During the summers of 1911 and 1912 the natural course of the writer's study of the climate of the past led him to attempt to learn from the big trees at least a part of their story. During the 3,000 or 4,000 years covered by recorded human history, as I have shown in "The Pulse of Asia," "Palestine and its Transformation," and various magazine articles, the climate of western and central Asia and of the countries around the Mediterranean Sea appears to have changed. On the whole the climate seems to have grown drier, so that regions which once were fertile have now become desert. Farther north, however, or in regions which are cold and damp because of high altitude, an opposite result has apparently been produced. The relatively dry and warm conditions of the present have changed lands which once were too cold for the practice of agriculture into places where large numbers of people can live in comfort by means of that pursuit. Thus there appears to have been a change in the location of the regions best suited to human occupation. The change has not proceeded regularly, however, but in a pulsatory fashion. It seems to have been interrupted by centuries of exceptional aridity on the one hand and of exceptional moisture on the other. When these pulsations of climate are compared with the course of history a remarkable agreement is noticed. Among a mass of minor details this apparent relationship may be concealed, but the broad movements of races, the rise and fall of civilization, seem to show a degree of agreement with climatic changes so great that it scarcely seems possible to avoid the conclusion that the two are intimately related. Unfavorable conditions of climate, such as a change toward aridity in regions already none too well supplied with water, have apparently led to famines, epidemics, economic distress, the decline of trade, misgovernment, migrations, wars, and stagnation; while favorable changes have fostered exactly opposite conditions.
This theory strikes so profoundly at the roots of all historical interpretation and is of such fundamental importance in its bearing on the future of nations and of the human race as a whole that it demands most careful testing. The first step in carrying on the necessary tests is obviously to determine the exact degree of accuracy of our conclusions as to the dates and nature of climatic changes. Only when that has been done are we prepared to proceed to a fuller investigation of the relation of the changes to historic events.
After some years had been spent in a study of this great problem from various standpoints in Asia, the logical thing seemed to be to take up the same lines of work in some other continent and see how far the two agreed. Fortunately I was invited by Dr. D. T. MacDougal to cooperate with the Department of Botanical Research of the Carnegie Institution of Washington in a study of the climate of the southwestern part of the United States. In general the phenomena of ancient ruins, old strands of inclosed salt lakes, the gravel terraces of rivers, and the distribution and agriculture of the prehistoric population seemed to indicate that the climatic history of America has been the same as that of Asia. The results, however, were unsatisfactory in two respects. In the first place, previous to the time of Columbus we know almost nothing about the dates of events in America, and hence it is impossible to know whether the apparent climatic fluctuations of America agree in time with those of Asia. In the second place, a theory is a dangerous thing. Strive as he will, the author is apt to be partial to it and to interpret all that he sees in such a way as to fit his preconceived ideas. During my work in Arizona, New Mexico, and old Mexico I knew that when its results were announced critics would say, "That is all very interesting, but not convincing. You went out West expecting to find evidences of pulsatory changes of climate during historic times, and, of course, you found them. We will wait a while before we believe you."
Last Updated: 12-Feb-2007