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Climate change can increase the risk of conditions that exceedhuman thermoregulatory capacity1–6. Although numerous stud-ies report increased mortality associated with extreme heatevents1–7, quantifying the global risk of heat-related mortalityremains challenging due to a lack of comparable data onheat-related deaths2–5. Here we conducted a global analysisof documented lethal heat events to identify the climaticconditions associated with human death and then quantifiedthe current and projected occurrence of such deadly climaticconditions worldwide. We reviewed papers published between1980 and 2014, and found 783 cases of excess humanmortality associated with heat from 164 cities in 36 countries.Based on the climatic conditions of those lethal heat events,we identified a global threshold beyond which daily meansurface air temperature and relative humidity become deadly.Around 30% of the world’s population is currently exposedto climatic conditions exceeding this deadly threshold for atleast 20 days a year. By 2100, this percentage is projectedto increase to ∼48% under a scenario with drastic reductionsof greenhouse gas emissions and ∼74% under a scenario ofgrowing emissions. An increasing threat to human life fromexcess heat now seems almost inevitable, but will be greatlyaggravated if greenhouse gases are not considerably reduced.

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