Ecoscience: Population, Resources, Environment

the tropics, compressing the life zones that exist between the snow line and sea level (see Chapter 4).

It is worth mentioning again that the changes producing these conditions generally may have been -- and in some regions certainly were -- quite rapid. Although one tends to think of glaciation as a slow process involving the plastic flow of glacial ice (as described earlier), there is a much faster mechanism available for the advance of glaciers during the onset of ice ages: snow falls over a large region, fails to melt, and is compressed under the weight of new snow the following winter. Thus, the area under a semipermanent cover of ice and snow can increase enormously in a single season. This process of rapid glaciation has aptly been termed the snow blitz.⁴³

The evidence suggests that the departure of the Wisconsin glaciers was even more sudden than the onset. At the peak of the warming period that followed this most recent retreat of the ice sheets, average global temperatures rose to 2° or 3° C warmer than today's, sea level rose to today's level but apparently not above it, and the prevailing circulation patterns produced considerably more rainfall in the Sahara and the eastern Mediterranean lands than occurs today.⁴⁴ These conditions, called the postglacial optimum by climatologists, occurred between 5000 and 6000 years ago.

The fact that Earth has a long history of climate change offers small consolation, unfortunately, to today's human population, faced as it is with the prospect of further change in the future. Significant climate change in any direction -- hotter, colder, drier, wetter -- in the world's major food-producing regions would be likely to disrupt food production for years, and even decades, because the animals and crops now relied upon are relatively well adapted to existing climate conditions. The recent historical record and the nearness of present conditions to a temperature maximum, moreover, suggest that the most likely major trend to occur next is cooling. This almost certainly would disrupt food production for as long as the lowered temperatures persisted, by reducing the area and growing season available for some of the most important food crops. The dependence of agriculture on climate is explored further in Chapter 7 and Chapter 11. Other ecological effects of climate change could also have serious human consequences, which are treated in Chapter 11.

How and when human activities could themselves cause, accelerate, or prevent climate change is a complicated and imperfectly understood subject, which is also postponed until Chapter 11. What is most relevant to that discussion from the foregoing treatment of the machinery of climate and natural climate change are the complexity and probable instability of the patterns of energy flow in the ocean-atmosphere system, and the speed with which changes, once triggered, may spread and intensify.

Cloud, Preston, ed. 1970. Adventures in earth history. W. H. Freeman and Company, San Francisco. Many classic papers in the earth sciences.

Kuenen, P. H. 1963. Realms of water. Wiley, New York. Remarkably entertaining and informative treatment of water in the oceans, on land, and in the atmosphere. Splended sections on glaciers and sea ice.

Menard, H. W. 1974. Geology, resources, and society. W. H. Freeman and Company, San Francisco. Very readable text, especially good on recent climatic history.

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