Stefan Rahmstorf: Summer of extremes must be last wake-up call

This summer has been one of weather-related extremes in Russia, Pakistan, China, Europe, the Arctic - you name it. But does this have anything to do with global warming, and are human emissions to blame?

While it cannot be proven (or disproven) scientifically that global warming caused any particular extreme event, we can say that global warming very likely makes many kinds of extreme weather both more frequent and more severe.

For weeks, central Russia was gripped by its worst ever heat wave, which has caused probably thousands of fatalities. Russia's government has banned wheat exports, sending world grain prices soaring. Meanwhile, Pakistan is struggling with unprecedented flooding that has killed more than a thousand people and affected millions more. In China, flash floods have so far killed more than a thousand people and destroyed more than a million homes. .

This decade has been marked by stunning extremes. In 2003, the most severe heat wave in living memory caused 70,000 deaths in Europe. In 2005, the most severe hurricane season ever witnessed in the Atlantic devastated New Orleans. In 2007, unprecedented wildfires raged across Greece. And the North-west Passage in the Arctic became ice-free for the first time in living memory. Last year, more than a hundred people were killed in bush fires in Australia, following drought and record-breaking heat.

These events could be merely an astonishing streak of bad luck. But that is extremely unlikely. This is far more likely to be the result of a warming climate — a consequence of this decade being, worldwide, the hottest for 1,000 years.

All weather is driven by energy, and the Sun ultimately provides this energy. But the biggest change in Earth's energy budget by far over the past 100 years is due to the accumulation of greenhouse gases, which limit the exit of heat into space. Owing to fossil-fuel emissions, there is now a third more carbon dioxide in the atmosphere than at any time in a million years, as ice drilling in Antarctica has revealed.

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

l Stefan Rahmstorf is professor of physics of the oceans at Potsdam University, and a member of the German Advisory Council on Global Change.