretation. Isotope stratigraphy, applicable only to the marine realm,
makes use of the fact that the record of oxygen isotope variations —
APPENDIX A 143
which reflects chiefly the global ice volume — has distinctive char-
acteristics that permit the correlation of previously undated sequences.
The application of paleomagnetic correlation techniques has revolu-
tionized our approach to the climatic history of the past several million
years. Their importance stems from the fact that the principal magnetic
reversal boundaries, which have occurred irregularly about every
400,000 years, are recorded in both marine and continental sedimentary
sequences.
Regularities in Climatic Series
On the assumption that climatic changes are more than just random
fluctuations, paleoclimatologists have long sought evidence of regu-
larities in proxy records of the earth's climatic history. Many have
found what they believe to be firm evidence of order and refer to the
chronological patterns as "cycles." Although the number of records
is limited, and hard statistical evidence is sometimes lacking, it is never-
theless convenient to describe some of the larger climatic changes in
terms of quasi-periodic fluctuations or cycles with specified mean wave-
lengths or periods, in the sense that they describe the apparent repetitive
tendency of certain sequences of climatic events. For example, many
aspects of the global ice fluctuations during the last 700,000 years
may be summarized in terms of a 100,000-year cycle [see Figure
A.2(e)]. Each such period is marked by a gradual transition from a
relatively ice-free climate (or interglacial) to a short, intense glacial
maxima and followed by an abrupt return to ice-free climate. No two
such cycles are the same in detail, however, and should not be construed
as indicating strict periodicities in climate.
Some paleoclimatic cycles may be periodic, or at least quasi-periodic,
and rest on evidence that is exclusively or mainly chronological. The
best example is the approximate 100,000-year cycle found from the
spectral analysis of time series, such as that shown in Figure A. 4. For
the 100,000-year cycle, as well as some of the higher-frequency fluctua-
tions that modify it, there is circumstantial evidence to suggest that
these have in some way been induced by secular variations of the
earth's orbital parameters, which are known to alter the latitudinal pat-
tern of the seasonal and annual solar radiation received at the top of
the atmosphere. For the 2500-year (and shorter) fluctuations suggested
by some proxy data series, the causal mechanism is unknown.
With the possible exception of the approximately 100,000-year quasi-
periodic fluctuation referred to above, the quasi-biennial oscillation (of
144
UNDERSTANDING CLIMATIC CHANGE
PERIOD IN THOUSANDS OF YEARS
100 50 33 25 20 17 14 12 11
T
10
.
CYCLES PER 100,000 YEARS
FIGURE A.4 Power spectrum of climatic fluctuations during the last
600,000 years according to Imbrie and Shackleton (1974). The data
analyzed are time-series observations of 5180 in fossil plankton in
the upper portion of a deep-sea core in the equatorial Pacific, inter-
polated at intervals of 2500 years (Shackleton and Opdyke, 1973).
This ratio reflects fluctuations in global ice-volume.
L
APPENDIX A 145
2-3 year period) is the only quasi-periodic oscillation whose statistical
significance has been clearly demonstrated. This is not to say that other
such fluctuations in climate are absent but rather that much further
analysis of proxy records is required. A question of equal importance
is the shape of the continuum variance spectrum of climatic fluctua-
tions. A uniform distribution of variance as a function of frequency
(or "white noise") would imply a lack of predictability in the statistical
sense or a lack of "memory" of prior climatic states. A "red-noise"
spectrum, on the other hand, in which the variance decreases with in-
creasing frequency, implies some predictability in the sense that suc-
cessive climatic states are correlated. The existence of nonzero auto-
correlations in such a spectrum implies that some portion of the climatic
system retains a "memory" of prior states. In view of the relatively
short memory of the atmosphere, it seems likely that this is provided by
the oceans on time scales of years to centuries and by the world's major
ice sheets on longer times scales.
An initial estimate of the variance spectrum of temperature has been
made from the fluctuations on time scales from 1 to 10,000 years by
Kutzbach and Bryson (1974) and is shown in Figure A. 5. This spectrum
has been constructed from a combination of calibrated botanical, chemi-
cal, and historical records, along with instrumental records in the North
Atlantic sector. As may be seen in Figure A.5(a), the variance spectral
density increases with decreasing frequency (increasing period) over
the entire frequency domain but is most pronounced for periods longer
than about 30 years. In Figure A.5(b), the spectrum of the same time
series is shown with frequency on a logarithmic scale and the ordinate
as spectral density (V) times frequency (/), so that equal areas repre-
sent equal variance. Again, for periods longer than about 30-50 years,
the observed temperature spectrum is seen to depart significantly from
the white-noise continuum associated with the high-frequency portion of
the spectrum. The determination of the character of the variance spec-
trum of the various climatic elements remains largely a task for the
future.
We will use the term "cycle" in the following paragraphs to designate
such quasi-periodic sequences of climatic events, since there appears
to be no other word or phrase that conveys the concept of a series of
generally similar events spaced at reasonably regular intervals in time.
Although our knowledge of the record of past climates has improved
greatly during the last decade, a much broader paleoclimatic data base
is clearly required. Only then can adequate spectral analyses be per-
formed and the spatial and temporal structure of paleoclimatic variations
firmly established.
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148
UNDERSTANDING CLIMATIC CHANGE
CHRONOLOGY OF GLOBAL CLIMATE
Period of Instrumental Observations
A variety of meteorological indices have been used to characterize the
climate and its temporal variations during the past century or more
of extensive observations. Global- or hemisphere-averaged indices such
as the surface temperature index shown in Figure A. 6 are often used for
this purpose. This index clearly suggests a worldwide warming begin-
ning in the 1880's, followed by a cooling since the 1940's. The warming
may be recognized as the last part of a complex but recognizable trend
that has persisted since the end of the seventeenth century [see
Figure A.2(b)].
The geographic patterns of temperature change during these overall
warming and cooling epochs show considerable variability, with the
largest changes concentrated in the polar regions of the northern
hemisphere. Mitchell (1963) has shown that the pattern of temperature
change during recent decades is consistent with concomitant changes in
the large-scale atmospheric circulation as reflected in sea-level pressure.
Less attention has been given to the more complex relationships be-
tween circulation variation and changing precipitation patterns, although
Kraus (1955a, 1955b) and Lamb (1969) have considered this aspect
of the problem.
Lamb and Johnson (1959, 1961, 1966) and Lamb (1969) have
made an extensive analysis of certain features of atmospheric circulation
I960
FIGURE A.6 Recorded changes of annual mean temperature of the
northern hemisphere as given by Budyko (1969) and as updated after
1959 by H. Asakura of the Japan Meteorological Agency (unpublished
results).
APPENDIX A 149
based on the observed and historically reconstructed surface pressure
maps for individual months since about 1750. They have extracted such
indices as the strength of the zonal and meridional flow, the position
and wavelength of trough-ridge patterns, and the position and strength
of subtropical pressure systems. The year-to-year and decade-to-decade
changes in these indices reflect changing large-scale circulation pat-
terns, which in turn are associated with changing patterns of tempera-
ture and precipitation. From the instrumental era for the North Atlantic
sector, the typical variability over 20- to 30-year intervals of the low-
level westerlies is ±1-2 m sec-1, and that for the planetary-scale
circulation features (such as large-scale troughs and ridges) is ±1-2
deg latitude and ± 10-20 deg longitude.
Although changes in the position, pattern, and intensity of the gen-
eral circulation are interrelated, such empirical studies suggest that
longitudinal shifts have the most significant effects on the climatological
temperature and precipitation patterns, at least for middle and higher
latitudes. Examples of such shifts are shown in Figure A. 7. In tropical
and subtropical latitudes, on the other hand, latitudinal shifts appear
to be more closely related to regional climatic variations, as indicated
by the data of Figure A. 8.
The Last 1000 Years
To obtain an indication of the climate in the northern hemisphere for
the last 1000 years, Lamb (1969) has compiled manuscript references
on the character of European weather and has developed an index of
winter severity, as shown in Figure A.2(b) and A. 9. Although different
longitudes show somewhat different results, the trends shown by this
index (the excess number of unusually mild or unusually cold winter
months over months of opposite character) for the period since about
1700 have been validated by comparison with thermometer records.
Other portions of the record have been cross-checked with data on
glacial fluctuations, oxygen isotope variations, and tree growth, so that
the main characteristics of European climate during this period are
reasonably well known. LaMarche (1974) has constructed temperature
and moisture records from the ring-width variations in trees at high-
altitude arid sites in California [see Figure A.9(c)]. Comparisons of his
data with those from Europe shown in Figure A.9(d) indicate a degree
of synchrony in the major fluctuations of temperature between the west
coast of North America and Western Europe during the last 1000 years.
The early part of the last millenium (about a.d. 1100 to 1400) is
sometimes called the Middle Ages warm epoch but was evidently not as
150
UNDERSTANDING CLIMATIC CHANGE
1790-1829 r
1800-1839
1810-1849
1820-1859 |-
1830-1869
1840-1879 -
1850-1889 -
1860-1899
1870-1909
1880-1919
1890-1929
1900-1939
1910-1949
1920-1959 hr
Trough
60 °W
20UE
1780-1819
1800-1839
1820-1859
1840-1879 ~
1860-1899 -
1880-1919 -
1900-1939 -
1920-1959
Trough
Ridge
J II I L
70 W 60
50
40 30 20 10 0 10 °E
FIGURE A.7 Forty-year running means of the longitudes of the semipermanent
surface pressure troughs and ridges in the North Atlantic (Lamb, 1969). (a) at
45 °N in January; (b) at 55 °N in July.
APPENDIX A
151
2-3
120.
II 100-
80
120
ll
s 100-
(a)
- (b)
i i ! I I i I 1 ' ' F ' ' I ' I ' ' I
« I 1 1 1 ii 1 r— » 1 r 1 —
1900 10 20 30 40 50 60 70
YEAR
FIGURE A.8 Twenty-year running means of selected climatic indices (Winstanley, 1973).
(a) Frequency (days per year) of westerly weather type over the British Isles (from Lamb,
1969); (b) winter-spring rainfall (mm) at 14 stations in North Africa and the Middle East;
(c) summer monsoon rainfall (mm) at eight stations in the Sahel of North Africa and
northwest India.
warm as the first half of the twentieth century. The period from about
1430 to 1850 is commonly known as the Little Ice Age, and some
records indicate that this period had cold maxima in the fifteenth and
seventeenth centuries. From such evidence we infer that the atmospheric
circulation may have been more meridional than at present and char-
acterized in western Europe and western North America by short, wet
summers and long, severe winters.
During the Little Ice Age many glaciers in Alaska, Scandinavia, and
the Alps advanced close to their maximum positions since the last
major ice age thousands of years ago. A visual impression of these
events in the French Alps was shown in Figure A.l. The expansion of
the Arctic pack ice into North Atlantic waters caused the Norse colony
in southwest Greenland to become isolated and perish; and in Iceland,
grain that had grown for centuries could no longer survive.
152
UNDERSTANDING CLIMATIC CHANGE
PARIS - LONDON GREENLAND ICE CORE
HISTORIC 8 O18 (O/OO)
WINTER SEVERITY
COLD WARM -30 -29 -2~
1900
1700
1500
1300-
TREE
RING WIDTH
MM
I .3 4 .5 .6 .7
1900
1700
^CT WHITE
- <T MTNS.
