The Paris Olympics opened with rain on its parade, then blistering heat and, finally, a week of pleasant sunshine. As it comes to a close on Sunday, temperatures are expected to again soar up to 95 degrees Fahrenheit, or 35 degrees Celsius.

The only certainty about Summer Olympics weather is that there’s really no certainty at all.

Extreme heat is a growing threat for elite athletes, with cases of heat exhaustion and heatstroke becoming more common as fossil fuel pollution pushes temperatures and humidity levels up. Spectators, especially those those who fly in from cooler climates, are vulnerable to extreme heat, as well.

Most of the world’s cities will be unable to host the Games during summer in the coming decades as they blow past the threshold of safe humid heat, according to a CNN analysis of data from CarbonPlan, a climate science and analytics-focused nonprofit group.

The big picture

Heat stress can be measured with something called wet-bulb globe temperature — a combination of heat, humidity, windspeed, sun angle and cloud cover. CarbonPlan found that by 2050, heat stress in almost all cities in the eastern part of the US would shoot well past the 82.1-degree limit, beyond which experts recommend canceling sporting events.

In other words, holding the Summer Games in these cities would be a huge health risk for the athletes.

The super-humid states around the Gulf of Mexico, from Florida to the eastern half of Texas, would be well off the table. The Games held in 1996 in Atlanta simply wouldn’t be possible in 2050.

Much of eastern China, including Beijing and Shanghai, would be well above the limit, as would Hong Kong and huge swaths of Southeast Asia.

Suggestions to change the timing of the Summer Olympics so it doesn’t coincide with peak heat are growing louder, and it has been done before. Sydney, which swelters in the summer, held the 2000 Games in September and October during the Southern Hemisphere’s spring. Brazil’s Rio de Janeiro held the 2016 Games in August, when its winter temperatures average a comfortable 70 degrees or so.

Cities in northwest Europe — like London, Oslo and Stockholm — may become more attractive for the event, while Mediterranean cities — including Palermo in Sicily and Spain’s Seville — are mostly over the threshold. High-elevation South American cities could also become more appealing as global temperatures warm.

Bidders for the 2036 Games

The next Summer Games will be in Los Angeles, a city whose temperature is pleasantly moderated by the cool Pacific Ocean. The Games of 2032 are set for Brisbane in the northern Australian state of Queensland, a city that gets so hot in the summer, it will hold the event during its winter in late July — ideal, given it’s summer then for much of the world. A Brisbane winter doesn’t drop far below 50 degrees.

More than 10 countries have expressed interest in hosting the 2036 Summer Olympics, according to reports, but only six have made their bids public or official: India is bidding its western city of Ahmedabad and Indonesia its under-constuction new capital, Nusantara. Qatar is pitching Doha, while Turkey is bidding Istanbul. Poland and Chile are putting forward their capitals, Warsaw and Santiago.

Nearly all of them will, at some point, breach the heat stress limit, CarbonPlan’s data shows. Only Santiago is below the threshold year-round, including at the hottest part of summer. Ahmedabad and Doha would be well over the limit in the summer months, but could potentially pitch for their cooler seasons.

“Throughout much of the world, the worst heat of the year unfortunately coincides with when the Summer Olympics are typically held,” Oriana Chegwidden, a climate scientist with CarbonPlan, told CNN. “And heat could indeed pose significant risks in the countries bidding for the Olympics in 2036.”

She said that if these locations were chosen, heat risks could be reduced with some simple measures. “For example, planners could mitigate heat risks by starting before or after the peak of summer, or by holding events at night or in the early morning when it is cooler.”

She added that countries might consider bidding for cities with cooler climates, like those at higher elevations.

They hosted once. They may not again

Some of the cities that have already hosted the Summer Games will be way beyond safe temperatures by 2050.

Beijing, which hosted in 2008, would be much too hot and humid, with heat stress forecast to soar past 90 degrees. Athens, Rome, Atlanta, Tokyo and Seoul would also be too hot, as would Barcelona.

The Southern Hemisphere cities, Sydney and Brisbane in Australia, as well as Rio de Janeiro, all technically hit the too-hot list, but could still host in their cooler seasons.

It’s not as if the world will only see this happen mid-century. Tokyo 2020 — held in 2021 after the pandemic delayed the Games — were the hottest on record, with heat stress shooting well past the safety threshold to above 89 degrees.

Around 1 in 100 athletes suffered heat-related illnesses in Tokyo. Extraordinarily, not a single person was hospitalized, in part due to Japan’s preparations.

As it became clear extreme hit would hit the Japanese capital, a densely built city of skyscrapers that trap urban heat, organizers moved the marathon and walking events to the cooler, mountainous city of Sapporo. Still, it was too hot and humid, and six runners and walkers suffered exertional heatstroke, which is brought on by intense activity in extreme heat and can lead to organ failure and death.

Yuri Hosokawa, an assistant professor of sport sciences at Waseda University in Japan,  headed the heat response plan in Sapporo for the event.

“Exertional heatstroke, the most severe form of heat illness, when that happens, we need to make sure that the person is treated immediately, ideally within 30 minutes of collapse,” she told CNN. “And in order for us to do that, we needed to establish a protocol, where these individuals are not transported to hospital, but they’re actually treated on site.”

Hosokawa and her teams treated cases of heatstroke and heat exhaustion by immersing people in ice baths to bring down their body temperatures. Someone suffering heatstroke is unable to thermoregulate — to bring down their body temperature without external cooling.

“Cold water immersion, or an ice bath, is the most efficient way of cooling someone rapidly,” Hosokawa said. But some athletes in Tokyo were hesitant to take an ice bath, and if they were cohesive and not yet at the point of heatstroke, they were treated with rotating towels that had been soaked in ice-cold water.

Hosokawa is one of many experts calling on sports communities globally to change how sports are scheduled but also to relax certain rules. Allowing more substitutions in a football match to prevent athletes from dangerous overexertion is a simple example.

“If we just cancel sporting events when it gets hot, that could seriously take away an opportunity from everyone,” she said. “I love sports, and I want sports culture to stay, and I want my children to also play sports when they grow up. But if we want to do that, we need to seriously think about rule changes and scheduling.”