Characteristics of surface urban heat islands in global cities of different scales: Trends and drivers

Given the increasing severity of the surface urban heat island (SUHI) phenomenon, urban residents face heightened heat stress. Consequently, mitigating the effects of SUHI becomes critically important to improve urban livability. Previous studies often overlooked the influence of city size on SUHI, focusing on the correlational between SUHI and drivers and sidelining the synergistic effects of multiple factors on SUHI. To address this issue, we introduced a novel method, "geographical convergent cross-mapping", coupled with the random forest and Shapley value theory. Our results indicated that: Globally, SUHI exhibited a decreasing trend during day and night, with significant spatiotemporal disparities, and in cities of varying sizes, the cooling capacity of vegetation for SUHI showed significant variation. Locally, in arid cities spanning 150-200 km², vegetation contributed to cooling SUHI by up to 61%, but this effect lessened in larger cities. Notably, in equatorial cities larger than 200 km², this capability dropped drastically during summer, from 49% to 7%. Long-term trends showed an increase in daytime SUHI in equatorial regions and large cities along the southeastern coasts of China and the United States. This study provides key insights for precise urban climate adaptation and sustainable planning in cities of different sizes.