1500
<
UJ 1300
<=Samarche
1100
900
CHANGE IN
MEAN ANNUAL
TEMPERATURE
00 0.5 1.0 1.5 „
- 2001
CENTRAL
ENGLAND] 400
LAMB
600 m
800
1000
FIGURE A.9 Climatic records of the
past 1000 years, (a) The 50-year moving
average of a relative index of winter
severity compiled for each decade from
documentary records in the region of
Paris and London (Lamb, 1969). (b) A
record of 5180 values preserved in the
ice core taken from Camp Century,
Greenland (Dansgaard et al., 1971). (c)
Records of 20-year mean tree growth
at the upper treeline of bristlecone
pines, White Mountains, California (La-
Marche, 1974). At these sites tree
growth is limited by temperature with
low growth reflecting low temperature,
(d) The 50-year means of observed and
estimated annual temperatures over
central England (Lamb, 1966).
The Last 5000 Years
As indicated in Figure A.2(c), the period from 7000 to 5000 years ago
was marked by temperatures warmer than those that prevail today [and
is thus sometimes known as the hypsithermal interval (Flint, 1971)].
The last 5000 years is characterized by generally declining temperatures
and a trend toward more extensive mountain glaciation (but not ice
sheets) in all parts of the world (Porter and Denton, 1967). Close
APPENDIX A
153
TREE GROWTH
FLUCTUATIONS AT
UPPER TREELINE
HOLOCENE
GLACIER
FLUCTUATIONS
(Denton a Karien,l973)
3000
2000
1000
B.C. A.D.
1000
FIGURE A.10 Climatic records of the past 5000 years, (a) Average (100-year mean) ring
widths of bristlecone pine at the upper treeline in the White Mountains of California
(LaMarche, 1974). Positive growth departures indicate warm-season (April-October)
temperatures above the long-term mean, with a total temperature range of about 4°F.
(b) Records of the advance and retreat of Holocene Alaskan glaciers (Denton and
Karlen, 1973).
examination of the records of mountain glaciers, treelines, and tree
rings suggests that this general cooling trend was itself punctuated in
many parts of the world by cold intervals centered at about 5300, 2800,
and 350 years ago, as shown in Figure A. 10. Much further analysis
of proxy climatic records during this period is needed, including the
evidence available from historical sources.
The Last 25,000 Years
The climatic record of the last 25,000 years is largely concerned with
the present interglacial interval (or Holocene) and the terminal phases
of the last major glaciation [see Figure A. 2(d)]. Although the maxi-
mum ice extent occurred between about 22,000 and 14,000 years ago
(see Figure A.ll) the curves of ice accumulation and decline are not
identical for the various ice sheets. The Laurentide ice sheet (which
covered parts of eastern North America) and the Scandinavian ice
sheet (which covered parts of northern Europe) reached their maximum
extent between 22,000 and 1 8,000 years ago, while the Cordilleran ice
sheet achieved its maximum only 14,000 years ago. The maximum areas
of the northern hemisphere ice sheets during the past 25,000 years
were about 90 percent of the maxima during the last million years of
the Pleistocene (see Table A.2).
Widespread deglaciation began rather abruptly about 14,000 years
154
UNDERSTANDING CLIMATIC CHANGE
VARIATIONS IN
CARIBBEAN
PLANKTON
(Core VI2-I22)
Faunal Index Tw
gg 25
FLUCTUATION IN THE MARGINS OF
NORTHERN HEMISPHERE ICE SHEETS
THREE
(Erie Lobe of Lauren tide ice sheet)
2000 1500 1000 500 0
I I I I I
(Cordilleron Ice Sheet in Fraser-Puget
Lowland)
i I I i I L_J
(Eastern Sector of Scandinavian Ice Sheet
i i i i i_
DISTANCE (km) FROM CENTER OF OUTFLOW
• =C14 dates in VI2-I22
(ice margin fluctuation chronology controlled by numerous C14 data)
FIGURE A.ll Climatic records of the past 40,000 years, (a) Fluctuations in Caribbean
plankton (core V12-122) interpreted as a record of sea-surface temperature in C. (b)
Fluctuations in the isotopic composition of Caribbean plankton interpreted as a record of
changing global ice-volume. Both records are from Imbrie et al. (1973). Curves (c), (d),
and (e) are time-distance plots of changes in the margins of three northern hemisphere
ice sheets. Curve (c) is from Dreimanis and Karrow (1972), curves (d) and (e) are due to
G. H. Denton, University of Maine (unpublished). The chronology of curves (a) and (b)
is controlled by 14C dates shown by solid circles; the ice-margin curves are controlled
by numerous a4C dates.
ago, and the waning phases of the continental ice sheets were char-
acterized by substantial marginal fluctuations (Dreimanis and Karrow,
1972), as shown in Figure A.ll. The Cordilleran ice sheet, which had
just attained its maximum extent, melted rapidly and was gone by
10,000 years ago. The Scandinavian ice sheet lasted only slightly
longer and retreated at the rate of about 1 km per year between about
10,000 and 9000 years ago. The climatic instability suggested by these
fluctuations in the margins of the northern hemisphere's major ice sheets
is corroborated by the records from fossil pollen, deep-sea cores, ice
cores, and sea-level variations, as shown in Figure A. 12, and by
lacustrine records in western North America and Africa. By 8500 years
ago the ice conditions in Europe had reached essentially their present
APPENDIX A
155
TABLE A.2 Characteristics of Existing Ice Sheets and of the Maximum
Quaternary Ice Cover "
Area
(1012m2)
Existing Glaciers
Greenland
1.80
Spitsbergen + Iceland
0.07
Canadian Archipelago
0.15
North America
0.08
Europe + Asia
0.17
South America
0.03
Antarctica
rAL AREA
12.59
TOT
14.99 (3% of earth's surface)
TOTAL ICE
VOLUME 6
2.5 x 107km3
EQUIVALENT SEA-LEVEL
CHANGE
70 m
Maximum Quaternary Glaciation
Greenland
2.30
Spitsbergen + Iceland
0.44
Alaska
1.03
Cordillera
1.58
Laurentide
13.39
Scandinavia
6.67
Europe
0.09
Asia
3.95
South America
0.87
Antarctica
13.81
Other
AL AREA
0.04
TOT
44.17 (9% of earth's surface)
TOTAL ICE
VOLUME 6
7.5xl07km3
EQUIVALENT SEA-LEVEL
CHANGE
210 m
"From Flint (1971).
6 Based on the present ice thickness of 1700 m in Greenland and Antarctica.
state, and in North America the ice sheets had shrunk to about their
present extent by about 7000 years ago.
How widespread and synchronous these fluctuations were is not yet
known, but evidence is growing that there were several periods of
widespread cooling and glacial expansion in the regions bordering the
Atlantic Ocean [see Figure A. 2(c)], spaced about 2500 years apart.
One of these glacial advances (the Younger Dryas event, about 10,800
to 10,100 years ago) was a climatic event of unparalleled abruptness
in Europe, establishing itself within a century or less and lasting for
some 700 years. Northern forests that had advanced during the pre-
156
UNDERSTANDING CLIMATIC CHANGE
U 0
CO
MINNESOTA
POLLEN CORE
63 66 69
NORTH ATLANTIC
CORE V-23-81
? 8 '?
GREENLAND
ICE CORE
DATED SHORE-
LINE FEATURES
38 -33 -28 25 50 75 100
1 I Tl i 1 -h l-i o
10
15
63 66 69 72
Floral Index
Ti
6 9 12 15
Faunal Index
Ts
20
J*25
-43 -38 -33 -28 25 50 75 100
80,8(%o)
(% rise since 18,000 YBP )
SEA LEVEL
FIGURE A.12 Climatic records of the last 25,000 years, (a) A floral index reflecting
changes in vegetation in Minnesota, as documented by pollen counts in a bog core
(Webb and Bryson, 1972). The index is an estimate of July air temperature in °F. (b) A
faunal index reflecting changes in foraminiferal plankton in a core west of Ireland, from
C. Sancetta, Brown University (unpublished). The index is an estimate of August sea-
surface temperature in C. (c) Values of 5lsO in the ice-core of Camp Century, Greenland
(Dansgaard et al., 1971). The isotope ratio is judged to reflect air-temperature variations
over the ice cap, with the more negative values associated with colder temperatures,
(d) Generalized curve of numerous sea-level records (Bloom, 1971). Chronology of curves
(a) and (b) is established by "C dates (solid circles) and stratigraphic correlation with
14C dates (open circles). Chronology for curve (c) above arrow (12,700 years ago) taken
from Dansgaard et al., (1971); below arrow, the chronology of Dansgaard et al. has been
modified by stratigraphic correlation with dated records in North Atlantic deep-sea cores
(Sancetta et al., 1973). Curve (d) is controlled by numerous "C dates.
ceding warm interval were destroyed in many places. Such vegetation
records suggest that by the end of the Younger Dryas event, European
climate had returned to about its present state.
The rise in sea level during the last 18,000 years indicated in
Figure A. 12(d) is generally ascribed to the melting of northern hemi-
sphere continental ice sheets. Details of the sea-level curve, however,
do not correspond to the chronology of deglaciation just described:
while the continental ice sheets had essentially disappeared by about
7000 years ago, the worldwide stand of sea level has reached its maxi-
mum only during the last few thousand years or is still slowly rising
APPENDIX A
157
(Bloom, 1971). One possibility is that the West Antarctic ice sheet is
unstable and has been disintegrating during the entire interval in ques-
tion. Further research is clearly needed to settle this question, although
it serves to illustrate the global interrelationships among the elements
of the climatic system.
The Last 150,000 Years
In order to find an ancient counterpart to the warm, ice-free condi-
tions of the past 10,000 years (the Holocene or present interglacial),
it is necessary to go back some 125,000 years to an interval known as
the Eemian interglacial (see Figure A.2). As shown by the proxy data
of Figure A. 13, the warmest part of this period lasted about 10,000
NORTH ATLANTIC MEDITERRANEAN
PLANKTON (Core V2M2) POLLEN
0
5 8
II
14
1 ■ J
15
30
45-
60-
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90"
105
120
135
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) (Camp Century Core) (Core P6304-9)
O BO SO 90 -44 -38 -32 -28 0.5 -02 -09 -L6 -100-70-40-10
CARIBBEAN SHORELINE FEATURES
P*y<IfN«iS0J0PE (Bermuda.Barbadoe.New Guinea)
FAUNAL INDEX
Te
0 30 60 90
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POLLEN
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ICE CORE %.S0,8in
SO18 (%•) PLANKTON SHELL
100-70 -40 -K>
SEA LEVEL
METERS
FIGURE A.13 Climatic records of the last 135,000 years, (a) A faunal index reflecting
changes in foraminiferal plankton in a core west of Ireland. The index is an estimate of
August sea-surface temperature in C (Sancetta et al., 1973). (b) The percentage of
tree pollen accumulated in a Macedonian lake. High values indicate warmer and some-
what dryer conditions (van der Hammen et al., 1971). (c) Oxygen isotope ratio expressed
as 5lsO in an ice core at Camp Century, Greenland. This is interpreted as indicating
changing air temperatures over the ice cap (Dansgaard et al., 1971). (d) Oxygen isotope
ratio in skeletons of planktonic foraminifera in a Caribbean core, interpreted as changes
in global ice volume. High negative values reflect the melting of ice containing isotopically
light oxygen (Emiliani, 1968). (e) Generalized sea-level curve. Portion younger than
20,000 years is representative of a large number of sites (Bloom, 1971); see also Figure
A.12. Older segments are from elevated coral reef tracts on Barbados, Bermuda, and
New Guinea (Mesolella et al., 1969; Veeh and Chappell, 1970). Chronology of curves
(a) to (d) controlled by 14C dates (solid circles) and by stratigraphic correlation with dated
horizons (open circles). Curve (e) is controlled by 14C dates for the portion younger than
20,000 years and by uranium growth methods for the older segments.
