Award: OCE-1426981

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 (SLR). Salt marshes are intertidal habitats that provide a buffer for coastal communities to SLR and are also valued for many other ecosystem services, including wildlife habitat, nutrient cycling, carbon sequestration, aesthetics, and tourism. They are highly dynamic systems that have kept pace with changes in sea level over millennia. However, projected rates of SLR and increased human modification of coastal watersheds and shorelines may push marshes past a tipping point beyond which they are lost. This project examined the comparative vulnerability of salt marshes to SLR in three U.S. Atlantic coastal sites that vary with respect to sediment supply, tidal range and human impacts. It leveraged the long-term data, experiments and modeling tools at three Atlantic Coast Long-Term Ecological Research sites (in MA, VA, GA), and addressed the broad interdisciplinary question "How will feedbacks between marsh response to SLR and human adaptation responses to potential marsh loss affect the overall sustainability of the combined socio-ecological systems?" As a sub-contract to this project, researchers at the Virginia Institute of Marine Science created a new numerical model that predicts how salt marshes will change in response to sea level rise and human impacts. We applied the model to three coastal regions and discovered that the initial elevation of the marsh and the availability of open land adjacent to marshes were the two biggest factors influencing marsh survival. Our model predicts that marshes that begin at elevations relatively high above sea level (MA, GA) survive beyond 2100, regardless of the biophysical characteristics. This is illustrated by marsh survival under all sea level scenarios despite measured and modeled accretion rates that are less than the rate of sea level rise. In contrast, our model predicts at least partial marsh submergence for marshes that begin at low elevations above sea level (VA). Marshes here must migrate inland to survive sea level rise, and at the fastest rates of sea level rise are unable to do so fast enough to compensate for widespread loss of existing marsh. Together, the results suggest that there are typically two phases of marsh response to sea level rise, where marshes expand under low sea level rise rates before drowning under faster rates of sea level rise. Expansion occurs if marsh migration into adjacent uplands occurs faster than the drowning of existing marsh, but does not ensure ultimate long-term marsh survival. These model predictions were confirmed with observations of marsh migration into retreating coastal forests up and down the Atlantic coast and motivated a new scientific field related to sea-level driven land conversion. Global modeling of marshes and mangroves indicated that the ability to migrate inland in the face of growing anthropogenic flood-control structures is the single biggest factor influencing wetland response to sea level rise. Therefore, our work indicates that marsh survival in the face of sea level rise could be enhanced by facilitating marsh migration into adjacent uplands, rather than protecting existing marshland alone. This research supported 3 postdocs, 2 graduate students, and several undergraduate students. The results were published in more than 10 scientific journals, presented at scientific meetings, discussed in stakeholder meetings that included land managers and policy makers, and distributed to the general public through more than 10 popular media articles (e.g. New York Times, Time Magazine). Data from the project are available in the VCR Data Catalog (https://www.vcrlter.virginia.edu/home2/?page_id=105) and through BCO-DMO. Last Modified: 01/11/2021 Submitted by: Matthew Kirwan