What is “Blue” carbon?

“Blue Carbon” is a term used to describe the carbon stored in ocean and coastal habitats such as mangrove forests, seagrass meadows, saltwater marshes, kelp, and even the biomass of marine creatures. All of these habitats are complex, dynamic, and provide a range of ecosystem services, not just carbon storage.

Non-carbon services – such as acting as a nursery for fishes, reducing erosion or flood risk, or providing food for human users – are commonly referred to as “co-benefits” and vary between different habitat types. Blue carbon ecosystems also tend to be critical to local and global biodiversity. While there is a lot of interest in blue carbon, the reality is there are not yet successfully accredited carbon projects in all of these systems.


What are the Blue Carbon ecosystems, and which can produce credits?

Seagrass: Grass-like plants growing intertidally and subtidally in shallow seawater, seagrass can form extensive underwater meadows. Found anywhere except polar extremes, the global extent of seagrass has not yet been accurately mapped.

There is an accepted methodology for generating carbon offsets from seagrass, which has so far been successfully trialed in Virginia, with the project pending registration under the Verra VCS standard. In Kenya, there is currently research under way to explore whether it's feasible to add a seagrass component to the successful Mikoko Pimoja project, under the Plan Vivo standard.


Saltmarshes: Scrub habitat in the upper intertidal or subject to episodic saltwater intrusions. Verra have published a methodology for carbon accreditation of saltmarsh and large NGOs including The Nature Conservancy have begun feasibility studies of trial sites.

As far as we are aware, there are not yet any registered tidal marsh offsetting projects offering credits on the voluntary market, or any in an advanced stage of development where they are likely to soon.


Kelp (and other macroalgaes): As of August 2022 there is not yet an accepted methodology for generating offsets from the carbon incorporated in kelp and other seaweeds as they grow. There remains the challenge of what to do with the macroalgae once it’s harvested to ensure that carbon remains stored and is not released back into the environment. Sinking in the deep sea has been proposed as one solution, however the environmental impact of this approach is unknown.

Research and trials continue, with a draft methodology expected to be under review in 2022. Any kelp credits offered on the market today have not yet been verified or accredited by any carbon standard, although this should be possible in the near future. 


Ocean Biomass: Fish, whales, and other creatures all hold carbon in the physical mass of their bodies. Under natural conditions, a large amount of carbon in the waste products and carcasses of deceased marine fauna would enter the deep sea. Numerous species in the deep sea also migrate vertically over a 24 hour cycle, feeding higher in the water column at night before returning to depth by day. With the continual removal of marine biomass by fishing, these natural processes are disturbed and reduced.

While quantifying how changes in ocean management can reinvigorate portions of the oceanic carbon pump is important to inform policy decisions, it is unlikely to result in the private production of carbon offsets in the near future. One potential exception lies in assigning a carbon value per tonne in some licensed fisheries, and compensating licence holders for not catching their assigned quota. This approach still remains theoretical.


Mangroves: Trees and shrubs which have evolved to tolerate exposure to saltwater and grow in the intertidal zones along equatorial and tropical coastlines. Mangroves store carbon in the biomass of the trees, but also capture particulate matter in their complex root systems, sequestering carbon in deep sediments for hundreds of years. More information on mangroves is available here.

The science behind mangrove carbon projects is well established, with the first project certified in 2012 and a choice of accepted methodologies and accrediting standards. However, due to the cost of expertise and time required, after ten years there are still only a handful of certified projects.



Mangrove carbon research in Singapore. Photos by Mark Beeston


Fair Carbon's first modular guide presents current best practices for the design, implementation, and ongoing management of carbon offset projects in mangrove ecosystems. Guides to accreditation in other blue carbon ecosystems will be developed and added as new methodologies are published and data from successful trial sites become available.