Antarctic sea ice may not be expanding as rapidly previously thought – with much of the increase potentially down to a data error, scientists have said.
While Arctic sea ice has been melting significantly in recent years, satellite observations suggest sea ice cover in the southern hemisphere has been increasing and has reached record highs in the past few years.
The increase in Antarctic sea ice despite rising global temperatures has puzzled scientists and been highlighted by climate change sceptics as evidence of flaws in the theory of man-made global warming.
The hole in the ozone layer, increases in freshwater from melting glaciers, winds and natural variability are among the theories put forward for the growth in Antarctic sea ice, which appears to be increasing at a third of the rate the Arctic’s sea ice is retreating.
But now research published in the European Geosciences Union journal The Cryosphere suggests that much of the measured expansion could be down to a previously undocumented error in the way satellite data was processed.
A 2007 international assessment of climate science by the Intergovernmental Panel on Climate Change (IPPC) found that Antarctic sea ice cover remained constant between 1979 and 2005.
The newer assessment gave larger rates of sea ice expansion than the old one in any given period
But more recent research and the latest assessment by the IPCC, published in the last year, suggests that southern hemisphere sea ice extent increased at a rate of about 16,500 square kilometres a year between 1979 and 2012.
The difference has been attributed to adding more years to the data.
But lead author of the new paper, Ian Eisenman of the Scripps Institution of Oceanography at the University of California, San Diego, US, said: “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.”
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.”
The data for sea ice cover is sourced from observations by a number of different instruments on a number of satellites, with scientists using a mathematical formulae and further processing to calculate what the extent of the sea ice is from the readings.
The researchers compared two datasets, calculated with different versions of the formula, which were used in the two IPCC assessments.
The study shows a difference between the datasets, relating to a change in satellite sensors in December 1991 and the way the data from the two instruments was calibrated. Eisenman said: “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 said it is not readily apparent which record contains the step change.
But by comparing the datasets and calculating Antarctic sea ice extent for each of them, the researchers discovered that the newer assessment gave larger rates of sea ice expansion than the old one in any given period.
They found the large increase in the reported rate of Antarctic sea ice expansion since the IPCC assessment in 2007 was down to the effect of a change in the way the observations were processed, rather than due to adding more years of data.
But the researchers said they could not tell if the change corrected a problem or introduced one.
If it introduced a problem, the rate of Antarctic sea ice expansion could have been overestimated – potentially solving the puzzle of why Antarctic sea ice has been recorded as increasing to such an extent despite rising global temperatures.