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مدیر وبلاگ : علیرضا اسحاقی
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سه شنبه 17 مرداد 1391 :: نویسنده : علیرضا اسحاقی
greenland


Greenland’s ice is on the hot seat again.

A heat wave, possibly the biggest in a century, washed over the frozen island in mid-July. Around 97 percent of the surface ice melted temporarily. Slush even appeared at Greenland’s highest, coldest spot.

The massive ice cap also darkened a lot this year, so it absorbs more of the sun’s energy and melts still faster. Overall, more of Greenland’s ice melted in June and July than in any previous year during the satellite era, says Marco Tedesco of the City University of New York.

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In The Cryosphere Discussions, Tedesco and other ice experts predicted just before the July melt that an island-wide thaw could happen within the decade. “We could kind of see it coming,” says team leader Jason Box, a glaciologist at Ohio State University. “It’s not hard to make these predictions, because the odds are stacked in favor of warming.”

Weather and climate patterns conspired this year to produce what Box calls a “one-two-three-four-five punch.” Among other things, a strong warming trend shifted much of the upper snowpack closer to the melting point. When a dome of particularly warm air began moving over Greenland on July 8, things were primed for nearly all of the snow on top to thaw.

At the Summit Camp research station more than 3,200 meters above sea level, the thermometer soared above freezing for several days straight. It was the first significant melt at the site since 1889, says Mary Albert, a cryosphere expert at Dartmouth College.

At a second site 700 kilometers to the northwest, “it was wet and slushy, which made it really hard to walk around on the ice without falling knee-deep into the snow,” says Dartmouth graduate student Kaitlin Keegan. Flags planted in the snow began to topple. Supply planes were unable to land on the once-icy runways.

Ice cores from that site show evidence of similar melt in 1946 (though to a lesser degree) and in 1889, says Dorthe Dahl-Jensen of the University of Copenhagen. “So it is rare indeed,” she says, especially when temperatures began soaring again during the last week of July. The air was warm and foggy, and the ice-block-throwing competition scheduled as part of the “Icelympics,” polar scientists’ companion to the London Games, had to be canceled because the blocks were melting.

Over the past millennium, melt layers appear in cores drilled through Summit ice less than once per century, says Richard Alley, a glaciologist at Penn State University. “Nature could have caused this [year’s] melt event by chance,” he says. “But humans made it more likely with greenhouse gases.”

By measuring microwave energy coming off of Greenland, satellites can see how much meltwater there is versus how much ice. During the peak of the July melt, Summit’s uppermost snow contained about one-tenth as much melt as that seen in areas closer to the southwest coast, where melting has been most pronounced in recent years, say Tedesco and his colleague Xavier Fettweis, of the University of Liège in Belgium.

Overall, Greenland has been losing close to 300 billion metric tons of ice yearly. At Summit, meltwater refreezes in place, but at lower elevations it runs off into the oceans and contributes to sea level rise.

The two regions most sensitive to rising surface temperatures are southwestern and northwestern Greenland, Tedesco says. “Those are the two exposed nerves,” he says. In the southwest, raging meltwaters washed out a bridge in the town of Kangerlussuaq on July 11. In the northwest, the Petermann glacier broke off a massive chunk of ice on July 16, the second huge iceberg calving in two years.

But not all such headline-making events can be linked to surface warming, Kurt Kjaer of the University of Copenhagen and his colleagues write in the Aug. 3 Science.

Kjaer’s team used old aerial photographs to study how surface elevation had changed in northwestern Greenland since the mid-1980s. Looking closely at glaciers along the northwest coast, the scientists found two periods in which more ice than usual flowed into the ocean. Roughly 26 billion metric tons were lost annually in short-lived speedups from 1985 to 1993, and again from 2005 to 2010, the researchers report. Neither of these two speed-ups were predictable, they say. “This finding challenges predictions about the future response of the Greenland ice sheet to increasing global temperatures,” they write.

With temperatures heating up, though, the answers should soon become clearer





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برچسب ها : Greenland enters melt mode،


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