The rapidly-produced study is the latest effort by climate and health researchers to quickly link the death toll during heatwaves to global warming -- without waiting months or years to be published in a peer-reviewed journal.
The estimated deaths were not actually recorded in the European cities, but instead were a projection based on methods such as modelling used in previously peer-reviewed studies.
Death tolls during heatwaves are thought to be vastly underestimated because the causes of death recorded in hospitals are normally heart, breathing or other health problems that particularly affect the elderly when the mercury soars.
To get a snapshot of this summer, a UK-based team of researchers used climate modelling to estimate that global warming made temperatures an average of 2.2 degrees Celsius hotter in 854 European cities between June and August.
Using historical data indicating how such soaring temperatures drive up mortality rates, the team estimated there were around 24,400 excess deaths in those cities during that time.
They then compared this number to how many people would have died in a world that was not 1.3C warmer due to climate change caused by humans burning fossil fuels.
Nearly 70 percent -- 16,500 -- of the estimated excess deaths were due to global warming, according to the rapid attribution study.
This means climate change could have tripled the number of heat deaths this summer, said the study from scientists at Imperial College London and epidemiologists at the London School of Hygiene & Tropical Medicine.
The team had previously used similar methods to find a similar result for a single European heatwave that started in late June.
The researchers said they were not able to compare their estimates to actual excess deaths recorded in European cities this summer because most countries take a long time to publish that data.
"It's impossible to get real-time statistics right now," however the estimates are "in the right ballpark," study co-author Friederike Otto told a press conference.
- 'Even more alarming' -
The estimates did reflect previous peer-reviewed research, such as a Nature Medicine study which determined there were more than 47,000 heat-related deaths during the European summer of 2023.
Numerous prominent climate and health researchers also backed the study.
"What makes this finding even more alarming is that the methods used in these attribution studies are scientifically robust, yet conservative," said atmospheric science researcher Akshay Deoras at the UK's University of Reading.
"The actual death toll could be even higher."
The study said that Rome had the most estimated deaths attributed to climate change with 835, followed by Athens with 630 and Paris with 409.
More than 85 percent of the estimated excess deaths were among people aged 65 or over.
The researchers emphasised the study did not represent Europe as a whole because some areas -- such as the Balkans -- were not included.
"An increase in heatwave temperature of just 2-4C can be the difference between life and death for thousands of people -- this is why heatwaves are known as silent killers," study co-author Garyfallos Konstantinoudis said.
This year was Europe's fourth-hottest summer on record.
Experts warn deaths from wildfire smoke to surge in decades ahead
Paris (AFP) Sept 18, 2025 -
Global deaths linked to wildfire smoke could rise six-fold to 1.4 million people a year by 2100 even with moderate levels of global warming, according to research published Thursday.
It adds to warnings of the significant and growing health impacts from climate-stoked wildfires, which belch out smoke that can drift across continents and oceans.
This latest study, published in Nature, used machine learning and modelling to project the level of wildfire emissions and premature deaths to the end of the century.
"Our key finding is that climate change will substantially increase the health burden from wildfire smoke, even under moderate warming," co-author Bo Zheng, of China's Tsinghua University, told AFP.
Researchers predicted that between the period 2010-2014 and 2095-2099, global fire carbon emissions could rise by 23 percent under mid-range emissions scenarios.
And by the end of the century they found that wildfire smoke could cause about 1.4 million premature deaths annually -- around six times today's level.
Zheng said this underscored the "urgent need" to reduce greenhouse gas emissions and adapt to climate change.
Fires are becoming more frequent and ferocious as human activities and climate change drive warmer conditions and more frequent droughts.
Meanwhile, ageing populations stand to be more vulnerable to the health impacts of air pollution.
The health burden would probably fall unequally, the study found, with poorer countries suffering the highest rises in premature deaths linked to fire smoke.
The heaviest toll was projected in Africa, where the number of fire-related deaths could rise 11-fold.
Sub-Saharan Africa would be the hardest hit.
Zheng said Africa already had the world's largest burnt area across its savannas, forests, and grasslands.
"This extensive burning results in disproportionate smoke exposure and health impacts," he said, adding that fires were often concentrated in areas where access to healthcare could continue to be limited.
Deaths in Europe and the United States could stay roughly at the same level or as much as double by the end of the century.
To project wildfire emissions, the authors used machine learning to estimate the future area burned under different climate scenarios.
They then used computer modelling to simulate harmful PM2.5 pollutants, enabling them to estimate exposure and premature deaths.
Zheng said that the finding of 1.4 million premature deaths by 2100 was "our best understanding".
But he noted there was still a large range of uncertainty, calling for more measurements of wildfire PM2.5 and the development of better health models to improve precision.
PM2.5 pollutants in wildfire smoke are fine, airborne particles small enough to enter the bloodstream through the lungs, and are linked to higher rates of chronic bronchitis, lung cancer and heart disease.
The authors also noted a lack of international cooperation, with wildfire smoke able to cause health alerts in other countries and even continents.
A recent study, also published in Nature, found that Canada's record-breaking 2023 wildfires exposed more than 350 million people in North America and Europe to air pollution that likely contributed to tens of thousands of deaths.
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