The Arctic's "Surprising" Double-Dip Melt (Update)

A double-dip melt means that 2010 could rank in a statistical tie with 2008 as having the second-lowest extent of Arctic sea ice left after the annual melt season.

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Through early summer, the extent of Arctic sea ice seemed to be melting to another record-low, but late summer moderation in temperatures and Arctic wind patterns mean the annual melt is likely to result in the third-lowest extent of sea ice ever recorded, according to the latest report from the National Snow and Ice Data Center. Researchers thought the melt season ended September 10, but a double-dip emerged that should be ending with the Autumnal Equinox Wednesday. The result could be a statistical tie with 2008 for the second-lowest extent of sea ice left after the melt season, according to Walt Meier, a research scientist with the National Snow and Ice Data Center at the University of Colorado-Boulder. But it will likely go in the record books as the third-lowest, with 2008 edging it out and 2007 still ranking as the most extreme melt in the 32-year satellite record.

"It's not unheard of, but a little surprising," Meier said of the late-season dip. While researchers haven't analyzed anything but the satellite data that informs daily estimates, he said wind patterns and warm seas likely contributed to the double dip. Hurricane Igor, which is approaching Greenland, isn't expected to have any effect on sea ice, since most ice in its path has long-since melted.

Update 9/29: The National Snow and Ice Data Center now report that the annual minimum was reached Sept. 19, and that 2010 ranks as the third-lowest minimum ever recorded, just 14,000 square miles behind the second-lowest year, 2008. While 14,000 square miles isn't nothing (about the size of Maryland and Delaware combined) it represents less than 1% of the ice left in the Arctic at minimum, so the difference between the ice minimum in 2008 and 2010 is, well, minimal.

Through the end of the melt season, the center reports, "the Northwest Passage and the Northern Sea Route are largely free of ice, allowing the potential for a circumnavigation of the Arctic Ocean. At least two expeditions are attempting this feat, the Norwegian explorer Borge Ousland and the Peter I yacht from Russia."

Enough ice remained in the Arctic through August, on average, to cover 80% of the continental United States (2.31 million square miles). In an average August, there's enough Arctic ice to cover the continental U.S. and then some (2.96 million square miles). For August, the extent of ice was the second-lowest ever recorded, but the annual melt is expected to finish third overall, with a little more ice left than there was in 2007 or 2008. Compared to the historical record, 2009 would have set a record, too, if it weren't for the extremes of 2007 and 2008.

"The bigger picture is not so much 'is this number three or number two,'" Meier said. "It's really the long-term trend that we have: 32 years of data, showing a very significant downward trend, a 10% annual downward trend in September ice extent. That's the signal we're interested in. That's the response to the changing climate."

Satellite records date to 1979, but ice cores and other data indicate that the rate of melting is unprecedented and can only be explained by the heat-trapping accumulation of greenhouse gases in the atmosphere. As ice melts, the darker water that is revealed absorbs more heat than the reflective ice, reinforcing the melting trend.

Charts: The ice extent in August is graphed at left. The area of ice left in the Arctic in September 2010 is mapped at right, with the median ice extent from 1979-2000 marked with an orange line. (National Snow and Ice Data Center)

As the ice melts, polar bear, Pacific walrus, ribbon seals and other species struggle for survival. Meanwhile, the potential for human use grows – for shipping through once ice-locked channels, and for oil and gas drilling. Environmental groups, galvanized by the BP oil spill in the Gulf of Mexico, are warning that an oil spill in the Arctic could yield even greater damage, given the fragile ecosystem, harsh conditions for a cleanup and remoteness from rescue equipment.

More concerning for humans is this: As it becomes clear that the worst-case melting scenarios are taking place, does that mean that the worst-case predictions for other aspects of global warming are inevitable? Wildfires, droughts, crop failure, sea-level rise, massive rates of species extinctions, extreme weather close to home, heat waves ... each could appear more quickly, and have more severe impacts than the public expects.

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