Summer heatwaves caused the deaths of 263 Londoners, according to stark new estimates - as a week-long hot weather health alert for the capital kicks in.
About 171 of these excess deaths were added by climate change, which made temperatures in the capital nearly 4C higher than they would have been, according to scientists at Imperial College London and the London School of Hygiene and Tropical Medicine (LSHTM).
In the first rapid study to estimate the number of deaths linked to climate change in a heatwave, researchers found human-driven global warming was responsible for around 65% of the deaths that occurred across 12 cities, including London, Paris, Madrid, Barcelona and Rome.
They found around 1,500 of the 2,300 estimated heat deaths in those cities were the result of climate change - equating to a tripling of the number of deaths in the heatwave due to global warming.
The scientists behind the study warned heatwaves were “quietly devastating” and their research showed how dangerous climate change already was with just 1.3C of warming, particularly for older and more vulnerable people.
The assessment focused on 10 days of heat from June 23 to July 2, when a “heat dome” high pressure system over Europe trapped hot dry air and pushed up temperatures, as well as pulling hot air from North Africa, intensifying the heatwave.
London recorded the UK’s hottest day of 2025 on July 1, when temperatures reached 34.7C in St James’s Park, while parts of Essex and Kent also topped 33C.
The capital has already seen two official heatwaves this summer - when temperatures reach 28C or higher on at least three consecutive days.
And forecasters say a third is likely to begin as early as Wednesday. The Met Office is predicting 28C for London, with temperatures expected to reach 32C on Friday and Saturday and not dropping below the heatwave threshold before Tuesday of next week.
A yellow heat health alert has been issued for the southern half of England from 10am on Wednesday until 10am next Tuesday.
The alert, issued by the UK Health Security Agency (UKHSA), covers London, the East Midlands, West Midlands, East of England, South East and South West.
“Significant impacts” are likely across health and social care services due to higher temperatures over the period.
The analysis estimates that climate change tripled the number of heat-related deaths across European cities in the recent heatwaves.
Global warming, caused primarily by humans burning fossil fuels and cutting down forests, made the searing heat that gripped much of Europe in late June and early July much more intense, researchers found.
The heatwaves were up to 4C hotter across cities compared to a world without climate change, the study from the World Weather Attribution group of researchers said.
The searing temperatures triggered an amber health alert for London, led schools to close in parts of France, outdoor working to be banned during the hottest parts of the day in Italy, and raised the risk of wildfires.
The researchers used weather data to assess the intensity of the heatwaves over their hottest five-day period in a world which has seen 1.3C of warming and compared it the cooler pre-industrial climate.
The analysis showed the heatwaves were around 1-4C higher than in a world which had not warned 1.3C due to climate change.
The scientists also used existing research on the relationship between heat and the number of daily deaths in the cities to estimate the number of excess deaths due to the heatwaves and compared it to a “counterfactual” of the numbers of deaths in heatwave temperatures without climate change.
Most of the deaths were in older age groups, the researchers said, highlighting the growing risk older people in Europe face from dying prematurely due to longer, hotter and more frequent heatwaves.
Dr Ben Clarke, researcher at the Centre for Environmental Policy at Imperial College London, said: “Heatwaves don’t leave a trail of destruction like wildfires or storms.
“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.
“Our study shows how dangerous climate change already is with just 1.3C of warming.
“However, we could reach 3C this century, unless countries speed up the transition from fossil fuels to renewable energy.
“That would bring fiercer heatwaves to Europe, causing more deaths and placing greater pressure on health systems.”
Dr Pierre Masselot, research fellow at LSHTM, said: “At the rate the world is warming, heatwaves are not going away and we have to prepare for their public health impacts.
“Cities can adapt by planting trees, reducing the space given to cars and caring for the most vulnerable.
“But ultimately, the best way to avoid dire consequences is to aggressively reduce greenhouse gases emissions.”
And Dr Friederike Otto, professor in climate science at the Centre for Environmental Policy at Imperial, said: “This study highlights a simple fact: burning more oil, coal and gas will kill more people.
“The only way to stop European heatwaves from becoming even deadlier is to stop burning fossil fuels,” she warned.
