Growth in sea ice may be slower than reported
FRISCO — Global warming deniers have long been using the observed expansion of Antarctic sea ice as a way to try and confuse the public about the reality of global warming, but some new research by scientists with the Scripps Institution of Oceanography at University of California San Diego suggests the rate of expansion is not as dramatic as reported.
The findings, published in The Cryosphere (European Geosciences Union) acknowledge that there has been some expansion recently, but that some of the reported ice gain may be due to inconsistencies in computer models used to measure Antarctic sea ice.
Major scientific research institutions, including NOAA’s National Snow and Ice Data Center, have been reporting new record Antarctic sea ice extent recently, and scientists have attributed that gain to shifts in wind patterns that allow the ice to spread out. Other recent studies suggest that the expansion may soon end, and that Antarctic sea ice will decline significantly in coming decades.
The growth in Antarctic sea ice has been regional in nature, with big gains in some open ocean areas, but declines in other areas, for example around the rapidly warming Antarctic Peninsula. It’s also important not to confuse Antarctic sea ice with the meltdown of several land-based Antarctic ice shelves, with recent research suggesting that major sections of the West Antarctic ice shelf poised for disintegration.
The problem may be a difference between two datasets and the way they were calibrated during a transition in satellite sensors in 1991, said Scripps researcher Ian Eisenman.
“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,” Eisenman said.
Eisenman’s research highlights potential inconsistencies in climate reports from the IPCC. The 2005 climate assessment reported that Antarctic sea ice cover remained more or less constant between 1979 and 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 kilometres per year. Scientists assumed the difference to be a result of adding several more years 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,” said Eisenman, who set out to figure out what was wrong. Both assessments can’t be correct, he added.
“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,” he said.
Scientists have used satellite data to measure sea ice cover for 35 years. But the data doesn’t come from a single instrument, orbiting on a single satellite throughout this period. Instead, researchers splice together observations from different instruments flown on a number of different satellites. They then use an algorithm – the most prevalent being the Bootstrap algorithm – and further processing to estimate sea ice cover from these data.
“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. 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 explained.
With the exception of the longer time period covered by the most recent dataset, the two records were thought to be nearly identical. But, by comparing the datasets and calculating Antarctic sea ice extent for each of them, the team found that there was a stark difference between the two records, with the current one giving larger rates of sea ice expansion than the old one in any given period.
If the error is in the current dataset, the results could contribute to an unexpected resolution for the Antarctic sea ice cover enigma.