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Carbon Sequestration

Blue carbon

Coastal wetlands -- mangroves, salt marshes, seagrass meadows -- store more carbon per hectare than tropical rainforests. They are also among the fastest-disappearing ecosystems on Earth.

What blue carbon is

Blue carbon is the carbon stored in coastal and marine ecosystems, particularly mangroves, salt marshes, and seagrass meadows. The term distinguishes this carbon from terrestrial "green carbon" stored in forests and soils on land.

What makes blue carbon systems unusual is where their carbon goes. In a forest, most carbon is stored in wood and leaf litter, where it is relatively accessible to decomposers. In coastal wetlands, carbon is buried in deep, waterlogged sediments where the absence of oxygen slows decomposition almost to a halt. Carbon that enters these sediments can remain stored for hundreds to thousands of years.1

2-5x
more carbon per hectare than tropical forests
~50%
of global coastal wetlands lost since the 19th century
19%
of deforestation emissions from coastal wetland loss

The three blue carbon ecosystems

Mangroves

Mangroves are salt-tolerant trees and shrubs that root in intertidal zones across tropical and subtropical coastlines. Their interlocking root systems trap sediment and organic material, building up the carbon-rich soils that make them some of the most carbon-dense ecosystems on the planet. Carbon is stored both in the trees themselves and, to a far greater degree, in deep sediment layers that have accumulated over centuries.2

Salt marshes

Salt marshes grow in sheltered coastal areas at temperate latitudes -- estuaries, tidal inlets, and the sheltered edges of bays. Their dense root mats bind sediment and contribute thick layers of organic-rich mud over time. Like mangroves, they trap and hold carbon in conditions that resist decomposition, making them highly efficient long-term stores.3

Seagrass meadows

Seagrasses are the only flowering plants that grow fully submerged in seawater. Their meadows cover shallow coastal floors across every continent except Antarctica, and they bury carbon in sediments while simultaneously supporting some of the most productive marine food webs on Earth -- providing nursery habitat for fish species that sustain coastal fisheries worldwide.4

Why blue carbon matters for climate

The climate significance of blue carbon comes from two facts taken together: these ecosystems store carbon exceptionally well, and they are being destroyed at an alarming rate.

When a mangrove forest is cleared for a shrimp farm, or a salt marsh is drained for development, the carbon that has accumulated over centuries in its sediments is exposed to oxygen and begins to oxidize rapidly. The ecosystem shifts from a carbon sink to a carbon source. This release of legacy carbon is what makes coastal wetland loss so disproportionately damaging from a climate perspective -- it accounts for up to 19% of global emissions from deforestation, despite covering a fraction of the land area.5

The restoration opportunity

Because blue carbon ecosystems store carbon so densely and recover relatively quickly when conditions allow, coastal restoration is one of the highest-return investments in nature-based climate solutions. A restored mangrove begins accumulating carbon within years of replanting, and continues doing so for decades.

Blue carbon and coastal rewilding

Blue carbon ecosystems are natural targets for coastal rewilding because restoring them delivers immediate, measurable results across multiple goals simultaneously. Reinstating tidal flow, removing barriers to sediment movement, and replanting mangroves and seagrasses can recover ecological function within years rather than decades.6

Ecosystem engineers play a role here too. Oyster reefs stabilize sediment and improve water clarity, helping seagrasses re-establish. In estuarine systems, changes to hydrology that mimic the work of beavers in freshwater systems can rebuild conditions that favour marsh recovery.

Blue carbon in global climate policy

Blue carbon has moved steadily into formal climate frameworks over the past decade. It is recognized by the IPCC and the UNFCCC, eligible for voluntary carbon market credits in a growing number of jurisdictions, and supported by coordinated international programs including the Blue Carbon Initiative -- a partnership between the Intergovernmental Oceanographic Commission, Conservation International, and the IUCN -- that develops monitoring methodologies and financing mechanisms.7

Countries with extensive coastlines -- Indonesia, Australia, the United States, Kenya -- have begun incorporating blue carbon into national climate plans. Indonesia alone holds an estimated 3.4 million hectares of mangrove, more than any other country, making its coastal ecosystems significant in global carbon accounting.8

Frequently asked questions

Why are blue carbon ecosystems better at storing carbon than forests?
Their waterlogged, oxygen-poor sediments prevent the microbial decomposition that releases carbon in terrestrial systems. Carbon that enters these sediments can stay there for centuries without being broken down.
What happens when blue carbon ecosystems are destroyed?
The stored carbon in their sediments oxidizes and enters the atmosphere as CO2. This makes their destruction especially costly from a climate perspective -- the damage is not just the loss of future sequestration, but the release of centuries of accumulated carbon.
Can blue carbon be used in carbon markets?
Yes. Voluntary carbon markets accept blue carbon credits in many jurisdictions. International initiatives are working to standardize methodologies so that credits are consistent, verifiable, and comparable across projects.
How does blue carbon relate to rewilding?
Restoring coastal wetlands is one of the most impactful forms of coastal rewilding. It recovers both carbon storage and biodiversity, often quickly, and generates co-benefits for coastal communities through flood protection and fisheries recovery.
References
  1. Mcleod, E. et al. (2011). A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Frontiers in Ecology and the Environment, 9(10), 552-560. doi:10.1890/110004
  2. Donato, D.C. et al. (2011). Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience, 4, 293-297. doi:10.1038/ngeo1123
  3. Chmura, G.L. et al. (2003). Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemical Cycles, 17(4). doi:10.1029/2002GB001917
  4. Duarte, C.M. et al. (2010). Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows. Global Biogeochemical Cycles, 24. doi:10.1029/2010GB003793
  5. Pendleton, L. et al. (2012). Estimating global "blue carbon" emissions from conversion and degradation of vegetated coastal ecosystems. PLOS ONE, 7(9). doi:10.1371/journal.pone.0043542
  6. Ocana, F.A. et al. (2021). Blue carbon as a nature-based climate solution. One Earth, 4(9), 1224-1236.
  7. Blue Carbon Initiative (2023). About the Blue Carbon Initiative. Conservation International / IOC-UNESCO / IUCN. thebluecarboninitiative.org
  8. Murdiyarso, D. et al. (2015). The potential of Indonesian mangrove forests for global climate change mitigation. Nature Climate Change, 5, 1089-1092. doi:10.1038/nclimate2734