Changes in climate extremes and their impacts on the natural physical environment
2012
This chapter addresses changes in
weatherand
climateevents relevant to extreme impacts and disasters. An
extreme(
weatheror
climate) event is generally defined as the occurrence of a value of a
weatheror
climatevariable above (or below) a threshold value near the upper (or lower) ends (‘tails’) of the range of observed values of the variable. Some climate extremes (e.g., droughts, floods) may be the result of an accumulation of
weatheror
climateevents that are, individually, not extreme themselves (though their accumulation is extreme). As well,
weatheror
climateevents, even if not extreme in a statistical sense, can still lead to extreme conditions or impacts, either by crossing a critical threshold in a
social,
ecological, or physical system, or by occurring simultaneously with other events. A
weathersystem such as a
tropical cyclonecan have an extreme impact, depending on where and when it approaches
landfall, even if the specific cyclone is not extreme relative to other
tropical cyclones. Conversely, not all extremes necessarily lead to serious impacts. [3.1] Many
weatherand
climateextremes are the result of natural climate variability (including phenomena such as
El Nino), and natural decadal or multi-decadal variations in the climate provide the backdrop for anthropogenic climate changes. Even if there were no anthropogenic changes in climate, a wide variety of natural
weatherand
climateextremes would still occur. [3.1] A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of
weatherand
climateextremes, and can result in unprecedented extremes. Changes in extremes can also be directly related to changes in mean climate, because mean future conditions in some variables are projected to lie within the tails of present-day conditions. Nevertheless, changes in extremes of a climate or
weathervariable are not always related in a simple way to changes in the mean of the same variable, and in some cases can be of opposite sign to a change in the mean of the variable. Changes in phenomena such as the
El Nino-Southern Oscillationor monsoons could affect the frequency and intensity of extremes in several regions simultaneously.
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