Megacity heatwaves are now a fatal nightmare

Daytime and night-time heatwaves are coinciding, leaving no opportunity to cool down.

- David Fickling

Most people in reasonable health could probably complete a 100m run without serious ill effects. Get them to jog non-stop for 24 hours, and they might be in serious danger.

That’s a useful way to think about how urbanisation on our rapidly warming planet is risking the well-being of billions of people.

We’re used to measuring dangerous heatwaves in terms of daytime highs, such as the scorching 48.2 deg C in a town in India’s Uttar Pradesh province in May. We should probably be more worried about the 32.5 deg C night-time low in Delhi the following week, though.

The sprint of one sweltering afternoon is survivable for most people. The marathon of days on end with no night-time respite can be fatal.

Scientists are only just starting to understand this phenomenon, and data on the subject is still sparse. But it’s all pointing in one direction.

A 2023 study of 25 million deaths in Japan between 1973 and 2015 found that mortality was as much as 10 per cent higher on hot nights, even after controlling for daytime heat.

Another found that such conditions raised the risk of death in parts of Switzerland by as much as a third. The same effects have since been observed in the US and 28 East Asian cities.

When daytime and night-time heatwaves coincide, leaving no opportunity to cool down, the effects could be even worse.