US researchers say a reported expansion of sea ice in the Antarctic may be due to an error in the way satellite data is processed.
The ice is actually retreating at a dramatic rate, they now say.
The findings are published today in The Cryosphere, a journal of the European Geosciences Union, in an article called “A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated?”.
Previously, satellite observations suggested that sea ice cover in the Antarctic is expanding at a moderate rate, reaching record highs in recent years.
What’s causing Southern Hemisphere sea ice cover to increase in a warming world has puzzled scientists since the trend was first spotted.
Now, a team of researchers has suggested that much of the measured expansion may be due to an error, not previously documented, in the way satellite data was processed.
This implies the 2007 and 2013 assessment reports from the Intergovernmental Panel on Climate Change can’t both be correct.
“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.
This chart shows how different the data sets are, one showing a sharp fall in sea ice, the other a big increase:
One set of data showed that, between 1979 and 2012, Southern Hemisphere sea ice increased at a rate of about 16.5 thousand square kilometres per year.
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.
In the study published in The Cryosphere, Eisenman and collaborators compare two datasets for sea ice measurements. The most recent one was generated using a version of Bootstrap updated in 2007, while the other is the result of an older version of the algorithm.
The researchers 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.
They say: “We cannot be certain whether the change that caused the increase in the trend corrected a problem or introduced one.”
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