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increasing, and they could occur again. The panel knows of no
convincing attempt, however, to compute the probability of
cataclysmic changes such as the stopping of the current that warms
Europe. Because the probability and nature of such unexpected
changes are unknown, the panel cannot project their impacts or
devise adaptations to them.
As discussed in Chapter 3, a rise in global average temperatures
in the next century above those of any period in the last 200,000
years cannot be excluded. Unfortunately, there currently is no way
to reliably determine the effects of such global changes for
particular regions. These changes will probably be gradual. People
in the United States likely will have no more difficulty adapting
to such future changes than to the most severe conditions in the
past, such as the Dust Bowl.* Other countries may have more
difficulty, especially poor countries or those with fewer climate
zones. Some natural systems of plants and animals would be stressed
beyond sustainability in their current form, a prospect some people
may find unacceptable. The stronger the concern about these various
changes, the greater the motivation to slow greenhouse warming.
In addition, the panel has not found it possible to rule out or
rule in such major disturbances as sudden and major changes in
regional climates, ocean currents, atmospheric circulations, or
other natural or social phenomena. At present, it is not possible
to analyze their likelihood or consequences.
Human societies and natural systems of plants and animals change
over time and react to changing climate just as they react to other
forces. It would be fruitless to try to maintain all human and
natural communities in their current forms. There are actions that
can be undertaken now, however, to help people and natural systems
adjust to some of the anticipated impacts of greenhouse warming.
The panel recommends action now (see Chapter 9) based on gradual
climate change. Such action would be more important if climate
change proved to be sudden and unanticipated rather than smooth and