Historical Data Show Arctic Melt of Last Two Decades Is 'Unprecedented'

Sea ice melting since 1979 is 'enormously outside the bounds of natural variability' and clearly linked to humans burning fossil fuels, research shows.

While satellite images of the Arctic clearly show that sea ice in the region has been on a steady decline since those images began in 1979, the relatively short span of that history has been seized on by some climate denialists to discount its significance in concluding humans are warming the planet.

Now, scientists have compiled the most detailed study to date of sea ice records going back more than a century and a half.  The data show that the rapid meltdown that satellites have been documenting since 1979 is unprecedented since at least 1850 and coincides with the buildup of heat-trapping greenhouse gases in the atmosphere from burning fossil fuels.

Arctic sea ice has not been at levels as low as today's for at least 5,000 to 7,000 years, according to Julienne Stroeve, a researcher with the National Snow and Ice Data Center (NSIDC), who was not involved in the study.  "It may have been sometime during the mid-Holocene, based on driftwood found in Greenland that came from Siberia," she said.  "Some other studies have suggested at least 800,000 years."

"Any reconstruction back before the satellite data record is extremely helpful in putting today's changes in context.  None of the models show this decline without including the observed record of greenhouse gases.  This helps to confirm that rising levels of GHGs is causing the sea ice to decline," Stroeve said.

The researchers, with the NSIDC, The University of Alaska, Fairbanks and the University of Illinois, compiled and digitized sea ice measurements from 14 historical sources.  Those included hand-drawn ice-boundary maps, and documents like logbooks from the U.S. whaling fleet, records from the Danish Meteorological Institute and reports by U.S. Navy oceanographers, from 1850 through 1978.  This analog data was digitized to make it compatible with satellite records starting in 1979.  The work yielded a finely gridded dataset that can be used by climate models to project how sea ice will change in the decades ahead.

The corresponding paper was published July 11 in the journal Geographical Review.

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