The findings underscore how the continued burning of fossil fuels is harming health and well-being on every continent, with the effects especially under-recognized in developing countries, the authors said.
"With every barrel of oil burned, every tonne of carbon dioxide released, and every fraction of a degree of warming, heat waves will affect more people," said Friederike Otto, a climate scientist at Imperial College London and co-author of the report.
The analysis -- conducted by scientists at World Weather Attribution, Climate Central, and the Red Cross Red Crescent Climate Centre -- was released ahead of global Heat Action Day on June 2, which this year spotlights the dangers of heat exhaustion and heat stroke.
To assess the influence of global warming, researchers analyzed the period from May 1, 2024 to May 1, 2025.
They defined "extreme heat days" as those hotter than 90 percent of temperatures recorded at a given location between 1991 and 2020.
Using a peer-reviewed modeling approach, they then compared the number of such days to a simulated world without human-caused warming.
The results were stark: roughly four billion people -- 49 percent of the global population -- experienced at least 30 more days of extreme heat than they would have otherwise.
The team identified 67 extreme heat events during the year and found the fingerprint of climate change on all of them.
The Caribbean island of Aruba was the worst affected, recording 187 extreme heat days -- 45 more than expected in a world without climate change.
The study follows a year of unprecedented global temperatures. 2024 was the hottest year on record, surpassing 2023, while January 2025 marked the hottest January ever.
On a five-year average, global temperatures are now 1.3 degrees Celsius above pre-industrial levels -- and in 2024 alone, they exceeded 1.5C, the symbolic ceiling set by the Paris climate accord.
The report also highlights a critical lack of data on heat-related health impacts in lower-income regions.
While Europe recorded more than 61,000 heat-related deaths in the summer of 2022, comparable figures are sparse elsewhere, with many heat-related fatalities misattributed to underlying conditions such as heart or lung disease.
The authors emphasized the need for early warning systems, public education, and heat action plans tailored to cities.
Better building design -- including shading and ventilation -- and behavioral adjustments like avoiding strenuous activity during peak heat are also essential.
Still, adaptation alone will not be enough. The only way to halt the rising severity and frequency of extreme heat, the authors warned, is to rapidly phase out fossil fuels.
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