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[交流] 【Share】Thaw of Earth's icy sunshade may stoke warming

OSLO (Reuters) - Shrinking ice and snow cover in the Northern Hemisphere is reflecting ever less sunshine back into space in a previously underestimated mechanism that could add to global warming, a study showed.

Satellite data indicated that Arctic sea ice, glaciers, winter snow and Greenland's ice were bouncing less energy back to space from 1979 to 2008. The dwindling white sunshade exposes ground or water, both of which are darker and absorb more heat.

The study estimated that ice and snow in the Northern Hemisphere were now reflecting on average 3.3 watts per square meter of solar energy back to the upper atmosphere, a reduction of 0.45 watt per square meter since the late 1970s.

"The cooling effect is reduced and this is increasing the amount of solar energy that the planet absorbs," Mark Flanner, an assistant professor at the University of Michigan and lead author of the study, told Reuters.

"This reduction in reflected solar energy through warming is greater than simulated by the current crop of climate models," he said of the findings by a team of U.S.-based researchers and published in the journal Nature Geoscience Sunday.

"The conclusion is that the cryosphere (areas of ice and snow) is both responding more sensitively to, and also driving, stronger climate change than thought," he said.

As ever more ground and water is exposed to sunlight, the absorbed heat in turn speeds the melting of snow and ice nearby.

Arctic sea ice, for instance, has shrunk in recent decades in a trend that the United Nations panel of climate scientists blames mainly on greenhouse gases from mankind's burning of fossil fuels in factories, power plants and cars.

Many studies project that Arctic sea ice could vanish in summers later this century in a trend that would undermine the hunting cultures of indigenous peoples and threaten polar bears and other animals, as well as adding to global climate change.

ICE SHRINKS

But Flanner said that it was impossible to draw conclusions from the study about the rate of future melting, for instance of Arctic sea ice, since it was based on only 30 years of data.

"There are a lot of other things that determine climate ... this is just one of them," he said.

Other factors include whether there will be more clouds in a warmer world -- whose white tops also reflect sunlight. Or there could be more water vapor that traps heat in the atmosphere.

The study estimated that each degree Celsius (1.8 degree Fahrenheit) rise in temperatures would mean a decline in solar energy reflected out to space of between 0.3 and 1.1 watts per square meter from the Northern Hemisphere's snow and ice.

Temperatures in the Northern Hemisphere have risen by about 0.75 degree Celsius in the past three decades. The study did not look at the Southern Hemisphere, where Antarctica has far more ice but is much colder and shows fewer signs of warming.

"On a global scale, the planet absorbs solar energy at a rate of about 240 watts per square meter averaged over a year. The planet would be darker and absorb an additional 3.3 watts without the Northern Hemisphere cryosphere," Flanner said.

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