Marshes are among the most productive ecosystems on Earth, delivering services that protect coastlines, clean water, support fisheries, and store enormous amounts of carbon. Despite covering a relatively small area of the planet, their outsized contributions to both human economies and natural systems make them far more valuable than their appearance might suggest. Yet globally, salt marshes alone shrank by an area double the size of Singapore between 2000 and 2019, lost at a rate of 0.28% per year.
Carbon Storage That Outpaces Forests
Marshes accumulate organic carbon at an average rate of about 250 grams per square meter per year. That’s five times higher than the maximum rate found in any other terrestrial ecosystem, which tops out around 50 grams per square meter annually. Even marine ecosystems like seagrass beds fall short, averaging 120 to 200 grams. This makes marshes one of the most efficient carbon sinks on the planet, locking away CO₂ in waterlogged soils where it can remain stored for centuries.
The key is the waterlogged environment itself. In a forest, dead plant material decomposes relatively quickly in the presence of oxygen, releasing carbon back into the atmosphere. In a marsh, saturated soils create low-oxygen conditions that slow decomposition dramatically. Carbon-rich sediments build up layer by layer over time, essentially burying carbon underground. When marshes are drained or destroyed, that stored carbon gets exposed to air and re-enters the atmosphere, turning a carbon sink into a carbon source.
Natural Flood and Storm Protection
Marsh vegetation acts as a physical buffer against incoming waves. During storm surge conditions, up to 60% of wave reduction can be attributed directly to the plants themselves, according to experiments published in Nature Geoscience. Dense stems and leaves create friction that absorbs wave energy before it reaches the shore, lowering wave heights and reducing the force of water hitting coastal communities.
The financial value of this protection is staggering. During Hurricane Sandy, coastal wetlands in the northeastern United States prevented an estimated $625 million in direct flood damages. In Ocean County, New Jersey alone, marshes contributed 36% of the state’s total wetland-related savings. Modeling with synthetic storms in that county found that salt marshes reduce average annual flood losses by 16%, with even greater protection at lower elevations where flooding hits hardest.
Traditional hard infrastructure like seawalls and bulkheads might seem like a stronger defense, but they come with serious tradeoffs. These structures disconnect marshes from the intertidal zone, interrupt natural sediment movement, and deflect wave energy to adjacent shorelines, often worsening erosion nearby. Seawalls support 23% less biodiversity and 45% fewer organisms than natural shorelines. Living shorelines that incorporate marsh vegetation trap sediment, reduce erosion, and continue to provide habitat and carbon storage at the same time.
Water Filtration at Scale
Marshes function as natural water treatment systems. As water flows through marsh soils and vegetation, plants and microorganisms pull excess nutrients out of the water column. A 12-year study of treatment wetlands in the midwestern United States found that these systems removed 71 to 85% of dissolved phosphorus from agricultural runoff. Nitrogen removal follows a similar pattern, with marsh soils hosting bacteria that convert harmful nitrogen compounds into harmless gas that returns to the atmosphere.
This matters because excess nitrogen and phosphorus from farms, lawns, and wastewater are the primary drivers of dead zones in coastal waters. When these nutrients reach bays and estuaries unchecked, they trigger algal blooms that deplete oxygen and kill marine life. Marshes positioned between agricultural land and open water intercept much of this pollution before it reaches sensitive ecosystems. Losing marshes doesn’t just mean losing habitat. It means losing a filtration system that would cost billions to replicate with engineered infrastructure.
Fisheries and Wildlife Habitat
More than 75% of the commercial fish and shellfish harvest in the United States depends on coastal wetlands for food or habitat at some point during the animals’ life cycles. For recreational fishing, that number climbs to 90%. Shrimp, blue crabs, flounder, and many other species spend their juvenile stages in the shallow, sheltered waters of marshes, where dense vegetation provides cover from predators and abundant food from decomposing plant material. Without healthy marshes, these populations lose the nursery habitat they need to sustain themselves.
Marshes also support species found nowhere else. The light-footed clapper rail, California least tern, and salt marsh bird’s beak (a rare flowering plant) are all federally endangered species tied directly to salt marsh habitat. The Belding’s Savannah sparrow, a state-listed endangered bird, nests exclusively in coastal marshes. These species can’t simply relocate when marshes disappear. Their survival depends on the specific conditions that only intact marsh ecosystems provide: tidal flooding patterns, particular vegetation structure, and the food webs that develop within them.
Shoreline Stabilization and Erosion Control
Fringing salt marshes hold coastlines in place through a combination of root systems and sediment trapping. Marsh grasses send dense root networks into the soil, binding it together and making it resistant to the pull of tides and waves. Above ground, stems slow water flow enough that suspended sediment settles out and accumulates, gradually building the marsh surface higher. This process creates a self-reinforcing cycle: more sediment means more elevation, which supports more vegetation, which traps more sediment.
This ability to grow vertically is also what allows marshes to keep pace with rising sea levels. NOAA research shows that marshes in study areas along the U.S. coast are currently gaining elevation at rates between 2.3 and 9 millimeters per year. With current sea level rise at roughly 2.5 millimeters per year, many marshes can still keep up. But this balance is fragile. If sea levels rise faster than marshes can build, or if sediment supply is cut off by dams or development, marshes drown and the shoreline protection they provided vanishes.
Economic Value of Marsh Ecosystems
The global monetary value of natural wetland ecosystem services is estimated at $47.4 trillion per year, representing 43.5% of the value of all natural biomes combined. Coastal wetlands, despite making up only 15% of global wetland area, deliver 43.1% of that total value, roughly $20.4 trillion per year. These figures capture the combined worth of flood protection, water purification, carbon storage, fisheries support, recreation, and other services that would otherwise need to be provided through human-built infrastructure or simply lost.
These numbers can feel abstract, but the Hurricane Sandy example brings them into focus. A single storm, a single region, and wetlands prevented $625 million in property damage. Scale that across every hurricane season, every coastline, every year, and the economic case for marsh conservation becomes difficult to argue against. Restoring or protecting an acre of marsh is almost always cheaper than building the seawalls, water treatment plants, and fish hatcheries that would be needed to replace its functions.
Why Marshes Keep Disappearing
Between 2000 and 2019, the world lost a net 1,453 square kilometers of salt marsh. Coastal development, agricultural conversion, pollution, and rising seas all contribute. In many areas, hard infrastructure like seawalls blocks the natural landward migration that marshes need to survive as water levels rise, squeezing them between rising seas on one side and development on the other.
Sediment starvation is another growing problem. Dams on rivers trap the sediment that marshes need to build elevation. Without fresh sediment input, marshes can’t keep pace with sea level rise and gradually convert to open water. Once a marsh is gone, the carbon it stored begins releasing into the atmosphere, the species it supported lose their habitat, and the coastline it protected becomes exposed. Rebuilding a marsh from scratch is possible but slow, expensive, and rarely produces an ecosystem as functional as the original.