【英语时差8,16】全球变暖的元凶--煤烟（在线收听）

 Soot was cause for a different concern in another recent study. Scientists with the American space agency, NASA, suggest it as a major cause of global warming. The study says the problem is how soot interacts with snow and ice. Snow and ice have highly reflective surfaces. A lot of the sunlight that hits them is forced back up toward the sky. This helps prevent melting. But the scientists say the problem develops when snowflakes pick up fine particles of soot as they fall. The black carbon in soot reduces the ability of snow and ice to reflect sunlight. Instead, the black soot absorbs the energy and warmth, and causes melting. The scientists say the soot causes the melting season of glaciers to begin earlier and last longer. This has a large effect, they say, because wet snow is much darker than dry snow. So the problem increases. The scientists estimate that soot is two times as effective as carbon dioxide in changing surface air temperatures. But they say the good news is that cleaner diesel engines and other technologies are being developed to reduce soot.