122 MISCELLANEOUS GEOLOGICAL CONSIDERATIONS 



But what does this mean? Ice Ages and Heat Ages indicate strong, 

 regional, climatic variations. But they indicate strong climatic varia- 

 tions for part of the earth only. None of the latest Ice Ages changed 

 the tropics to any extent. Neither the tropical rain forest nor the 

 central zone of the coral seas was affected. Ice Ages and Heat Ages 

 did not affect all of the earth. Never did the whole surface of the 

 earth freeze over or become unbearably hot. Ice Ages held sway 

 in higher latitudes, in what are now polar and temperate regions. 

 Heat Ages predominantly affected lower latitudes, such as the present 

 tropical and subtropical regions. Ice Ages and Heat Ages are the 

 result of a lowering or a rising of the mean global annual temperature 

 over less than ten degrees centrigrade only. 



So, to conclude, notwithstanding the occurrence of Ice Ages or 

 Heat Ages, the mean temperature of the surface of the earth showed 

 only very slight variations over that period of 2 billion years. Now 

 the important thing is that this period represents an extreme length 

 of time, even if measured against the long times normal in geology 

 and astronomy. 



GLASSHOUSE EFFECT OF THE ATMOSPHERE 



The mean temperature at the surface of the earth is the sum of a 

 number of independant variables, the most important being solar 

 radiation, heat flow from the interior of the earth, and heat retention 

 by the atmosphere. Each of these is in itself complex. 



Solar radiation is the main heat source for the surface of the earth. 

 Now it seems pretty well established that apart from sunspot cycles 

 and such minor variations, solar radiation has been extremely con- 

 stant. Not only has it been very constant over the short time of some 

 hundred years or so during which meteorologists have actually meas- 

 ured solar radiation, but it seems quite certain that it must also have 

 been constant over lengths of time of several billion years. The simple 

 nuclear reactions generating heat in the sun are reasonably well 

 understood by now, and can be calculated and extrapolated back into 

 time with relative ease. So if we take account only of the solar 

 radiation, a constant mean temperature of the earth's surface is not 

 so amazing at all. 



The heat flow from the earth's interior, on the other hand, is very 

 imperfectly known. It may have shown important fluctuations in 