158 UNDERSTANDING CLIMATIC CHANGE
years and was followed abruptly by a cold interval of substantial glacial
growth lasting several thousand years. The interval between this post-
Eemian event (c. 115,000 years ago) and the most recent glacial maxi-
mum 18,000 years ago was characterized by marked fluctuations
superimposed on a generally declining temperature. An intense glacial
event about 75,000 years ago is sometimes used to separate the interval
into an older and generally nonglacial regime and a more recent glacial
one.
A remarkable feature of the climatic record of the past 150,000
years is that both the present and the Eemian interglacials began with
an abrupt termination of an intensely cold, fully glacial interval. Be-
cause these catastrophic episodes of deglaciation have left such a
strong imprint on the climatic record, they have been named (in order
of increasing age) termination I and termination II (see Figure A. 14
and Broecker and van Donk, 1970) .
The Last 1,000,000 Years
For at least the last 1,000,000 years the earth's climate has been char-
acterized by an alternation of glacial and interglacial episodes, marked
in the northern hemisphere by the waxing and waning of continental
ice sheets and in both hemispheres by periods of rising and falling
temperatures. How clearly these fluctuations are stamped on the various
proxy data records is shown in Figure A. 14. The most prominent
features of the isotope curve shown here are seven terminations, mark-
ing a transition from full glacial to full interglacial conditions. All but
one (termination III) of these changes are relatively rapid monotonic
swings and provide an objective basis for defining a climatic "cycle"
for at least the last 700,000 years. As shown in Figure A. 14, these
same fluctuations can be recognized in diverse and widely distributed
records, including the chemical composition of Pacific sediments, fossil
plankton in the Caribbean, and the soil types in central Europe. These
"cycles," identified as A to E by Kukla (1970), are found in each of
the records shown in Figure A. 14 and may be grouped into a climatic
"regime" covering the last 450,000 years (designated a). The earlier
records (regime (3) show higher-frequency fluctuations with less co-
herence among the various proxy climatic recorders.
The Last 100,000,000 Years
Although continuous and detailed records are lacking for these earlier
times, at least the broad outline of this period of climatic history may
APPENDIX A
159
ISOTOPIC COMPOSITION OF
PACIFIC PLANKTON
(CORE V28-238)
CHEMICAL COMPOSITION
OF EQUATORIAL PACFIC
TAXONOMIC COMPOSITION
OF CARIBBEAN PLANKTON
(CORE VI2-I22)
CENTRAL EUROPEAN
SOIL RECORD
Observation
-1.5 -2.0 -2.5 100 80 60 40 36.2 36.0 35.8 35.6
»°'8 <°/~l CoCO, (%) FAUNAL INDCX (S) SOIL TYPE
Interpretation. DECREASING GLOBAL INCREASING CoCOs DECREASING SALINITY TEMPERATE CLIMATE
ICE VOLUME
DISSOLUTION
FIGURE A.14 Climatic records of the last 1,000,000 years, (a) Oxygen-isotope curve in Pa-
cific deep-sea core V28-238, interpreted as reflecting global ice volume (Shackleton and
Opdyke, 1973). The relatively rapid and high-amplitude fluctuations are taken to indicate
sudden deglaciations and are designated as the terminations I to VII. (b) Calcium
carbonate percentage in equatorial Pacific core RC1 1-209 (Hays et al., 1969). Low values
are taken to indicate periods of rapid dissolution by bottom waters, (c) Faunal index
reflecting changing composition of Caribbean foraminiferal plankton, calibrated as an
estimate of sea-surface salinity in parts per thousand (Imbrie et al., 1973). Glacial periods
are marked by the influx of plankton preferring higher-salinity waters (Prell, 1974). (d)
Sequence of soil types accumulating at Brno, Czechoslovakia (Kukla, 1970). Type 1 is a
wind-blown loess with a fossil fauna of cold-resistant snails or gley soils indicating
extremely cold conditions; type 2 includes pellet sands and other hillwash deposits,
partly interbedded with loess; type 3 includes brownearth and tschernosem soils; type 4
includes para brownearth (lessive) soils; type 5 are soils of temperate savannas, including
brownlehms, rubified brownlehms, and rubified lessives with large, hollow carbonate con-
cretions. The duration of each soil type at this locality is plotted in proportion to the
maximum thickness observed. Note that each record shown here reflects a climatic
fluctuation or "cycle" averaging about 100,000 years. This is particularly true during the
last 450,000 years (climatic regime a). Chronology of the curves is obtained by linear
interpolation between indicated control points, shown by solid circles.
be discerned. From the climatic point of view, perhaps the most strik-
ing aspect of world geography at the beginning of this interval was the
essentially meridional configuration of the continents and shallow
ocean ridges, which must have prevented a circumpolar ocean current
in either hemisphere. In the south this barrier was formed by South
America and Antarctica (which lay in approximately their present
160 UNDERSTANDING CLIMATIC CHANGE
latitudinal positions); by Australia (then a north-eastward extension
of Antarctica); and by the narrow and relatively shallow ancestral
Indian Ocean (Dietz and Holden, 1970). About 50,000,000 years ago
the Antarctica-Australian passage began to open (Kennett et ah, 1973),
and as Australia moved northeastward, the Indian Ocean widened
and deepened. Both paleontological and sedimentary evidence suggest
that about 30,000,000 years ago the Antarctic circumpolar current
system was first established. This must be considered a pivotal event in
the climatic history of the past 100,000,000 years, and when the evi-
dence of global plate movements is complete, it may well be possible
to account for much of the secular climatic changes of this period as
a response to the changing boundary conditions imposed by the distribu-
tion of land and ocean.
During the last part of the Mesozoic era (from 100,000,000 to
65,000,000 years ago) global climate was in general substantially
warmer than it is today, and the polar regions were without ice caps.
About 55,000,000 years ago numerous geologic records (Addicott,
1970; Flint, 1971) make it clear that global climate began a long
cooling trend known as the Cenozoic climatic decline (see Figure A.15).
Evidence from the marine record indicates that about 35,000,000 years
ago Antarctic waters underwent a substantial cooling (Douglas and
Savin, 1973; Shackleton and Kennett, 1974a, 1974b). There is direct
evidence that ice reached the edge of the continent in the Ross Sea area
some 25,000,000 years ago; and during the Oligocene epoch, roughly
35,000,000 to 25,000,000 years ago, global climate was generally
quite cool (Moore, 1972). During early Miocene time (20,000,000 to
15,000,000 years ago) evidence from low and middle latitudes indicates
a warmer climate, but isotopic evidence and faunal data indicate that
this warming did not affect high southern latitudes.
About 10,000,000 years ago there is widespread evidence of further
cooling, substantial growth of Antarctic ice (Shackleton and Kennett,
1974a, 1974b), and growth of mountain glaciers in the northern
hemisphere (Denton et al, 1971). For general descriptive purposes the
present glacial age may be defined as beginning at this time. Indirect
evidence from marine sediments indicates that about 5,000,000 years
ago the already substantial ice sheets on Antarctica underwent rapid
growth and quickly attained essentially their present volume (Shackle-
ton and Kennett, 1974a, 1974b). This evidence is generally consistent
with direct records from the Antarctic continent, which show that
between 7,000,000 and 10,000,000 years ago a large ice sheet existed
in West Antarctica, and that by about 4,000,000 years ago the ice sheet
in East Antarctica had developed to essentially its present volume
APPENDIX A
161
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162
UNDERSTANDING CLIMATIC CHANGE
(Denton et al., 1971; Mayewski, 1973). The present Antarctica ice
mass is equivalent to about 59 m of sea level.
Although the behavior of the smaller, West Antarctic ice sheet is
incompletely known, the available evidence indicates that it has under-
gone considerable fluctuation, and that its variations are roughly syn-
chronous with the northern hemisphere glacial-interglacial cycle. This
may be due to the fact that while the East Antarctic ice sheet is solidly
grounded on the continent, much of the West Antarctic ice mass is
grounded on islands or on the sea floor and could therefore be sig-
nificantly influenced by sea-level variations due to glacial changes in
the northern hemisphere. Such continental ice sheets first appeared
in the northern hemisphere about 3,000,000 years ago, occupying lands
adjacent to the North Atlantic Ocean (Berggren, 1972b), and during at
least the last million years the ice cover on the Arctic Ocean was never
less than it is today (Hunkins et al, 1971 ) .
The Last 1,000,000,000 Years
Our knowledge of the climatic events over this time range consists
principally of evidence of glaciations as preserved in the geological
record. This may be seen in perspective with that for the more recent
periods discussed above in Figure A. 15. The present glacial age is seen
to be at least the third time that the planet has suffered widespread
continental glaciation. The Permo-Carboniferous glacial age (about
300,000,000 years ago) occurred at a time when the earth's land masses
were joined in a single supercontinent (Pangaea). The area of this
continent was distributed in roughly equal proportions between the
hemispheres, with a concentration of land in the midlatitudes (Dietz
and Holden, 1970). Glaciated portions of Pangaea included parts of
what are now South America, Africa, India, Australia, and Antarctica.
One or more earlier glacial ages are known from late Precambrian
times (about 600,000,000 years ago), from the indications of glacia-
tion in deposits now widely scattered over the globe, including Green-
land, Scandinavia, central Africa, Australia, and eastern Asia (Holmes,
1965).
Although other glacial ages may have occurred besides those recog-
nized in Figure A. 15, none has left such a clear and widespread imprint
on the geological record (Steiner and Grillmair, 1973). While the evi-
dence is far from complete, it may be that each of the earth's major
glacial ages — including the present one — resulted from crustal move-
ments that permitted the development of sharp thermal gradients over
a continental land mass that includes a pole of rotation. To establish
APPENDIX A 163
this or other hypotheses of long-period climatic changes, however,
will require the assembly of a much more complete geological record
and the performance of appropriate climate modeling experiments.
GEOGRAPHIC PATTERNS OF CLIMATIC CHANGE
While the chronology of certain features of climatic change may be
revealed by the analysis of instrumental and paleoclimatic data at in-
dividual sites, the geographic pattern of these changes is an equally
important characteristic. From what we know of the behavior of the
(present) atmosphere, it would be remarkable if there were not a defi-
nite spatial structure to the variations on all climatic times scales. The
search for these patterns requires synoptic data for the various climatic
elements, and this is presently available only from the records of
modern observations and from a few marine proxy sources.
Structure Revealed by Observational Data
The task of describing the spatial and temporal structure of climatic
variations from the observations of the instrumental era is far from
complete. Most studies have therefore focused primarily on local or
regional climatic changes. Lamb and Johnson (1961, 1966) have made
comprehensive analyses of intrahemispheric and interhemispheric cli-
matic indices, and the statistical structure of these circulation variations
has been studied by Willett (1967), Wagner (1971), Iudin (1967),
Brier (1968), and Kutzbach (1970). Such analyses, especially of
hemispheric pressure data, reveal that the year-to-year and decade-to-
decade variations have a spatial structure that may be associated with
amplitude and phase changes of the long planetary waves in the
atmosphere.
