What Is Blue Carbon?

October 2022

What do the experts think...

Most people know that trees are fantastic at sequestering carbon from the atmosphere, but did you know the real powerhouses of carbon storage come in the form of coastal ecosystems such as mangroves, sea grasses and salt marshes, collectively referred to as ‘blue carbon ecosystems’?

What is blue carbon?

‘Blue carbon’ is carbon dioxide that is captured and stored by the world’s oceans and coastal ecosystems.

The ocean captures carbon through both:

  1. The process of photosynthesis by plants and plant-like organisms (e.g. phytoplankton), and
  2. The simple chemistry of carbon dioxide dissolving into water (creating carbonic acid).

[FUN FACT] How the temperature of water impacts its carbon storage capability

Did you know that…the solubility of carbon dioxide varies based on salinity and temperature of the water and there is a finite amount that the water can absorb? 

The colder the water, the more CO2, as demonstrated in this video: https://youtu.be/wK4reyh86w0 

Now think about what this means if the temperature of our oceans is increasing…!

What’s the difference between ‘blue carbon’ and ‘green carbon’?

While blue carbon is captured and stored in marine ecosystems, green carbon is the more widely known and understood process that occurs across terrestrial habitats (i.e. inland trees and forests).

Although green carbon ecosystems such as tropical rainforests are well known for their carbon sequestration abilities, blue carbon ecosystems are in fact even more effective at capturing and storing carbon. Unlike “green carbon” rainforests, which store carbon in biomass, and therefore release it when the trees die, mangroves & seagrass store most of the carbon in their soil and sediment. Blue carbon ecosystems not only store carbon for a millennia (if undisturbed), they also capture carbon approximately 40 - 50x faster!

Blue carbon ecosystems sequester carbon from the atmosphere and store it deep in the soil, as opposed to green carbon ecosystems which hold most of their carbon in their roots and other plant parts. Credit: CEAB.

What are blue carbon ecosystems and how do they work?

There are three main types of blue carbon ecosystem:

  1. Salt marshes (sometimes referred to as ‘tidal marshes’)
  2. Mangrove forests
  3. Seagrass meadows

All three ecosystems very efficiently capture carbon via the process of photosynthesis, growing much faster than their green carbon counterparts. In this process, plants use the carbon dioxide captured to grow plant parts such as stems, roots and leaves. The carbon is predominantly stored in the soil, not in above-ground plant materials as is the case with tropical forests. One reason coastal wetlands are particularly good at storing carbon is because the soils are largely anaerobic, which means they lack oxygen.  This lack of oxygen slows the decomposition of plant matter, meaning the carbon stored in its parts stays intact for longer (rather than being released back into the atmosphere).

Salt Marshes

A salt marsh in the Yangtze River estuary (Jiuduansha Wetland) on December 25, 2006. Credit: Lin Zheng via Eos

Salt marshes are coastal wetlands that are flooded and drained by salt water brought in by the tides. The ‘marsh’ is usually made up of decomposing plant matter that is very deep and has very low oxygen levels, leading them to have a sulphur rotten-egg smell. Despite their unpleasant odour, salt marshes have many positive benefits such as providing essential habitat for many fish species, protecting shorelines, reducing flooding and protecting water quality.

Mangrove Forests

Mangrove forests. Credit: World Bank.

Mangrove forests are groups of trees or shrubs found on coastlines, with large roots that usually lie semi-submerged in slow-moving water (salt or freshwater) and/or soils with very low oxygen levels. Like salt marshes, mangroves’ dense root systems prevent erosion and reduce coastal erosion, while providing essential habitat for estuary wildlife.

Seagrass Meadows

The Posidonia australis seagrass meadow in Australia's Shark Bay on Aug. 7, 2019. Credit: Rachel Austin via NBC News.

Seagrass meadows are underwater ecosystems formed by seagrass, typically found on sheltered, shallow, continental shelves (the underwater areas of land surrounding each continent). Known to have an abundance of phytoplankton, they provide shelter and food to an incredibly diverse community of animals, from tiny invertebrates to large fish, crabs, turtles, marine mammals and birds and importantly, sheltered refuges and feeding areas for juvenile marine animals.

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[FUN FACT] Did you know… The world’s largest plant is an ancient seagrass discovered in Western Australia?

Where are blue carbon ecosystems found?

Blue carbon ecosystems can be found along most coastlines in the world.

