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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.
The author discusses lake levels in the southwestern United States in order to
选项 A、illustrate the mechanics of the relationship between lake level, evaporation, and precipitation.
B、provide an example of the uncertainty involved in interpreting lake levels.
C、prove that there are not enough ancient lakes with which to make accurate correlations.
D、explain the effects of increased rates of evaporation on levels of precipitation.
E、suggest that snowline elevations are invariably more accurate than lake levels in determining rates of precipitation at various points in the past.
答案B
解析 作者为什么讨论美国西南部湖水位问题?A．讲述水位、蒸发、降水间关系。过具体，文中讨论的中心问题是古气候研究的方法。B．正确。提供分析湖水位方法不确定性的一个例证。即原文L45—46“the interpreta—tion of such levels is ambiguous”。C.“not enough ancient lakes”不是这个例子所说内容。D．蒸发增加对降水影响。无。E．雪线比湖水位要更精确。无。文中提到雪线只是说明湖水位测定有误差，而并没有把它提升到与测湖水位法平等的地位上。
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