Original research articleScrutinizing the carbon cycle and CO2 residence time in the atmosphere
Graphical abstract
Introduction
The carbon cycle can be understood as a series of carbon reservoirs in the Earth-Atmosphere-System (EASy), which are connected to each other by exchange fluxes of carbon and its main bio-chemical compounds. For climate considerations especially atmospheric CO2 as the main atmospheric phase of the global carbon cycle is of great importance due to its infrared active properties and its classification as the most dangerous greenhouse gas. Therefore, particularly the increase of CO2 in the atmosphere, which climate scientists mainly trace back to growing anthropogenic emissions as well as a reduced uptake of CO2 by oceans and land vegetation, are in the focus of many investigations.
In the 5th Assessment Report (AR5, 2013) of the Intergovernmental Panel on Climate Change (IPCC) we can read (AR5-Chap.12-FAQ 12.3, p. 1107): “Global temperature would not respond quickly to the greenhouse gas concentration changes... Eliminating CO2 emissions only would lead to near constant temperature for many centuries (commitment from past emissions)... As a consequence of the large inertia in the climate and carbon cycle, the long-term global temperature is largely controlled by total CO2 emissions that have accumulated over time, irrespective of the time when they were emitted.”
So, the IPCC assumes that not only the Earth as a large heat storage but also the atmosphere as a big storage for CO2, cumulating this greenhouse gas over many centuries, is responsible for a slow response of the global temperature. But obviously this response is assumed to work only in one direction. While the CO2 increase of 100 ppm over the last century is made liable for a relatively fast increase of the temperature of about 0.8 °C over this period, eliminating further emissions are expected to lead to near constant temperatures for many centuries. The IPCC explains this with ‘extremely long time scale processes involved into the removal of anthropogenic CO2 emissions into the atmosphere, which makes the concept of a single, characteristic atmospheric lifetime not applicable to CO2’ (AR5-Chap.6-Box-6.1).
Because of the IPCC's interpretation of an extremely long atmospheric residence time together with a high climate sensitivity CO2 is supposed to be the most dangerous component destabilizing our climate. Our own assessment of global warming by CO2 (Harde, 2013, Harde, 2014, Harde, 2016) shows a less dramatic influence of CO2 on the climate, yielding an equilibrium climate sensitivity (temperature increase at doubled CO2) almost a factor of five smaller than published in AR5, and also a closer inspection of the residence time gives significantly different values than presented by the IPCC.
Therefore, it seemed worthwhile to scrutinize the carbon cycle and the different accounting schemes for the residence time with their individual assumptions in more detail and to identify the fundamental distinctions of these concepts. For a better comparison and deeper understanding we have tried to reproduce the IPCC's accounting scheme for the carbon cycle (see AR5-Chap.6) as far as possible, only supplemented by some own contemplations. This is presented in Section 2, while in Section 3 we contrast this to an alternative description, which is based on the balance equation and the empirical evidence that uptake rates scale proportional with the CO2 concentration, in agreement with the observed exponential decay of 14C in the atmosphere. The balance equation is a fundamental law that must be obeyed by any legitimate model of CO2. In addition, we consider temperature dependent natural emission and absorption rates, by which the paleoclimatic CO2 variations and the actual CO2 growth rate can well be explained. For these studies we have applied the IPCC's own estimates of natural absorption and emission, not because they are necessarily correct, but to demonstrate that, with those estimates, governing physical laws, lead to an explanation of increased CO2 entirely different to the one advocated by the IPCC.
Previous critical analyses facing the IPCC's favored interpretation of the carbon cycle and residence time have been published, e.g., by Jaworowski et al. (1992), Segalstad (1998), Dietze (2001), Rörsch et al. (2005) or Essenhigh (2009), and more recently by Humlum et al. (2013), or Salby, 2013, Salby, 2016. Although most of these analyses are based on different observations and methods, they all derive residence times (in some cases also differentiated between turnover and adjustment times) in part several orders of magnitude shorter than specified in AR5. As a consequence of these analyses also a much smaller anthropogenic influence on the climate than propagated by the IPCC can be expected.
Section snippets
CO2 emission-absorption-balance
The total carbon emission rate is supposed to be between 200 and 220 GtC/yr, which corresponds to a CO2 emission rate of ET = 734–807 Gt/yr (transfer from C → CO2 is a factor of 3.67). For our further considerations we calculate with a mean rate of ET = 760 Gt/yr = 760 Pg/yr. The IPCC estimates, that from this total rate a fraction EA = 32.7 Pg/yr (8.9 PgC/yr: 7.8 PgC/yr fossil fuels + 1.1 PgC/yr net land use change) results from anthropogenic sources, while the rest with EN = 727.3 Pg/yr originates from natural
Balance equation and CO2 residence time
As already outlined in the preceding Section changes of CO2 in the atmosphere on the one hand depend on the total emission rate ET of CO2 into the atmosphere and on the other hand on the re-absorption by plants or by the uptake in water. Since natural cycles like unsaturated absorption or decay processes are always characterized by an exponential relation (see, e.g., Lambert-Beer's law, scattering and decay processes), different to Eq. (1) here we assume an absorption rate, which naturally
Conclusions
Climate scientists assume that a disturbed carbon cycle, which has come out of balance by the increasing anthropogenic emissions from fossil fuel combustion and land use change, is responsible for the rapidly increasing atmospheric CO2 concentrations over recent years. While over the whole Holocene up to the entrance of the Industrial Era (1750) natural emissions by heterotrophic processes and fire were supposed to be in equilibrium with the uptake by photosynthesis and the net ocean-atmosphere
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgement
We thank Prof. Murry Salby, formerly Macquarie University Sydney, for many helpful discussions when preparing the paper. We also thank the editor as well as the reviewers for critically reading the manuscript and important advices.
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