The impending retirements of Roger Federer, Serena Williams and Rafael Nadal — winners of a combined 65 major singles titles — have some worried about the end of the glory days of tennis. But a bigger existential crisis is facing the sport: climate change.

All sports, if they haven’t already, are expected to suffer in a warming world. Sea-level rise could flood arenas and stadiums. Greater use of artificial snow could lead to more serious injuries for skiers and biathletes. Stronger storms and wildfires could wreak havoc on schedules across leagues. But few sports are likely to fare worse than tennis. The sport follows the sun 10 months of the year, and more than 80 percent of its tournaments are played outdoors. And there are no substitutions in tennis: Players spend hours on the court without teammates ready to take their place while they rest.

In some ways, tennis could suffer just as much as endurance events such as marathon running, where athletes are always moving and consistently exposed to the heat, said Debra Stroiney, a professor of kinesiology at George Mason University.

“Running a marathon, you’re out there for hours, constantly working,” she said. “But tennis, too, they’re out there sometimes three, four, five hours, depending on the match, and yeah, you’re sitting down once in a while, not hitting the ball once in a while. But they’re also running around, and they’re constantly moving.”

To investigate tennis’s future in a warming world, we used forecasts of maximum temperature and relative humidity from five climate models produced as part of the Sixth Assessment Report of the United Nations Intergovernmental Panel on Climate Change. These forecasts all assume that we continue on our current fossil-fuel-intensive trajectory, which is expected to lead to between about 6 to 10 degrees Fahrenheit of warming from pre-industrial levels to the end of this century. (The planet has already warmed about 2.2 degrees Fahrenheit since the pre-industrial era, the mid- to late 1800s.)

By using forecasts from multiple-climate models that span a range of expectations, we could determine the highest temperature predicted by any of the models for each day. We calculated the average maximum temperature and extreme maximum temperature, or a “maximum of maximum temperatures,” to provide a sense of what we might expect if a heat wave occurs during a future tournament. For instance, what might be the hottest temperature during the 2050 Australian Open in Melbourne?

To be clear, these daily forecasts are not weather forecasts. But they provide a window into how climate change could shift what we now consider normal weather toward new extremes.

In the 2050 Australian Open finals, it could feel as hot as 147 degrees Fahrenheit,⁴⁴ with an air temperature of 105.4 and relative humidity of 58.2 percent. When Ash Barty made Australian tennis history there earlier this year, the high air temperature in Melbourne Park was more than 30 degrees lower, at 71.

At the 2050 French Open in Paris, players could experience a heat index of 113 degrees, with the temperature potentially reaching 90. The high when Nadal won his record-extending 14th Roland Garros title earlier this month? 72 degrees.

At Wimbledon, the groundskeeping crew will have to work extra hard to keep the lawns lush and tidy as it could feel like 102 degrees in London in 2050. But that might feel like a reprieve compared with the 2050 U.S. Open in New York, where the heat index could rise to 145 degrees.

Tennis has already experienced the perils of a warmer planet. Ivan Dodig of Croatia wondered if he “could maybe even die” before retiring from a 2014 Australian Open match. “Impossible to play in this heat..it’s only about surviving,” Elena Vesnina from Russia tweeted during the tournament.

Last year at the Tokyo Olympics, Russian Daniil Medvedev voiced similar worries: “I’m a fighter, I will finish the match, but I can die,” he said to the chair umpire during play. “If I die, is the ITF [International Tennis Federation] going to take responsible?”

At the 2018 U.S. Open, Roger Federer was upset in the fourth round amid some of the muggiest conditions he could recall. “I just struggled in the conditions tonight. It’s one of the first times it’s happened to me,” he said. “At some point also I was just happy that the match was over.”

Players have already been endangered by the collateral damage from climate change. Smoke from wildfires that scientists say were more likely to have occurred due to the rise of global temperatures caused Dalila Jakupović of Slovenia to retire from the 2020 Australian Open qualifying match that she was leading. “I just couldn’t breathe anymore, and I just fell to the floor,” she said.

The ATP, WTA and Grand Slam tournaments all have extreme-heat policies or ways to give players more breaks in dangerous conditions. But short of more frequent stops during matches, how can tennis survive on a warmer planet?

The key factor will be players’ core body temperatures. Our bodies are typically at about 98.6 degrees Fahrenheit, of course, but when they warm to between 101 and 104 degrees Fahrenheit, the risk of heat illnesses rises, said Ethan Hill, a professor in the School of Kinesiology and Physical Therapy at the University of Central Florida. At 104 degrees or above, we risk suffering heat stroke and potential organ failure.

How a person’s body reacts to heat depends on the individual — and crucially whether the player is well hydrated and acclimated to the heat. Acclimated players typically sweat sooner, helping their bodies stay cooler, Hill said. But, generally speaking, Hill and Stroiney said, tennis players can expect to see more heat illnesses in such extreme weather. “We have that known limit,” Stroiney said. “Our bodies are going to say ‘no,’ or the core temperature increases to a point where our brain will actually say ‘stop.’”

Tennis has ways to mitigate that risk, including hosting more indoor tournaments, Stroiney said, and fewer matches during the heat of the day, Hill said.

Tennis also could use innovative ways to cool players down, Hill said. For instance, during changeovers or set breaks, cooling vests and fans could be deployed to bring down players’ core temperatures. Tournaments often give players ice towels and fans in extreme heat conditions.

Hill is also encouraged by research into using sodium and glycerol supplementation to help electrolyte and hydration levels, although he stressed that more research is needed, especially in extreme heat environments.

“It’s going to be incredibly important that we begin to develop strategies to cool people down,” Hill said. “This obviously will become more important as the planet continues to warm and as we continue to compete in these warm climates.”

Tennis officials understand the risks to the players and emphasize such strategies as maintaining hydration and encouraging players to acclimate. “When players consistently train and compete in higher temperatures, the body adapts through acclimatization, and [that] allows the body to perform at higher levels,” Todd Ellenbecker, the ATP’s vice president of medical services, said through a spokesperson. “Careful planning, preparation, coupled with optimal fitness levels, all help athletes to adapt to sports performance in the heat.”

Hill and Stroiney agree that acclimation will be crucial for athletes. Within a week, acclimated athletes can retain 2 to 3 more liters of water and maintain a lower heart rate while exerting themselves, Hill said.

But it’s not as simple as applying well-worn strategies and expecting similar results. In more intense extreme heat, both Hill and Stroiney wonder how our typical physiological responses will fare, or whether new solutions might somehow be needed. For instance, it generally takes athletes two weeks to fully acclimate, Hill said. “Now as the planet warms, that could be longer,” he said.

The body also naturally moves hot blood from the core to the skin to help cool the body, he said. “That’s a pretty efficient process that improves with heat acclimation,” Hill said. “But … when the planet gets so warm, will that physiological response still be effective?”

Stroiney has similar doubts, and it’s personal. She has played tennis and run 10 marathons. But she, like some tennis players, doesn’t do well in the heat. How will she and other athletes perform in ever-increasing temperatures?

Hill is optimistic that tennis, or the world, will change before waves of heat illnesses exhaust the sport. “I don’t think it’s all doom and gloom,” he said. “We just need to be very cognizant that it’s going to be warmer.”

So nearly 30 years from now, when Federer, Williams and Nadal are sharing their Grand Slam championship stories, they may also be telling tales of the long-forgotten days of outdoor tennis.