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to locations where it is adequate to produce required supplies of food and fiber. Early agricultural societies like those of the Nile delta were built around seasonal variations in water flow, which affected their technology, their social organization, and even their religious beliefs. In many modern societies, a hazard insurance industry and programs of disaster transfer payments from government have arisen to help offset social and economic loss from the extreme weather conditions that are part of a variable climate.

Knowledge about climate is used not only to respond to extreme events—by reducing risk and exploiting climatic "windfalls"—but also to make minor adjustments to improve efficiency when variations are less extreme. In the United States, for example, an entire industry of consulting climatologists has developed to provide tailored climatic information routinely to clients in sectors such as the hydroelectric power industry, which can use this information to make incremental adjustments to planning and operations.

When Climate Becomes Hazardous

Climate does not always stay within the limits that social institutions plan for, and human adjustment is not perfect. One-hundred-year floods occasionally occur in consecutive years in the same watershed. Killing frosts occurring days, even weeks, after the "95 percent probability of last frost date" may happen two years in three. In such situations, when conditions fall outside the range of the expected, climate can become a hazard. An additional recent concern is the possibility that global climate change may increase the frequency or magnitude of extreme climatic events such as heat waves and major storms, making the systems that societies have put in place to cope with such events no longer adequate.

Climatic hazards come in many forms, from rapid-onset, short-lived events such as hurricanes, hail storms, and blizzards to slow-onset, long-lived fluctuations such as droughts. When climatic knowledge is poor, preparedness is low, and coping systems inadequate, climatic hazards exact severe social, economic, and environmental costs. By the same token, departure from normal climatic conditions can create new opportunities to be exploited.

Climate Sensitivity and Vulnerability

The sensitivity of human well-being to climatic variation is the extent to which important outcomes change as a function of that variation. Sensitivity is mainly indirect, in that climatic effects on human health and socioeconomic systems are in large part mediated by climate-sensitive



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