In the past four years, significant progress has been made in the modeling of climate and climatic changes, in the understanding of mechanisms of climatic change, and in the diagnosis of climatic changes, both present and past. At the same time, it is apparent that the ‘need to know’ has become a matter of rapidly increasing practical importance and that the frontier of unsolved problems is expanding. Climatic change and environmental change have emerged as items of high priority [see, for example, CEQ, 1970]. A concern that human activities may be rivaling natural processes in affecting the stability of the atmosphere-hydrosphere-biosphere system is attested to by recent reports. For example, a symposium on global effects of environmental pollution [AAAS, 1970] and a report on critical environmental problems [SCEP, 1970] provide surveys of the climatic and ecological effects of human activities. Many of the points reviewed briefly here receive detailed consideration in these reports. The SCEP report includes lists of recommendations for action. Whereas attention focused formerly on describing and explaining past climates, increased attention is focused currently on predicting possible future climates [Bryson and Wendland, 1970; Fletcher, 1969a; Mitchell, 19707semi; Robinson, 1970; University of Wisconsin, 1970].