1.2.3 Extreme Events
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.