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land or in an estuary taking decades or centuries, and it comes from evolution taking aeons. Climate change will not likely make land barren except at the arid extremes of existing climates. What is likely are changes in the composition of ecological communities in favor of those species able to move rapidly and far and the disappearance of some species that move slowly. The impacts of such changes on the functioning of ecological systems, and the consequent impacts on human society, cannot be predicted with confidence.

The Impacts of Some Conceivable Climate Changes are Large but cannot be Assessed

Although we know of no way to compute the probability of cataclysmic changes on the planet, like the reversal of the ocean current that warms Europe, large changes of climate have happened. New diseases and prolonged wars have caused calamitous centuries. Desperate masses have fled drought in places with marginal farming and growing populations. These disasters were not necessarily related to greenhouse gases, but they could be exacerbated by rapid future climate changes. Clearly, trends of economic and social development have lessened the vulnerability of many societies to climate. At the same time, trends such as increasing populations in river flood plains and low coastal areas have increased the vulnerability of some regions and nations. So, even if the United States could, by and large, adapt to climate change, the misfortunes of others unable to do so could substantially affect the United States and other industrial countries.

Over the next 50 years some nations will probably reduce their vulnerability to climate change, but others may become more vulnerable. Although migration is possible and domestic tranquility can sometimes be maintained, major climate changes could overtax societal capabilities, especially of poor countries. The probability and nature of such unexpected changes are unknown. Therefore, we cannot predict their impacts or devise adaptations to them.

References

Intergovernmental Panel on Climate Change. 1990. Climate Change: The IPCC Impacts Assessment, W. J. McG. Tegart, G. W. Sheldon, and D. C. Griffiths, eds. Canberra: Australian Government Publishing Service. Available, in the United States, from International Specialized Book Service, Portland, Oregon.

Smith, J. B., and D. A. Tirpak, eds. 1989. The Potential Effects of Global Climate Change on the United States. Washington, D.C.: U.S. Environmental Protection Agency.

United Nations Environment Programme and The Beijer Institute. 1989. The Full Range of Responses to Anticipated Climatic Change. Nairobi: United Nations Environmental Programme.



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