Bottom-up accounting of landfills across 346 cities reveals overlooked carbon stocks

Abstract Landfills, one of the final destinations of all kinds of materials used by human beings, are vital for understanding the carbon cycle of cities. Global Methane Pledge at COP26 highlighted the significance of landfill, which was the third-largest source of methane emissions in the world. High temporal- and spatial-resolution datasets of landfill organic carbon stocks and flows are crucial for formulating national carbon reduction strategies; however, current research is very limited. Here, a quantitative-based solid-water-gas coupling transformation model of organic carbon cycles in landfills was established, and the landfills of 346 cities in China were chosen as the studied case to show its temporal shift and spatial distribution of organic carbon stock and greenhouse gas (GHG) emissions from 2001 to 2030. In the past twenty years, the organic carbon stock in municipal solid waste landfills in China was 503.3 ± 4.2Tg, with 6.4% and 0.3% of the organic carbon input transferred into the atmosphere and aquatic environment, respectively. The organic carbon stock in landfills has the dual attributes of resource value and environmental impacts, showing significant differences among regions. In the Northeast China and Northwest China, the soil-like resource and renewable energy potential from landfills were higher, while in South China, the methane emission from landfills was greater than that in other regions. Our scenarios analysis finds that raising the methane collection rate, minimizing landfilling rate, and implementing landfill mining and eco-remediation measures can reduce the GHG emissions by 4.6%, 13.4%, and 53.0%, respectively, compared to that in the business-as-usual scenario, by 2030. The GHG reduction by 2030 is 836.2Tg, accounting for 1.1%-3.6% and 1.2%-8.7% of the GHG emission gaps for China to achieve 1.5℃ and 2℃ warming targets under landfill-remediation scenario. Landfills can be potentially a carbon-negative sector if all of the policy-interventions are applied, and thus contribute to the carbon-neutral strategy of China by 2030.