TART III — CLIMATIC CHANGE 



Figure 111-8 — OBSERVED LAGGED TEMPERATURE 

 VARIATION OF THE NORTHERN HEMISPHERE 



The observed temperature variation of the northern hemisphere has here been 

 corrected (see solid line) for the time lag of the ocean-atmosphere-soil system and 

 the system's response to factors that cause the variation of temperature. A half- 

 response-time of ten years was used. The broken line is the smoothed curve of 

 Figure 111-4 repeated for comparison. 



feed the human population are sig- 

 nificant. 



The theory of climate is so poorly 

 developed that we cannot predict ac- 

 curately whether the climatic trend 

 will continue, or how the distribution 

 of rainfall and frost will change if it 

 does. Clearly this knowledge is a 

 national and international need of 

 high priority. Clearly, too, we must 

 know whether any or all of the recent 

 fluctuation in climate is man-made, 

 and whether it can be man-controlled. 

 Lacking an adequate theoretical basis 

 for prediction, we can only look to 

 the past to see what kinds of changes 

 are possible, with what rapidity they 

 may occur, and what the causal fac- 

 tors might have been. 



Basic Balances 



Although numerical models of the 

 atmospheric circulation are still too 

 crude to simulate the climatic pattern 

 within an error small enough to be 

 less than the occasional ecologically 

 significant variations, certain basic 



relations may be identified that can 

 yield information on some of the fac- 

 tors important to climatic change. 



Ultimately the sun drives the at- 

 mosphere. The fact that we have 

 water in gaseous, liquid, and solid 

 states in the proportions we do is de- 

 pendent on our distance from the sun 

 and the fraction of the sunlight that 

 the earth absorbs. In the long run, 

 there must be the same amount of 

 heat re-radiated to space from the 

 atmosphere as is absorbed by the 

 earth. 



The receipt of solar radiation occurs 

 on the cross-sectional area of the 

 earth, but re-radiation takes place 

 from four times as large an area — 

 i.e., the entire area of the globe. Thus, 



SttR 2 (1 - a) = 4ttR- I, 

 where 5 is the solar constant, 



R is the radius of the earth, 

 a is the albedo, or "reflec- 

 tivity," of the earth, 

 and It is the mean outward in- 

 frared radiation flux from 

 the earth to space. 



Satellite data show that It is fairly 

 uniform over the earth, on the annual 

 average, and that the albedo of the 

 earth is such that the above equation 

 is approximately balanced. The out- 

 ward radiation measured from space 

 is smaller on the average than that 

 emitted by the earth's surface, so that 



S (1 - a) = 4 (to-To 4 - AI) 

 where t is the emissivity of the 

 earth's surface, 



a is the Stef an-Boltzman con- 

 stant, 



To is the surface tempera- 

 ture of the earth, 



AI is the difference between 

 the heat radiated upward 

 by the earth's surface and 

 that leaving the top of the 

 atmosphere for space — 

 the "greenhouse effect," 

 and the overbar on c<jT,, 4 indi- 

 cates an average over the 

 whole surface of the 

 globe. 



The above equation is crude, but 

 it provides an insight into the factors 

 that might affect the general tempera- 

 ture state of the earth. Clearly, fluc- 

 tuations in solar intensity, the fraction 

 of incoming solar radiation that is 

 "reflected" or scattered away before 

 reaching the ground, and the "green- 

 house effect" are the major causes 

 of variation in the mean temperature 

 of the earth. A change of one or 2 

 percent in any one of these variables 

 is enough to produce a significant cli- 

 matic change, yet none of them is 

 known with this accuracy except per- 

 haps the solar intensity. 



Albedo — The temperature of the 

 earth is most sensitively dependent on 

 the albedo of the earth-atmosphere 

 system — an increase of a few per- 

 cent would cool the earth to ice-age 

 temperatures. This variable — reflec- 

 tivity — can be measured by mete- 

 orological satellites, but not yet with 

 sufficient accuracy. The albedo is also 

 a variable that can be changed by 

 human activity, primarily by changes 

 in the transparency of the atmosphere 



70 



