Sediment dynamics drive global patterns of salt-marsh vulnerability.

Salt marshes protect coastlines, support biodiversity, and store substantial blue carbon, yet their rates of gain and loss vary markedly around the world. Although relative sea-level rise (RSLR) is widely recognized as a major threat, contemporary global patterns of marsh change do not simply track RSLR, suggesting additional controls mediate vulnerability. Here, we combine satellite-derived global salt-marsh change with datasets describing hydro-sedimentary regimes, human pressure, and extreme events to identify the processes underlying this spatial heterogeneity. Tidal range, suspended sediment concentration, and wave-sediment coupling are the strongest first-order predictors of contemporary marsh trajectories, outperforming RSLR in explaining global patterns. These results show that the effects of RSLR are mediated by local sediment availability, delivery, and redistribution. Building on this framework, the Salt Marsh Vulnerability Index identifies coastlines globally where physical conditions favor persistence or increase the risk of marsh degradation, providing a foundation for conservation, restoration, and blue-carbon planning.