PART III — CLIMATIC CHANGE 



Modeling of the atmospheric heat 

 engine is complicated by the existence 

 of another, more sluggish but massive 

 heat engine — namely, the oceans. 

 While the ocean does not move as fast 

 as the atmosphere, its tremendous 

 heat capacity more than offsets its 

 slow movement. The ocean circula- 

 tion is coupled to that of the atmos- 

 phere, and nearly as much heat is 

 transported from equator to pole in 

 the oceans as in the atmosphere. 



The key to this atmosphere-ocean 

 system, the ultimate driving force, is 

 the solar radiation that is absorbed, 

 mostly in the equatorial regions, and 

 the infrared radiation that is emitted 

 back to space at all latitudes. One 

 cannot consider the heat involved in 

 radiation, however, without also con- 

 sidering the internal heat released into 

 the atmosphere bv the condensation 

 of water vapor. In fact, most of the 

 heat that is transported from the 

 equator to the middle latitudes is in 

 the form of the latent heat of water 

 vapor, heat that is released whenever 

 it rains or snows. 



Experimental general -circulation 

 models of the atmosphere that have 

 been run on large computers at the 

 National Center for Atmospheric Re- 

 search (NCAR), the Geophysical Fluid 

 Dynamics Laboratory of the National 

 Oceanic and Atmospheric Adminis- 

 tration (NOAA), and the University 

 of California at Los Angeles also take 

 into account the effect of the moun- 

 tain ranges of the world, the rotation 

 of the earth, and the complex proc- 

 esses that exchange heat, moisture, 

 and momentum vertically by means 

 of convection, particularly in the 

 tropics. All these processes can be 

 related to each other by a set of dif- 

 ferential equations that involve time. 

 A model is made to "run" by integrat- 

 ing these equations in small time- 

 steps, and the result is a model of a 

 moving fluid system that behaves 

 very much like the real atmosphere. 



Future Refinements — With these 

 general-circulation models we can, in 



principle, do "experiments" to learn 

 how the atmosphere would change 

 with time if there were a change, for 

 example, in the ability of the atmos- 

 phere to transmit solar radiation due 

 to smoke, haze, or smog, or how it 

 would change if there were a growth 

 or shrinking of the size of the great 

 polar ice-caps. 



Actually, however, we are still a 

 long way from realizing a model that 

 is adequate for such experiments in 

 "climatic change." The current gen- 

 eral-circulation models are designed 

 to show the hour-to-hour, day-to-day, 

 and week-to-week changes; we would 

 run out of computer time if we used 

 them to study really long-term 

 changes. Long-term changes in this 

 system would certainly involve 

 changes in the ocean. Hence, it would 

 not be enough to consider only the 

 circulations of the atmosphere. Never- 

 theless, there is hope that, in time, we 

 will be able to develop theoretical 

 numerical models with which to con- 

 duct experiments on the atmosphere- 

 ocean climate and how it will change 

 with changes in the heat available to 

 the system. These models will require 

 a considerable effort in developing 

 quasi-statistical shortcuts and the 

 availability of larger computers than 

 we have now. 



The Radiation Budget 



As mentioned, radiation is the ulti- 

 mate source of energy to drive the 

 complex atmosphere-ocean system. In 

 order to gain an idea of the role that 

 radiation plays in keeping the system 

 in motion, we can perform a simple 

 calculation of the rate of energy input 

 from the sun as compared to the 

 amount of energy that the atmos- 

 phere contains at any time. The solar 

 radiation absorbed by the system is 

 about 600 calories per cirr per day, 

 and the average total thermal heat 

 energy of the atmosphere is about 

 60,000 calories per crrr. This means 

 that if the solar radiation were cut off 



abruptly, about 10 percent of the 

 energy of the atmosphere would dis- 

 appear within ten days. This is enough 

 to cause an appreciable change in the 

 circulation. Such a rough calculation 

 indicates that the atmosphere will re- 

 spond to a change of heat input in a 

 week or less. 



Determinants of the Earth's Albedo 

 — The solar radiation that reaches 

 the earth is partly reflected back into 

 space, partly absorbed by the atmos- 

 phere, but mostly absorbed by the 

 surface. (See Figure III-7) In the 

 1940's it was estimated that about 

 40 percent of the solar radiation was 

 reflected back to space, but more re- 

 cent estimates, based largely on satel- 

 lite observations, have been lowered 

 to about 30 percent. This average re- 

 flectivity is referred to as "the earth's 

 albedo." The fact that there has been 

 such a large uncertainty as to the 

 magnitude of the albedo is testimony 

 to our general uncertainty about the 

 amount of energy available to the 

 system. 



Cloud Cover — Another important 

 variable is the cloud cover, since 

 clouds are generally much more highly 

 reflecting than the surface of the 

 earth. The same is true of snow and 

 ice. An increase in cloud cover in- 

 creases the albedo. For example, a 

 change of 5 percent in the average 

 cloudiness of the equatorial zone, an 

 amount that would go unnoticed, 

 would change the albedo of the earth 

 by about 1.5 percent. This would 

 represent an appreciable decrease in 

 the energy available to drive the 

 atmosphere-ocean system. 



The same effect as a decrease in 

 amount of cloud cover would be 

 achieved by a decrease in the reflec- 

 tivity of the clouds, since both would 

 decrease the net albedo of the earth. 

 Clouds moving over regions with in- 

 dustrial pollution, such as Europe, 

 show a decrease in reflectivity from 

 about .95 (for pure water clouds more 

 than half a kilometer thick) to .80 or 

 .85. This much reduction in cloud 



66 



