Four billion people, about half the world’s population, experienced at least one extra month of extreme heat due to human-caused climate change from May 2024 to May 2025, scientists say.
The extreme heat caused illness, death, crop losses, and strained energy and healthcare systems, according to the analysis from World Weather Attribution, Climate Central and the Red Cross. “Although floods and cyclones often dominate headlines, heat is arguably the deadliest extreme event,” the report said.
Many heat-related deaths are unreported or are mislabeled by comorbid conditions like heart disease or kidney failure. Heat waves are silent killers, said Friederike Otto, associate professor of climate science at Imperial College London, one of the report's authors.
“People don’t fall dead on the street in a heat wave, people either die in hospitals or in poorly insulated homes and therefore are just not seen.” Low-income communities and vulnerable populations, such as older adults and people with medical conditions, suffer the most from extreme heat.
The scientists used peer-reviewed methods to study how much climate change boosted temperatures in a heat wave and calculated how much more likely its occurrence was due to climate change. In almost all countries in the world, the number of extreme heat days has at least doubled. The report says strategies to prepare for heat waves include monitoring and reporting systems for extreme temperatures, providing emergency health services, cooling shelters, updated building codes, enforcing heat safety rules at work, and designing cities to be more heat-resilient.
While these strategies help people adapt to the increasingly frequent and severe heat waves, “only comprehensive mitigation, through phasing out of fossil fuels, will limit the severity of future heat-related harms,” the study said.
