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发布时间：2006-06-22 13:27     点击：

    Climatic conditions are delicately adjusted to the composition of the Earth's atmosphere. If there were a change in the atmosphere-for example, in the relative proportions of atmospheric gases-the climate would probably change also. A sight increase in water vapor, for instance, would increase the heat-retaining capacity of the atmosphere and would lead to a rise in global temperatures. In contrast, a large increase in water vapor would increase the thickness and extent of the cloud layer, reducing the amount of solar energy reaching the Earth's surface.

    The level of carbon dioxide, CO2, in the atmosphere has an important effect on climatic change. Most of the Earth's incoming energy is short- wavelength radiation, which tends to pass through atmospheric CO2 easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation, which CO2 absorbs and then remits toward the Earth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet. An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of CO2 ,whose surface temperatures have been measured at 430℃. If the CO2 content of the atmosphere is reduced, the temperature falls. Ac- cording to one respectable theory, if the atmospheric CO2 concentration were halved, the Earth would become completely covered with ice. Another equally respectable theory, however, states that a halving of the CO2 concentration would lead only to a reduction in global temperatures of 3℃.

    21. The passage supplies information for answering which of the following questions?

    (A) Why are projections of the effects of changes in water vapor levels on the climate so inaccurate?

    (B) What are the steps in the process that takes place as CO2 absorbs long- wavelength radiation?

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