Global blue carbon losses from salt marshes exceed restoration gains.

Salt marsh soil organic carbon (SOC) is a key blue carbon pool affected by both disturbance and restoration; yet its long-term global dynamics remains poorly understood. Here we provide the global assessment of surface SOC changes in salt marshes from 2002 to 2019, combining multi-source remote sensing imagery with the machine learning calibrated by field observations. We find a net global SOC loss of 0.52 million tonnes, primarily driven by declines in North America and Oceania, which are only partially offset by gains in Asia and South America. The United States alone accounts for ~60% of the global loss, equivalent to 6.2 million tonnes of CO2 if fully released. Losses are concentrated in mature salt marshes with large SOC storage, while gains occur primarily in newly formed salt marshes with relatively low SOC density. These patterns suggest that global restoration efforts are failing to keep pace with degradation. To avert irreversible climate and ecological damage, the protection of mature, carbon-rich salt marshes must become a core component of global climate strategies.