THE GENERAL ENVIRONMENT 



83 



Minor fluctuations of climate continued. 

 In the first century A.D., climatic condi- 

 tions were similar to those found today. 

 From near the end of the second to the 

 middle of the fourth centuries, the climate 

 was wet. The fifth century was dry, and 

 the seventh was both dry and warm, so 

 that passes in the Alps were in use that are 

 now closed by glaciers. Heavier rainfall 

 came in Europe near the start of the ninth 

 century, but Nile floods were low until 

 about 1000 A.D. 



Warmer, drier conditions returned to Eu- 

 rope during the tenth and eleventh cen- 

 turies. Greenland was settled in 984 A.D. 

 and was abandoned at the beginning of 

 the fifteenth century. During that period its 

 cUmate is generally thought to have been 

 milder than it is today. In Europe, the thir- 

 teenth and fourteenth centuries were cold 

 and wet. Amelioration must have set in, for 

 the glaciers of Chamonix were small in 

 1580, but advanced rapidly until the 

 middle of the seventeenth century; then a 

 retreat began that lasted until 1770, when 

 they again advanced up until the middle 

 of the last century. Since that time the 

 glaciers have retreated approximately to 

 the positions held in the sixteenth century 

 (Brooks, 1926; Russell, 1941). 



The latter part of this somewhat detailed 

 summary is often condensed as follows: 



1. The Boreal period: warm, dry, continental 

 climate; birch and pine were dominant trees. 



2. The Atlantic period: warmer, moist, 

 oceanic climate; oaks were dominant trees. 



3. The sub-Boreal period: warm, dry con- 

 tinental climate; oaks continuing dominant. 



4. The sub-Atlantic period: cool, very wet, 

 oceanic climate; beech and spruce were dom- 

 inant trees (Clements and Chaney, 1936). 



The scheme may be still more simplified 

 to give only three stages (von Post's hy- 

 pothesis) of postglacial climates, namely: 



1. A period of increasing warmth, 



2. A period of maximum temperature, and 



3. A period of fluctuating, but, on the whole, 

 decreasing temperature. 



Climates in other parts of the world may 

 or may not follow the European pattern. 

 The climatic sequence in eastern North 

 America can be correlated in a general way 

 with that of Europe. The correlation is as 

 close as could well be expected, since east- 

 ern North America gets its climate from 

 the interior, while, in contrast, western 



Europe is under strong marine influence. In 

 addition, European climates have been 

 much affected by the complicated history 

 of the Baltic Sea. The three stages of the 

 relatively simple von Post's hypothesis cor- 

 respond fairly well on the two sides of the 

 North Atlantic, and perhaps a still closer 

 correlation exists, as shown in depth pro- 

 files of pollen preserved in bogs; this is out- 

 hned in Table 4. 



Table 4. Possible Climatic Correlation between 



Western Europe and Eastern North America 



(From Deevey, 1944, after Sears) 



Shifts in the location of the tree line to 

 the south of the tundra and in mountains 

 also gives evidence of general climatic 

 trends. According to this criterion, there 

 seems to be evidence that, at present, trees 

 are advancing in Alaska, retreating in 

 southeastern Mackenzie, and apparently re- 

 treating in eastern Canada. The resulting 

 picture of current trends is by no means 

 clear (Raup, 1941). 



Two generalizations stand out as a result 

 of this hasty review of past climates. The 

 first is the reiterated statement that the 

 present zonal climate, which our experience 

 and records indicate is normal, is highly 

 unusual when viewed with geological per- 

 spective. Through long geological eras there 

 has been climatic cosmopolitanism rather 

 than present day climatic provincialism. The 

 second generalization, a corollary of the 

 first, is to the effect that modern cUmates 



