WASHINGTON — Blistering record heat in Europe this summer helped kill more than 50,000 people, with nearly two-thirds of that death toll blamed on human-caused climate change, a new study found.

Researchers modeled heat-related deaths for different age groups in 854 European cities during repeated heat waves in June, July and August and found that 50,391 people died from warmer temperatures, and of those, 32,619 died because of the extra heat caused by the burning of coal, oil and gas, according to a study in an upcoming issue of Nature Health.

That's 71% higher than the previous deadliest summer, which was 29,480 in 2022, according to the same researchers, who produced a study earlier this month tracking yearly summer heat-related deaths in those European cities. And this summer's death figures are underestimates because they don't count a deadly heat wave in May and they only look at deaths in those 854 cities, which are about 40% of Europe's population, study authors said.

About one-third of the deaths were from June 17 to July 1, a particularly intense heat wave when temperatures were 9.5 degrees Fahrenheit (5.3 degrees Celsius) above normal, said study co-author Garyfallos Konstantinoudis, a biostatistician at Imperial College London.

"Much of Western and Northern Europe, our infrastructure is just not designed to cope with the sort of temperatures that we are seeing now because of the effects of human-caused warming," said lead author Clair Barnes, a climate scientist at Imperial College London. She said some European cities this summer had temperatures 21.6 F (12 C) warmer than normal.

Of those 50,000, more than 32,000 died from the added heat of human-caused climate change, the study also found. Italy, France, Spain and Germany had the highest death tolls, the study said. Of all the European capitals, Rome had the highest heat death total from 2000 to 2025, according to the team's study earlier this month.

An extra dose of heat from fossil fuel burning
This would've already been an abnormal natural hot spell on top of a huge amount of climate change, Barnes said.

She said Europe is the fastest-warming continent — although the Arctic, which is not a continent, is warming even faster — and is 4.5 F (2.5 C) hotter than pre-industrial times.

"Just the underlying warming from human activity, from mostly the burning of fossil fuels, it's increasing all of our temperatures by what sounds like quite a small amount," Barnes said. "For many people, it's certainly the case that that extra two-and-a-half degrees (Celsius) of warming is what's going to tip those temperatures from uncomfortable to dangerous and potentially deadly."

More than half of the people who died in Europe's heat were age 85 or older, but the stifling temperatures also killed more than 800 people between the ages of 20 and 44, Konstantinoudis said.

How the death numbers are calculated
The team used an established and accepted scientific method to figure out how many of the 50,000 heat deaths were from natural high summer heat and how many were due to what heat-trapping gases have done to the climate. They ran computer simulations of a world without climate change and compared it to the deaths that happened this summer. The difference is what was caused by global warming.

The percentage of heat deaths attributable to global warming has grown from about 45% in the early 2000s to 65% now, Barnes said.

"Summers are getting hotter, and the analytical tools for attributing deaths to heat are getting more accurate," said Dr. Howard Frumkin, former director of the National Center for Environmental Health in the U.S., who wasn't part of the study. "Together, these two trends yield increasingly high and increasingly accurate counts of heat-related deaths."

The scientific team also had to use statistical modeling to come up with the base count of 50,000 heat deaths because those deaths are underreported and often not listed in death certificates, despite being a major factor, Konstantinoudis said.

Several outside experts agreed with him, calling the methods state-of-the-art.

The researchers ran a complex mathematical equation that has been used in several previously peer-reviewed studies, Barnes said, which factors in specific age and temperatures for each of the 854 cities based on a localized optimal temperature that coincides with the fewest deaths. When it's warmer or colder, the equation calculates excess deaths above lowest-death conditions.

Researchers then checked their calculations against real-time estimates from government agencies. Mostly — but not always — they matched fairly well, Konstantinoudis said. The team's methods for May and June British heat waves calculated 2,700 excess deaths and the United Kingdom health statistics agency said it was 2,800, he said.

Cooling and adapting are needed, but they're not enough
"We need to prepare for the heat we cannot now avoid, with urgent investments in heat early-warning systems, equitable access to low-carbon cooling, and neighborhood buddy systems that check on and support people most at risk — while rapidly reducing the fossil fuel use driving the crisis," said Canadian emergency room physician Dr. Courtney Howard, president-elect of the Canadian Medical Association, who wasn't part of the research.

Adapting is only part of the answer, said Dr. Jonathan Patz of the University of Wisconsin-Madison, who wasn't part of the research but also praised it.

"We can't just be mopping up the mess on the floor without thinking about turning the faucet off," said Patz, a professor of health and the environment. "So we need prevention at different levels. We need better cooling and protecting vulnerable populations from inevitable increased frequency and extreme heat waves. But we also need to go to the root of the problem and absolutely address the burning of fossil fuels."