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things being equal, the uncertainty of scenarios reduces the rank of climate change as an issue.

The findings in Part Four about impacts of climate change generally agree with those of other U.S. and international investigations (Smith and Tirpak, 1989; United Nations Environment Programme and The Beijer Institute, 1989; Intergovernmental Panel on Climate Change, 1990). We, however, direct most of our attention to adaptations rather than to impacts.

Enormous uncertainties attend any analysis of climate change and adaptation to it. The present report is necessarily only a statement of present knowledge and is thus a beginning. One of its functions is encouraging further assessments, especially of the indirect costs of adaptation.

An activity that is affected by a change in the weather tomorrow could be insensitive to climate change if it were adaptable and its renewal were faster than the rate of climate change prolonged through decades. If we ignore adaptations, we imagine the climate near the middle of the next century imposed on the people of today, the way they live, and their current natural environment. So, adaptation can change the sensitivity to climate change as time passes and thereby change its rank as a policy issue. The reader will read below that human activities can change fairly rapidly and natural ecosystems more slowly, whereas evolutionary adaptation by genetic changes in populations of organisms is generally even slower.

Humanity and Nature Have the Potential to Adapt

Human adaptability is shown by people working in both Riyadh and Barrow and seeking out both Minneapolis and Galveston. Recent American migration has on average been toward warmth.

There are limits on the speed of human responses. These limits make not only the direction but also the rates of climate change crucial. People need time to adapt in situ to a new climate or to move to a region of preferred climate. If they move, they must find places where the other components of the environment, like soil and water, also fit them. Although time is taken to adapt managed things like farming, the historical evidence suggests that American farmers can keep up with gradual climate change of the magnitude the panel assumes.

The capacity of humans to adapt is evident in the rapid technological, economic, and political changes of the past 90 years. The average renewal period for machinery and equipment and the average age of buildings are one to three decades. So, through continuing normal investment, humanity's business activities have the potential to adapt during the next half century to the types of changes upon which our analysis is predicated.

Another factor that may limit adaptation is water. Some activities, like