Global asymmetric mangrove-edge shifts reveal coastal squeeze under sea-level rise

Changes at the land–sea interface reveal whether mangrove systems advance seaward, retreat landward, or undergo coastal squeeze as their seaward and landward boundaries converge. Yet global assessments have largely relied on area change, with less focus on these directional processes. This study applies a global coastline-normal transect framework to Global Mangrove Watch data for 1996, 2008, and 2020. By quantifying the seaward and landward edges together with width and centroid position, we distinguish expansion, directional migration, and contraction across multiple spatial scales. Globally, mangroves lost an average width of 77.2 m: the seaward edge retreated landward by 47.5 m, a pattern consistent with marine forcing including relative sea-level rise and coastal erosion, while the landward edge shifted seaward by 53.7 m, compatible with restricted accommodation space. Together, these opposing movements reveal widespread asymmetric contraction. Among regions exhibiting clear directional migration, 65.2% shifted seaward, including advances of up to 2.5 km in Marajó Bay, Brazil, whereas less frequent landward migration reached 5.6 km in the northern Gulf of Mexico. After 2008, the prevalence of landward migration increased, particularly across Asia and Africa. The edge trajectories constrain the set of plausible controls but do not uniquely attribute change to sea-level rise, sediment supply, topography, land use, or restoration. Conservation should therefore protect potential landward migration pathways while accounting for uncertainty in the mechanisms producing individual local trajectories.