32 president's address. 



cient to penetrate the obstructing atmospheric blanket. The less 

 of the efficient heat-trapping carbon dioxide and water vapour 

 are contained in the atmosphere, the lower will be the surface 

 temperature necessary to produce a state of equilibrium, and 

 hence, the colder will be the climate at the earth's surface, and, 

 conversely, the more carbon dioxide and water present in the air, 

 the higher will be the temperature. The carbon dioxide may be 

 considered as the controlling factor in determining the absorp- 

 tive power of the atmosphere for heat, for, although water 

 vapour has probably a greater actual effect, it depends entirely 

 on temperature for its presence, while carbon dioxide is not 

 directly affected by the temperature changes which it itself 

 induces. When carbon dioxide is removed the temperature falls, 

 and with fall in temperature the proportion of water vapour 

 decreases; such decrease is followed by a further fail in tempera- 

 ture which again robs the atmosphere of more water vapour, and 

 this process goes on until the lowest temperature which the carbon 

 dioxide will permit is reached, and a condition of thermal equili- 

 brium is set up. When carbon dioxide is increased the tempera- 

 ture rises, and with rise of temperature the capacity of the 

 atmosphere for holding water is augmented, and thereby a further 

 rise in temperacure is brought about until by alternate action 

 and reaction thermal equilibrium is again established. We thus 

 see that the carbon dioxide is the dominant element, and in what 

 follows I will for the sake of simplicity speak of the temperature 

 changes as if entirely due to carbon dioxide variation. 



The greatest step in advance within recent years, in the 

 development of the carbon dioxide hypothesis, is due to Professor 

 Arrhenius, who, as the result of an extremely able and laborious 

 mathematical examination of the problem, has shown that a 

 certain reduction in the proportion of carbon dioxide now present 

 in the atmosphere would, in so far as can be seen, be competent 

 to bring about a sufficient fall in the average temperature at the 

 earth's surface to produce glaciation to latitudes as low as to be 

 well within the tropics.* Arrhenius bases his calculations on 



* Arrhenius, Phil. Mag. (Ser. 5), Vol.xli. 1896, •p.2;37. 



