The “borrowed-size” effect in carbon reduction: Does the spatial structure of urban agglomeration matter?

The existing literature has yet to examine whether and how the top-down urban agglomeration policy influences the spillover effect for environmental benefits. Furthermore, it remains unexplored how a region’s spatial structure shapes this policy-induced environmental spillover, creating a critical gap in understanding the role of spatial structure in sustainability-oriented regional planning. Based on a sample of 195 cities in China from 2004 to 2019, this paper examines whether the urban agglomeration policy affects the spillover effect of carbon emissions and how the spatial structure of urban agglomeration affects this relationship. Using a high-speed rail weighted Spatial Durbin Difference-in-Differences method, our results show that urban agglomeration policy enhances the “borrowed-size” effect in carbon reduction, exhibiting a carbon reduction spillover effect from leading cities to peripheral cities. Furthermore, we demonstrate that spatial structure plays an important role in shaping the patterns of the spillover effect in carbon emissions by showing that the “borrowed-size” effect is weakened in monocentric urban agglomerations. This study extends the “borrowed-size” framework into the environmental domain and establishes spatial structure as a key determinant of sustainability-oriented regional planning.