PART III — CLIMATIC CHANGE 



simple hydrostatic heat-balance 

 models (as refined, for example, 

 by Manabe and used by him to 

 estimate the thermal effect of 

 variable CO-, stratospheric wa- 

 ter vapor, surface albedo, and 

 the like). Manabe himself has 

 often stressed the limitations of 

 such models, the most impor- 

 tant of which are: that they do 

 not take account of atmospheric 

 dynamics other than purely 

 local convective mixing; and 

 that they do not take into ac- 

 count changes of atmospheric 

 variables other than the varia- 

 ble that is explicitly controlled 

 as a parameter of the calcula- 

 tion (plus water vapor in those 

 experiments stipulating a con- 

 stant relative humidity). 



2. Climatic changes caused by nat- 

 ural agencies, and those possi- 

 bly caused by human agencies, 

 are not necessarily additive. For 

 example, by analysis of past 

 data on CO- accumulation in 

 the atmosphere, roughly 50 per- 

 cent of all fossil COj added to 

 the atmosphere appears to have 

 been retained there. Using pub- 

 lished United Nations projec- 

 tions of future fossil CO- 

 production, together with a 

 constant 50 percent retention 

 ratio, it can be predicted that by 

 a.d. 2000 the total atmospheric 

 CO- load will have exceeded its 

 nineteenth-century baseline by 

 more than 25 percent. As 

 pointed out by Machta, how- 

 ever, recent atmospheric CO- 

 measurements at Mauna Loa 

 and other locations indicate that 

 the atmospheric CO- retention 

 ratio has been dropping steadily 

 since 1958, to a present value 

 of only about 35 percent. 



It may be significant that the 50 

 percent retention figure applied to a 

 time when world average tempera- 

 tures were rising, and that the ob- 

 served decline since 1958 applies to a 

 time when world average tempera- 

 tures have been falling. It is conceiv- 



able, though certainly not proven, 

 that the reversing trend of world cli- 

 mate in recent years has somehow 

 altered the rate at which the oceans 

 can absorb fossil CO2. If this is the 

 case, we are witnessing an interactive 

 effect whereby climatic changes pro- 

 duced by one agency (presumably a 

 natural one) are at least temporarily 

 reducing the climatic impact of an- 

 other agency (in this case, an inad- 

 vertent human one). Such interactive 

 effects are very poorly understood, 

 and yet they may be a very important 

 element in the evolution of present- 

 day climate. 



The Use of Advanced Mathematical 

 Models — To return to our question 

 of what rationale we should follow in 

 our study of contemporary climatic 

 change and of human influences on 

 climate, we are left with little choice. 

 We have to rely on the development 

 of advanced mathematical models of 

 the global atmosphere that will be 

 suitable for long-term integration to 

 generate stable climatological statistics 

 and will be capable of simulating 

 many dynamic and thermodynamic 

 processes in the atmosphere and at 

 the earth's surface. Relatively sophis- 

 ticated models of these kinds have 

 already been developed, at least one of 

 which has been expanded to deal with 

 coupled atmosphere-ocean systems. 

 Experiments with such models have 

 begun to lay a solid foundation for a 

 quantitative theory of global climate 

 and have elucidated the climate- 

 controlling influence of the general 

 atmospheric and oceanic circulations. 

 There appears to be no limit to the 

 refinement possible in such models, 

 other than the limits imposed by com- 

 puter capacity and speed. 



The manner in which such numeri- 

 cal experiments bear on the study of 

 climatic change is essentially twofold: 



1. The experiments verify that a 

 wide range of environmental 

 factors have a bearing on the 

 global pattern of atmospheric 

 circulation and climate. They 



confirm that the most impor- 

 tant factors in this respect are: 

 (a) solar emittance; (b) the 

 geometry of the earth-sun sys- 

 tem including the orbital and 

 axial motions of the earth; 

 (c) the distribution of oceans 

 and land masses; (d) the state 

 of the ocean surface which, 

 along with the juxtaposed at- 

 mospheric state, governs the 

 fluxes of energy, moisture, and 

 momentum across the surface; 

 (e) the state of the land surfaces 

 with respect to albedo, thermal 

 capacity, water and ice cover, 

 relief, and aerodynamic rough- 

 ness; and (f) the gaseous and 

 aerosol composition of the at- 

 mosphere itself. To the extent 

 that all of these factors may 

 vary with time, either slowly 

 or rapidly, in response to forces 

 other than the contemporary at- 

 mospheric state itself, all such 

 factors are automatically to be 

 regarded as potential causes of 

 climatic change. 



2. In the numerical experiments, it 

 is possible to simulate the be- 

 havior of circulation and cli- 

 mate as a function of arbitrarily 

 chosen boundary conditions 

 and atmospheric constituency, 

 which enter the experiments as 

 controllable parameters. This 

 makes it possible to vary any 

 of the environmental factors 

 listed above and determine how 

 the circulation and climate re- 

 spond. In this way, various 

 theories of climatic change can 

 be tested in terms of their mete- 

 orological consistency. With the 

 further refinement of joint at- 

 mosphere-ocean models, the 

 more realistic modeling of con- 

 tinents and ocean basins, and 

 the introduction of ice-cap in- 

 teractions into the models, the 

 range of factors in climatic 

 causation that are amenable to 

 this kind of study will eventu- 

 ally became almost exhaustive 

 of all reasonable possibilities. 



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