ICE AGES — SIMPSON 295 



CAUSE OF THE PLEISTOCENE ICE AGE 



A very large number of theories have been propounded to explain 

 the cause of the ice age. It is becoming more and more generally 

 accepted that no change located in the earth itself— such as a change 

 in the distribution of land and water — or in the earth's atmosphere — 

 such as a change in the amount of carbon dioxide or volcanic dust — 

 can explain the sequence of climatic changes associated with the 

 Pleistocene Ice Age, and we are being thrown back onto changes 

 outside the earth. The most obvious source of climatic change would 

 appear to be solar radiation. This seems such a simple solution. If 

 we assume that the sun's radiation can change, all climates would 

 appear to become possible ; an ice age would be produced by reducing 

 the radiation and a warm period by increasing it. Unfortunately, 

 things do not go in this simple way. As the result of a study which 

 I made a few years ago to find what happens to the solar radiation 

 when it falls on the earth, and how the earth returns to space the 

 energy which it receives from the sun, I was led to the conclusion that 

 the last ice age was not caused by a decrease of solar radiation but 

 by an increase. I must now try to explain this paradox. 



The sun sends out a stream of energy which we call sunshine. The 

 earth intercepts a quantity of this energy which on absorption warms 

 up the surface and the atmosphere. If the earth had been a "black 

 body" it would have warmed up to a temperature, which can be easily 

 calculated, when it would have emitted just as much radiation as 

 it received from the sun. In this way the temperature of the earth 

 would have increased and decreased with an increase and decrease 

 of the solar radiation. One of the first results of my investigation 

 was to show that the earth does not react to changes in solar radiation 

 like a "black body." I was able to show that if the solar radiation 

 were to increase, the temperature of the earth's surface would not 

 increase to anything like the extent that one would expect from the 

 increase of solar radiation; but that the cloud would increase and 

 return the greater part of the additional solar radiation without 

 warming up the surface of the earth. 



In this way the balance between the incoming and outgoing radia- 

 tion is maintained more by the amount of cloud than by the tempera- 

 ture. In present conditions just about half the sky is covered by 

 cloud; the amount is slightly different in the different latitudes, but 

 not to any large extent. The clouds appear bright because they 

 reflect the sun's light. The light which the clouds reflect cannot be 

 used to warm up the earth; it is just returned to space, and to that 

 extent the solar radiation is reduced. The amount of solar radiation 

 returned directly by the clouds has been measured and proves to be 

 43 percent of the total incoming solar radiation. That is, nearly 



