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

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