CAUSES Ol 



bodies the idea that the average 

 atmospheric state is virtually indis- 

 tinguishable from an equilibrium 

 state, which in turn is uniquely con- 

 sistent with the earth-environmental 

 conditions at the time; in this view, 

 the atmosphere requires a relatively 

 short time to adjust to its new equilib- 

 rium state when the earth-environ- 

 mental conditions change. 



The "Conspirator" Concept — An- 

 other interpretation can be called the 

 "conspirator" concept of climatic 

 change. This concept considers that 

 the average atmospheric state is in- 

 fluenced as much by its own past 

 history as by contemporary earth- 

 environmental conditions, that there 

 may be more than one equilibrium 

 state that is consistent with those en- 

 vironmental conditions, and that the 

 choice of equilibrium state approxi- 

 mated by the actual atmospheric state 

 at any given time depends upon the 

 antecedent history of the actual state. 



Sliarp Distinctions — The distinc- 

 tions between these two concepts is 

 sharp for long-period climatic change, 

 such as the change from Tertiary to 

 Quaternary times. On such a time- 

 scale, the dynamic and thermody- 

 namic time-constants of atmospheric 

 processes are infinitesimal, even if one 

 chose to include the oceans and the 

 polar ice-caps as coupled "atmos- 

 pheric" processes. As now seems 

 plausible, earth -environmental 

 changes included gradual sea-floor 

 spreading and continental drift, to- 

 gether with a gradual increase of 

 average continental elevation. It is 

 usually assumed that the climate acted 

 in keeping with the "slave" concept 

 throughout and that, after a certain 

 point in the course of continental drift 

 was reached (perhaps when the Arc- 

 tic Ocean was isolated), the equilib- 

 rium climate was transformed in a 

 deterministic manner from a glacial- 

 inhibiting pattern to a glacial-stimu- 

 lating pattern. 



On the other hand, it is possible to 

 argue, following Lorenz, that the 



actual climate of the Quaternary was 

 not necessarily preordained by its 

 contemporary environmental state; 

 that the evolution of climate to its 

 Quaternary mode was not a deter- 

 ministic evolution but a probabilistic 

 one that might have turned out very 

 differently under identical conditions 

 of continental drift and other environ- 

 mental change. The different Quater- 

 nary outcomes (two or more) would 

 have followed from differences in the 

 precise course of the climate itself, 

 due either to transient environmental 

 disturbances or perhaps to "random" 

 excursions of atmospheric state along 

 the way. 



Subtle Distinctions — With regard 

 to relatively rapid climatic change, 

 however, the distinction between the 

 "slave" and the "conspirator" con- 

 cepts of change is much more subtle 

 in character, and perhaps unrecog- 

 nizable within present bounds of 

 either theory or observation. The rea- 

 son for this is to be found in the inti- 

 mate dynamic and thermodynamic 

 coupling that exists between the at- 

 mosphere and the oceans, and to a 

 lesser extent in the coupling between 

 the atmosphere, the oceans, and the 

 polar ice-caps. These couplings intro- 

 duce long time-constants into the 

 changes of atmospheric state, and re- 

 sult in various forms of autovariatiou 

 in the total system, on the time-scale 

 of decades and centuries. In the 

 course of such autovariation, the at- 

 mosphere itself may be said to obey 

 the "slave" principle. But in a rela- 

 tively limited period of years, the 

 coupled atmosphere-ocean system 

 would exhibit changes of state that 

 are not independent of its initial state. 

 In this case, the system is more prop- 

 erly described as obeying the "con- 

 spirator" principle. To complicate 

 matters further, it is conceivable that 

 the autovariation of the atmosphere- 

 ocean system is riding on top of a 

 transient of the Lorenz type already 

 mentioned. 



In the presence of Lorenz-type 

 transients, the effect of systematic en- 



vironmental changes on p 

 climate (changes, for exam] 

 ing secular increases of carbc 

 oxide (CO:;) or other consequences ot 

 human activities) might be so badly 

 confounded as to be totally unrecog- 

 nizable. Even without such transients, 

 however, atmosphere-ocean autovari- 

 ation could effectively obscure the 

 effect of systematic environmental 

 changes that we are seeking to dis- 

 cover. 



Rationale for the Isolation of 



Human from Natural Factors in 



Climatic Change 



What rationale, then, are we to fol- 

 low in establishing the climatic effects 

 of systematic environmental change 

 on the scale of decades and centuries? 

 More specifically, how do we go about 

 the task of isolating the contribu- 

 tion of man's activities to twentieth- 

 century climatic change? 



First of all, there seems no real pos- 

 sibility of detecting Lorenz-type tran- 

 sients in present-day climate, so we 

 will have to proceed on the assump- 

 tion that they are not now occurring 

 nor are they likely to be induced in 

 the foreseeable future by further 

 environmental change from human 

 activities. 



Second, while we should not hesi- 

 tate to use presently available esti- 

 mates of the climatic effects of atmos- 

 pheric pollution and other forms of 

 environmental change as an interim 

 guide in assessing the potential cli- 

 matic hazards of various human ac- 

 tivities, we should also remember that 

 such estimates are highly tentative. 

 We should take pains not to put 

 undue confidence in them. 



Shortcomings of the Present Data 

 Base — In this connection, there are 

 two important points to consider: 



1. Most present estimates of the 

 climatic impact of human activ- 

 ities are based on relatively 



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