CAUSES OF C ! \NGE 



resulting from particulate pollution. 

 According to Angstrom, a 7 percent 

 increase in the turbidity of the atmos- 

 phere will produce a one percent 

 change in albedo and a 1 -centigrade 

 change in world mean temperature. 



The "Greenhouse Effect" — The 

 other variable that can be changed by 

 human activity is the "greenhouse 

 effect." This depends on such things 

 as the water-vapor content of the air, 

 dustiness, cloudiness, and, especially, 

 the carbon dioxide content. The car- 

 bon dioxide content of the atmosphere 

 has risen 11 percent or so in the past 

 century, and it is widely believed that 

 the rise is due to human activity in the 

 burning of fossil fuels and greater 

 exposure of soil humus and the like to 

 oxidation. (See Figure I1I-9) 



In times past, changes in vegetation 

 and land distribution and elevation 

 affected the earth's albedo, as did 

 short-time changes in cloud and snow 

 cover. Volcanic activity, then as now, 

 produced a variable input of particu- 

 lates to the atmosphere, as did blow- 

 ing dust from desert areas — which 

 in turn affected the albedo. (See Figure 

 111-10) In earlier times, the distribu- 

 tion of land and sea, volcanic activitv, 

 the elevation of the land and nature 

 of the biota probably affected the 

 magnitude of the greenhouse effect. 

 The sun's intensity may also have 

 varied, though there is no evidence. 

 In addition, there are complex feed- 

 back mechanisms, such as additional 

 water vapor in the air at higher tem- 

 peratures, that increase the green- 

 house effect which in turn increases 

 the water-vapor content still more. 

 While increased volcanic activity 

 makes the atmosphere more turbid 

 and thus tends to depress the tem- 

 perature, it may also contribute to the 

 greenhouse effect and thus tend in 

 part to counteract the temperature 

 effect. The complete equation for re- 

 lating these effects is not known, but 

 it appears that the effect of turbidity 

 on the greenhouse effect is only about 

 10 percent of its effect on the albedo. 



We do know, however, that there 

 is something new under the sun — 

 a population of humans sufficiently 



numerous to modify the whole albedo 

 of the earth and the magnitude of the 

 greenhouse effect through their sheer 



Figure 1119 — LAGGED TEMPERATURE CURVE FOR 

 THE NORTHERN HEMISPHERE CORRECTED FOR CO, 



In this graph, the mean observed temperature variation for the northern hemisphere 

 has been adjusted for the time lag shown in Figure 111-8 and for the warming effect 

 of carbon dioxide (CO.). It can be seen that the increase of variation due to the 

 "greenhouse effect" of CO. is small compared with the variation of temperature 

 corrected for system lag. (Compare values of Figures 111-8 with 111-9) Only about 

 3 percent of the variance can be explained by the presence of CO.. 



Figure III -10 — LAGGED TEMPERATURE CURVE FOR THE 

 NORTHERN HEMISPHERE CORRECTED FOR CO, AND DUST 



The mean observed temperature variation for the northern hemisphere has here 

 been adjusted for the time lag of the system, the warming effect of CO,, and the 

 effect of both stratospheric (volcanic) and tropospheric dust. The dust effect ex- 

 plains 80% of the variance of the adjusted temperature, with 63% due to strato- 

 spheric and 17% due to tropospheric dust. The resulting curve shows what tempera- 

 tures would be observed under conditions of direct solar radiation with cloudless 

 skies, although some residual errors remain. (Compare Figures III-4, 8, and 9) 



71 



