Scientists see pervasive and permanent changes in the last five years

By Janet Raloff

Web edition: December 2, 2011

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UNFROZEN NORTH

Melt ponds form on the frozen surface of the Chukchi Sea northwest of Alaska in July 2010. These ponds can increase light transmission up to tenfold compared with bare ice, new data show.

Credit: K. Frey, Clark Univ.

The northern polar region’s climate has materially changed over the past five years, a team of 121 scientists from 14 nations concludes in a December 1 Arctic report card. Compared with 2006 and earlier, they note, the Arctic is warmer and less icy.

Sufficient observational data now exist “to indicate a shift in the Arctic Ocean system since 2006,” says Jacqueline Richter-Menge of the U.S. Army’s Cold Regions Research and Engineering Laboratory in Hanover, N.H., a coeditor of the new analysis. “This shift is characterized by the persistent decline in the thickness and summer extent of sea-ice cover and by a warmer, less salty upper ocean.”

Triggering that turning point, Richter-Menge says, were unusually warm Arctic temperatures in 2006 together with a persistent weather pattern that pushed ice across the Arctic and into the North Atlantic through the Fram Strait east of Greenland: “We like to call it the perfect storm of the Arctic.”

Once extreme events foster the loss of old, multi-year ice, “we seldom go back to where we were before,” says James Overland of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle.

“We’ve got a new normal,” concludes Don Perovich of CRREL.

Sea-ice loss in 2011 was the second most severe in the 32-year satellite record of Arctic monitoring. “The past five years have had the five smallest September ice extents,” Perovich says, “showing that Arctic sea ice has not recovered from the large decrease observed in 2007.”

Regional warming and melting of land ice cover have also continued at a record pace, the new report finds. For instance, satellite data show that the loss of ice from Greenland during 2010 to 2011 was the largest since monitoring began in 2002. In the Canadian Arctic, the duration of 2011 lake-ice cover was shorter by four to five weeks compared to what had been the average between 1997 and 2010. 

For a decade, monitoring systems have detected continuous warming at Arctic sites near the coast, accompanied by a greening of the landscape as reduced snow cover has allowed small shrubs to grow bigger and seeds of trees and other plants to germinate in formerly frozen soils.

This year, new record highs were witnessed at 20 meters depth at every permafrost observatory on the North Slope of Alaska, where measurements began more than three decades ago. The most recent data suggest this warming “has begun to propagate south towards the northern foothills of the Brooks Range, where a noticeable warming in the upper 20 meters of permafrost has become evident since 2008,” the report says.

The wholesale melting has substantially darkened the sea and landscape, making both better absorbers of solar energy and accelerating the region’s warming.

“In an environment that is inherently icy, you have to ask yourself: Once it begins to melt and we get these feedbacks, like the darkening, how would it be possible to actually recover?” says Richter-Menge. “It’s hard to come up with a scenario where that would happen.”

Some regional darkening has nothing to do with melting, says Jason Box of Ohio State University in Columbus. Albedo is the reflectivity of a surface, which is ordinarily highest on ice and new snow. He points out “that we’re seeing a significant albedo reduction over the upper elevations of Greenland, where there is rarely melting.” He attributes it to a change in the shape — the rounding — of ice crystals in response to warmth.

And although Greenland’s ice sheets used to get new blankets of snow year-round, Box reports “there’s been little summertime snowfall over the ice sheets since about 2006.”

With the Arctic a potent driver of climate across the globe, change at the top of the world will have repercussions elsewhere. Altered wind patterns appear to be one of the earliest symptoms, Richter-Menge says.

Over the past several winters, she notes, “we’ve seen where the cold Arctic air isn’t staying in the Arctic any more, but breaking out to head south” into temperate regions. Meanwhile, the more southerly air it’s displacing has been drawn northward, leading to unusually warm Arctic winters.

The report’s big take-home message, she says: “These changes represent a persistent condition” — with consequences far beyond the Arctic.