DES MOINES, Iowa — Paul Grinvalds has crossed the finish line of 114 marathons across all 50 states, a testament to his endurance and commitment to the sport. Yet, even with decades of experience, the 68-year-old West Des Moines resident remains acutely vulnerable to the one opponent he cannot outrun: the rising mercury. For Grinvalds, temperatures above 60 degrees Fahrenheit (15.5 Celsius) signal a physiological red line.
"One thing about marathoners is typically we’re kind of stubborn," Grinvalds says. "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."
Grinvalds is not an outlier; he is a representative of a global community of runners grappling with a new reality. As climate change, fueled by the burning of fossil fuels, continues to drive record-breaking global temperatures, the traditional "marathon season" is being forced into a radical, mandatory evolution. From shifting race dates to offering "early exit" medals, organizers are scrambling to protect participants from a sport that is becoming increasingly perilous in the heat.
The Physiology of Failure: Why Heat is the Enemy
To understand the crisis facing the sport, one must look at human biology. Running a marathon requires the body to move with high-intensity efficiency. However, only about 25% of the energy produced during this exertion is used for forward motion; the remaining 75% is converted into metabolic heat.
In moderate conditions, the body dissipates this heat through sweat and blood flow to the skin. But as ambient temperatures climb, the body faces a physiological conflict. "As you get to warmer temperatures, you end up having a bigger competition between blood supplying your muscles to keep running and blood flow to your skin for thermal regulation," explains exercise physiologist Dr. Chris Minson.
When the thermometer pushes into the upper 50s and 60s, this internal tug-of-war intensifies. If humidity is added to the mix, the danger spikes exponentially, as sweat can no longer evaporate to provide the necessary cooling effect. Without these mechanisms, the risk of heatstroke, organ failure, and death rises, turning a recreational pursuit into a high-stakes health gamble.
A Chronology of Change: Adaptation in Real-Time
The shift in how marathons are managed is no longer a theoretical exercise; it is an active response to recent disasters.
- 2023: The Twin Cities Marathon in Minneapolis-St. Paul was forced into a devastating cancellation. After four decades of tradition, the race was called off due to record-shattering heat and humidity, leaving thousands of runners stranded.
- 2024: The Sioux Falls Marathon in South Dakota was cancelled due to extreme heat, prompting organizers to permanently move their race from the sweltering heat of August to the cooler, more stable climate of September.
- 2025: The Twin Cities Marathon returned, but with temperatures near 70 degrees Fahrenheit at the start, the event served as a final warning. Organizers analyzed years of climate data and concluded that the old tradition of the first Sunday in October was no longer sustainable.
- 2027 and Beyond: The Twin Cities Marathon will permanently shift its race date to later in October. While this introduces the risk of frost or even snow, race director Ed Whetham notes that the trade-off is clear: "We’ll take a 39-degree (4 C) start over a 70-degree (21 C) start any day."
Similar shifts are occurring globally. 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, attempting to beat the heat of the day.
Supporting Data: A Warming World
The necessity of these changes is backed by cold, hard data. The contiguous United States recently experienced its warmest July on record, following 2024, which went down as the hottest year in recorded history. According to the World Meteorological Organization, the global average temperature is expected to rise by more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels between 2025 and 2030.
The climate research group Climate Central analyzed 221 marathons and found that warming is already degrading conditions for participants. Their study suggests that by 2045, the "optimal" temperature range for recreational runners—typically in the low 40s Fahrenheit—will become increasingly rare for 46 of the 51 major U.S. marathons studied.
"The ability to go outside, to exercise, to take advantage of your human body and being healthy—that’s something that’s getting challenged by climate change," says Andy Pershing, who led the research.
Official Responses and Safety Measures
The race industry is struggling to balance tradition with duty of care. In Los Angeles this past March, organizers introduced a controversial measure: allowing runners to stop at the 18-mile mark and still receive a finisher’s medal. While fewer than 5% of the 27,000 participants took the offer, the move was seen as a vital safety valve.
"We welcome the feedback, and it’s certainly something that caused a lot of discussion," says 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."
Douglas Casa, a professor of kinesiology and leader of the Korey Stringer Institute, which tracks sports-related deaths, argues that such measures are a necessary concession to human psychology. "If someone was smart enough to say, ‘OK, it’s a little hotter today than usual, I’m going to run a 7-minute mile instead of a 6:30,’ then they won’t get any hotter than normal," Casa says. "It’s just that runners are sometimes very locked into what they normally do."
Implications for the Future of Physical Activity
The implications of this warming trend extend far beyond the finish line of a 26.2-mile race. Jessica Murfree, who leads a laboratory at the University of North Carolina-Chapel Hill studying the impact of climate change on sports, warns that we are entering an era where the basic human act of outdoor exercise is under threat.
"If we can’t walk, if we can’t exercise or go for runs, it’s too hot—can we still stay physically active while remaining cool?" Murfree asks. She predicts a mass migration toward indoor gyms, climate-controlled facilities, and water-based sports like swimming, as outdoor exercise becomes a luxury of the early morning or late evening.
For some, the answer is adaptation through extreme training. John Seals, another West Des Moines runner, embraces heat waves as a training tool. He intentionally runs in the midday heat and spends time in saunas to prepare for the 135-mile ultramarathon in Death Valley. "We had a heat wave this week, and for me, that’s a great thing," he says.
However, for the vast majority of the population—casual joggers, school athletes, and aging runners like Grinvalds—the strategy is less about conquering the heat and more about survival. The "stubborn" nature of the athlete, once considered a virtue, is now a liability. As the world warms, the most successful runners will not necessarily be the fastest, but those who are the most willing to listen to their bodies and accept that the old records were set in a climate that no longer exists.
The sport of running is not dying, but it is undoubtedly changing. The marathon, once a celebration of human potential in the open air, is becoming a strategic exercise in navigating a planet that is increasingly inhospitable to endurance.
