Geography of Ohio 205 



Practically the only source of the surface heat of the earth, 

 worth considering, is the sun, and according to present ideas of 

 earth origin, this has always been the great source of heat. Sun- 

 light warms the surfaces exposed to it. How^ long the heat 

 remains in these surfaces depends upon their capacity for radiat- 

 ing heat. Some rocks hold heat longer than others; water holds 

 the heat longer than any rock. The sun's rays, coming through 

 the atmosphere, do not appreciably warm the atmosphere, but 

 heat radiated from the warm surfaces of the earth does raise the 

 temperature of the atmosphere. If the heat thus radiated were 

 kept near the earth, or if the atmosphere were kept quite con- 

 stantly warm, climatic conditions would alter. 



Some gases act as blankets and keep radiated heat from passing 

 cjuickly through them. Water vapor and carbon-dioxide belong 

 to this class. It is supposed, therefore, that if the present content 

 of carbon-dioxide and water vapor in the atmosphere were in- 

 creased, the mean annual temperature of the earth woud be raised 

 accordingly. One estimate says that two or three times the pre- 

 sent content of carbon dioxide would raise the average tempera- 

 ture 8 or 9 degrees; and that if the present content of carbon- 

 dioxide were reduced one-half to two-thirds it would lower the 

 average temperature 4 or 5 degrees. The former condition of 

 higher temperature is thought to represent the climate of the 

 Miocene period; the latter condition, that of the Pleistocene. 

 Studies of the rocks have led students to conclude that there has 

 been a variation in the amount of carbon-dioxide in the earth's 

 envelope during the past, and it is further concluded that the 

 same causes will bring about in the earth's future a repetition of 

 this variation. To discuss this latter point completely, would 

 involve much detail that would hardly be in place in these chap- 

 ters. Those interested are referred to the literature where the 

 matter is fully considered.'' 



This theory of change in the earth's atmosphere makes a 

 stronger appeal to the students of the present day than do any of 

 the other theories advanced to account for the glacial period. 



^ Chamberlin and Salisbury, Geology, vol. iii, (1906), pp. 432-445. 



