Beyond 2100: Elevation capital disguises salt marsh vulnerability to sea-level rise in Georgia, USA

Abstract

Salt marshes rely on sufficient sediment inputs and room for lateral migration to maintain vertical and lateral stability under sea-level rise. As the global rate of sea-level rise accelerates, marshes unable to keep pace become vulnerable to drowning. We evaluated the long-term response of a salt marsh in Georgia, USA, to historical (1935–2018) and future projected rates of sea-level rise. We expected the marsh to be resilient because it receives high sediment inputs and has room to migrate landward. However, sediment cores show marsh accretion (1.55 mm y−1) is lower than the historical rate of sea-level rise (3.25 mm y−1) and that rates are independent of elevation. Results from a vertical accretion model show that while marsh area is stable through 2100 under historical and high sea-level rise scenarios, the marsh relies on elevation capital to maintain its extent under a high rate of sea-level rise. The marsh rapidly loses area beyond 2100 as it depletes its elevation reserve. By 2160, only 12% of the initial marsh area remains. Our results demonstrate that while elevation capital can extend the period of time a marsh maintains its areal extent, it does not remove the long-term threat of drowning when marsh accretion cannot keep pace with sea-level rise.