Heat waves are a growing global threat to human health, well-being and livelihoods. Across 12 major European cities during the summer of 2025, a 10-day period of extreme heat led to 2,300 deaths—1,500 of them were attributed to climate change amplifying temperatures by 1°C–4°C. Heat waves were responsible for nearly half a million global deaths every year from 2000 to 2019.