Evolutionary characteristics of the relationship between urban intensive land use and carbon emissions in China's three major coastal urban agglomerations

Under mounting global pressure for carbon emission mitigation, China—currently the world's largest carbon emission contributor—confronts the critical challenge of reconciling emission reduction targets with sustained economic growth and progressive enhancement of citizens' living standards. Adopting a land-intensive utilization framework, this paper systematically investigates the relationship between urban intensive land use (UILU) and carbon emissions (CEs) in China's three major eastern urban agglomerations—the Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD). Methodologically, standard deviation ellipse analysis, the HR coordination model, and the Environmental Kuznets Curve (EKC) were employed to identify viable emission reduction pathways. Key findings from the 2005–2021 study period reveal four principal patterns: (1) UILU levels across the three urban agglomerations demonstrate an overall upward trajectory; Inter-agglomeration disparities exhibit phased transitional characteristics, following a “contraction-expansion-contraction” sequence, whereas intra-agglomeration disparities manifest persistent widening trends. Spatial analysis through standard deviation ellipses demonstrates distinct orientation patterns: the BTH exhibits a predominant northeast-southwest alignment; the YRD displays northwest-southeast orientation; and the PRD maintains an approximate east-west axis. (2) CEs across the three urban agglomerations demonstrated an overall increasing trajectory, stabilizing during the final phase of the study period. Spatiotemporal analysis revealed distinct orientation patterns: the BTH's standard deviation ellipse maintained a northeast-southwest alignment, while the YRD and PRD exhibited northwest-southeast orientations. (3) The HR coordination degree displayed moderate fluctuation with an overall ascending trajectory, demonstrating hierarchical coordination levels: YRD > PRD > BTH. (4) EKC analysis delineates distinct morphological patterns: The EKC curves for UILU and CEs in the BTH and PRD demonstrated an inverse N-shaped pattern, with turning points at 0.14 and 0.49 for the BTH, 0.13 and 0.44 for the PRD, respectively. In contrast, the EKC relationship for the YRD follows a U-shaped curve with a turning point at 0.20.