Kirwan, M.L., Temmerman, S., Skeehan, E.E., Guntenspergen, G.R., and Fagherazzi, S., 2016, Overestimation of marsh vulnerability to sea level rise, Nature Climate Change 6, 253-260, DOI: 10.1038/NClimate/2909.
Salt marshes (Figure 1.) are prominent ecosystems where the land meets the sea along calm shorelines. They are characterized by low, broad topography, often situated behind the shelter of barrier islands or estuaries. As a result, salt marshes are intertidal, such that portions are flooded by seawater daily with the flow and ebb of tides (Figure 2.). The plants that call salt marshes home are uniquely adapted to the presence of saltwater, and are dominated by grasses and low shrubs.
Humans have historically relied and continue to rely on marsh ecosystems. Many marshes have been highly developed due to the combination of aesthetic beauty of the coastline and the natural protection from erosion and waves. Additionally, marshes are important for commercial fisheries and play a prominent role in carbon sequestration.
Recognizing the societal and ecological importance of salt marshes, scientists have spent three decades studying instances of marsh loss. The inevitable question has arisen: Will salt marshes survive sea level rise? Previous studies have emphatically answered “No!” suggesting a 20-50% loss by the year 2100. However, Kirwan and colleagues have challenged this notion, finding that salt marshes are not nearly as fragile as previously thought, and that salt marshes will not succumb to the rising sea without a fight!
The researchers begin by describing a major discrepancy. Previous investigators describe salt marshes as fragile ecosystems despite contrary evidence. For example, marshes in the United States were found to be entirely stable (not experiencing loss) between 2004-2009. Additionally, worldwide marsh elevation was found to be increasing. To drive the point home, dynamic process-based models suggest marshes can survive in places experiencing significant sea level rise (more than 1 centimeter per year compared to current global average of 0.37 millimeters per year!).
So why the discrepancy? Common assessments underestimate marsh resilience, not considering increased accretion (vertical accumulation of sediment and vegetation) or inland migration in the face of sea level rise.
To further explore this discrepancy the researchers did an analysis of previous marsh accretion rates from previous studies in North America and Europe. The goal of this analysis was to determine if the marsh sites were submerging, a criterion met when the vertical accretion of the marsh site is being outpaced by the rate of sea level rise. Each site was matched to a tide gauge, which provided site-specific sea level rise rates. This analysis alone confirmed what many previously had feared: 19-36% of the marshes are submerging!
But wait! This analysis is the same as the common assessment and greatly simplifies a very dynamic ecosystem. A critical finding from the analysis was the difference in marsh accretion rates in sites characterized as low marsh and high marsh (Figure 2.). The low marsh is the portion of a salt marsh that is flooded by seawater with daily tidal changes, whereas the high marsh sits above the high tide line, and is only flooded during very high spring tides or during storms. Low marsh sites were found to accrete (build up) at rates of 6.9 mm/year, more than double that of high marsh sites. Rising sea level will cause higher high tides, a change that transition high marsh areas to low marshes. At first glance this may seem harmful to marsh survival. But remember, low marshes accrete at rates more than double that of high marshes! As a result, marshes will be more resilient to sea level rise by building up much faster than rising sea level (recall the current global average SLR rate is 3.7 mm/year). Restricting their analysis to only low marsh sites, scientists found that fewer than 5% of marshes are submerging.
A major finding from the analysis is that many conventional models underestimate the resiliency of salt marshes to sea level rise. Salt marshes lend their adaptability to ecogeomorphic feedbacks, the complex interactions between the physical shape and size of a marsh, the availability of sediment and the nutrients and plants that grow on the marsh. A simple example of ecogeomorphic feedback can be conceptualized by a sand dune (Figure 3.). Grasses growing on a sand dune help to trap sand carried by the wind, causing the dune to grow in size. In the case of a marsh, increasing rates of sea level rise and flooding of the marsh cause increased sediment deposition (causing the marsh to build vertically). More seawater flooding also makes plants more productive, in turn improving the capture of more sediment and reducing wave energy and erosion of the marsh. Besides vertical accretion, marshes are able to retreat inland in the presence of rising sea level, transitioning previously forested areas to high marshes. The best models for predicting how salt marshes respond to future sea level rise account for these complex ecogeomorphic feedbacks.
The study also finds that marshes will remain resilient against sea level rise through the end of the century. Models predict that the rates at which marshes vertically accrete will match the accelerations in sea level rise predicted by the latest IPCC reports, which by the end of the century, may be more than 20 mm/year (webpage).
Are any marshes vulnerable? The researchers described several scenarios, which put marshes at risk for submergence. In the presence of very high rates of sea level rise, greatly exceeding the projections by the IPCC, marshes may be unable to keep up with rising sea level. Other scenarios involve the failure of ecogeomorphic feedback conditions, where sediment availability is very low, or human-made structures prevent inland migration of the marsh. Lastly, salt marshes with very low tidal range are also at a higher risk for being submerged.
Salt marshes are extremely important ecosystems. Humans have relied on the protective nature of salt marshes for seaside development, supporting tourism and commercial fisheries. Many species, including threatened shorebirds, use salt marshes as nesting grounds. Any threats to salt marsh stability should be carefully evaluated due to the economic and ecologic significance of this ecosystem. It is rare to report something positive when discussing future sea level rise, however, this study finds that salt marshes are more resilient to an inevitable future than previously reported.