Before describing the climatic changes that occurred during the Pleistocene or Quaternary Ice Age, it must be pointed out that the earth’s geological history is divided into four main periods: Pre-Cambrian (Archaeozoic and Proterzoic); Palezoic; Mesozoic; and Cenozoic (Tertiary and Quaternary). The form of life that existed in different periods of geological time.

A persual of makes it clears that the geological record opens with the archaezoic era. Before that period there had been no record that can be deciphered. However, there is evidence of many variations and fluctuations of climate.

In view of the limited space, the present discussion has been confined to the climatic changes which took place after the appearance of man roughly a million years ago, at the beginning of the Quaternary.

Quaternary Ice Age:

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The Quaternary or Pleistocene Ice Age witnessed the advance of huge glaciers or ice-sheets from different centers. In North America, great glaciers spread out from centers near the Hudson Bay which buried all of eastern Canada, New England, and much of the Middle West under a thick sheet of ice estimated to be one and a half kilometer thick.

Other such centers were located in the Canadian Rockies and the Cordilleras in western North America. Glaciers spreading out from these centers covered Alaska, Western Canada, Washington, Idaho, and Montana.

The most important ice centers in Europe were located in Scandinavia and the Alps. Glaciers from Scandinavia and Scotland engulfed most of Great Britain, Denmark, and parts of northern Germany.

Besides, a smaller icecap centered on the Alps covered Switzerland, Austria, Italy, France and Germany. Minor centers of glaciations were located in Spitzbergen, Iceland, Ireland, Scotland, and northern England, the Pyrennes, the Caucasian range, the Himalayas and the mountain ranges of Central Asia.

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In the southern hemisphere, small ice sheets developed over Argentina, New Zealand, and parts of Australia. The highest mountains of equatorial Africa and New Guinea were also covered by glaciers and ice sheets.

In all, the ice covered about 13 million square miles of land where there is no ice today. Of the total area occupied by land ice, there were 5 million square miles in the Antarctic, 4,500,000 sq. miles in North America, 1,250,000 sq. miles in Europe and least as much in Asia, and more than 8,00,000 sq. miles in Greenland.

At present 6,000,000 square miles of land is occupied by ice. In addition to glaciers and ice sheets encroaching upon the land, there were great extensions of ice sheets which floated in the North Atlantic and Antarctic.

On the whole, about 10% of the earth’s surface must have been covered with ice. The actual sea level must have lowered by about 260 feet.

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Pluvial periods:

During the glacial periods the precipitation is estimated to be greater than during the interglacial periods in the tropical and subtropical regions. The lakes of the Great Basin in North America spread out to form large inland seas.

In East Africa a large chain of lakes are correlated with the Alpine glaciations its different phases: Kafuan is connected with Gunz and Mindel, Kamasian with the Riss, and Gambian with the three maximums of the Wurm. The pluvial periods were separated by interpluvial periods when most of the lakes dried up.

Postglacial period:

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There were alternating periods of rainy and dry climates during the postglacial period. This is clearly indicated by the alternation of rapid growth and drying up of trees, which phenomena are borne out in peat bog sections.

Investigations and researches regarding the succession of rainy and dry periods were done by Norwegian Axel Blytt, G. Erdtman and H. Godwin.

The rainy Atlantic period was followed by the sub-boreal which was rather cooler and drier with long droughts making the surface of the peat dry. However, this was followed by the return of more rainy conditions.

To trace out the climatic history of the postglacial period, scientists have placed much reliance on verve analysis, pollen analysis, archaeological and historical records and radio-carbon dating.

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Even the direct measurement of temperature and precipitation has been taken for the most recent centuries to work out the past climatic conditions.

It is now commonly agreed that the main dry periods are dated about 2200-1900, 1200-1000 and 700-500 B.C. The last dry period was so developed that scientists prefer to describe it as a dry heat wave lasting for about two centuries.

As a result of these dry periods, the lakes shrank in area, and in certain localities trees grew on dry lake floors below the outlet. From a detailed geological study of certain lakes in Ireland, Germany and Austria, it may be concluded that the rainfall must have been about 50% of what it has been in the recent past.

However, the steady development of forests was not interrupted because the droughts were not so severe. No doubt, it led to large-scale migration of peoples from drier to more humid sites. This dry period ended at about 500 B.C. Soon afterwards there was a rapid change in climatic conditions.

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Sphagnum bogs grew rapidly and the lake settlements were flooded. This is called the sub-Atlantic period. It may be pointed out that many of the conclusions are based on inferences and are open to challenge.

However, because of the scientific techniques employed in deciphering the past climates, there is ample scope for cross-checking and confirmation.

The difficulty of the last lies in the fact that those who can judge the evidence (biologists, geologists) cannot judge the conclusions, and those who can judge the conclusions (meteorologists), cannot judge the evidence.