Unlocking the Carbon Mitigation Potential of Green Infrastructure: Evidence from China's Sponge City Program.

Carbon mitigation and nature-based green infrastructure (GI) have become central themes in global efforts to address climate and urban sustainability. The net carbon benefits of GI and the factors shaping them remain poorly understood due to data and methodological gaps. This study draws on 679 engineering cases from 15 demonstration Sponge Cities in China to conduct multiscale life-cycle carbon accounting of GI. Overall, sponge retrofitting shifted conventional urban surfaces from a net source to a sink. At the infrastructure scale, carbon benefits varied widely across different types of GI, mainly due to differences in construction materials and operational efficiency. At the project scale, all project types achieved net carbon benefits, with variations primarily driven by infrastructure configuration. Regionally, carbon payback periods ranged from 7.5 to 28.6 years, depending on rainfall volume control rates and infrastructure composition, with wetter regions achieving higher net benefits. To maximize cost-effective runoff, pollution, and carbon reduction, NSGA-II favored green roofs and sunken green spaces without considering landscape functions. Ensuring stable carbon benefits under climate change and overdesign storms requires future GI to adopt sustainable and resilient pathways. By integrating life cycle assessment with multiobjective optimization, this study establishes a novel framework for decoupling carbon emissions from infrastructure growth, offering actionable pathways for global carbon-neutral urban design.