Sea ice cover around Antarctica may not be growing as quickly as previously believed, with flawed analysis of satellite data a possible cause of the overestimation, a new study indicates.
While sea ice in the Arctic is retreating at a significant rate, satellite observations of Antarctica have suggested sea ice coverage there is expanding slowly and has reached record levels in recent years.
Writing in The Cryosphere, a journal of the European Geosciences Union, the authors of a new study suggest the increase may be illusory, caused by a previously undocumented error in the manner satellite data is processed.
"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," says lead study author Ian Eisenman at the Scripps Institution of Oceanography of the University of California, San Diego.
The AR4 assessment showed Antarctic sea ice extent more or less constant between 1979 and 2005, whereas AR5 indicated an increase in sea ice in the Southern Hemisphere from 1979 to 2012 of around 6.3 thousand square miles a year.
The addition of seven more years of satellite observational records was assumed by scientists to be the reason for the counterintuitive increase.
"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.
The sea ice measurements are created from a number of instruments aboard a lot of different satellites, with a computer algorithm used on the combined data to create sea ice cover estimates, he says.
However, the AR4 and AR5 assessments were created using two difference versions of the algorithm, which was upgraded in 2007, the study found.
That created a difference in the two datasets linked to a 1991 transition in satellite sensors and the manner in which calibration was performed on data recorded by two separate instruments, Eisenman says.
"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," he explains.
However, he added, it is uncertain which data set contains the error because natural variability -- "noise" in the data from month to month -- makes it difficult to isolate the algorithm error.
"When we subtract one record from the other, though, we remove most of this noise, and the step-like change in December 1991 becomes very clear," he says.
Further research could resolve the apparent enigma of Antarctic sea ice cover, the researchers said.
