What is blue carbon?

Carbon captured and stored by coastal and marine ecosystems — principally saltmarsh, mangroves and seagrass — which sequester carbon at rates far exceeding most terrestrial habitats, per unit area.

Why these habitats are so effective:

They accumulate carbon in sediment, not only in living plant material. Terrestrial forests store most of their carbon above ground, where it is released when the trees die or burn. Coastal habitats trap organic matter in waterlogged, oxygen-poor sediment, where decomposition is extremely slow.

The store keeps growing downward. Sediment accumulates over centuries and millennia, so the carbon stock is not capped by how much can stand above ground — a fundamental structural advantage.

Trapped material comes from elsewhere too. These habitats capture particles carried by tides, so they store carbon they did not fix themselves.

Very high sequestration rates per hectare, frequently cited as substantially exceeding tropical forest on a per-area basis, though total global storage is smaller because the habitats cover far less ground.

The three main habitats:

Mangroves, in the tropics and subtropics, which also provide storm protection and nursery habitat for fisheries.

Saltmarsh, in temperate coasts including Britain, which provides flood defence and habitat.

Seagrass meadows, extensively lost and now the subject of active restoration.

Why loss is doubly damaging. Disturbing these habitats — through dredging, drainage, development, aquaculture conversion or anchoring — releases stored carbon that accumulated over centuries. The habitats have been lost at high rates globally, with large proportions of seagrass and mangrove extent gone.

The co-benefits are substantial and frequently the stronger argument: coastal protection against storm surge and erosion, which is increasingly valued as sea levels rise; fisheries productivity, since many commercial species depend on these nurseries; and water quality.

The complications: measuring stocks and fluxes accurately is difficult; some habitats also emit methane, complicating the net accounting; permanence depends on the habitat surviving; and including blue carbon in carbon markets raises the same additionality questions as other offsets.

Kelp forests are sometimes included, and their carbon accounting is more contested since they do not store in sediment in the same way.