Onset of runaway fragmentation of salt marshes

•The sea level rise threshold for marsh drowning is lower than previously thought•No equilibrium after crossing the local drowning•Resilience of microtidal marshes depends weakly on inorganic sediment supply•Predicted widespread loss through runaway fragmentation We use a simplified formulation transport across to explain why are most vulnerable in environments (>60% salt US). find that fraction receives scales positively with tidal range, so only small portions receive significant sediment. Although vulnerability has traditionally been thought depend flooding frequency and suspended concentration channels, we show must instead rely accumulation plant roots build soil. This organic maximum rates comparable or current relative measured at many landscapes. Thus, currently tipping point global regardless supply. 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