Blue carbon functional collapse precedes physical marsh retreat

Abstract Blue carbon ecosystems are critical for climate mitigation, yet their persistence is threatened by global coastal erosion. Conservation strategies often rely on observing shoreline retreat as an indicator of ecosystem vulnerability, implicitly assuming geomorphic stability ensures carbon retention. Here we show that the functional collapse of tidal marsh carbon sinks can occur significantly before physical retreat becomes evident. Combining field evidence from the dynamic Jiangsu coast with process-based modelling, we reveal that fringing effects (i.e., intense hydro-geomorphic disturbances that trigger cascading feedback) drive an unexpected transition from carbon sink to source where carbon losses outpace burial (up to 119%). Crucially, we identify a lag of months to a decade between marsh retreat and the sink–source transition, suggesting that apparent biogeomorphic stability masks a growing climate debt. Global carbon budgets may overestimate coastal storage and effective management requires pre-emptive action prior to development of a net local source.