Sea Ice Expansion in Antarctica: Product of Satellite Data Calibration Error?

Arctic sea ice is retreating at a dramatic rate. In contrast, satellite observations have been suggesting that sea ice cover in the Antarctic is expanding – albeit at a moderate rate – and that sea ice extent has reached record highs in recent years.

It has been a puzzle for scientists to figure what may be causing two such opposing responses to climate at the two poles. More so, how can sea ice cover be increasing in a warming world?

Now, a team of researchers suggest that much of this 'measured' expansion may be due to an error in the way satellite data was processed. Perhaps, there is no sea ice expansion in Antarctica.

The findings are published in The Cryosphere, a journal of the European Geosciences Union (EGU).

Ice trends were reported way back in IPCC's AR4 and AR5 reports during 2007 and 2013 that produced conflicting observations.

"Our findings show that the data used in one of the reports contains a significant error. But we have not yet been able to identify which one contains the error," says lead-author Ian Eisenman of the Scripps Institution of Oceanography at University of California San Diego in the US.

The AR4 reported that Antarctic sea ice cover remained more or less constant between 1979 and 2005. On the other hand, AR5 which looked at the period between 1979 and 2012 noted a sea ice expansion in the Southern Hemisphere at a rate of about 16.5 thousand square kilometres per year.

Scientists had then assumed this discrepancy as stemming from the fact that several more years were added to the observational record.

"But when we looked at how the numbers reported for the trend had changed, and we looked at the time series of Antarctic sea ice extent, it didn't look right," says Eisenman, who set out to figure out what was wrong.

Instruments play a big role today in science. But sometimes, too much of a good thing can be bad too as realised by scientists trying to make long-term measurements with multiple instruments.

Scientists used satellite data to measure sea ice cover for 35 years. But the data comes from various instruments flown on a number of different satellites. They use an algorithm on the data to estimate sea ice cover. Two different versions of the algorithm were used in the two instances.

Investigating researchers have 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. Again, there was a problem with the calibration, they found. What ought to have been noticed was missed on account of natural variability expected.

Eisenman said his findings have introduced two possibilities. One is that the new, updated dataset is wrong. In that case, Antarctic ice would be expanding, but not nearly as quickly as scientists believed. Or, the error was with the pre-2007 version. In that case, studies that have used that version of the data would need to be reassessed.