Nowadays, when there's a killer heat wave or serious drought somewhere, people wonder: Is this climate change at work? It's a question scientists have struggled with for years. And now there's a new field of research that's providing some answers. It's called "attribution science" - a set of principles that allow scientists to determine when it's a change in climate that's altering weather events -- and when it isn't.

 

The principles start with the premise that, as almost all climate scientists expect, there will be more "extreme" weather events if the planet warms up much more: heat waves, droughts, huge storms.

 

But then, there have always been periodic bouts of extreme weather on Earth, long before climate change. How do you tell the difference between normal variation in weather — including these rare extremes — and what climate change is doing?

 

That sort of discernment is difficult, so scientists have had a rule, a kind of mantra: You can't attribute any single weather event to climate change. It could just be weird weather.

 

Then they took a close at last year's heat wave in Australia.

 

The chances that the continent's extreme temperatures reflected normal variation is "almost impossible," says Peter Stott, a climate scientist at the Hadley Center of the Met Office, in Exeter, Great Britain. "It's hard to imagine how you would have had those temperatures without climate change," he says.

 

Stott is one of a group of researchers analyzing the patterns of "extreme weather" events in the past and comparing them with the patterns Earth is experiencing now. The intensity of last year's Australian heat wave was statistically "off the charts," he says. Climate change had to be behind it.

 

 

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