An estimated 1,500 people across 12 European cities were killed by human-caused climate change during the latest heatwave, a quickfire study has found. Burning fossil fuels has made heatwaves up to 4°C hotter in Europe, tripling the number of heat-related deaths that occurred between 23 June and 2 July, according to scientists at Imperial College London and the London School of Hygiene & Tropical Medicine. It is the first time that a rapid analysis - a type of study that measures the influence of climate change on an extreme weather event - has been carried out for a heatwave. Since the analysis covers only a dozen cities, researchers estimate that the true death toll across Europe stretched into the tens of thousands. "It shows that climate change is an absolute game changer when it comes to extreme heat, but still very much under-recognised," says Dr Friederike Otto, Professor in Climate Science at the Centre for Environmental Policy, Imperial College London. "If we continue to follow the wishes of the fossil fuel industry and delay serious mitigation further, more and more people will lose their lives, for the financial benefit of only a tiny, rich, loud, influential minority." To estimate how many excess heat deaths were caused by climate change, the researchers first analysed historical weather data to see how intense the temperatures would have been in a world that hadn't been warmed by 1.3°C. Climate change - caused primarily by the burning of oil, coal, and gas, and to a much lesser extent deforestation - made the heatwave 1-4°C hotter, they found. It's also making heatwaves strike earlier in June. Separate research from the EU's climate change service Copernicus, also published today, shows that June 2025 was the fifth hottest June on record in Europe, marked by two significant heatwaves. During the second event between 30 June and 2 July, surface air temperatures exceeded 40°C in several countries, spiralling up to 46°C in Spain and Portugal. The World Weather Attribution team then used previous research on the relationship between heat and the number of daily deaths, regardless of cause, in the 12 cities. They estimated the number of heat-related deaths in both the recent heatwave and a hypothetical cooler event over ten days. The study estimates that about 2,300 people died in the extreme temperatures across the cities, from Lisbon to Budapest. If the climate hadn't been heated up, there would be about 1,500 fewer excess deaths, meaning climate change is behind 65 per cent of these excess deaths. These numbers represent real people who have lost their lives in the last days, due to the extreme heat. "While the number of heatwave deaths are estimated, as it is impossible to get real time statistics, they are in the right ballpark - as has been shown in many peer-reviewed studies," Dr Otto told reporters. "These numbers represent real people who have lost their lives in the last days, due to the extreme heat. And two-thirds of these would not have died if it wasn't for climate change." Climate change was behind 317 of the estimated excess heat deaths in Milan, 286 in Barcelona, 235 in Paris, 171 in London, 164 in Rome, 108 in Madrid, 96 in Athens, 47 in Budapest, 31 in Zagreb, 21 in Frankfurt, 21 in Lisbon and 6 in Sassari, the findings show. This means the likely death toll was higher than other recent disasters, including last year's Valencia floods (224 deaths) and the 2021 floods in northwest Europe (243 deaths). Although the greatest number of excess deaths occurred in Milan, the highest proportion is estimated to be in Madrid: 90 per cent, due to the large increase in heat that pushed temperatures past a threshold where heat deaths increase rapidly. One reason for this is Madrid's central position in Spain, the researchers explain. The further away from the coast, the stronger the "climate change signal" is in extreme heat - because the ocean warms slower than the land. Lisbon, by contrast, benefited from being coastal. "This study shows that every fraction of a degree of warming makes a huge difference - whether it is 1.4, 1.5 or 1.6°C," says Dr Garyfallos Konstantinoudis, lecturer at the Grantham Institute – Climate Change and the Environment, Imperial College London. "These seemingly small changes will result in hotter heatwaves and huge surges in heat deaths." People aged 65 and over made up 88 per cent of the deaths linked to climate change, the study reveals, highlighting how those with underlying health conditions are most at risk of premature death in heatwaves. In contrast to climate-driven floods and wildfires, heatwaves are regarded as a silent killer. "Most people who die in heatwaves pass away at home or in hospitals as their bodies become overwhelmed and give in to pre-existing health conditions [such as heart disease, diabetes, and respiratory problems]," says Dr Malcolm Mistry, Assistant Professor at the London School of Hygiene & Tropical Medicine. But people of all ages need to take the threat seriously - contrary to what some media images of people playing on beaches might suggest. "A lot of people might feel they are invincible, but they are not," says Dr Otto. Good progress has been made in Europe on developing heat action plans, the researchers note. These national plans set out the actions that can be taken before and during dangerous high temperatures, and authorities have got better at spreading the message widely. However, to protect people from more dangerous heatwaves, long-term strategies to reduce the urban heat island effect are crucial - such as expanding green and blue spaces – as well as short-term measures like cooling centres and support systems for vulnerable citizens, they add. Ultimately, the best and most efficient measure of all is to drastically cut our greenhouse gas emissions. "The only way to stop European heatwaves from becoming even deadlier is to stop burning fossil fuels," says Dr Otto. "A warming climate sure as hell makes heatwaves worse," comments Richard Allan, Professor of Climate Science at the UK's University of Reading, who was not involved with the "forensic" analysis. "Communities need to adapt to an increasingly dangerous world through more resilient infrastructure and improved warning systems, yet it is only with rapid and massive cuts in greenhouse gases through collaboration across all sectors of society that worsening of weather extremes can be reined in." Dr Chloe Brimicombe, climate scientist at the Royal Meteorological Society, adds that, "Research like this is important and being used more in climate litigation cases where groups take countries and companies to court over climate change." — Euronews
