DES MOINES, Iowa — For Paul Grinvalds, a 68-year-old veteran of the pavement who has successfully completed 114 marathons across all 50 states, the challenge of the 26.2-mile distance is supposed to be about endurance, training, and mental fortitude. But in recent years, Grinvalds has found himself battling a more formidable opponent than fatigue: the thermometer.
Even with decades of experience, Grinvalds admits that his performance—and his safety—starts to deteriorate once temperatures climb beyond 60 degrees Fahrenheit (15.5 Celsius). He is far from alone. As the planet experiences its most sustained period of warming in recorded history, the world of long-distance running is undergoing a profound transformation. What was once a sport defined by the grit of the runner is now being dictated by the volatility of the climate.
"One thing about marathoners is that we are typically, by nature, kind of stubborn," Grinvalds said from his home in West Des Moines. "If we have a goal and it’s hot, a lot of times we’re still going to try to pursue that goal. That is, quite frankly, just stupid, because you are almost guaranteed to have a very painful experience."
The Changing Landscape of Distance Racing
The reality of climate change—driven largely by the combustion of fossil fuels like coal and petroleum—is no longer a theoretical concern for race directors; it is a present-day operational crisis. From the iconic city marathons to smaller regional races, the blueprint for a successful event is being redrawn. Organizers are now routinely forced to shift race dates to cooler months, mandate earlier start times, and implement "exit strategies" for runners to safely withdraw mid-race.
The shift is perhaps most visible in the Midwest. The Twin Cities Marathon, a staple of the Minneapolis-St. Paul athletic calendar for over 40 years, announced it would permanently shift its traditional first-Sunday-in-October date to later in the month starting in 2027. This decision follows a harrowing 2023 season, where record heat and humidity forced a last-minute cancellation, followed by a 2025 event that saw dangerous starting temperatures near 70 degrees Fahrenheit.
"Canceling the 2023 race was devastating for our runners and for the organization," said Ed Whetham, the marathon’s race director. "It prompted an extensive analysis of historical weather patterns that made it clear we needed to adapt to the new normal."
Whetham acknowledges the trade-off: moving the race deeper into October increases the risk of cold snaps or even early-season snowfall. However, he remains pragmatic. "We will take a 39-degree (4 Celsius) start over a 70-degree (21 Celsius) start every single time. It is a necessary shift to ensure the safety of our participants."
A Global Trend: From Sioux Falls to Southern California
The trend of "chasing the cool" is nationwide. In Sioux Falls, South Dakota, organizers moved their annual race from August to September following a 2024 cancellation due to excessive heat. Similar adjustments are being made across the country:
- The Air Force Marathon (Dayton, Ohio): Now features earlier start times to beat the midday sun.
- The Little Rock Marathon (Arkansas): Has shifted its operational schedule to avoid peak heat hours.
- The Long Beach Marathon (California): Has adjusted start times to minimize heat exposure.
In Los Angeles, a city already accustomed to warm climates, organizers took a bold step in March. They offered runners the option to "stop" at the 18-mile mark, providing them with a finisher’s medal despite not completing the full distance. While the move sparked some debate on social media, the data told a different story: fewer than 5% of participants took the offer, and only 14 out of 27,000 runners required hospital treatment.
"We welcome the feedback, but when we grade ourselves, it comes down to how safe our runners were on that day," said Meg Treat, a spokesperson for the event. "We are very happy with that outcome."
The Science of Heat Stress
To understand why the marathon is particularly vulnerable to climate change, one must look at human physiology. At the 26.2-mile distance, the body generates massive amounts of internal heat. Under ideal conditions, this is managed through blood flow to the skin, where heat is dissipated.
"As temperatures rise into the upper 50s and 60s, the body begins a losing battle," explains Dr. Chris Minson, a leading researcher in thermal regulation. "You end up having a massive competition between the blood supply required to keep your muscles moving and the blood flow required to reach the skin for cooling."
Only about 25% of the energy produced during a run is used to move the body forward; the remaining 75% is converted into heat. When the ambient temperature is high—and particularly when humidity prevents sweat from evaporating—the body’s cooling mechanism fails. This creates a dangerous feedback loop that can lead to heat exhaustion or, in extreme cases, heatstroke.
Implications for the Future of Physical Activity
The warning signs are backed by hard data. The contiguous United States recently experienced its warmest July on record, and 2024 stands as the hottest year in global history. The World Meteorological Organization projects that between 2025 and 2030, the global average temperature will likely rise by more than 1.5 degrees Celsius compared to pre-industrial levels.
A study of 221 races by the climate research group Climate Central offers a grim forecast: by 2045, the "optimal" running conditions (low 40s Fahrenheit) will be a relic of the past for 46 of the 51 major U.S. marathons analyzed.
"The ability to go outside, to exercise, to take advantage of your human body—that is something being challenged by climate change," said Andy Pershing, who led the Climate Central research.
Jessica Murfree, who directs a lab at the University of North Carolina-Chapel Hill focused on the intersection of climate change and sports, suggests the problem extends far beyond the marathon. "If we cannot walk, if we cannot exercise, or go for runs because it is too hot, what happens to public health?" she asks. "We are seeing a shift where more people are forced to exercise indoors or pivot to activities like swimming, which provide natural cooling."
The "Stubborn" Runner’s Dilemma
Despite the warnings, the culture of the sport remains inherently driven by "toughing it out." Experts like Douglas Casa, a professor of kinesiology and head of the Korey Stringer Institute, argue that education is the most critical tool for survival.
"If a runner is smart enough to acknowledge the heat and adjust their pace—say, running a 7-minute mile instead of a 6:30—they won’t get any hotter than they would on a cool day," Casa says. "The problem is that runners are often locked into their goals and their data. They feel that if they aren’t hitting their splits, they aren’t doing it right."
For some, however, the response to climate change is extreme adaptation. John Seals, another runner from West Des Moines, trains by running in the midday heat and using saunas to force his body to acclimate. He is preparing for an annual 135-mile ultramarathon in Death Valley. "We had a heat wave this week," Seals said, "and for me, that’s a great thing."
Conclusion: A New Era of Athletic Responsibility
As the Earth continues to warm, the relationship between the athlete and the environment is changing. For race organizers, the priority has shifted from facilitating a personal record to ensuring that every runner crosses the finish line in one piece. For the individual runner, the challenge is no longer just about beating the clock—it is about respecting the limitations of the human body in an increasingly volatile climate.
The era of "running through it" is fading. In its place is an era of data-driven decisions, flexible race logistics, and a newfound humility toward the elements. As climate change continues to rewrite the record books, the running community is discovering that the greatest victory is not a fast time, but a safe return.
