Declining urban density attenuates rising population exposure to surface heat extremes.

In the past two decades, global cities have experienced a decline in urban population density, marked by urban land expansion outpacing population growth. The impact of this trend on urban population exposure to surface urban heat island (SUHI) has not been fully examined by considering the intra-city spatial variations in both population distribution and surface temperature. In this study, we examine the temporal trends of population-weighted SUHI extremes (as a proxy for spatial heat exposure) in the 1,000 largest global cities using high-resolution data from 2003 to 2020. Our findings reveal that cities with declining population densities experience a slower increase in population-weighted SUHI extremes compared to the rise in the area-average values, by approximately 0.5 ± 0.04 K per decade for every 1,000 persons/km/year decrease in density (R = 0.71; P < 0.01). Conversely, cities undergoing densification face greater challenges in mitigating rising heat exposure. We further show that vegetation changes have a moderate association with changes in population-weighted heat exposure (R = 0.56), whereas variations in surface albedo exhibit a negligible relationship (R = 0.03). These results underscore the need for strategic urban planning to balance the benefits of densification with the imperative of reducing heat exposure risks. Urban greening efforts may offer some mitigation potential, while albedo-based interventions require further investigation to assess their effectiveness at scale. As cities worldwide pursue sustainability and economic growth through increasing density, it is critical to integrate heat exposure consideration into urban planning to enhance climate resilience.