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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
Humanity and Nature Have the Potential
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