PART III — CLIMATIC CHANGE 



numbers and control of energy. Thus 

 man can, and probably has, modified 

 the climate of the earth. 



The Climates of the 

 Past Century 



From late in the nineteenth century 

 until the middle of the twentieth, the 

 mean temperature of the earth rose. 

 During this time the carbon dioxide 

 content of the atmosphere rose enough 

 to explain the global temperature rise 

 — apparently the first global climatic 

 modification due to man. At the same 

 time, local production of particulate 

 pollution was starting to increase 

 rapidly due to mechanization and 

 industrialization. By the middle of the 

 twentieth century, these trends — 

 amplified by a general population ex- 

 plosion and a renewal of volcanic 

 activity — increased the worldwide 

 particulate load of the atmosphere to 

 the point where the effect of these 

 particulates on the global albedo more 

 than compensated for the carbon di- 

 oxide increase and world temperatures 

 began to fall. 



The total magnitude of these 

 changes in world or hemispheric mean 

 temperature is not impressive — a 

 fraction of a degree. However, the 

 difference between glacial and non- 

 glacial climates is only a few degrees 

 on the worldwide average. 



Actually, it is not the mean tem- 

 perature of the earth that is impor- 

 tant, but rather the circulation pattern 

 of the atmosphere. This is stronglv 

 dependent on the temperature differ- 

 ence from the tropics to the poles. 

 The same man-modifiable factors that 

 affect the mean temperature of the 

 globe-albedo and carbon dioxide — 

 even if applied uniformly over the 

 globe — will have the effect of chang- 

 ing the meridional temperature gradi- 

 ent and thus the circulation pattern 

 and resultant weather pattern. It is 

 this change of pattern that is of prime 

 concern. Dzeerdzeerski in the Soviet 

 Union, Kutzbach in the United States, 



and Lamb in England have all pro- 

 duced different kinds of evidence that 

 the circulation patterns have changed 

 in the past two decades. In turn, the 

 local climates show change — some 

 regions wetter, some drier, some 

 colder, some warmer — though some 

 remain unchanged. 



The most striking changes have 

 been where the effects of the change 

 are cumulative, such as the slightly 

 changed balance between evaporation 

 and precipitation in East Africa which 

 has caused the level of great lakes 

 such as Victoria to rise markedly. 

 Another case is the balance between 

 ice wastage and production that has 

 changed enough in the last decade to 

 bring drift-ice to the Icelandic shores 

 to an extent unknown for a century. 

 It would be most useful to know what 

 the cumulative ecological effect of 

 these local or regional changes might 

 be. Since biological selection in re- 

 sponse to environmental changes usu- 

 ally requires a number of generations 

 to show the total effect of the change, 

 it is probably too soon to know the 

 total ecological impact of the present 

 change. Here we can only look to the 

 past to see what is possible. 



The Lesson of History 



The advent of radiocarbon dating 

 has given a new dimension to the 

 study of the variety of paleobotany 

 known as palynology. It is now pos- 

 sible to put an absolute time-scale on 

 the record of environmental change 

 contained in the pollen assemblages 

 recovered from bogs and lake sedi- 

 ments. In the context of the present 

 discussion, the most startling result 

 is the rapidity with which major envi- 

 ronmental changes have taken place. 



If we examine the most carefully 

 studied and best-dated pollen profiles, 

 we find that the pollen frequencies 

 often show a quasi-exponential change 

 from, for example, an assemblage 

 that might indicate boreal forest to an 

 assemblage typical of mixed hard- 



woods. Calling the time required for 

 half the change to occur the half-life 

 of the transition, it appears that such 

 major changes in vegetation may have 

 half-lives of a couple of centuries or 

 less. (Greater specificity must await 

 analyses with much finer time-resolu- 

 tion than has been generally used.) 

 Since the plants integrate the climate, 

 the half-life of the climatic change 

 must be shorter still! 



With the agricultural land use of 

 the world still reflecting the climatic 

 pattern almost as closely as the native 

 vegetation did, a major shift in cli- 

 matic pattern within a century could 

 be disastrous. Unlike the past, migra- 

 tion into open lands is not possible: 

 there are none, and forcible acquisi- 

 tion of agricultural land with a favor- 

 able climate is not acceptable. Only 

 in a few nations would a combination 

 of regional variety and advanced tech- 

 nology allow an accommodation to a 

 major climatic change. 



What We Need To Know 



Faced with the possibility that we 

 are well into a climatic change of ap- 

 preciable magnitude, of man's mak- 

 ing, there appear a number of ques- 

 tions to which answers are urgently 

 needed. 



Since in the past there have been 

 rapid changes in climate due to natural 

 causes, such as major changes in vol- 

 canic activity, what is the probability 

 of increased volcanism in the next few 

 decades adding to the pollution of the 

 atmosphere made by man and thus 

 speeding up the present climatic 

 change? 



How far will the present climatic 

 change go? It appears that the change 

 from a glacial climate to a nonglacial 

 climate occurred with great rapidity. 

 Would the opposite change occur as 

 fast? What chance is there, on a rela- 

 tively short time-scale, to control the 

 sources of turbidity? 



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