Blue carbon is a term used in climate change efforts to describe how carbon moves and is stored in marine areas that can be managed. It most often refers to the ability of tidal marshes, mangroves, and seagrass meadows to store carbon. These ecosystems help reduce the effects of climate change and support ecosystems that can adapt to it. However, when these ecosystems are damaged or destroyed, they release stored carbon into the atmosphere, increasing greenhouse gas emissions.

Methods to manage blue carbon are part of a group called "ocean-based biological carbon dioxide removal (CDR) methods." These methods involve plants and organisms capturing carbon from the air. Scientists are working to improve the ability of these ecosystems to store carbon. However, whether blue carbon can effectively remove carbon dioxide over a long period is still being studied.

Another term, "deep blue carbon," describes storing carbon in the deep parts of the ocean.

Definition

Blue carbon is described by the IPCC as "carbon movement and storage in ocean systems that can be managed through living processes." Another explanation says blue carbon is "carbon captured and stored by oceans and coastal areas, especially in vegetated coastal regions like seagrass meadows, tidal marshes, and mangrove forests."

Coastal blue carbon focuses on "plants that grow in coastal areas, such as tidal marshes, mangroves, and seagrasses." Seagrass, salt marshes, and mangroves are sometimes called "blue forests" to contrast them with "green forests" on land.

Deep blue carbon is found in the high seas beyond the control of any single country. It includes carbon in "continental shelf waters, deep-sea waters, and the sea floor beneath them," and makes up 90% of all ocean carbon. Deep blue carbon is generally seen as "harder to manage" because there is limited information about "how long the carbon remains stored in these areas."

Role in climate change context

The term "blue carbon" was first used in 2009. It was created to show that coastal ecosystems, such as mangroves, salt marshes, and seagrass beds, play a much bigger role in storing carbon than their size suggests. Some people use the term to describe all the carbon stored in the ocean, including areas far from the coast. Today, blue carbon is widely recognized for its importance in helping to reduce and adapt to climate change.

Vegetated coastal ecosystems, like tidal marshes, mangroves, and seagrasses, store carbon very quickly. This happens because these ecosystems trap carbon in their soils and sediments. These ecosystems help reduce the effects of climate change and support ways to adapt to it. However, when these ecosystems are damaged or destroyed, the stored carbon is released back into the atmosphere.

Mangroves, salt marshes, and seagrasses are highly effective at storing carbon. They take carbon dioxide from the air and store it in the sediments below them, as well as in their roots, stems, and dead plant material.

Even though these coastal ecosystems cover less land and have less visible plant life than forests, they can greatly affect how much carbon is stored over time, especially in the soil.

A major concern is that these ecosystems are disappearing faster than any other type of habitat on Earth, even faster than rainforests. Scientists estimate that 2–7% of these ecosystems are lost each year. This loss not only reduces carbon storage but also removes important habitats that help manage climate effects, protect coasts, and support health.

When these carbon-storing habitats are damaged or destroyed, the stored carbon is released into the atmosphere, increasing the speed of climate change. Losses of these habitats are happening around the world.

Measuring how quickly these ecosystems are disappearing is difficult, but scientists have estimated that if current trends continue, 30–40% of tidal marshes and seagrass beds, and nearly all mangroves, could be lost in the next 100 years.

The decline of these ecosystems is caused by human activities, such as changes in land use, droughts, building dams, farming, and rising sea levels due to climate change. These activities reduce the amount of habitat available and increase the release of carbon from the soil.

As human impacts and climate change worsen, the ability of blue carbon ecosystems to store carbon will decrease, leading to more carbon dioxide being released into the atmosphere. Scientists are working to better understand how much carbon is being released, but current data is not complete. Loss of plant roots and underground parts can turn these habitats from carbon storage areas into sources of carbon emissions.

In some cases, mangroves and seagrass ecosystems have shown increased carbon storage when they receive more nutrients, either intentionally or from human waste.

Studies of mangrove soil in the Red Sea found that adding more nutrients to the soil did not increase the release of carbon dioxide. This neutral effect was not true for all types of mangrove forests. In some forests, increased nutrients led to faster growth of mangroves, which in turn increased carbon storage. In other forests, increased nutrients caused more carbon dioxide to be released, but the growth of mangroves also increased by up to six times the normal rate.

Carbon storage by type of biome

Tidal marshes are found around the world along coastlines from the Arctic to the subtropics. These wetlands are areas between the high and low tide lines where plants with soft stems, like grasses, grow. In tropical regions, mangroves often replace marshes as the main type of coastal vegetation.

Marshes are highly productive ecosystems. Much of their plant growth happens underground, forming layers of organic material that can be up to 8 meters deep. These wetlands support many species, including plants, birds, and young fish. They also help protect coasts from flooding, reduce pollution in nearby waters, and store carbon in the soil. Like mangroves and seagrass, marshes trap carbon in underground plant material because of slow decomposition and high sediment buildup. Globally, salt marshes cover between 22,000 and 400,000 square kilometers, storing about 210 grams of carbon per square meter each year.

