Urban Shrinkage and the Urban Heat Island Effect: Developing a Population–Function–Environment Framework

Urban thermal environments are an important component of urban climate research, and the development of urban heat islands (UHI) differs markedly between contexts of urban expansion and urban shrinkage. To examine urban thermal environments in shrinking cities, this study developed a progressive cross-domain search strategy, screened the relevant literature based on conceptual relevance, and combined a scoping review, systematic literature analysis, and bibliometric analysis. The review identified the following characteristics of existing research: (1) UHI and urban shrinkage research have an intersection that has gradually shifted toward an urban phenomenon-oriented perspective; (2) no stable direct relationship exists between population and UHI intensity, while population and economic changes may affect urban thermal environments through intermediate aspects such as urban functions and spatial conditions; and (3) existing cross-domain studies examine population, urban functions, and environmental responses separately, with limited connections among these dimensions. Based on systematic literature synthesis and conceptual integration, this study proposes a Population–Function–Environment (P-F-E) conceptual framework hypothesis to organize these relatively dispersed dimensions and provide a conceptual basis for further examination of their potential relationships.