单选题 Soot（煤烟灰）and Snow: a Hot Combination New reasearch from NASA scientists suggests emissions of black soot after the way sunlight reflects off snow. According to a computer simulation, black soot may be responsible for 25 percent of observed global warming over the past century. Soot in the higher latitudes（维度）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. Soot 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 increase, as the soot becomes more concentrated on the snow surface."This provides a positive feedback, as glaciers and ice sheets melt, they tend to get even dirtier." said Dr. James Hansen, a researcher at NASA's Goddard Institute for Space Studies, New York. Hansen found soot's effect on snow albedo（反照率）, which may be contributing to trends toward early springs in the Northern Hemisphere, such as thinninbg Arctic sea ice and melting glaciers permafrost. Soot also is believed to play a role in changes in the atmosphere above the oceans and land. "Black carbon reduces the amount of energy reflected by snow back into space, thus heading the snow surface more than if there were no black carbon, " Hansen said. Soot's increased absorption of solar energy is especially effecntive in warming the world's climate." This forcing is unusually effective, causing twice as much global warming as a carbon-dioxide forcing of the same magnitude." Hansen noted. Hansen 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 bi the largest climate forcing for the rest of the century. The researchers found that observed warming in the Northern Hermisphere was large in the winter and spring at middle and high latitudes. These observations were coherent with the researchers' climate model situations, which showed some of the largest warming effects occurred when there were heavy snow cover and sufficient sunlight.