Crossing the Threshold: A Global Analysis of 277 Cities Reveals Unequal and Increasing Urban Heat Risk

• In Event: Multi-Level Climate Adaptation and Governance Across Individual, Household, Community, and City Levels

Abstract

Heat Related Illness (HRI) is widely acknowledged as a significant and growing threat to human health and welfare under current climate change scenarios. As governments struggle to adapt to increasing temperatures, scientists have pioneered a wide range of metrics to measure heat stress and predict its effects on human welfare. However, governments rarely adopt these more specialized heat stress metrics when activating extreme heat response protocols, preferring instead to rely on more easily interpretable measures, creating inconsistency and confusion when practitioners try to decide on “safe” heat levels for their communities. Additionally, most studies on this topic are done on either a small (single city) or large (entire region) scale, with little in between. In this paper, we build on a physiological approach pioneered by Vanos et al. (2023) to measure heat stress, which takes into account differences in age, activity level, and underlying climate conditions. We analyze 30 years of climatological data within the administrative boundaries of 277 cities across the globe, reporting results for two age groups (younger and older adults) and two activity levels (no movement and basic daily tasks). For each day in our time frame, we calculate whether or not the threshold was crossed for each group and activity level, and analyze trends over time by climate zone, socioeconomic grouping, and actuarial breakdown. We find that the liveability threshold for older adults was crossed most often (sample average of 32.9 days per year), followed by the liveability threshold for younger adults (sample average of 1.3 days per year). The survivability threshold for older adults was crossed the least often (sample average of 0.5 days per year). As expected, results varied significantly by city climate zone and type of threshold crossing. Results show a persistent increase in threshold crossings for all three threshold categories from 1995 to 2024. Results showed no significant pattern with regard to Scaled Human Development Index (SHDI). Certain cities stand out as particularly dangerous for younger or older adults, and those cities with higher proportions of the at-risk population and a higher number of threshold crossings were examined in more detail. We believe this measure provides a simple, flexible, and accurate way to measure the danger of heat stress on an urban scale – one that is tied to administrative boundaries and therefore actionable for both government officials and the general public.

Author

• Isabel Bukovnik, University of North Carolina at Chapel Hill
  Presenting Author