What Is Blue Carbon? How Ocean Ecosystems Store CO₂

Blue Carbon refers to the carbon dioxide captured and stored by the world’s ocean and coastal ecosystems. Think of it as the ocean’s version of a carbon-storing forest; but instead of trees on land, the plant life in the sea includes mangroves, seagrass, and salt marshes.

Although they cover less area than forests, coastal blue carbon ecosystems are more efficient at sequestering carbon. Pound for pound, blue carbon ecosystems can absorb carbon up to 10 times faster than forests. Much of this carbon is in their soils, where it decomposes slowly, allowing blue carbon ecosystems to store twice as much carbon per equivalent area.

Read on to learn why these coastal ecosystems are carbon sequestration superstars and how they protect shorelines and support life both above and below the surface.

Key Takeaways Blue carbon definition: the carbon stored by ocean and coastal ecosystems, such as mangroves, seagrass meadows, and salt marshes. These ecosystems act as long-term carbon sinks, absorbing carbon from the atmosphere and storing it in plants and soils for thousands of years. When they are destroyed, stored carbon is released, contributing to additional warming. Blue carbon ecosystems also sustain biodiversity, support local livelihoods, and protect coastal areas. Conserving and restoring them is important for climate mitigation and healthy coastlines; carbon offsets help fund these efforts.

The Big Three Blue Carbon Ecosystems 

While the entire ocean absorbs CO2​, the term “blue carbon” most often refers specifically to the carbon stored in just three coastal ecosystems: mangroves, seagrass, and salt marshes. These are considered blue carbon ecosystems due to their incredible efficiency in absorbing and storing carbon – both within the plants and in the sediment below, where it is buried and locked away for hundreds to thousands of years. 

These vegetated coastal habitats grow along coastlines in intertidal and shallow coastal zones where the sea meets the land. Their plants are rooted in waterlogged soils saturated with saltwater from tides or ocean flows. Mangroves are saltwater trees with above-ground roots, salt marshes are coastal wetlands, and seagrasses are aquatic flowering plants. Each has unique adaptations that allow it to survive in these wet, high-salinity environments and withstand water movements. Those same traits, along with the mucky soils beneath them, also make them effective at storing carbon.

Blue carbon ecosystems are found on every continent except Antarctica, covering approximately 126 million acres (51 million hectares) of coastline. Mangroves dominate warmer tropical and subtropical coasts, salt marshes thrive in temperate zones, and seagrasses grow across the globe.

What About Phytoplankton, Kelp, or Coral Reefs?

Not all ocean ecosystems are classified as blue carbon. Scientists focus on mangroves, seagrass meadows, and salt marshes because they’re relatively straightforward to measure and monitor. These habitats grow close to shore in shallow, waterlogged soils where it’s easier to observe how carbon is captured, buried, and stored over time.

Beyond the “Big Three,” many other marine organisms play a role in the ocean carbon cycle, but they don’t meet the official blue carbon definition because the carbon they store doesn’t last long enough or is hard to measure over time. Scientists continue to study these systems, and as understanding expands, more ecosystems may eventually be recognized for their blue carbon potential.

Tiny phytoplankton act as the ocean’s primary CO2​ removers, sucking up around a quarter of CO2 from the air. The problem is that they die and decompose quickly, putting the CO2 right back into the water, with only a small portion sinking deep for permanent storage. However, researchers are exploring ways to encourage the long-term sinking of phytoplankton’s carbon remains, which could add huge amounts of permanent storage.

Giant underwater kelp forests and other seaweeds are great at capturing carbon as they grow. But because wild kelp attaches to rocks instead of rooting in muddy soils, its carbon must drift out to sea before it can be buried. However, this challenge is changing: new research shows that seaweed farming in sediment-based environments can bury carbon at rates similar to other blue carbon habitats. Restoring and managing kelp forests could potentially remove tens of millions of tons of carbon globally.

Lastly, coral reefs pose a unique issue. Corals absorb carbon to build their hard skeletons, forming massive reef structures over millions of years. But the same process that helps them grow also releases CO2​ back into the water. Due to this combination of long-term storage and short-term release, scientists have long debated whether coral reefs are a net carbon source or carbon sink. The picture is becoming even more complex as ocean acidification slows coral growth, reducing CO2​ release, but also signaling severe stress for reef ecosystems. 

Although coral reefs, seaweed, and phytoplankton aren’t technically considered blue carbon ecosystems, they are foundational to ocean health and protect adjacent carbon-sequestering ecosystems, including mangroves, seagrasses, and tidal marshes.

Why Blue Carbon Matters for the Climate and Coastal Communities

As their name implies, blue carbon ecosystems help stabilize the climate through coastal carbon storage. They act as natural carbon sinks, capturing carbon from the atmosphere and storing it for thousands of years. By removing CO₂, they help slow the pace of climate change.

