Our short memory causes us to think of our climate as dependable. It has been winter and it shall be spring; from year to year the differences are small and transient. This is the seasonal cycle of climate. We shall meet many climate cycles, but only this one is fully understood. Other climate changes are not so obviously cyclic. Not long ago the climatic backdrop to human history was not like today at all. I do not refer here to the glacial world of prehistoric cave dwellers, but to Viking days, to the golden 17th century, to Manifest Destiny and the Oregon Trail only a century and a half ago.

Climate varies on many scales, and the smaller variations close to us help us understand the large swings of the geological past. What do they do, what might they mean and, as far as we so far understand it, how do they come about?

THE INCONSTANT CLIMATE

We derive our confidence that the climate will not change much from its constancy and generally favorable state in the first half of this century. Before 1920 drought and famine could be counted on to devastate India on the average once every 8.5 years. That risk dropped to half between 1920 and 1960, and the population, previously controlled largely by famine, rose accordingly. Early in this century Californians expected a truly dry winter about once in seven years, but for the next 50 years droughts arrived less than half as often.