Spatial Patterns of Historical Changes in Human Heat Stress Disagree Across Metrics

A robust estimation of how heat stress is changing worldwide is complicated by the variety of heat stress metrics in use. This study compares heat stress changes between 1979–2000 and 2001–2023 across five commonly used metrics calculated with ERA5 reanalysis data. We identify regions where all metrics indicate significant increases in heat stress, highlighting a high-certainty need for urgent adaptation efforts. Conversely, we also find regions where metrics disagree, even on the direction of change. The substantial inter-metric spread in population heat exposure is comparable to the spread across five reanalysis products and 17 CMIP6 climate models. We attribute these inter-metric discrepancies to differing temperature-humidity relative weight across metrics. Our findings highlight metric choice as a significant source of uncertainty in heat stress projections and emphasize the need for a better understanding of the suitability of different metrics for specific climate regimes and impacts.