Blue carbon

Blue carbon is primarily associated with vegetated coastal ecosystems that transfer atmospheric carbon into long-lived organic matter. Mangroves grow along tropical and subtropical shores, tidal marshes occupy temperate and polar intertidal zones, and seagrass meadows occur in shallow marine waters worldwide. Although these habitats cover a small fraction of the seafloor, they can accumulate carbon-rich sediments at rates that exceed those of many terrestrial systems.¹

Carbon is stored in living stems, roots, and leaves, but the largest reservoir is often waterlogged soil or seabed sediment. Low oxygen, slow decomposition, and repeated burial help preserve organic matter. Coastal wetlands also provide nursery habitat, shoreline protection, water filtration, and cultural and economic benefits, so blue-carbon conservation is both a climate measure and an ecosystem-management strategy.²

The central climate mechanism is the movement of carbon from the atmosphere into biomass and sediment, followed by long-term retention. Tides and plant roots trap particles, while below-ground production adds organic material to soils. Some carbon is exported into nearby waters as dissolved or particulate organic carbon; part of that material may later be buried in coastal or offshore sediments, though its final fate is difficult to measure.

Blue-carbon ecosystems can also release greenhouse gases. Waterlogged soils may produce methane and nitrous oxide, and emissions vary with salinity, temperature, nutrients, and human disturbance. Consequently, climate accounting must evaluate net greenhouse-gas effects rather than count carbon burial alone. Protecting intact habitat usually avoids immediate emissions from clearing and drainage, while restoration can rebuild storage gradually and may not fully reproduce the original carbon balance.³

Reliable blue-carbon accounting combines habitat maps, vegetation surveys, sediment cores, measurements of carbon density, and estimates of greenhouse-gas fluxes. Researchers must define the project boundary and account for carbon stored above ground, below ground, in dead material, and in adjacent sediments. Remote sensing can identify mangrove extent and monitor change, but field sampling remains necessary for carbon stocks and soil depth.

Under the Paris Agreement, countries may include coastal wetlands in national greenhouse-gas inventories when methods and data support defensible estimates. Carbon-credit projects have also developed around avoided conversion and restoration, but permanence, leakage, additionality, land tenure, and community rights require careful safeguards. A project that protects one marsh while displacement causes another marsh to be cleared has not delivered the claimed net benefit.

Blue-carbon storage is not limited to visible plant tissue: deep marsh peat and mangrove soils can contain carbon accumulated over centuries, while seagrass roots stabilize sediments that would otherwise be resuspended. Conversely, disturbing a wetland can expose buried organic matter to oxygen and accelerate carbon dioxide emissions; drainage can also create conditions for substantial nitrous-oxide release.⁴

Some coastal systems are carbon sources under particular conditions, including heavily eutrophic seagrass beds, eroding shorelines, and wetlands altered by dams or changed sediment supply. Restoration therefore depends on local hydrology, salinity, sediment delivery, and species composition rather than planting vegetation alone. Indigenous peoples and local communities often hold essential knowledge of coastal change, and durable projects generally combine ecological monitoring with secure access, fair benefits, and protection from displacement. Blue carbon is thus a field spanning oceanography, wetland ecology, soil science, and environmental governance.

These sources provide scientific assessments and methodological guidance for blue-carbon ecosystems, greenhouse-gas accounting, and coastal-wetland management.

Carbon burial

The incorporation of organic carbon into sediment where decomposition and oxidation are sufficiently slow for long-term storage.

Additionality

The requirement that claimed climate benefits would not have occurred without the intervention or project.

Leakage

A shift of emissions or habitat loss outside a project boundary that reduces its net climate benefit.

Permanence

The duration for which stored carbon remains out of the atmosphere, including the risk of later reversal.