阅读下面这篇短文，短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为第2～5段每段选择1个正确的小标题；(2)第27~30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。请将答案涂在答题卡相应的位置上。
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 visi-ble 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 in- coming solar radiation. If there were no carbon dioxide in the atmosphere, heat would es-cape 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 the forests have increased atmospheric carbon dioxide by a- bout 15 percent in the 1st hundred years, and we continue to add carbon dioxide to the at- mosphere. 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 kilometers above the Earth. Below this altitude (called the radiating level), the tem- perature increases by 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 capaci- ty of the atmosphere to absorb infrared radiation increase, the radiating level and the. tem- perature 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 atmos- phere '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. There- fore, more infrared radiation would be absorbed and reradiated back to the Earth's sur- face. The resultant warming at the surface could be expected to melt snow and ice, reduc- ing the Earth's reflectivity. More solar radiation would then be absorbed, leading to a fur-ther increase in temperature.
23. Paragraph 1
24. Paragraph 3
25. Paragraph 4
26. Paragraph 5
A. The prediction of the model
B. The influence on temperature
C. The effect of carbon dioxide
D. The potential problem
E. The import of the assumption
F. The serious consequences
27. Nowadays, too much carbon dioxide
28. The radiating level and the temperature of the surface must rise because of the
29. Only if the amount of water vapor in the atmosphere increases as the temperature rises, the atmosphere' s relative humidity
30. An increase in the amount of carbon dioxide means that there are more molecules of carbon dioxide
A. will be constant
B. to absorb infrared radiation
C. is becoming the potential problem
D. prepare ourselves to be chemists, physicist
E. increasing capacity of the atmosphere to absorb infrared radiation.
F. escape from the earth much more easily
参考译文
二氧化碳分子在地球大气中起着单向屏障的作用，影响了地球的热平衡。尽管这些分子微粒允许可见光波长的辐射穿过(这些分子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。为了使地球保持稳定的平均温度，这种来自地球的辐射必须与射入的太阳辐射保持平衡。假设大气中没有二氧化碳，热量将会更加容易地从地球上散失，地表温度将会非常低，以致海洋可能变成固体——冰块。
然而，今天潜在的问题是二氧化碳过多。矿物燃料的燃烧和森林的砍伐使大气中的二氧化碳在第一个百年中增加了大约15％，而且，我们不断地向大气中排放二氧化碳。二氧化碳的增加会导致全球的平均温度升高吗?这种上升会给人类社会带来严重后果吗?数字模型使我们得以计算二氧化碳增加与气温升高的关系，结果表明，答案很可能是肯定的。
在目前的环境下，在海拔五六千米的高度可以观测到一18℃的气温，这一高度被称作辐射水平面。在这一高度以下，距离地面每下降一千米温度就升高6℃，到达地面的时平均温度约为15℃。二氧化碳数量的增加就意味着有更多二氧化碳分子吸收红外线辐射。当大气吸收红外辐射的容量增加时，地面的辐射水平和温度也必定增加。
数字模型预言大气中增加一倍的二氧化碳将会使全球的平均地表温度升高2.5℃。这一模型假定大气的相对湿度保持不变，温度随着海拔升高以每千米6．5℃的比率降低。假定相对湿度恒定很重要，因为大气中的水蒸气是红外线波长的辐射的另一高效的吸收体。因为热空气比冷空气会含有更多湿气，只有当大气中水蒸气含量随着温度的升高而增加时，相对湿度才是恒定的。因此，更多的红外辐射会被吸收并且散射回地球表面。在地面上，气候变暖的结果是使冰雪熔化，地球的反射率由此降低了，结果能够吸收更多的辐射，气温也进一步上升
责任编辑：sealion1986
