Is global warming to blame for the March heat?
No, it’s not your imagination. It’s been freakishly hot across much of the United States this spring, with more than 7,000 temperature records broken since March 12. So Andrew Freedman asked a bunch of climate scientists whether global warming was to blame. Here was one hedged reply:
Gabi Hegerl, Chair of Climate System Science at the University of Edinburgh, said there is evidence that extreme heat events have become more common and more severe, including at the regional level in parts of the U.S. “This is consistent with observing more and stronger heat waves,” she said.
Hegerl said that in order to draw conclusions about global warming’s role in this particular heat wave, one would need to conduct modeling studies where you compare the odds of this event occurring with and without added greenhouse gases in the atmosphere, such as carbon dioxide, “to see how much the warming has changed the odds.”
That’s not a very satisfying answer. But, then again, it’s a complicated topic. As the planet heats up, the odds of extreme heat waves increase. This isn’t terribly controversial. Indeed, it already appears to be happening. A 2009 study from the National Center for Atmospheric Research found that the ratio of record highs to record lows has been increasing in the United States over the past four decades. (During the 2000s, the United States saw twice as many record highs as record lows.) That’s a solid indication that, for all the ups and downs of daily weather, things are getting steadily hotter overall.
But it still remains very difficult for climate scientists to point to a particular heat wave and say that man-made global warming is responsible for it. After all, as Freedman details, quite a few things can cause March temperatures to spike — things such as high-pressure blocking patterns or a lack of snow cover. What we want to know is whether these record highs would have occurred without global warming. And, as Hegerl notes above, that takes time and effort to pin down.
Yet researchers are trying to answer this question; as I wrote in an earlier story, this is one of the cutting-edge areas of climate research. Last October, for instance, Stefan Rahmstorf and Din Coumou published a paper in the Proceedings of the National Academies of Sciences that examined the heat wave in Moscow in 2010, when record temperatures killed hundreds. They examined more than a century of data and used statistical techniques to tease out natural variability. They concluded that there was an 80 percent likelihood that Moscow’s heat wave would never have occurred without man-made global warming.
That still, of course, leaves the challenge of sorting out just how much blame to assign global warming for any given freak occurrence. “Imagine you have a lot of historical data, and you’ve decided that a heat wave can never get bigger than 9°C,” Martin Hoerling of the National Ocean and Atmospheric Administration told me. “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.”
So why does any of this matter? Isn’t it enough to know that the planet is warming and that unseasonably sweaty conditions are the sort of thing we’re likely to see more and more frequently if we continue belching greenhouse gases into the air?
Perhaps. Although better attribution studies could shake up the legal landscape down the road. As Peter Stott of Britain’s Met Office Hadley Center has pointed out, it’s quite possible that we could see more lawsuits 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.
Obviously no one’s going to sue over the fact that the cherry blossoms in D.C. are blooming earlier and earlier each year. But they might sue over deadly heat waves in Moscow. Or other disasters. The science of weather attribution isn’t quite at that point yet, but it’s progressing rapidly.