Extreme heat is a killer. A recent heat wave shows how much more deadly it’s becoming
Extreme heat is a killer and its impact is becoming far deadlier as the human-caused climate crisis supercharges temperatures, according to a new study saying global warming tripled deaths in recent European heat wave.
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(CNN) — Extreme heat is a killer and its impact is becoming far, far deadlier as the human-caused climate crisis supercharges temperatures, according to a new study, which estimates global warming tripled the number of deaths in the recent European heat wave.
For more than a week, temperatures in many parts of Europe spiked above 100 degrees Fahrenheit. Tourist attractions closed, wildfires ripped through several countries, and people struggled to cope on a continent where air conditioning is rare.
The outcome was deadly. Thousands of people are estimated to have lost their lives, according to a first-of-its-kind rapid analysis study published Wednesday.
A tourist drinks water during a heatwave in Thessaloniki on July 8, 2025. A four-day heatwave confirmed by meteorologists began in Greece on July 6, is the second to grip Greece since late June. Temperatures are expected to reach 42 Celsius (107.6 Fahrenheit), with a maximum of 38 Celsius in Athens and increase as the week continues. (Photo credit: Sakis Mitrolidis/AFP/Getty Images via CNN Newsource)
A team of researchers, led by Imperial College London and the London School of Hygiene and Tropical Medicine, looked at 10 days of extreme heat between June 23 and July 2 across 12 European cities, including London, Paris, Athens, Madrid and Rome.
They used historical weather data to calculate how intense the heat would have been if humans had not burned fossil fuels and warmed the world by 1.3 degrees Celsius. They found climate change made Europe’s heat wave 1 to 4 degrees Celsius (1.8 to 7.2 Fahrenheit) hotter.
The scientists then used research on the relationship between heat and daily deaths to estimate how many people lost their lives.
They found approximately 2,300 people died during ten days of heat across the 12 cities, around 1,500 more than would have died in a world without climate change. In other words, global heating was responsible for 65% of the total death toll.
“The results show how relatively small increases in the hottest temperatures can trigger huge surges in death,” the study authors wrote.
Heat has a particularly pernicious impact on people with underlying health conditions, such as heart disease, diabetes and respiratory problems.
People over 65 years old were most affected, accounting for 88% of the excess deaths, according to the analysis. But heat can be deadly for anyone. Nearly 200 of the estimated deaths across the 12 cities were among those aged 20 to 65.
Climate change was responsible for the vast majority of heat deaths in some cities. In Madrid, it accounted for about 90% of estimated heat wave deaths, the analysis found.
The thermometer of a drugstore shows the temperature of 39 degrees Celsius (102 degrees Fahrenheit) during a heat wave, in Rome, Tuesday, July 1, 2025. (Photo credit: Andrew Medichini/AP via CNN Newsource)
The study’s focus on 12 cities makes it just a snapshot of the true heat wave death toll across the continent, which researchers estimate could be up to tens of thousands of people.
“Heatwaves don’t leave a trail of destruction like wildfires or storms,” said Ben Clarke, a study author and a researcher at Imperial College London. “Their impacts are mostly invisible but quietly devastating — a change of just 2 or 3 degrees Celsius can mean the difference between life and death for thousands of people.”
The world must stop burning fossil fuels to stop heat waves becoming hotter and deadlier and cities need to urgently adapt, said Friederike Otto, a climate scientist at Imperial College London. “Shifting to renewable energy, building cities that can withstand extreme heat, and protecting the poorest and most vulnerable is absolutely essential,” she said.
Akshay Deoras, a research scientist at the University of Reading who was not involved in the analysis, said “robust techniques used in this study leave no doubt that climate change is already a deadly force in Europe.”
Richard Allan, a professor of climate science at the University of Reading who was also not involved in the report, said the study added to huge amounts of evidence that climate change is making heat waves more intense, “meaning that moderate heat becomes dangerous and record heat becomes unprecedented.”
It’s not just heat that’s being supercharged in out hotter world, Allan added. “As one part of the globe bakes and burns, another region can suffer intense rainfall and catastrophic flooding.”
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