The essentially two-dimensional character of climate is masked in
studies of zonally averaged parameters, although these may be use-
ful for other purposes. An example of the importance of both zonal
and nonzonal spatial variability of the atmospheric circulation is
provided by the first eigenvector pattern (or empirical orthogonal
function) of hemispheric pressure for January, shown in Figure A. 16,
as well as by the patterns of pressure, temperature, and rainfall variabil-
ity shown in Figures A. 17, A. 18, and A. 19. These data suggest an
association between the changes in the monthly average intensity and
position of the Aleutian and Icelandic lows. For example, during the
first two decades of this century there has been a tendency for decreased
intensity and westward extension of the Aleutian low, coupled with an
164
UNDERSTANDING CLIMATIC CHANGE
JANUARY
RRST EIGENVECTOR
FIGURE A.16 The first eigenvector of northern hemisphere sea-level pressure, based on
the individual mean January maps for 1899-1969 (Kutzbach, 1970). This spatial function
accounts for 22 percent of the total inter-January variance.
increased intensity and northeastward shift of the Icelandic low. Lamb
(1966) and Namias (1970) have described important regional changes
in temperature and precipitation associated with these circulation
changes. The opposite tendency has prevailed since the mid-1 950's,
and Lamb (1966), Winstanley (1973), and Bryson (1974) have
described the possible relationships between the changing midlatitude
circulation patterns of the 1960's, the equatorial shift of the subtropical
highs, and the increasing frequency of droughts along the southern
fringes of the monsoon lands of the northern hemisphere (see Figure
A.8). These changes appear to reflect an equatorward extension of
the westerly wave regime and a contraction of the Hadley circulation,
APPENDIX A
165
FIGURE A.17 Standard deviation (mbar) of monthly mean pressure at sea level, 1951-
1966 (Lamb, 1972). (a) December, northern hemisphere; (b) July, southern hemisphere.
166
UNDERSTANDING CLIMATIC CHANGE
T
December
FIGURE A.18 Standard deviation (°C) of monthly mean surface air temperature in the
northern hemisphere, 1900-1950 (Lamb, 1972). (a) July; (b) December.
APPENDIX A
167
Rainfall variability
The figures denote percentage departures from normal
Under 10 10-15 15-20 20-25 25-30 30-40 Over 40%
FIGURE A.19 The variability of mean annual rainfall for the world (adapted from
Petterssen, 1969).
although much further analysis is clearly required to confirm such a
conjecture.
Interhemispheric relationships of climatic indices have been (and
remain) less amenable to study because of the general lack of observa-
tions from the southern hemisphere. Observations are sufficient, how-
ever, to show that the circulation in the southern hemisphere is some-
what stronger and steadier than that in the northern hemisphere. Whether
this results in the southern hemisphere circulation leading that in the
northern hemisphere, or whether variable features in the equatorial
circulation influence both hemispheres similarly, is not presently known
(Bjerknes, 1969b; Fletcher, 1969; Lamb, 1969;Namias, 1972a, 1972b).
It is likely that interhemispheric relationships of one sort or another
are important for the understanding of climatic variations, and that
our ability to describe them will require the availability of much more
comprehensive data than now exist from the southern hemisphere, the
equatorial region, and the oceanic and polar regions of the northern
hemisphere.
The present accumulation of upper-air data, especially in the north-
ern hemisphere since the early 1950's, however, has permitted a
beginning of the study of the three-dimensional spatial and temporal
variability of the general circulation. A foundation of basic statistics is
168
UNDERSTANDING CLIMATIC CHANGE
provided by calculations of the means and variances of standard
meteorological variables (see, for example, Crutcher and Meserve, 1970;
Taljaard et al, 1969) and by atlases of energy budgets (Budyko, 1963).
The covariance structure of circulation patterns at 700 mbar in the
northern hemisphere is treated by O'Connor (1969), and other aspects
of the tropospheric circulation have been considered by Gommel (1963)
and Wahl (1972). The most comprehensive analysis of atmospheric
circulation statistics, however, is that based on the period 1958-1963
as undertaken by Oort and Rasmusson (1971). While this work docu-
ments the monthly, seasonal, and annual variations of many features
of the observed general circulation (in the northern hemisphere), it
does not directly address many of the variables of primary climatic
interest. Using the same data set, however, Starr and Oort (1973)
have reported an unmistakable downward trend of the mean air
temperature in the northern hemisphere of 0.6 °C over the five-year
interval shown in Figure A. 20. Diagnostic studies of this type represent
great investments of time and effort but are essential steps toward the
monitoring of climate and an assessment of the mechanisms of climatic
variation.
A complete description of climatic changes from instrumental records
must also include studies of the momentum and energy budgets of the
atmosphere and oceans and their variability with time over many years
and decades. While this must remain largely a task for the future,
several efforts have established the existence of significant interannual
variations in the atmosphere. Krueger et al. (1965) have discussed the
May
1958
April
1959
April
1960
April
1962
April
1963
1959
1960
1961 1962
1963
FIGURE A.20 Monthly mean mass-averaged values of the northern hemisphere tempera-
ture for the period May 1958 to April 1963 (Starr and Oort, 1973). The consecutive
monthly averages are plotted on the scale marked at the top; the bottom scale shows
the beginning of each calendar year.
APPENDIX A 169
interannual variations of available potential energy, and Kung and Soong
(1969) have described the fluctuations of the atmospheric kinetic
energy budget. As noted previously, the interannual variations of pole-
ward angular momentum and energy fluxes has been studied compre-
hensively by Oort and Rasmusson ( 1971 ). A measure of this variability
is shown in Figure A. 21 and amounts to about ±30 percent of the
mean transports.
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January as shown by five years data (Oort and Rasmusson, 1971). The solid curve is the
5-year mean for 1959-1963. (a) Angular momentum transport; (b) sensible heat transport.
170
UNDERSTANDING CLIMATIC CHANGE
The unique global potential of satellite-based measurements has
been exploited by Vonder Haar and Suomi (1971), who have sum-
marized the satellite measurements of planetary albedo and of the
planetary radiation budget for the five years 1962 to 1966. They found
large interannual variations in the zonally averaged equator-to-pole
gradient of the net radiation as shown in Figure A. 22. This forcing
function can now be monitored routinely by meteorological satellites
and opens the door to more detailed studies of atmospheric energetics
than heretofore possible (Winston, 1969). Vonder Haar and Oort
(1973) have combined satellite measurements of the earth's radiation
budget with atmospheric energy transport calculations to produce a new
estimate of the poleward energy transport by the northern hemisphere
0.3
N.H.
7 0.2
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z
o
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FIGURE A.22 Seasonal (dashed lines) and interannual
variation (vertical bars) of the equator-to-pole gradient of
net radiation as measured from satellites (Vonder Haar
and Suomi, 1971). (a) Northern hemisphere; (b) southern
hemisphere.
APPENDIX A
171
oceans. They find that the oceanic heat transport averages about 40 per-
cent of the total in the 0-70 °N latitude band and accounts for more
than half at many latitudes. Another example of the use of satellite-
derived measurements of climatic indices is given by Kukla and Kukla
(1974). Their measurements of the interannual changes in the area
of snow and ice cover in the northern hemisphere are shown in Figure
A. 23 and reveal year-to-year fluctuations of the order of a few percent.
Note, however, the relatively large change during 1971 and the subse-
quent maintenance of extensive snow and ice coverage and an associated
increase of the reflected solar radiation.
Time variations of the surface energy budget on a global scale are
1969
1970
1971
1972
1973
FIGURE A.23 Twelve-month running means of snow and ice cover in the
northern hemisphere (upper curve) and the estimated reflected solar
radiation disregarding variations of cloudiness (lower curve), as reported
by Kukla and Kukla (1974). The averages are plotted on terminal dates, with
the years marking January 1.
172 UNDERSTANDING CLIMATIC CHANGE
not available from direct observations and must be inferred from the
conventional measurements of temperature, humidity, cloudiness, wind,
and radiation. Fletcher (1969) has drawn attention to the variations in
the energy budget of polar regions as a function of variable sea-ice con-
ditions, while Sawyer (1964) has noted the possible role of fluctuations
in the surface energy budget as a cause of interannual variations of the
general circulation itself.
A number of observational studies of large-scale interaction between
the ocean and the atmosphere have illustrated the complexity and im-
portance of this mechanism; see, for example, Weyl (1968) and Lamb
and Ratcliffe (1972). Bjerknes (1969b) has considered the response
of the North Pacific westerlies to anomalies of equatorial sea-surface
temperature and variations in the Hadley circulation, while Namias
(1969, 1972b) has described positive feedback relationships between
large-scale patterns of ocean-surface temperature in midlatitudes and
the circulation of the overlying atmosphere. Such modes of atmosphere-
ocean coupling may be important parts of climatic fluctuations and must
be given further study.
In summary, we may say that observational data at the earth's sur-
face show that during the period 1900 to 1940 the northern hemisphere
as a whole warmed, although some areas (mainly the Atlantic sector of
the Arctic and northern Siberia) warmed far more than the global
average, some areas became colder, and others showed little measurable
change (Mitchell, 1963). In the time since 1940, an overall cooling
has occurred but is again characterized by a geographical structure;
cooling since 1958 has occurred in the subtropical arid regions and in
the Arctic (Starr and Oort, 1973). There is also some evidence that
the northern hemisphere oceans are cooling (Namias, 1972b), although
the oceanic data base necessary to confirm this has not yet been
assembled.
Structure Revealed by Paleoclimatography
Most of the work done to date on climatic change beyond the time
frame encompassed by meteorological observations represents a study
of time series taken at specific sites. This lack of synoptic data on the
longer-range climatic changes is a serious handicap to the portrayal
and understanding of the mechanisms involved. In order to underscore
these points, and to encourage further research, we present here ex-
amples of the few proxy data that have been assembled to reveal a
spatial structure of climatic change.
APPENDIX A
173
Distribution of Ice Sheets
The continental margins of the northern hemisphere ice sheets at their
maximum extension during the last million years are clearly marked
by the debris deposits in terminal moraines, while the extent of sea
ice is recorded by features preserved in marine sediments. Figure A. 24
140
160
180
160
140
FIGURE A.24 Maximum extent of northern hemisphere ice cover during the present
glacial age (modified after Flint, 1971). Continental ice sheets, indicated by the dotted
area, are B, Barents Sea; S, Scandinavian; G, Greenland; L, Laurentide; C, Cordilleran.
Sea ice is indicated by the cross-hatched pattern. The boundary mapped is the southern-
most extent of the ice margin that occurred in any sector during the last million years.
The last glacial maximum, about 18,000 years ago, occupied about 90 percent of the
area shown here (see also Table A.2).
174 UNDERSTANDING CLIMATIC CHANGE
shows the distribution of maximum ice cover, and Table A. 2 gives
statistics of the areas of the individual continental ice sheets. In North
America the ice extended as far south as 40 °N and spanned the entire
width of the continent, while in Europe the ice sheet extended only to
about 50 °N. Note that large regions in eastern Siberia were unglaciated.
Sea-Surface Temperature Patterns
The north-south migration of polar waters in the North Atlantic in
response to major cycles of glaciation is shown in Figure A.25. During
glacial maxima these waters were found as far south as 40 °N, well
beyond the present extent of polar waters. A synoptic analysis of the
ocean surface temperatures of 18,000 years ago (at about the time of
the last glacial maximum) is shown in Figure A. 26. These temperature
estimates have been derived by multivariate statistical techniques applied
to planktonic organisms as preserved in about 100 deep-sea cores in
scattered locations across the North Atlantic (Mclntyre et al., 1974).
The most striking feature of this glacial-age map is the extensive south-
ward displacement of the 10 to 14°C water, while the warmer water
was found in nearly its present position. In parts of the Sargasso Sea
the glacial-age ocean was, if anything, slightly warmer than it is today.
Because the atmosphere receives much of its heat from the sea, such
estimates of sea-surface temperature are likely to be important in de-
veloping a satisfactory reconstruction of past climates, and it is there-
fore important to consider their reliability. Berger (1971), for ex-
ample, has suggested that carbonate dissolution on the sea bed may
distort the taxonomic composition of the fossil fauna on which such
paleotemperature estimates are based. Kipp (1974), on the other hand,
shows that when the statistical transfer functions are calibrated on ma-
terials that incorporate the dissolution effects, an unbiased estimate of
such parameters as the sea-surface temperature can be obtained. The
temperature reconstruction in Figure A.26(b) is based on the statistics
of the foraminiferal fauna distribution and encompasses 91 percent of
the variance of the data (Mclntyre, 1974). The 80 percent confidence
interval of each of the cores is ±1.8°C (Kipp, 1974). Shackleton and
Opdyke (1973), using a revised isotopic method based on the differ-
ence between lsO values in benthic and planktonic species, have pro-
vided an independent confirmation of the sea-surface temperature
estimates of Imbrie et al. (1973) for a portion of the glacial-age
Caribbean.
