Critical transition in barrier island’s state and the loss of barrier resiliency

Barrier islands cover a large portion of US coasts, support unique ecosystems and beach communities, and protect inland infrastructure from direct storm impacts. Based on their elevation, they can exist in three possible states: relatively protected “high-elevation” barriers with well-developed coastal dunes, vulnerable “low-elevation” barriers without dunes, and “mixed” barriers with scattered dunes separated by overwash fans. We analyzed data from 16 barriers along US coasts and found that the island state is controlled by the barrier ‘elevation capital’, loosely defined as the elevation of the barrier excluding dunes. We find a critical value of the elevation capital, around 0.5m, below which eroded dunes cannot recover and the barrier remains low-elevation, a value consistent with predictions from a recent analytical model of the barrier’s stochastic dynamics. Under the current sand supply, we find that several barriers already reached the tipping point for a potentially permanent “low-elevation” state. We predict this transition to become widespread under expected trends in sea level rise, with important implications for the survival of the whole barrier system.