Climate change amplified heatwave by 2 to 4°C in June, study finds

Climate change caused by the burning of fossil fuels has made the recent heatwave in western Europe up to 4°C hotter in many cities, exposing thousands of vulnerable people to dangerous heat stress, according to a "rapid study" published Wednesday.

Between late June and early July, temperatures soared well above 40°C in many European countries during an exceptional and early heatwave, triggering numerous health alerts. "We estimate that global warming amplified the heatwave by about 2 to 4°C in most of the cities" studied, including Paris, London, and Madrid, said Ben Clarke of Imperial College London, who led the study with the London School of Hygiene and Tropical Medicine.

The episode likely resulted in a far higher number of heat-related deaths than would have been the case without the influence of climate change, concluded this "rapid study" , conducted by more than a dozen researchers from five European institutions, pending official results in several weeks.

To assess the influence of climate change, the scientists simulated the intensity of this episode in a world without the massive burning of coal , oil, and gas, using historical weather data. They concluded that the heatwave "would have been 2 to 4°C less intense" without climate change in 11 of the 12 cities studied. These additional degrees significantly increased the health risk for the 30 million residents of the cities studied, including Paris, London, and Madrid.

"This puts certain groups of people in a more dangerous situation," said researcher Ben Clarke of Imperial College London. "For some, it's still warm and pleasant weather. But for a large part of the population, it's becoming dangerous," he said.

The study attempts for the first time to estimate the number of deaths attributable to the heatwave in the 12 cities studied and the proportion attributable to climate change. Based on peer-reviewed scientific methods and established research on heat and mortality, the study estimates that the heatwave likely caused approximately 2,300 premature deaths between June 23 and July 2 in these cities.

And about 1,500 deaths, or about two-thirds, would not have occurred without the degrees added by human climate disruption.

The authors, from institutions in the UK, the Netherlands, Denmark and Switzerland, stressed that this estimate was only a snapshot, ahead of any official count.

Heatwaves are particularly dangerous for the elderly, the sick, young children, outdoor workers, and anyone exposed to high temperatures for long periods without respite, especially during consecutive hot nights. "For thousands of people, an increase of just 2 or 4°C can mean the difference between life and death," said Garyfallos Konstantinoudis of Imperial College London.

"This is why heatwaves are known as silent killers: most deaths occur in homes and hospitals, out of sight, and are rarely reported," he said. Officials estimate it will take several weeks to establish a definitive death toll. A succession of similar episodes has already caused tens of thousands of premature deaths across Europe in previous summers.

Meanwhile, the European Copernicus service announced on the same day that June 2025 was the hottest June ever recorded in Western Europe, as "extreme" temperatures hit the continent in two consecutive early heatwaves.