Heat, Humidity, and the Marathon: How Runners Are Adapting to a Warming Climate

When Paul Grinvalds laced up for his 114th marathon, he didn’t just think about mileage—he thought about the temperature. The 68‑year‑old from West Des Moines, Iowa, admits that even a high‑50s Fahrenheit day feels “a bit warm” for a 26.2‑mile run. “If we have a goal and it’s hot, a lot of times we’re still going to try to do that goal, which is just stupid because you’re almost guaranteed to have a very painful experience,” he says, shaking his head.

Grinvalds isn’t alone. Most marathoners know the hard truth: the human body struggles to cool itself when you’re pushing it for hours, and added heat makes that battle even tougher. That reality is nudging race directors across the country to rewrite the rulebook.

Take the Twin Cities Marathon. After a sweltering, record‑breaking heat wave forced the 2023 race to be cancelled, organizers decided—starting in 2027—to move the event from the first Sunday in October to later in the month. “We’ll take a 39‑degree start over a 70‑degree start,” race director Ed Whetham joked, but the sentiment is earnest. A later October date promises cooler mornings, even if there’s a chance of an early snowflake.

South Dakota’s Sioux Falls Marathon made a similar pivot, slipping its annual race from August to September after a scorching 2024 edition was called off. And it’s not just the Midwest. The Air Force Marathon in Dayton, the Little Rock Marathon in Arkansas, and the Long Beach Marathon in California are all nudging start times earlier in the morning, hoping to catch the day’s coolest hour.

Los Angeles tried something different. In March, when temperatures surged, officials gave runners the option to stop at 18 miles and still receive a finisher’s medal. Less than five percent took the shortcut, and only 14 of roughly 27,000 participants needed medical attention—none stayed in the hospital overnight. Still, the move sparked a flurry of online chatter, some of it skeptical, but the marathon’s spokesperson, Meg Treat, defended the decision: “We’re grading ourselves on safety, and we’re happy with the outcome.”

The science backs up these adjustments. Climate Central’s study of 221 U.S. marathons found that by 2045, the sweet‑spot temperature for recreational runners—low 40s Fahrenheit—will be a rarity at 46 of the 51 major U.S. races they examined. And the broader climate picture is grim: the United States just logged its warmest July on record, while 2024 was the planet’s hottest year ever. The World Meteorological Organization warns that between 2025 and 2030 the globe could warm by more than 1.5 °C compared with the 1800s.

“The ability to go outside, to exercise, to stay healthy—that’s getting challenged by climate change,” says Andy Pershing, who led the Climate Central research. Universities are listening, too. Jessica Murfree at the University of North Carolina‑Chapel Hill is studying how shifting weather patterns affect endurance sports, from marathons to bike races.

For runners like Grinvalds, the message is clear: preparation now means paying attention not just to mileage, but to the thermometer. “If it’s hotter than 60 degrees, I’m thinking twice,” he admits, wiping sweat from his brow. “Maybe it’s time we stop being stubborn and listen to what our bodies are telling us.”