Coastal salt marshes are among the most ecologically important and vulnerable coastal ecosystems, supporting carbon storage, coastal protection, fisheries, and biodiversity. Accelerating sea-level rise and altered sediment supply could influence the position and extent of marsh systems. However, a systematic understanding of where and how marshes are relocating, rather than simply expanding or contracting, at the global scale remains largely lacking. Current global assessments predominantly rely on area-based change accounting. However, area alone does not fully capture marsh dynamics, as similar changes in area can reflect very different spatial changes.

To address this gap, we develop a geometry-based framework for quantifying marsh dynamics. Rather than relying solely on changes in area, each marsh system is characterized by its position relative to a shoreline reference. Applying this framework to an annual 30 m global wetland map spanning 2000–2022, we quantify these properties for every mapped marsh system worldwide and estimate their long-term trends. The resulting global assessment reveals patterns of marsh relocation and quantifies their long-term trajectories.