It's hot. Maybe too hot.

High-stakes geopolitics aren’t the only external factor threatening to hijack the tournament. Perhaps ironically for a competition hosted by a U.S. president who is highly skeptical about climate change and says assertions about rising temperatures have been made “by stupid people,” the heat is very likely to be a problem. Heat waves have become a persistent part of Northern Hemisphere summers — each one made hotter, longer and more likely to occur as a result of man-made global warming. The locations of several stadiums across the U.S. and Mexico, as well as the peak-summer timing of the World Cup, are expected to put players and fans at risk of overheating. The problem isn’t just heat, but also humidity. The combination of the two feels far hotter and is measured with wet-bulb temperature, which mimics how the human body cools off through sweating. A wet-bulb temperature of 95 degrees Fahrenheit can be fatal even to healthy people; the football players’ union FIFPRO says wet-bulb temperatures above 79 degrees — which can be reached through a combination of 86 degree heat and 50 percent humidity, for example — will affect performance and health, and 82 degree heat should prompt the postponement of a match. When scientists last month ran the numbers, they found that 26 of 104 matches are expected to take place in conditions of at least 79 degree wet-bulb temperature. Five matches are estimated to breach the 82 degree wet-bulb barrier. And a peer-reviewed study found that during last year’s FIFA Club World Cup in the U.S., average wet-bulb temperature exceeded 82 degrees in 31 of 57 matches analyzed by scientists. That study also found that high temperatures were associated with players covering less ground, forcing a change of tactics. Exhaustion sets in faster under high temperatures — at the Club World Cup, 10 players asked to be substituted in a single match. But heat doesn’t just affect gameplay. At the 2024 Copa America, an assistant referee collapsed in the he