Blue Carbon Guide

Expert author: Tiffany Potter – Head of Origination, Americas

This guide to blue carbon covers: 

  • What is blue carbon?
  • Blue carbon ecosystems and their co-benefits 
  • How salt and tidal marshes store carbon and deliver co-benefits
  • How seagrass meadows store carbon and deliver co-benefits
  • How mangroves store carbon and deliver co-benefits
  • How kelp and seaweed store carbon and deliver co-benefits
  • Threats to blue carbon ecosystems
  • Blue carbon and the international voluntary carbon market 


What is blue carbon?

Blue carbon is the term used to describe carbon that is stored by our oceans and coastal ecosystems in algae, seagrasses, macroalgae, mangroves, salt marshes and other plants in coastal wetlands. 

Blue carbon projects are those activities that involve seas, waterlands and oceans to store carbon. Oceans play a significant role in the carbon cycle, absorbing around a third of all carbon dioxide in the atmosphere and soaking up 90% of excess heat produced by global warming

Coastal habitats such as seagrass meadows, tidal marshes, and mangrove forests are some of the world’s largest active carbon sinks. Although they make up less than 2% of the sea floor, these habits capture around half of the sea’s carbon at a rate 40 to 50 times higher than tropical forests.


Blue carbon ecosystems and their co-benefits

The four main blue carbon ecosystems are: 

  • Salt and tidal marshes 
  • Seagrass meadows 
  • Mangrove forests 
  • Kelp forests 

Each has a unique way of storing carbon and delivering a range of co-benefits. 

From helping to protect cities from flooding and acting as natural filtration systems to providing shelter and nursery areas for fish and other marine life, blue carbon ecosystems are vital for ensuring a healthy planet.  


How salt and tidal marshes store carbon and deliver co-benefits

Salt and tidal marshes are coastal wetlands which are flooded and drained by daily tidal cycles and consist of soil rich in organic matter and mineral sediment. The composition of this land makes tidal marsh ecosystems an excellent carbon store, accumulating large stores of carbon deep within the soil having the ability to store this carbon for thousands of years. Salt and tidal marshes cover approximately 140 million hectares of the earth’s surface and their annual carbon sequestration rate is estimated to be equivalent to six to eight tons CO2/ ha

Beyond carbon sequestration, tidal marshes provide a number of other benefits to the environment. Marshes help to protect cities and towns from flooding through absorbing storm water and acting as a buffer between land and sea. According to the National Oceanic and Atmospheric Administration (NOAA), one acre of salt marsh can absorb up to 1.5 million gallons of floodwater compared to areas where salt marshes have been lost. 

Marshes act as a natural filtration system, filtering out trace metals and other pollutants allowing high quality water to run into coastal bays, sounds, and estuaries. They also provide shelter and nursery grounds to a wide variety of marine and terrestrial life including shrimp, crabs, finfish and birds.  


How seagrass meadows store carbon and deliver co-benefits  

Seagrass meadows are underwater ecosystems made up of flowering seagrass, which absorbs CO2 through the process of photosynthesis and stores carbon in its roots under the sea bed. Although this process is similar to the carbon capture trees perform, carbon decomposes at a much slower rate under water compared to on land which means the carbon seagrass meadows absorb can be stored for thousands of years. 

Seagrass meadows can be found in shallow, sheltered coastlines in most continents and although these meadows only cover 0.2% of the sea floor, annually they are responsible for sequestering 10% of the ocean’s carbon. Studies also suggest that seagrass meadows can capture carbon 35 times faster than tropical forests. 

Similar to tidal marshes, seagrass meadows provide non-carbon benefits. The seagrass meadows’ deep roots help stabilise the sea floor from high currents and storms. Seagrasses also act as filters by trapping sediment and particles suspended in the water. 

The shelter that seagrasses provide makes these meadows an excellent nursery area for juvenile fish as well as being a source of food for larger marine life including sea turtles, manatees and dolphins. 


How mangrove forests store carbon and deliver co-benefits

Mangroves are trees that grow along sheltered coastlines in the tropics and subtropics and they are able to store up to five times more carbon than upland tropical forests. Even though they only make up 0.7% of the world’s tropical forests, it is estimated that these carbon-rich mangroves store more carbon per unit area than any other ecosystem on the planet. Their high productivity and slow soil decomposition rates make mangroves well adapted to capturing and storing carbon. 

