82 



ANALYSIS OF THE ENVIRONMENT 



nual layers of clay interspersed with coarser 

 materials deposited on the bottom of lakes. 

 These are called varves. The finer clays 

 settle slowly in the quiet water under the 

 ice in winter, while coarser stuflF is held 

 back until spring and summer. The varves 

 in the Scandinavian lakes have been fol- 

 lowed for some 13,700 years. 



Tree rings have also been studied for the 

 light they throw on cHmatic history. As yet, 

 tree ring analysis covers a much shorter 

 period of time. Tree rings must be inter- 

 preted with care, since they represent, not 

 annual rings necessarily, but merely alter- 

 nating periods of rapid and slow growth. A 

 severe midseason drought following a good 

 growing period would produce a good 

 growth ring; another good growing season 

 in the same year would produce another 

 supposed annual ring. Also, we know that 

 damage caused by insects, lightning, fire, 

 frost, intense heat, excessive snow, sleet, 

 wind, and so forth, as well as drought, may 

 aflFect the rate of growth of trees and so 

 tend to modify the width of the rings of 

 growth (Antevs, 1938). 



Past climates can also be reconstructed in 

 part from the succession of plant types in 

 peat, from ecological evidence of the shift- 

 ing position of the tree line in mountains or 

 in the far north, from the recovery of re- 

 sistant pollen grains in bogs, from the study 

 of tools, weapons, bones, and kitchen mid- 

 dens of men. Finally, there is the brief 

 period covered by more or less trustworthy 

 human documents. 



Humphreys (1942) has a brief word to 

 say about one cause of long-time climatic 

 changes. At present the earth is nearest the 

 sun during the first week in January and 

 farthest away during the first week in July. 

 The difi^erence in distance, if long contin- 

 ued, would modify the temperature on the 

 earth about 4 degrees. If conditions were 

 reversed, as they actually were about 

 10,500 years ago and will be again in about 

 that period of time, the temperature con- 

 trast between summer and winter would 

 be definitely greater than it now is, espe- 

 cially in the northern hemisphere, which 

 contains most of the land mass of the earth. 

 Under present conditions of this long cycle, 

 winters in the northern hemisphere are 

 shorter and milder and summers are longer 

 and also milder, and the climate in general 

 is more equitable in our part of the globe 

 than would be so in any other earth posi- 



tion with respect to this motion of the peri- 

 helion and precession of the equinoxes. 



The study of climatic history since che 

 last glacial retreat, the Recent epoch ot 

 geologists, has been most pursued in 

 Europe. A frequently accepted summary of 

 the existing evidence, the so-called Blytt- 

 Sernander hypothesis, follows: The retreat 

 of the ice begun some thirty to forty thou 

 sand years ago and continued fairly steadily 

 until about 12,000 B.C.* This time of gla- 

 cial recession was followed by a sub-Arctic 

 period that lasted about 4000 years until 

 near 8000 B.C., when the ice had re- 

 treated suflBciently to allow sea water to 

 enter the then fresh- water Baltic lake. 



Then came a warmei Boreal period char- 

 acterized in the Baltic area by the develop- 

 ment of the so-called Yoldia fauna (or com- 

 munity), in which the bivalve mollusk 

 Yoldia arctica was prominent (today this 

 species is restricted to salt waters that have 

 a temperature of 0° C. or lower). On land 

 the Boreal period was marked by a north- 

 ward movement of forests. About 5000 B.C. 

 the Baltic began to support animals that 

 live today in waters warmer than those we 

 now find in the Baltic Sea. This is called the 

 Littorina period, so named for the snail that 

 is prominent in the deposits of the place 

 and time; several species of this genus now 

 inhabit the shores of the north temperate 

 ocean. This Atlantic period lasted until 

 about 3000 B.C. The climate was gener- 

 ally warm and moist; all the mountain gla- 

 ciers disappeared from Europe and from 

 much of North America. The Atlantic peri- 

 od marks the climax in amelioration to date 

 since the last glacial retreat. 



A drier sub-Boreal period followed that 

 came down to about 1000 B.C., but was 

 interrupted by floods some 300 years earlier. 

 It is supposed, according to the Blytt- 

 Sernander hypothesis, to have given way to 

 a milder sub-Atlantic period, which was 

 in typical development between about 850 

 and 300 B.C. The existence of the sub- 

 Atlantic period is questioned by some who 

 think that there has been a general deterio- 

 ration of climate from the Atlantic period 

 to the present, which, however, has been 

 interrupted by relatively small swings in 

 temperature and rainfall (Sears, 1935; 

 Trewartha, 1940). 



* Deevey (1944) follows DeGeer in giving 

 a somewhat different time scale. 



