PART III — CLIMATIC CHANGE 



2. CAUSES OF CLIMATIC CHANGE 



Basic Factors in Climatic Change 



It is useful to introduce the prob- 

 lem of climatic change by considering 

 the definition of climate. Practical 

 definitions of the term "climate" vary 

 in their specifics from one authority to 

 another. All are alike, however, in 

 distinguishing between climate and 

 weather (and between climatology 

 and meteorology) on the basis that 

 climate refers to "average" atmos- 

 pheric behavior whereas weather 

 refers to individual atmospheric events 

 and developments. On the face of it, 

 then, it might seem that we are left 

 simply with the decision of what time 

 interval to choose over which to aver- 

 age the observed weather into "the 

 climate." By "average" is meant aver- 

 age statistical properties in all re- 

 spects, including means, extremes, 

 joint frequency distributions, time- 

 series structure, and so on. 



Climatic Change as a Fundamental 

 Attribute of Climate 



Were atmospheric behavior to pro- 

 ceed randomly in time, the problem 

 of defining climate would reduce to a 

 straightforward exercise in statistical 

 sampling. We could make our esti- 

 mate of climate as precise as we wish 

 merely by choosing an average inter- 

 val that is sufficiently long. One diffi- 

 culty arises immediately because our 

 knowledge of past atmospheric be- 

 havior becomes less and less detailed 

 (and less and less reliable) the further 

 back in time we go. But there is an- 

 other, more important difficulty: If 

 our knowledge of past climates is im- 

 precise, it is at least good enough to 

 establish that long-term atmospheric 

 behavior does not proceed randomly 

 in time. Changes of climate from one 

 geological epoch to another, and ap- 

 parently also those from one millen- 



nium to another, are clearly too large 

 in amplitude to be explained as ran- 

 dom excursions from modern norms. 



When one examines modern recon- 

 structions of the paleoclimatic rec- 

 ord, one might be led to suppose that 

 geological changes of climate — such 

 as those associated with the alternat- 

 ing glacials and interglacials of the 

 Pleistocene ice age — are smoothly 

 varying functions of time, readily dis- 

 tinguishable from the much more 

 rapid variability of year-to-year 

 changes of atmospheric state. In 

 other words, one might suppose that 

 each part of a glacial cycle has its own 

 well-defined climate, just as each sea- 

 son of the year is revealed by modern 

 meteorological data to have its own 

 well-defined climate. In such a case, 

 the averaging interval needed to ob- 

 tain a stable estimate of present-day 

 climate should be long enough to sup- 

 press year-to-year sampling variabil- 

 ity, but short in comparison to the 

 duration of a glacial cycle. 



If we succumbed to the foregoing 

 rationale for defining climate, we 

 would probably be living in a fool's 

 paradise. The reason is simple 

 enough: the apparent regularity of 

 atmospheric changes in the geological 

 past is only an illusion, attributable to 

 the inadequate resolving power of 

 paleoclimatic indicators. Most such 

 indicators act to one degree or another 

 as low-pass filters of the actual cli- 

 matic chronology. If our more recent 

 experience — based on relatively 

 higher-pass filters such as tree-rings, 

 varves, ice-cap stratigraphy, and pol- 

 len analysis applicable to post-glacial 

 time — is any guide, the state of the 

 atmosphere has varied on most, if not 

 all, shorter scales of time as well. 



In other words, the variance spec- 

 trum of changes of atmospheric state 

 is strongly "reddened," with low- 

 frequency changes accounting for rel- 

 atively large proportions of the total 

 variance (in the broadband sense). 

 At the same time, important gaps in 

 the spectrum of climatic change have 

 yet to be identified and may not even 

 exist. Taken together, these circum- 

 stances imply that there may be no 

 such tiling as an "optimum" averag- 

 ing interval, and therefore no assur- 

 ance that we can define (let alone 

 measure) a unique, "best" estimate of 

 what constitutes average behavior of 

 the atmosphere. 



To summarize, atmospheric state is 

 known to vary on many scales of 

 time, and it cannot be ruled out from 

 present knowledge that it varies on all 

 scales of time (from billions of years 

 all the way down to periods so short 

 that they are better defined as mete- 

 orological variability). Thus it can be 

 argued that the very concept of cli- 

 mate is sterile as a physical descriptor 

 of the real world as long as it adheres 

 to the classical concept of something 

 static. In any event, present-day cli- 

 mate is best described in terms of a 

 transient adjustment of atmospheric 

 mean state to the present terrestrial 

 environment. 



The Problem of Causes 



If climate is inherently variable, as 

 here suggested, different interpreta- 

 tions can be lent to the variability. 



The "Slave" Concept — One inter- 

 pretation is the conventional one, 

 which can be called the "slave" con- 

 cept of climatic change. This em- 



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