GLASSHOUSE EFFECT OF THE ATMOSPHERE 123 



the past for all we know. We can do no more than put on record 

 that its variations have never been so excessive as to endanger the 

 continuation of life on earth. 



The fact is that climatologists find it difficult to explain even 

 minor changes in mean temperature, such as have led to the last 

 Ice Ages. This stems from the fact that the heat retention of our 

 atmosphere is a very complex factor, possessing various feedback 

 mechanisms which counteract either cooling or heating by variations 

 in the primary factors of heat flow. 



For example, a warming of the earth's surface either by stronger 

 solar radiation or by a greater heat flow from the interior, would 

 lead to increased evaporation, and hence to more cloud formation. 

 This would in its turn directly affect the reflectivity of the atmos- 

 phere, the so-called albedo of the earth, resulting in a smaller part 

 of the solar radiation reaching the surface of the earth. Other, more 

 complex, variations, such as a shift in altitude of the tropopause, 

 would also enter into the process, all tending to buffer the effect of 

 any primary variation in solar radiation or terrestrial heat flow. 



Consequently, climatologists are busily studying the mechanism 

 which could have led to such minor variations in mean temperature 

 through which Ice Ages originated. They are not so much concerned 

 with the reasons why, even over the long periods of geologic history, 

 no stronger variations have occurred which could possibly have been 

 obnoxious to life on earth. 



Shapley (1953), for instance, gives six conditions for planetary 

 life, four of which directly apply to our problem. These are: 



1. 'The controlling star must not be variable by more than 4 or 

 5 per cent; it must not be a double star and, of course, not subject 

 to catastrophic explosions like those of the novae.' It follows that 

 our controlling Sun has followed these conditions for the last several 

 billion years at least. 



2. 'The orbital eccentricity of the planet must be low, to avoid 

 excessive differences in insolation as the planet moves from perihelion 

 to aphelion and back (most cometary orbits would be lethal for 

 organisms).' 



3. 'The planet must have a suitable rotation period, so that nights 

 do not overcool, nor days overheat.' Our planet Earth accordingly 



