【正确答案】
B
【答案解析】[听力原文]
Carbon dioxide in the atmosphere has an important effect on world climate. It lets through most of the short-wave energy directed at the earth by the sun, but retains a high proportion of the long-wave energy radiated back by the earth out toward space. This is known as the "greenhouse effect", by analogy with the garden greenhouse whose glass windows admit solar radiation, but partially block the infrared radiation reradiated from within. This causes the temperature in a greenhouse to rise to a higher level than that of the free air outside. Because of the rise in carbon dioxide concentration in atmosphere (from 265 parts per million in 1850 to 340 parts per million in 1985), due primarily to the burning of fossil fuels such as coal, oil and natural gas, scientists predict that significant changes in climate patterns will occur by the end of the century. They foresee a rise in average global temperature of between 3 and 5 Centigrade before the year 2010.
Temperature increases in the polar regions could be three times the global average, which would result in rapid melting of the polar ice caps. Sea levels could rise by as much as 5 to 7 meters, causing flooding of vast areas of low lying land.
Trees normally act as a sink for carbon dioxide, which they "breathe in", returning oxygen as waste to the atmosphere as they "breathe out". However, when trees are cleared or burned, the carbon they contain, as well as some of the carbon in the underlying soil, is oxidized and released to the atmosphere. Since 1860 some 90 to 180 thousand million tons of carbon have been released to the atmosphere as a result of forest clearing, as compared with 150 to 190 thousand million tons from the burning of coal, oil and natural gas.
Many scientists consider the warming of the atmosphere as putting forward one of the most environmental threats to man. And forest clearance is doubly harmful since not only is the carbon "sink" effect of trees lost, but more carbon is released to the atmosphere.