Nonlinear associations and potential pathways of urban polycentricity and transportation carbon emissions in the Yangtze River Delta China

Transportation carbon emissions (TCEs) are central to the low-carbon transition of cities, yet the role of urban polycentricity (UP) remains contested. Using data for 41 Yangtze River Delta (YRD) cities in 2005, 2010, 2015, and 2020, this study identifies urban centers from LandScan gridded population data, measures morphological polycentricity by integrating population agglomeration, the relative size of centers, and inter-center spatial distance, and examines its relationship with TCEs using a spatial Durbin model, mechanism tests, and heterogeneity analysis. The results show pronounced spatial heterogeneity in both UP and TCEs. UP exhibits a significant U-shaped association with TCEs. Below the turning point, higher UP is associated with lower TCEs; beyond the turning point, this negative association gradually weakens and becomes positive. Mechanism analysis shows that UP is systematically associated with changes in road network density, patch density, edge density, and the aggregation index. These urban-form variables may serve as potential pathways linking polycentric development to TCEs, while population size and economic development alter the strength of this association. These associations also vary by city size, policy region, and development stage. The findings indicate that a polycentric urban structure is not inherently low-carbon and that differentiated strategies are required for low-carbon spatial planning.