CAUSES OF CLIMATIC CHANGE 



Figure MI- 



FACTORS IN THE RADIATION BALANCE OF THE EARTH 



SELECTIVE ♦_, . 



ABSORPTION t Y ^' 



nJ^t AND EMISSION ~-~yL~~ / ] ' 



f\ ^7\thermal 



1 - FMISSION 



^ 



RAYLEIGH SCATTERING 



i 



EMISSION 

 FROM CLOUD 



THERMAL 

 EMISSION OF 

 ATMOSPHERE 



( ~ 



J'h CtOUD REFLECTION 



rV~j AND ABSORPTION 





SELECTIVE ABSORPTION 



•;\ AND 



EMISSION 



CLOUD 

 ABSORPTION 4~ 

 AND 5 



SCAHERING - ■ 

 ABSORPTION AND EMISSION 



THERMAL EMISSION 

 FROM EARTH SURFACE 



ABSORPTION AND REFLECTION 

 BY EARTH SURFACE 



The diagram indicates the major components in the global radiation balance. The 

 albedo, or reflectivity, is composed of the radiation reflected from the ground, 

 clouds, aerosols, and other materials that might scatter incoming solar radiation. 



reflectivity, in the unlikely event that 

 all clouds were so affected, would 

 have about the same effect as a 5 per- 

 cent reduction in cloud amount. 



Budyko, at Leningrad, and Sellers, 

 at the University of Arizona, have 

 taken this energy calculation one step 

 further, arguing that a decrease of 

 only 1.6 to 2.0 percent in the solar 

 radiation available to the earth would 

 lead to an unstable condition in which 

 continental snow cover would ad- 

 vance all the way to the equator, with 

 the albedo raised by the greater snow 

 cover to the point where the oceans 

 would eventually freeze. Lest this 

 rather frightening calculation be taken 

 too seriouslv, it should be mentioned 

 that there is no evidence that a mech- 

 anism for a change of as much as 1.5 

 percent actually exists, or ever has in 

 the history of the earth. The model 

 nevertheless illustrates the delicacy of 

 our planet's thermal balance. 



Aerosols — The aerosols that fill 

 the atmosphere — natural haze, dust, 



smoke, smog, and so on — probably 

 play an important role in the radiation 

 balance of the earth, but this is one of 

 the great uncertainties in the theory 

 of how the atmosphere behaves. 

 Aerosols in cloudless air probably in- 

 crease the albedo to some extent, 

 and they absorb sunlight themselves. 

 Also, as we have noted, they can 

 change the reflectivity of clouds. We 

 are quite certain that variations in the 

 solar radiation absorbed by the earth's 

 atmosphere and surface, due to 

 changes in turbidity or total aerosol 

 content of the atmosphere, are signifi- 

 cant. Furthermore, as will be noted 

 below, aerosols in the atmosphere can 

 be greatly affected by man and vol- 

 canic activity. 



Factors Affecting Loss of Terrestrial 

 Heat — On the other side of the 

 ledger by which we keep track of the 

 amount of heat into and out of the 

 atmosphere-ocean heat engine is the 

 loss to space of terrestrial heat by in- 

 frared radiation. Over a period of a 

 year or so, the amount of radiation 



lost by infrared radiation must almost 

 exactly balance the amount of solar 

 radiation absorbed by the earth and 

 its atmosphere. If this did not hap- 

 pen, the earth would rapidly heat or 

 cool. 



As a general principle, any sub- 

 stance in the atmosphere that absorbs 

 infrared radiation will slow the cool- 

 ing of the surface. The reason for this 

 is that the energy radiated from the 

 surface is absorbed by the absorbing 

 substance in the atmosphere, thus 

 heating the atmosphere which in turn 

 radiates back toward the ground. In 

 effect, an absorbing layer acts as a 

 radiation blanket, and its presence 

 will result in a higher surface tem- 

 perature. 



An auxiliary effect of this absorb- 

 ing blanket will be an increase in the 

 stability of the lower part of the at- 

 mosphere, between the surface and 

 the absorbing layer. This increase in 

 stability will reduce convection in the 

 lower layers. The ability of the at- 

 mosphere to stir itself by convection 

 is a principal source of cumulus 

 clouds, so that a decrease in convec- 

 tion would also decrease precipitation. 



Infrared Absorbers — There are 

 two main classes of infrared absorbers 

 in the atmosphere: trace gases (water 

 vapor and carbon dioxide (CO;;) being 

 the most important in the lower at- 

 mosphere) and aerosols of all kinds, 

 including clouds. Various estimates 

 have been made of the effect of in- 

 creasing COj in the atmosphere, since 

 man has in fact been able to raise 

 the total amount through burning fos- 

 sil fuels. Since 1900, the amount of 

 CO2 has increased an average of 10 

 to 15 percent, and this trend has 

 usually been cited to account for the 

 observed rise in the average surface 

 temperature of 0.2 centigrade up to 

 1940. The theoretical calculations of 

 Manabe and Weatherald indicate that 

 a doubling of the COj content in the 

 atmosphere would have the effect of 

 raising the temperature of the atmos- 

 phere (whose relative humidity is as- 



67 



