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that face the risk of extreme negative outcomes as a result of climatic events that overwhelm the adaptations they have in place. Vulnerability, like sensitivity, is a function of both climatic events and human adaptation. We use separate terms to reflect the special importance most societies give to the risk of catastrophic (i.e., extreme negative) outcomes. It is important to recognize that, as with sensitivity, human activities can increase or decrease vulnerability. For instance, urban development in hurricane-prone coastal areas increases the risk from hurricanes even when the frequency of hurricane events remains unchanged. Increasing population and affluence in the arid western United States have stimulated rising demand for essentially fixed water supplies; this has increased the risk from drought apart from fluctuations in precipitation. Systems of flood-control dams decrease vulnerability to flood damage from most major storms, but they may increase the damage caused by the most extreme ones. Actions that affect the distribution of income also affect the vulnerability of human populations to extreme negative climatic events by altering the resources people have to prepare and respond.

Sensitivity and vulnerability to climate variability constantly change over time. Some reduction or increase in sensitivity, and particularly in vulnerability to extreme events, may be the unintended result of fundamental structural social changes accompanying social development. For example, as the general level of affluence and technological sophistication rises in a developing country, changes in food preferences (for example, wheat over millet, meat over grain) may lessen (or strengthen) dependence on resources that are directly affected by seasonal-to-interannual climate variability. As people depend increasingly on world markets for food, their well-being becomes less sensitive to local climate variations, but perhaps more sensitive to distant climatic events that may threaten their supply lines.

The Potential Usefulness of Climate Forecasts

Climate forecasting can benefit people by allowing them to change the things they do to anticipate climatic events, thus reducing their sensitivity to negative events and perhaps increasing their sensitivity to positive events. The potential value of skillful climate forecasts may or may not be greatest in those regions where the predictive skill is the greatest. The greatest value may be found in the regions where climate variability has the largest economic impacts (positive or negative), or where vulnerability is greatest and adequate coping mechanisms can be provided. In regions where impact or vulnerability is very large, even a small increase in forecast skill may be of great value, even if the predictions are not as certain as in other regions. Therefore, a focus on improving forecast skill