Scientists say 4 billion people, about half the world’s population, experienced at least one extra month of extreme heat because of human-caused climate change from May 2024 to May 2025.
The extreme heat caused illness, death, crop losses, and strained energy and health care 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 other conditions like heart disease or kidney failure.
The scientists used peer-reviewed methods to study how much climate change boosted temperatures in an extreme heat event and calculated how much more likely its occurrence was because of climate change. In almost all countries in the world, the number of extreme heat days has at least doubled compared with a world without climate change.
Caribbean islands were among the hardest hit by additional extreme heat days. Puerto Rico, a territory of the United States, endured 161 days of extreme heat. Without climate change, only 48 would have occurred.
“It makes it feel impossible to be outside,” said Charlotte Gossett Navarro, chief director for Puerto Rico at Hispanic Federation, a nonprofit focused on social and environmental issues in Latino communities, who lives in the San Juan area and was not involved in the report.
“Even something as simple as trying to have a day outdoors with family, we weren’t able to do it because the heat was too high,” she said, reporting feeling dizzy and sick last summer.
When the power goes out, which happens frequently in Puerto Rico in part because of decades of neglected grid maintenance and damage from Hurricane Maria in 2017, Navarro said it is difficult to sleep. “If you are someone relatively healthy, that is uncomfortable, it’s hard to sleep … but if you are someone who has a health condition, now your life is at risk,” Gossett Navarro said.
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,” he said.
Low-income communities and vulnerable populations, such as older adults and people with medical conditions, suffer the most from extreme heat.
The high temperatures recorded in the extreme heat events that occurred in Central Asia in March, South Sudan in February and in the Mediterranean last July would have not been possible without climate change, according to the report. At least 21 people died in Morocco after temperatures hit 118 degrees Fahrenheit (48 degrees Celsius) last July. People are noticing temperatures are getting hotter but don’t always know it is being driven by climate change, said Roop Singh, head of urban and attribution at the Red Cross Red Crescent Climate Centre, in a World Weather Attribution statement.
“We need to quickly scale our responses to heat through better early warning systems, heat action plans, and long-term planning for heat in urban areas to meet the rising challenge,” Singh said.
City-led initiatives to tackle extreme heat are becoming popular in parts of South Asia, North America, Europe and Australia to coordinate resources across governments and other agencies. One example is a tree-planting initiative launched in Marseille, France, to create more shaded areas.
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.
But without phasing out fossil fuels, heat waves will continue becoming more severe and frequent and protective measures against the heat will lose their effectiveness, the scientists said.
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