Rapidly urbanizing megaregions are pivotal for achieving land-based climate-mitigation targets, yet the mechanisms governing their ecosystem carbon dynamics remain poorly understood. In this study, we quantify the spatiotemporal evolution of carbon storage in the Guangdong–Hong Kong–Macao Greater Bay Area (GBA), a globally recognized example of rapid coastal urbanization, and diagnose the drivers behind an emerging regional “carbon deficit”. We couple a patch-generating land-use simulation (PLUS) model, a process-based carbon accounting model (InVEST-C), and interpretable machine learning (SHapley Additive exPlanations) to (i) reconstruct land-use and carbon changes from 2000 to 2020 and (ii) project carbon trajectories under three contrasting development pathways to 2030. Total ecosystem carbon storage decreased from 5.45×10 8 Mg in 2000 to 5.11×10 8 Mg in 2020, with the loss rate during 2010–2020 being 18 times higher than during 2000–2010, coinciding with the phase of megaregional integration. Scenario analysis shows that a business-as-usual “natural development” pathway, which extends 2010–2020 land-use trends without additional policy constraints, would lead to a further loss of 2.79×10 7 Mg by 2030. In contrast, a policy-driven sustainable development pathway, which constrains construction-land expansion through urban growth boundaries and ecological conservation redlines and actively promotes ecological restoration, could avoid more than 83% of this potential decline. SHAP-based interpretation reveals that topography provides the essential natural capital for carbon storage, whereas population pressure is the decisive driver of loss. We further identify a strong non-linear interaction in which the ecological cost of urban expansion is greatly amplified on steep, ecologically fragile slopes. Our findings provide a transferable framework for diagnosing carbon deficits in rapidly urbanizing megaregions and deliver spatially explicit evidence to support strict ecological conservation redlines (ECRs) and urban growth boundaries (UGBs) that reconcile urban development with climate and ecological resilience. • PLUS–InVEST–SHAP quantifies and explains carbon dynamics in a mega-region. • 2000–2020 urban expansion drove a sharp carbon deficit, accelerating post-2010. • Sustainable-development scenario mitigates > 83% of projected carbon loss by 2030. • Elevation, slope, and population dominate; slope × population amplifies losses. • UGBs/ECRs protect cropland and forest sinks in peri-urban agricultural belts.
Li et al. (Fri,) studied this question.