The essential role of blue carbon

The essential role of blue carbon

While carbon sequestration is often associated with terrestrial forests, the equally vital role of marine and coastal ecosystems is often overlooked. Mangroves, seagrass beds and salt marshes are immense carbon sinks, playing a crucial role in mitigating global warming and preserving biodiversity.

Matthieu Duault

Matthieu Duault

Climate Copywriter

Update :

When we talk about carbon and global warming, we are often inclined to think about the essential role of forests in carbon sequestration, and how to counter the threats to these ecosystems. However, on a planet 72% covered by water, marine ecosystems, and particularly coastal ones, play a role at least equivalent to, if not more important than, that of terrestrial ecosystems in carbon sequestration.

With this in mind, we're going to talk about the essential role of blue carbon.

What is blue carbon?

The term "blue carbon" refers to the capacity of ocean and coastal ecosystems, such as mangroves, seagrass beds and coastal wetlands, to store and fix carbon from the atmosphere and oceans. These ecosystems act as carbon sinks, helping to mitigate the effects of climate change. They transform carbon dioxide into biomass through photosynthesis and sequester it in sediments. Most of this carbon comes from the decomposition of organic matter (plants, decomposing animals, etc.).

Mangroves, for example, are coastal ecosystems made up of plants that live in intertidal zones. These wetlands act as carbon reservoirs, storing carbon dioxide (CO2) from the atmosphere and helping to prevent its accumulation, thus helping to reduce the greenhouse effect.

At the same time, blue carbon ecosystems provide numerous ecosystem services in terms of biodiversity and coastal protection.

The use of the term "blue carbon" highlights the importance of conserving and restoring coastal ecosystems as part of strategies to combat climate change.

The 3 types of blue carbon ecosystems

There are 3 main types of ecosystem that can store blue carbon. They are distributed over all the world's coastlines, with the exception of the Antarctic.

Worldwide distribution of blue carbon ecosystems
Distribution of blue carbon ecosystems worldwide (Source : Blue Carbon Initiative) 

Seagrass beds

Seagrass beds or seagrass meadows are made up of algae and non-vascular marine plants. They are found in shallow coastal areas where sunlight can penetrate sufficiently to enable photosynthesis.

Essential to the survival of many species, seagrasses are both a habitat for numerous marine species and a source of food. They also play an important role in stabilizing the seabed, oxygenating and purifying water, and preventing coastal erosion by reducing the speed of marine currents. They are also capable of storing 2 times more carbon than terrestrial forests.

Although they represent only 0.2% of the ocean's surface, seagrasses are estimated to store over 10% of the ocean's carbon. Their surface area is shrinking by 1.5% per year, and to date they have lost almost 30% of their global surface area.


Mainly found in tropical and subtropical zones, these ecosystems, located in sheltered coastal areas, are made up of plants and trees, including the famous mangrove, perfectly adapted to the brackish waters of intertidal zones.

Their capacity to store carbon is 4 to 5 times greater than that of a forest. One hectare of mangrove can store up to 3,754 tonnes of carbon (Unesco). Most of the carbon stored by mangroves is found in their soils (83%), with the remainder stored in the biomass of the mangrove trees. Beyond their usefulness in carbon sequestration, mangroves also provide shelter for numerous species and play a key role in filtering water and protecting the coastline against erosion and bad weather.

At the current rate, the area occupied by mangroves is shrinking by 2% every year. It is estimated that human activities have led to the destruction of 30% of their original surface area. Their capacity to absorb CO2 is such that it isestimated that the destruction of mangroves is responsible for 10% of CO2 emissions linked to deforestation, even though they only cover 0.7% of the earth's surface.

Salt marshes

Salt marshes, or schorres, are intertidal zones located along coastlines and mostly found in temperate zones around the globe. These wetlands are characterized by the presence of saltwater from the ocean or sea, mixed with freshwater from streams, rivers or lakes, and are often subject to tides, with variable water levels and salinity. They are made up of halophilic plants, i.e. those adapted to high-salinity environments.

Like mangroves, these coastal marshes are a formidable reservoir of carbon, both in the biomass and in the soil, and play an essential role in protecting coasts, preserving biodiversity (especially for migratory birds) and filtering water.

Salt marshes are disappearing by 1 to 2% every year. Although they still cover 140 million hectares worldwide, they have already lost more than half their historic surface area.

Why is blue carbon so important?

Blue carbon plays an essential role in the fight against global warming. Its capacity to store organic carbon is unmatched by any other ecosystem on the planet.

The vast majority of this carbon is found in sediments, up to 6 meters underground, and is therefore stored over the very long term. Moreover, the rate of carbon burial in sediments is up to 55 times faster than in tropical rainforests.

