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.
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
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
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