Antarctic Sea Ice Expansion May Have Been Overestimated

Antarctic sea ice may not be expanding as fast as previously thought, according to new research. A simple processing error in satellite data could be to blame for this overestimation.

Arctic sea ice is retreating at a dramatic rate. In contrast, satellite observations suggest 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.

Scientists have long puzzled over what exactly is causing Southern Hemisphere sea ice cover to increase in a warming world. Now, a study published in the journal The Cryosphere suggests that much of the measured expansion may be due to an error that researchers failed to notice until now.

"This implies that the Antarctic sea ice trends reported in the IPCC's AR4 and AR5 [the 2007 and 2013 assessment reports from the Intergovernmental Panel on Climate Change] can't both be correct: 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," lead author Ian Eisenman, of the Scripps Institution of Oceanography at University of California, San Diego, said in a statement.

The aforementioned AR4 report said that Antarctic sea ice cover remained relatively constant from 1979-2005. On the other hand, recent literature and the AR5 indicate that, between 1979 and 2012, Southern Hemisphere sea ice extent increased at a rate of about 16.5 thousand square kilometers per year.

Scientists had assumed that the difference between the two reports was the result of adding several more years to the observational record; but a second looked showed otherwise.

"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," Eisenman said.

While satellite data has been used by scientists over the past 35 years to measure sea ice cover, it's not without its flaws. Combined observations from several different instruments flown on a number of different satellites are used, followed by an algorithm and further processing.

Researchers implemented this technique for both the AR4 and AR5 reports, but this new study reveals that one of them contained a calibration error in the data for December 1991. Apparently, this slip-up significantly influenced the overall, long-term trend set.

Eisenman and his colleagues have yet to identify which report is the weak link, but if the error is in the current dataset, the results could contribute to an unexpected resolution for the Antarctic sea ice cover enigma.

"You'd think it would be easy to see which record has this spurious jump in December 1991, but there's so much natural variability in the record - so much 'noise' from one month to the next -- that it's not readily apparent which record contains the jump," Eisenman explained.