Soren Waerenskjold of the Uno-X Mobility team competing in the 2026 Tour de France

Jasper Jacobs/Belga/Shutterstock

The world’s top athletes routinely push themselves in an effort to make history – but it is climate change that is breaking all the records this year, with Europe suffering unprecedented early-season wildfires and its hottest and most humid heatwave ever.

On 12 July, the Tour de France was forced to shorten a stage of the cycling race for the first time in its 123-year history. As temperatures neared 40°C and riders stuffed ice down their jerseys, organisers cut out 30 kilometres of stage nine between Malemort and Ussel.

Amid this heat, though, the Norwegian-Danish team Uno-X Mobility has fared surprisingly well, despite hailing from chillier climes. Tobias Hallend Johannessen finished second in stage nine and Torstein Træen took the leader’s yellow jersey for two stages earlier in the competition.

In recent years, coach Olav Aleksander Bu – a pioneer of the data-intensive Norwegian method of endurance training – has become increasingly focused on adapting athletes to extreme heat, using limited but frequent exposure under closely monitored supervision to build up tolerance and boost performance in hot weather.

The Norwegian method, made famous by several Olympic and Ironman endurance winners, seeks to calibrate an athlete’s training by taking frequent finger-prick readings of lactate, which rises in the blood when muscles don’t get enough oxygen. Bu tells New Scientist how, by adding CORE sensors that monitor body temperature to this approach – and shunning air conditioning and cold beverages – he helps endurance athletes perform their best in extreme temperatures.

Editor’s note: The methods in this article are experimental and conducted under close supervision and monitoring. They should not be attempted at home.

Alec Luhn: It’s been the hottest Tour de France and one of the hottest FIFA World Cups ever. How is heat changing sport, and what has it been like living through that in France?

Olav Aleksander Bu: Maybe the closest comparison is the [1968] Olympics in Mexico City, where people were caught by surprise because the sprinters set new records, while everybody that was in endurance mostly underperformed heavily. And what this led to was deeper research and understanding of how altitude affects performance. Heat is not that much different. It’s an external stressor which normally will cause underperformance.

Olav Aleksandr Bu (left) with champion triathlete Kristian Blummenfelt

Santara / CORE

We knew before the Tokyo Olympics [in 2021] that it would be the then-warmest Olympics on record. With my athletes, I started do quite a bit of research on what had been done in sports [and] in the military. We are even more adapted to perform in heat than most animals are, and that is because we can sweat. On the African continent, tribes go out to hunt buffalo in daytime. [The buffalo] outruns humans very easily. But because it gets exhausted much quicker in the heat, they basically [play an] endurance game with this buffalo. Eventually, it gets so exhausted that they can walk up and kill it.

This became the foundation for our strategy leading up to Tokyo, which we are now implementing in cycling.

Can you outline your heat training strategy for elite athletes?

When you start exercising in heat, you build it up gradually. The first thing you do is try to stay away from cold. So: less exposure to cold beverages, less exposure to air con, less exposure to everything that cools down your body and makes you dependent on that.

Then, gradually, you face shorter, low-intensity sessions where you ride in the heat, and then gradually build it up to more intense and more enduring sessions, where you start to build a higher core temperature.

In some cases, we are training in Norway, and that means they put on this painter’s suit, and you are limiting the body’s ability to get rid of heat. Another way is we very often use a small room, put in a heater there. If you’re used to 20°C, bring it up to 30°C, and then gradually bring it up from there.

CORE heat suits as used by the Uno-X Mobility Tour de France team

Aline Barre / CORE

All athletes are a little bit heat-adapted already because, for every calorie that you burn, 70 to 80 per cent of that is heat, and only 20 per cent approximately goes into forward propulsion. If you are pedalling at 200 watts on your bike, that means you’re producing already 800 watts of heat, and that is the same as putting a heater in a room.

How long does it take an athlete to train for high temperatures?

We actually adapt much quicker to heat than we do to other things. With a good heat-training protocol, you will get most of the benefit already after seven to 10 days. One of the key biomarkers we’re looking for is blood volume and plasma expansion, because plasma acts as a coolant. As your body’s temperature comes up, instead of bringing blood into your muscles to enrich the muscles with oxygen, it will start to redirect more blood towards the skin to transport heat away from the body, like a radiator. When you do heat training over one week, you get quite a significant increase in coolant – we can observe that directly by changes in plasma volume.

How important has heat training been to Uno-X Mobility’s success at the Tour de France?

It is critical, because if you don’t train for heat, then you will underperform. One of the simplest ways you can observe this is [via] variable temperature sensors, like CORE, which you can attach to your body. When you get out into the heat and you do a training session, you will see that the temperature [reading] goes up much quicker. But over one week of training in warmer conditions, you can start to increase your speed. The average distance you are capable of running comes back almost to the same normal level again, but without the temperature going up.

This is key because if you don’t do it, for example, what we will see is that if you bring up temperature, they are not acclimatised. In the most extreme cases, performance drops by more than 30 to 40 per cent.

Saunas can provide repetitive heat exposure – is that useful for your athletes?

Active acclimatisation will always be more powerful. However, passive acclimatisation, like using a sauna, can be a good tool. Let’s say you want to go out and run two, three times a week like you normally do, and now it’s really hot. You can run those days when you can run, and the other days you can take a hot tub or a sauna to increase the temperature in the body. You don’t really have to overcomplicate it.

A lot of what you’re doing hasn’t been conclusively examined by peer-reviewed research. How can you be sure it’s working?

The performance speaks for itself. The best peer review you can have is basically performing in the conditions you are racing in. Whether it’s perfect is a completely different thing, but at least it is a step up, an improvement compared to what we have done before.

And [we] have 50 athletes – that’s already starting to become a quite significant population size. Month after month, year after year, [we] repeat and see that we are able to reproduce it, which is what science really is about.

Could you imagine the future where regular people, not just athletes, are wearing body-temperature monitors, just to stay safe?

For most people, [a high core body temperature is] something that we very quickly feel in our body, like when we have a fever. But I would think that, as temperatures get more critical, it’s a natural step that this technology gets integrated into a lot of the sensors that we already are wearing today.

Greenhouse gas emissions are still rising. How many years can the Tour de France and other competitions go on in their current form?

We might have to go to more drastic measures in sport, and that doesn’t only apply to the Tour de France and the Olympics. That goes for sports in general. However, what needs to change is how we approach heat. Heat will need to become an even broader, bigger topic that more people need to really understand.