Coastal wetlands, also known as “blue carbon” ecosystems, include mangroves, tidal salt marshes and seagrass meadows. As the Nature Conservancy reports, there are 13.8 million hectares of mangroves, but there are still gaps in the data for mapping the extent of salt marshes and seagrasses: the estimated total cover of these three ecosystems is between 35 and 120 million hectares globally – less than 1% of the world’s total land area. Seagrass beds, mangroves and tidal marshes store large amounts of carbon. They draw in carbon as they grow, and much of this is later transferred into the rich organic soils held by their roots. That carbon can remain in the soil for thousands of years, making it one of the longest-term climate mitigation solutions, making wetland protection, management, and restoration important natural climate solutions.
Coastal wetlands are some of the most endangered habitats on the planet, despite providing valuable climate adaptation services such as flood attenuation and wastewater treatment services, erosion control, buffering against sea level rise and storm damage. They also support biodiversity, and have significant social, socio-economic and environmental co-benefits.
Around the globe and throughout the United States, many coastal wetlands are converted for agriculture, aquaculture or urban development. The loss of healthy wetlands releases stored carbon into the atmosphere. Polluted run-off can also degrade the health of wetlands, leading to an eventual release of carbon trapped in the soil.
Avoiding coastal wetland conversion is a low-cost climate mitigation pathway, leading to avoided greenhouse gas emissions. Many interventions such as establishing protected areas, improving land tenure and enforcing land-use laws can be put into place immediately. Preventing conversion and maintaining the health of coastal wetlands will allow these areas to continue storing and absorbing carbon from the atmosphere.
Targeted adaptive management efforts can aim to protect and enhance complementary wetland ecosystem functions and services including fish and wildlife production, habitat for rare and endangered species, shoreline protection against wind and waves, aesthetics and green space, water storage for flood protection, groundwater recharge, water filtration and pollution control; and carbon sequestration. As the American Society for Wetland Managers reports, all types of wetlands are carbon sequestering systems (aka “carbon sinks”), from temperate freshwater wetlands to boreal peatlands. That means that wetlands have the ability to store excess carbon (via photosynthesis) from the atmosphere – one of the primary components of greenhouse gases and a driver of climate change. Drainage and degradation of wetlands can release significant amounts of this stored carbon back into the atmosphere in the form of methane and reduce the ability of wetlands to sequester additional carbon.
Better management practices can help protect these stores of carbon and the ability of wetlands to sequester it. These practices can include re-vegetation, enhanced buffers, and planned retreat, as well as nutrient control. As USDA details, some wetlands can be sources of greenhouse gas emissions, but interventions such as control of nitrogen, maintaining hydrology, and silvicultural water management in wetlands can lead to higher biomass production, which may partially offset increased soil organic matter oxidation and direct N2O emissions. While carbon accounting is relatively new in wetland management projects, long-standing wetland stewardship initiatives are in place across the country that support a variety of objectives, including improving water quality, providing habitat, and protecting wetland systems from direct human pressures and disturbances.
Many wetland systems are degraded and in need of restoration. Efforts to restore mangroves, salt marshes and seagrasses are already underway in many regions, and there are large areas, particularly of abandoned or unproductive aquaculture where restoration would yield rapid returns in both carbon and co-benefits. Coastal wetlands such as mangroves, tidal marshes, or seagrass beds can be restored by reducing pollution, replanting lost vegetation and/or by repairing the natural flow of water. While seagrass restoration is largely dependent on improving on-shore watershed and nutrient management practices, which relies on policies can be expensive and often take many years to implement fully, increasing data indicates these projects may also result in localized ocean acidification reductions, yielding additional benefits for nearby marine and terrestrial systems.
As USDA reports, the effect of restoring both forested and grass wetlands will lead to carbon sequestration and characteristic CH4 emissions. The extent to which carbon sequestration, organic matter turnover, and gas fluxes return to rates typical for the wetland type depends on many factors, particularly the degree of alteration, time since restoration, hydrology, and development of the vegetation. However, in general, restored sites will be carbon sinks due to sequestration in the developing biomass (e.g., forest stand) and soils.
Restoring wetlands can be straightforward from a technical point of view. Mangroves, for instance, are easily planted, but in many cases restoring the hydrology alone is all that is needed to allow natural recolonization. However, the opportunity cost for such restoration is sometimes high where former wetlands are now developed, or used in productive aquaculture. Re-establishing coastal wetlands can be a relatively high-cost pathway although it varies according to ecosystem and geography. For example, mangrove restoration in developing countries is low cost compared to tidal marsh restoration in the United States. Despite high costs associated with tidal marsh restoration, some land trusts such as the Elkhorn Slough Foundation and the Land Trust of Santa Cruz County are already engaging in wetland restoration projects for a variety of management aims. Where priorities align, opportunities exist to incorporate carbon sequestration benefits into existing coastal wetland restoration efforts.
For salt marshes in particular, which have high carbon sequestration rates, there is great restoration potential in the U.S., which contains more than 30% of the world’s salt marshes. Restoring these systems can yield numerous benefits to ecosystem services. In the Lower Mississippi River Valley, The Conservation Fund’s Go Zero program has focused on restoring deciduous forested wetland ecosystems. While these systems serve critical roles in the watershed by reducing the risk and severity of flooding to downstream communities by providing areas to store floodwater, carbon sequestration benefits are also a positive result from these ongoing restoration efforts. You can learn more about land trust projects underway to restore wetlands for numerous management objectives here.