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  3. The molecules of carbon dioxide in the Earth’s atmosphere affect the heat balance of the Earth by acting as a one-way screen. Al

The molecules of carbon dioxide in the Earth’s atmosphere affect the heat balance of the Earth by acting as a one-way screen. Al

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问题     The molecules of carbon dioxide in the Earth’s atmosphere affect the heat balance of the Earth by acting as a one-way screen. Although these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concentrated, to pass through, they absorb some of the longer-wavelength , infrared emissions radiated from the Earth’s surface, radiation that would otherwise be transmitted back into space. For the Earth to maintain a constant average temperature, such emissions from the planet must balance incoming solar radiation. If there were no carbon dioxide in the atmosphere, heat would escape from the Earth much more easily. The surface temperature would be so much lower that the oceans might be a solid mass of ice.
    Today, however, the potential problem is too much carbon dioxide. The burning of fossil fuels and the clearing of forests have increased atmospheric carbon dioxide by about 15 percent in the last hundred years, and we continue to add carbon dioxide to the atmosphere. Could the increase in carbon dioxide cause a global rise in average temperature, and could such a rise have serious consequences for human society? Mathematical models that allow us to calculate the rise in temperature as a function of the increase indicate that the answer is probably yes.
    Under present conditions a temperature of - 18°C can be observed at an altitude of 5 to 6 kilometers above the Earth.Below this altitude (called the radiating level) , the temperature increasesby about 6°C per kilometer approaching the Earth’s surface, where the average temperature is about 15°C. An increase in the amount of carbon dioxide means that there are more molecules of carbon dioxide to absorb infrared radiation. As the capacity of the atmosphere to absorb infrared radiation increases, the radiating level and the temperature of the surface must rise. One mathematical model predicts that doubling the atmospheric carbon dioxide would raise the global mean surface temperature by 2.5°C： This model assumes that the atmosphere’s relative humidity remains constant and the temperature decreases with altitude at a rate of 6. 5 °C per kilometer. The assumption of constant relative humidity is important, because water vapor in the atmosphere is another efficient absorber of radiation at infrared wavelengths. Because warm air can hold more moisture than cool air, the relative humidity will be constant only if the amount of water vapor in the atmosphere increases as the temperature rises. Therefore, more infrared radiation would be absorbed and reradiated back to the Earth’s surface. The resultant warming at the surface could be expected to melt snow and ice, reducing the Earth’s reflectivity. More solar radiation would then be absorbed, leading to a further increase in temperature.
The primary purpose of the passage is to______.
选项 A、warn of the dangers of continued burning of fossil fuels
B、discuss the significance of the amount of carbon dioxide in the atmosphere
C、demonstrate the usefulness of mathematical models in predicting long-range climatic change
D、describe the ways in which various atmospheric and climatic conditions contribute to the Earth’s weather
答案B
解析 文章第一段讲述了大气中二氧化碳的存在对于保持地表温度平衡的重要作用，接着讲述了如今由于化石燃料的燃烧，使大气中的二氧化碳增多，从而对全球气温以及人类社会的影响，主要是为了强调二氧化碳在大气中的重要作用，因此B项最符合题意。A、C、D三项在文中都有所体现，但都不是文章主旨，因此不选。
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