Other reconstructions of paleo-ocean surface temperatures have been
based on data from radiolaria, coccoliths, and foraminifera; and al-
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APPENDIX A 177
though some discrepancies are revealed where independent data are
available, the derived ocean temperatures show considerable spatial
coherence (Mclntyre, 1974). Such estimates of past sea-surface tem-
perature will prove useful in climatic simulations with numerical general
circulation models (see Appendix B).
Patterns of Vegetation Change
Figure A.27 illustrates the use of fossil pollen data to record the changes
in vegetation accompanying the deglaciation of eastern North America
during the interval 11,000 to 9000 years ago. At the beginning of
this time, pine species occupied sites in the southeastern Appalachians,
but as the ice retreated, the pine moved farther north and west to
colonize newly uncovered areas. A relatively complete chronology of
the retreat of the Laurentide ice sheet itself is given by radiocarbon
dating (Brysonef al, 1969).
Patterns of Aridity
For only four desert areas in the world do we have enough information
to plot aridity as a function of time, and even in these areas the record
extends back only a few tens of thousands of years. As shown in
Figure A.28, the data suggest a degree of synchroneity between the two
African regions and the Great Basin, while the records from the Middle
East are quite different. None of the data from closed-basin lakes show
significant correlation with the glacial record, and we are clearly a long
way from understanding the response of arid regions to glacial cycles.
More generally, insufficient research has been devoted to the role of
desert regions in the processes responsible for the climate of the earth.
Patterns of Tree-Ring Growth
Changes of thickness of the growth rings added by trees each year
reflect environmental change in a complex way. By appropriate calibra-
tion, such data may be made to furnish significant climatic information
for the past several hundred to several thousand years. Studies of many
tree-ring series over a wide geographic area can, moreover, provide
accurately dated synoptic evidence of regional climatic patterns (Fritts,
1965).
Fritts et al. (1971) have demonstrated the feasibility of reconstruct-
ing the anomalies of sea-level pressure and temperature from the
spatial patterns of tree growth over western North America. Examples
178
UNDERSTANDING CLIMATIC CHANGE
95 85 75 65
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FIGURE A.27 The distribution of pine pollen at selected times dur-
ing the deglaciation of eastern North America (Bernabo et al., 1974).
Contours are lines of pollen frequency, expressed as a percent of
total pollen. Control points representing radiocarbon dated cores
are indicated by the open circles. The approximate margins of the
Laurentide ice sheet are indicated by the stippled pattern (after
Bryson et al., 1969).
APPENDIX A
179
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FIGURE A.28 Variations in the size of closed-basin lakes, as indicated by the degree
of aridity found from radiocarbon dating of shorelines and bottom sediments. Higher
rainfall and lower evaporation may be inferred at the times of larger water surfaces,
(a) From Broecker and Kaufman (1965); (b) from Kaufman (1971); (c) from Butzer et al.
(1972); (d) from Servant et al. (1969).
of such synoptic maps based on average decadal growth are given in
Figure A. 29. Although such reconstructions show considerable varia-
tion in the year-to-year climatic states, the inferred variations in the
intensity of Icelandic and Aleutian lows, for example, are similar to
those described in the modern record (Kutzbach, 1970). The develop-
ment of an expanded network of tree-ring sites could significantly
broaden our knowledge of the patterns of climatic fluctuations over the
past several centuries.
SUMMARY OF THE CLIMATIC RECORD
In this survey of past climates, the characteristic time and spatial
structures of climatic variations have been discussed as though there
were sufficient data to document large regions of the globe. This is true
only for the more recent parts of the instrumental period, as there are
large gaps in the presently available historical and proxy climatic
records. With these limitations in mind, it is nevertheless useful to
summarize the general characteristics of the climatic record:
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180
APPENDIX A 181
1. The last postglacial thermal maximum was reached about 6000
years ago, and climates since then have undergone a gradual cooling.
This trend has been interrupted by three shorter periods of more marked
cooling, simliar to the so-called Little Ice Age of a.d. 1430-1850, each
followed by a temperature recovery. The well-documented warming
trend of global climate beginning in the 1880's and continuing until the
1940's is a continuation of the warming trend that terminated the
Little Ice Age. Since the 1940's, mean temperatures have declined and
are now nearly halfway back to the 1 880 levels.
2. Climatic changes during the past 20,000 years are as severe as
any that occurred during the past million years. At the last glacial maxi-
mum, extensive areas of the northern hemisphere were covered with
continental ice sheets, sea level dropped about 85 m, and sea-surface
temperatures in the North Atlantic fell by as much as 10°C. At
northern midlatitude sites not far from the glacial margins (locations
now occupied by major cities and extensive agricultural activity), air
temperatures fell markedly, drastic changes occurred in the precipita-
tion patterns, and wholesale migrations of animal and plant communi-
ties took place.
3. The present interglacial interval — which has now lasted for about
10,000 years — represents a climatic regime that is relatively rare during
the past million years, most of which has been occupied by colder,
glacial regimes. Only during about 8 percent of the past 700,000 years
has the earth experienced climates as warm as or warmer than the
present.
4. The penultimate interglacial age began about 125,000 years ago
and lasted for approximately 10,000 years. Similar interglacial ages —
each lasting 10,000 ±2000 years and each followed by a glacial maxi-
mum— have occurred on the average every 100,000 years during at
least the past half million years. During this period fluctuations of the
northern hemisphere ice sheets caused sea-level variations of the order
of 100 m. In contrast, the East Antarctic ice sheet has apparently varied
little since reaching its present size about 5 million years ago, while
the West Antarctic ice sheet appears to have been disintegrating for
many thousands of years.
5. About 65 million years ago global climates were substantially
warmer than today, and subsequent changes may be viewed as part of
a very long-period cooling trend. For even earlier times, the proxy
climatic evidence becomes increasingly fragmentary. The best docu-
mented records suggest two previous extensive glaciations, occurring
about 300 million and 600 million years ago.
182 UNDERSTANDING CLIMATIC CHANGE
FUTURE CLIMATE: SOME INFERENCES FROM PAST BEHAVIOR
The overall picture of past climatic changes described in this survey
suggests the existence of a hierarchy of fluctuations that stand out
above the "white noise" or random fluctuations presumed to exist on all
time scales. In addition to the dominant period of about 100,000 years,
there are apparent quasi-periodic fluctuations with time scales of about
2500 years and shorter-period fluctuations on the order of 100-400
years. Each of these explains progressively less of the total variance
but may nevertheless be climatically significant. No periodic component
of climatic change on the order of decades has yet been clearly estab-
lished, although significant excursions of climate are observed to occur
in anomalous groups of years.
In view of the limited resolving power of most climatic indicators,
especially those for the relatively remote geological past, it is difficult
to establish whether the apparent fluctuations are quasi-periodic or
whether they more nearly represent what are basically random Markov-
ian "red-noise" variations. In the case of the longer-period variations
(of 100,000-year and 20,000-year periods), there is circumstantial evi-
dence to suggest that these may have been induced in some manner
by the secular variations of the earth's orbital elements, which are known
to alter the seasonal and latitudinal distribution of solar radiation re-
ceived at the top of the atmosphere. In other cases, the observed varia-
tions have yet to be convincingly related to any external climatic control.
The mere existence of such variations does not necessarily mean that
changes in the external boundary conditions are involved, however. The
internal dynamics of the climatic system itself may well be the origin
of some of these features. Whether forced or not, climatic behavior of
this type deserves careful study, as the conclusions reached bear directly
upon the problem of inferring the future climate.
The prediction of climate is clearly an enormously complex prob-
lem. Although we have no useful skill in predicting weather beyond a
few weeks into the future, we have a compelling need to predict the
climate for years, decades, and even centuries ahead. Not only do we
have to take into account the complex year-to-year changes possibly
induced by the internal dynamics of the climatic system, and the likely
continuation of the (yet unexplained) quasi-periodic and episodic
fluctuations of the last few thousand years discussed above, but also
the changes induced by possibly even less predictable factors such as the
aerosols added to the atmosphere by volcanic eruptions and by man
himself (Mitchell, 1973a, 1973b). These questions lie at the heart of
APPENDIX A 183
the problem of climatic variation and are given consideration elsewhere
in this report.
In the face of these uncertainties, any projection of the future climate
carries a great risk. Nevertheless, we may speculate about the possible
course of global climate in the decades and centuries immediately
ahead by making certain assumptions about the character of the major
fluctuations noted in the climatic record. In the following paragraphs
we attempt to draw together these considerations into an overall assess-
ment of the probable direction and magnitude of present-day climatic
change, taking into account the risk of a major future change associated
with the seemingly inevitable onset of the next glacial period.
Potential Contribution of Sinusoidal Fluctuations of Various Time
Scales to the Rate of Change of Present-Day Climate
Estimates of the amplitudes of all the principal climatic fluctuations
identified in this report are listed in Table A. 3 (where they have been
made consistent with the data presented in Figure A. 2 and are expressed
in terms of the total range of temperature between maxima and minima).
On the assumption that all of these fluctuations can be approximated
by quasi-periodic sine waves, the ratio of the amplitude (A) to the
period (P) of each fluctuation becomes proportional to the maximum
contribution of that fluctuation to the rate of change of climate. By
considering also the phase of each fluctuation, as inferred from the
paleoclimatic record, the contribution of each fluctuation to the present-
day rate of change can be estimated (see Table A.3).
Estimation of the phase of each sinusoidal fluctuation (indicated by
the estimated dates of the last temperature maximum in Table A.3)
permits an assessment of the sign and magnitude of the contribution of
each fluctuation to the total rate of change of globally average tempera-
ture in the present epoch. The sum of these individual contributions
( -0.015°C/yr) agrees reasonably well with the observed rate of change
of — 0.01°C/yr during the past two decades, as determined from
analyses of surface climatological data by Reitan ( 1971 ) and by Budyko
(1969). It should be noted that this trend is dominated by the shortest
fluctuations, and especially by the fluctuations of the order of 100 years
(see Figure A.6).
The estimated maximum rate of change associated with all time
scales of climatic fluctuation shown in Figure A. 2 is plotted as a con-
tinuous function of wavelength in Figure A. 30. The family of curves
also shown in this figure indicates the relationship between maximum
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APPENDIX A
185
.001 -
.0001
.00001
100,000
10,000 1,000 100 10
PERIOD OF FLUCTUATION (years)
FIGURE A.30 Relative (maximum) rate of change of climate contributed by climatic
fluctuations, as a function of characteristic wavelength. The family of parallel curves
shows the expected relationship in Markovian "red noise" as characterized by the serial
correlation coefficient at a lag of one year. The dashed line is a conservative estimate
of actual climate as inferred from the data of Figure A.2 and from additional data on the
shorter-period fluctuations from Kutzbach and Bryson (1974). The dotted curve shows
the modifications to be expected if the principal fluctuations identified in Table A. 3 were
actually quasi-periodic.
rate of change and wavelength in Markovian "red noise," for various
degrees of "redness" characterized by the value of the serial correla-
tion coefficient at a time lag of one year (Gilman et al., 1963). By com-
parison with these curves, it is suggested that the observed shorter-
period climatic fluctuations (i.e., fluctuations of the order of 100 to
200 years) are not clearly distinguishable from random fluctuations,
whereas the longer-period fluctuations (especially those with periods
of 20,000 years or more) may be appreciably larger in amplitude than
would be expected in random noise. The contributions of the longer-
period fluctuations to present-day climatic change are seen nonetheless
186 UNDERSTANDING CLIMATIC CHANGE
to be relatively small. Should the longer-period fluctuations be non-
sinusoidal (or episodic) in form, rates of change perhaps ten times
larger than the magnitudes shown in Figure A. 30 could be possible.
