A hotter world is quietly changing one of the simplest things we do for our health – moving our bodies. For many people, a walk in the park, a jog around the neighbourhood or a cycle to work is becoming harder, and sometimes unsafe, as temperatures rise.

Scientists are beginning to understand how heat affects physical activity and why this matters for long-term health. A new modelling study in The Lancet Global Health suggests that if rising temperatures lead to sustained reductions in activity, the knock-on effects could contribute to hundreds of thousands of premature deaths each year by the middle of this century.

The underlying behaviour is familiar. As temperatures climb, many people cut back on outdoor exercise. There is no single threshold, but activity often becomes noticeably less comfortable somewhere in the high 20s celsius, especially in humid conditions. Running, cycling or even brisk walking can feel more strenuous.

This is because the body has to work harder to stay cool. More blood is diverted to the skin and sweating increases, which can lead to earlier fatigue, dizziness and dehydration. Faced with this, people may slow down, shorten their exercise or avoid it altogether. Across large populations, this can translate into less movement,........

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