Global Distribution of Blue Carbon Ecosystems. Credit: The Blue Carbon Initiative

Seagrasses are found in meadows along the shore of every continent except Antarctica.

Mangroves are most often found straddling the equator (between 25° North and South latitude). About 42% of the world's mangroves are found in Asia, with 21% in Africa, 15% in North and Central America, 12% in Australia and the islands of Oceania, and 11% in South America.

Salt marshes are found in the upper coastal intertidal zone between land and salt water or ‘brackish’ water (which has more salinity than freshwater, but not as much as seawater) in temperate and high latitudes where there isn't strong wave action and therefore sediments are able to build up. They are found on all continents other than Antarctica.

What are the benefits of blue carbon ecosystems?

We’ve covered some of the benefits of blue carbon ecosystems already, including capturing carbon, acting as coastal protection from erosion and flooding, and supporting biodiversity by providing wildlife habitat. 

They also underpin the health of our fisheries and therefore the nutrition of hundreds of millions of people, especially in developing nations where fish is a primary source of protein. These benefits and how they support animal and human life are referred to as ‘ecosystem services’.

[QUOTE] “The value of nature to people has long been recognised, but in recent years, the concept of ecosystem services has been developed to describe these various benefits. An ecosystem service is any positive benefit that wildlife or ecosystems provide to people.” National Wildlife Federation

Blue carbon ecosystems do a lot of ‘heavy lifting’ when it comes to global carbon sequestration, sequestering more than half of the ocean’s CO2 despite only occupying approximately 0.2% of the seafloor. It’s safe to say that we need these ecosystems to thrive in order to avoid runaway climate change!

In Australia, some governments such as the NSW State Government and SA State Government have recognised the importance of blue carbon and developed strategies to encourage their protection, which is a promising sign.

There are now also blue carbon offset projects emerging to help finance projects that protect these ecosystems. The demand for and development of these projects has rapidly increased in recent years, with organisations such as UNESCO reporting the huge potential for blue carbon credits to help fund many important conservation and restoration initiatives across marine Heritage Sites and beyond.

The first widely recognised blue carbon methodology was released by Verra in 2015, however with a limited scope focusing only on tidal wetland and seagrass restoration. In 2020, the first blue carbon conservation methodology to be approved under any major GHG program was released by Verra, opening up even greater opportunities for these project types to be developed.

What happens if (or when) we lose blue carbon ecosystems? 

These ecosystems are amongst the most threatened in the world, and when they degrade we’re not limiting their potential to sequester carbon but also making them a source of greenhouse gases because they’re sitting on carbon that they’ve drawn down and preserved for thousands of years.

Blue carbon ecosystems are threatened by coastal development – damaged by farming, harmful fishing practices and pollution – so protecting and restoring them can be expensive and difficult. An estimated 25 to 50% of vegetated coastal habitats have already been lost, equating to the release of up to one billion tonnes of CO2 annually according to the Environmental Justice Foundation.

Because of human-driven increased levels of carbon dioxide in the atmosphere, there is more CO2 dissolving into the ocean than ever before, leading to greater ‘ocean acidification’. As the ocean continues to absorb more CO2, its pH will decrease, disrupting the environment for ocean life that is not accustomed to or able to adapt to more acidic conditions. While some studies have shown that the plants critical to blue carbon ecosystems (e.g. seagrasses) can thrive in environments with higher levels of CO2, however without these plants to capture the CO2, ocean acidification will only increase.

What are some initiatives to protect blue carbon ecosystems?

Sadly, currently there are limited official protections in place for the ocean and these habitats. As at June 2021, less than 20% of all countries with blue carbon habitats along their coastlines referenced the protection and restoration of these areas in their climate mitigation plans. Activists are calling on world leaders to develop specific, legally binding targets to protect and restore blue carbon environments as part of their mandated updates under the Paris Agreement.

Blue carbon offset projects in Australia

According to experts, Australia is well positioned  to develop high quality blue carbon offset projects, as there are many coastal ecosystems in need of restoration and there is a good level of scientific research in the field.

In January 2022, the Australian Government announced its own ‘Blue Carbon Method’ - a new process to create Australian Carbon Credit Units (ACCU’s) through the protection or development of blue carbon ecosystems. Due to the relatively recent introduction of this Method, currently there are limited Australian-based blue carbon credits available.

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