Blue Carbon: Mangroves, Salt Marshes and Seagrass
Mangrove forests, salt marshes and seagrass meadows cover a thin strip of the world’s coastline, but they hold carbon in a way most land ecosystems cannot: below ground, in waterlogged sediment where it breaks down slowly. Lose the ecosystem and that carbon can return to the air. Restore it and the coast starts drawing CO₂ down again.
Blue carbon is the carbon these ocean and coastal ecosystems capture and store, and the reason restoring them counts as carbon removal.
Blue carbon is carbon captured and stored by ocean and coastal ecosystems, mainly mangroves, tidal salt marshes and seagrass meadows. Restoring them removes CO₂ for decades to centuries, at an estimated $11–300 a tonne, cheapest for mangroves.
Definition
The IPCC defines blue carbon as “biologically driven carbon fluxes and storage in marine systems that are amenable to management.” In practice the term almost always means coastal blue carbon: the carbon taken up and stored by vegetated coastal ecosystems with rooted plants, namely mangroves, tidal salt marshes and seagrass meadows.
The name sets it apart from “green” carbon held by forests and soils on land. In carbon removal terms, the State of CDR (2026) counts coastal wetland restoration as a conventional method of carbon dioxide removal, alongside afforestation and soil carbon: established, land-based and storing carbon for decades to centuries.
Blue carbon is not the same as the ocean’s natural CO₂ uptake. The open ocean absorbs a large share of human CO₂ emissions through chemistry, but that is not something a project can manage or claim. Engineered ocean methods such as ocean alkalinity enhancement are marine carbon removal, but they are usually treated as their own methods rather than as blue carbon.
The Three Blue Carbon Ecosystems
| Ecosystem | Where it grows | Restoration cost position |
|---|---|---|
| Mangroves | Salt-tolerant trees and shrubs on tropical and subtropical coasts | Low end of the $11–300 a tonne range |
| Tidal salt marshes | Grasses and herbs on sheltered temperate coasts flooded by tides | Within the range |
| Seagrass meadows | Flowering plants growing fully underwater in shallow coastal seas | High end of the range |
Cost positions from the State of CDR (2026, p.247), drawing on grey-literature estimates for 2050.
How Coastal Ecosystems Store Carbon
- Plants take up CO₂. Mangroves, marsh plants and seagrasses photosynthesise like any plant, fixing carbon into leaves, stems and roots.
- Carbon goes below ground. Much of it ends up in roots and in organic material trapped in the sediment, along with material carried in by tides and rivers.
- Waterlogged sediment slows decay. Sediment that is flooded and low in oxygen breaks organic carbon down slowly, so it can stay stored for decades to centuries.
NOAA reports that mangroves and coastal wetlands take up carbon each year at a rate about ten times greater than mature tropical forests, and store three to five times more carbon per area. Unlike in tropical forests, most of it sits in the soil rather than in the plants.
The reverse is also true. Draining, dredging or clearing these ecosystems exposes the stored carbon to oxygen and can release it as CO₂, which is why their protection matters as much as their restoration.
Restoration vs Protection: Removal or Avoidance?
Blue carbon projects do two different things, and they are counted differently:
- Restoration (replanting mangroves, re-opening tidal flow to a drained marsh, reseeding seagrass) creates new carbon uptake. That is carbon removal.
- Protection (stopping a mangrove forest from being cleared) keeps carbon that is already stored from being released. That is avoided emissions, not removal.
Both can be worthwhile, but they support different claims; see carbon removal vs avoidance credits. The State of CDR (2026) warns that published potentials for coastal wetlands often combine the two, along with removals expected in future, and that these are hard to separate.
Cost, Potential and Durability
| Measure | Figure |
|---|---|
| Cost, US$ per tonne CO₂ (2050 estimates) | 11–300; mangroves lowest, seagrass highest |
| Technical potential by 2030 (Macreadie et al. 2022) | 0.6–1.1 billion tonnes CO₂e a year |
| Potential by 2050 (latest IPCC assessment) | Under 0.1 to 0.8 billion tonnes CO₂e a year |
| Of which available at up to $100 a tonne | Under 0.2 billion tonnes CO₂e a year |
| How long the carbon stays stored | Decades to centuries, in plants and sediment |
The cost estimates in the grey literature are two to four times lower than those in peer-reviewed studies, possibly because they count lifecycle emissions and capital and operating costs differently. Costs are expected to fall by more than a third between 2030 and 2050 as seeding and measurement improve. The lower potential for 2050 than for 2030 may reflect growing scarcity of sites to restore.
Blue Carbon Today
No one yet knows how much CO₂ coastal wetland restoration removes worldwide. The State of CDR (2026) leaves blue carbon out of its 2.2 billion tonne estimate of current removal, because the models it uses do not include wetlands. A few countries report small wetland sinks in their national greenhouse gas inventories; the United States is the only one reporting a substantial sink from vegetated coastal wetlands, about 12 million tonnes of CO₂ a year up to 2022, and even there the removal from human action cannot be separated from the total.
On the voluntary carbon market, coastal wetlands are grouped with peatlands. That combined category issued about 78,000 credits and retired about 1.26 million in 2025 (State of CDR 2026, Table 4.2).
