For each degree Celsius of warming, 7 percent more water is available to rain down from saturated air masses. Drought risk also increases with warming: even where rainfall does not decline, increased evaporation dries out the soils.

Carbon-dioxide

The carbon-dioxide effect can also change the preferred patterns of atmospheric circulation, which can exacerbate extremes of heat, drought, or rainfall in some regions, while reducing them in others.

The problem is that a reduction in those extremes to which we are already well-adapted provides only modest benefits, whereas the new extremes to which we are not adapted can be devastating, as recent events in Pakistan show.

The events of this summer show how vulnerable our societies are to weather-related extremes. But what we see now is happening after only 0.8 degrees Celsius of global warming. With swift and decisive action, we can still limit global warming to a total of 2 degrees Celsius or a bit less.

Even that much warming would require a massive effort to adapt to weather extremes and rising sea levels, which needs to start now.

With weak action, like that promised by governments in Copenhagen last December, we will be on course for 3-4 degrees Celsius of global warming.

This is bound to outstrip the ability of many societies and ecosystems to adapt. And, with no action at all, the planet could even heat up by 5-7 degrees Celsius by the end of this century - and more thereafter. Knowingly marching down that road would be insane.

We must face the facts: Our emissions of greenhouse gases probably are at least partly to blame for this summer of extremes. Clinging to the hope that it is all chance, and all natural, seems naive.

Let us hope that this summer of extremes is a last-minute wake-up call to policy makers, the corporate world, and citizens alike.

(The author is professor of physics of the Oceans at Potsdam University, and a member of the German Advisory Council on Global Change. His latest book is "The Climate Crisis "(co-written with David Archer). Copyright: Project Syndicate, 2010.www.project-syndicate.org)