Some weather extremes simply lie beyond Gaussian distribution

When assessing the climate system it doesn’t hurt to include some geography to your statistical assumptions. After all, without mountains, oceans and coastlines there would be no weather, just one boring climatic average – and therefore no ground to create that normal distribution in the first place…

As for instance illustrated by the 2007 IPCC report extreme weather events are likely to increase more rapidly than any seemingly small shift in average values would lead to suspect.

IPCC 2007: increasing extremes according to Gaussian probability distribution

That is a general characteristic of the Gaussian or normal probability distribution – as shown to the right.

But new research indicates the physical reality may not bend so neatly to this simple statistical model. Under the current climate change skewness of the graph could also increase. And then there is the chance that some weather events simply show little relation to the climatic average.

That is what a research group of the University of California in Los Angeles hints at – after examining an established dataset of US temperature measurements. In their publication in Geophysical Research Letters they show the tails of the Gaussian curve could be longer than we thought.

Good news?

This would imply extreme events are more natural to climate than we may have thought. Also, when the graph would be steeper, an increase in average values would not necessarily translate to an equally high increase in the probability of extreme weather [although that does require a statistical definition of extreme, like >2SDs].

But let’s use common sense here. As the graph above shows in the climate of the US such long tails occur either at the minimal or at the maximum extreme end of the curves. In Texas for instance it is usually mild to warm (winter/summer), but it can suddenly be much colder. In California on the other hand it is normally agreeable, but it can also be really hot.

That last situation we call an El Niño, the first situation are cold winds blowing in from the north, along that long N-S corridor that is protected by the Rocky Mountains. And indeed, in winter the Canadian climate has little in common with the Texas average.