Climate through the Ages: A Study of the Climatic Factors and Their Variations

Climate through the Ages: A Study of the Climatic Factors and Their Variations

Climate through the Ages: A Study of the Climatic Factors and Their Variations

Climate through the Ages: A Study of the Climatic Factors and Their Variations

Excerpt

During some hundreds of millions of years with which we are acquainted through the records of the rocks, the surface of the earth has passed through some strange climatic vicissitudes. At least three, probably four times in its history great ice-sheets have spread out from various centres, covering the plains and even filling the shallow continental seas. There have been other periods of somewhat less intense climatic stress, when perhaps only small glaciers were able to develop among the high hills. These strenuous episodes have been of great importance in the development of the earth's living beings, but always they have been brief, and always after them the earth has returned to more genial conditions, which have endured for long periods. Hence we may regard these genial conditions as the normal state of affairs on this our earth, and the glacial periods as episodes disturbing the normal climate for a brief time, as at long intervals a passing cyclone disturbs the peaceful life of a tropical island. Many references to these warm periods will occur in the next few chapters, and it is necessary to have some preliminary knowledge of the climatic conditions which characterised them. I am therefore beginning this study, not with the stir and strife of an ice-age, but with the everyday life of the genial periods. We are not yet in a position to discuss their meteorology, but we can see what the geologists have to say about their general climate.

The first point to notice about these periods, however, is not climatic, although we shall see later that it has a very great bcaring on their climatology; it concerns the absence of mountain ranges. Warm periods were without exception periods of low relief, while glacial . . .

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