IMPORTAISXE OF STUDYING CLIMATIC CHANGES 479 



Page 



The effect of a Permian cloud blanket in low latitudes 580 



The blanketing effect of clouds at present 581 



The Permian desert in the north 587 



Conclusion 589 



Introduction 



111 the study of glaciation and of other ancient climatic phenomena it 

 has been tacitly assumed that changes of climate in the geological past 

 have been due to causes which are not now in operation, or whose effects 

 during the past few generations have been so small that they have es- 

 caped observation. .This is a natural assumption. In the first place, the 

 phenomena of the last Glacial period, which may serve as the standard 

 example for all the main climatic changes, are so vastly greater than 

 anything that we can now observe that the mind naturally supposes them 

 to have been due to some cause of correspondingly grand proportions. 

 In the second place, the phenomena of that period have appeared to be 

 wholly different from anything now occurring. No one, for instance, has 

 hitherto described any modern occurrences corresponding to the glacial 

 accumulation of snow in Labrador and Scandinavia, or to the apparently 

 coeval aridity which gave rise to loess in the Mississippi Valley and cen- 

 tral Europe. In the thirfl place, meteorologists have insisted that their 

 records give no support to the idea of any recent changes of climate 

 greater than the little fluctuations which every one notices from decade 

 to decade. They have expanded this into the doctrine that the present 

 causes of climatic instability are not competent to produce anything more 

 than temporary variations, which disappear within a few years. In view 

 of this, geologists have apparently had no choice except to assume that 

 Glacial periods, epochs of aridity, and the other main climatic vicissi- 

 tudes of the remote past have been phenomena distinct from anything 

 that we can now actually observe. 



Geological Importance of Study of Present climatic Changes 



Two lines of recent study suggest that there may possibly be ground 

 for modifying this commonly accepted opinion. In the first place, mete- 

 orologists and climatologists have of late become more and more con- 

 vinced of the instability of all climatic conditions. Cycles of 11 and 35 

 years are now accepted by numerous students. Many phenomena in cen- 

 tral and western Asia, the American Southwest, California, North Africa, 

 and Central America seem to suggest that cycles lasting hundreds of years 



