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Gareth at Hot Topic has blogged about the range of recent extraordinary weather events around the world (see Fire and rain). And there have been plenty of news reports bringing this to our attention. The Russian heat wave and fires. The floods in Pakistan and China.

Many are asking if this is just coincidental. Or should we attribute them, at least in part, to global warming resulting from human activity. Peter Stott at the Guardian (Climate change: how to play our hand?) and John Scott at Discovery News (Russian Heat, Asian Floods Share Common Cause) say we should.

However, no one is making a direct attribution. Rather they are say that global warming is likely to increase the incidence of extreme weather events. And perhaps that is what we are seeing now.


Climate as “average weather”

Climate is not the same thing as weather. It’s more the “average weather.” As has been said “A single rainy day does not make a wet climate”. Nor does an extra cold winter “prove” that global warming has stopped. Although I have noticed that a cold winter does bring out the climate change deniers in droves.

When we talk about climate change we are referring to trends averaged over many years. Usually 20 – 30 years, but even longer when data is sparse or “noisy”.

So a few extreme weather events in themselves don’t indicate global climate change. It’s just that when the frequency of such events increase as they currently appear to, one suspects this is the result of the changed climate.

The reports of the Intergovernmental Panel on Climate Change (IPCC) point out that:

“simple statistical reasoning indicates that substantial changes in the frequency of extreme events (and the maximum feasible extreme, e.g., the maximum possible 24-hour rainfall at a specific location) can result from relatively small shift of the distribution of a weather or climate variable.”

The last IPCC report illustrated this with the figure below (see Box TS.5 Figure 1 – AR4 WGI Technical Summary.):

These curves represent the distribution of probability of different weather events. Temperature is described here but other weather factors such as rainfall, etc. could be used.

So even a relatively small increase in average global temperature – represented by going form the lefts hand curve to the right hand one – will increase the frequency of extreme events. In this case hot weather.

Of course, this figure is idealised. The probability distributions may be more complex. And regional effects may come into play. For example, there is a suggestion that increased global temperatures could cause a reduction in the ocean currents such as the Gulf Stream. This could actually bring colder temperatures to Europe.

A dangerous future – politically as well as weather-wise

Jean-Pascal van Ypersele, vice-president of the IPCC, describes the current extreme weather events as ones which:

“reproduce and intensify in a climate disturbed by greenhouse gas pollution. Extreme events are one of the ways in which climatic changes become dramatically visible.”

This was one of the main messages I got from James Hansen‘s recent book Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity (see Thinking of our grandchildren). The effects of climate change will not be just a gradual increase in global temperatures. It will also be increased frequency of extreme weather events. This, together with the projected increase in sea levels, will inevitably cause widespread disruption to human societies.

Personally I think this, and the inevitable political and social disruption it will cause globally, is the most frightening aspect of climate change.

See also: The dire probabilities of unusual weather

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