A standardized indicator reveals sharper increases in heat related mortality in temperate zone cities worldwide.

Global warming is intensifying heat risks globally, yet understanding of these rising risks on a global scale remains limited. We introduce a standardized heat-sensitivity indicator (ΔRR)-the change in relative risk (RR) per 1 °C above the minimum mortality temperature (MMT)-which enables like-for-like comparisons across heterogeneous city studies. Using ΔRR, we quantify how sensitively heat-related mortality responds to incremental warming rather than only to extremes. This study reveals that over a latitude of around 40°N, development inequality disrupts the patterns of heat risk variability, and high-risk cities primarily cluster in ranges with temperatures between 10 °C and 22 °C. These findings emphasize the urgent need for more focus on cities in temperate zones. Our results also position ΔRR as a transferable metric for global surveillance, comparison, and planning of heat-health adaptation.