250 



SCIENCE 



[N. S. Vol. XXXI. No. 790 



PRINCIPLES EELATING TO CLIMATE 



The history of past climates affords 

 problems which are among the most ob- 

 scure of paleogeography. 



On the one hand climate at any particu- 

 lar epoch has been determined by the dis- 

 tribution of land and sea and the corre- 

 sponding movements of the winds and 

 positions of the great cyclonic and anti- 

 cyclonic centers. Given a map showing the 

 oceans and lands of the northern or south- 

 ern hemisphere, the climatologist may 

 apply the principles deduced from the 

 present relations of atmospheric activity 

 to surfaces that affect the temperature of 

 the atmosphere in different degrees and he 

 may arrive at a reasonable conclusion in 

 regard to the distribution of temperatures 

 and precipitation. 



Such a conclusion is based, however, 

 upon existing climatic conditions and can 

 be only a rough qualitative approximation 

 to the very different conditions of earlier 

 ages. The geologic record yields abundant 

 evidence to show that our present climates 

 are unusual in the extreme differentiation 

 of climatic zones. No previous age offers 

 evidence of equal polar refrigeration, and 

 none has as yet shown proof of deserts of 

 equal extent and general distribution. On 

 the contrary, it would seem that climate in 

 the past has been generally more uniform 

 from pole to pole and around the earth 

 than it is now. 



We may attempt to explain this result 

 of observation by recognizing that the 

 present diversity of climates is connected 

 with extreme conditions of mountain 

 growth. Mountain ranges are to-day more 

 general and of greater altitude than they 

 have commonly been in the past and the 

 condition of the low lands, which has at 

 times prevailed over the greater part of the 

 continent, has been favorable to uniformity 

 just as the converse is favorable to diver- 



sity of climate. But this explanation falls 

 far short of satisfying the requirements of 

 the problem. 



We may supplement the reasoning by 

 appeal to the reversal of oceanic circula- 

 tion suggested by Chamberlin as a possi- 

 bility in view of the fact that equatorial 

 saline waters, even though warm, might 

 under certain conditions become denser 

 than fresher polar waters, even though 

 these be cold, and thus warm waters sink- 

 ing in the equatorial regions and flowing 

 toward the pole would carry with them the 

 higher temperatures of the tropics and 

 produce more genial climates in the polar 

 regions. This suggestion is extremely at- 

 tractive, and has a high degree of proba- 

 bility, particularly when we consider that 

 the present circulation of deep-lying cold 

 waters is largely due to the polar ice-caps, 

 which are themselves extraordinary fea- 

 tures. There is reason to believe that the 

 present oceanic circulation is abnormal and 

 the reversed circulation suggested by 

 Chamberlin has in past ages been the 

 normal condition. 



In recognizing the effectiveness of low 

 lands and reversed ocean currents to pro- 

 duce uniformity of climate such as the 

 geologic record requires, we arrive at a 

 working hypothesis which satisfies the im- 

 mediate condition of cei'tain climates that 

 characterized great periods of the earth's 

 history ; but we are yet far from an under- 

 standing of the processes which underlie 

 the change from one condition of climates 

 to another. There is some general cause, 

 so subtle that it has as yet eluded distinct 

 recognition, which affects the conditions of 

 climate more deeply than the local phe- 

 nomena suggest. It has been approached 

 by theories along astronomical lines and by 

 a single theory which connects climate with 

 the earth's internal forces. 



The astronomical causes may be shown 



