Humanity’s burning of fossil fuels directly led to the deaths of at least 1,500 people in a European heatwave last week, a landmark study said.

Scientists at Imperial and the London School of Hygiene and Tropical Medicine used peer-reviewed techniques to calculate that about 2,300 people in 12 cities likely died from the heat in last week’s bout of high temperatures, with nearly two-thirds of them dying because of the extra degrees that climate change added to the natural summer warmth.

The rapid analysis by World Weather Attribution calculated that climate change might have tripled the death toll from the event.