As we wind down from another meeting of the world’s greatest athletes at the Summer Olympics in Rio de Janeiro, a new report is warning that global warming may dramatically reduce the number of cities that can host the event in the future, with rising temperatures posing a safety concern to the participating athletes.

Using the marathon as a bellwether for athlete safety, due to the grueling, 26.2-mile (42.2 kilometer) run that the race entails, a joint research team from UC Berkeley and the University of Auckland drew on a previous study on climate change to determine the future of the marathon, and possibly the rest of the summer Olympics, in relation to a warming world. They found that by 2085, only eight Northern Hemisphere cities outside of western Europe are likely to be cool enough to host the summer games.

Some of the cities that were vying for the 2020 or 2024 summer games wound up on the researcher’s short list for a high likelihood of having to cancel their marathon events due to unsafe temperatures, including Budapest, Istanbul, Madrid, Paris, and Rome. Tokyo, scheduled to host 2020’s games, would also be at risk. In north America, only three cities — Calgary, San Francisco, and Vancouver — were seen as safe for competition. Africa and Latin America yielded no viable cities for the event, although Western Europe still had 25 viable sites.

While the study was looking at climate conditions a half-century from now, present-day marathons have already been affected by extreme temperatures, including the dropout of 30 percent of the competitors in the U.S. Olympic Team trials marathon in Los Angeles, and the mid-race cancellation of the 2007 Chicago Marathon, as participants succumbed to high temperatures. The research team is currently working on a broader study that will look at the impact of climate change on outdoor sports as a whole, to be released in the near future.