Human-caused climate change added an average of 30 days of extreme heat for about half of the world’s population over the past year, according to a new study.
That amounts to four billion people exposed to prolonged, dangerous temperatures. And the findings single out fossil fuel emissions as the cause of the blistering heat.
The study, published ahead of Heat Action Day on 2 June, comes from a collaborative attribution analysis by World Weather Attribution, Climate Central and the Red Cross Red Cross Climate Centre. Researchers examined temperature data from 247 countries and territories between May 2024 and May 2025 and found that, in 195 of them, climate change at least doubled the number of days classified as “extreme heat.”
“This study needs to be taken as another stark warning. Climate change is here, and it kills,” says Dr Friederike Otto of Imperial College London and World Weather Attribution (WWA).
What the researchers found and how they did it
To understand the influence of climate change, scientists first defined “extreme heat” as days when temperatures exceeded the 90th percentile of the averages between 1991 and 2020. In other words, the hottest 10 per cent of days during that period. Then they tallied how many such days occurred in each country between May 2024 and May 2025.
Next, they used climate models to simulate a world without human-induced warming. By comparing both sets of data, they were able to quantify just how many additional extreme heat days could be attributed directly to climate change.
The difference was staggering. In many parts of the world, global warming didn’t just increase temperatures. It made once-rare heatwaves a near-daily occurrence.
Aruba, for example, suffered 187 days of extreme heat in the past year. Without climate change, the island’s population would have endured just 45 such days.
Researchers identified 67 extreme heat events globally last year. They focused on four: Central Asia in March 2025, South Sudan in February 2025, Mexico, the US and Central America in June 2024 and the Mediterranean in July 2024.
In every case, they found that climate change made the back-breaking heat more likely or more severe.
“Climate change is clearly challenging life on every continent,” says Dr Mariam Zachariah, a WWA researcher at the Centre for Environmental Policy at Imperial College London.
“These frequent, intense spells of hot temperatures are associated with a huge range of impacts, including heat illness, deaths, pressure on health systems, crop losses, lowered productivity and transport disruptions,” she adds.
The toll is already clear in Europe
Europe is among the regions already suffering the most visible and deadly consequences of human-induced climate change.
Last summer, wildfires and heatwaves swept through southern Europe. Scientists warned that climate change made them three times more likely.
Greece endured deadly June heatwaves triggered by climate change that strained hospitals and caused mass evacuations from tourist destinations. Over the past two summers, extreme heat has led authorities to close the Acropolis – the country’s most-visited site, attracting millions each year – during the hottest hours of the day.
In July, southeastern Europe experienced its longest heatwave on record, lasting 13 consecutive days and affecting 55 per cent of the region.
Prolonged heat in Spain, France and Italy caused transport disruptions, strained power grids and forced schools to shut early.
In total, the record-breaking heat is thought to have killed more than 47,000 people in Europe in 2024.
The outlook continues to be grim, too. Climate experts project 23 million additional temperature-related deaths in Europe by 2099 if warming continues unchecked.
“The evidence for the link between climate change and heatwaves is undeniable,” says Roop Singh, head of urban and attribution at the Red Cross Red Crescent Climate Centre.
“Through our interactions, we know that people are feeling the rise in heat, but they don’t always understand that it’s being driven by climate change, and that it will continue to get much, much worse.”
A deadly, invisible threat
Despite the mounting pressure it places on global populations, heat remains one of the most underappreciated consequences of the climate crisis.
Climate change was found to have caused 41 additional days of dangerous heat in 2024, unleashing what researchers called “unrelenting suffering” across the globe.
That suffering isn’t always visible. Prolonged exposure to extreme heat has been linked to accelerated ageing and damage at the cellular level.
For the next generation, the picture is even more alarming.
A recent study estimated that 83 per cent of five-year-olds will live through far more frequent and intense heatwaves than any previous generation.
“There is no place on Earth untouched by climate change – and heat is its most deadly consequence,” said Dr Kristina Dahl, vice president for science at Climate Central.
“We have the science to quantify how fossil fuel emissions are reshaping our daily temperatures – and putting billions at risk.”
