January 03, 2008	Thursday	Zilhaj 23, 1428

PARIS: The dramatic loss of the Arctic ice cap may have been triggered by disruption to the thermal layers of atmosphere stacked over Earth’s far north, according to a Swedish research.

The study offers a new explanation for the rise in the Arctic’s surface temperature, which over the past century has been nearly two degrees Celsius (3.6 Fahrenheit), or twice the global average.

Until now, the big suspect in “Arctic amplification” has been reflectivity of sunlight.

When the Sun’s rays hit snow or ice, most of that solar energy bounces back into space — but as those melting surfaces give way to dark-blue sea, the heat is absorbed instead.This self-reinforcing process, called a feedback, is an established factor in accelerating warming in snow and ice.

But Stockholm University scientists led by Rune Graversen believe a possibly bigger cause for Arctic warming could be changes in heat transport in the middle of the troposphere, an atmospheric band that extends 10km above Earth’s surface.

In polar regions, the layers of relative heat above the surface are usually stable. But Graversen says that over the last two decades or so there have been changes in Arctic atmospheric circulation which have brought in heat and moisture.

The moisture is particularly important, as it helps form persistent low cloud over the Arctic.

Moisture-laden clouds at this altitude tend to absorb heat from the Sun, thus bringing a warming effect close to the surface. In contrast, high-altitude clouds, which mainly comprise icy crystals, reflect heat back into space, and thus cool the surface.The circulatory shifts have an especially big impact in summer, says Graversen.—AFP