Soot（烟灰）and Snow: a Hot Combination
①New research from NASA scientists suggests emissions（释放）of black soot alter the way sunlight reflects off snow. According to a computer simulation（模拟），black soot may be responsible for 25 percent of observed global warming over the past century.
②Soot in the higher latitudes（纬度）of the Earth，where ice is more common，absorbs more of 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.
③Soot in areas with snow and ice may play an important role in climate change. Also，if snow and 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，they tend to get even dirtier，" said Dr. James Hansen，a researcher at NASA's Goddard Institute for Space Studies，New York.
④Hansen found soot’s effect on snow albedo（反照率）（solar energy reflected back to space），which may be contributing to trends toward early springs in the Northern Hemisphere，such as thinning Arctic sea ice，melting glaciers and permafrost（永冻层）.Soot also is believed to play a role in changes in the atmosphere above the oceans and land.
⑤”Black carbon reduces the amount of energy reflected by snow back into space，thus heating the snow surface more than if there were no black carbon，"Hansen said. Soot’s increased absorption of solar energy is especially effective in warming the world's climate. “This forcing is unusually effective，causing twice as much global warming as a carbon-dioxide（二氧化物）forcing of the same magnitude（量，大小），”Hansen noted.
⑥Hansen cautioned , although the role of soot in altering global climate is substantial , it does not 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.
⑦The 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 were heavy snow cover and sufficient sunlight.
【第21题】 In the twentieth century , soot_____.A.produces much more global warming than a carbon-dioxide forcing of the same magnitude.;B.contributed to 25 percent of observed global warming;C.winter and spring;D.the warming effects increase .;E. Still surpass soot in warming the world’s climate during the last century;F.can be seen mostly in the Northern Hemisphere
【正确答案】：B
【题目解析】：段落句子补充信息类的题目需要回归到该问题所出现的位置，根据上下文进行寻找、进行提炼或转换近义的词句。以下同此。本题定位到第一段的最后一句，可以找出黑色烟灰也是全球气候变暖的因素之一，其所占的比重为25%。