Although salt marshes are not as large as forests, they store carbon much faster. Studies show that marshes can bury carbon at rates up to 87.2 billion tons per year, which is more than tropical rainforests, which store about 53 billion tons annually. Since the 1800s, human activities like development and farming have damaged marshes. A 25% loss of marshes has reduced their ability to store carbon and released stored carbon into the atmosphere. This damage leads to less carbon in soil, less plant growth, less photosynthesis, and faster erosion.

Humans have altered tidal marshes for centuries, including farming, building ports, creating salt ponds, and developing aquaculture. Pollution from oil, chemicals, and excess nutrients also harms marshes. Long-term threats include invasive species, rising sea levels, river dams, and reduced sediment flow. These changes can weaken the ability of marshes to store carbon.

Globally, mangroves stored about 4.19 billion tons of carbon in 2012, with Indonesia, Brazil, Malaysia, and Papua New Guinea holding more than half of this total. About 2.96 billion tons of this carbon is in the soil, and 1.23 billion tons is in living plants, with 0.41 billion tons in roots and 0.82 billion tons in aboveground parts. Mangrove forests cover between 83,495 and 167,387 square kilometers globally, with Indonesia containing about 30% of all mangrove areas. Mangroves store about 10% of global carbon, burying approximately 174 grams of carbon per square meter each year.

Mangroves, like seagrass, store large amounts of carbon. They account for 3% of carbon stored by tropical forests and 14% of carbon buried in coastal oceans. A 2023 study found that restoring mangrove areas stores about 60% more carbon per hectare over 40 years than planting on non-mangrove tidal flats. Restoring all possible deforested mangrove areas could store up to 672–689 billion tons of carbon equivalent over 40 years.

Mangroves naturally face challenges like floods, storms, and disease, but human activities like urban growth, aquaculture, and overharvesting of resources harm them. Mangroves provide important ecosystem services and carbon storage, making them vital to protect. Dams and coral reef destruction also affect mangrove health by altering water flow and wave energy.

Seagrass covers only 0.1% of the ocean floor but stores 10–18% of oceanic carbon. Global seagrass meadows hold up to 19.9 billion tons of organic carbon. Studies show that seaweed forests near coasts can store carbon by transporting dead plant material into the deep ocean. Carbon accumulates in marine sediments, which lack oxygen and preserve organic material for long periods. Seagrass burial rates average about 138 grams of carbon per square meter each year.

Seagrass is threatened by pollution, rising temperatures, sediment buildup, and coastal development. Seagrass loss has increased from 0.9% per year before 1940 to 7% per year by 1990, with about one-third of global seagrass lost since World War II. Factors like drought, poor water quality, agriculture, invasive species, and climate change contribute to this decline. Scientists recommend protecting seagrass to maintain carbon storage and other ecosystem benefits.

Restored seagrass meadows begin storing carbon in sediments within about four years, once the plants grow dense enough to trap sediment. The deep ocean holds large amounts of carbon because CO₂ dissolves and sinks to the seafloor at depths greater than 3 kilometers. Models suggest that storing CO₂ in deep ocean sediments could provide long-term carbon removal. Other methods being studied include seaweed farming, ocean fertilization, and storing CO₂ in basalt rock.

The term "deep blue carbon" has been used since 2017 to describe carbon stored in the deep ocean. Research groups like the Ocean Frontier Institute highlight deep blue carbon as a key strategy for achieving climate goals. Deep blue carbon could store 10–20 times more carbon than coastal blue carbon, offering significant potential for reducing atmospheric CO₂.

Example projects

• In 2023, Microsoft and Running Tide signed a two-year agreement to remove up to 12,000 tons of carbon using an ocean-based system designed to capture carbon.
• In Canada, the North Atlantic Carbon Observatory (NACO) project is measuring how well the ocean can continue absorbing carbon, with a focus on deep blue areas of the ocean.
• In Denmark, the "Greensand" project is capturing carbon at its source and storing it in deep parts of the North Sea. This project aims to store up to eight million tonnes of CO₂ each year by 2030.
• A restoration project in South Australia will cover 2,000 hectares (4,900 acres) of mangroves, salt marshes, and seagrass in the St. Vincents Gulf and Spencer Gulf. The project will also explore ways to protect these large blue carbon ecosystems.
• In South Korea, large seaweed, such as kelp, is being used in a climate change program. The country has created the Coastal CO₂ Removal Belt (CCRB), which includes both natural and artificial ecosystems to capture carbon.
• Marine permaculture projects in Tasmania and the Philippines use seaweed forests to capture carbon. These projects may be used in both tropical and temperate ocean areas.