Less well understood is the additional heat stress experienced by billions of people as a result of those extreme heat events. The study analysed more than 70 years of climate data stored in the ERA5-HEAT data set, which includes heat stress data going back to 1940. Around 70 per cent of the world’s population now experiences at least 90 days of strong heat stress annually, compared with 55 per cent in the 1970s. They found that hazardous heat stress has intensified across every continent over the last five decades, including in places historically unaccustomed to such heat, like parts of Europe. Extreme heat causes more human deaths than any other extreme weather event; children and the elderly are particularly vulnerable.