The 2026 World Cup will be the hottest in human history, Katharine Hayhoe, chief scientist at The Nature Conservancy, said during an extreme heat webinar Monday. “We know from really good analyses done by the World Weather Attribution that a number of the host cities will face dangerous conditions during match time,” she said. The chances that the WetBulb Globe Temperature, which assesses physical heat stress on the body, could trigger mandatory water breaks for the athletes are very high, she said, and there’s “not a zero chance” that high temperatures could cause games to be halted. For host cities, extreme heat is another layer that must be folded into event and crisis planning for the games, which start in 11 U.S. cities Thursday. But extreme heat is a potential public safety crisis in all urban areas, the panelists said. “We have to be thinking differently about this, because we've been taking for granted … if we plan such-and-such an event, we can just look at the average conditions, and that's what it will be like,” Kayhoe said. “But our average conditions are changing, and when it comes to heat risk, changing faster and faster.” Cities should be focusing on the following factors, the experts said. Most stadiums aren’t built for heat. More than half of all major sports venues in the U.S. are in areas projected to experience dangerous heat days with temperatures above 95 degrees Fahrenheit within the next 25 years, Kayhoe said. Only a handful of World Cup stadiums are climate-controlled. Stadium operators need to start making smart design choices, the panelists said. In World Cup host city Dallas, for example, a roof was added to the baseball stadium more than a decade ago so the venue could be air-conditioned, Kayhoe said. Jennifer Vanos, associate professor at