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Did Climate Change Make That Freak Weather Even Worse?

In June 2003, while he was still a graduate student, Noah Diffenbaugh attended a scientific conference with his adviser in Trieste, Italy. That month, the average daily high temperature there was 88 degrees Fahrenheit; typically, highs in Trieste at that time of year are about 10 degrees cooler. “People were saying, ‘This is really hot. This is really not what usually happens,’” he recalls.Diffenbaugh, now a climate scientist at Stanford University, had caught the leading edge of the 2003 heat wave—the hottest European summer since the 16th century. (That record has since been broken multiple times, most recently this past summer.) It was hard not to link the near-unprecedented temperatures, which are thought to have killed over 70,000 people across Europe, with the inexorable creep of climate change. But back in 2003, no scientist would stand up to make that connection. “‘It’s impossible to ever attribute any particular event to global warming’—that was the predominant public stance at the time,” Diffenbaugh says.According to Daniel Swain, a climate scientist at UCLA, there were some good reasons for this reticence. Unseasonable weather sometimes happens by chance, and scientists worried that tying weather too closely to climate would allow climate deniers to use cold weather as ammunition. In 2015, US senator James Inhofe (R-Oklahoma) did just that when he brought a snowball onto the Senate floor in an attempt to disprove climate change.But, Swain says, the idea that weather and climate can be separated is illusory. “Climate is nothing but weather in aggregate,” he says. The global mean temperature—which, according to the IPCC’s Sixth Assessment Report, has already risen by over 1 degree Celsius —is a convenient scientific construct. Averaging temperature measurements over the entire world helps scientists ignore the random vicissitudes of weather when determining the overall trajectory of climate change. But it’s not global mean temperature that kills people. People die when floods overwhelm urban infrastructure , or when unheard-of temperatures and humidities persist in specific locations for days on end. “No human, no ecosystem on Earth, will ever experience the global mean temperature,” Swain says.So in 2004, a group of scientists led by Peter Stott at the United Kingdom’s national weather service decided to quantify the extent to which climate change had contributed to the 2003 heat wave. “It is an ill-posed question whether the 2003 heat wave was caused, in a simple deterministic sense, by a modification of the external influences on climate,” the group wrote in their subsequent paper, “because almost any such weather event might have occurred by chance in an unmodified climate.” Instead, they asked a different question: How much more likely had greenhouse gas emissions made the deadly heat wave?Using climate models, the team simulated what the world would look like with and without those emissions. Essentially, they simulated weather conditions on two alternative Earths—one in which humans had pumped enormous volumes of carbon dioxide and methane into the atmosphere, and one in which they hadn’t. And in the world with these emissions—the world we live in—a record-breaking European summer heat wave was, on average, about four times as likely.