THE GENERAL ENVIRONMENT 



81 



low average level of land masses, and wide- 

 spread mild temperatures have character- 

 ized the earth during most of geological 

 time. Seas were more extensive and some- 

 what warmer, and the Arctic Ocean was 

 ice-free even in winter. Precipitation was 

 probably less, but thanks to the higher tem- 

 perature of the greater proportion of water 



The Pleistocene ice age is of more direct 

 importance for present day ecology than 

 are the several major glaciations of long- 

 past geological eras. The absence of a 

 Pleistocene "continental" glacier from 

 Siberia and much of Alaska not only af- 

 fected biotic distribution at the time, but 

 has had important influence upon the loca- 



Lower 

 Proterozoic 



Upper 

 Pro'crozoic 



Hercynian 



Alpine 



\ Climate 



Upper 

 Carboniferous 



Quaternary 



Fig. 4. Periods of mountain building and glaciation through the ages. ( Redrawn from Brooks. ) 



surface, the humidity of the air was higher. 

 If we can trust the generahzations based on 

 a correlation of red soil and salt deposits 

 with aridity, extensive midcontinental des- 

 erts were also characteristic. On this point 

 there is a controversy, and perhaps we may 

 think of these early deserts as being of a ra- 

 ther mild variety. The intense aridity of 

 modern deserts seems to be associated with 

 the high-standing land masses and the zonal 

 climate to be found in periods of geological 

 revolutions. 



During the more usual conditions the 

 land areas of the earth probably had a cli- 

 mate much like that of present day tropical 

 lowlands, with forests along the coasts and 

 tropical grasslands in the interior. Toward 

 the poles, that is, above 55 to 60 degrees 

 north latitude, climatic zones became evi- 

 dent, but the shores of the perenniallv open 

 Arctic and Antarctic Oceans experienced 

 only mild winters. 



The change from a normal geologic cli- 

 mate to a glacial one is marked for prac- 

 tical purposes by the formation of a polar 

 ice cap. An increase on the order of 1.1° C. 

 in the general temperature of the earth to- 

 day would eventually make the whole 

 Arctic ice mass unstable in summer, and, 

 if long continued, would probably clear the 

 Arctic seas of ice. Brooks has calculated 

 that an initial change of about 3° C. at 

 the critical temperature at latitude 50 de- 

 grees north would make the diflFerence be- 

 tween a nonglacial and glacial climate 

 (Fig. 5). 



tion of many plants and animals today. This 

 last glaciation was marked by four or five 

 main advances of the ice with intervening 

 interglacial periods. In the 30,000 to 

 40,000 years since the last ice retreated 

 from low-lying regions in the middle lati- 

 tudes of North America and Europe, the 

 climate of the northern hemisphere has not 

 shown a steady trend toward amelioration. 

 The record is read, in part, from the an- 



LATITUDE, DEGREES 



Fig. 5. Temperature di£Ference between non- 

 glacial and glacial climates. (Redrawn from 

 Brooks. ) 



