Climate as defined is associated with a certain probability distribution of weather events. Weather events with values far away from the mean (such as heat waves, droughts and flooding) are by definition less likely to occur. The least likely events in a statistical sense are called �extreme events�. Extreme weather in one region (e.g. a heat wave) may be normal in another. In both regions nature and society are adapted to the regional weather averaged over longer periods, but much less to extremes. For example, tropical African temperatures could severely damage vegetation or human health if they occurred in Northern Europe. Impacts of extreme events are felt strongly by ecosystems and society and may be destructive.

Small changes in climate may, but will not necessarily, have a large impact on the probability distribution of weather events in space and time, and on the intensity of extremes. Nature and society are often particularly ill prepared and vulnerable for such changes. This is the reason why since the SAR much more attention has been paid to observed and projected variations of extremes. Chapter 2 gives an assessment of the present knowledge.

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