The divergent effects of spatial structure of urban agglomerations on carbon emission reduction capacity

The adjustment of the spatial structure of urban agglomerations is an effective tool for improving carbon emission reduction. Given the complexity of sustainable environment and carbon emission reduction, urban low-carbon transformation faces challenges such as population agglomeration, factor mobility, and urban expansion. This study constructs a carbon emission reduction capacity indicator system and adopts Zipf's law to measure the centrality index of the spatial structure of urban agglomerations. The influence of spatial structure on carbon emission reduction capacity is verified using five Chinese national urban agglomerations in the Yellow River Basin. The results show that urban agglomerations are dominated by a polycentric spatial structure, with strong carbon emission reduction capacity in the middle and lower reaches and weak capacity in the upper reaches. Carbon emission reduction capacity presents an inverted U-shaped relationship with spatial structure, and functional specialization presents nonlinear mediating effects. Carbon emission reduction effects can be maximized through rational functional specialization of the spatial structure. The moderation of the increase in factor mobility shows a weakening effect. Thus, a moderately polycentric spatial structure of urban agglomerations should be optimized to promote production factor mobility through sustainable governance. Reasonable urban functional positioning and the coordinated management of carbon emission reduction among cities should be developed. • The effect of spatial structure on carbon emission reduction capacity is nonlinear. • The moderately polycentricity improves carbon emission reduction capacity. • Moderation of the increase in factor mobility shows a weakening effect. • Functional specialization is important to carbon emission reduction capacity. • Developing policies for synergistic management of carbon in urban agglomerations.