Nature-based Solutions can help restore degraded grasslands and increase carbon sequestration in the Tibetan Plateau

The global push towards carbon neutrality by 2050 (for major economies like the US and EU) and 2060 (for China) necessitates ambitious carbon sequestration strategies. While renewable energy and carbon-removal technologies are crucial, natural ecosystems, particularly grasslands, play a significant role in carbon storage. The Tibetan Plateau (TP), covering a quarter of China's land area, hosts diverse ecosystems with substantial carbon sink potential, estimated at 33.12 to 37.84 Tg C per year. However, human activities and climate change have led to significant grassland degradation, reducing carbon storage and even transforming grasslands from carbon sinks to sources. Since the 1970s, various ecological restoration practices have been implemented on the TP, but a more ambitious, coordinated approach, such as NbS, is urgently needed to enhance carbon sequestration and achieve carbon neutrality. NbS offers powerful tools to address multiple challenges, including climate change, food security, and disaster risk reduction. Globally, NbS has been successfully employed to restore vegetation, mitigate soil degradation, and enhance carbon sequestration, with the potential to save approximately 10 gigatons of CO2 equivalent per year. The application of NbS in the TP's grassland ecosystems, however, remains limited, highlighting the need for conceptual and quantitative research to assess their potential and inform policy decisions.

Existing literature demonstrates the potential of grassland restoration to improve carbon sinks. Human activities, primarily overgrazing and climate change, have caused extensive grassland degradation on the TP, leading to soil erosion and significant carbon loss (approximately 40% of grasslands degraded). This degradation has reversed the grassland's role from a carbon sink to a carbon source, resulting in the loss of 1.01 Pg of soil carbon since the 1980s. Studies show that restoration practices such as grazing exclusion and artificial grassland establishment significantly increase plant coverage, productivity, and CO2 sequestration. However, challenges remain, including a lack of comprehensive methods for evaluating restoration projects and inconsistencies in assessing the degraded state of grasslands. The existing restoration projects on the TP have often been top-down in nature, lacking the input and buy-in of local herders. This has resulted in conflicts and resistance from local communities.

This research employed a mixed-methods approach. A literature review and meta-analysis assessed the effectiveness of various grassland restoration techniques (optimizing grazing regimes, grazing exclusion, and artificial grassland establishment) in the TP. Data were collected from 284 published articles (English and Chinese) using keywords such as "grazing," "grazing exclusion," and "artificial grassland." The meta-analysis calculated response ratios (RR) of key variables (aboveground biomass, belowground biomass, soil organic carbon, microbial biomass carbon, and plant litter biomass) using MetaWin 2.1 software. Grassland degradation was quantified using Google Earth Engine (GEE) to acquire Normalized Difference Vegetation Index (NDVI) data from 1982 to 2020. Sen's slope estimation method and the Mann-Kendall (M-K) significance test were applied to classify grassland degradation levels. To project future carbon sequestration, the researchers used the Boreal Ecosystem Productivity Simulator (BEPS) to obtain Net Ecosystem Productivity (NEP) data (1981-2019). The ORCHIDEE-MICT model, driven by ISIMIP3b climate forcing data across three Shared Socioeconomic Pathways (SSPs), projected future carbon sequestration with and without NbS interventions. The potential carbon sequestration by NbS was calculated by subtracting the simulated carbon sequestration in 2060 without NbS from the maximum NEP (Cmax) for the period 1981-2019.

The meta-analysis revealed that heavy grazing negatively impacted soil organic carbon and aboveground biomass but not root biomass. Conversely, grazing exclusion significantly enhanced carbon storage in litter mass, aboveground, and belowground biomass. Artificial grasslands also showed enhanced carbon sequestration. The analysis of grassland degradation indicated that approximately 40% of grasslands on the TP are degraded to varying degrees. Model projections showed that NbS projects are expected to increase the carbon sink of the TP grassland ecosystems by 15–21 Tg C by 2060, depending on the socioeconomic pathway scenario. The simulations suggested that protection of lightly degraded grasslands (optimizing grazing), improved management of moderately degraded grasslands (grazing exclusion), and restoration of heavily degraded grasslands (artificial grassland construction) could contribute significantly to this carbon sequestration increase (10.5 Tg C, 2.5 Tg C, and 5.0 Tg C per year respectively). A conceptual framework for implementing NbS was proposed, highlighting the importance of integrating traditional Tibetan knowledge and a collaborative approach involving policymakers, scientists, and local herders.

The findings highlight the significant potential of NbS to enhance carbon sequestration in the TP's degraded grasslands. The integrated framework proposed in this study addresses the limitations of past top-down approaches by emphasizing collaboration with local communities and incorporating their traditional ecological knowledge. The quantitative projections, based on model simulations, provide crucial evidence for policy decisions related to grassland restoration and carbon neutrality targets. The results underscore the need for a holistic approach that considers the economic and social aspects alongside ecological goals. The substantial increase in carbon sequestration projected with NbS implementation strengthens the case for prioritizing these nature-based solutions in climate change mitigation strategies.

This study provides a comprehensive assessment of the potential of NbS to restore degraded grasslands and enhance carbon sequestration on the Tibetan Plateau. The findings highlight the significant carbon sequestration potential of NbS, which, when integrated with traditional knowledge and a collaborative governance approach, could significantly contribute to achieving carbon neutrality goals. Future research should focus on refining the NbS framework, conducting large-scale validation studies, and establishing detailed implementation guidelines for various grassland types and degradation levels.

The model projections rely on several assumptions, including the successful implementation of NbS projects and the ability of degraded grasslands to reach their optimal carbon sequestration capacity by 2060. Uncertainties associated with future climate change scenarios and the complexities of ecosystem dynamics might affect the accuracy of these projections. The meta-analysis was based on available literature, and the generalizability of findings could be influenced by potential publication bias and variations in methodologies across different studies.