Paul Grinvalds, 68, has completed 114 marathons across all 50 U.S. states, yet he still struggles whenever the temperature rises above 15.5 degrees Celsius. Like most runners, he finds that the body cannot cool itself well during hard exertion, and heat makes this even harder. Such conditions can be both painful and dangerous. This is a reality that marathon organizers increasingly must confront as climate change pushes temperatures higher, driven mainly by burning fuels such as coal and gasoline.

To find cooler weather, organizers are shifting event dates, starting earlier, and even letting runners cut their races short. Beginning in 2027, the Twin Cities Marathon in Minnesota will move its traditional race date from early October to later in the month, following a 2023 cancellation caused by record heat. Race director Ed Whetham said he would rather have a 4-degree Celsius start than a 21-degree Celsius start. Races in South Dakota, Ohio, Arkansas, and California have made similar changes.

In Los Angeles in March 2026, warm weather led organizers to let runners finish after 29 kilometers and still receive a medal. The wider trend worries some researchers: the mainland United States had its warmest July ever in 2026, and 2024 was the planet's hottest year on record. A study of 221 races predicted that by 2045, ideal running conditions would be less likely at 46 of the 51 U.S. marathons examined. This shows how a warming climate is making marathon conditions worse.