In recent years, there has been a flurry of restoration projects aimed at mitigating the impact of coastal storms using salt marshes and vegetated surfaces (called “living shorelines”). Based on a large dataset of salt marsh erosion and wave measurements collected all around the world, we find that erosion rates of marsh boundaries and incident wave energy collapse into a unique linear relationship. Our result clearly shows that long-term salt marsh deterioration is dictated by average wave conditions, and it is, therefore, predictable. Violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. This result is of high value for coastal restoration projects and the use of living shorelines to mitigate storms effect.