Even such rates, however, would contribute little over and above the
normal interannual variability of present-day global climate, and the
cumulative change of climate associated with the longer-period fluctua-
tions would remain relatively small until several centuries had elapsed.
Despite its simplistic view of climatic change, this exercise is an
instructive one in that it demonstrates how difficult it would be for
longer-period sinusoidal fluctuations to contribute substantially to the
changes of climate taking place in the twentieth century. If the longer-
period fluctuations are those that primarily determine the course of
the glacial-interglacial succession of global climate, it would seem
that the transition to the next glacial period — even if it has already
commenced — will require many centuries to accumulate to a drastic
shift from present climatic conditions.
In assessing such projections, however, we must keep in mind that
our ability to anticipate the locally important synoptic pattern of climatic
variations is limited. The work of Mitchell (1963), for example, has
shown that while the northern hemisphere average air temperatures
rose only about 0.2 °C during the period 1900 to 1940, there were
many areas that deviated markedly from this hemispheric average
trend. Parts of the eastern United States, for example, exhibited a
1.0 °C rise in average temperature (5 times the hemispheric average),
parts of Scandinavia and Mexico showed temperature increases of
2.0°C (10 times the hemispheric average), while in Spitsbergen the
warming was 5°C (25 times the hemispheric average). The correspond-
ing data on other climatic elements are sparse but may be expected to
exhibit comparable or even greater spatial variance.
Likelihood of a Major Deterioration of Global Climate in the
Years Ahead
As noted above, the longer-period climatic fluctuations seem to be
associated with larger amplitudes of change than those consistent with
Markovian "red-noise" behavior. The same cannot be said, however, of
the shorter-period fluctuations. For the moment let us suppose that
all the fluctuations described in this report are actually random fluctua-
tions, in the sense that transitions between successive maxima and
minima may occur at random (Poisson-distributed) intervals of time
rather than at more or less regular intervals. The probability that one
or more transitions of a fluctuation will occur in an arbitrarily specified
APPENDIX A
187
length of time may then be calculated from the negative binomial distri-
bution. Following this approach, we can assess the risk of encountering
a change of climate in the years ahead as rapid as the maximum rate
of change otherwise associated with sinusoidal climatic fluctuations
on each of the characteristic time scales noted above. Such a measure
of risk, for time intervals between 1 year and 1000 years into the future,
can be inferred by interpolation between the curves of transition prob-
ability in Figure A.31. The proper interpretation of this figure will
be apparent from the following examples :
0.01
0.001
10 100
WAITING TIME(YEARS)
1000
FIGURE A.31 Probability of onset of climatic transitions analogous to the changes
between maxima and minima in climatic fluctuations of arbitrarily selected characteristic
wavelengths (interior numbers, in years), as a function of elapsed time after present.
Dashed curves denote probability of one transition; solid curves denote that of one or
more transitions. Based on the assumption that intervals between transitions are strictly
random (Poisson distributed).
188 UNDERSTANDING CLIMATIC CHANGE
1. The curve labeled 100,000 in the figure indicates the probability
of a major transition of climate (in either direction) that is normally
associated with climatic fluctuations on the time scale of 100,000 years
(a change of global average temperature of up to perhaps 8°C in a
total time interval of 50,000 years or less). The curve indicates that
if successive transitions of this kind recur at random time intervals as
assumed here, the onset (or termination) of such a transition will occur
in the next 100 years with a probability of about 0.002 and in the next
1000 years with a probability of about 0.02.
2. The dashed curve labeled 100 in the figure indicates the prob-
ability of one transition of climate (in either direction) that is normally
associated with climatic fluctuations on the time scale of 100 years (a
change of up to perhaps 0.5 °C in a total time interval of about 50 years
or less). Such a transition is indicated to have a probability of about
0.02 of occurring in the next year, a probability of about 0.16 of occur-
ring in the next 10 years, and a probability of about 0.35 of occurring
in the next 50 years. The solid line labeled 100 in the figure indicates
the probability of one or more transitions of the same kind, which
rises from about 0.2 in the next 10 years to about 0.8 in the next
100 years. If it can be assumed that the typical duration of such a
transition (when it occurs) is not less than four or five decades, and
that only one such transition can occur at the same time, then the dashed
curve would be the appropriate guide for estimating such probabilities
in the next few decades. Otherwise, the solid curve would be a more
appropriate guide.
When Figures A. 30 and A. 31 are considered together, it is suggested
that whether climatic fluctuations are or are not quasi-periodic, those
that are most relevant to the course of global climate in the years and
decades immediately ahead are the shorter-period (historical) fluctua-
tions and not the longer-period (glacial) fluctuations. Even if the phase
of the longer-period changes is such as to contribute to a cooling of
present-day climate, the contribution of such fluctuations to the rate
of change of present-day climate would seem to be swamped by the
much larger contributions of the shorter-period (if more ephemeral)
historical fluctuations. We must remember, however, that this analysis
assumes a simple model of climatic change in which climatic fluctua-
tions of various periods are independent and therefore additive. The
paleoclimatic record presented here does not preclude the possibility
that relatively sudden climatic changes could arise through interactions
between fluctuations of different periods.
One may still ask the question: When will the present interglacial
end? Few paleoclimatologists would dispute that the prominent warm
APPENDIX A 189
periods (or interglacials) that have followed each of the terminations
of the major glaciations have had durations of 10,000 ±2000 years. In
each case, a period of considerably colder climate has followed im-
mediately after the interglacial interval. Since about 10,000 years has
elapsed since the onset of the present period of prominent warmth, the
question naturally arises as to whether we are indeed on the brink of a
period of colder climate. Kukla and Matthews (1972) have already
called attention to such a possibility. There seems little doubt that the
present period of unusual warmth will eventually give way to a time
of colder climate, but there is no consensus with regard to either the
magnitude or rapidity of the transition. The onset of this climatic de-
cline could be several thousand years in the future, although there is
a finite probability that a serious worldwide cooling could befall the
earth within the next hundred years.
What is the nature of the climatic changes accompanying the end
of a period of interglacial warmth? From studies of sediments and soils,
Kukla finds that major changes in vegetation occurred at the end of the
previous interglacial (Figure A. 14). The deciduous forests that covered
areas during the major glaciations were replaced by sparse shrubs, and
dust blew freely about. The climate was considerably more continental
than at present, and the agricultural productivity would have been
marginal at best. The stratification of fossil pollen deposits in eastern
Macedonia (Figure A.13) also clearly shows a marked change in vege-
tative cover between interglacial warmth and the following cold periods.
The oak-pine forest that existed in the area gave way to a steppe shrub,
and grass was the dominant plant cover. Other evidence from deep-sea
cores reveals a substantial change in the surface water temperature in
the North Atlantic between interglacial and glacial periods (Figure
A.13), and the marine sediment data show that the magnitude of the
characteristically abrupt glacial cooling was approximately half the total
glacial to interglacial change itself.
The question remains unresolved. If the end of the interglacial is
episodic in character, we are moving .toward a rather sudden climatic
change of unknown timing, although as each 100 years passes, we have
perhaps a 5 percent greater chance of encountering its onset. If, on
the other hand, these changes are more sinusoidal in character, then
the climate should decline gradually over a period of thousands of years.
These are the limits that we can presently place on the nature of this
transition from the evidence contained in the paleoclimatic record.
These climatic projections, however, could be replaced with quite
different future climatic scenarios due to man's inadvertent interference
with the otherwise natural variation (Mitchell, 1973a). This aspect of
190 UNDERSTANDING CLIMATIC CHANGE
climatic change has recently received increased attention, as evidenced
by the smic report (Wilson, 1971). A leading anthropogenic effect is
the enrichment of the atmospheric C02 content by the combustion of
fossil fuels, which has been rising about 4 percent per year since 1910.
There is evidence that the ocean's uptake of much of this C02 is
diminishing (Keeling et al., 1974), which raises the possibility of even
greater future atmospheric concentrations. Man's activities are also con-
taminating the atmosphere with aerosols and releasing waste heat into
the atmosphere, either (or both) of which may have important climatic
consequences (Mitchell, 1973b). Such effects may combine to offset a
future natural cooling trend or to enhance a natural warming. This
situation serves to illustrate the uncertainty introduced into the prob-
lem of future climatic changes by the interference of man and is occur-
ring before adequate knowledge of the natural variations themselves
has been obtained. Again, the clear need is for greatly increased re-
search on both the nature and causes of climatic variation.
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APPENDIX B
SURVEY OF THE CLIMATE SIMULATION
CAPABILITY OF GLOBAL CIRCULATION MODELS
INTRODUCTION
Much of the present effort within garp, as well as other research pro-
grams in the atmospheric and oceanic sciences, is aimed toward the
development of a quantitative understanding of the behavior of the
atmosphere, with the immediate objective of improving the accuracy of
weather forecasts. Other research efforts and plans, and the research
program proposed in this report in particular, are directed to the
longer-range objective of understanding the physical basis of climate
and climatic change. Essential to both of these objectives are the dy-
namical models of the global atmospheric and oceanic circulation.
These general circulation models (or gcm's) have been developed
over a number of years, in parallel with the growth of computing
capability and the increase of atmospheric data coverage. The several
atmospheric and oceanic gcm's have now reached the point where
reasonably accurate simulations of the global distribution of many im-
portant climatic elements are possible and where their coupling into a
single dynamical system is now feasible. This therefore seems to be a
useful time to survey briefly these models' climate simulation capabilities.
Here we have not attempted to present a detailed discussion of the
various gcm's, as such descriptions are readily available both in the
literature and in documents describing special models. Model reviews
have recently been prepared by Robinson (1971), Willson (1973),
Smagorinsky (1974), and Schneider and Dickinson (1974), and general
196
APPENDIX B 197
discussions of the use of such models for weather prediction and for
studies of the general circulation are available [see, for example, the
review by Smagorinsky (1970) and also Haltiner (1971) and Lorenz
(1967)]. A survey of the physical and mathematical structure of both
regional and global atmospheric models is also in preparation for garp
(1974). What has not been assembled heretofore is the comparative
climatic performance of the various models, and this Appendix is an
initial effort to fill this need for both the atmospheric and oceanic
global gcm's.
In general, any formulation that relates variables of the climatic
system to the external or boundary conditions may be considered a
climatic model. We can thus identify basically empirical and statistical
climatic models, as well as' those that rest on the system's dynamical
equations.
Within the dynamical climate models, a wide variety of the type
and degree of parameterization may be seen. At one extreme are the
vertically and zonally averaged atmospheric models that address the
mean heat balance at the earth's surface, such as those of Budyko
(1969) and Sellers (1973). In such models, the transport of heat is
parameterized in terms of mean zonal variables, which are in turn re-
lated to the surface temperature. At the other extreme are the high-
resolution global general circulation models or gcm's. In these models,
the details of the transient cyclone-scale motions are resolved, along
with the global distribution of the elements of the heat and hydrologic
balances. Even these models, however, parameterize certain physical
processes, in that they employ empirical or statistical representations
of some of the subgrid scale processes in the surface boundary layer
and in the free atmosphere and open ocean, such as the effects of
diffusion and convection.
