June heatwave may have killed around 20,000 people in Europe

These numbers are much higher than the direct counts announced so far, but this isn’t surprising because it takes time for data on deaths to be collected and analysed. “This figure is a modelled estimate rather than a final count, and it will be some months before the true toll is confirmed, in part because heat rarely appears on a death certificate,” says Raquel Nunes at the University of Warwick in the UK. For instance, on 28 June, the head of the World Health Organization, Tedros Adhanom Ghebreyesus, said more than 1300 excess deaths had been reported so far. This number is largely based on a statement from Public Health France reporting around 1000 more deaths in the country than expected from 24 to 26 June. However, that statement made it clear that this number is based on a computerised death certificate system that is far from complete. It records 80 per cent of hospital deaths, 45 per cent of deaths in long-term care facilities and 25 per cent of deaths at home. “Mortality will consequently be higher than these initial figures suggest,” the statement said. Even so, other experts think Callahan may have overestimated the numbers. “Twenty-thousand for a single week seems very large,” says Dann Mitchell at the University of Bristol in the UK. “We’d have to look into details of the modelling to be more sure.” While Callahan’s method is sound, the main issue is that he used data from 2015 to 2019 to calculate the relationship between heat and deaths, says Marcin Walkowiak at Poznań University of Medical Sciences in Poland. People may now be less vulnerable due to ongoing adaptations, such as increased access to air conditioning, his team’s work suggests. Walkowiak’s back-of-the-envelope calculation is that if this is taken into account, the actual number of deaths would be around 15,000. Callahan is sticking to his guns. “We don’t have very strong evidence that the relationship between temperature and mortality dramatically changed over time,” he says. “So it’s not obvious it’s different now than it was 10 years ago.” “In general, we find that our sort of broader statistical estimates give higher numbers than direct reporting on the ground, because that direct reporting can often miss people who die from heat where it’s not obvious that heat was the cause,” he says. Extreme heat is now making cities unlivable. How can we survive it? On the flip side, Walkowiak says that Callahan hasn’t taken into account the fact that heatwaves of the same temperature are more deadly in early summer than in late summer. “In late summer, part of the especially vulnerable population is already long gone,” he says.