Climate Science
What Is Blue Carbon?
The carbon stored by coastal wetlands — mangroves, salt marshes, seagrasses — is called blue carbon. These ecosystems sequester CO₂ up to 50 times faster than tropical forests, and lock it away for thousands of years.
The Definition
Carbon by color: why “blue” is different
Green Carbon
Carbon stored by terrestrial ecosystems — forests, grasslands, soils. Stored in living biomass and surface soils. Can be released quickly when burned or decomposed.
Brown Carbon
Carbon stored in peatlands — partially decomposed organic matter accumulated over centuries. Contains enormous reserves but is highly vulnerable to drainage and burning.
Blue Carbon
Carbon stored by coastal and marine ecosystems — mangroves, seagrasses, salt marshes. Stored in deep, permanently waterlogged anaerobic sediments where it can persist for millennia.
What makes blue carbon uniquely valuable is the mechanism of storage. On land, when a tree dies, decomposition releases its carbon back into the atmosphere within years or decades. But in the waterlogged, oxygen-depleted sediments beneath mangroves and salt marshes, decomposition is so slow that organic carbon can accumulate for thousands of years — building up carbon densities that dwarf anything in a terrestrial ecosystem.
The Blue Carbon Initiative — a collaboration between Conservation International, IUCN, and IOC-UNESCO — estimates that coastal wetlands cover less than 2% of Earth's ocean surface yet may store as much as 20–30 gigatonnes of carbon in their sediments, accumulated since the last Ice Age. Protecting this ancient carbon store may be one of the highest-leverage climate actions currently available.
Carbon Density Comparison
How blue carbon ecosystems compare
Mangroves
1,500 t CO₂/ha1,000–2,500 t CO₂/ha stored in biomass and sediment
Salt Marshes
620 t CO₂/haUp to 620 t CO₂/ha — primarily in deep soil carbon
Seagrass Meadows
830 t CO₂/haUp to 830 t CO₂/ha — including ancient buried sediment
Tropical Rainforest
380 t CO₂/ha~380 t CO₂/ha — almost entirely in living biomass
Temperate Forest
200 t CO₂/ha~200 t CO₂/ha — soil carbon minimal by comparison
Grassland
80 t CO₂/ha~80 t CO₂/ha — low carbon density per unit area
Key insight: Mangroves store up to 4× more carbon per hectare than tropical rainforests — but nearly all of it is in deep sediment, not living trees. This means cutting down a mangrove forest and counting only the above-ground biomass dramatically underestimates the true climate cost of mangrove destruction.
The Science
How coastal wetlands sequester carbon
Photosynthesis
Mangroves, marsh grasses, and seagrasses absorb CO₂ from the atmosphere through photosynthesis, converting it into organic carbon — leaves, roots, and woody tissue.
Burial in Sediment
Unlike land forests, coastal wetlands accumulate organic material in waterlogged sediments where oxygen is limited. Without oxygen, decomposition is extremely slow — so carbon doesn't re-enter the atmosphere.
Long-term Storage
In anaerobic (oxygen-free) sediments, organic carbon can be preserved for thousands to tens of thousands of years. The oldest mangrove sediment carbon pools are over 8,000 years old — formed at the end of the last Ice Age.
Continuous Accumulation
As sea level rises, mangroves and salt marshes can build their sediment surface upward, continuously trapping more carbon. Healthy coastal wetlands are not static — they are actively growing carbon sinks.
Destruction = Emission
When coastal wetlands are cleared or drained, the accumulated carbon in their sediments is exposed to oxygen and rapidly released as CO₂ and methane — sometimes within weeks. A single hectare of cleared mangrove can emit 700–3,000 tonnes of CO₂.
When Blue Carbon Is Destroyed
The hidden cost of coastal destruction
Mangrove Clearing for Shrimp Farms
Industrial shrimp aquaculture clears mangroves — releasing thousands of years of accumulated carbon in one event — then abandons the ponds after 5–10 years, leaving acid-sulfate wastelands that continue emitting greenhouse gases for decades.
Peatland Burning in Southeast Asia
Indonesia drains and burns peatlands for palm oil plantations. In years with major peat fires (2015, 2019), Indonesia's greenhouse gas emissions from peat burning alone exceeded those of most developed nations. Each tonne of peat burned releases ~2,700 years of accumulated carbon.
Saltmarsh Diking and Draining
Coastal salt marshes in temperate regions have been extensively diked, drained, and converted to farmland or urban development. When drained, their rich sediment carbon — accumulated over centuries — oxidizes and releases CO₂, while the wetland's future carbon sequestration capacity is permanently lost.
The Path Forward
Blue carbon as a climate solution
Blue Carbon Credits
The voluntary carbon market now recognizes mangrove and salt marsh restoration as a verified carbon offset mechanism. Projects like Mikoko Pamoja in Kenya sell carbon credits directly to companies, funding ongoing community-led mangrove restoration.
Learn more: Ramsar Convention →Nationally Determined Contributions (NDCs)
Over 70 countries now include coastal blue carbon in their Paris Agreement NDCs — committing to protect and restore mangroves, salt marshes, and seagrasses as part of their national climate strategies. But implementation lags far behind commitment.
Learn more: UNFCCC →High Integrity Carbon Markets
The Verra Verified Carbon Standard (VCS) and Gold Standard frameworks include specific blue carbon methodologies, enabling coastal wetland restoration projects to access carbon finance. Over 40 mangrove carbon projects are currently registered globally.
Learn more: Verra →Community-Based Restoration
The most cost-effective and durable blue carbon projects combine restoration with local community economic development. Projects in Kenya, Madagascar, and the Philippines demonstrate that community stewardship produces both higher survival rates for planted mangroves and long-term carbon permanence.
Learn more: Global Mangrove Alliance →Blue carbon is one of the best climate solutions we have.
Protecting it is cheaper than any alternative. The question is political will.