Today, it is estimated that blue carbon ecosystems cover 49 million hectares and store between 10,450 and 25,070 million tonnes of CO2 in the first metre of soil alone(Ramsar).

It is therefore essential to preserve these wetlands:

  • to mitigate the effects of climate change linked to anthropogenic emissions
  • to protect these immense stocks of carbon sequestered in biomass and sediments

The destruction of blue carbon ecosystems therefore has a devastating effect on the climate. It prevents the absorption of CO2 by these ecosystems and, worse still, can lead to the release of billions of tonnes of additional carbon into the atmosphere, accelerating global warming.

In addition to their crucial role in carbon storage, these ecosystems are also essential to the environment through their numerous ecosystem services:

  • They make a significant contribution to the protection of marine and terrestrial biodiversity by serving as shelters, nursery areas and food reserves.
  • They help combat coastal erosion and protect shorelines from bad weather.
  • They filter water and preserve its quality
Ecosystem services of blue carbon
Ecosystem services linked to blue carbon (Source: The Blue Carbon Handbook - Ocean Panel)

Ecosystems under threat

Awareness of the importance of these ecosystems in combating global warming and mitigating its effects is relatively recent.

In the meantime, they have been and continue to be victims of human activity and the consequences of climate change, increasing their vulnerability.

Despite the launch of numerous preservation programs, the degradation of blue carbon ecosystems continues, and the surface area they occupy continues to shrink.

One of the main causes of the degradation of these ecosystems is eutrophication, caused by human activities, particularly certain agricultural practices. The multiplication of algae in marine environments leads to the asphyxiation of blue carbon ecosystems.

These ecosystems are found in maritime areas where human activity is densest and therefore has the greatest impact.

Water pollution and coastal development are also among the main sources of destruction of coastal blue carbon ecosystems.
Seagrass meadows are also victims of trawling or anchoring by fishing boats or pleasure craft. 

Added to this are natural disasters whose pace and power are amplified by global warming: droughts, storms, rising sea levels...

According to the Blue Carbon Initiativebetween 340,000 and 980,000 hectares of blue carbon ecosystems are lost every year. We have already lost at least 67% of mangroves, 35% of salt marshes and 29% of seagrass beds. As we have said, these degradations not only damage carbon storage capacities, but also result in net emissions of carbon and other greenhouse gases.

Growing interest in blue carbon

For a surprisingly long time, the oceans and their ecosystems remained outside the scope of international climate agreements. It was not until COP26, with the Glasgow Agreement, that the importance of ocean protection in the fight against global warming and environmental protection in general was finally acknowledged. Since then, many countries have incorporated the protection of the oceans and their ecosystems into their environmental policies, and have set up various carbon contribution projects.

Awareness of the importance of blue carbon goes hand in hand with the implementation of preservation projects that generate dedicated carbon credits.
The capacity of these ecosystems to store huge quantities of carbon over the very long term, coupled with their crucial role in preserving biodiversity, has led to a clear interest in these projects, making it easier to finance their development.

As part of its low-carbon label, France has included methods for labeling and therefore financing projects to protect and restore mangroves and posidonia meadows, which are particularly threatened in the Mediterranean basin.

The Blue Carbon Initiative, a coalition to protect blue carbon

Numerous international initiatives have been launched in recent years to draw attention to the risks facing these ecosystems and to take concrete action to protect them. 

Unesco, the IUCN (International Union for Conservation of Nature) and the NGO Conservation International have founded the Blue Carbon Initiative. This coalition aims to :

  • Developing and promoting scientific knowledge about blue carbon
  • Contribute to projects to restore and protect blue carbon ecosystems around the world
  • Support institutions in implementing policies to protect and restore these ecosystems

Unesco already counts at least 21% of the world's surface area of blue carbon ecosystems as part of its World Marine Heritage, enabling it to better protect these areas of special status.

blue carbon ecosystems in UNESCO World Heritage sites
Reserves in megagrams of carbon stored by the blue carbon ecosystems of UNESCO World Heritage sites on a logarithmic scale (Source: "UNESCO Marine World Heritage: custodians of the globe's blue carbon assets")

By bringing together a variety of players (governments, NGOs, research institutes, etc.), this coalition aims to coordinate international efforts in favor of blue carbon and to promote the importance of this resource in mitigating climate change and protecting biodiversity, while involving the various stakeholders, including local communities, who are heavily impacted by the destruction of these ecosystems.

Their main missions include accounting, for each country, for the carbon stocks sequestered and emissions generated by blue carbon ecosystems, improving the management of blue carbon ecosystems in protected marine and coastal areas, and developing carbon contribution projects.

Sources : 

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