The saturation effect of carbon emissions from urban economic activities: Empirical evidence from nighttime lights

Cities serve as both major contributors to carbon emissions and key actors in the implementation of emission reduction strategies. Drawing upon the EKC theory, we constructed an analytical framework integrating economic, energy, and spatial dimensions. Using calibrated nighttime light data, multi-source statistical records, and remote sensing imagery from 298 Chinese cities spanning 2011 to 2023, we empirically assess the saturation effect of urban economic activity on carbon emission. Findings reveal that a significant inverted U-shaped relationship between urban economic development and carbon emission, with most cities positioned on the curve upward-sloping segment, characterized by diminishing marginal returns. Mechanism analysis indicates that energy consumption growth and urban spatial expansion serve as two core mediating pathways, transmitting the positive impact of economic scale on carbon emission through flow-based energy consumption and stock-based spatial lock-in dimensions, respectively. Heterogeneity analysis further shows that the effect of nighttime light intensity on carbon emission is stronger in eastern cities, non-major urban centers, and service-oriented cities. This research offers theoretical insights and empirical support for understanding the nonlinear characteristics of urban carbon emission evolution, offering scientific basis for formulating differentiated low-carbon governance policies for cities.