Award: OCE-1427282

Nearly half of the world’s population lives within 100 km of the coast, the area ranked as the most vulnerable to climate-driven sea-level rise. Accelerated rates are projected to increase sea levels by 0.5 – 2 m by 2100. Such massive changes could transform both the ecological and social dynamics of low-lying coastal areas. In this project, we used long-term data and modeling to examine the vulnerability of salt marshes to sea-level rise in three U.S. Atlantic coastal sites that vary with respect to sediment supply, tidal range and human impacts. To keep pace with sea-level rise, salt marshes rely on sufficient sediment supply to increase vertical elevation and landward migration. Marshes unable to keep pace become vulnerable to drowning. Key findings of the project include : (1) Marshes with "elevation capital" (high initial elevation relative to sea level) can remain stable through 2100 even if vertical sediment accumulation rates are lower than sea-level rise rates. However, elevation capital does not remove the long-term threat of marsh drowning beyond 2100. This is illustrated by model simulations out to 2300 that indicate widespread drowning, despite their stability in the near and intermediate future. Together these results suggest that topography (initial marsh elevation relative to mean tidal level, elevation of adjacent uplands) is the most important variable driving the fate of wetlands out to 2100, though more typical assumed biophysical drivers (e.g. sediment supply, plant biomass) likely determine vulnerability over longer time scales. The ability to migrate inland is the single biggest factor influencing marsh success in response to sea-level rise on century time scales. (2) Storm surge events are important in controlling rates of sediment deposition and the vertical accretion of marshes. High wind events disproportionately promote sediment deposition, which has implications for how we project marsh vulnerability. Changes in storm characteristics or frequency, something typically not captured in marsh deposition models, may play an outsized role in marsh vulnerability to climate drivers. Ultimately, placing marsh evolution in the context of the sediment budget for the entire intertidal landscape is critical to accurately forecasting marsh vulnerability to sea-level rise. (3) Economic models suggest that current actions to preserve marsh migration zones may be sub-optimal, with too little emphasis being given to the preservation of higher-elevation agricultural land that is expected to allow effective marsh migration at higher sea-levels. (4) Marsh ecosystem services broadly categorized as "cultural" were most salient across all three LTER domains, and were recognized by diverse stakeholders as more important than the economic benefits of marshes. These cultural services are broadly characterized as those that support experiences and cognitive processes that contribute to personal well-being or communal fulfillment. The combined data and model development in this project has advanced our understanding of the mechanisms underlying salt marsh vulnerability to accelerated sea-level rise and enhanced our ability to predict and manage long-term change. Last Modified: 01/10/2021 Submitted by: Karen Mcglathery