By: Molly Bost, Jenny Davis, and Leanne Poussard (NOAA NCCOS)
Global carbon emissions have significantly increased in the last century, largely due to industrial processes and the burning of fossil fuels.1 Emissions of carbon and other greenhouse gasses have increased global mean surface temperature, and alarmingly, 2013–2021 rank among the ten warmest years on record.2 This warming trend has fueled international focus on limiting emissions and increasing the number and capacity of natural “carbon sinks”: ecosystems that take up and store atmospheric carbon dioxide (CO2). Coincidentally, there is growing interest in the restoration of coastal vegetated habitats like salt marshes for their ability to provide protection against coastal flooding and shoreline erosion. When utilized for their protective benefits, these habitats are often referred to as “natural infrastructure.” Coastal vegetated habitats are exceptionally efficient at taking up and burying CO2; as a result, their use as natural infrastructure may have the added benefit of increasing carbon burial. The National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Coastal Ocean Science (NCCOS) are working to assess the carbon implications of natural infrastructure by measuring the impacts of salt marsh restoration on carbon uptake and storage.
Salt Marshes as “Blue Carbon” Ecosystems
“Blue carbon” ecosystems include a variety of coastal vegetated habitats like seagrass meadows, salt marshes, and mangrove forests. While it is well known that these ecosystems provide numerous ecosystem services, they have been recently recognized for their potential role in climate mitigation due to the unusually high efficiency with which they take up and store carbon. Salt marshes account for about one-third of carbon stored in soil globally, despite covering less than 10% of the earth’s land surface.3 In addition to their value as carbon sinks, salt marshes also provide erosion control and flood protection for coastal communities, are habitat for numerous aquatic and terrestrial species, and help to enhance water quality. In recent decades, the total extent of blue carbon habitat has declined due to coastal development and sea level rise. As these habitats disappear, so do the valuable services that they provide.
For a marsh to keep up with rising sea levels, it must build elevation to avoid drowning. Marshes do this by trapping sediment particles that high tides naturally deliver to the marsh surface and by producing root material, which contributes to soil volume. Through a combination of these processes, many marshes have been able to keep up with sea level rise for hundreds of years and, in the process, have built deep, carbon-rich soils. This capacity to continuously accumulate carbon-rich soil is what makes wetlands so valuable from a carbon-burial perspective. Rates of sea level rise are predicted to accelerate in the coming decades, and this acceleration will make it harder for marshes to build elevation at a fast enough pace to keep up. When marshes do not keep pace with sea level rise, they eventually become so waterlogged that the plants drown and the carbon-rich soils erode, releasing the stored CO2 into the atmosphere. For this reason, the April 2022 report from the Intergovernmental Panel on Climate Change (IPCC) strongly endorses reducing ecosystem destruction and promotes restoration actions as a way to mitigate further carbon emissions.
Restoration of Degraded Marshes Using Dredged Sediment
Marshes that are challenged to keep pace with sea-level rise may be aided by the application of dredged sediments. This approach involves spreading sediments dredged from navigation channels over the surface of low-lying marshes to raise the elevation to a level that is more favorable for plant growth. Restoring marshes through sediment application has been shown to decrease their vulnerability to future sea level rise. However, the carbon-burial implications of sediment addition to marshes is not currently understood. To help fill this knowledge gap, NOAA’s NCCOS and the US Army Corps of Engineers’ (USACE) Engineering With Nature® program (EWN®) are partnering to study the carbon storage capabilities of a marsh restored through the application of dredged sediments at Deal Island, Maryland.
Deal Island, MD
In the fall of 2024, a deteriorating marsh site at Deal Island, MD, will be restored using sediment dredged from the nearby lower Wicomico River. Scientists have spent the last few years collecting baseline data at the site, including sediment cores from the marsh platform. From a sediment core, scientists can determine how quickly sediment and organic matter have been accumulating over the last few hundred years. This information will offer insight into how the marsh has evolved through time and provide a sense of its “natural state” against which to compare postplacement conditions. Continued monitoring of carbon burial at the site after sediment placement will allow for an evaluation of the impact of restoration through beneficial use on the future value of this marsh as a carbon sink. The understanding gained through this effort will be fundamental to estimating the potential value of natural-infrastructure approaches for offsetting carbon emissions.
NCCOS and EWN® work collaboratively to harness the power of nature to achieve a broad suite of engineering, environmental, and socio-economic benefits. This collaboration focuses on the use of natural infrastructure, also known as natural and nature-based features (NNBF), such as beaches, dunes, islands, wetlands, and reefs, to increase the resilience of coastal regions to storms and sea level rise. Together, NCCOS and EWN are providing the research, science, engineering, and guidance to help coastal communities know how, where, and when to best construct NNBF. See the Project Page on the NCCOS website for more information.
Contacts and Related Links
1. Boden, T.A., G. Marland, and R.J. Andres. 2017. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. https://doi.org/10.3334/CDIAC/00001_V2017.
2. NOAA National Centers for Environmental Information, Monthly Global Climate Report for Annual 2021, published online January 2022, retrieved on December 13, 2022 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202113.
3. Melton, J.R. et al. 2012. Present state of global wetland extent and wetland methane modelling: conclusions from a model intercomparison project (WETCHIMP). Biogeosciences Discuss 9(11577-11654). https://doi.org/10.5194/bg-10-753-2013.