问答题
The term "Ice Age" may give a wrong impression. (46) The epoch that spanned the 1.5 to 2.0 million years prior to the current geologic epoch was not one long continuous process, but a period of fluctuating climate with ice advances interrupted by times of climate not very different from the climate experienced now. Ice sheets that derived from an ice cap centered on northern Scandinavia reached southward to Central Europe. And beyond the margins of the ice sheets, climatic oscillations affected most of the rest of the world; for example, in the deserts, periods of wetter conditions (pluvials) contrasted with drier, interpluvial periods. (47) Although the time involved is so short, about 0.04 percent of the total age of the Earth, the amount of attention devoted has been incredibly large, probably because of its immediacy, and because the epoch largely coincides with the appearance on Earth of humans and their immediate ancestors.
There is no reliable way of dating much of the Ice Age. Geological dates are usually obtained by using the rates of decay of various radioactive elements found in minerals. Some of these rates are suitable for very old rocks but involve increasing errors when used for young rocks; others are suitable for every young rocks and errors increase rapidly in older rocks. Most of the Ice Age spans a period of time for which no element has as appropriate decay rate.
(48) Nevertheless, researchers of the Pleistocene epoch (更新世)have developed all sorts of more or less fanciful model schemes of how they would have arranged the Ice Age had they been in charge of such geological events. For example, an early classification of Alpine glaciation suggested the existence there of four glaciations. (49) This succession was based primarily on a series of deposits and events not directly related to these periods, rather than on the more usual modern method of studying biological remains found in ice beds. Yet this succession was forced willy-nilly onto the glaciated parts of Northern Europe, where there are partial successions of true glacial ground moraines and interglacial deposits, with hopes of ultimately piecing them together to provide a complete Pleistocene succession. Eradication of the Alpine nomenclature is still proving a Herculean task.
There is no conclusive evidence about the relative length, complexity, and temperatures of the various glacial and interglacial periods. We do not know whether we live in a postglacial period or an interglacial period. The chill truth seems to be that we are already past the optimum climate of postglacial time. (50) Studies of certain fossil distributions and of certain temperate plants suggest decreases of a degree or two in both summer and winter temperatures and, therefore, that we may be in the declining climatic phase leading to the Ice Age and extinction.