The sub–two–hour marathon has long been a dream for elite runners.
But the window to break this elusive record is shrinking – thanks to climate change.
Previous studies have shown that for elite men, the optimal running temperature is 4°C, while for elite women, it’s 10°C.
However, a new analysis by scientists from Climate Central shows that these optimal conditions are slipping away.
The researchers looked at how temperatures are changing on 221 popular global race courses – including London, Berlin, and Boston.
Worryingly, their analysis shows that a whopping 86 per cent of these races will see a decline in odds of optimal running temperatures by 2045.
‘Climate change isn’t just about races becoming harder; it’s about knowing that record-breaking performances could soon be out of reach if conditions keep getting hotter,’ said Mhairi Maclenna, the fastest British finisher at the London Marathon 2024.
‘As athletes, we push limits — but there’s only so much we can do when emissions keep temperatures rising.’
Previous studies have shown that for elite men, the optimal running temperature is 4°C, while for elite women, it’s 10°C. However, a new analysis by scientists from Climate Central shows that these optimal conditions are slipping away
The world record for the fastest 26.2 mile (42.2 km) run is 2:00:35, as set by the late Kenyan athlete Kelvin Kiptum at the 2023 Chicago Marathon
According to a 2012 study, published in PLoS One, men run best in cooler conditions (4°C for elite runners and 6°C for recreational runners), while women fair best in slightly warmer weather (10°C for elites and 7°C for recreational runners).
In their new analysis, the Climate Central team set out to understand how often these optimal conditions will be seen at marathons around the world.
| Marathon | Elite men (2025) | Elite men (2045) | Elite women (2025) | Elite women (2045) |
|---|---|---|---|---|
| Tokyo | 69% | 57% | 78% | 85% |
| Boston | 61% | 53% | 79% | 82% |
| London | 22% | 17% | 87% | 81% |
| New York City | 19% | 17% | 71% | 66% |
| Chicago | 14% | 14% | 57% | 54% |
| Berlin | 1% | 0% | 40% | 29% |
| Sydney | 0% | 0% | 31% | 21% |
The researchers focused on 221 marathons worldwide, and calculated the probability of optimal conditions in 2025, 2035, and 2045.
The results suggest that if elite athletes want to break the sub-two-hour record, their window of opportunity is rapidly shrinking.
From 2025 to 2045, the largest decline in ideal conditions is for elite men at the Tokyo Marathon, dropping from 69 per cent to just 57 per cent.
Meanwhile, elite women will only have a 29 per cent of optimal conditions at the Berlin Marathon in 2045.
However, it’s not all doom and gloom.
Global warming will actually slightly increase the odds of optimal race day conditions for elite women at the Boston Marathon and the Tokyo Marathon.
The researchers focused on 221 marathons worldwide, and calculated the probability of optimal conditions in 2025, 2035, and 2045 (% values represent the likelihood of optimal temperatures on race day)
‘Climate change has altered the marathon,’ said Catherine Ndereba, former Marathon Record Holder.
‘Dehydration is a real risk, and simple miscalculations can end a race before it begins.
‘We’re not just training to run anymore; athletes have to adapt how they deal with the conditions, including in how they eat and hydrate.’
The researchers hope the findings will highlight the importance of slashing greenhouse gas emissions immediately.
‘Around 1.1 million people finish a marathon each year, but as the planet warms due to climate change, the cool, comfortable race-day conditions that help runners perform their best are becoming harder to find,’ Climate Central explained.
‘For most recreational runners, the odds of racing in perfect conditions are already slim for some races.
‘For elite athletes chasing records, they’re facing some races where optimal temperatures are nearly impossible.
‘A different future will require significant and lasting emissions cuts to minimize carbon pollution.’
The world record for the fastest 26.2 mile (42.2 km) run is 2:00:35, as set by the late Kenyan athlete Kelvin Kiptum at the 2023 Chicago Marathon.
His rival, Eliud Kipchoge, actually beat the two-hour milestone at a race in Vienna back in 2019.
However, this was not recognised as a record, due to the presence of pacemakers, delivery of hydration by bicycle, and the lack of open competition.
