Surface urban heat island in China

Abstract

Surface urban heat island (SUHI), which reflects high land surface temperature in urbanized areas as compared with neighboring non-urban areas and thereby being regarded as an urban environmental problem threatening human society, is commonly measured using extensive remotely sensed data. Advancements in remote sensing and spatial science in the last few decades have improved SUHI studies, but there still exist significant gaps in the field. Using China as the case study area, this thesis attempts to provide additional insights focusing on three knowledge gaps, including SUHI intensity measurement, regional SUHI evaluation, and nationwide mapping of thermal inequity. In the first study, a new indicator of SUHI intensity (SUHII) is created, which offers an innovative approach to more accurately measure SUHII. This new indicator could take into account the inter-pixel latent thermal interactions, in which the homogeneous/heterogeneous land use and associated thermal properties of neighboring pixels could affect the thermal characteristics of center pixels via either reinforcement or mitigation of heat load. It could, therefore, mitigate potential biases in SUHII measurement brought by the non-representativeness of urban and rural pixels and ignorance of pixel-level thermodynamics associated with the heterogeneous urban landscape. It contributes to rigorous assessment of potential heat risks at the micro-pixel scale and informs cognate mitigation strategies. The second study employs panel data analysis and random forest algorithm to better understand how various socioeconomic factors and biophysical attributes affect SUHII. Using the Pearl River Delta region as the case study area, the relative importance and dynamic contribution of land-use cover, socioeconomic and climate factors to regional-scale SUHI variation during 2000-2020 is investigated. The empirical findings suggest that the extent of build-up area is the most important factor exacerbating SUHII significantly, while urban green spaces could only slightly mitigate SUHII. These results call for better urban landscape design regarding the extent of both greening and built-up areas so that green spaces can be constructed to make more contributions to SUHI mitigation. The third case study focuses on thermal inequity in China. Two groups of population across 248 cities (including urban population being affected by SUHI and those being protected by the cooling effect of urban green spaces) are specified, combining spatiotemporal data of population and SUHI distribution during 2005 and 2019. The analytical results reveal explicitly the existence of thermal inequity. Wealthy cities have a large portion of the population to be protected, in contrast, poor cities have a large portion of the population being affected by intense SUHI. The green area is negatively associated with the portion of the population exposed to SUHI. Given an overall increasing trend of the population being affected by SUHI and global warming, more policy efforts should be devoted to addressing thermal inequity in China. The three case studies provide a comprehensive evaluation of SUHI and associated thermal inequity in China’s unique socioeconomic context, enriching empirical insights pertinent to SUHI and providing practical implications for designing and constructing healthy cities for an ever-increasing urban population.