A new report by the Grantham Institute has found that climate change tripled the number of heat-related deaths during the extreme heatwave that struck 12 major European cities from June 23 to July 2, 2025.
Across cities like Paris, Madrid, Milan, Athens, and London, researchers estimate that 2,305 people died due to the extreme heat, with 1,504 deaths directly attributable to human-induced climate change. Over 80% of those deaths were among people aged 65 and older.
The heatwave was fueled by a “heat dome” trapping hot air over Western and Southern Europe. In Italy, outdoor work was banned during peak hours; in France, schools were closed. Madrid saw over 90% of its excess heat deaths linked to climate change, while Milan suffered the highest number of absolute fatalities (317 deaths).
This extreme heat came unusually early — typically, such temperatures hit in late July or August. Scientists warn that early-season heatwaves are deadlier because populations aren’t acclimatized yet. Climate data show that June heatwaves are now intensifying faster than those in July.
Without global warming, cities would have seen 1-4°C cooler temperatures, and such heatwaves would have been far rarer. For instance, Lisbon’s heat event is now considered a 1-in-150-year occurrence, but is becoming more frequent.
