Nonlinear impacts of urban size and vegetation cover on global surface urban heat: Insights from 6022 cities

Urban overheating, a confluence of urban heat island and climate change, is escalating alongside the rapid global urbanization. While previous studies have examined how urbanization and vegetation influence surface urban heat islands (SUHI), their nonlinear effects across climate zones remain insufficiently understood. Here, we present a globally consistent assessment of 6022 cities using MODIS Aqua data (MYD11A2) from the summer of 2019, validated with multi-year records (2017–2021), through a self-developed, scalable SUHI quantification method that enables cross-climate comparisons. Our results reveal distinct rapid- and slow-growth zones in SUHI intensification with urban size, with the fastest increase occurring in small cities below the top 20% of global urban size. This uneven rise in SUHI intensity stems from the synergistic effects of urban expansion and vegetation loss. Vegetation cooling exhibits a clear saturation beyond an inflection point, with the equatorial zone showing both weaker cooling efficiency and earlier saturation onset. Using a dual-perspective framework that integrates absolute and relative temperature metrics, we further show that Global South cities experience compounded thermal stress—featuring not only 3.37 ± 0.14 °C higher absolute temperatures due to their geographical setting, but also 0.24 ± 0.05 °C greater SUHI intensity than cities in the Global North. Together, these findings demonstrate that the effectiveness of heat mitigation strategies varies across climates and urbanization stages, underscoring the heightened vulnerability of smaller cities and the need for context-specific, climate-sensitive planning interventions. This study provides a globally integrated yet regionally differentiated understanding of surface urban heat and establishes a planning-relevant framework to guide targeted and climate-sensitive urban heat mitigation strategies.