Coastal Wetland Resilience, Accelerated Sea‐Level Rise, and the Importance of Timescale

Abstract

Coastal wetlands (marshes and mangroves) are among the most valuable ecosystems on the planet (Barbier, 2019; Costanza et al., 2014), yet they are threatened by accelerated sea-level rise and other human impacts. Coastal wetlands occupy extensive portions of low-elevation coastal zones (LECZs; commonly defined as <10 m above mean sea level) that are home to all or portions of 21 of the 33 largest megacities (population >10 million) worldwide (https://digitallibrary.un.org/record/3799524). Recent work has shown that LECZs are considerably lower in elevation than previously assumed (Kulp & Strauss, 2019) and several LECZs may subside more rapidly than commonly thought (Keogh & Törnqvist, 2019). Given that the acceleration of global sea-level rise will continue well into the future (Oppenheimer et al., 2019), predicting the extent and health of coastal wetlands is a topic of great import. A recent global-scale prediction of coastal wetland extent for the remainder of this century (Schuerch et al., 2018) has suggested that even under pessimistic climate scenarios, and provided that there is room for landward migration, coastal wetland area will generally increase, echoing previous model studies that have suggested that coastal marshes can keep up with rates of sea-level rise as high as 10-50 mm yr −1 (Kirwan et al., 2016). These results stand in stark contrast with studies based on the geologic record that show tipping points for marsh drowning in the Mississippi Delta (USA) at rates of relative sea-level rise (RSLR) of ∼3 mm yr −1 (Törnqvist et al., 2020) and an inability of mangroves worldwide to initiate sustained accretion when rates of RSLR exceed ∼6 mm yr −1 (Saintilan et al., 2020). The main purpose of this Commentary is to examine these seemingly contradictory outcomes. We first discuss the concept of accommodation that plays an increasingly important role in models that seek to predict coastal wetland change until the end of this century (e.g., Schuerch et al., 2018). We then make the case that these models must be constrained by the stratigraphic record (i.e., observations over centennial to millennial timescales) and we discuss key reasons why instrumental observations (i.e., annual to decadal timescales) cannot fully capture the outcome Abstract Recent studies have produced conflicting results as to whether coastal wetlands can keep up with present-day and future sea-level rise. The stratigraphic record shows that threshold rates for coastal wetland submergence or retreat are lower than what instrumental records suggest, with wetland extent that shrinks considerably under high rates of sea-level rise. These apparent conflicts can be reconciled by recognizing that many coastal wetlands still possess sufficient elevation capital to cope with sea-level rise, and that processes like sediment compaction, ponding, and wave erosion require multidecadal or longer timescales to drive wetland loss that is in many cases inevitable. Plain Language Summary The rapid, climate-driven acceleration of global sea level thr...

Reconciling models and measurements of marsh vulnerability to sea level rise

“…Our focus on vertical accretion and SSC represents a common, but imperfect, approach to assessing wetland vulnerability. Volumetric sediment fluxes are potentially better metrics of wetland vulnerability, and its dependence on sediment supply, because they account for spatial gradients within marshes and the source of suspended sediment (Ganju et al 2017;Törnqvist et al 2021). SSC is a poor predicter of marsh vulnerability and sediment supply in systems where sediment cannot reach the interior of marshes Duran Vinent et al 2021) and in systems with significant resuspension and edge erosion (Ganju et al 2013).…”

“…Numerical models typically focus on basic feedbacks between inundation and sediment transport that allow projections of elevation building through time in response to changing environmental conditions (Fagherazzi et al 2012;Kirwan et al 2016). Yet, models are inherent simplifications of realworld process that often rely on basic treatment of vegetation, nonvolumetric sediment budgets, lack of spatial resolution, and sensitivity to poorly constrained parameters such as the concentration and settling velocity of suspended sediment (Wiberg et al 2020;Törnqvist et al 2021). Field measurements directly measure rates of vertical accretion influenced by a more complete suite of processes (DeLaune et al 1978;Jankowski et al 2017;Parkinson et al 2017), but can be difficult to apply to other sites.…”

Migration and transformation of coastal wetlands in response to rising seas

“…In figs. S1 and S2, we illustrate the potential for loss of tidal saline wetlands and freshwater wetlands across the conterminous United States under a worst-case scenario, where saltwater intrusion leads to the collapse of freshwater wetlands (39)(40)(41) and biogeomorphic feedbacks are not able to compensate for high rates of sea level rise (19)(20)(21), partially due to the inability to fill emerging accommodation space (12). The risk of catastrophic, landscape-scale wetland loss is especially high along the Gulf of Mexico and south Atlantic coasts, with hot spots in the Mississippi River Delta, Everglades, Albemarle-Pamlico, and Chesapeake Bay estuaries.…”

Resilience of U.S. coastal wetlands to accelerating sea level rise

“…Protecting or creating space for lateral migration (accommodation) faces significant legal and land use zoning challenges. More fundamentally, the potential for accommodation is ultimately limited by sediment supply, so this study's implicit assumption of independence between accommodation and SEC is unlikely to hold in the long run; under accelerating SLR, sustained net gains from wetland migration should only be possible with increasing sediment supply (reflected in SEC rates) (Törnqvist et al 2021).…”