Blue Carbon

Blue carbon refers to the carbon captured and stored by coastal and marine ecosystems, including mangroves, tidal marshes, and seagrasses. These ecosystems act as significant natural carbon sinks, removing carbon dioxide from the atmosphere and sequestering it for long periods in both plant biomass and sediment. Blue carbon is increasingly recognised as a vital component in mitigating climate change due to its high efficiency in carbon storage relative to terrestrial forests. The concept has also gained prominence in environmental policy, conservation strategies, and carbon offset initiatives.

Background and Definition

The term blue carbon emerged in the early 2000s to describe the role of marine and coastal ecosystems in regulating the global carbon cycle. Unlike terrestrial forests, which store carbon primarily in above-ground biomass, blue carbon ecosystems sequester substantial amounts in underwater soils and sediments, which remain undisturbed for millennia if left intact. These habitats are highly productive, capturing atmospheric carbon through photosynthesis and depositing organic matter in anaerobic conditions that slow down decomposition.
The three main ecosystems involved are:

• Mangroves: Salt-tolerant forests found in tropical and subtropical coastlines.
• Seagrasses: Underwater flowering plants forming dense meadows in shallow waters.
• Tidal marshes: Coastal wetlands periodically inundated by tides, typically found in temperate and boreal regions.

Carbon Sequestration Potential

Blue carbon ecosystems are among the most effective natural carbon sinks. Studies indicate that they can sequester carbon at rates up to four times higher than terrestrial tropical forests. For example, mangroves store large quantities of carbon in their below-ground root systems and soil. Seagrass meadows, despite their limited spatial extent, account for about 10–18% of total oceanic carbon burial annually. Tidal marshes also contribute significantly, with carbon-rich soils that can trap organic matter for thousands of years.
However, the loss or degradation of these ecosystems releases previously stored carbon back into the atmosphere, intensifying greenhouse gas emissions. This phenomenon is known as blue carbon emissions, and it poses a major risk given the ongoing pressures on coastal habitats.

Global Distribution

Blue carbon ecosystems are distributed across the globe but vary in concentration:

• Mangroves are mainly located in Southeast Asia, Africa, the Caribbean, and parts of South America.
• Seagrasses are widespread in the Mediterranean, Australian waters, the Caribbean, and temperate coastlines.
• Tidal marshes dominate in the United States, Northern Europe, and Australia.

Together, these ecosystems cover less than 2% of the ocean’s surface but store nearly half of the carbon buried in marine sediments.

Threats and Degradation

Human activity poses severe threats to blue carbon ecosystems. Coastal development, aquaculture, agriculture, pollution, and rising sea levels contribute to their decline. Mangroves are cleared for shrimp farming and urban expansion, seagrass meadows are damaged by dredging and nutrient loading, and tidal marshes are drained for agriculture or altered by dykes and embankments.
The destruction of these habitats not only diminishes biodiversity but also releases centuries of stored carbon. It is estimated that 0.5–3% of global blue carbon ecosystems are lost each year, resulting in emissions of approximately 0.15–1.02 gigatonnes of carbon dioxide annually.

Role in Climate Policy and Mitigation

Blue carbon is gaining recognition in international climate agreements. The Paris Agreement (2015) acknowledges the role of nature-based solutions, including coastal ecosystems, in climate mitigation and adaptation. Countries are encouraged to incorporate blue carbon strategies into their Nationally Determined Contributions (NDCs).
Projects involving blue carbon are also emerging in carbon markets, where the restoration and protection of mangroves or seagrasses can generate tradable carbon credits. These projects not only offset emissions but also provide co-benefits, such as coastal protection, fisheries support, and tourism development.

Ecosystem Services Beyond Carbon Storage

Beyond carbon sequestration, blue carbon ecosystems offer several ecosystem services that enhance their value:

• Coastal protection: Mangroves and marshes buffer shorelines against storm surges and erosion.
• Biodiversity habitat: These ecosystems serve as nurseries for fish, shellfish, and other marine species.
• Water purification: Seagrass meadows filter pollutants, enhancing water quality.
• Livelihood support: Many coastal communities depend on these habitats for fishing, fuelwood, and ecotourism.

Conservation and Restoration Efforts

Conservation initiatives focus on protecting existing ecosystems, restoring degraded habitats, and integrating blue carbon into marine spatial planning. Key organisations, such as the International Union for Conservation of Nature (IUCN) and UNESCO, promote blue carbon projects globally. Techniques such as replanting mangroves, seagrass transplantation, and managed realignment of tidal marshes are being implemented with varying success.
Community engagement plays a critical role in ensuring sustainable outcomes. For example, local participation in mangrove restoration projects not only secures livelihoods but also strengthens ecosystem resilience against climate impacts.

Case Study: Sundarbans Mangroves

A study found that the Sundarbans mangrove forests are rapidly losing their capacity to absorb CO2 from the atmosphere. Key reasons identified:

• Increased water salinity due to reduced freshwater flows into rivers like the Matla
• Mangrove deforestation reducing forest extent and tree heights
• Shrimp farming and pollution placing pressure on the ecosystem

Challenges and Criticism

Despite its potential, the concept of blue carbon faces challenges:

• Measurement difficulties: Quantifying carbon storage and sequestration rates requires advanced methodologies and long-term monitoring.
• Policy integration: Incorporating blue carbon into national climate policies is still limited, particularly in developing nations.
• Market limitations: Carbon credits from blue carbon projects often face uncertainties in valuation and verification.
• Risk of overemphasis: Critics argue that focusing too heavily on blue carbon may overshadow the need for reducing fossil fuel emissions and protecting other ecosystems.

Significance in the Future

As the global community intensifies efforts to combat climate change, blue carbon ecosystems are likely to play a greater role in nature-based climate solutions. Their capacity to store vast amounts of carbon, coupled with co-benefits for biodiversity and human societies, underscores their ecological and socio-economic importance. Protecting and restoring these habitats could represent a crucial pathway towards achieving carbon neutrality and enhancing coastal resilience in the coming decades.

International Blue Carbon Initiative

The International Blue Carbon Initiative is a coordinated, global program focused on mitigating climate change through the conservation and restoration of coastal and marine ecosystems. The Blue Carbon Initiative focuses on mangroves, salt marshes and seagrasses, which are found on every continent except Antarctica.