The Arctic saw a record loss of summer sea ice in 2012, and the 2013 melt is off to a faster start than a year ago. Another record is uncertain, but warming has sapped the ice's staying power.
Saunders Island and Wolstenholme Fjord are shown in this picture taken during an Operation IceBridge survey flight in April. Sea ice coverage in the fjord ranges from thicker, white ice seen in the background, to thinner grease ice and leads showing open ocean water in the foreground.
Michael Studinger/NASA/Reuters
EnlargeAfter a record loss of summer sea ice in the Arctic Ocean last year, the 2013 melt season has begun at the top of the world, with ice vanishing in April at a faster pace than it did this time last year.
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Summer sea ice ? a key player in Earth's climate system and one whose decline is widely taken as a prominent sign of global warming ? has been shrinking in extent since satellites first started to build a consistent record of the ice in late 1978. Ice losses in 2007 set a melt-season record, only to be eclipsed by last year's decline. Ice volume and thickness also have been declining during the past 34 years.
Whether the start to the 2013 thaw presages another record melt is unclear. Sketching expectations for the end of the melt season based on pre-season indicators and one month's activity can be as dicey as predicting that the Boston Red Sox, with the best record in baseball coming out of April, will sweep the World Series next fall.
At the least, ice conditions entering this year's melt season appear to have set the stage for another significant retreat, compared with the 1979-2000 average.
After ice losses set the previous record in 2007, summer ice-extent recovered somewhat in each of the next three years, notes Claire Parkinson, a researcher studying climate and polar sea ice at NASA's Goddard Space Flight Center in Greenbelt, Md.
Still, the long-term decline has increased the ice's vulnerability to sharp summer declines. Resilient multi-year ice floes, once about 60 percent of the Arctic Ocean's summertime ice cover, have dwindled to some 30 percent of summer sea ice, according to the National Snow and Ice Data Center (NSIDC) in Boulder, Colo. It has been replaced by thinner, weaker ice.
This shift in ice quality has helped "precondition" the region for continued losses, Dr. Parkinson says, adding, "the very fact that last year was a record low certainly means [the ice] was further preconditioned."
Much depends on the wind patterns that set up over the spring and summer, she adds. Decades ago, when most of the summer ice was mostly densely packed, multi-year ice, typical spring and summer wind patterns had relatively little effect on ice loss. These days, those same wind patterns can do far more damage because more of the ice is less-densely packed and more fragile.
At the peak of this past winter's freeze in March, Arctic sea ice covered 5.8 million square miles, some 270,000 square miles below the 1979-2000 average, according to NSIDC. That peak occurred after a record rebound in ice extent from last summer ? the record set because last summer's melt-back had been so vast.
The ice that returned is so-called seasonal ice ? ice with no melt season under its belt yet.
Moreover, by the end of March, the Arctic Ocean had lost additional multi-year ice to the North Atlantic as ice bergs, while multi-year ice elsewhere had migrated to the Beaufort Sea off of the north coasts of Canada and Alaska. This area has become a graveyard for significant amounts of multi-year ice over the past several summers, according to the NSIDC.
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