CAN WE ASSESS THE IMPACTS OF CLIMATIC CHANGES? JESSE H. AUSUBEL Board on Atmospheric Sciences and Climate, National Research Council, 2101 Constitution Ave., Washington D.C. 20418, U.S.A. Abstract. Assessing the impact of a given human-induced climatic change, like a global warming brought about by carbon dioxide, involves balanced examination of the implications of the specific environmental change and the other implications of the assumptions which generate the COz in the first place. There will be many, simultaneous impacts on ecosystems; these are illustrated by looking at effects on photosynthesis and on marine biota. For several reasons economics is quite limited in the evaluation it can provide of such effects. Nevertheless, useful 'scenarios' can be constructed by combining description of possible CO2-induced impacts with the other implied changes, for example, the increases in world mining, trading, and burning of coal. Several, often divergent pictures are likely to persist because of inherent uncertainties and cultural bias. 1. Introduction The title of this paper is a question, 'Can we assess the impacts of climatic changes?' In fact this title raises at least two kinds of questions. The first is the familiar one of what are biological, agricultural, and economic effects of the atmosphere becoming considerably warmer and enriched with CO2 over the next fifty to one hundred years? The second is how many 'we's' are there? How many sets of facts will there be? How many interpreta- tions of the sets of facts? This paper offers some brief comments on both kinds of questions. The first three sections treat the more conventional side of assessment, discussing in turn a couple of specific ecological impacts, namely CO2 fertilization and marine effects, some of the limitations of economics in relation to such impacts, and the need to assess the CO2-induced impacts jointly with other implications of the assumptions which create the altered environment. Then the paper turns to the problem of 'the several we's'. 2. Ecological Impacts I Discussion of ecological impacts of C02 and climatic change must start with a disclaimer. The amount of systematic research that has been conducted on ecological aspects of climatic change is small. Impacts of climatic changes on individual, isolated organisms remain quite uncertain, and net impacts for whole communities or ecosystems are also very uncertain. Forces at play may act simultaneously in different directions. Nevertheless, the magnitude of the changes contemplated is large from an ecological perspective. One must go back thousands, or even tens of thousands, of years to find change of such magnitude. Based on geographical and paleoclimatic evidence, one can conclude that a temperature change like that expected to accompany a doubling of C02 could by itself shift ecosystem boundaries polewards by hundreds of kilometers. In combination with changes in rainfall and C02 fertilization, the consequence may be Climatic Change 5 (1983) 7-14. 0165-0009/83/0051-0007501.20. Copyright 9 1983 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.