This article shows how the storage of blue carbon helps to limit global warming and protect against sea level rise. It also explains why the biggest threat to the blue carbon ecosystem comes from humans.
Blue Carbon Explained
Blue carbon is a relatively new term used to describe carbon stored underwater, in the world’s coastal and estuarine ecosystems. The term was first used in an United Nations report in 2009, as a way of differentiating underwater carbon from ‘green carbon’, that is, carbon stored on land in forests, plants and soil. 1
Recent research has identified blue carbon ecosystems as an important net carbon sink, due to their ability to remove and store large amounts of carbon dioxide (CO2) from the atmosphere. This helps to combat global warming by reducing the greenhouse effect, which is powered by gases like CO2.
As a result, blue carbon has attracted the attention of scientists focused on climate change mitigation, as well as those committed to marine conservation.
Where Is Blue Carbon Found?
Mangroves and seagrasses, as well as salt marshes (also known as tidal marshes) account for the majority of blue carbon. Primarily found along the coast or in estuaries, they account for between 50-70 percent of the ocean’s carbon storage. In total, sources of blue carbon cover about 49 million hectares and are found around every continent except Antarctica.
Why Is Blue Carbon Important?
The blue carbon ecosystem is a tried and tested carbon capture and storage system that’s completely free for us to use as long as we like. And, it’s much more efficient than the system employed by plants on land.
Vegetated coastal ecosystems are brilliant at sucking CO2 out of the atmosphere, a process known as carbon sequestration. According to environmental scientist Arianna Sutton-Grier at NOAA, their carbon capture rates are ten times higher than most land forested systems, which makes them a key link in the carbon cycle of the ocean.
So, while ocean plants cover far less land mass than terrestrial plants, they still manage to sequester an equivalent amount of carbon.
Seagrasses for example, occupy only 0.1% of the ocean surface but are considered one of the largest carbon reservoirs in the world. 2 The global seagrass organic carbon reservoir is estimated at anywhere between 4.2 and 19.9 billion tonnes. 3
Seagrass meadows bury 95 percent of their carbon in their soil and, if left undisturbed, the carbon accumulates over thousands of years. But plants and trees on land store their carbon in living biomass, like leaves, roots, stems and trunks. When the plant dies, the carbon is gradually released into the atmosphere.
Mangroves perform equally well, absorbing up to four times as much CO2 as trees and plants on land. In fact, mangrove forests are believed to store more carbon per hectare than any other ecosystem. The total reservoir of blue carbon stored by mangroves is currently estimated at 11.7 billion tonnes. 4
Seagrasses are flowering plants that live underwater and they are one of the most productive plant communities on Earth. As they require sunlight, they are mostly found in shallow waters. Florida for example, has over 2.5 million acres of seagrass meadows.
If you want to help protect these communities, encourage your local government to embrace practices that reduce water pollution from source – such as pipe discharges from factories and businesses. And from other sources where human activities like using fertilizers on farms and gardens cause pollutants to escape in stormwater runoff.
Protection For People Living Along Coastlines
Maintaining a healthy coastal ecosystem also helps provide flood protection and food security for millions of people. Mangroves, grasses and marshes act as natural flood barriers. As sea levels rise, maintaining these habitats must be a priority for any climate change adaption policy.
Mangroves, for example, are estimated to be worth over US $1.6 billion each year in services that support the ‘blue economy’ such as fisheries and tourism.
What’s The Problem?
When blue carbon ecosystems are damaged or lost, they stop sequestering CO2. Worse still, they can release thousands of years’ worth of very old stored carbon into the atmosphere – usually within the first few months or years after a disturbance.
Unfortunately, mangroves are being lost at a rate of 2 percent per year, mainly in Southeast Asia. Carbon emissions from mangrove deforestation account for up to 10 percent of emissions from deforestation globally, despite accounting for less than 1 percent of land coverage.
Salt marshes are disappearing at a rate of 1-2 percent a year, and have lost over 50 percent of their historical global coverage, according to The Blue Carbon Initiative.
The Problem is Us
The threat to blue carbon ecosystems mainly comes from people – that’s us! According to the World Wildlife Fund (WWF), loss of mangrove habitat is usually caused by coastal developments, such as shrimp farming and other forms of aquaculture. The damming of rivers and water pollution also degrade mangrove ecosystems. Climate change – in the form of droughts, ocean warming and marine heatwaves – is another growing threat.
The decline in seagrasses is also due to a variety of factors including sediment and nutrient runoff, drought, agricultural practices such as overuse of fertilizers, commercial fishing practices, overgrazing, algal blooms, and disease.
Changing weather patterns linked to global warming are also having a knock-on effect. Intense storms, ocean warming, rising sea levels and ocean acidification is unbalancing the marine ecosystem. For example, the loss of coral reefs increases wave heights in lagoons which leads to losses in both mangroves and seagrasses. 5
Climate Variability in the Oceans
Regional weather cycles – such as the Indian Ocean Dipole (IOD) between East Africa and SE Asia, and the El Nino-Southern Oscillation (ENSO) in the tropical Pacific – account for much of the climate variability in terms of extreme weather events like storms and cyclones. These events can impact directly on coastal mangrove forests and other blue carbon reservoirs.
Ocean Floor Disturbances
Blue carbon is also found in marine sediment along the ocean floor. If undisturbed, it remains there for hundreds of thousands or even millions of years 6
However, human exploitation of the oceans – dredging the ocean floor for the purposes of fishing, oil/gas exploration and land reclamation – is making this once semi-permanent stock of organic carbon vulnerable to remineralization, a process that will worsen future climate change.
What Is Remineralization?
In the deep ocean, where oxygen levels are low, carbon is stored as organic carbon (Corg) in marine sediments. When the sediments are shaken and disturbed, Corg is exposed to increased levels of oxygen which reconverts it back to CO2. The process is called remineralization. 7
So, What Can Be Done About It?
It is clear that a degraded ocean loses its capacity to support a healthy carbon cycle and act as a carbon sink. Many countries have adopted tree planting as part of their climate action plans, through relevant mechanisms such as Reducing Emissions through Decreased Deforestation (REDD). Most scientists say that blue carbon conservation should also be set alongside tree care. Perhaps carbon off-setting can be extended to include blue carbon conservation projects. 8
Bottom line: Mangroves, seagrasses and salt marshes provide us with a wonderfully effective carbon capture and storage system. So, instead of destroying them and then spending billions vainly trying to capture carbon from the air or from factory chimneys, why not look after the system we already have?
- A Rapid Response Assessment. United Nations Environment Programme (GRID-Arendal, 2009). Nellemann C., et al. (eds) Blue Carbon. This was the first report to use the term ‘blue carbon’.
- “Carbon stocks and accumulation rates in Red Sea seagrass meadows.” Nature. October 2018
- “High variability of Blue Carbon storage in seagrass meadows at the estuary scale.” Ricart, A.M., York, P.H., Bryant, C. et al. Sci Rep 10, 5865 (2020).
- “Total ecosystem carbon stocks of mangroves across broad global environmental and physical gradients.”
- “Persistent organic matter in oxic subseafloor sediment.” Estes et al. 2019. Nature Geoscience.
- “The Future of Blue Carbon Science.” Nature, September 2019.
- “Assessing the risk of carbon dioxide emissions from blue carbon ecosystems.” March 2020.
- Ocean Facts. National Ocean Service (NOAA)