PART III — CLIMATIC CHANGE 



of dust on solar and terrestrial radia- 

 tion is virtually unknown. 



In an historical context, it has been 

 suggested that the Rajasthan Desert 

 of northwest India may have origi- 

 nated largely through overgrazing, 

 with the resultant increase in atmos- 

 pheric dust content leading to condi- 

 tions that further decreased rainfall. 

 Correct identification of natural and 

 man-made tendencies is vital in such 

 instances if attempts are to be made 

 to reverse the processes. 



Evaluation of Current Status 



Data — The data base available for 

 the study of climatic fluctuations last- 

 ing less than 200 years is limited in 

 a number of respects: 



Spatial coverage of climatic data 

 covering approximately the last cen- 

 tury and a half is restricted. Direct 

 observations are particularly limited 

 for the southern hemisphere gener- 

 ally, the oceans, the high arctic, 

 mountain areas in general, and parts 

 of the tropics. Fortunately, more ex- 

 tensive records are available for the 

 European-North Atlantic sector, the 

 area where climatic fluctuations have 

 been pronounced. 



Climatic data available for a sub- 

 stantial length of time is restricted to 

 only a few categories, however — 

 mainly temperature, precipitation, and 

 pressure. Indices of volcanic activity 

 and dust since the late seventeenth 

 century are available. But records of 

 solar radiation, atmospheric CO-, and 

 other dust content, for example, exist 

 only for shorter periods and provide 

 a more restricted spatial coverage. 



Reconstruction of changes over the 

 past two centuries is now possible, 

 using (a) snow/ice cores from Green- 

 land and Antarctica, which provide 

 records of O 18 changes with good 

 time-resolution over thousands of 

 years, and (b) tree-ring indices in 

 selected areas — the arid margins 

 for moisture changes and the arctic 



(or alpine) margins for temperature 

 changes. Other techniques for recon- 

 structing past climates do not allow 

 the necessary degree of time-resolu- 

 tion, mainly because of inherent limi- 

 tations in available dating methods. 



Data on the extraterrestrial and ter- 

 restrial variables that may cause fluc- 

 tuations are even more limited. Car- 

 bon dioxide, for example, has been 

 measured in a few places over the 

 past 100 years but regular monitoring 

 is very recent. The monitoring of tur- 

 bidity has only just begun in a limited 

 way, and extensive and reliable meas- 

 urements are similarly available for 

 only about a decade. Fluctuations in 

 solar radiation will only be deter- 

 mined from satellite data, although 

 there are several centuries of sunspot 

 records. 



Changes in other terrestrial vari- 

 ables such as sea-surface tempera- 

 tures, extent of snow cover, pack-ice 

 and frozen ground, cloudiness, and 

 total atmospheric vapor content can- 

 not be assessed with sufficient accu- 

 racy from available (or foreseeable) 

 ground networks. Satellite monitor- 

 ing will again be indispensable. 



The fact that "artificial" climatic- 

 changes due to man's activities mav 

 obscure, or accentuate, natural trends 

 further complicates efforts to study 

 climatic changes over the past 200 

 years. 



Theoretical Formulations — The 

 development of operative numerical 

 models of the atmosphere and oceans 

 which account for the major observed 

 features of global climate represents 

 a significant recent advance. Theoret- 

 ical formulations are generally avail- 

 able as far as atmospheric-circulation 

 models are concerned, although theo- 

 ries of "almost intransitive" systems 

 need further development. Tidal phe- 

 nomena in the atmosphere have been 

 a subject of much recent study, but 

 their possible implications for climatic 

 fluctuations have not yet been estab- 

 lished. Some phenomena — e.g., the 



scattering/absorption properties of 

 aerosols; interactions between strato- 

 spheric ozone and the general circula- 

 tion — still present important theo- 

 retical problems. 



Interactions — The possible impact 

 of climatic fluctuations on man's 

 activities — agriculture, fisheries, do- 

 mestic heating, transportation, con- 

 struction industries, and so on — 

 appears to have been generally neg- 

 lected, particularly in terms of mod- 

 eling and long-range planning. Eco- 

 system studies of the International 

 Biological Program will provide some 

 information pertinent to these prob- 

 lems, but the difficulty with all short- 

 term programs of this type is that 

 climate tends to be regarded as an 

 environmental constant. 



Some Controversial Topics — Con- 

 cerning the stability of the arctic 

 pack-ice, would it re-form under pres- 

 ent climatic conditions if attempts 

 were made to remove it? Data short- 

 comings for this area and the problem 

 of ocean and atmospheric advection 

 of heat have prevented resolution of 

 this question. 



It has been argued that the appar- 

 ent recent increase of atmospheric 

 turbidity may account for the down- 

 turn of temperature since about 1940. 

 If this were to be confirmed, a contin- 

 ued deterioration could be expected, 

 other things remaining constant. 



The problem of changes induced by 

 turbidity is related to the more gen- 

 eral, and equally important, problem 

 of distinguishing between "natural" 

 and man-induced climatic change. 

 This is especially significant in assess- 

 ing the actual and potential effects 

 of large-scale, long-term weather/ 

 climate modification programs. 



Instrumentation — The technical 

 aspects of required instruments are, 

 in general, adequately covered. With 

 respect to determinations of atmos- 

 pheric turbidity, however, the ap- 

 plication of LIDAR (light detection 



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