State-of-the-art and challenges in global grassland degradation studies

• Grassland ecosystems have seriously deteriorated globally. • Grassland degradation definitions and assessment methods are diverse. • Grassland degradation is affected by climate change, social systems and policy. • This review provides insights into achieving grassland ecosystems sustainability. Grassland ecosystems are experiencing severe deterioration due to ongoing climate fluctuation and human disturbance. Although numerous research centers on the patterns, processes, and functioning of degraded grassland, there is still a lack of standards for defining and assessing degraded grassland, which restricts the cognition of the degraded grassland mechanisms and restoration practices. Therefore, we review current grassland degradation research for the sake of the common definitions and assessment methods worldwide. Grassland degradation definitions are divided into three stages, including biotic/abiotic factors, ecosystem functions, and ecosystem services/sustainability, and further combine the concept of “nature’s contributions to people” with the definition of grassland degradation. Moreover, grassland degradation assessment methods and indicators are diverse across scales. Additionally, we systematically explore the climate change and social system factors that affect grassland degradation, and reveal that grassland management policies play an essential role in grassland degradation and restoration. Overall, this review advances our understanding of grassland degradation and calls for a unified and effective global definition and assessment criteria, which will contribute to the sustainable management of the grassland ecosystem.