PART III — CLIMATIC CHANGE 



sumed to be fixed) by about 2 centi- 

 grade, an appreciable change. 



The role of aerosols in the radiative 

 balance cannot be calculated with 

 anything like the certainty of that for 

 carbon dioxide. Various estimates 

 have been made of the effect of 

 aerosols, with conflicting results. The 

 principal effects of aerosols are to in- 

 crease the scattering of sunlight in the 

 atmosphere and also to absorb sun- 

 light, the two effects being about 

 equal. Thus, Robinson, in England, 

 reports an average decrease of 25 per- 

 cent in the amount of sunlight reach- 

 ing the surface due to aerosols, and 

 presumably at least half of this 

 amount went into heating the atmos- 

 phere. In clear air, such as that found 

 in the polar regions, the effect of 

 aerosols is much less, but in the 

 tropical zone the turbidity of the at- 

 mosphere, probably due primarily to 

 natural haze from vegetation, is high 

 all the time. 



Man-Made Aerosols — Aerosols 

 should be taken into account in any 

 calculation of the radiative balance of 

 the earth-atmosphere system, but the 

 fact is that we do not yet know how 

 to do this with certainty. Further- 

 more, there is the practical question 

 of how man-made aerosols compete 

 with natural aerosols. 



The haze observed in many parts of 

 the world far from industrial sources 

 originates chiefly in the organic mate- 

 rial produced by vegetation, with 

 large contributions from sea salt from 

 the ocean and dust blown from dry 

 ground. At times, volcanic activity in 

 the tropics produces a worldwide in- 

 crease of the aerosol content of the 

 high atmosphere. It is estimated by 

 Budyko, for example, that the solar 

 radiation reaching the ground after 

 the 1963 eruption of Mt. Agung, in 

 Bali, was reduced in the Soviet Union 

 by about 5 percent, a significant at- 

 tenuation whose total effect on the 

 global radiation balance is not clear. 



In contrast to these natural aero- 

 sols, man has overwhelmed nature in 



certain parts of the world where in- 

 dustrial smog and smoke have an evi- 

 dent effect on the clarity of the atmos- 

 phere. Observations in a few cities, 

 such as Washington, D. C, and Uccle, 

 Belgium, have documented the in- 

 crease in turbidity and the decrease in 

 solar radiation reaching the surface 

 over the past few decades, even 

 though progress has been made in the 

 United States and Europe in reducing 

 the production of smoke from coal- 

 burning heat sources. 



An additional complication, a pos- 

 sible effect of man-made contaminants 

 in the atmosphere, is the observed 

 reduction of the albedo of clouds due 

 to contaminants absorbed in cloud 

 droplets. This effect must also be 

 taken into account in a complete cal- 

 culation of the radiation budget and 

 man's effects on it. 



Needed Scientific Activity 



In view of the uncertainties in the 

 many factors involved in the radiation 

 balance of the earth, and the possibil- 

 ity that man is significantly affecting 

 the radiation balance by his introduc- 

 tion of aerosols and his increase in the 

 COi; content, it is necessary to inten- 

 sify our studies of the effects of these 

 factors on the climate. 



Models — The key to such studies 

 is the development of adequate clima- 

 tological models on which experi- 

 ments can be run. One would, for 

 example, study the change in the 

 average temperature in various re- 

 gions of the globe for certain changes 

 in the optical characteristics of the 

 atmosphere resulting from aerosols 

 and carbon dioxide. There are many 

 feedbacks in this system, and the 

 model should take as many as pos- 

 sible into account. A major feedback, 

 already referred to, is that due to 

 changing ice and snow cover in the 

 polar regions; another is due to 

 change of cloud cover; the two prob- 

 ably react in the opposite direction to 

 a change in average temperature. 



Since the oceans are important in 

 the long-term heat balance of the sys- 

 tem, a climatic model must certainly 

 include oceanic circulations, even 

 though they are largely secondary to 

 atmospheric circulations in the sense 

 that the atmosphere drives the sur- 

 face currents. Progress in modeling 

 oceanic circulation has been made in 

 a number of places, notably the Geo- 

 physical Fluid Dynamics Laboratory 

 of NOAA, NCAR, Florida State Uni- 

 versity, and The RAND Corporation. 

 The challenge, eventually, will be to 

 combine the atmospheric and oceanic 

 circulations in one model. 



Monitoring — It is not sufficient to 

 develop a theory without being aware 

 of changes actually taking place in the 

 real atmosphere. For this reason it 

 will be necessary to continue to moni- 

 tor the climate, as is being done in a 

 number of stations throughout the 

 world. In addition to the usual pa- 

 rameters of temperature, wind, and 

 precipitation, the composition of the 

 atmosphere and its turbidity need to 

 be monitored better than they are 

 now. This is not a simple task, since 

 quantitative measurements of trace 

 gases require fairly elaborate tech- 

 niques, while measurements that de- 

 scribe the aerosol content of the at- 

 mosphere should provide information 

 on the optical properties of these 

 aerosols as well as their concentration. 

 It is necessary to know how these 

 aerosols affect incoming solar radia- 

 tion and outgoing infrared radiation. 

 This has not been done adequately, 

 except on a few occasions using spe- 

 cial equipment. 



Satellites have been useful in many 

 ways in obtaining new information 

 about the global atmosphere, and they 

 can contribute significantly to the 

 monitoring task. Except for cloud 

 cover, however, observations to date 

 have not been sufficiently quantita- 

 tive. Cloud cover can and should be 

 monitored by satellites. Satellites can 

 also monitor snow and ice cover, al- 

 though there is a problem during the 

 polar night when pictures cannot be 

 taken in the usual manner. This situa- 



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