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根据下面资料，回答题 Soot ( 烟灰) and Snow..a Hot Combination
① New research from NASA scientists suggests emissions (释放) of black soot alter the waysunlight reflects off snow. According to a computer simulation(模拟), black soot may be responsiblefor 25 percent of observed global warming over the past century.
② Soot in the higher latitudes(纬度) of the Earth,where ice is more common,absorbs moreof the sun' s energy and warmth than an icy,white background. Dark-colored black carbon, or soot,absorbs sunlight, while lighter colored ice reflects sunlight.
③ Soot in areas with snow and ice may play an important role in climate change. Also, if snowand ice covered areas begin melting, the warming effect increases, as the soot becomes more concentrated on the snow surface. "This provides a positive feedback, as glaciers and ice sheet smelt, theytend to get even dirtier," said Dr. James Hansen, a researcher at NASA' s Goddard Institute forSpace Studies, New York.
④ Hansen found soot' s effect on snow albedo (反照率) (solar energy reflected back tospace), which may be contributing to trends toward early springs in the Northern Hemisphere, suchas thinning Arctic sea ice,melting glaciers andpermafrost(永冻层). Soot also is believed to play arole in changes in the atmosphere above the oceans and land.
⑤ "Black carbon reduces the amount of energy reflected by snow back into space, thusheating the snow surface more than if there were no black carbon," Hansen said. Soot' s increasedabsorption of solar energy is especially effective in warming the world' s climate. "This forcing isunusually effective, causing twice as much global warming as a carbon-dioxide(二氧化物) forcing ofthe same magnitude (量，大小)," Hansen noted.
⑥Hansen cautioned, although the role of soot in altering global climate is substantial,it doesnot alter the fact that greenhouse gases are the primary cause of climate warming during the past century. Such gases are expected to be the largest climate forcing for the rest of this century.
⑦ The researchers found that observed warming in the Northern Hemisphere was large in the winter and spring at middle and high latitudes. These observations were consistent with the researchers'climate model simulations, which showed some of the largest warming effects occurred when there wereheavy snow cover and sufficient sunlight.
In the twentieth century, soot________.
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