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The subtle — but very real — link between global warming and extreme weather events

President  Obama passes an image of a hurricane during a tour of the National Hurricane Center in Miami last week.  (Reuters/Kevin Lamarque)

Last week, some people got really mad at Bill Nye the Science Guy. How come? Because he had the gall to say this on Twitter:

Billion$$ in damage in Texas & Oklahoma. Still no weather-caster may utter the phrase Climate Change.

Nye’s comments, and the reaction to them, raise a perennial issue: How do we accurately parse the relationship between climate change and extreme weather events, as they occur in real time?

It’s a particularly pressing question of late, following not only catastrophic floods in Texas and Oklahoma, but also a historic heatwave in India that has killed over 2,000 people so far, and President Obama’s recent trip to the National Hurricane Center in Miami, where he explicitly invoked the idea that global warming will make these storms worse (which also drew criticism).

As the Nye case indicates, there is still a lot of pushback whenever anyone dares to link climate change to extreme weather events. But we don’t have to be afraid to talk about this relationship. We merely have to be scrupulously accurate in doing so, and let scientists lead the way.

Take the floods. One exemplary voice here has been Texas Tech climate researcher (and evangelical Christian) Katharine Hayhoe, who took to Facebook to explain the science. As Hayhoe noted, climate change doesn’t “cause” individual extreme events, in this case or in others. But “just like steroids make a baseball player stronger, climate change EXACERBATES many of our weather extremes, making many of them, on average, worse than they would have been naturally,” she said.

Thus, Hayhoe treated the link between a changing climate and the floods not as a matter of simple causation, but as a matter of context. She notes that overall, “heavy rainfall and flood risk is increasing,” due to the fact that warming charges the atmosphere with more water vapor, which is then more available to fall in individual precipitation events. (Texas state climatologist John Nielsen-Gammon has made similar remarks. )

And indeed, Texas lies in a region of the  country that has seen, overall, a 16 percent increase in the amount of rain or snow that falls in the heaviest 1 percent of precipitation events, according to the National Climate Assessment:

And what about India’s extreme heat? Here again, we must bear in mind that extreme weather events are not directly caused by climate change. Indeed, weather extremes can occur  — and weather records can break — due solely to natural climate variability.

Nonetheless, and as with past major heat extremes, such as Australia’s 2012-2013 “angry summer,” the odds of an event like this one occurring may have shifted. Indeed, meteorologist Jeff Masters of the Weather Underground has directly stated that the heat wave “was made much more probable by the fact that Earth is experiencing its hottest temperatures on record.”

Precisely how the odds of an event like this one have changed will, no doubt, soon be formally studied by climate researchers, who use a sophisticated statistical methodology. We don’t know how such an inquiry will turn out, but it’s worth noting that after the deadly 2003 European heatwave, scientists estimated that “human influence has at least doubled the risk of a heatwave exceeding this threshold magnitude.” Indeed, a recent study found that 75 percent of all “moderate” heat extremes on Earth — events that would only happen one out of every 1,000 days in a normal climate — are now more likely to occur because of climate change.

[Study: Global warming has dramatically upped the odds of extreme heat events]

Overall, all of this is consistent with a finding from a recent special report on extreme events by the U.N.’s Intergovernmental Panel on Climate Change, which noted that “a changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events.”

And then, finally, there are the hurricanes. Speaking at the National Hurricane Center on the eve of hurricane season, Obama waded into this highly contentious issue — and didn’t flinch. He said:

The best climate scientists in the world are telling us that extreme weather events like hurricanes are likely to become more powerful.  When you combine stronger storms with rising seas, that’s a recipe for more devastating floods.
Climate change didn’t cause Hurricane Sandy, but it might have made it stronger.  The fact that the sea level in New York Harbor is about a foot higher than a century ago certainly made the storm surge worse.

This, too, is scientifically backed. For instance, a quite cautious fact sheet on hurricanes and global warming from the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory affirms that the average storm should be more intense in the future. And on Friday a new study found that typhoons (hurricanes in the western Pacific Ocean) could grow 14 percent stronger over the course of this century due to warming oceans.

The claim about rising seas is even easier to support. Even if storms didn’t change at all, rising seas would amplify their destructive power, allowing their storm surges to penetrate further inland than they would have in a world featuring lower seas.

Just to make sure about this, I asked for a reaction to Obama’s remarks from MIT’s Kerry Emanuel, one of the world’s leading hurricane experts.

“In his remarks at the National Hurricane center, President Obama presented a balanced view of how climate change is expected to affect hurricanes,” said Emanuel by e-mail. “His statement that warming is expected to make storms more powerful while sea level rise is already contributing to increased risks associated with storm surges is  in accord with the consensus view of climate scientists.”

The relationship between a warming world and individual events will never be easy to describe, and one chief reason is our brains. Just as we have trouble conceptualizing relativity, or vast periods of time, so we’re also wedded to thinking in terms of simple and direct causation. That’s very different from concepts like dice loading, odds shifting, and statistically increased risk.

Indeed, in a very influential 2012 paper, National Center for Atmospheric Research climate scientist Kevin Trenberth sought to reframe the entire discussion that occurs when someone asks whether global warming “caused” a particular extreme event. “In reality the wrong question is being asked: the question is poorly posed and has no satisfactory answer,” Trenberth wrote. “The answer is that all weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”

In none of the cases above do we see climate scientists claiming that global warming directly “caused” an event. Rather, they’ve made clear that you can’t, or shouldn’t, say that. But this hasn’t left them speechless — just nuanced.

Also in Energy & Environment:

EPA proposes a biofuels compromise — and makes nobody happy

This is climate skeptics’ latest argument about melting polar ice — and why it’s wrong

This is why people are so clueless about how much energy they use

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