Dynamical climate models also display a wide variety of parameter-
ization with respect to time. This ranges from equilibrium or steady-
state models, such as that of Saltzman and Vernekar (1971), to the
gcm's that explicitly calculate the time dependence of the circulation
in steps of a few minutes. With respect to their treatment of both space
and time, therefore, a wide range of models exists, and each is suited
to the investigation of particular aspects of the climatic problem. The
gcm's (of both the atmosphere and ocean) provide the most detailed
representation of the physical processes involved but require large
amounts of computation. These models have therefore been used up
to the present time to study only the climatic variations on time scales
of the order of years (for the atmosphere) to centuries (for the oceans).
The more highly parameterized models, on the other hand, provide
198 UNDERSTANDING CLIMATIC CHANGE
less detail but are capable of treating the longer-period climatic varia-
tions with much less computation. Once they are adequately calibrated
with respect to observations, an important use of the gcm's will be to
generate detailed climatic statistics, from which parameterizations ap-
propriate to the various statistical-dynamical models may be prepared.
In the remainder of this Appendix we give our attention to the princi-
pal atmospheric and oceanic general circulation models, for the pur-
pose of indicating their present capability to simulate climate. Before
presenting these results, however, it is useful to review briefly the
historical development of numerical modeling in general.
DEVELOPMENT AND USES OF NUMERICAL MODELING
The basis for the mathematical modeling of the behavior of the at-
mosphere was first unambiguously stated by V. Bjerknes in 1904. It is
only in the last 20 years or so, however, that the means for carrying
out such modeling on a practical basis have become available. These
include adequate observations for model calibration and verification,
a knowledge of the important physical processes and their parameteriza-
tion, and the computers and numerical methods necessary to perform
the calculations.
The observational base for numerical modeling of the atmosphere
has grown steadily since the 1940's and early 1950's, when the global
radiosonde network began to take shape. The igy provided further ex-
pansion, but the observational coverage still needs augmentation, espe-
cially over the oceanic regions. The real breakthrough toward the
global measurements necessary for numerical modeling has come from
the remote-sensing capabilities of meteorological satellites; with the aid
of suitable surface (ground-truth) observations, these are capable of
providing the first truly worldwide observations of the air and ocean
surface temperature, moisture and cloudiness, and elements of the heat
and hydrologic balance. By using the numerical models diagnostically,
there is then the prospect of deducing the accompanying global distribu-
tions of other variables, such as the wind velocity. Such a scheme is
the observational basis of the proposed First garp Global Experiment
(fgge) in 1978.
The physical and theoretical basis for numerical modeling has grown
significantly with the development of the theory of baroclinic instability,
the parameterization of moist convection, and advances in our knowledge
of the behavior of the stratosphere and the planetary boundary layer.
Our growing understanding of these processes has increased the pros-
pects for improved weather forecasts. These hopes are bounded, how-
APPENDIX B
199
ever, by the realization that the atmosphere possesses limited predict-
ability, i.e., that there is a time range beyond which the local variations
of weather appear as random fluctuations as far as their explicit pre-
diction by numerical models is concerned. Present indications are that
this limit lies at about two weeks' time.
The key physical processes that control the longer-period variations
of the atmosphere — those that are properly associated with climate —
are largely unknown, although we are beginning to recognize the im-
portance of a number of feedback relationships, such as the air-sea
coupling and cloudiness-temperature feedback. Numerical models that
incorporate such effects are our best tool to develop a quantitative
understanding of their role, in climate and climatic variation.
The computational base for numerical modeling has grown during
the last 20 years in parallel with the development of successive genera-
tions of high-speed computers, as shown in Figure B.l. This overview
makes clear the interrelated development of numerical models, theory,
and computer speed. Numerical weather prediction may be considered
to have begun with the first successful numerical integration of the
U H i
c
Computer speed
•i-i t3 o. Atmospheric model
Oceanic model
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1950
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Wind-driven barotropic
Barotropic (vorticity)
models
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Baroclinic atmospheric models
1
Primitive equation models
First baroclinic models
1955
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TI-ASC (8)
1 M 1 >
ILLIAC-4 (15)
1 —
FIGURE B.l Highlights in the development of numerical modeling of the atmosphere
and ocean.
200 UNDERSTANDING CLIMATIC CHANGE
vorticity equation (Charney et al., 1950), with the demonstration of
the ability of baroclinic models to forecast cyclonic development
(Charney and Phillips, 1953), or with the commencement of operational
numerical weather prediction in 1955. Numerical general circulation
studies may be considered to have begun with the simulation of the
atmospheric energy cycle in an idealized model with sources and sinks
of energy and momentum (Phillips, 1956), with the first successful
hemispheric circulation experiments (Smagorinsky, 1963), or with the
first extended global integration (Mintz, 1965). Numerical climate
models for the atmosphere may be considered to have begun with the
global simulation of the seasonal and interannual variation of the
primary climatic elements (Mintz et al, 1972), although the modeling
of climate by other methods has a much longer history. The numerical
modeling of climatic variation, on the other hand, which addresses the
coupled ocean-atmosphere climatic system, has only just begun (Bryan
et al, 1974; Manabe et al, 1974a, 1974b) .
The development during the past decade of numerical methods whose
stability and accuracy can be suitably controlled has made it possible
to carry out such calculations for extended periods of time. Even with
today's fastest computers, however, the solution of the more detailed
global numerical models proceeds only at a rate between one and two
orders of magnitude faster than nature itself, and our ability to per-
form the large number of numerical integrations required for the system-
atic exploration of climate and climatic change requires the continued
development and dedication of new computer resources.
A similar pattern of development has occurred in the numerical
modeling of the oceans, except that the rate of progress has been
slower due principally to a lack of suitable oceanic observations. The
data base for the oceans is fragmentary in comparison with that for
the atmosphere, and there is no oceanic counterpart of the radiosonde
or weather station network. The bathythermograph has been widely
used to measure the thermal structure of the ocean's surface layer for
the past few decades, but even this has not been done on a synoptic basis.
The bulk of the data for oceanic temperature, salinity, and currents
has been obtained in the course of occasional oceanographic expeditions
or special observational programs. Even so, the number of direct
velocity measurements is quite small, and our knowledge of the oceanic
circulation is largely based on geostrophic estimates from conventional
hydrographic observations.
Our knowledge of the dynamics of the ocean circulation is also less
complete than is that for the atmosphere. While the character of the
vorticity balance of the ocean was first established by Sverdrup (1947)
APPENDIX B 201
and Stommel (1948), the role of the thermohaline circulation was
demonstrated with a numerical model only a few years ago (Bryan and
Cox, 1968), and the effects of bottom topography have been established
even more recently (see, for example, Holland and Hirschman, 1972).
Numerical models are proving of great value in the study of time-
dependent behavior of the oceanic general circulation and in the analysis
of oceanic mesoscale motions such as those now being revealed by the
mode observations. The structure of these eddies and the role that they
play in the oceanic heat balance is one of the principal unsolved prob-
lems in physical oceanography. Other important questions concern the
nature of vertical mixing in the ocean, especially in the surface layer,
and the mechanics of the formation of deep and bottom water. Each of
these can perhaps be most fruitfully studied with appropriate regional
numerical models, in order to lay the foundation for their parameteriza-
tion in three-dimensional models of the world ocean. But perhaps the
most important problem of all from the viewpoint of climate is the
interaction between the ocean and the atmosphere; the numerical
modeling of this coupled system offers our best hope of achieving a
quantitative understanding of the dynamics of climatic variation.
Numerical models thus lie at the heart of the modern study of climate
and climatic change: they complement (and may even be regarded
as a part of) the observing system, they serve as tools for climatic
analysis and diagnosis, and they offer the most rational way of assessing
the course of future climatic events. Whether or not climate forecasting
in the time-dependent sense ever becomes feasible, the use of numerical
models to simulate the average or equilibrium climates of the past
and the likely climatic consequences of various natural or anthropogenic
effects in the future will justify their development.
ATMOSPHERIC GENERAL CIRCULATION MODELS
Formulation
All general circulation models are based on the fundamental dynamical
equations that govern the large-scale behavior of the atmosphere. This
system consists of the equation of motion (expressing the conserva-
tion of momentum), the thermodynamic energy equation (expressing
the conservation of heat energy), the equations of mass and water
vapor continuity, and the equation of state. When geometric height
(z) is the vertical coordinate, these equations can be written in vector
form as follows :
202 UNDERSTANDING CLIMATIC CHANGE
dV — ■*- dV — ■*■ 1 —
-^ + V-VV + w-^- + 2nxV + — Vp = F, (1)
g+P? = 0, (2)
^ + V-pF + |^(pw)=0, (4)
f + f.V, + .||=5, (5)
P=p/«r. (6)
Here K is the horizontal velocity, w is the vertical velocity, n is the
rotation vector of the earth, p is the density, p is the pressure, g is the
gravitational acceleration, 6 is potential temperature [which is related
to the ordinary temperature T by the relation 6=T(p0/p)K, where
/?o=1000 mbar and * = 0.286 is the ratio of the specific heats], q is
the water vapor mixing ratio, R is the gas constant for (moist) air,
and V is the horizontal gradient operator.
The terms F, Q, and S on the right-hand sides of Eqs. (1), (3), and
(5) represent the sources and sinks of momentum, heat, and water
vapor due to a variety of physical processes in the atmosphere and must
be either prescribed or parameterized in terms of the primary dependent
variables in order to close the system (l)-(6). The net frictional force
F consists of the frictional drag at the earth's surface and the internal
friction in the free atmosphere, as well as the changes of large-scale
momentum due to smaller-scale processes. The net (diabatic) heating
rate Q consists of the latent heat released during condensation, the
heating due to the exchange of both long-wave and shortwave radia-
tion, and the sensible heating of the atmosphere by turbulent heat fluxes
from the underlying surface. The net moisture addition rate S consists
of the difference between the evaporation rate (from both the surface
and from cloud and precipitation) and the condensation rate.
An important contribution to each of these source terms is the vertical
flux of momentum, heat, and moisture, which accompanies cumulus-
scale convection in the atmosphere. We may note that such convective-
scale processes are not governed by the system (l)-(6) and must be
represented in terms of the larger-scale variables. This parameterization
is particularly critical for the net heating, because most of the latent
heating in the atmosphere is accomplished by convective motions, which
are also responsible for much of the cloudiness (see Figure 3.2).
APPENDIX B 203
The various atmospheric gcm's are each formulated in slightly dif-
ferent ways and employ different treatments of the source terms. There
is at present insufficient evidence to decide which particular formulation
is the most satisfactory, and there is even more uncertainty regarding
the most correct parameterization of the unresolved physical processes
contained within F, Q, and S. A summary of some of the features of
the better-known atmospheric general circulation models is given in
Table B.l. Each of the models shown here uses generally similar pro-
cedures to determine the ground-surface temperature (from an assumed
heat balance over land and ice), the surface hydrology (with runoff
permitted after saturation of the surface soil), and the occurrence of
convection (from vertical .stability criteria depending on the moist
static energy). Each of the models also incorporates the observed large-
scale distributions of terrain height, surface albedo, and sea-surface
temperature.
Solution Methods
All the atmospheric gcm's considered here employ finite-difference
methods of second-order accuracy, with the dependent variables gen-
erally determined on a spatially staggered grid with a resolution of
several hundred kilometers (see Table B.l). Time differencing is also
generally of second-order accuracy, with time steps between 5 and
10 min used to maintain (linear) computational stability. Long-term
(nonlinear) computational stability is inherent in some of the models'
space differencing schemes, while others employ eddy diffusion processes
to achieve this end. Various degrees of smoothing are also employed
in the models' solution, in addition to that inherent in the finite-difference
approximations themselves. Depending on the computer used, the
number of model levels, and the frequency with which the radiative
heating calculations are performed, global atmospheric gcm's gen-
erally run between 10 and 100 times faster than real time.
