296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



the evidence of fluctuations furnished by existing lakes, glaciers, tree 

 rings, etc., and the available records of measured precipitation and 

 stream flow, we stand impressed by the great and irregular climatic 

 variations of the immediate past. 



Thus at the end of the disastrous drought year of 1936, we in tliis 

 country look into the future somewhat bewildered and almost afraid, 

 the more so because we recognize that much of the productive part 

 of our country is not very far from the margin of semiaridity. Wo 

 must frankly admit that in spite of all our investigation we do not 

 know in which direction we are trending — toward greater humidity 

 or more severe droughts — in the ensuing year, decade, or century. 

 We can, however, make some predictions, wliich are in part reassuring 

 and in part otherwise. It is virtually certain that drought conditions 

 are not permanent but will be followed by years of abundant rainfall 

 and bounteous crops; on the other hand, it is almost equally certain 

 that the recent droughts are not abnormal but that in the course of 

 time other droughts of equal and even greater severity may be 

 expected. 



It is believed that the climatic fluctuations of the past have been 

 the underlying cause of much turmoil in human history. But it is 

 significant that the human race has not only managed to live through 

 the times of drought and the intervening cold and wet times, but also 

 that it has had its notable evolution in this Quaternary period of 

 strongly fluctuating climate. The climate of the present, as the 

 climate of the past, challenges man to greater effort and achievement. 



The possible influence of artificial changes vpon precipitation. — 

 Wlien, during periods of wet j'^ears, the settlers moved into the semi- 

 arid region of our country and found to their delight that they could 

 raise good crops, they fondly developed the faith that rainfall follows 

 the plow. Now, after a series of years of drought and crop failure this 

 faith has been sorely tried; and we are tormented with the fear that 

 on account of the acts of man in plowing and draining, our country 

 is rapidly becoming a desert. In the presence of such intense pubhc 

 concern it is difficult to maintain a wholly judicial attitude. It is 

 reasonable to expect that fluctuations in humidity such as are known 

 to have characterized the past, should also occur, through wholly 

 natural causes, in the present and future. This logical inference, how- 

 ever, does not afford any reason for assuming that the very extensive 

 and radical changes which have been made by the white men on the 

 face of our country have produced no effects toward greater humidity 

 or aridity. Neither may it be assumed that such effects can be of no 

 practical consequence if they are obscured by natural fluctuations. 



If the average annual contribution to the precipitation upon a 

 continent from water evaporated out of the sea remains the same, 

 then the drainage of swamps should reduce the average annual pre- 



