(Gornitz et al., 1997). Finally, pollution of existing groundwater supplies is a persistent problem in many regions. Understanding the causes and consequences of these deficits and problems is critical for managing the future of water resources on earth (Kling, 2001).

Third, a nonlinear response exists between soil moisture and the production of greenhouse gases. As soil dries, microbes limited by water respire less soil organic matter, which produces less carbon dioxide and methane. As soil moisture increases, respiration increases and more greenhouse gases are produced. But if the soils are waterlogged, they become anoxic and overall microbial activity is again reduced (leading to increased net carbon storage). Thus knowing where any system lies on this nonlinear response curve is necessary to predict how greenhouse gas production will change as water availability changes (Kling, 2001).

Fourth, many biological systems are nonlinear in their response to water. Outbreaks of vector-borne diseases, such as cholera outbreaks linked to El Nino and water-temperature changes (Colwell, 1996; Pascual et al., 2000) and other unpleasant events are often linked to deviations in precipitation patterns (Balbus and Wilson, 2000). Both extremes in precipitation can be harmful, because some outbreaks are linked to precipitation increases and flooding, while others are associated with droughts. Both extremes lead to stresses on organisms and ecosystems, and as such the biological nonlinear and complicated response may have undesirable effects for humans.

There is virtually no direct research on the economics of water systems and abrupt climate change at this time (exceptions are Glantz, 1999; Arnell, 2000). Given the importance of water for many aspects of society, from agriculture to electricity production, as well as the documented record of periodic droughts, this should be one of the priority areas for research.

Human and Animal Health

Yet another area of potential concern is health—both of humans, of domesticated plants and animals, and of wildlife (National Research Council, 1999a). There is widespread appreciation of the potential for unwelcome invasions of new or exotic diseases in the human population, particularly of vector-borne diseases such as malaria. Similar concerns may arise for pests and diseases that attack livestock or agriculture. Another concern is diseases of wildlife.

Scenarios based on climate models for greenhouse warming indicate that changes will occur in the geographic distribution of a number of water-



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