Blue carbon credits: mangroves, wetlands and ocean carbon

The minute

• Mangroves, salt marshes and seagrass meadows store carbon in waterlogged soil, for centuries
• Per hectare, a mangrove can hold several times more carbon than a typical tropical land forest (one Indo-Pacific study measured an average of about 1,000 tonnes of carbon per hectare, mostly in the soil)
• Brazil has one of the world’s largest mangrove areas, but few blue carbon credit projects issued so far

Why it matters: Restoring mangroves delivers two gains that land forests do not deliver the same way, carbon storage and coastal protection against erosion and storm surge.

Why it stores so much

The waterlogged soil of mangroves is low in oxygen, which slows the decomposition of organic matter and keeps carbon locked in the sediment for centuries to millennia, instead of years or decades as in land forests. Most of the total carbon stock in a mangrove ecosystem sits below ground, in the sediment rather than in the trees themselves. Estimates vary by site, but the belowground share often accounts for the large majority of the total, which is why measurement protocols for blue carbon differ so much from those used in terrestrial forests.

How the credit works

Specific methodologies, such as Verra’s for blue carbon, measure carbon in soil and biomass, using the same baseline and additionality logic as other projects, but with a measurement protocol of its own, since most of the stock sits underwater.

The IPCC published a Wetlands Supplement in 2013 that gave national governments guidance on how to include coastal wetlands in greenhouse gas inventories. That supplement is also the scientific backbone that voluntary market methodologies rely on when quantifying blue carbon.

Practical steps for a project developer

A blue carbon project typically follows this sequence:

1. Site identification. The developer selects a degraded or threatened mangrove, salt marsh or seagrass area and confirms land tenure or use rights.
2. Baseline assessment. Field teams collect sediment cores (vertical soil samples, usually one meter deep or more) across the project area to measure the existing carbon stock. This is the most expensive and technically demanding step, because the sampling happens in flooded or tidally influenced terrain.
3. Methodology selection. The developer chooses a recognised methodology (Verra, Gold Standard or another registry) and builds the project design document, defining the baseline scenario, the project scenario and the expected carbon benefit.
4. Validation. An independent auditor reviews the project design before credits can be issued.
5. Monitoring and verification. At regular intervals (often every five years), the developer repeats sediment coring and biomass surveys. A second audit confirms the carbon gains, and credits are issued for the verified period.

Because sediment coring requires specialised equipment and trained personnel working in tidal conditions, the per-hectare cost of measurement in blue carbon projects tends to be higher than in upland forest projects.

The Brazilian case

Brazil has mangroves along its entire coast, notably in Maranhão and Pará, but most globally certified blue carbon projects today sit in Colombia, Indonesia and India.

One reason for the gap is legal. Under Brazil’s Forest Code (Lei 12.651/2012), mangroves are classified as Áreas de Preservação Permanente (APPs), meaning they are already protected by law. That legal protection creates a specific difficulty for carbon crediting: the developer must demonstrate that the project activity goes beyond what the law already requires. This is the additionality test. If the mangrove would have been conserved anyway because of existing regulation, the carbon benefit cannot be claimed as additional.

In practice, developers working in Brazil need to document that legal protection alone has not been enough to prevent degradation (for example, showing ongoing encroachment, shrimp farming or urban expansion despite the APP status). Where that evidence exists, a project can pass the additionality bar, but assembling it adds time and cost to project development.

The additionality problem in protected ecosystems

This challenge is not unique to Brazil. Any country that already grants legal protection to coastal wetlands faces the same tension: the stronger the existing law, the harder it is to prove that a carbon project adds something the law does not already deliver. Registries evaluate this on a case-by-case basis, and the outcome depends on whether the developer can show a credible threat that would occur without the project revenue.

What remains unresolved

Several open questions still shape the blue carbon market:

• Permanence risk from sea level rise. If the sea level rises faster than sediment accumulates, coastal wetlands can drown, releasing stored carbon. Most methodologies require a buffer pool of credits to cover reversal risk, but the long-term adequacy of those buffers under accelerating sea level rise is still debated.
• Standardisation of belowground measurement. Different registries and research groups use different coring depths, sampling densities and lab protocols. Until measurement is more standardised, comparing blue carbon credits across projects remains difficult for buyers.
• Integration with national climate commitments. As countries define which emission reductions count toward their own targets under the Paris Agreement, blue carbon credits sold on the voluntary market may face corresponding adjustment requirements, which can affect pricing and demand.

Common mistakes

The most frequent error among new project developers is underestimating the cost and timeline of sediment measurement. Unlike aboveground biomass, which can be estimated with satellite imagery and relatively quick field plots, belowground carbon requires physical soil extraction, lab analysis and repeated visits across tidal cycles. Projects that budget only for terrestrial-style monitoring often face cost overruns that delay or prevent credit issuance.

Frequently asked questions

Is blue carbon more expensive than regular forest credits?

It often carries a premium, due to the scarcity of certified projects and the co-benefit of coastal protection.

Does mangrove restoration count as REDD+?

Not exactly. It usually uses its own blue carbon (wetland restoration) methodology, distinct from forest REDD+.

Is Brazilian blue carbon on sale today?

Projects are in development, but the number of issued Brazilian blue carbon credits is still small.

Read next: what a carbon credit is.

Read next: the additionality test.

Primary sources: 2013 Supplement to the 2006 IPCC Guidelines: Wetlands (IPCC, 2014); Mangroves among the most carbon-rich forests in the tropics (Donato et al., Nature Geoscience, 2011); Forest Code, Law 12,651/2012, official text in Portuguese (Presidency of Brazil, 2012); VM0033 Methodology for Tidal Wetland and Seagrass Restoration, v2.1 (Verra, 2023).