DES MOINES, Iowa — For Paul Grinvalds, the marathon is more than a race; it is a life’s pursuit. Having completed 114 marathons across all 50 states, the 68-year-old resident of West Des Moines, Iowa, possesses an intimate knowledge of the physical toll that 26.2 miles exacts on the human body. Yet, even with decades of experience, Grinvalds remains acutely vulnerable to a force that no amount of training can fully negate: the rising mercury of a warming planet.
For Grinvalds and thousands of runners like him, any race day where the temperature climbs beyond 60 degrees Fahrenheit (15.5 Celsius) is a cause for concern. "One thing about marathoners is that we are typically stubborn," Grinvalds admits. "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."
This stubbornness, once a hallmark of the sport’s grit, has become a liability. As climate change—driven primarily by the combustion of fossil fuels—pushes global temperatures to record highs, the traditional marathon calendar is coming under intense pressure. From the Midwest to the West Coast, organizers are being forced to rethink everything from start times to the very feasibility of their events, marking a permanent shift in how the sport operates.
The Chronology of a Changing Calendar
The decision by the Twin Cities Marathon to push its race date back by two weeks starting in 2027 is perhaps the most prominent indicator of the "new normal." For over four decades, the event was a staple of the first Sunday in October. However, the cancellation of the 2023 race due to record-breaking heat and humidity, followed by a 2025 race that saw starting temperatures hover near 70 F (21 C), forced organizers to confront the reality that historical weather patterns are no longer a reliable guide for the future.
This pattern is playing out across the United States. In South Dakota, the Sioux Falls Marathon transitioned from an August slot to September after safety concerns led to a full cancellation in 2024. Similar pivots are occurring nationwide:
- The Air Force Marathon (Dayton, Ohio): Now employs earlier start times to beat the heat.
- The Little Rock Marathon (Arkansas): Has shifted its operational window to avoid the midday sun.
- The Long Beach Marathon (California): Has adjusted its logistical start to prioritize cooler, early-morning temperatures.
These changes are not merely administrative; they are reactive measures to a rapidly changing climate. While moving a race date involves complex negotiations with city officials, road closures, and volunteer coordination, race directors increasingly view these challenges as the cost of doing business in a warming world.
Data-Driven Decisions: The Science of Heat
The concern among race organizers is supported by overwhelming meteorological and scientific data. The contiguous United States recently experienced its warmest July on record, and 2024 stands as the hottest year in recorded human history. Projections from the World Meteorological Organization suggest that between 2025 and 2030, the global average temperature is likely to rise by more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) compared to the pre-industrial era of the 1800s.
A comprehensive study of 221 races conducted by the climate research group Climate Central highlights the impending crisis. The study found that by 2045, the "optimal" conditions for recreational runners—typically in the low 40s F (6–7 C)—will become increasingly rare at 46 of the 51 major U.S. marathons analyzed.
"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.
The physiological challenge is rooted in basic thermodynamics. According to Christopher Minson, a professor of human physiology, when ambient temperatures rise into the upper 50s and 60s, the body faces a brutal internal competition for blood flow. Muscles require oxygen-rich blood to maintain pace, while the skin requires that same blood to facilitate heat dissipation through sweat and radiation.
"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," Minson explains. To make matters worse, only about 25% of the energy produced by a runner is used for forward propulsion; the remaining 75% is converted into metabolic heat, which the body must then struggle to expel.
Official Responses and Strategic Adaptations
In March, the Los Angeles Marathon took a controversial but proactive step: it offered runners the option to end their race at the 18-mile mark while still receiving a finisher’s medal. While the initiative was met with some online derision from those who felt it undermined the spirit of the 26.2-mile distance, race officials defended the decision as a critical safety intervention.
"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." The results were telling: fewer than 5% of participants took the shortened option, and hospitalizations were kept to a minimum—just 14 of 27,000 runners, all of whom were released the same day.
At the Twin Cities Marathon, race director Ed Whetham views the permanent delay of the race as an exercise in prudence. "We’ll take a 39-degree (4 C) start over a 70-degree (21 C) start any day," he notes, acknowledging that while late October in Minnesota brings the risk of snow, the health and safety of the participants remain the paramount concern.
The Broader Implications for Physical Activity
The implications of this warming trend extend far beyond the finish line of a marathon. Jessica Murfree, who leads a research lab at the University of North Carolina-Chapel Hill focusing on the intersection of climate change and sports, argues that the issue is a public health crisis in the making.
"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 notes that the impact is being felt across all athletic disciplines, from youth soccer and college football to professional tennis. As humidity levels rise alongside temperatures, the body’s natural cooling mechanism—evaporative sweat—becomes less effective, leading to a higher risk of exertional heat stroke.
The solution, according to experts like Douglas Casa of the Korey Stringer Institute, is a combination of systemic change and individual education. "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 says. "It’s just that runners are sometimes very locked into what they normally do and what their goals are."
While some elite athletes, such as ultrarunner John Seals, are engaging in extreme heat-acclimation training—including running in midday heat and utilizing post-workout saunas—to prepare for events like the 135-mile race in Death Valley, such practices are not feasible or safe for the average recreational runner.
A Future in Flux
As we look toward the next decade, the relationship between human endurance and the environment will continue to be tested. The "stubbornness" that defined the marathoner of the 20th century is evolving into a more cautious, data-informed approach.
Whether it is through earlier start times, the implementation of "bail-out" points, or the wholesale rescheduling of events to avoid the most volatile months of the year, the racing industry is being forced to adapt. For Paul Grinvalds and the rest of the running community, the marathon remains a testament to human will. However, in an era of climate instability, the greatest test of that will may no longer be the distance of the course, but the ability to listen to the body and respect the limits imposed by an increasingly hostile climate.
The marathon will survive, but it will look different. As the seasons shift and the record books are rewritten, the runners who cross the finish line in the coming decades will likely be those who have learned that, in the face of a changing planet, flexibility is the ultimate strength.
