370 HUMPHREYS: PRODUCTION OF CLIMATIC CHANGES 



middle of 1912. Now all these decreases of insolation intensity, 

 amounting at times to 20 per cent of the average intensity, fol- 

 lowed violent volcanic eruptions that filled the isothermal region 

 with a great quantity of dust. 



Surface temperatures. It has been known for a long time that 

 the number of sun-spots and the average temperature of the 

 earth are roughly related in the sense that the greater the num- 

 ber of spots the lower the temperature, and the smaller the 

 number of spots the higher the temperature. But this relation 

 has many marked exceptions, every one of which, or at any 

 rate all the more important ones, occurred immediately after 

 violent volcanic explosions and presumably therefore were caused 

 by volcanic dust. 



The list on l^age 369 gives the more important of these dis- 

 crepancies, since the beginning of sun-spot records to the present, 

 and their probable causes. 



From the above it appears quite certain that volcanic dust 

 can lower the average temperature of the earth by an amount 

 that depends upon the quantity and duration of the dust, and 

 that it repeatedly has lowered it certainly from 1°F. to 2°F. 

 for periods of from a few months to fully three years. Hence 

 it certainly has been a factor, in determining our past climates, 

 and presumably may often be a factor in the production of our 

 future climates. Nor does it require any great volume of dust 

 to produce a marked effect. Thus it can be shown by a simple 

 calculation that less than the one thousandth part of a cubic 

 mile of rock spread uniformly thru the upper atmosphere as 

 volcanic dust would everywhere decrease the average intensity 

 of insolation received at the surface of the earth by at least 20 

 per cent and therefore would, presumably, if long continued, 

 decrease our average temperatures by several degrees. 



CONCLUSION 



It has been shown in the above, among other things, that 

 volcanic dust in the high atmosphere decreases the intensity of 

 solar radiation in the lower atmosphere, and therefore the aver- 

 age temperature of the earth, substantially as theory indicates 