But their climate-fighting powers go beyond emissions reductions. They also protect coastal communities from the effects of climate change, including rising sea levels, storm surges, and flooding. Mangrove roots weaken wave energy, while coastal vegetation absorbs water and buffers nearby communities from flood damage. Their extensive root systems also stabilize shorelines and help prevent erosion, keeping coasts intact even under greater pressures. 

Research estimates that mangroves protect 15 million people from flooding each year and prevent over $65 billion in property damage. Their root systems also reduce wave height by up to two-thirds, lessening the impact of tropical storms and coastal erosion. 

Additionally, blue carbon ecosystems provide important habitats for birds, marine life, and land animals, and support diverse, interconnected food webs. This abundance of life supports local jobs and food security through fishing and other marine-based livelihoods. Blue carbon ecosystems also create opportunities for recreation and ecotourism, from kayaking through serene marshes to spotting marine life while snorkeling in seagrass meadows.

In this way, their benefits extend far beyond carbon; they serve as life-support systems for both wildlife and people.

 

How Blue Carbon Ecosysystems Store So Much CO2

What makes blue carbon ecosystems remarkable isn’t just that they capture carbon — it’s how efficiently and how long they store it. These coastal habitats have evolved in wet, low-oxygen environments that act like natural vaults, burying and preserving what most ecosystems would quickly release back into the air. 

They hold more carbon per unit area than most “green carbon” systems (forests and other land-based plants), thanks to a powerful combination of rapid growth and slow decay. Their plants grow quickly, absorbing large amounts of carbon dioxide, while their waterlogged soils keep it locked away for centuries or even millennia. In this way, coastal ecosystems act as both a short-term sponge and a long-term reservoir for carbon.

Here’s how coastal blue carbon storage works:

Blue carbon ecosystems convert carbon dioxide into organic biomass through photosynthesis, storing it in their leaves, branches, roots, and soils. When leaves and organic matter fall to the seafloor, they’re trapped by roots and buried in sediment. Oxygen scarcity in the saturated soils slows decomposition. Layer by layer, carbon accumulates in these muddy soils. Some deposits reach up to six meters deep and remain buried for thousands of years.

Unlike forests, which store most of their carbon in living plant material, blue carbon ecosystems keep the majority underground. For example, seagrass meadows and salt marshes often store more than 95% of their carbon below the surface. Land-based ecosystems are also more vulnerable to disturbances, such as fire, drought, and agriculture, which can quickly release their stored carbon back into the atmosphere.

Similar long-term storage occurs in “teal carbon” ecosystems, such as peatlands and freshwater swamps, which also build up dense, carbon-rich soils over time.

Together, blue carbon ecosystems store around 12 billion metric tons of carbon worldwide, and each year they add an additional 81 million metric tons of carbon to their soils.  

How Scientists Measure Blue Carbon 

Understanding just how much carbon these ecosystems hold requires careful measurement and monitoring. Scientists employ a range of techniques to measure blue carbon stocks, including field measurements, remote sensing data, and modeling techniques.

Field studies involve collecting sediment from coastal ecosystems to analyze carbon content and determine the rate of carbon storage, while remote sensing uses satellite and drone imagery to map the areas these environments cover and estimate biomass. Scientists can then use this data to model the overall carbon storage capacity of mangroves, salt marshes, and seagrass beds.

These measurements not only quantify the amount of carbon being stored but also help verify the impact of restoration projects and carbon offset programs.

Conservation and Restoration

Despite their many benefits, blue carbon ecosystems are disappearing at an alarming rate. The rate of coastal ecosystem loss is estimated to be twice that of terrestrial forests, with 0.03 – 1% of their total area lost annually.

When mangroves, seagrass meadows, or salt marshes are destroyed, they release the carbon stored in their plants and soils and lose their ability to capture more from the atmosphere. Protecting and restoring them is therefore an important nature-based climate solution. Studies show that conserving existing blue carbon ecosystems could prevent the release of over 300 million metric tons of emissions per year, roughly the same as taking 70 million cars off the road.

However, restoration and long-term management require substantial funding. Blue carbon offsets help bridge this gap by channeling investment into conservation and restoration. Each carbon credit represents one metric ton of carbon dioxide that has been reduced or removed through a blue carbon project. These initiatives also strengthen community resilience, protect marine life, and support local livelihoods.

Around the world, blue carbon offset projects are already underway. In Italy’s Venetian Lagoon, the Valle Paleazza project is revitalizing traditional fishing valleys to restore salt marshes that store carbon and buffer coastal flooding. In Kenya, the Mikoko Pamoja project engages local villagers in protecting 290 acres of mangroves through replanting and forest monitoring.

How to Take Action for Blue Carbon

You can help safeguard these vital ecosystems. Here are two simple ways to get started:

• Travel Better: Download our Sustainable Marine and Coastal Tourism Tips to learn how to explore oceans and beaches responsibly and preserve delicate habitats.  
• Support Conservation Projects: Use our carbon calculator to offset your travel and fund projects that protect mangroves, seagrass, salt marshes, and other climate-critical ecosystems. 
• Become a Member: As a member, you’ll gain access to tools and resources that help your business protect nature and reduce its carbon footprint.