Steve Connor: It has been impossible to say these events were part of a bigger picture – until now


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The Independent Online

Meteorologists have always emphasised the difference between climate and weather, summing up the distinction by saying that "climate is what you expect, whereas weather is what you get".

In other words weather is the day-to-day stuff that comes out of the sky seemingly at random, whereas climate is what happens consistently, and more predictably, over many years, decades and even centuries.

Trying therefore to tease apart the climate change "signal" in any single extreme weather event has been a mug's game. And yet the basic physics of the climate suggests that this signal should be there if we can look hard enough. The world is getting hotter, and warmer air holds more moisture, which means that there there is a stronger probability of it falling back out of the sky in sudden, extreme downpours.

Warmer temperatures should also change the way the air circulates on large scales around the planet. For instance, warmer, drier air rising from the equatorial regions, once it has dumped its moisture in tropical downpours, should travel further north and south of the equator – increasing the risk of droughts spreading to areas such as the Mediterranean and the American South-west.

But this is theoretical. Linking extreme weather to global warming scientifically, whether it is a particularly vicious hurricane in the Caribbean or a drought in the Horn of Africa, has been considered nigh on impossible. Until now.

Kevin Trenberth of the US National Centre for Atmospheric Research in Colorado is one of several climate scientists who believes it is time to look again at extreme storms, floods and droughts to see if a change signal can be detected.

He believes, for instance, that Hurricane Katrina, which devastated New Orleans in 2005, was made much deadlier by the extra heat of the ocean and added moisture in the air.

"Instead of the observed 12 inches of rain they had in New Orleans [due to Katrina] they would have had 11 inches, but frequently these days the little bit extra is the straw that breaks the camel's back, it's that little bit extra that breaks the levy," Dr Trenberth said.

So too, he believes, with this year's deadly tornado season in the south-eastern part of the US which was continuously fuelled by very warm, moist air streaming inland from the Gulf of Mexico.

"It's a chain of events. You have got more warm, moist air near the surface that provides the fuel for thunderstorms. Converting an ordinary thunderstorm cell into a superstorm cell increases the risk of tornadoes being formed," Dr Trenberth said. "It's not a direct effect of climate change, but the environment in which tornadoes are spawned is in turn spawned by the environment that climate change is producing for thunderstorms."

This type of thinking will infuriate the climate doubters who dismiss any link of an extreme storm, flood or drought with climate change. But there is little doubt some climate scientists now want to go on the offensive, believing global warming really is beginning to have an impact on people's lives.

Linking extreme weather with climate change is now to be investigated scientifically. As Peter Stott of the Met Office said: "It's not helpful just to brush off this question."