How much carbon does coastal erosion move from US marshes into the ocean? More than 660,000 metric tonnes each year along the Gulf and Atlantic coasts, according to a NASA-supported study. New marsh growth offsets part of that movement, but the researchers still estimate a net annual loss of about 380,000 tonnes.
What did the study measure?
The researchers combined Landsat satellite imagery with US Geological Survey lidar elevation data to track changes in coastal wetlands from Texas to Maine between 1985 and 2022. The long record allowed them to identify where marshes disappeared, where new marsh formed and how much carbon was likely displaced as shorelines changed.
Coastal wetlands store carbon in living plants and in waterlogged soils. When an eroding edge collapses or vegetation is submerged, some of that stored material is carried into nearby waters. Measuring this lateral movement is essential because it sits between the land, atmosphere and ocean—the boundaries where carbon accounting is often most difficult.
What are the headline numbers?
- Estimated gross carbon moved into the ocean: more than 660,000 metric tonnes per year.
- Estimated net loss after accounting for new marsh growth: about 380,000 tonnes per year.
- Study area: the US Gulf Coast and Eastern seaboard, from Texas to Maine.
- Observation period: 1985 to 2022.
The difference between gross and net figures matters. Marshes do not only vanish; they can also expand or shift. Counting erosion without new growth would overstate the system-wide loss, while looking only at total marsh area could miss the carbon moved through constant shoreline turnover.
Does all of that carbon become atmospheric carbon dioxide?
No. NASA’s summary emphasises that most of the displaced carbon enters the ocean rather than being emitted directly into the air. Once in the water, it can follow several paths: burial in sediments, transport offshore, consumption by organisms or conversion into dissolved gases.
That uncertainty is why the result should not be described as an annual atmospheric emission. It is a measurement of carbon leaving eroded marsh terrain and entering a more complex coastal-ocean system. Researchers at NASA’s Goddard Institute for Space Studies are developing models to better represent these exchanges.
Why did the Mississippi River Delta stand out?
The study found major regional differences. The Mississippi River Delta moved more coastal carbon than the entire Eastern seaboard, reflecting both its enormous marsh area and its rapid landscape change. The researchers identified increases in erosion after hurricanes Katrina in 2005 and Ida in 2021, when storms uprooted or submerged extensive vegetation.
A hurricane does not create every underlying vulnerability. Subsidence, sea-level rise, altered sediment delivery and development can shape how a coast responds. But the satellite record can reveal abrupt changes associated with major events and place them within decades of slower change.
Why are satellites useful for coastal carbon research?
Field sampling provides detailed information at individual sites, but coastlines are too extensive and dynamic to measure everywhere from the ground. Landsat offers repeated observations across decades, while lidar supplies precise elevation information. Combining them helps researchers distinguish low-lying marsh from open water and track shifting boundaries consistently.
The approach also creates a repeatable baseline. Future imagery can show whether restoration, storms or rising seas alter the pace of loss, although estimates still depend on assumptions about how much carbon different marsh soils and vegetation contain.
What does the finding change?
Coastal wetlands are often discussed as “blue carbon” stores because they can accumulate organic material for long periods. The new work highlights the other side of that balance sheet: erosion can export stored carbon across a shoreline even when it does not immediately reach the atmosphere.
Better estimates of that movement can improve Earth-system models and help restoration planners understand where protecting a marsh edge may preserve both habitat and stored carbon.
What is the practical takeaway?
The study does not say that 660,000 tonnes of carbon instantly becomes carbon dioxide. It shows that coastal erosion is a substantial, measurable pathway in the carbon cycle—and one that varies sharply by region and storm history. Tracking where the material goes next is the crucial remaining question.
Source note: Read NASA Science’s 24 August 2026 research brief.