A change in algorithms leads to a potential spurious increase in sea ice. Figure from Eisenman, Meier, and Norris, distributed under the Creative Commons 3.0 License.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.
First, we should understand how challenging this problem is. Satellites can measure microwave emission from the ground level. The energy emitted by sea ice differs from the energy emitted by open water. Furthermore, the energy emitted by a surface changes with temperature. As a consequence, it becomes difficult to distinguish between cooler ice temperatures and warmer water temperatures, for example. So, different algorithms have been developed to make these distinctions and the algorithms have changed over time. The image below shows that moving from one algorithm to another (red to blue) caused a shift in 1991 and a significant difference of ice extent from the IPCC AR4 (black square in right image) to the more recent AR5 (black circle in right image).
A change in algorithms leads to a potential spurious increase in sea ice. Figure from Eisenman, Meier, and Norris, distributed under the Creative Commons 3.0 License.
An easier way to view the impact of the algorithm switch is shown below which plots the difference between the methods. There, the 1991 jump is clearly evidence.
Time evolution of differences in ice extent from two algorithms. Figure from Eisenman, Meier, and Norris, distributed under the Creative Commons 3.0 License.
Posted by John Abraham on Friday, 1 August, 2014
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