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Planet-heating pollution tripled the death toll from the “quietly devastating” heatwave that seared Europe at the end of June, early analysis covering a dozen cities has found, as experts warned of a worsening health crisis that is being overlooked.
Scientists estimate that high heat killed 2,300 people across 12 major cities as temperatures soared across Europe between 23 June and 2 July. They attributed 1,500 of the deaths to climate breakdown, which has heated the planet and made the worst extremes even hotter.
Milan was the hardest-hit city in absolute terms, with 317 out of 499 heat deaths attributed to climate breakdown, followed by Paris and Barcelona. London had 273 heat deaths, 171 of which the researchers attributed to human influence on the climate.
“This study demonstrates why heatwaves are known as silent killers,” said Malcolm Mistry, an epidemiologist at the London School of Hygiene & Tropical Medicine and co-author of the study. “While a handful of deaths have been reported in Spain, France and Italy, thousands more people are expected to have died as a result of the blistering temperatures.”
The rapid analysis from the World Weather Attribution group, which used established methods but has not yet been submitted for peer review, blames climate breakdown for two-thirds of the deaths.
Older people had the highest mortality, the study found, with 88% of the climate-driven deaths in people over the age of 65. The researchers said extreme heat was an “underappreciated” threat as most victims died out of public view in homes and hospitals, and with little media coverage.
“Heatwaves don’t leave a trail of destruction like wildfires or storms,” said Ben Clarke, a climate scientist at Imperial College London and co-author of the study. “Their impacts are mostly invisible but quietly devastating. A change of just 2 or 3C can mean the difference between life and death for thousands of people.”
The scientists used epidemiological models to estimate heat-related mortality in cities such as Paris, London, Madrid and Rome over a 10-day-period, and compared the death toll with that of a hypothetical world in which humans had not heated the planet by burning fossil fuels or destroying nature.
They cautioned that the relationships between temperature and death they used in their models were derived from local mortality data up to 2019, and so may not fully capture how people in each city have adapted to hotter weather over time.
They found climate breakdown pushed temperatures in some cities up to 4C higher, resulting in 1,500 extra deaths. The death toll was greater than that of other recent weather disasters that were made worse by fossil fuel pollution, such as the floods that killed 224 people in Spain in 2024 and the floods that killed 243 people across north-west Europe in 2021.
Previous studies have estimated that about 44,000 people die from heat in Europe each year, averaged over the past few decades. The scientists suggested the vast death toll of 2,300 people from a single heatwave in just 12 cities could make this summer particularly dangerous.
The EU’s Earth observation service, Copernicus, said last month was the third hottest June on record globally and that an “exceptional” marine heatwave developed in the western Mediterranean. The average daily sea surface temperature was the highest ever recorded for the region in June at 27C.
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Copernicus also found a large increase in dangerous “tropical nights”, where night-time temperatures do not drop below 20C and people struggle to rest. Parts of Spain had as many as 24 tropical nights last month, 18 more than the average for June.
Samantha Burgess, a deputy director of the Copernicus climate change service, said the record temperatures in the Mediterranean made the heat stress that large parts of Europe experienced “much more intense”.
She said: “In a warming world, heatwaves are likely to become more frequent, more intense and impact more people across Europe.”
Analysis by Mercator Ocean, a nonprofit research organisation that runs Copernicus’s marine service, found nearly two-thirds of the Mediterranean was hit by marine heatwaves that were classed as strong or worse, the greatest extent ever recorded.
The high temperatures are known to disturb fish and kill some of the plants they feed on. Mass-mortality events have repeatedly struck the Mediterranean in recent years as marine heatwaves have grown hotter.
Karina Von Schuckmann, a scientist at Mercator Ocean, said: “One particular aspect that is quite concerning … is this repeat emergence of heat stress. If you repeat the heat stress over time, the vulnerability of these specific ecosystems increases.”
