Rethinking Urban Heat Islands in Polycentric Metropolitan Systems: A Bibliometric and Systematic Review of Networked Heat Dynamics

Rapid urban expansion is reshaping large metropolitan regions into polycentric systems in which multiple centers interact through transport, infrastructure, land-use, economic and ecological networks. Urban heat island (UHI) research has traditionally relied on single-city or core–periphery models; these remain useful for explaining heat contrasts within individual cities, but are insufficient for explaining how thermal loads form, propagate and accumulate across interconnected metropolitan regions. This study combines bibliometric analysis and a PRISMA-guided systematic review to synthesize research on UHI processes in polycentric cities, mega-urban regions and metropolitan systems. The bibliometric corpus comprises 468 Scopus-indexed records published in 2020–2025, while 35 full-text studies were retained for qualitative synthesis. The results show strong publication growth from 54 records in 2020 to 124 in 2025, an annual growth rate of 18.09%, and an interdisciplinary evidence base led by environmental science, social science, Earth-system science and engineering. Three spatial patterns recur across the core studies: multi-core hotspots, corridor-based heat propagation and peripheral thermal expansion. The review contributes a network-based interpretation of UHI as a nested metropolitan process in which node morphology, functional hierarchy, transport connectivity, blue–green infrastructure (BGI) and governance coordination jointly shape heat intensity, footprint and exposure. Rather than displacing single-city or core–periphery interpretations, the proposed framework extends them by positioning local heat analysis as one layer within a larger multiscale heat-governance architecture.