Sunday, 3 August 2014

Growing understanding of Antarctic sea ice growth

One common climate change denier arguments is to point at Antarctic sea ice's growth as if it proves warming isn't happening. Cairns's "leading" wingnut letter writer, Bill Schutz, is one of a few who has included this in his letters to the Cairns Post. Such authors ignore the Antarctic land ice melt and the effect the water from that melt has on sea ice formation, and also the effects of a change in circumpolar winds. If recent work proves right, another factor emerges. Writing at the Guardian, John Abraham describes new work showing that a change in measurement calibration is another cause of the apparent increase:
There has been a lot of attention on ice at the southern pole of the Earth. To be clear, the Earth’s climate is changing and the Earth is getting warmer. This means that the oceans are warming, the atmosphere is warming, sea levels are rising, and ice is melting. In fact, the Earth’s ice is melting almost everywhere. In the Arctic, sea ice is in a long-term retreat, the Greenland ice sheet is melting, so is the Antarctic ice sheet as are the world’s glaciers. But, there is a perplexing anomaly. The sea ice (ice floating on water) that surrounds the Antarctic appears to be growing. Scientists want to know why.

There are many hypotheses, and my colleague Dana Nuccitelli has written about this recently, but here I add a few emerging points. For instance, we know that there is an enormous amount of ice atop the Antarctic ice sheet that is melting each year. Since ice is much fresher than sea water (less salty), the resulting freshwater is creating a fresher zone of water surrounding the continent. The presence of fresh water affects how easily ice can form.

Another view has looked at the quality of the measurements themselves. Could some of the increase be a spurious trend in the measurements themselves? This view was investigated in a very recent publication by Ian Eisenman and colleagues. What the authors found was that a change in sensor calibration caused a shift that has been interpreted as ice acceleration. In the abstract, the authors state,
Specifically we find that a change in the intercalibration across a 1991 sensor transition when the data set was reprocessed in 2007 caused a substantial change in the long-term trend. Although our analysis does not definitively identify whether this change introduced an error or removed one, the resulting difference in the trends suggests that a substantial error exists in either the current data set or the version that was used prior to the mid-2000s… furthermore, a number of recent studies have investigated physical mechanisms for the observed expansion of the Antarctic sea ice cover. 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 satellite observations. 
Read more at the Guardian

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