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  3. In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

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问题     In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal criteria. First, the material— rocks, lakes, vegetation, etc—on which the method relies must be widespread enough to provide plenty of information, since analysis of material that is rarely encountered will not permit correlation with other regions or with other periods of geological history. Second, in the process of formation, the material must have received an environmental signal that reflects a change in climate and that can be deciphered by modern physical or chemical means. Third, at least some of the material must have retained the signal unaffected by subsequent changes in the environment. Fourth, it must be possible to determine the time at which the inferred climatic conditions held. This last criterion is more easily met in dating marine sediments, because dating of only a small number of layers in a marine sequence allows the age of other layers to be estimated fairly reliably by extrapolation and interpolation. By contrast, because sedimentation is much less continuous in continental regions, estimating the age of a continental bed from the known ages of beds above and below is more risky.
    One very old method used in the investigation of past climatic conditions involves the measurement of water levels in ancient lakes. In temperate regions, there are enough lakes for correlations between them to give us a reliable picture. In arid and semiarid regions, on the other hand, the small number of lakes and the great distances between them reduce the possibilities for correlation. Moreover, since lake levels are controlled by rates of evaporation as well as by precipitation, the interpretation of such levels is ambiguous. For instance, the fact that lake levels in the semiarid southwestern United States appear to have been higher during the last ice age than they are now was at one time attributed to increased precipitation. On the basis of snowline elevations, however, it has been concluded that the climate then was not necessarily wetter than it is now, but rather that both summers and winters were cooler, resulting in reduced evaporation.
    Another problematic method is to reconstruct former climates on the basis of pollen profiles. The type of vegetation in a specific region is determined by identifying and counting the various pollen grains found there. Although the relationship between vegetation and cli- mate is not as direct as the relationship between climate and lake levels, the method often works well in the temperate zones. In arid and semiarid regions in which there is not much vegetation, however, small changes in one or a few plant types can change the picture dramatically, making accurate correlations between neighboring areas difficult to obtain.
It can be inferred from the passage that paleoclimatologists have concluded which of the following on the basis of their study of snowline elevations in the southwestern United States?
选项 A、There is usually more precipitation during an ice age because of increased amounts of evaporation.
B、There was less precipitation during the last ice age than there is today.
C、Lake levels in the semiarid southwestern United States were lower during the last ice age than they are today.
D、During the last ice age, cooler weather led to lower lake levels than paleoclimatologists had previously assumed.
E、The high lake levels during the last ice age may have been a result of less evaporation rather than more precipitation.
答案E
解析 研究美国西南部雪线，古气候学家做出什么结论?A．冰期多雨。这是作者反对的错误结论，见L46—57的论述。B．上个冰期比现在降雨少。文中只说不一定比现在更湿润L53—55，推不出此选项。C．湖面在上个冰期比现在低。和原文L46—51叙述正相反。D．同上“lower lake levels”说反了。E．正确。上一个冰期的高水位可能是低蒸发量的结果，而不是更多降水。见L51—57。
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