CAUSES OF CLIMATIC CHANGE 



Factors related to human activi- 

 ties would, of course, be in- 

 cluded. 



Other Requirements — As neces- 

 sary as such model experiments may 

 be to the study of climatic change, it 

 is important to realize that they are 

 not sufficent to solve the problem of 

 climatic change. It is not enough that 

 we develop the ability to measure the 



response of climate to varying envi- 

 ronmental conditions. If we are to 

 decipher past climatic changes or to 

 predict future changes, it is necessary 

 to determine which environmental 

 controls of climate have been (or will 

 be) doing the varying, at what rate, 

 and in what direction. 



At present, there is a deplorable 

 lack of understanding about the vari- 



ability of our environment. There can 

 be no guarantee that the necessary 

 understanding will ever be acquired in 

 full, for that in turn may depend on 

 unknowable past events and unpre- 

 dictable future events. But we should 

 learn what we can, for in no other 

 way can we be certain whether the 

 climatic changes of the twentieth cen- 

 tury are or are not causally related to 

 man's activities. 



The Radiation Balance 



It has become evident that man can 

 change the entire atmosphere of his 

 planet in certain subtle ways. And he 

 can modify large regions in rather 

 obvious ways — for example, with 

 smoke and smog. There are now new 

 dimensions to his leverage on the at- 

 mosphere as he flies large jet aircraft 

 even higher in the stratosphere, and 

 as booster rockets of the Saturn class 

 introduce hundreds of tons of exhaust 

 into the thin reaches of the upper 

 atmosphere. 



The effects of these changes on the 

 environment are diverse. We have re- 

 cently become aware of the evident 

 and sometimes acute effects of pollu- 

 tion in the world's cities. Now we 

 realize that man may even be able to 

 change the climate of the earth. This 

 is one of the most important questions 

 of our time, and it must certainly rank 

 near the top of the priority list in 

 atmospheric science. 



General-Circulation Models 

 of the Atmosphere 



In recent years, it has been possible 

 to create fairly realistic numerical 

 models of the global atmosphere that 

 behave very much the way the real 

 atmosphere does. (See Figure III-6) 

 The atmosphere is a great heat engine 

 that runs on solar energy, taking ad- 

 vantage of the greater amount of heat 



that reaches the equatorial zone. The 

 function of the heat engine is to trans- 

 port this heat from the equator to the 



poles. In the process, the atmosphere 

 moves with the patterns of the winds 

 seen on any weather map. 



Figure 111-6 — COMPUTER SIMULATION OF SEA-LEVEL PRESSURE FIELD 



Sea level pressure distribution for the western hemisphere has been simulated by 

 a numerical model developed by the National Center for Atmospheric Research. 

 The time is 42.83 days into the simulation and represents a moment in a typical 

 January. The contour interval is 5 millibars. White areas represent clouds. The 

 influence of these clouds is taken into account in the radiation calculations. Areas 

 of high pressure are indicated by H, areas of low pressure by L. 



65 



