Running a marathon means endurance, perseverance and preparation. But, in addition to training and determination, another decisive factor influences performance: the climatic conditions of the race location. According to a recent report published by Climate Central, entitled “Running Out of Cool Days”, global warming is drastically reducing the chances of running in optimal weather conditions [1].

The researchers who authored the study analysed 221 marathons around the world, including the seven Abbott World Marathon Majors (Tokyo, Boston, London, Berlin, Chicago, New York and Sydney) [2], and the results are clear. By 2045, 86% of these races will experience a decrease in the likelihood of ideal running temperatures, affecting both elite athletes and amateur runners.

According to the analyses carried out by the scientists, marathon runners’ performance depends heavily on the surrounding temperature. There is a sort of “sweet spot”, a temperature range in which the body can dissipate heat without excessive effort. For professionals, the optimal temperature is around 4°C for men, while for women the value rises to 10°C [1]. Amateur runners, on the other hand, tend to perform better at around 6-7°C, regardless of gender.

When the thermometer moves away from these values, performance drops. Too much heat means overheating, dehydration and an increased risk of heatstroke; too much cold means muscle stiffness and breathing difficulties [1]. The problem in recent years is that days with ideal temperatures are disappearing at an ever-increasing rate. Climate projections show that if greenhouse gas emissions continue to grow according to the SSP3-7.0 scenario (one of the most pessimistic in terms of emissions [3]), many cities that currently host famous marathons may no longer be able to offer suitable conditions for their athletes.

Figure 1: Tired Marathon pro-athlete. Image by Erdenebayar Bayansan from Pixabay.

In 2025, the Tokyo Marathon was the “luckiest” of the World Majors, with a 69% chance of offering ideal conditions for professionals and an 80% chance for amateur runners. However, by 2045, this figure could fall to 57%. Even so, Tokyo experienced one of the hottest editions in its history on the 2nd of March this year, with an average temperature of 15.2°C in the Japanese capital, eight degrees above the seasonal norm. According to the Climate Shift Index (the indicator of how climate change has altered the frequency of daily temperatures in any location in the world, every day), this temperature anomaly was made three times more likely by human-induced climate change [1].

The Berlin Marathon also recently suffered from these extreme conditions. In September 2025, runners had to compete with an average temperature of 20.7°C, almost 7 degrees above normal, an event that science attributes without a shadow of a doubt to the effects of global warming.

The study describes a worrying picture for many of the world’s most prestigious races [1].

• Berlin, loved by runners for its flat, fast course, will see the probability of ideal temperatures drop to 0% by 2045.
• London will go from 22% to 17%, while New York will stabilise at its minimum values (from 19% to 17%).
• Sydney is the most penalised, with the chances of running in optimal conditions already almost nullified today.

A small positive note comes from Boston and Tokyo, where female professionals, who are more tolerant of heat, could see a slight increase in favourable conditions. However, this does not compensate for the general deterioration of many other participants.