Due to human-induced climate change, half of the world’s population experienced at least one extra month of extreme heat last year, according to a new study released on Friday. The findings highlight how the continued burning of fossil fuels is harming health and well-being across every continent—especially in developing countries, where such impacts often receive less attention, the authors of the study stated.
“Every barrel of oil burned, every ton of carbon dioxide emitted, and every fraction of a degree in temperature rise will lead to more people being affected by heatwaves,” said Friederike Otto, a climate scientist at Imperial College London and co-author of the report.
This analysis, conducted by scientists from World Weather Attribution, Climate Central, and the Red Cross Red Crescent Climate Centre, was released on June 2 ahead of Global Heat Action Day. The study highlights the growing risks of heat exhaustion and heat stroke this year.
To assess the impact of global warming, researchers analyzed the period from May 1, 2024, to May 1, 2025. They defined “extreme heat days” as those with temperatures hotter than 90 percent of the days recorded in a given location between 1991 and 2020.
Using a peer-reviewed modeling approach, the researchers compared the number of extreme heat days with a simulated world without human-caused temperature rise.
The results were striking: 49 percent of the global population—around 4 billion people—experienced at least 30 more extreme heat days than they would have in a world without climate change.
The team identified 67 extreme heat events during the study period, all of which bore the signature of climate change.
The Caribbean island of Aruba was the most affected, recording 187 extreme heat days—over 45 days more than would have been expected in a world without climate change. This study comes after an unprecedented year of global temperatures. The year 2024 surpassed 2023 to become the hottest year on record, and January 2025 was the hottest January ever recorded.
On a five-year average, global temperatures are now 1.3°C above pre-industrial levels, and in 2024 alone, they temporarily crossed the symbolic 1.5°C threshold set by the Paris Climate Agreement. The report also highlights a severe lack of data on heat-related health impacts in low-income regions.
While Europe recorded over 61,000 heat-related deaths during the summer of 2022, comparable statistics are rarely available elsewhere. Many heat-related deaths are mistakenly attributed 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. Practical adaptations like well-designed buildings with shade and ventilation, and avoiding strenuous activity during extreme heat, are also crucial. However, they warned that adaptation alone will not be enough. The only way to prevent the increasing severity and frequency of extreme heat is the rapid phase-out of fossil fuels.
