Heat-related mortality risk is becoming a central concern in climate and public health research. While past studies have shown that cold-related deaths far outnumber heat-related ones—up to a 9:1 ratio—this pattern is rapidly shifting as global temperatures rise. The difference stems from the frequency of moderately cold days, which, while less severe, cumulatively contribute to a higher number of deaths. However, extreme heat poses a much higher risk on a per-day basis.

For example, in Barcelona between 2000 and 2018, cardiovascular death risk on the coldest day rose by 50%, while on the hottest day, it spiked by 240%. As very hot days become more common, the number of heat-related deaths is projected to increase dramatically.

Crucially, the exposure–response curve shows that increases in heat-related mortality outpace potential reductions in cold-related deaths. Recent summers, like 2022 in Spain, already demonstrate this trend, with triple the number of extreme hot days leading to nearly five times more heat-attributable deaths.

This evolving risk profile makes it essential to prioritize policies and research focused on mitigating the impacts of extreme heat. Effective climate adaptation strategies must address this growing public health threat urgently.