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ment has typically coupled long-term structural measures (e.g., dams and levees) and flood plain management with short-term warnings of impending flood events and emergency responses to actual floods.

Recent large flood events have spurred considerable rethinking of flood management policies and infrastructure design in the United States (Mount, 1995; Changnon, 1996; Pielke, 1996). Systems of dams and levees, which limit flood damage in most years, have been blamed for exacerbating the devastation caused by great floods, such as the Mississippi flood of 1993. Flood damages have increased in inflation-adjusted terms over the course of this century; however, it is not clear whether per capita or wealth-adjusted vulnerability to flooding has increased or decreased (Pielke, 1996).

The United States and other developed countries typically manage flooding as a hazard to be avoided and controlled; some societies in the developing world have designed their agricultural activities to make use of annual cycles of flooding. Villagers along the Senegal River, for example, plant their crops on bottomlands as the annual flood waters recede (Magistro, 1998). In such cases, more accurate forecasts of the timing and extent of annual flooding might help such societies to anticipate good and bad agricultural years, giving them additional lead time to implement such coping strategies as migration to cities to seek additional income.

More accurate long-term forecasts of regional flood probabilities might allow more effective planning and deployment of emergency flood management and relief operations and perhaps improved prioritization of federal levee repair and maintenance investments. However, currently available long-term flood outlooks are neither well understood nor effectively used by many public and private decision makers (Changnon, 1996; Pielke, 1997).

Human Health

Human health is sensitive to several types of climatic variation. Some sensitivities are to extreme events. Extreme temperatures cause hypothermia or heat stress in unprotected individuals, and precipitation shortfalls can bring droughts that reduce crop yields, resulting in famine and malnutrition. Climate can also affect human health more indirectly through its effect on ecosystems. An important instance is changes in the ecology of infectious disease organisms or their vectors that can precipitate disease outbreaks. An illustration is the story of the hantavirus outbreak of 1993. A prolonged drought in the U.S. Southwest in the early 1990s reduced the populations of animals such as owls, coyotes, and snakes that prey on rodents. When the drought yielded to intense rains associated with the 1992-1993 El Niño, the grasshoppers and piñon nuts

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