The modeled death toll is 71% higher than the previous summer record in the researchers’ city data: 29,480 deaths in 2022. The new estimate does not include a deadly heat wave in May and covers cities representing about 40% of Europe’s population, so the full toll could be higher.

The deadliest stretch came from June 17 to July 1, when temperatures averaged 5.3 degrees Celsius (9.5 degrees Fahrenheit) above normal. About one-third of the summer’s estimated heat deaths occurred during those two weeks, said study co-author Garyfallos Konstantinoudis, a biostatistician at Imperial College London.

Italy, France, Spain and Germany recorded the highest totals. More than half of those who died were 85 or older, but heat also killed more than 800 people ages 20 to 44, Konstantinoudis said.

The researchers estimated deaths by comparing this summer’s conditions with computer simulations of a world without human-caused climate change. Their model accounts for local temperatures and age-specific risks in each city, calculating excess deaths against temperatures associated with the fewest deaths. Heat is often not recorded as a cause on death certificates, making statistical modeling necessary, Konstantinoudis said.

Lead author Clair Barnes, a climate scientist at Imperial College London, said much of Western and Northern Europe lacks infrastructure designed for the temperatures now occurring. Some cities experienced temperatures as much as 12 degrees Celsius (21.6 degrees Fahrenheit) above normal, she said.

Europe has warmed about 2.5 degrees Celsius (4.5 degrees Fahrenheit) since preindustrial times, Barnes said. The share of heat deaths attributed to global warming has risen from about 45% in the early 2000s to roughly 65% today, she added.

Researchers compared some model estimates with government figures. For British heat waves in May and June, their analysis estimated 2,700 excess deaths; the U.K. health statistics agency reported 2,800, Konstantinoudis said. Outside experts described the research methods as state of the art.

The findings point to both immediate health protections and the need to curb the fossil-fuel pollution driving additional warming. Canadian emergency physician Courtney Howard, who was not involved in the study, called for heat warning systems, fair access to low-carbon cooling and neighborhood networks to check on people at greatest risk. University of Wisconsin-Madison professor Jonathan Patz said cities also need to address the cause of the crisis, not only respond to its consequences.