The extreme heat wave that gripped Europe in late June and sent temperatures soaring to 114 degrees Fahrenheit was made at least five times more likely by global warming, scientists with the World Weather Attribution group said Tuesday.
It was a quick and unambiguous finding, a judgment that in past times would have been harder to declare without heavy hedging.
The week of heat threatened vulnerable older people, damaged roads and railroad tracks and forced a rescheduling of national school exams in France. The Swiss meteorological service called it one of the most intense heat waves in that country's history and said the heat had a clear climate change signal.
"The normally hottest part of the summer is yet to come," World Weather Attribution's Geert Jan van Oldenborgh warned on Twitter.
The World Weather Attribution scientists zoomed in on France while the heat wave was still being felt to assess the impact of global warming.
Using climate models and historical temperature records, they compared heat waves with and without the effects of human-caused greenhouse gases. They calculated that global warming had made the extreme June heat event at least five times more likely—and said the probability was likely even higher.
"Without considering climate model results, the observed temperature record suggests that a heat wave like the one in June is now at least 10 times more likely than in 1901, and possibly 100 times or more, and that maximum heat wave temperatures are about 4 degrees Celsius [about 7°F] warmer now than in 1901," said co-author Robert Vautard, a climate researcher at the Laborataire des Sciences du Climat in France.
"Every heat wave occurring in Europe today is made more likely and more intense by human-induced climate change," the scientists wrote in the latest study. "How much more depends very strongly on the event definition: location, season, intensity and duration."
As the heat wave spread across western and central Europe this past week, temperatures spiked to between 10°F and 18°F above normal in France, Germany, northern Spain, northern Italy, Switzerland, Austria and the Czech Republic. Scores of cities and towns set monthly and all-time highs.
Europe as a whole saw its hottest June on record, a full 1.8°F (1°C) above its previous high set in 1999. Copernicus, the European climate service, announced on Tuesday that the entire Earth had just experienced its warmest June on record, topping the reading from June 2016 that followed a warming El Niño.
'We're Locked Into a Large Amount of Warming'
The heat wave lingered in Europe for days, with hot nights that didn't allow buildings or humans a chance to cool down. Heat wave deaths often outnumber deaths from all other natural disasters annually, but they don't get as much public attention because they typically don't appear in the statistics until after the event.
People will have to rethink how they live as heat waves intensify, said Friederike Otto, acting director of the Environmental Change Institute at the University of Oxford and a co-author of the study.
"All over Europe, we need to build much better," she said. "We need well-insulated buildings that keep cool in summer and warm in winter. Without that, we will suffer in heat waves, but importantly also not reach net zero carbon emissions."
Extreme heat, especially when prolonged over days or weeks, can also stress crops, disrupt pollination and trigger flooding, as was reported along the Inn River in Austria in early June, when record warmth at high elevations quickly melted the winter's accumulated snow.
The attribution study's findings about the heat wave are in line with other recent studies, including major reports from the Intergovernmental Panel on Climate Change (IPCC) and the National Climate Assessment, said Daniel Swain, a climate scientist with UCLA and the National Center for Atmospheric Research.
Of all global warming's dangerous impacts, scientists are most certain that the build-up of greenhouse gases will continue to send temperatures soaring to unprecedented spikes more often.
In a 2017 study published in the Proceedings of the National Academy of Sciences, Swain and other scientists found human fingerprints on 80 percent of all heat waves in areas with reliable temperature records. In a few decades, every summer will see temperatures hotter than today's, he said.
Researchers studying the causes of the heat waves are eyeing changes in the jet stream and other ocean atmospheric patterns, but "just the temperature rise of global warming is enough to drive unprecedented heat waves," Swain said.
"If you have the same exact weather patterns and you add 1 to 2 degrees Celsius warming on top of that, you're going to increase the likelihood of unprecedented heat waves," he said. "Everything will be unprecedented. Summer is like a whole new season of heat for much of the world.
"That is scary, because you can imagine a whole summer that is hotter than the extremes right now. We are locked into a large amount of additional warming even in an optimistic emissions scenario," he said. "What we do in the next few years matters."
Wildfires, Arctic Warming & Melting Greenland
The June heat wave also set the stage for a massive, fast-moving wildfire in Spain, said University of Reading (UK) climate researcher Hannah Cloke, who is working on ways to improve warnings for people about extreme events by combining data from wildfires, including smoke impacts, with heat stress data.
"This is actually really getting quite scary," she said. "It's a human problem, not a scientific problem.
"We are deep into the red and there's not really a way back from that."
Like last year, heat waves have been occurring across the Northern Hemisphere this summer.
Early in June, a heat wave in Greenland caused widespread melting across the surface of the ice sheet. Alaska is also setting high temperature records, sea ice around its coast is melting faster than normal, and wildfires have burned nearly half a million acres already this year.
Some of those fires are above the Arctic Circle, along with many more in Siberia. There was an "unprecedented" level of wildfire activity in the Arctic during June, said Mark Parrington, a senior scientist with Copernicus ECMWF.
India and Pakistan also sweltered in a deadly heat wave in early June that contributed to the depleting of a major municipal water reservoir in Chennai, a city of about 10 million people.