The coach of Poland's Olympic handball team wipes sweat from his face. Ben Curtis/AP

By 2085, rising temperatures will mean that just 33 cities can safely host the games.

It may be winter in Rio right now, but Olympians are competing in temperatures that run between 70 and 90 degrees Fahrenheit. And environmental health researchers now say the forecast for future games is looking even hotter—perhaps too hot for Olympic athletes to handle.

In a commentary published last week in the journal The Lancet, a team of public-health researchers warns that, if global temperatures continue to climb, only a handful of cities will be cool enough to host the summer Olympics by 2085.

“With temperatures and humidity rising all over the world, nearly everywhere is going to be risky to do the Olympics as we've always done it,” says Kirk Smith, a professor of global environmental health at University of California, Berkeley, and the the lead author of the paper.

In fact, the researchers conclude that almost 90 percent of the cities in the Northern Hemisphere might not be able to hold the games without putting athletes at risk of heat stress and heat exhaustion. Most of the cities that can still host the Olympics will be in Western Europe, while none will be in Latin America or Africa.

Kirk and his colleagues looked the climate conditions of 645 cities in the Northern Hemisphere that are eligible to host the Olympics. Cities that had fewer than 600,000 in population were excluded, as were those that exceeded 1,600 meters (or roughly 5,250 feet) in elevation. They used data from two standard climate models to calculate the temperatures and humidity of those cities over the next century, assuming the levels of greenhouse gas emissions would remain high. With those numbers, they then estimated each city’s wet-bulb globe temperature (WBGT), a measure of heat stress that takes into account temperature, humidity, heat radiation, and wind.

Cities are considered to be of high to medium risk if their WBGTs exceed 26 degree Celsius (or 78.8 degree Fahrenheit), which the researchers say is the maximum temperature to safely hold marathons, considered by some to be the most demanding events in the Olympics. (That’s actually a conservative measure; a 2010 study put the temperature threshold of risky marathons at 70 degrees Fahrenheit. Another analysis, based on data from more than 2 million marathoners, found the ideal temperature to be as low as 40 to 50 degrees.)

According to Runner’s World, excessive heat causes a runner to slow down as the body battles dehydration, increased heart rate, and reduced blood flow (and oxygen) to the muscles. For every 10-degree increase in air temperature above 55 degrees, the average finishing time can increase by 1.5 to 3 percent. Not only that, runners also face the risk of overheating. Kirk and his colleague write in their study that any city with even a 10 percent chance of canceling a marathon event would not be considered a viable option to host the summer Olympics.

Only 8 out of the 33 cities with WBGT temperatures that are of “low risk” to Olympians lie outside of Western Europe. (The Lancet)

By their measure, the researchers conclude that only 33 cities are of “low risk” to athletes, including London, San Francisco, Vancouver, and Dublin. Of those cities, only eight lie outside Western Europe. Looking even further, into the 22nd century, the researchers write, that list will dwindle to include just four cities: Belfast, Dublin, Edinburgh, and Glasgow. The commentary comes ahead of a larger study that the researchers plan to publish this fall or early next year.

Of course you can run the marathon indoors and it would be fine,” says Kirk. Marathons could also be moved to the winter Olympic games. Or, the Olympics could arguably be hosted in just one city, like Vancouver, or on a dedicated island (preferably somewhere in the cool, northern region), but that comes with its own set of challenges. In short, “It wouldn't be the same the Olympics that we've considered,” Kirk says.

Heat exhaustion is just one of the many obstacles the global warming will bring to the games. There may not be any reports of Olympians fainting at this year’s Olympics in Rio, but the effects of climate change have been apparent—most notably in the fear over the spread of Zika. As The Atlantic explains, the virus-carrying mosquitoes could enjoy a greater reach as temperatures warm up. Then there are the threats of more extreme weather and air pollution, which are typically associated with climate change.

But Kirk admits this isn’t really about the elite athletes. The biggest impacts will be on the poor living in these cities, many of whom make a living by spending their days outdoors on farms or on construction sites with little to no breaks. “Very few athletes are going to die [from climate change],” he says. “But if they are the fittest people in the world and we still have to worry about them, are there programs to protect people in not just poor countries, but in the U.S. even?”

And if you think 2085 is a long way off, consider this: “Babies born today are going to be alive in 2085, so it’s not far in the future,” Kirk says. “We're not talking about some future unknown generation. My daughter is pregnant and when her son is born, he's a part of this.”

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