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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. A

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

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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 low 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℃ can be observed at an altitude of 5 to 6 kilometres above the Earth. Below this altitude(called the radiating level), the temperature increases by about 6℃ per kilometre approaching the Earth’ s surface, where the average temperature is about 15℃. 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℃. This model assumes that the atmosphere’s relative humidity remains constant and the temperature decreases with altitude at a rate of 6.5℃ per kilometre. The assumption of constant relative humidity is important, because water vapour 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 vapour 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.
According to the passage, the greatest part of the solar energy that reaches the Earth is______.
选项 A、concentrated in the infrared spectrum
B、concentrated at visible wavelengths
C、absorbed by carbon dioxide molecules
D、absorbed by atmospheric water vapour
答案B
解析 细节题。根据第一段Although these molecules allow radiation at visiblewavelengths，where most of the energy of sunlight is concentrated，to pass through，theyabsorb some of the longer-wavelength，infrared emissions radiated from the Earth’ssurface，radiation that would otherwise be transmitted back into space．“尽管这些分子微粒允许可见光波长的辐射穿过，这里集中了大部分的日光能量，但是，它们吸收一些来自地球表面具有较长波长的红外线辐射，这些辐射原本是应该传回太空的。”可知大部分的日光能量集中在了可见光波长，句中sunlight和题干solar互为对应点，所以B选项表达正确。
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