Europe has had five 500-year summers in 15 years. And now this

Another deadly heat wave has Europe in its sweaty grip this week. Record temperatures topped 100 degrees Fahrenheit (37.8 degrees Celsius) in parts of France, Germany, Poland and Spain, with hotter days to come. The same thing happened last year—record-breaking heat was responsible for 700 deaths in Sweden and more than 250 in Denmark, countries that have never needed air conditioning before this new era of climate-change-driven extreme events.

Europe’s five hottest summers in the past 500 years have all occurred in the last 15 years, not including this summer. All have been deadly. The 2003 heat wave was the worst, having led to the deaths of over 70,000 people; in 2010, 56,000 died in Russia alone.

These extreme heat events are all connected to a slower jet stream that locks weather systems into place, says Michael Mann of Penn State University. Mann co-authored a study last year that linked the slowdown in the jet stream—the band of high-altitude winds that sweep around the globe from west to east—to last summer’s unprecedented droughts, heat waves, wildfires, and flooding events across the entire Northern Hemisphere. And it is likely behind India’s weak monsoon rains and the widespread flooding in the U.S. Midwest this year.

“My colleagues at PIK (Potsdam Institute for Climate Impact Research) have verified that this is what we’re seeing right now in Europe,” Mann said in an email.

The loss of sea ice in the Arctic is amplifying warming in the northernmost regions of our planet, and that is disrupting the natural jet stream patterns, said Dim Coumou of Vrije Universiteit Amsterdam and PIK. Jet stream winds are driven by the temperature difference between the icy air of the Arctic and hot air from the tropics. A rapidly warming Arctic—this past winter it saw its lowest ever ice cover—reduces that temperature difference and slows the jet stream.

Like a slow-moving river, a slower jet stream forms deep meanders, which can stall during the summer, sometimes for weeks. Weather patterns stall with them, whether they be heat waves or torrential rains.

While temperatures in Europe are nowhere near as hot as India’s current month-long heat wave—temperatures on the Asian subcontinent have reached 123°F (51°C)—most Europeans, particularly in the north, are unused to anything over 85°F. Air-conditioning remains rare. It’s found in less than five percent of homes in France, for example, and less than two percent of German homes.

Hot city nights

The number of heat wave days in European cities are nearly double those of the surrounding suburban and rural landscape because of the urban heat island effect, said Jürgen Kropp of PIK. Concrete and asphalt absorb heat during the day and release it at night, keeping urban areas hotter. Without major cuts in carbon emissions, the number of heat wave days in cities will increased 10-fold by the end of this century, Kropp said. “Wednesday was the hottest day ever for June, here in Berlin.”

There is a real debate about what to do about the increasing heat, he went on. Air-conditioning increases energy use, which will increase Germany’s carbon emissions, making climate change worse. The majority of Germans want more action on climate, Kropp said—but window AC units are in high demand right now.

Europe has learned from the 2003 heat wav, which killed more than 70,000 across the continent, said Richard Keller, a University of Wisconsin-Madison professor of medical history. The death toll should be more limited this year, said Keller, the author of Fatal Isolation, a book on the Paris heat wave of 2003 that killed thousands. “France is much better prepared, emergency services are in place, and awareness of the dangers is much higher,” Keller said.

Some schools in France are closed — hardly any have air-conditioning, Keller said. Cooling areas and temporary water fountains have been set up in busy city areas; parks and swimming pools are being kept open later. In Paris, older cars are banned from the city to counteract the fact a heat wave aggravates the city’s pollution.

A reduction in pollution levels will be a silver lining for women’s World Cup soccer fans who will pack Paris’s Parc des Princes stadium Friday to watch the U.S. national team play host team France.

“The heat won’t be a problem for the players,” Keller said. “They’re top-notch athletes who can cope. It’s the fans who are at risk of heat stroke.”