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When should we blame climate change for natural disasters?

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The Intergovernmental Panel on Climate Change has a new report out today concluding that global warming will make heat waves, droughts, floods, and other extreme weather events much more common in the decades to come. (It’s less clear how climate change will affect hurricanes and tornadoes.) Already in 2010, the United States saw a record number of natural disasters costing $1 billion or more, and the IPCC is warning us not to be shocked if we see even more destructive weather as the Earth heats up.

Athit Perawongmetha/Getty

Can we sue coal plants for this yet? Yet many climate scientists have been trying to go a step further than the IPCC in recent years. It’s one thing, after all, to say that a warming world will “load the dice” and make extreme weather events more likely. But is it possible to pinpoint specific disasters that are occurring — say, last year’s deadly heat wave in Moscow — and say with confidence that global warming is causing those events? That they’re not just freak occurrences? And, if so, could that lead to more climate-related lawsuits in the years to come?

It’s a surprisingly tricky question. On the one hand, scientists are quite confident that climate change is now causing an uptick in extreme weather events across very broad areas. A recent study in Nature, for instance, found that the extra greenhouse gases that humans are putting into the air are driving heavier rainfall patterns in the Northern hemisphere. A warmer planet means more water vapor in the atmosphere, which in turn makes storms stronger. Human influence is clear there. But what about specific disasters? Is it possible to get that precise?

That’s what a lot of new research is trying to answer. In October, Stefan Rahmstorf and Dim Coumou published a paper in the Proceedings of the National Academies of Science that looked at at the 2010 heat wave in Moscow, when temperatures hit record highs and killed hundreds. The researchers examined more than a century of data and used statistical techniques to tease out natural variability from the long-term warming trend. They concluded there was an 80 percent likelihood that Moscow’s heat wave would never have occurred without man-made global warming.

“Our study was of a loaded-dice type,” Rahmstorf told me. “We’re not attributing an individual event to a cause. We’re saying that this sort of weather event has become five times more likely.” But, he added, “in practice, when you say it’s 80 percent likely, then it’s very close to a single-event attribution. The dice is loaded so strongly that you can say it’s almost certain” that global warming caused the heat wave. The PNAS paper introduced a new methodological technique — using what’s known as Monte Carlo simulations — to make these sorts of attributions, which opens up new avenues of exploration.

Similarly, a recent peer-reviewed paper by researchers at the National Ocean and Atmospheric Administration concluded that climate change was partly responsible for a series of recent wintertime droughts in the Mediterranean region. “The magnitude and frequency of the drying that has occurred is too great to be explained by natural variability alone,” said NOAA’s Martin Hoerling, the lead author of the study, in a news release.

In a phone interview, Hoerling explained the challenges of sorting out just how much blame to assign global warming for any given disaster: “Imagine you have a lot of historical data and you’ve decided that a heat wave can never get bigger than 9°C,” he says. “That’s the biggest heat wave possible from natural factors. So then you find that, because of human influences, temperatures have warmed 1°C. And now along comes a 10°C heat wave. You can say that this never would have happened without human influence. But you could also say that 90 percent of it was due to natural variability.”

Hoerling notes that it’s a lot easier to separate out the climate signal in heat waves, since temperature data is fairly straightforward. It’s much harder, however, to assign blame for individual storms, since natural variability for precipitation is stronger and more erratic. “That involves a lot more complexity: how does this storm track, how do changes in ocean temperature factor in?” he said. Even though we know that humans are having an overall influence on precipitation patterns, getting precise is still a challenge.

We’re likely to see more climate research along these lines in the future. But why does it matter? As Peter Stott, of UK’s Met Office Hadley Center, has pointed out, it’s quite possible that we could see more legal suits being brought against polluters for climate-caused natural disasters in the years ahead. In that case, he notes, “there would be a requirement for objective and scientifically robust information” on the causes of specific disasters. The science of weather attribution isn’t quite at that point yet, but it’s progressing rapidly.

Still, even if we can’t always assign blame for an isolated disaster, it’s becoming amply clear that a warming world will, on the whole, see more extreme weather. Joe Romm has a roundup of even more recent science (and charts) on this phenomenon.

Related: My colleagues Jason Samenow and Andrew Freedman have great posts on this topic.

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