to remain obscure until climate-dynamics research finally progresses to a point where it can confirm that merit. By then, the modeling framework available to illuminate climatic dynamics would make possible a much more comprehensive assessment of reality.
If society is to embrace any deterministic climate projection as a basis for sound future planning, it should be clear that two criteria have to be met. First, the scientific basis of the projection must be rational. Second, the projection must be accompanied by a suitably objective and unbiased measure of its own reliability. No deterministic projections of future climate now available can be said to meet both of these criteria.
Making reliable projections of future climate of any kind will likely remain an elusive goal until our knowledge of climate dynamics is adequate to provide the answers to two sets of fundamental questions.
The first set of fundamental questions to be answered concerns the natural climate system: (1) Were man’s various impacts on environment to be held in the future at their present-day levels, how would the earth’s climate evolve from its present state in the decades and centuries ahead? (2) Is the answer to (1) knowable? Or is it the case that the natural evolution of climate is a probabilistic process in time, such that, after a relatively short interval, the state of global climate will be essentially unrelated to its present state? (3) To the extent that the answer to (1) is knowable, to what further extent is it within man’s power to find that answer?
The second set of fundamental questions to be answered concerns man himself in relation to his potential capacity for interfering in the workings of the natural climate system: (4) On the basis of each of a variety of credible alternative scenarios as to future increases and proliferation of his impacts on environment, can man assess the consequences of those changing impacts on the natural evolution of climate in the decades and centuries ahead? (5) Can man establish which, if any, alternative scenarios would lead to “unacceptable” climatic consequences and are therefore to be avoided? (6) If the answer to (5) is yes, will man come into possession of such knowledge in time to avert possible calamity?
The very essential matter of climate predictability is addressed by question (2). Should climate turn out to be inherently unpredictable after a short interval of future time, then the capacity of science ever to provide answers to the other questions posed here will be limited, perhaps severely so.
Up to this point we have mainly been referring to deterministic projections of climate, by which we mean statements as to the “expected” climate in a specific future interval of time or in each of a series of future intervals. Also to be considered are probabilistic projections of climate, in which the conditions to be projected for specified future intervals of time are not the “expected” climate as such but rather the probability density distribution of the full range of outcomes of climate that the statistics of past climates indicate to be possible. Such probabilistic projections are normally assessed through statistical inference rather than physical inference and are based essentially on past experience rather than physical understanding with regard to climatic variability. Projections of this kind are always meaningful, in the sense that even if no organized time-series structure whatever is evident in past climatic behavior, future projections can still be made. In that case, the projections would reduce to unconditional statements to the effect that the future probability of any given climatic condition (at any time in the future) is identical to that of the same climatic condition in the past.
The record of past climates deserves our very close scrutiny, to learn the full extent of organization in the time-series structure of climatic variations that could be used to frame improved probabilistic projections of future climate. To date, surprisingly little has been done along these lines.
Viewing past climates in the perspective of events of the past million years, we can at least venture the conclusion that, in the long run, natural forces will be more likely to relax our global climate toward a glacial condition than to maintain climate indefinitely in its present, unusually warm interglacial condition. When the return to a glacial condition may begin and at what rate global climate will be altered when it does begin are by no means obvious from the paleoclimatic record.
Viewing past climates in the somewhat shorter perspective of the postglacial period, the sequence of “neoglacial” events appears to be a dominant feature. It would be tempting to suggest that the last neoglacial event, the “Little Ice Age” of the fifteenth to nineteenth centuries, is behind us and that the next comparable event will not arrive for another thousand years or more. On the other hand, we should keep in mind evidence that the Little Ice Age was appreciably shorter than earlier neoglacial events. This invites an alternative view of the comparative warmth of the twentieth century, as being not a recovery from the Little Ice Age but perhaps only an interruption of it.
Beyond such vague statements as these, disturbingly little can be said about the probable course of natural climate in the decades and centuries ahead. Moreover, a number of man’s activities are being ingested by the climate system, adding further uncertainty to an already uncertain future of climate. The need for better understanding of the climate system, in all its aspects, is a clear and present one.