Climate change found to have spurred worldwide heatwaves

But floods and droughts have less certain links to planetary warming.

 

The weather has been crazy lately. California, described recently in the New Yorker as the nation's "fruit basket, salad bowl and dairy case," is starting to look more like its day-old bread shelf, growing staler with each day that passes without rain. Communities in Colorado had the opposite problem around this time last year, with the rain coming on too strong for too long, flooding roads, homes and parks. Not long after that, South Dakota enjoyed a stretch of unusually balmy 80-degree days only to be whiplashed by 70-mile-per-hour winds and snow that buried or simply chilled to death thousands of cattle. 

There is an inevitable question that now comes with each wild weather event like these: What's climate change got to do with it? It's a really tough question to address in the immediate aftermath, when people most crave answers. But thanks to rapid advances in a field known as "attribution science," researchers, given a little time, are starting to be able to say "a little," "a lot," "not much" or "we're still not sure."

Colorado floods
Flooding in Boulder County, Colorado in 2013. Photo courtesy of Colorado National Guard.

A collection of studies published last week in the Bulletin of the American Meteorological Society did just that for the California drought, and last year's floods and blizzard in Colorado and South Dakota, as well as other wacky weather events around the globe, like the 2013 Australian heat wave.

The results were mixed. The most conclusive came from five studies that used different methods to tease apart the role of manmade climate change in the sweltering spells that Australians sweated out last year. Climate scientist David Karoly told the New York Times that 2013's hot temperatures in Australia were "virtually impossible without climate change." If those sound like strong words, it's because they are, wrote reporter Justin Gillis: "It is perhaps the most definitive statement climate scientists have made tying a specific weather event to global warming."

Climate change was also found to have played a role in the other heat waves studied, though not quite as large a role as it appears to have played in Australia. The Colorado floods and South Dakota blizzard, on the other hand, were not found to have been made more likely by climate change, nor could its fingerprint be conclusively seen in the California drought. Researchers said that the role climate change plays in extreme precipitation events and drought is in need of further study. 

What to make of all this? Perhaps the most important takeaway is that climate change is no longer a future threat. It's here and affecting the weather we experience today.

Another is that climate models — which are used both to project future conditions, and to parse out the roles of manmade warming and natural variability in current weather events — are at this point much better at dealing with temperature than precipitation or drought. In part, says Stephanie Herring, the lead editor of the recent studies, that's because the observational and paleo record is really robust for temperature. We have a really good sense of how hot and cold it's been thousands of years in the past, which makes it easier to judge whether temperature swings now are, in fact, "extreme." 

Plus, precipitation and drought are much more complicated processes to model. "The dynamics and physics of rainfall are much harder to capture," says Herring, who works out of Boulder, Colorado. "We had really bad flooding in Boulder last year, but I live in a town where our river never crested. Jamestown, 15 minutes from here, was destroyed."

Drought, she adds, is the result of a combination of a whole bunch of environmental conditions and atmospheric processes. In the recent studies, for instance, scientists ran models incorporating different sea surface temperatures to see if a warming ocean raised the likelihood of the drought. They found it didn't. Another group looked at the influence of climate change on something they call the "ridiculously resilient ridge," a high-pressure pattern in the North Pacific that has been stubbornly hanging out and blocking storms from reaching California. They found climate change had increased the chance of such a ridge forming. But the question of how exactly the pattern influenced the amount of precipitation California has or hasn't received wasn't addressed in the paper, says Herring, making it tough to offer a definitive statement about climate change's role in the Golden State's withering.

So why bother spending time picking apart the recent past? "By understanding what's causing extreme events today, it can help us understand what might cause extreme events tomorrow," Herring says. "Climate change is not necessarily driving these things one way or another. That impacts the way communities might prepare for future events." 

Cally Carswell is an HCN contributing editor based in Santa Fe, New Mexico. 

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