Key Takeaways: Blue Carbon Explained
- Blue carbon is carbon captured by ocean and coastal ecosystems, including mangroves, tidal and salt marshes, and seagrasses. When undisturbed, it can stay locked away for centuries in waterlogged, oxygen-poor sediments.
- Mangroves are powerful carbon sinks, storing 3–4 times more carbon per equivalent area than terrestrial ecosystems.
- Coastal degradation releases a significant amount of carbon back into the atmosphere. When coastal ecosystems are drained, converted or destroyed, their carbon-rich soils can lose the waterlogged conditions that keep carbon locked away.
- Blue carbon projects are a significant opportunity to increase climate change resilience and absorb CO₂ already in the atmosphere, yet they are not a silver bullet.
- An effective approach is a balanced one: restoring degraded coastal ecosystems, including mangroves, conserving what already exists, and creating sustainable alternative livelihoods for coastal communities.
Every spring, the waters around the small Bahamian island of Bimini witness an unlikely homecoming: pregnant lemon sharks, some of whom have traveled hundreds of miles, arrive at shallow, mangrove-lined coasts to give birth where they themselves were born. Known as “natal philopatry”, this life strategy draws lemon sharks back to the exact mangrove nursery where they were born. With little to no parental care, newborn shark pups find, among gnarled roots and heavy silt, a safe place to learn and grow for up to seven years.
Lemon sharks are not alone in having a close relationship to mangroves. Based on a model that estimated the density of 37 commercially important fishery species, the 2024 edition of The State of the World’s Mangroves report estimated that each year, mangroves support nearly 800 billion juvenile fishes, prawns, bivalves and adult crabs.
At the same time that mangroves are sheltering an extraordinary diversity of marine life, they are performing an ecosystem service that is globally consequential. “One of the oldest carbon capture and storage technologies is the mangrove forest, which we've just discovered not only stores organic carbon from itself but also grabs carbon from all around it and buries it in the soil for centuries” said Dr. Chris Fulton, lead research scientist at the Australian Institute of Marine Science.
While mangroves grow on just 0.1% of Earth's land surface, they can store 3–4 times more carbon per equivalent area than terrestrial ecosystems.
Scientists call this hidden climate power “blue carbon.”
What is Blue Carbon?
In a nutshell, blue carbon is carbon that gets captured by the world’s ocean and coastal ecosystems. While terrestrial forests are known for absorbing around 30% of the CO₂ emitted globally, the role of coastal ecosystems — specifically mangroves, tidal and salt marshes, and seagrasses — is increasingly being recognized.
The “blue” in the name is a clue about how this type of carbon storage works. Coastal vegetation pulls carbon dioxide from the atmosphere and uses it to build its biomass. Dead leaves, branches, and roots make their way into the soil, which is often covered by tidal waters.
Unlike terrestrial environments — where carbon is stored in organic material that typically breaks down quickly when exposed to oxygen-rich soil (with notable exceptions, such as peat bogs) — coastal wetlands are both anaerobic and water-logged. This lack of oxygen acts as a preservative, preventing the rapid decay of plant matter and keeping carbon securely stored.
Some of that carbon gets released back into the atmosphere through respiration during natural plant processes and by soil microbes, but the vast majority ends up in soils, sediments, and the ocean deep — where it can remain locked away for centuries.
Why Blue Carbon Ecosystems Are Carbon Sequestration Powerhouses
Thanks to their ability to straddle land and sea, blue carbon ecosystems act as carbon storage powerhouses. Although they occupy a relatively small footprint compared to the world’s forests, they are incredibly dense and productive. A square meter of seagrass can remove about half a pound of carbon from the atmosphere every year — roughly triple that of a tropical rainforest and ten times that of a grassland of the same size.
Incredible as it is, that’s only part of the story. Found growing on mangrove surfaces, colonizing aboveground roots, and carpeting portions of the forest floorIn the ocean, macroalgae fix vast amounts of CO₂ and, as they get pulled into the deep ocean by currents, help power a vertical “conveyor belt,” known as the biological carbon pump, of sinking particles and long‑lived dissolved inorganic carbon that carries a portion of it into the deep ocean, where it can remain isolated from the atmosphere for centuries to millennia.
Coastal currents and tides form a second “conveyor belt” running horizontally across the seafloor. As tides ebb and flow, they move plant debris and rich sediments from mangroves, marshes, seagrasses, and nearby waters onto the coastal shelf, where that material can be buried in deeper, largely unvegetated sediments. This lateral export is thought to account for a significant proportion of carbon a year, although global totals remain uncertain.
Taken together, these ocean pathways mean that blue‑carbon ecosystems and the surrounding coastal ocean are responsible for a large share of the carbon that gets buried in coastal ocean areas each year, acting as a massive, mostly hidden conveyor belt for climate stabilization.
And of course, these ecosystems do much more than sequester and store carbon. They form critical natural infrastructure, filtering water, supporting global fish stocks, increasing food security, and shielding coastlines from erosion and the impact of storm surges.
How Mangrove Loss Contributes to Climate Change
Despite their importance, coastal ecosystems are under severe pressure. Current estimates suggest we are losing mangroves at a rate as high as 3% annually, while other coastal habitats are struggling even more — with seagrass meadows vanishing at rates nearing 7% per year. Every time a mangrove forest is lost, we lose a vital nursery for species like the lemon shark, but we also lose a powerful climate change solution.
Blue carbon ecosystems are highly effective at storing carbon when undisturbed. However, when those same ecosystems are drained, cleared, or developed, the waterlogged conditions that keep it locked away are lost, exposing carbon-rich soils to oxygen. This triggers rapid microbial decomposition, transforming places that were once long-term carbon sinks into a significant source of greenhouse gas emissions. A landmark global analysis, still widely cited, estimated that the conversion and degradation of mangroves, tidal marshes, and seagrasses releases roughly 0.15–1.02 Pg (billion tons) of CO₂ each year. More recent studies have refined how we map and account for these losses, but they broadly support the conclusion that coastal development drives hundreds of millions of tons of CO₂ emissions annually.
The Realities of Blue Carbon Projects
Blue carbon initiatives offer real promise in a changing climate, but they are not a silver bullet. “Blue carbon ecosystems and related components can help over long timescales but cannot replace short-term ambitious efforts to eliminate anthropogenic greenhouse gas emissions.” said Hans‑Otto Pörtner, physiologist, marine biologist and coauthor of the European Marine Board’s Policy Brief in 2023 on Blue Carbon.
At the same time, projects face steep costs, legal complexity, and the need for rigorous, science-based monitoring to verify impact. To succeed, these efforts need strong policy support, long-term community stewardship, and a dual commitment to restoring damaged ecosystems and preventing further loss.
What do we know for sure? The same mangroves that shelter lemon sharks and thousands of other species also help stabilize the climate — making their restoration a powerful win for both biodiversity and the planet.
