Extreme heat kills hundreds of thousands of people annually, with an estimated 210,000 more heat-related deaths occurring each year between 2012 and 2021 than from 1990 to 1999, according to reporting published in Nature. In Asia, heatwaves caused roughly 75,000 deaths per hot season between 1990 and 2019, while record temperatures in Europe caused nearly 14,000 deaths in June.
Death certificates rarely list heat as a direct cause of death, even though high temperatures increase heart attack risks and worsen existing health conditions. Kristie Ebi, a climate epidemiologist at the University of Washington in Seattle, noted that accurate death counts remain vital for prevention, stating that nobody needs to die in a heatwave and that preventive action depends on knowing how many died.
Public health agencies primarily calculate mortality using the excess-deaths method. This approach calculates the difference between total deaths during a heatwave and expected deaths during a baseline period with normal temperatures. Antonio Gasparrini, a biostatistician and epidemiologist at the London School of Hygiene and Tropical Medicine, explained that while the excess-deaths method is fast, it cannot distinguish heat deaths from other causes and cannot forecast upcoming heatwave impacts because it operates only after an event ends.
Researchers also use exposure-response functions, which model the statistical correlation between specific temperature levels and mortality rates. These models can predict real-time or future impacts—such as estimating that a 5 °C temperature rise will increase a city's weekly mortality rate by 3%—though they depend on accurate weather forecasts and can be skewed by non-heat mortality shifts.
The UK Health Security Agency reported an estimated 2,877 heat-related deaths in England across May and June heatwaves using the excess-deaths method. Applying exposure-response modeling to England and Wales for the same period, Gasparrini and his colleagues calculated 2,736 deaths, attributing nearly half of those fatalities to human-induced climate change.
