Climate Change Equals Hot Summers. Case Closed.

An unusually explicit climate scientist drops the hedging and says what more and more people believe

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A man pushes a cart of bottled water during warm weather in New York City on July 6, 2012

Every time a major heat wave, drought or hurricane occurs, journalists call up climate scientists to ask if global warming is the cause. Nearly every climate scientist has the same stock response: while global warming can make extreme events more likely or more powerful, we can’t say climate change is causing that string of 100°F days or that Category 5 hurricane. This includes other extreme events like the severe dryness that has left more than three-fifths of the continental U.S. in some form of drought. Sure, we can expect more heat waves and extended dry periods in a global-warming future, but no reputable scientist would say that what’s exceedingly likely to unfold over time is directly responsible for what’s happening in 2012.

But James Hansen is different. The NASA climate scientist is one of the world’s foremost global-warming researchers, and almost certainly its boldest. It was Hansen who testified before Congress on climate change during the landmark hearings in 1988 — during another hot, dry summer — warning of the risks of man-made greenhouse gases. And it was Hansen who, over the past several years, has embraced the role of the scientist-advocate: marching in protests against the proposed Keystone XL oil sands pipeline, calling for a carbon tax as a way to cut greenhouse-gas emissions and authoring the popular science book Storms of My Grandchildren. Hansen has no reluctance to speak out about why man-made climate change is so dangerous — and how it should be fixed.

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So it shouldn’t be surprising that Hansen is willing to go further than most of his colleagues in tying extreme weather to global warming. In a new study published in the Aug. 6 issue of Proceedings of the National Academy of Sciences, he uses relatively straightforward math and analysis to reach an explicit conclusion: the recent extreme heat and drought of the sort that has sacked much of the U.S. is so unusual that the only thing that could be causing it is man-made climate change. No hedging, no yes-but, no cautious qualifying.

“Our analysis shows that it is no longer enough to say that global warming will increase the likelihood of extreme weather and to repeat the caveat that no individual weather event can be directly linked to climate change,” Hansen wrote in an op-ed in the Washington Post last weekend. “To the contrary, our analysis shows that, for the extreme hot weather of the recent past, there is virtually no explanation other than climate change.”

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Hansen and his team looked at the recent past — rather than trying to model the future — to see if they could find the signature of man-made climate change through day-to-day and season-to-season weather. They used the period of 1951–80 as a base because it was a meteorologically stable stretch that also had a wealth of global weather data, unlike earlier periods. During that time period, extremely hot summers — like the one much of the U.S. is experiencing now — occurred only in 0.1% to 0.2% of the globe in a given year. But since 1981, extremely hot summers have baked about 10% of the earth’s land area annually — and in recent years, that percentage has been even higher.

That means the odds of experiencing an extreme summer have risen from 1 in 300 during the 1951–80 period to nearly 1 in 10 now, according to Hansen’s calculations. “I don’t want people to be confused by natural variability,” he said in a statement. “We now know the chances these extreme weather events would have happened naturally — without climate change — is negligible.”

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Hansen’s math isn’t new, but he is much more willing to connect — in a peer-reviewed paper — extremely high temperatures to climate change instead of more natural weather phenomena like the El Niño and La Niña oscillations. Hansen also argues that global warming has made the climate more variable: while extremely hot weather is more likely to occur during the summer, about 15% of the summers over the past 30 years have actually been cooler than the 1951–80 average. To take the title from a new book by the nonprofit Climate Central — which includes the work of TIME contributor Michael D. Lemonick — what we may be experiencing is as much “global weirdness” as global warming.

For the public, the details of fingerprinting extreme-weather events may matter less than the fact of heat and drought itself. And this summer’s outlandishly tough weather — I’m beginning to expect that Oklahoma will simply burst into flames one of these days — is clearly having more of an impact on public opinion than a scientific paper ever could. A recent study found that for every 3ºF that local temperatures had risen above the norm in the week before a survey on climate change, belief in climate change rose by 1%. A July study by the University of Texas found that 70% of Americans surveyed believed the climate was changing, up from 65% in the cooler months of March. If there’s one thing we can take from Hansen’s work, it’s that those temperatures are more and more likely to hit extremes in the years to come, which means belief in climate change should continue to rise as well. Now we just need to do something about it.

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