Urban vegetation cooling capacity was enhanced under rapid urbanization in China

Abstract

The cooling capacity of urban vegetation, which forms “urban cooling islands” (UCI), can play an important role in mitigating urban heat islands (UHI). However, the spatiotemporal changes in UCI intensity at the national scale have not been fully explored under rapid urbanization in China. In this study, based on 23,081 Landsat images from the Google Earth Engine, spatiotemporal UCI was obtained for 320 cities in China from 2000 to 2018 and correlated with meteorological and urbanization parameters to uncover the factors driving UCI changes. The direct and indirect effects of the driving factors on UCI were investigated using structural equation modeling. Our results showed that China experienced 12.38%, 181.23%, 83.6%, and 22.63% increases in population density, gross domestic product (GDP), the impervious surface, and leaf area index (LAI), respectively, between 2000 and 2018. Furthermore, we found more rapid urbanization in the temperate continental climate (TCC) zone than in the temperate monsoon climate (TMC) and subtropical monsoon climate (SMC) zones in China. The national averaged UCI was 2.47 °C, with the largest UCI recorded in small cities and TCC zones. From 2000 to 2018, over 70% of cities showed an increasing trend in UCI intensity, mainly concentrated in TMC and megacities. At the national scale, the increase in UCI was likely explained by a combination result of the intensified wind speed, decrease in urban areas, and enhanced LAI. We also identified that the driving mechanisms of UCI were very different in different climatic zones. In TCC, precipitation had a major impact on UCI intensity at a threshold of 78.92 mm. In TMC and SMC, in addition to temperature, urbanization factors significantly influenced UCI. The main factors in TMC were LAI, with a specific threshold of 1.15. These results deepen the understanding of UCI formation and provide new insights into the regional adaptation of urban vegetation to mitigate urban heat waves.