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biophysical systems. For example, human nutrition is sensitive to climate mainly because crop production is sensitive to climate, and crop production is sensitive because of climatic effects on such factors as local rainfall and the spread of crop pests and diseases. So both biophysical and socioeconomic systems may be sensitive to climate, and many of the socioeconomic effects are due in part to the biophysical ones.

The human consequences of climatic variation depend on the behavior of social systems as well as on biophysical events. To the extent that a society or social group understands or accurately anticipates climatic events and their biophysical effects, it may be able to buffer the negative effects of these events and take advantage of climatic opportunities, thus decreasing sensitivity on the downside while exploiting it on the upside. Modern production agriculture and those whose livelihoods depend on it remain sensitive to variability in temperature and precipitation despite decades of technical and social innovation aimed at reducing sensitivity by controlling access to water; limiting infestations of pests, weeds, and diseases; insuring against catastrophic loss; developing drought-tolerant and disease-resistant seed varieties; and the like.

Often sensitivity is greatest at ecological, economic, and social margins. Faunal and floral communities in areas straddling the margins (boundaries) of ecosystems—natural and managed—are less stable with respect to climate variability than communities safely in the interiors (Blaikie and Brookfield, 1987). Similarly, the poor, the elderly, the infirm, and other marginal segments of society often bear a disproportionate share of the total social costs of climatic variability (Blaikie and Brookfield, 1987). In such cases, a relatively minor climatic fluctuation may cause disproportionately large consequences. With appropriate policies in place, the most affected groups may therefore gain great benefits from the use of climate forecasts.

Our definition of sensitivity includes human efforts to adapt to climate in that it refers to outcomes after taking into account things people do to cope with expected climatic variations. This definition contrasts with that employed by some other writers, whose concept of sensitivity presumes that the human consequences of climatic events can be meaningfully analyzed independently of adaptive behavior. We do not find this approach useful because, as we elaborate in Chapter 3, human societies, and particularly the conduct of weather-sensitive activities, has coevolved with climate and has always included a range of adaptive strategies. Thus, sensitivity—a measure of the functional relationship between climatic events and human outcomes—is a property of human groups or activities that have particular adaptations in place. Changing the adaptations can change sensitivity.

We use the term "vulnerable" to refer to human groups or activities

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