Selected Climatic Simulations
In order to display the level of accuracy characteristic of present-day
atmospheric gcm's in the simulation of climate, we have here assembled
the results of model integrations drawn from recently published (and in
some cases as yet unpublished) sources. To facilitate comparison, these
are presented in a common format, along with the corresponding ob-
served distributions.
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APPENDIX B 205
Sea-Level Pressure
Although the various gcm's differ greatly in their resolution of the
vertical structure of the atmosphere, each simulates the distribution of
a number of climatic variables at the earth's surface. Of these, perhaps
the distribution of sea-level pressure is the most familiar; it is shown
here as simulated by four different models for the month of January.
In Figure B.2 the average sea-level pressure simulated by the 11-level
gfdl atmospheric model is shown for the months of December, January,
and February (Manabe et ah, 1974b). Figures B.3, B.4, and B.5 show
the corresponding average January sea-level pressure simulated by the
six-level ncar model (Kasahara and Washington, 1971), by the two-
level Rand model (Gates, 1972), and by the nine-level giss model
(Somerville et al., 1974). In each case the observed average January
sea-level pressure distribution is also shown. While the models' results
differ in a number of details, these results generally show a useful level
of accuracy. As might be anticipated, the largest errors (and the greatest
differences among the models) occur in the middle and higher latitudes
of the northern hemisphere where cyclonic activity is the most frequent.
It should be recalled, however, that sea-level pressure alone is by no
means a complete indicator of climate.
Tropospheric Temperature and Pressure
In Figure B.6 the average January 800-mbar temperature simulated by
the two-level Rand model (Gates, 1972) is shown, along with the ob-
served distribution. Although systematic errors may be noted over the
continents, the simulated large-scale temperature distribution clearly
reflects the positions of the major thermal perturbations in the lower
troposphere. The average January 500-mbar height simulated by the
nine-level giss model (Somerville et al, 1974) is shown in Figure B.7,
along with the observed distribution. These results also clearly show
that the mean position and intensity of the long waves in the westerlies
are portrayed reasonably well in the simulation.
Cloudiness and Precipitation
Among the more difficult climatic elements to simulate accurately in a
gcm are the cloudiness and precipitation. This is doubtless due to the
fact that a substantial portion of the total cloudiness and precipitation
observed occurs in connection with convective-scale motions, especially
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218
UNDERSTANDING CLIMATIC CHANGE
in the lower latitudes. As noted earlier, these processes must be parame-
terized in the gcm's, and their accurate calibration is relatively difficult.
In Figure B.8, the average January total cloudiness simulated by the
six-level ncar model (Kasahara and Washington, 1971) is shown,
along with a composite observed distribution for January and for De-
cember, January, and February. With the exception of the equatorial
region and the low latitudes of the northern hemisphere, the large-scale
areas of maximum and minimum cloudiness are reasonably well
simulated.
In Figure B.9, the annual average precipitation simulated by the
11-level gfdl model (Manabe et al., 1974b) is shown, along with the
corresponding observed distribution. In addition to the large-scale
precipitation pattern in middle latitudes, this simulation also portrays
a number of the smaller-scale features, including the zone of heavy
precipitation near the equator. Although this comparison is for a some-
what longer time period than the others shown here, the difficulty of
correctly parameterizing the precipitation process makes the skill of this
simulation impressive.
OCEANIC AND COUPLED ATMOSPHERE-OCEAN GENERAL
CIRCULATION MODELS
Estimates based on observed data show that the heat transported by
ocean currents plays a major role in the global heat balance (Vonder
Haar and Oort, 1973). A model that is to be useful for the study of
climatic variation must therefore include the ocean as well as the
atmosphere. As suggested by the simulations just reviewed, the speci-
fication of a fixed ocean surface temperature in atmospheric gcm's is
a strong boundary condition and may mask weaknesses in the models'
simulation of the heat balance. The problem of climatic variation there-
fore furnishes a major motivation for the accelerated development of
numerical models of the oceanic general circulation.
Relative to numerical models of the atmosphere, numerical modeling
of the ocean is still in a primitive state. As previously noted, this is
primarily due to the lack of sufficient data to perform a careful verifica-
tion of the models and to parameterize properly the effects of the
smaller-scale motions. The only large body of data presently available
for verifying ocean circulation models is the collection of measurements
of density structure. While these data were sufficient to calibrate the
earlier analytic theories of the ocean thermocline, global numerical
models require a much more extensive data base for adequate verifica-
tion.
APPENDIX B 219
It is now recognized that many of the earlier studies, such as those
by Bryan and Cox (1967) and Haney (1974) for idealized basins,
as well as the higher-resolution simulations of Cox (1970) for the
Indian Ocean and of Friedrich (1970) for the North Atlantic, represent
transient rather than equilibrium solutions for the boundary conditions
imposed. The extended integration of even more detailed numerical
models will be necessary in the future, in order to design and calibrate
adequately other simpler models. Such models will require less calcula-
tion and thereby allow more freedom to carry out the large number
of numerical experiments required. General reviews of numerical model-
ing of the ocean circulation are given in the proceedings of a recent
symposium (Ocean Affairs Board, 1974) and by Gilbert (1974).
Formulation
The principal dynamical components of an oceanic general circulation
model are similar to those of its atmospheric counterpart, namely, the
equations of motion, conservation equations for potential temperature
and salinity, the continuity equation, and an equation of state. In addi-
tion, an oceanic model should contain equations for the growth and
movement of pack ice.
In some problems of oceanic circulation, it is not necessary to treat
the temperature and salinity separately, and these variables can be
combined into a single density variable. In climatic studies, however,
we are interested in the heat transported by ocean currents explicitly;
and in many regions of the world ocean, particularly the polar seas,
the density and temperature are not proportional. In these regions at
least, it is therefore necessary to predict salinity as a separate inde-
pendent variable. A changing salinity structure in the ocean may pro-
vide the basis of climatic change mechanisms that have not yet received
sufficient attention.
In an ocean model, the equation of motion (1) may be simplified
by treating the density P as a constant po (Boussinesq approximation),
while the hydrostatic equation (2) remains unchanged. The thermo-
dynamic energy equation (3) and the water vapor continuity equation
(5) are represented in the ocean by conservation equations for po-
tential temperature 6 and salinity s of the form
-|^ (6,s) + V-V(B,s)+w^(6,s) = (Q,<r), (7)
where Q and o- denote source functions. The continuity equation (4)
220
221
223
224 UNDERSTANDING CLIMATIC CHANGE
may be simplified by considering the ocean to be incompressible, in
which case we may write
^ + V-F=0. (8)
oz
The oceanic equation of state may be written symbolically as
P=P(0,s,p), (9)
where the actual expression is a polynomial of high order, whose co-
efficients have been determined by laboratory experiments. To close
the system, expressions must be chosen for F [in the simplified form of
Eq. ( 1 )] and for Q and o- in terms of the dependent variables. As in the
case of atmospheric models, this closure is an important problem in
the formulation of oceanic models and includes the parameterization
of the mesoscale oceanic eddies.
Solution Methods
The predictive equations for momentum, temperature, and salinity
given in the previous section are generally approximated by centered
differences of second-order accuracy, with care taken to conserve both
linear and quadratic quantities. The numerical methods that have been
used successfully for large-scale models of the atmosphere are usually
further modified by the exclusion of external gravity waves from the
system. This permits the use of a time step 50 to 100 times larger than
is possible for the atmosphere. This is accomplished by requiring the
total, vertically integrated flow to be divergence-free, in which case
it is possible to specify the total transport by a stream function.
The numerical time integration of an oceanic gcm formulated in
this manner proceeds by a combination marching and jury process, in-
volving the explicit prediction of 0,s and V, and the iterative solution
for the total transport stream function. Takano (1974) has recently
introduced the implicit treatment of Rossby waves, which allows a con-
siderably longer time step with little loss in accuracy for problems in
which the emphasis is on low-frequency oceanic phenomena.
Selected Climatic Simulations
To illustrate the characteristic climatic performance of global oceanic
gcm's, we here present comparative solutions from the recent models
of Takano et al. (1974), Cox (1974), and Alexander (1974). A
number of characteristics of these models are given in Table B.2. These
APPENDIX B 225
TABLE B.2 Characteristics of Recent Global Ocean Circulation Models
Feature
UCLA a
GFDL "
Rand '
Number of levels
5
9
2
Horizontal spacing
A0 = 4°
A0 = 2°
A</> = 4°
AX = 2.5°
A\ = 2°
AX = 5°
Salinity
No
Yes
No
Depth
4 km
Actual
300 m
Horizontal mixing d
Am=W
^, = 2x10"
/4if=7xl0°
(cm3 sec-1)
An = 2.5xl07
A,,= W
A„ = 5x\07
Initial condition
Isothermal
Observed T,s
Observed T
Time span of experiment
30 yr
2.5 yr
1.5 yr
Upper boundary condi-
Momentum flux,
Momentum flux,
Momentum flux,
tion
thermal forcing
T,s specified
heat flux
"Takano et al. (1974); see also Mintz and Arakawa (1974) and Takano (1974).
6 Cox (1974); see also Bryan et al. (1974).
c Alexander (1974).
d Here AM and An denote the eddy coefficients for momentum and heat, respectively.
models are currently undergoing further development, and similar
oceanic models are under construction at ncar and at giss. It is a
general characteristic of all such oceanic models that the circulation is
dominated by the large values of viscosity, and further efforts are re-
quired to extend the solutions into the less viscous and more nonlinear
range.
Surface Current
The annual surface current simulated by the nine-level gfdl model
(Cox, 1974) is shown in Figure B.10, along with the observed currents
for February and March. The February surface currents simulated by
the five-level ucla model (Takano et al, 1974) and the March 1 sur-
face currents simulated by the two-level Rand model (Alexander,
1974) are similarly shown in Figures B.ll and B.12. In each case the
overall pattern of the large-scale circulation is simulated successfully,
although in general the strength of the equatorial and major western
boundary currents is underpredicted. We may note, however, that the
ucla model's solution represents a 30-year integration, the gfdl solu-
tion is for 2.5 years, and the Rand solution is for 1.5 years. Closer
examination reveals that the simulated surface currents diverge from
the equator somewhat more than do those observed, due to the models'
effective averaging over the depth of the surface Ekman layer.
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236 UNDERSTANDING CLIMATIC CHANGE
Sea-Surface Temperature
The February sea-surface temperature simulated by the five-level ucla
model (Takano et al, 1974) is shown in Figure B.13, along with the
observed distribution. To some extent the agreement of the simulation
with observation is due to the use of observed components in the sur-
face heat balance condition. The prediction of low surface temperatures
at the equator, however, is a feature entirely due to the model's internal
dynamics.
Coupled Ocean-Atmosphere Models
As has been previously noted, a dynamical model adequate for the
study of climatic variation should include the coupling of the ocean
and atmosphere. The first attempt at such coupling was made by Manabe
and Bryan (1969) for an idealized ocean basin and later extended by
Wetherald and Manabe (1972). In such a joint model, the net fluxes of
heat, moisture, and momentum at the air-sea interface are determined
by the atmospheric model, while the ocean model in turn provides the
sea-surface temperature as a lower boundary condition for the at-
mosphere.
These studies at gfdl have recently been extended to the entire world
ocean, and the results of a coupled numerical integration are now
available (Manabe et al, 1974a; Bryan et al, 1974). In this study, the
nine-level gfdl atmospheric model was integrated for 0.85 of a year
simulated time, while a twelve-layer ocean model was integrated for
256 years' time. The annual sea-surface temperatures simulated in this
joint model are shown in Figure B.14, along with the observed distribu-
tion. The general level of accuracy may be considered satisfactory,
especially in view of the absence of any specification of observed quanti-
ties at the air-sea interface. Much further development and testing of
such coupled models is required so that their potential for the study of
global climatic variations may be realized.
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