Monday, 21 May 2012

What is a change in extreme weather?

What is a change in extreme weather?

The reason for changes in extremes can be divided up into two categories: changes in the mean (see panel a of the figure below) and other changes in the distribution (simplified as a change in the variance in panel b). Mixtures are of course also possible (panel c).

If you are interested in the impacts of climate change, you do not care why the the extremes are changing. If the dikes need to be made stronger or the sewage system needs larger sewers and larger reservoirs, all you need to know is how likely it is that a certain threshold is reached. Much research into changes in extreme weather is climate change impact research and thus does not care much about this distinction.

If you are interested in understanding the climate system, it does matter why the extremes are changing. Changes in the mean state of the climate are relatively well studied. Interesting questions are, for instance, whether a change in the mean changes the distribution via feedback processes or whether the reduced temperature contrasts between the poles and the equator or between day and night cause changes in the distribution.

If you are interested in understanding the climate system also the spatial and temporal averaging scales matter. If rain fronts move slower, they may locally produce more extreme daily precipitation sums, while on a global scale or instantaneously there is no change in the distribution of precipitation.

I hope scientists will distinguish between these two different ways in which extremes may change in future publications and, for example, not only compute the increase in the number of tropical days, but also how many of these days are due to the change in the mean and how many are due to changes in the distribution. I think this would contribute to a better understanding of the climate system.

Figure is taken from Real Climate, which took it from IPCC (2001).

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