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  3. Climatic conditions are delicately adjusted to the composition of the Earth’s atmosphere. If there were a change in the atmosphe

Climatic conditions are delicately adjusted to the composition of the Earth’s atmosphere. If there were a change in the atmosphe

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问题     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 atmosphere gases--the climate would probably change also. A slight increase in water vapor, for instance, would increase the hem-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 mount of solar energy reaching the Earth’ surface.
    The level of carbon dioxide, CO2 in the atmosphere has 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 a 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 the 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. According 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℃.
    If, because of an increase in forest fires or volcanic activity, the CO2 content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relies on both the warmth and the availability of CO2, would probably increase. As a consequence, plants would use more and more CO2. Eventually CO2 levels would diminish and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO2 would thereby be returned to the atmosphere and gradually the temperatures would rise again. Thus, if this process occurred, there might be a long-term oscillation in the amount of CO2 present in the atmosphere, with regular temperature increase and decrease of a set magnitude.
    Some climatologists argue that the burning of fossil fuels has raised the level of CO2 in the atmosphere and has caused a global temperature rise of at least 1℃. But a supposed global temperature rise of 1℃ may in reality be only several regional temperature increase, restricted to areas where there are many meteorological stations and caused simply by shifts in the pattern of atmospheric circulation. Other areas, for example the Southern Hemisphere oceanic zone, may be experiencing an equivalent temperature decrease that is unrecognized because of the shortage of meteorological recording stations.
The author refers to the Venus primarily in order to ______.
选项 A、show the inherent weakness of the greenhouse effect theory
B、show that the greenhouse effect works on other planets but not on the earth
C、show the extent to which Earth’s atmosphere differs from that of the Venus
D、support the argument that the CO2 level in the atmosphere has a significant effect on climate
答案D
解析 本题的四个选项中，只有D项为正确答案。这可从文中第二段的第一句话“The level of carbon dioxide，CO2，in the atmosphere has important effect On climatic change．”推知，即大气中CO2低含量会对气候造成重大影响是作者提到的论点。作者提到金星，主要是为了证明这一论点。
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