Strategic urban afforestation in the Greater Bay Area towards 2050: Spatiotemporally explicit assessment of potential and trade-offs

Urban afforestation is increasingly recognized as a cost-effective, nature-based solution for climate change mitigation and adaptation. However, despite a surge in ambitious afforestation initiatives and related studies worldwide, a comprehensive understanding of its future potential and the inherent trade-offs among its benefits remains lacking. This study provides spatiotemporally explicit insights into future urban afforestation potentials and their associated benefits towards 2050, explicitly accounting for the impacts of future urban land-use changes. We incorporated Sentinel-2 images, the Continuous Change Detection and Classification (CCDC) algorithm, and the Patch-generating Land Use Simulation (PLUS) model to map future land-use changes and pinpoint locations suitable for afforestation across different Shared Socioeconomic Pathways (SSPs) from 2020 to 2050. Using spatial prioritization, we then estimated two key afforestation benefits—enhancing carbon sequestration and improving equitable exposure to trees—and evaluated their trade-offs across alternative afforestation strategies. Our analysis reveals substantial afforestation potential in the Greater Bay Area (GBA) by 2050, from 4,698 km² (SSP2) to 5,388 km² (SSP4), representing a 9-25% increase compared to the 2020 baseline. Crucially, we demonstrate the critical role of peri-urban areas (i.e., 20-60% impervious surface fraction), which encompass at least 30% of all the areas suitable for afforestation and account for over half of the projected increase during 2020-2050. Afforesting all the identified areas in GBA could result in a 15% increase in tree exposure and a maximum carbon sequestration potential of 12,000 MgC per year. Compared to a randomized afforestation strategy, spatial prioritization significantly enhances tree exposure while offering a modest improvement in carbon sequestration. Our findings highlight the growing potential of urban afforestation as a vital nature-based solution and underscore the need to explicitly incorporate land-use change uncertainties into planning frameworks to facilitate more effective and sustainable urban development.