Page:Popular Science Monthly Volume 76.djvu/584

580 fluctuations in the quantity of this gas may go far to explain the noteworthy glacial episodes of later and earlier geological times, and those warm climates which at other periods have spread so widely over the earth.

Thus we have geographic hypotheses, and astronomic hypotheses, so it seems appropriate that we should have atmospheric hypotheses, in the laudable effort to understand and explain that great series of geologic climates, which may indeed seem remote, but with the latest of which, we should remember, we ourselves have to do.

Our interest in the evolution of the atmosphere and of climate is of no theoretical sort. We are not in the grip of forces which are in a despotic way our masters. We have a large control of organic life on the earth and of the disposition and character of all land waters. Through these means we also largely regulate the processes of denudation and we may thus in some measure modify the very constitution of the atmosphere. Van Hise, on what he regards as a moderate estimate of the coal the human race will burn per annum during the present century, estimates that in 812 years the amount of carbon dioxide in the atmosphere would be doubled. According to the view of Arrhenius such a change would greatly ameliorate the climate of the world. This view of the heat-holding effects of an increase of CO2 is not undisputed, but so large a change in the constitution of the atmosphere, by the hand of man himself, may well cause him to investigate, with serious persistence, the terrestrial consequences of his own deeds.

Van Hise has impressively set forth the work of man in lowering the level of the ground water. We do this by deforestation, by cultivation, by irrigation, by the sinking of artesian wells and by mining. For so many purposes man needs water, or needs to get rid of water, that actual and serious lowering of the water table has taken place, and will be brought about more and more with growing density of population. Lowering the ground water, as we have seen, increases the contact of the atmosphere with the rocks, and sets in motion a chain of actions which may have consequences for good or ill, quite outside our present knowledge and inviting expert investigation for many years to come. These considerations have a bearing, possibly an imminent bearing, upon all our conservation enterprises in the United States.

Scientific truth often gives us no inkling of astonishing practical results which are about to flow from it. Thus meteorology is no restricted theme for the curious. It is not merely a science of climate, though this would give it the highest interest for science and for life: it is profoundly related to the history of our planet and it is an essential part of physical, biological and human geography.