Additional Reading Climatic Change

Weather we expect to vary, both from day to day and from season to season. Nor are we surprised when one year has a colder winter or a drier summer than the one before. Less familiar are changes in climate. Even though climate represents averages in weather conditions over periods of, say, 20 or 30 years, there is abundant evidence that it, too, is not constant but instead undergoes quite marked fluctuations over long spans of time. The most dramatic such fluctuations were the ice ages of the past.

The last ice age reached its peak about 20,000 years ago when huge ice sheets hundreds of meters thick in places covered much of Europe and North America. Then the ice began to retreat and climates became progressively less severe; in a period of 12,000 years the average annual temperature of central Europe rose from -4°C to +9°C (24°F to 48°F). By about 6,000 years ago average temperatures were a few degrees higher than those of today. A time of declining temperatures then set in, reaching a minimum in Europe between 2,500 and 3,000 years ago.

A gradual warming up followed that came to a peak between 1,200 and 800 years ago; so generally fine were climatic conditions then that the Vikings established flourishing colonies in Iceland and Greenland from which they went on to visit North America. The subsequent deterioration led to cool summers, exceptionally cold winters, and extensive freezing of the Arctic Sea from 700 to 300 years ago. So extreme was the weather about 350 years ago that it has been called the "Little Ice Age." Greenland became a much less attractive place than formerly and the colony there disappeared, the coast of Iceland was surrounded by ice for several months per year (in contrast to a few weeks per year today), and glaciers advanced farther across alpine landscapes than ever before or since in recorded history.

During the last century a trend toward higher temperatures became evident which has led to a marked shrinkage of the world's glaciers. In the first half of this century especially pronounced temperature increases took place whose most noticeable consequences were milder winters in the higher latitudes. In Spitzbergen, for instance, January temperatures averaged from 1920 to 1940 were nearly 8°C (14°F) higher than those averaged from 1900 to 1920, and Greenland became less inhospitable than before. \ Alas, these balmy conditions seem to have peaked about 1945, and since then the worldwide average annual temperature has been falling steadily (Fig. 5.13). The total drop in the past 30 years has been less than 0.5°C, which does not seem like very much, but the effects have been dramatic. What has happened has been a shift toward the equator of the various wind and climatic zones. In the Northern Hemisphere this shift has had a variety of effects. Siberia is growing colder as the polar front moves south. The northern rim of Africa, formerly in the dry zone of the horse latitudes, now receives unaccustomed rain as the cyclonic weather systems of the westerlies sometimes sweep over it. The horse latitudes have moved farther south, depriving vast areas of sub-Sahara Africa, the Middle East, India, and southern Asia of the moist tropical air that formerly brought them abundant rain. Famines have been the result. In North America, the pattern of air flow has changed so as to bring colder winters and more precipitation to western states.