Human Sun–Shade Adaptation Reshapes Global Outdoor Thermal Comfort Assessments

Abstract

Outdoor thermal comfort assessments typically assume static exposure to either sun‐exposed or shaded conditions, neglecting the common behavior of moving between sun and shade to regulate thermal stress during non‐occupational outdoor activities. This simplification may influence estimates of climate‐driven changes in outdoor thermal comfort where such behavioral adjustment is feasible. Here we introduce a sun–shade adaptive exposure framework and assess global patterns and long‐term trends in comfort days and population exposure using the Universal Thermal Climate Index derived from hourly ERA5 data. Accounting for this adaptive behavior substantially alters estimates of both the annual frequency of comfort days and their long‐term trends. Relative to adaptive exposure, static assumptions can produce differences in estimated comfort days by more than 2 months annually in subtropical regions and in estimated long‐term trends by more than 6 days per decade in low latitudes under sun‐exposed conditions and by more than 4 days per decade in many subtropical regions under shaded conditions. These differences also extend to global population exposure, resulting in different dominant exposure categories. By incorporating sun–shade adaptation into thermal comfort assessments, our results show that widely used static assumptions may not fully capture the evolution of outdoor thermal comfort where behavioral adaptation is possible. This framework complements conventional static exposure assumptions by providing an alternative behavioral perspective for evaluating climate impacts on non‐occupational outdoor activities and informing urban design and heat adaptation strategies.