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 predictable.
*See dissenting statement by panel member Jessica Mathews at the end of Part One.