What Blue Carbon Really Is
Blue carbon is often described in terms of coastal vegetation. The climate value sits somewhere else entirely.
When I first heard the term blue carbon, I pictured coastal vegetation. Mangroves rising out of tidal water, seagrass meadows filtering light, salt marshes fringing an estuary. But that is not where the carbon is.
In blue carbon systems, the climate value sits in the sediment beneath the water. Mangroves, tidal marshes, and seagrass meadows accumulate organic matter in waterlogged, oxygen-poor soils where decomposition slows almost to a stop. Carbon that enters those sediments can stay there for centuries. In mangroves, up to 98 percent of the ecosystem’s carbon stock sits belowground, in sediment rather than in biomass. The canopy is visible. The carbon is not.
This is what makes blue carbon different from terrestrial carbon accounting, which tends to focus on aboveground woody biomass. In a forest credit, the trees are the asset. In a blue carbon system, the trees are incidental. The asset is what lies beneath them, built up slowly through centuries of organic accumulation in anaerobic sediment.
The density of that accumulation is striking. Coastal blue carbon ecosystems store roughly two to five times more carbon per unit area than terrestrial ecosystems, including tropical rainforests. Mangrove carbon stocks average around 740 tonnes of carbon per hectare and can exceed 1,000 tonnes in some settings. Seagrass meadows, which cover a fraction of the ocean floor, hold a global carbon pool estimated between 4 and 20 petagrams of carbon depending on methodology. These are not marginal numbers. They represent a significant and largely invisible part of the global carbon balance.
That invisibility is also part of why these ecosystems have been so poorly protected. A mangrove cleared for a shrimp farm does not look like a carbon disaster. The canopy comes down, the roots are gone, and the economic activity begins. But the sediment below, which took centuries to accumulate, can release the majority of its stored carbon once the ecosystem is disturbed and oxygen reaches the soil. In some documented cases, conversion has released up to 92 percent of original carbon stocks. The loss is largely irreversible on human timescales.
What keeps blue carbon stored is stability. The sediment pool is durable as long as the ecosystem remains physically and biologically intact, hydrology is maintained, and the habitat is not converted or degraded. This makes governance and stewardship as important as the carbon science itself. A blue carbon credit is only as good as the long-term management of the place it comes from.
For blue finance, the relevance is direct. These ecosystems sit at the intersection of climate, biodiversity, coastal protection, and community livelihoods. They are also among the most threatened coastal habitats on the planet, under pressure from development, aquaculture expansion, and sea level rise. Financing their protection and restoration is not a niche environmental concern. It is a question of whether the natural infrastructure that stabilises coastlines, supports fisheries, and stores centuries of accumulated carbon remains intact or is converted to other uses.
The instruments for financing that protection are still developing, particularly in Canada. But understanding what is actually being protected, sediment carbon in living coastal systems, is the starting point for any serious engagement with the field.
