DES MOINES, Iowa — For Paul Grinvalds, the marathon is not just a test of endurance; it is a lifelong pursuit. Having completed 114 marathons across all 50 U.S. states, the 68-year-old West Des Moines native is a veteran of the road. Yet, despite his extensive experience, he finds himself increasingly vulnerable to a silent, growing adversary: rising global temperatures.
For Grinvalds, even a mild race day with temperatures hovering in the 50s Fahrenheit (10 degrees Celsius) feels warm. When the mercury climbs beyond 60 degrees Fahrenheit (15.5 Celsius), his performance—and his safety—begin to deteriorate. He is not alone. As climate change continues to drive record-breaking heat waves, the sport of long-distance running is undergoing a radical, often painful transformation.
The Physiological Toll of Rising Mercury
The mechanics of human performance are fundamentally compromised by heat. During intense physical exertion, the body acts as a biological engine, producing massive amounts of heat as a byproduct of muscle contraction. To prevent internal overheating, the body relies on a sophisticated cooling system: diverting blood flow away from the muscles and toward the skin, where it can be cooled through sweat evaporation.
However, as ambient temperatures rise, this thermoregulatory process hits a wall. When the mercury climbs, the heart must work overtime, simultaneously trying to supply oxygen-rich blood to the working muscles while pushing blood to the skin to facilitate cooling.
"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.
The math is unforgiving. A runner’s body is only about 25% efficient at converting energy into forward motion. The remaining 75% of energy is released as heat. In high-humidity environments, this process becomes even more perilous, as sweat cannot evaporate effectively, leading to rapid core temperature spikes. For the "stubborn" marathoner, ignoring these biological warning signs often leads to disaster.
"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 admits.
A Chronology of Climate-Induced Cancellations
The threat posed by heat is no longer theoretical; it is a logistical reality that has forced race directors across the United States to rethink decades-old traditions.
- 2023: The Twin Cities Marathon in Minneapolis-St. Paul was abruptly canceled due to record-shattering heat and humidity. The event, a staple of the autumn racing calendar for over 40 years, was shuttered to protect runners from dangerous conditions, a move race director Ed Whetham described as "devastating."
- 2024: The Sioux Falls Marathon in South Dakota was forced to cancel its race entirely due to extreme heat. Following this, organizers made the permanent decision to shift the race date from August to September to find a cooler window.
- 2025: The Twin Cities Marathon saw starting temperatures near 70 F (21 C), prompting officials to conduct a massive, multi-year analysis of local weather patterns.
- 2027 and Beyond: The Twin Cities Marathon will permanently shift its race date from the first Sunday in October to later in the month. While this increases the risk of cold or snowy weather, organizers argue it is a necessary trade-off. "We’ll take a 39-degree (4 C) start over a 70-degree (21 C) start," Whetham noted.
This trend is mirrored globally and nationally. From the Air Force Marathon in Ohio to the Little Rock Marathon in Arkansas and the Long Beach Marathon in California, organizers are consistently moving start times to the pre-dawn hours, hoping to finish before the sun reaches its zenith.
The Data: A Hotter Future for Racing
The necessity of these changes is backed by sobering meteorological and climate data. The contiguous United States recently endured its warmest July on record, and 2024 stands 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 races and found that the warming trend is actively eroding the viability of traditional racing schedules. Their projections suggest that by 2045, the "optimal" conditions for recreational runners—typically in the low 40s Fahrenheit (6–7 C)—will be increasingly rare at 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 Climate Central research.
Official Responses and Safety Innovations
In response to these findings, the sports industry is shifting its risk management protocols. In Los Angeles, the March 2025 marathon offered participants the option to stop at the 18-mile mark and still receive a finisher’s medal—an attempt to curb the health risks associated with the final, most grueling miles in rising heat.
While the decision drew some criticism from purists, Meg Treat, a spokesperson for the event, defended the move. "We welcome the feedback, but when we’re grading ourselves, it has to do with how safe our runners were on the day of the event, and we’re very happy with the outcome," she said. Only 14 out of 27,000 runners required hospital care that day, all of whom were released shortly after.
Kinesiology experts, such as Douglas Casa of the Korey Stringer Institute, argue that these adjustments are not just prudent—they are vital. "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 mile,’ then they won’t get any hotter than normal," Casa explains. "It’s just that runners are sometimes very locked into what they normally do and what their goals are."
Broader Implications: The Future of Physical Activity
The impact of climate change on organized athletics is merely the tip of the spear. Jessica Murfree, who leads a research lab at the University of North Carolina-Chapel Hill, warns that the concern extends well beyond the professional or hobbyist racing circuit.
"If we can’t walk, if we can’t exercise or go for runs because it’s too hot, can we still stay physically active while remaining cool?" she asks. Murfree notes that climate change is effectively pushing the public toward indoor exercise facilities or water-based activities like swimming, creating an equity issue for those who lack access to climate-controlled environments.
For the most extreme athletes, the response has been to double down on acclimation. Some, like West Des Moines runner John Seals, intentionally train in the heat of the day and utilize saunas to force their bodies to adapt. Seals, who is training for a 135-mile ultramarathon in Death Valley, sees heat waves as a training tool rather than a deterrent.
"We had a heat wave this week, and for me, that’s a great thing," Seals said.
However, as the baseline temperature of the planet continues to climb, the window for safe, outdoor activity is shrinking for the average person. For those like Paul Grinvalds, the sport they love is becoming a game of chasing shadows and cooler seasons—a race against a climate that is, in every sense, turning up the heat.
The challenge for the future is clear: how to maintain a culture of human movement and physical health in an environment that is becoming increasingly hostile to the very act of being active. As organizers, athletes, and scientists continue to navigate these shifts, the "stubborn" nature of the marathoner may need to evolve into a more flexible, science-based approach to survival.
