New research from NASA scientists suggest black soot(煤烟) change the way sunlight reflects off snow. Black soot may be responsible for 25 percent of observed global warming over the past century.

       Soot 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, while background. Dark—colored black carbon, or soot, absorbs sunlight, while lighter colored ice reflects sunlight.

       Soot in areas with snow and ice may play all important role in climate change. Also, if snow and ice covered areas begin melting, the warming effect increases, as the soot become s more centralized on the snow surface.“this provides a positive result, as glaciers（冰川） and ice sheet melt, they tend to get dirtier.”said Dr 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 were more easily-seen in early springs in the Northern Hemisphere, such as thinning Arctic sea ice. 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 heating the snow surface more than if there were no black carbon,”Hansen said. Soot’s increased absorption of sunlight is especially effective in warming the world's climate.“This greenhouse effect causes twice as much global warming as a carbon dioxide（二氧化碳）greenhouse effect of the same quantity,”Hansen noted.

       The researchers found that observed warming in the Northern Hemisphere was large in the winter and spring at middle and high latitudes. These observations are in agreement with the researchers'climate study, which showed some of the largest warming effects occurred when there were heavy snow cover and enough sunlight.

19. What is the main idea for the third paragraph?

    A. The melting of glacier and ice sheet.

    B. Dr James Hansen's space studies.

    C. Explanation of increased warming effect caused by Soot.

    D. Effort to Reduce Snow Albedo.

20. What are the two key words which best explain the reasons of the effect of soot to the earth?

    A. color and sunlight      B. energy and warmth

    C. ice and latitude        D. greenhouse and carbon dioxide.

   16—20 DCCAD