detailed characteristics and distribution within smaller and smaller regions (mesoclimate and microclimate).

Generally speaking, the size of the region over which the climate (or some characteristic of climate) is systematically influenced by a given factor—whether natural or man-made—bears a rather close relationship to the geographical dimensions of the factor itself. For example, the distribution of solar radiation over the whole earth, dictated by the seasonally varying geometry of the earth’s orbit and axis of spin relative to the sun, affects climate (together with atmospheric circulation) on the scale of the whole earth. On the other hand, the climatic influence of a small lake, for example, extends over an area not much larger than the lake itself. Thus we can conclude that, in general, global-scale qualities of climate are governed by global-scale environmental factors and not by local-scale factors, except under special circumstances for which local-scale factors are operating in unison over large regions of the earth. When considering the climatic impacts of power-plant cooling towers, for example, questions of the geographical scale of such impacts arise that have to be kept in mind.

2.2
THE RECORD OF PAST CLIMATES

For more than a century it has been known from a variety of geological evidence that the earth’s climate was not always what it is today. It has been only in the last few years, however, that highly significant developments in the new and rapidly developing science of quantitative paleoclimatology have begun to provide us with insights into many details of the earth’s climatic history, especially that of the last million years. These insights lend valuable perspective to the climate of the present.

Briefly stated, the climate of the earth is now known beyond any doubt to have been in a more or less continual state of flux. Changeability is evidently a characteristic of climate on all resolvable time scales of variation, from that of aeons down to those of millennia and centuries. The lesson of history seems to be that climatic variability must be recognized and dealt with as a fundamental quality of climate and that it would be potentially perilous for modern civilization to assume that the climate of future decades and centuries will be free of similar variability.

With this lesson in mind, some salient features of the earth’s climatic history are worth outlining here.

Geological evidence leaves little doubt that, during the past billion years or so, the prevalent condition of global climate was one of relative warmth—as much as 10°C warmer than now—and almost totally free of polar ice (see Figure 2.1). T his warm condition was, however, punctuated by relatively short ice-age intervals of the order of 10 million years’ duration and separated by a few hundreds of millions of years. Begin-

FIGURE 2.1 History of glacial ages over the last 1,000,000,000 years. Intervals when extensive polar ice sheets occurred are indicated as glacial ages on the left. An outline of significant events in the Cenozoic climate decline is given in the middle, and the significant climate events during the last major glacial-interglacial cycle are given at right, Source: U.S. Committee for the Global Atmospheric Research Program, Understanding Climatic Change: A Program for Action, National Academy of Sciences, Washington, D.C., 1975.



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