Climate Change Is Rewriting the Marathon Calendar

If you have run a late-September or October marathon, you know the pattern. The first half feels crisp and controlled. Somewhere past mile 18, the sun clears the buildings, the pavement starts radiating, heart rate drifts up without any change in effort, and the final 10K becomes a negotiation.

Runners like to call that bad luck. The data says it is a trend.

The window is shrinking

Climate Central, a nonpartisan climate research group, analyzed long-term weather data for 221 marathons worldwide. The headline finding: by 2045, 86 percent of those races are projected to face reduced odds of ideal race-day conditions, and all seven Abbott World Marathon Majors—including Boston and New York City—are among them. In the United States, 46 of the 51 marathons analyzed face declining odds of optimal temperatures.

The odds are already thin. Climate Central estimates the chance of a typical recreational U.S. marathoner racing in optimal temperatures at roughly 27 percent—and defines “optimal” narrowly. Elite men run fastest around 39°F, elite women near 50°F, and recreational runners do best in the low 40s. By the time conditions reach 60°F, the physiology is working against you; studies of marathon performance show finishing times slow progressively as weather warms, and the effect compounds with humidity and sun.

There is also an equity problem in the data. A 3-hour runner spends far less time exposed to the afternoon heat than a 5-hour runner. As race days warm, the back of the pack absorbs the most heat exposure precisely when they are most fatigued.

How organizers are responding

Reporting from the Associated Press in August 2026 documented the concrete changes:

• Moving race dates. The Twin Cities Marathon, a more-than-40-year fixture on the first Sunday of October, will shift later into the month starting in 2027—a decision that followed the race’s 2023 cancellation for record heat and humidity and a 2025 start near 70°F. The Sioux Falls Marathon moved from August to September after canceling its 2024 edition for hot conditions.
• Starting earlier. The Air Force Marathon in Dayton, the Little Rock Marathon, and the Long Beach Marathon all moved starts earlier in the morning to buy back cooler hours.
• Shortening options. Some races now offer runners official ways to cut runs short when conditions turn dangerous—reversing the old culture of finish-at-all-costs.

None of this is new at the elite level. Tokyo 2020 moved its marathon 500 miles north to Sapporo to escape the Japanese summer, and World Athletics events run on WBGT-based heat plans that can change start times or shorten races. What is new is ordinary regional marathons treating heat as a planning assumption rather than a surprise.

What this means for your race day

Research the race, not the city. Ignore the brochure line about “average race-day temperatures.” Look up hourly historical data for the specific date: how cold does the night get beforehand, and how fast does it warm between 9 a.m. and noon? A course with a nighttime low of 70°F produces a muggy start no matter what the “average” says.

Set goals in tiers before the expo. Decide in advance what the plan becomes at 40°F, at 55°F, and at 65°F. A reasonable rule: once the start approaches the mid-50s, move goal pace out by 15 to 35 seconds per mile and switch your attention from the clock to effort. Use a tune-up race or test run to see what warm-weather pace actually feels like before you commit to a number.

Race by effort in the heat. Heart rate and perceived effort tell the truth when temperature corrupts pace. If your watch says marathon pace but your body says tempo, believe your body—and understand that in heat this drift is expected, as the summer training guide explains.

Spend the aid stations on cooling, not just fuel. Ice in the cap and down the spine, sponges on the neck and wrists, water over the head. These are not comfort luxuries; they are pace-preservation tools that let you keep running when others slow.

Cooperate with the safety plan. If a race shortens the course, delays the start, or invokes a heat flag system, that decision is made on wet-bulb physics, not on your training block. The consensus recommendations for endurance events in heat give organizers exactly this authority for good reason.

The honest reframe

Marathon training already requires adaptability—weather scrambles workouts, taper weeks never feel right, and race-day variables stack up. Warming just made that adaptability part of the sport’s baseline. Choosing a cooler-odds race date, training through summer heat, and building a layered race plan are no longer fringe strategies. They are how modern marathoners protect both the PR and the season.

Runners like to say you cannot control the weather. True—but you can control when you race, how you pace, and when you choose to back off. The runners who internalize that are the ones still lining up, healthy, year after year.

Sources

• Declining odds of cooler, faster U.S. marathons, Climate Central, 2025.
• Hot weather forces marathons to consider changes to help runners, Associated Press, August 2026.
• Impact of weather on marathon-running performance, Medicine & Science in Sports & Exercise, 2007.
• Consensus recommendations on training and competing in the heat, British Journal of Sports Medicine, 2015.