Characteristics of surface heat source changes in China’s typical urban agglomerations during 1979–2021 and their influence on compound hot extremes

Urban agglomerations are increasingly emerging as the primary form of new urbanization and a crucial vehicle for modernization. The climatic effects induced by urbanization have become a research frontier with significant scientific and practical relevance. As a direct energy source for the atmosphere, the surface heat source undergoes changes profoundly influenced by alterations in land surface characteristics and human activities during urbanization. This is particularly critical in urban agglomerations, where compound high-temperature events frequently occur, exerting substantial impacts on urban habitability and societal functioning. However, a systematic understanding of the relationship between surface heat sources and these compound events is still lacking. Based on high-resolution CMA and ERA5 reanalysis data, this study analyzes the spatiotemporal distribution and evolution characteristics of surface heat sources in four major Chinese urban agglomerations: the Pearl River Delta, Yangtze River Delta, Chengdu-Chongqing region, and Beijing-Tianjin-Hebei region. It compares their regional differences and explores the mechanisms linking them to compound high-temperature events. The results indicate that the surface heat sources in these four urban agglomerations generally exhibit a seasonal pattern of “strong in summer and weak in winter.” In some seasons, the urban surface shifts from being a heat source to a heat sink, with densely urbanized areas prone to forming extreme value centers for heat source/sink. Changes in surface heat sources significantly affect the frequency of compound high-temperature events, while their influence on intensity is relatively limited. Furthermore, the identification of compound high-temperature events is highly sensitive to the choice of the baseline period; differences in the climatic background reflected by the baseline period are key to inconsistent trend judgments. This study provides new insights for addressing the environmental challenges posed by urbanization, enhancing urban climate resilience, and improving residents’ quality of life.