¡¡¡¡tions that existed in the past£¬ paleoclimatologists invoke
¡¡¡¡four principal criteria. First£¬ the material-rocks£¬ lakes£¬
¡¡¡¡vegetation£¬ etc.-on which the method relies must be
¡¡¡¡£¨5£© 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 environ-
¡¡¡¡£¨10£© mental 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 deter-
¡¡¡¡£¨15£© mine 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 extrapola-
¡¡¡¡£¨20£© tion and interpolation. By contrast£¬ because sedimenta-
¡¡¡¡tion is much less continuous in continental regions£¬ esti-
¡¡¡¡mating 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
¡¡¡¡£¨25£© 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
¡¡¡¡£¨30£© distances between them reduce the possibilities for corre-
¡¡¡¡lation. Moreover£¬ since lake levels are controlled by rates
¡¡¡¡of evaporation as well as by precipitation£¬ the interpreta-
¡¡¡¡tion of such levels is ambiguous. For instance£¬ the fact
¡¡¡¡that lake levels in the semiarid southwestern United
¡¡¡¡£¨35£© 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 snow-line eleva-
¡¡¡¡tions£¬ however£¬ it has been concluded that the climate
¡¡¡¡then was not necessarily wetter than it is now£¬ but rather
¡¡