Blue Carbon Credits
Blue carbon credits are issued under registry methods for wetland and seagrass restoration:
- Verra VM0033, Methodology for Tidal Wetland and Seagrass Restoration, v2.1, which covers restoring tidal wetlands (including mangroves and salt marshes) and seagrass meadows.
- Isometric Mangrove Restoration protocol, v1.1.
The EU has no certification method for coastal wetlands. Its carbon farming methods adopted in July 2026 cover farming and agroforestry on mineral soils, rewetting peatlands and other organic soils, and afforestation. Because blue carbon storage lasts decades to centuries and can be reversed, credits from it usually carry monitoring requirements and a buffer pool against reversal.
Measurement and Reversal Risks
Reversal. Carbon stored in coastal ecosystems can be released again if the site is cleared, drained, dredged or damaged, so its permanence is decades to centuries, not millennia. Measurement. Much of the carbon is below ground and some arrives from elsewhere with tides and rivers, so separating new removal from carbon that would have been stored anyway takes sediment sampling and models. Mixed claims. Potentials and project figures often combine avoided emissions, removal and expected future removal, which the State of CDR (2026) notes are difficult to disentangle.
Common Confusions
- Calling protection removal. Stopping a mangrove from being cleared avoids emissions; only restoration and new growth remove CO₂.
- Counting the ocean’s natural uptake as blue carbon. The open ocean’s chemical CO₂ uptake is not a managed, claimable removal.
- Treating blue carbon as permanent. Storage is decades to centuries and can be reversed; it is not comparable to CO₂ stored in rock.
- Quoting one cost. Restoration costs range from about $11 to $300 a tonne, and seagrass costs far more than mangroves.
- Mixing up the potentials. Many large figures combine avoided emissions and removal, and 2030 and 2050 estimates differ.
Sources
- The State of Carbon Dioxide Removal, 3rd edition (June 2026; errata July 2026), Edwards, Geden, Gidden, Lamb, Minx, Nemet and Smith (eds.), stateofcdr.org, CC BY 4.0. Chapter 7, p.165 (coastal wetlands not estimated; US inventory sink); Chapter 10, pp.246–247 (potential and cost of coastal wetland restoration); Chapter 4, p.97 (Table 4.2, credits); Chapter 11, p.255 (overlap of removal and avoidance); Figure 1.3, p.29 (storage time). Read 30 October 2026.
- NOAA National Ocean Service, Coastal blue carbon (sequestration rate and storage compared with tropical forests). Read 30 October 2026.
- IPCC, Climate Change 2021: The Physical Science Basis, Annex VII Glossary (“blue carbon”).
- Registry methods: Verra, VM0033; Isometric certified protocols (Isometric Standard v2.2). Read 30 October 2026.
- European Commission, certification methodologies for carbon removals and carbon farming. Read 30 October 2026.
Cost, removal and scale figures marked from the State of CDR are served from the GreenCalculus MasterBrain (keys offsets.cdr.*), each with the report page it was read from. We re-check them when the State of CDR publishes a new edition, and the registry methods every quarter.
Frequently Asked Questions
Blue carbon is carbon captured and stored by ocean and coastal ecosystems. The IPCC defines it as biologically driven carbon fluxes and storage in marine systems that are amenable to management. In practice it usually means coastal blue carbon: the carbon held by mangroves, tidal salt marshes and seagrass meadows, much of it in their waterlogged sediment.
Because a small area stores a lot of carbon. NOAA reports that mangroves and coastal wetlands take up carbon each year at about ten times the rate of mature tropical forests and store three to five times more carbon per area, mostly in their soil. When these ecosystems are drained or cleared, that stored carbon can be released, so protecting and restoring them both matter for the climate.
Three vegetated coastal ecosystems: mangrove forests on tropical and subtropical coasts, tidal salt marshes on sheltered temperate coasts, and seagrass meadows growing underwater in shallow seas.
Restoring blue carbon ecosystems is. The State of CDR (2026) counts coastal wetland restoration as a conventional carbon dioxide removal method, storing carbon for decades to centuries. Protecting existing mangroves or marshes from being cleared is different: it keeps stored carbon from being released, which is avoided emissions, not removal.
Estimates vary. One study puts the technical potential of coastal wetland restoration at 0.6 to 1.1 billion tonnes of CO₂e a year by 2030; the latest IPCC assessment gives under 0.1 to 0.8 billion tonnes a year by 2050, of which under 0.2 billion tonnes is available at up to $100 a tonne (State of CDR, 2026). Many published figures mix removal with avoided emissions.
Estimates for 2050 range from about $11–300 a tonne of CO₂, with mangrove restoration at the low end and seagrass meadows at the high end (State of CDR, 2026). Peer-reviewed studies give costs two to four times higher than the grey literature.
Yes. Verra’s VM0033 methodology (v2.1) covers tidal wetland and seagrass restoration, and Isometric certifies a Mangrove Restoration protocol (v1.1). On the voluntary market, coastal wetland and peatland restoration together issued about 78,000 credits and retired about 1.26 million in 2025 (State of CDR, 2026). The EU has no certification method for coastal wetlands yet.
Decades to centuries, according to the State of CDR (2026), in plants and sediment. That storage can be reversed if the ecosystem is cleared, drained or damaged, so blue carbon is less durable than CO₂ stored in rock, which lasts more than ten thousand years.
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