The population exposure risk of urban heat island effect: From the perspective of urban spatial expansion in China

Urban spatial expansion is a critical aspect of urbanization, but it must occur in a manner that ensures the sustainable development of the urban climate. The primary objective of this study was to assess the population exposure risk of urban heat island (UHI) effect from the perspective of urban spatial expansion. Additionally, the population weighted urban heat island intensity (UHII) index (Ep) was proposed to compare the population exposure risk of UHI effects in different types of urban spatial expansion in China during the summer of 2005–2020. It was found that, in China, urban land increased by an area of 74,894 km2 (1.01-fold) from 2005 to 2020. Leapfrogging emerged as the predominant pattern of urban spatial expansion and was primarily driven by the conversion of cropland to urban land. It witnessed a significant increase in the area covered by UHIs of 107,119 km2 (1.20-fold), accompanied by a rise in the UHII of 0.12°C in China. Notably, the UHII for infilling urban expansion was significantly higher at 2.03°C, in contrast to 1.66°C for edge expansion and 1.47°C for leapfrogging. Correspondingly, the Ep was markedly larger under the infilling urban expansion (2.10°C) in contrast to edge expansion (1.75°C) and leapfrogging (1.67°C). However, edge expansion dominated the newly developed urban land, contributing to 18.60% of the Ep, which was still far less than the contribution of the pre-existing urban land to Ep (61.24%). Importantly, the dominant driver of the increased Ep was the increasing UHII. This study established a connection between inevitable urban spatial expansion and the population exposure risk of UHI effect, providing a strong theoretical basis and practical guidance for the active adaptation and mitigation of urban environmental hazards in urban regions.