The mangroves provide nutrient rich habitats for a host of wildlife including endangered species such as proboscis monkeys and dugongs. Mangroves provide habitat to spawning fish and young shrimp species which is the life blood of the seafood industry. They also protect coastal communities from flooding and minimise erosion.  

Mangroves also increase water quality as their roots hold onto sediment, allowing the plants and seagrass meadows that coexist with them a better chance at photosynthesising. 


How kelp forests and seaweed store carbon and deliver co-benefits

Kelp is a large, brown species of seaweed which covers a quarter of the world’s coastlines. It absorbs CO2 through photosynthesis and due to its rapid growth rate (up to 2 feet a day), kelp forests have a huge capacity for carbon sequestration. Research has suggested that kelp forests can store approximately 200 million tons of CO2 annually and at a rate 20 times faster than land forests.

Kelp forests provide benefits similar to seagrass meadows. These forests provide shelter and nursery areas to hundreds of commercially important fish species as well as urchins, sea otters and sea lions. They protect coastal areas and shorelines through slowing ocean currents, filtering pollutants and acting as a buffer between the land and sea. Kelp forests also make up a large proportion of the global seaweed farming industry. Kelp is harvested for a range of commercial purposes including pharmaceuticals, textiles, food and alternative energy.


Threats to blue carbon ecosystems

From the impacts of urban development to rising temperatures and sea levels, the world’s blue carbon ecosystems face a range of threats. When blue carbon ecosystems are destroyed, some of the earth’s most impressive carbon sinks are lost and the carbon they have stored is released back into the atmosphere. Since the 1900’s, it is estimated that over 50% of coastal wetlands worldwide have been damaged or destroyed resulting in more than 1 billion tons of CO2 released into the atmosphere annually.

It is estimated that approximately 50% of the world’s original tidal and salt marshes have already been degraded or destroyed due to urban development, eutrophication, land reclamation, agriculture, rubbish dumping and waste water. The rise in global temperatures and sea levels are also drowning the marshes in places where there isn’t adequate undeveloped adjacent land to allow them to migrate. 

Seagrass meadows are declining due to rising sea levels, rising ocean temperatures, ocean acidification, pollution from sewage outfalls, agriculture and increasing coastal developments. The UN environment programme has estimated that 7% of seagrass habitat is being lost worldwide each year, and at least 22 of the world’s 72 seagrass species are in decline.

Mangroves forests are being impacted by unsustainable coastal development, poor farm and water management, irresponsible fishing and agriculture practices, timber harvesting and the effects of climate change. A report by the UNEP found that mangrove forests are being cleared at least three to five times faster than terrestrial forests, costing the world as much as $42 billion in economic damages every year. 

Rising ocean temperatures are also impacting kelp forests, which requires cold water to grow. Coupled with overfishing of predatory fish, which sees herbivorous species dominate the ecosystem and destroy vast quantities of kelp forests, and the impact of some agricultural practices, Kelp forests are in rapid decline.  

In the face of these threats, the voluntary carbon market is playing a vital role in channelling funds to projects that help to protect and restore blue carbon ecosystems. 


Blue carbon and the international voluntary carbon market

The ocean’s ecosystems can play a key role in helping the world achieve its climate targets. A report by the World Resources Institute suggests that protecting and restoring seagrass, mangroves and salt marshes could reduce greenhouse gas emission by 1.4 billion tons annually. Damage to coastal ecosystems not only contributes to climate change but also has additional negative impacts including a greater risk of flooding in coastal areas, threat to biodiversity, and depletion of exploitable living resources which these ecosystems provide. 

The international voluntary carbon market (VCM) is driving the development of many innovative nature-based solutions designed to protect our blue carbon ecosystems. Often community-led projects, these activities protect and restore blue carbon ecosystems with the carbon offsets generated bought and sold on the VCM. 

From supporting mangrove restoration activities in Mozambique to innovative seaweed harvesting projects in North America, Viridios Capital has a diverse portfolio of blue carbon projects around the world. To learn more about our blue carbon projects or to discuss your own blue carbon project, contact our origination team at: [email protected].