In Naples, an emergency doctor named Mario Guarino watched visits to his hospital's emergency room climb by 15% during the hottest weeks of the summer. He had no way of knowing that his city was about to be named the hardest-hit on the continent: 815 people per million died from the heat there, according to a new study. Across 854 European cities, researchers estimate that heat killed a record 50,000 people between June and August. Using historical relationships between temperature and health, they calculated that about 32,000 of those deaths were added by climate change, meaning that it was responsible for around two in every three.

The toll was nearly twice as high as in each of the two next most dangerous years. Hot days pushed vulnerable bodies into overdrive, the authors explain, while warm nights denied them the chance to recover. The geography of the deaths is uneven. Malta, Italy and Hungary recorded the highest rates, while Italy, France, Spain, Germany and the UK counted the most deaths in absolute terms.

Britain alone is estimated to have lost 4,354 people. Yet the figures are hard to see in official records, because death certificates rarely mention heat as a contributing factor. A stroke victim or a lung cancer patient pushed past their limits by dangerous temperatures will usually be recorded as having died of the illness, so families seldom learn that heat and human activity hastened the loss. For the researchers, the most troubling finding is the trend rather than the total.