Uncovering the Impacts of 2D and 3D Urbanization on Urban Heat Islands in 384 Chinese Cities

Rapid urbanization in China has exacerbated the urban heat island (UHI) effect, posing considerable challenges to urban sustainability and public health. Most UHI studies have focused on the impacts of two-dimensional (2D) urbanization, which involves outward city expansion and increased built-up area. However, as cities mature, they typically transition from horizontal expansion to vertical densification (3D urbanization), leading to increased material stock density. The implications of this shift for the UHI effect remain underexplored. This study compared the 2D and 3D urbanization-induced impacts on UHI across 384 Chinese cities from 2000 to 2020, using impervious surface and gridded material stocks. Our results surprisingly indicated that 2D urbanization lost explanatory power of the UHI intensity when the impervious surface area percentage exceeded 87%. Relative importance analysis utilizing a random forest algorithm revealed that the population, vegetation abundance, and precipitation significantly moderated the effects of 3D urbanization, emphasizing the crucial role of urban green spaces in mitigating thermal stress. This study examined the spatiotemporal dynamics of the UHI effect in China, emphasizing the key role of 3D urbanization. Our findings highlight the urgent need to incorporate 3D urbanization characteristics when devising UHI mitigation strategies.