Response of Land Use Carbon Emissions to Urban Expansion—Evidence from 11 Megacities with Population Exceeding 10 Million in China

Megacities represent an advanced stage of urban development where expansion-induced carbon debt increases environmental pressure. This study examined the spatial response of carbon stock change to urban expansion in megacities. Based on 2000–2020 land use/land cover data for 11 Chinese megacities, concentric ring analysis and spatial exploratory methods were employed to quantify urban expansion intensity and carbon stock change and to infer stage transition characteristics. The results indicate that: (1) urban expansion intensity declined over time as growth shifted from rapid edge or infill expansion toward smart-growth-oriented development; (2) carbon budget fluctuations weakened, and emission patterns evolved toward improved carbon balances and enhanced sink effects, with Beijing and Tianjin emerging as local carbon sinks; (3) the carbon effects of LUCC showed a stage transition from rapid expansion–intensive emission to slow expansion–moderate emission, and policy intervention reshaped this relationship. The carbon effects of megacity expansion are stage-dependent rather than linear, and policy intervention can shift this relationship before economic maturity, offering guidance for low-carbon planning in developing-country contexts.