Soot(烟灰)and Snow:a Hot Combination ①New research from NASA scientistssuggests emissions(释放) of black soot alter the waysunlight reflects off snow.According to a computer simulation(模拟) ,black soot may Ibe responsible for 25 percent of obseryed globalwarming over the past century. ②Soot in the higher latitudes(纬度) of the Earth,where ice ismore common,absorbs more of the sun's energy and warmth than an icy,whitebackground.Dark-colorecl black carbon,or soot,absorbs sunlight,while lightercolored ice reflects sunlight. ③ Soot in areas with snow and ice mayplay an important role in climate change.Also,if snow and ice covered areasbegin melting,the warming effect increases,as the soot becomes moreconcentrated on the snow suce.“This provides a positive feedback,as glaciersand ice sheet smelt,they tend to get even dirtier," said Dr.James Hansen,aresearcher at NASA's Goddard Institute for Space Studies,New York. ④Hansen found soot's effect on snowalbedo(反照率) (solar energy reflected back to space),which may be contributing to trends toward early springs in the NorthernHemisphere,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 abovethe oceans and land. ⑤“Black carbon reduces the amount ofenergy reflected by snow back into space,thus heating the snow suce morethan if there were no black carbon," Hansen said.Soot's increasedabsorption of solar energy is especially effective in warming the world'sclimate."This forcing is unusually effective,causing twice as much globalwarming as a carbon-dioxide(二氧化物) forcing of the samemagnitude(量，大小)。" Hansen noted. ⑥ Hansen cautioned,although the role ofsoot in altering global climate is substantial,it does not alter the fact thatgreenhouse gases are the primary cause of' climate warming during the pastcentury.Such gases are expected to be the largest climate forcing for the restof this century. ⑦The researchers found that observedwarming in the Nort.hem Hemisphere was large in the winter and spring at middleand high latitudes.These observations were consistent with the researchers'climate model simulations,which showed some of the largest warming effectsoccurred when therc were heavy snow cover and sufficient sunlight.21. In the twentieth century,soot_______.