This likely led to far more heat-related deaths than would have occurred without the influence of global warming, concluded a rapid study of the episode by over a dozen researchers from five European institutions.
Temperatures between late June and early July soared well above 40 degrees Celsius (104F) in many European countries as the first heatwave of the summer broke records and triggered health warnings.
The EU's climate monitor Copernicus on Wednesday said it was the hottest June on record in western Europe, where some schools and tourist sites were shuttered as the mercury soared.
To assess what role climate change played, scientists compared how intense a heatwave would have been in a world that had not warmed due to burning masses of fossil fuels.
Using historical weather data, they concluded the heatwave "would have been 2-4C cooler" without human-induced climate change in all but one of the 12 cities studied.
The added degrees greatly elevated the risk in these cities, which have a combined population of more than 30 million and include major capitals Paris, London and Madrid.
"What that does is it brings certain groups of people into more dangerous territory," said researcher Ben Clarke from Imperial College London, which co-led the study with the London School of Hygiene and Tropical Medicine.
"For some people it's still warm, fine weather. But for now a huge sector of the population, it's more dangerous," he told reporters.
- Life and death -
The study, for the first time, also sought to estimate the death toll from the heatwave in the 12 cities studied, and how many could be attributed to climate change.
Based on peer-reviewed scientific methods and established research on heat and mortality, the study concluded the heatwave likely caused about 2,300 deaths between June 23 and July 2 across the 12 cities studied.
But about 1,500, or roughly two thirds, of all these deaths would not have occurred had climate change not pushed temperatures to such dangerous highs, researchers said.
The authors -- from research institutions in the UK, Netherlands, Denmark and Switzerland -- stressed this estimate was just a snapshot of the wider heatwave, as no official count was yet available.
Heatwaves are particularly dangerous for the elderly, the sick, young children, outdoor workers, and anyone exposed to high temperatures for prolonged periods without relief.
The effect on health is compounded in cities, where heat is absorbed by paved surfaces and buildings, making urban areas much hotter than their surroundings.
Copernicus said large parts of southern Europe experienced so-called "tropical nights" during the heatwave, when overnight temperatures don't fall low enough to let the body recover.
"An increase in heatwave temperature of just two or four degrees can mean the difference between life and death for thousands of people," said Garyfallos Konstantinoudis, a lecturer at Imperial College London.
"This is why heatwaves are known as silent killers. Most heat-related deaths occur in homes and hospitals out of public view and are rarely reported," he told reporters.
Authorities say it could take weeks to tally a more definitive death toll from the recent heatwave, but similar episodes have claimed tens of thousands of lives in Europe during previous summers.
June was hottest on record in western Europe: EU monitor
Paris (AFP) July 9, 2025 -
Western Europe sweltered through its hottest June on record last month, as "extreme" temperatures blasted the region in punishing back-to-back heatwaves, the EU climate monitor Copernicus said Wednesday.
Globally, this past June was the third warmest on record, continuing a blistering heat streak in recent years as the planet warms as a result of humanity's emissions of greenhouse gases.
The previous hottest June was in 2024 and the second hottest was in 2023, the Copernicus Climate Change Service (C3S) said.
Sweltering extremes were particularly pronounced in Europe, which is warming several times faster than the global average.
Millions of people were exposed to high heat stress across parts of the continent as daily average temperatures in western Europe climbed to levels rarely seen before -- and never so early in the summer.
Several countries recorded surface temperatures above 40 degrees Celsius, with heat of up to 46C in Spain and Portugal, Copernicus said.
Samantha Burgess, the EU monitor's Strategic Lead for Climate, said the impact of the heatwaves in Europe was "exceptional", intensified by record sea surface temperatures in the western Mediterranean -- which hit an all-time daily maximum in June.
"In a warming world, heatwaves are likely to become more frequent, more intense and impact more people across Europe," she said.
The two heatwaves -- from June 17 to 22, and again from June 30 to July 2 -- were linked to heat domes trapping warm air over affected regions, prolonging the stifling weather, and worsening pollution and wildfire conditions.
Portugal, Spain, France, Italy, and much of the Balkans saw some of the hottest "feels-like" temperatures, which measure the impact on the human body by taking into account factors like humidity.
Maximum feels-like temperatures north of Lisbon hit 48C, around 7C above average and associated with "extreme heat stress", said Copernicus.
Sea surface temperatures across the western Mediterranean were "exceptionally high" in the month, some 5C above average in some areas, with temperatures surging to a record 27C on June 30.
The higher water temperatures reduced nighttime air cooling along the coasts, contributed to higher humidity, and harmed marine life, Copernicus said.
- Heat, fires and floods -
An AFP analysis based on Copernicus data, found that 12 countries and some 790 million people around the world experienced record heat last month.
Dangerous heat blanketed parts of the United States, while in China, 102 weather stations logged the hottest-ever June day, with some measuring temperatures above 40C, according to state media.
June saw a catalogue of weather extremes across the world.
Devastating wildfires blazed across parts of Canada and southern Europe, while deadly flooding swept areas of South Africa, China and Pakistan.
Copernicus said it was drier than average in parts of western Europe, while particularly arid conditions were seen in North America, eastern and southern Africa, the Arabian Peninsula, parts of central and eastern Asia and southern South America.
Parts of the world that saw wetter-than-average conditions included the southern US, parts of China and southern Brazil.
- Warming limits -
Global warming, driven largely by the burning of fossil fuels, is not just about rising temperatures but the knock-on effects of the extra heat on the atmosphere and seas.
Warmer air can hold more water vapour, and warmer oceans mean greater evaporation, resulting in more intense downpours and storms.
The Copernicus dataset draws on billions of measurements from satellites, ships, aircraft and weather stations.
It has recorded extraordinary heat over the last two years.
While this was partly stoked by warmer El Nino conditions, temperatures have remained at record or near-record levels even after that faded last year.
Copernicus said that June was 1.3C above the temperatures of the pre-industrial era -- roughly the estimated current level of overall global warming.
But temperature extremes in recent years mean that it is one of only three months in the last 24 to have dipped below 1.5C warming.
The Paris climate deal saw countries agree to try to limit long term global warming to 1.5C, beyond which major and lasting climate and environmental changes become more likely.
But many scientists now say it will be almost impossible to stay under that level, with a breach expected around 2030 or before, leaving the world potentially facing the unprecedented challenge of trying to bring warming back down.
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