The Melting Finish Line: How Climate Change is Rewriting the Rules of Long-Distance Running
DES MOINES, Iowa — For Paul Grinvalds, the rhythm of a marathon is a lifelong pursuit. Having crossed the finish line of 114 marathons across all 50 states, the 68-year-old West Des Moines native is no stranger to the physical toll of 26.2 miles. Yet, even with his seasoned experience, Grinvalds admits that his body now struggles to cope with a changing environment. When temperatures climb beyond 60 degrees Fahrenheit (15.5 Celsius), he finds himself wilting—a physiological reaction that is becoming a common, and increasingly dangerous, struggle for runners worldwide.
"One thing about marathoners is that we are, typically, a stubborn bunch," Grinvalds says, noting that he finds even temperatures in the 50s F (10 C) to be taxing. "If we have a goal and it’s hot, a lot of times we are still going to try to achieve it. That is often a mistake, because you are almost guaranteed a painful, potentially life-threatening experience."
Grinvalds’ struggle is not an outlier; it is a manifestation of a global shift. As human-induced climate change—driven primarily by the combustion of fossil fuels—pushes average global temperatures to record highs, the world of endurance sports is facing an existential crisis. Race organizers, once bound by tradition and historical weather averages, are now being forced to overhaul their schedules, shift dates, and adopt radical safety protocols to protect participants from the escalating risks of heat exhaustion and heatstroke.
The Chronology of a Changing Calendar
For decades, marathon organizers operated on the assumption that climate patterns were static. Now, the past few years have shattered that predictability, leading to a cascade of event cancellations and forced schedule changes.
The most prominent example of this shift occurred in the Twin Cities. For over 40 years, the Twin Cities Marathon in Minneapolis-St. Paul was held on the first Sunday of October. However, that tradition came to a crashing halt in 2023 when the race was canceled due to record-shattering heat and humidity. The organizers were dealt another blow in 2025, when the race day saw starting temperatures near 70 degrees Fahrenheit (21 C)—a dangerous threshold for high-exertion distance running.
In response, race director Ed Whetham announced that starting in 2027, the event will permanently shift to later in October. "Canceling the 2023 marathon was devastating for the runners and for our organization," Whetham said. "It prompted an extensive analysis of weather patterns that resulted in the decision to permanently delay the race by a couple of weeks." While the move carries the risk of late-autumn chills—or even the occasional snow—Whetham is clear on his priorities: "We’ll take a 39-degree (4 C) start over a 70-degree (21 C) start any day."
This ripple effect is visible nationwide. In 2024, the Sioux Falls Marathon in South Dakota was canceled due to extreme heat; organizers subsequently moved the race from August to September. Similarly, iconic races such as the Air Force Marathon in Ohio, the Little Rock Marathon in Arkansas, and the Long Beach Marathon in California have all moved their start times to the pre-dawn hours to leverage the cooler morning air.
The Science of Thermal Regulation
The physiological challenge posed by heat is rooted in basic biology. During a marathon, a runner’s body is a furnace. According to expert analysis, only about 25% of the energy produced by a runner is actually used to propel the body forward. The remaining 75% is converted into heat.
"As you get to warmer temperatures, you end up having a bigger competition between blood supplying your muscles—to keep them fueled and moving—and blood flow to your skin for thermal regulation," explains Dr. Chris Minson, a professor and researcher in human physiology.
When the air temperature rises, the body’s cooling mechanism—sweat evaporation—becomes less effective, particularly in humid conditions. If the body cannot shed heat efficiently, it diverts blood away from the muscles to the skin in a desperate attempt to cool the core. This leads to a precipitous drop in performance and, if ignored, the onset of exertional heatstroke, a medical emergency.
Data and Projections: A Warming Future
The urgency of these changes is backed by sobering data. The contiguous United States experienced its warmest July on record recently, and 2024 was globally the hottest year ever documented. The World Meteorological Organization projects that between 2025 and 2030, the Earth’s average global temperature is likely to exceed the 1.5-degree Celsius (2.7 degrees Fahrenheit) warming threshold compared to pre-industrial levels.
A study of 221 races conducted by the climate research group Climate Central highlights the long-term threat. By 2045, the "optimal" running window—typically in the low 40s Fahrenheit (6-7 C)—will become increasingly elusive. The study found that 46 out of 51 major U.S. marathons analyzed will likely see their chances of providing these conditions diminish significantly.
"The ability to go outside, to exercise, to take advantage of your human body and being healthy—that is something that is being challenged by climate change," says Andy Pershing, who led the Climate Central research.
Official Responses and Safety Protocols
As the data becomes clearer, event organizers are adopting "heat mitigation" strategies. In March 2025, the Los Angeles Marathon took a controversial step: organizers offered runners the option to stop at the 18-mile mark and still receive a finisher’s medal. While the move sparked online derision from some purists, the outcome spoke for itself. Less than 5% of participants took the early exit, and only 14 out of 27,000 runners required hospital treatment, with all being released the same day.
"We welcome the feedback, and it’s certainly something that caused a lot of discussion," said Meg Treat, a spokesperson for the event. "But when we’re grading ourselves on it, it has to do with how safe our runners were on the day of the event, and we’re very happy with the outcome."
This sentiment is echoed by experts like Douglas Casa, a kinesiologist and CEO of the Korey Stringer Institute, which studies sudden deaths in sports. Casa argues that while organizers have a responsibility to adjust start times and provide hydration, the onus also falls on the athletes. "If someone was smart enough to say, ‘It’s a little hotter today than usual, I’m going to run a 7-minute mile instead of a 6:30 mile,’ then they won’t get any hotter than normal," Casa said. "The issue is that runners are sometimes very locked into their goals, refusing to adapt to the reality of the environment."
Beyond the Marathon: Implications for Daily Life
The crisis facing marathons is a bellwether for broader public health concerns. Jessica Murfree, who leads a University of North Carolina-Chapel Hill lab studying the impact of climate change on sports, warns that the dangers are not confined to elite athletes.
"The concern extends beyond organized sports to whether the average person can even get outside on increasingly warm days," Murfree said. She notes that as heatwaves become more frequent and prolonged, populations may be forced to abandon outdoor exercise entirely in favor of indoor gyms or water-based activities like swimming.
"If we can’t walk, if we can’t exercise or go for runs because it’s too hot, we have to ask: can we still stay physically active while remaining cool?" she asks. The shift may force a fundamental change in how communities design public spaces, with a greater emphasis on shade, urban cooling, and climate-resilient recreational infrastructure.
Conclusion: Adapting to the New Normal
For some, the answer is aggressive acclimatization. Athletes like John Seals, who trains in West Des Moines, intentionally run in the midday heat and utilize saunas to prepare their bodies for extreme conditions—such as the 135-mile ultramarathon in Death Valley. "We had a heat wave this week, and for me, that’s a great thing," says Seals.
However, such intense preparation is not a universal solution. As the climate continues to warm, the "stubbornness" of the marathoner may no longer be a badge of honor, but a liability. The future of the sport, and perhaps of outdoor activity in general, depends on a collective acknowledgment that the environment has changed. Whether through moving race dates to the edge of winter, offering shorter, safer course options, or simply listening to the physiological signals of an overheated body, the human spirit of endurance must now learn to endure the reality of a warming planet.
