Shifting Layers Over Arctic Blamed for Ice Melt

Jan. 2, 2008 -- 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 Swedish research to be published Thursday.

The study, published in Nature, 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 10 kilometers (seven miles) 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.

In 2007, summer sea ice in the Arctic shrank to about four million square kilometers (2.4 million square miles), a 23 percent decrease from the previous record low of 5.3 million square kilometers in 2005.

A second study, also in Nature, meanwhile, shows that the capacity of vegetation to absorb carbon dioxide (CO2) appears to be ebbing, with potentially serious consequences for global warming.

Currently, about 50 percent of all the CO2 produced by burning fossil fuels is soaked up -- "sequestered" -- by land masses, mainly through forests, and by oceans.