Updated, July 23, 1:40 p.m. | A new study of methods used to track Antarctic sea ice trends has raised important questions about whether recent increases in ice there are, to a significant extent, an illusion created by flawed analysis of data collected by a series of satellites. The news release excerpted below offers a good overview of the paper — “A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated?” — which was just published in The Cryosphere. The authors are Ian Eisenman and Joel Norris of the Scripps Institution of Oceanography and Walt Meier of NASA.
Here’s an excerpt from the abstract:
Recent estimates indicate that the Antarctic sea ice cover is expanding at a statistically significant rate with a magnitude one-third as large as the rapid rate of sea ice retreat in the Arctic. However, during the mid-2000s, with several fewer years in the observational record, the trend in Antarctic sea ice extent was reported to be considerably smaller and statistically indistinguishable from zero.
Here, we show that much of the increase in the reported trend occurred due to the previously undocumented effect of a change in the way the satellite sea ice observations are processed for the widely used Bootstrap algorithm data set, rather than a physical increase in the rate of ice advance….
The results of this analysis raise the possibility that much of this expansion may be a spurious artifact of an error in the processing of the satellite observations.
I engaged a batch of ice analysts, including the authors, in an e-mail conversation about the work. I’ll be adding some input from that discussion here as time allows. One comment worth adding from the start came from John Turner of the British Antarctic Survey, who noted that a variety of independent analytical methods “show a clear and statistically significant increase in Antarctic sea ice extent since the late 1970s, with record annual mean extents and record daily maxima being observed over the last few years.” He added, “There is no credible scientific evidence that Antarctic sea ice extent is NOT increasing.”
Those who seem to spend much of their time trying to cast doubt on human-driven global warming had latched onto the expansion of Antarctic sea ice as undercutting arguments for global warming.
Defenders of global warming science have been as quick to offer explanations for the trend.
[Edit/Insert (see postscript), July 23 , 1:35 p.m. | Now they would do well to note that the trend may in fact not be as robust as earlier estimates concluded — and thus less easily explained in relation to ozone depletion, global warming or anything else.
In an e-mail message when I first reported on the study, Walt Meier of NASA said this:
If the trend is smaller, then there is less to explain and it increases the possibility that natural variability can explain things.
However, either way the trend is statistically significantly positive. Particularly with the last couple of years. So I think there still needs to be a plausible forcing mechanism to understand what is happening.]
Satellites have been an extraordinary boon for scientists trying to track shifts in sea ice at both ends of the Earth. But turning imagery of floating ice into reliable estimates of ice coverage remains a challenge.
The news release from the European Geosciences Union, which publishes the journal, has some detail:
Scientists have used satellite data to measure sea ice cover for 35 years. But the data doesn’t come from a single instrument, orbiting on a single satellite throughout this period. Instead, researchers splice together observations from different instruments flown on a number of different satellites. They then use an algorithm – the most prevalent being the Bootstrap algorithm – and further processing to estimate sea ice cover from these data.
In the study published in The Cryosphere, Eisenman and collaborators compare two datasets for sea ice measurements. The most recent one, the source of AR5 conclusions, was generated using a version of Bootstrap updated in 2007, while the other, used in AR4 research, is the result of an older version of the algorithm.
The researchers found a difference between the two datasets related to a transition in satellite sensors in December 1991, and the way the data collected by the two instruments was calibrated. “It appears that one of the records did this calibration incorrectly, introducing a step-like change in December 1991 that was big enough to have a large influence on the long-term trend,” explains Eisenman. [ Read on.]
Postscript, 1:10 p.m. | Mark Zastrow has written a nice overview for Nature News, including this:
The climate scientist who maintains the data set, Josefino Comiso of the NASA Goddard Space Flight Center in Greenbelt, Maryland, says he is confident that the current data set is correct. Comiso says that he inadvertently introduced a mistake into the record — known as Bootstrap — at some point after 1991, but corrected it unknowingly when he updated the file in 20082.
Comiso and other climate scientists reject the suggestion that his data set may overestimate the recent trend in Antarctic sea-ice growth — by as much as two-thirds, according to Eisenman’s analysis. Another NASA sea-ice data set, processed using the other standard algorithm, shows a growth trend similar to that in Comiso’s current data.
Update, July 23, 7:01 a.m. |
Walt Meier of NASA, one of the authors, sent this note to help make the paper’s point in another way:
We’ve been getting quite a few questions on the paper and I’ve been thinking about how to summarize what the paper shows. The key figure in my mind is Figure 1B, but this may be somewhat hard to interpret. I think a table form may be more illuminating. So below is Figure 1B with the actual trend numbers and some discussion (the numbers below are roughly estimated by me reading from the figure; Ian has the specific numbers, but I think the rough numbers are close enough for now to show the main points).
Published trends in peer-reviewed articles on Antarctic sea ice extent (all on annual average extent):
2000: ~2000 sq km per year
2001: ~5000 sq km per year
2006: ~5000 sq km per year
2007: ~11,000 sq km per year
2009: ~13,000 sq km per year
2012: ~16,000 sq km per yearSo the published literature suggests that the trend increased 8X over the course of a little more than a decade. For comparison, in the Arctic, the declining trend in September sea ice roughly doubled over the same time period (though of course, the magnitude of the Arctic trends are much larger than the Antarctic trends).
Until this paper came out, there was nothing in the published literature that explained this increase in trend, and particularly the 2X increase from 2006 to 2007. My assumption (and I think that of many scientists) was that this was due to rapid changes in Antarctic sea ice. This paper found that much of that increase (acceleration) in trend is an artifact in a data error in one of the versions of the data (presumably Version 1, though our analysis could not determine that). In reality, if Version 2 were used, the trend values would look like this:
2000: ~12000 sq km per year
2001: ~15000 sq km per year
2006: ~15000 sq km per year
2007: ~11,000 sq km per year
2009: ~13,000 sq km per year
2012: ~16,000 sq km per yearWhile not published in the literature, Figure 1B show that in 1998 Version 1 yields a trend of -5000 sq km per year; however, in Version 2, the 1998 trend is ~7000 sq km per year. In the new version, the Antarctic trends have been continuously positive since at least 1997 (with the lowest magnitude in 1998). To me, the latter numbers puts the Antarctic increase in a somewhat different light from what has been published, even though both now agree that there is a statistically significant increasing trend in the Antarctic.
Postscript, July 23, 1:35 p.m. | Via Twitter, Dana Nuccitelli of The Guardian and Skeptical Science rightly questioned the original phrasing at the edit/insert above, in which I suggested that folks interpreting the Antarctic sea ice expansion “stand down.” I also took into account Greg Laden’s comment below, in which he interpreted the original phrasing as confusing normal scientific debate with the torqued talking points of nay-sayers. I certainly don’t want to confuse people.