Studies of past climates show unequivocally what can happen to our atmospheric environment. From climatic history we can learn several very specific lessons.
The climate can change rapidly. Major climatic changes, such as the change from glacial to non-glacial conditions, can occur in a few centuries, even though the adjustment of biota and the wasting of continental ice-sheets may take longer. Smaller changes may take only decades.
Changed climatic states may persist for centuries or millennia. While the climatic excursion of this century appears to have lasted half-a-century or so, the ‘little ice-age’ was at least three centuries long (roughly 1550–1850 A.D.), a North American drought period lasted two centuries, and there was a monsoon failure of seven centuries. These have all been recorded in the evidence of Nature.
Climatic changes may occur in different senses in different parts of the world. This paper has outlined several cases where cooling in the sub-Arctic was associated with reduced rainfall or increased frequency of failure of the monsoon, and a case in which drought in North America was concomitant with wet weather in western Europe.
The climate we think of as normal is quite abnormal by the standards of the past few centuries, and the magnitude of the change since 1945 is significant by comparison with similar changes in the past.
Combining the nature of the recent climatic change with the present narrow margin of world food-grain reserves, an urgent need to consider and react to the possibility of continued climatic variation is indicated.