Investigation of the influence and mechanism of urban population agglomeration on carbon emission efficiency

As a major source of carbon emissions, cities bear a critical responsibility in mitigating global climate change. In the face of the crucial challenge of balancing economic development with carbon reduction, investigating the influence and mechanisms of urban population agglomeration on the carbon emission efficiency holds paramount importance in guiding the coordinated development of urban economies and carbon reduction efforts. Drawing upon the context of the Yangtze River Delta region in China, in this paper, we propose a theoretical framework and research hypotheses, and then, by employing spatial econometric models, we investigate the impact of population agglomeration on the carbon emission efficiency, as well as the intervening and moderating mechanisms that are in operation. The research findings reveal that (1) although population agglomeration contributes to a reduction in carbon emissions, it concurrently suppresses the carbon emission efficiency; that is, each 100 ​% increase in population agglomeration significantly decreases the carbon emission efficiency by 72.2 ​%. (2) Urban population agglomeration increases transportation carbon emissions (β ​= ​420.5335∗) and suppresses the proportion of the tertiary industry output value in the total output value (β ​= ​−23.9671∗∗∗). Through the negative impact of the transportation carbon emissions on the carbon emission efficiency (β ​= ​−0.0002∗∗) and the positive effect of the proportion of the tertiary industry output value in the total output value on the carbon emission efficiency (β ​= ​0.0177∗∗∗), population agglomeration ultimately leads to a decrease in and an increase in the urban carbon emission efficiency, respectively. (3) The urban gross domestic product significantly amplifies the inhibitory effect of population agglomeration on the carbon emission efficiency, with the interaction term coefficient reaching 0.9029∗∗ (p ​< ​0.05). This indicates that for every 100 ​% increase in the urban gross domestic product, the negative effect of population agglomeration on the carbon emission efficiency increases by 90.29 ​%. • Urban population agglomeration (PAI) reduces carbon emissions but significantly inhibits carbon emission efficiency (CEE); a 100% PAI increase drops CEE by 72.2%. • Traffic carbon emissions and tertiary industry output proportion mediate PAI’s effect on CEE: PAI raises TCE and lowers TIG, both reducing CEE. • Urban gross domestic product (GDP) moderates PAI’s inhibitory effect on CEE: higher GDP strengthens this inhibition, linking economic development to the PAI-CEE relationship.