Navigating carbon reduction in dense, multi-functional compact cities: Moderators and threshold effects of green space and aging population

Abstract

Compact-city planning is widely promoted as a pathway to curb urban carbon emissions by compressing land use, shortening travel distances and concentrating infrastructure. Yet its ultimate effectiveness hinges on how ecological and demographic structures interact with built form. Leveraging data for Chinese cities, this study integrates spatial-moderation models with XGBoost-based non-linear simulations to test whether—and how—urban green-space structure and population aging condition the compactness–emissions nexus. Results show that both moderators exert significant and directionally nuanced effects. Expansive green-space coverage and higher patch aggregation amplify the carbon-saving benefits of compactness, whereas excessive landscape connectivity can dampen them, plausibly by increasing fragmentation and mobility demand. Aging likewise matters: a larger share of older residents—especially older men—steadily enhances carbon efficiency, reflecting lower mobility and lifestyle-related energy savings in high-density settings. All moderating effects display pronounced spatial heterogeneity shaped by variations in built-environment intensity, ecological-infrastructure integration and demographic accessibility. Nonlinear analyses reveal clear thresholds for policy guidance: optimal mitigation occurs when green-space coverage reaches 60–65 %, patch connectivity density stabilizes at 15–20 patches km⁻², and the aggregation index exceeds 0.12. Demographically, an elderly-population share of 13–20 %—with male shares above 6–8 %—further strengthens emission reductions. Taken together, the findings underscore that effective decarbonization in dense, multi-functional cities requires synchronized planning of green infrastructure and demographic adaptation. By quantifying critical ecological and demographic thresholds, this study provides actionable evidence for precision carbon governance amid rapid urbanization and population aging.