by Sanjai PADYA

50 million years ago, the earth entered a phase of stepwise cooling with the formation of large ice sheets in eastern Antarctica. A second temperature fall occurred 11million years ago, when western Antarctica became glaciated, and a third fall took place around five million years ago, when large ice sheets formed in the northern polar regions. Around two million years ago, our climate entered the Pleistocene, an era of oscillations between long cold glacial periods (ice ages) lasting about 100,000 years and warm periods of variable length, like the one in which we have been for the last 10,000 years.

In the 1830s some climatologists first identified traces left by the flow of old glaciers (moraines) over Northern Europe and America. But the thinking then was that if ice had really existed in those places, it only meant that the earth had been colder at some time in the very distant past. The Ice Age, it was thought, must have lasted millions of years, and this had been an aberration from the equable climate in which all forms of life had evolved. Then, at the end of the nineteenth century, evidence turned up that the glacial epoch had been made up of several cycles of advance and retreat of ice sheets. There had been not just one long Ice Age a very long time ago, but a series of them. And the last one ended as recently as only 10,000 years ago.

For a better perspective, let us retrace the history of the preceding cycle. The world left the penultimate glacial period ? named the Saalian in North Western Europe and the Illinoian in North America ? to enter the warm period known as the Eemian in Northwestern Europe (and as the Sangamonian in the United States) around 130,000 years Before Present (B.P.), with a fairly rapid warming. Temperatures soon reached values two or three degrees higher than today, and moisture and perhaps rainfall were higher as well. After about 3 thousand years, this warm and moist climate suddenly (matter of decades) underwent a strong cooling, then continued to cool a little more gradually to values 10 degrees or so below the initial warm years. After a further two to three thousand years, the temperature returned to values very much like those of today. There were several rapid, short-lived cooling and warming events, but in general conditions remained relatively warm until 115,000years ago.

Beginning of the last Ice Age ? the Wisconsin

The descent into colder conditions then started again with a rapid cooling to temperatures a good deal cooler than our own, and stayed in a sort of middling range, between warm and glacial, for some 30 thousand years. Sudden changes to colder conditions ? lasting between a few centuries and a few thousand years ? continued to occur, and on the whole there was a very gradual cooling trend superimposed.

Northern forests were retreating. Snow falling in winter in the higher latitudes did not melt completely the following summer and slowly accumulated to form ice sheets, which thus grew thicker year after year. In areas not affected by ice, lower temperatures made the climate drier. Trees died out and were slowly replaced by scrub and grass that can survive in drier conditions. The drier regions in both high and low latitudes changed to semi-desert and desert.

The tropics were also generally cooler than present, and suffered from long periods of deficient rainfall. Tropical forests gradually contracted and fragmented because of the drier climate. The large Amazon forest, like many others, was reduced until it could only serve as a refuge for some plant and animal species waiting for better times. This process was continued until 70,000 years Before Present, by which time fully glacial conditions had set in. Most of northern Europe and Canada were covered by ice fields three kilometres or more in thickness. Large amounts of ocean water were used up in building the ice sheets, with the result that sea levels had gone down by over a hundred metres. Large parts of the North Sea and the English Channel were dry land, and Britain was not an island. On the other side of Eurasia, large areas of the ocean floor on both sides of the Bering Strait between Asia and North America were exposed, thus forming a wide land bridge between those two continents.

Milder conditions, with gradual cooling

Fifteen thousand years later, by around 55,000 B.P, there was another half-hearted attempt at warming, and some of the ice melted back. There followed another period in which the climate oscillated between colder and warmer phases. The transitions started with a sudden jump and each phase lasted a few centuries to a few thousand years. Then 30,000years ago, the earth?s climate entered a phase of gradual cooling. Temperatures fell, semi-deserts and deserts advanced until once again the forests had shrunk into refugia for survival. In this Late Glacial Cold stage (or Upper Pleniglacial), the ice extent and the aridity were at a maximum between 21,000 years ago and 17,000 years ago.

Around 14,000 years ago, there was a rapid global warming of the atmosphere and the oceans. The ice started melting and retreating, and forests were spreading back. The process of recovery was well under way when, 12,800 years ago, barely after about 1,200 years of a pleasant climate, all life on earth was faced with another cooling. Our ancestors the Homo Sapiens, who had been around for a little over a hundred thousand years, found themselves going back to full ice-age conditions in a matter of decades. This was a very short-lived ice age, from which the climate snapped back again around 10,500 years ago to enter the present warm period now known as the Holocene.

The Holocene and human civilisation

It would appear that soon after the temperatures of the oceans and atmosphere settled in their warm-period values man started his efforts to raise animals and grow food. The old rock and cave paintings in Southern Europe and elsewhere show that man was already familiar with abstract thinking and was communicating with other men as long as 40,000 years ago. Anthropologists have come to the conclusion that they would at that time be conversing very much as we do today. We can therefore assume that efforts at agriculture may also have been made in the warmer phases of the previous ice age, but would have been snuffed out by the frequent rapid climate changes.

The speed at which civilisation evolved gives the impression that earlier agricultural experiments had not been forgotten. Archaeologists hold that a settlement at Jericho in Babylon had grown into a small town around 9,000 years ago, not long after the thaw. About 8,500 years ago, a sudden fall of temperature did occur but did not proceed very far. By 8,000 B.P., the climate had recovered and become a little warmer than today, and until today has been subject only to changes of small amplitude

Scientists wonder whether a long span of equable climate occurring just while civilisation was developing is really a coincidence. An examination of temperature over the past 250 thousand years ? values obtained from proxy data in ocean sediments and polar ice cores - reveals that this is the first time in a quarter million years that rapid changes have been absent for such a long time. Could it be that slash and burn planting and the breeding of flocks of animals have been releasing so much carbon dioxide and methane into the atmosphere that rapid cooling events are no more possible?