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.

Remarkably, that percentage has remained stable even as the output of man-made greenhouse gases has increased.

Up to now scientists have assumed that longer growing seasons were a silver lining of climate change because the warmer temperatures gave photosynthesising plants more time to remove the most important of these gases -- CO2 -- from the atmosphere.

This view has been bolstered by satellite images showing a clearly visible "greening trend," notably in the northern hemisphere.

But Shilong Piao of France's National Centre for Scientific Research (CNRS), found that over the past 20 years the planet's biomass above the 20th parallel released nearly as much CO2 in the fall or autumn as it soaked up during the spring.

"If future autumn warming occurs at a faster rate than in spring, the ability of norther ecosystems to sequester carbon may be diminished earlier than previously suggested," he warns.

While Piao's study draws a clear link between rising temperatures and reduced carbon uptake, projecting future trends is very difficult, cautions John Miller, a expert on carbon cycles at the University of Colorado, in a commentary, also published in Nature.

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