Role of Blue Carbon in Mitigating Climate Change

Blue carbon (BC) is the carbon dioxide (CO2) that Earth’s marine ecosystems, such as the deep sea, and coastal ecosystems, such as seagrasses, capture. Specifically, when bodies of water and coastal lands are healthy and protected, they have incredible carbon capture and sequestration (CCS) capabilities—capturing CO2 and storing it. The National Oceanic and Atmospheric Administration (NOAA) states that oceans alone absorb 31% of the atmosphere’s CO2. In order to lessen the effects of climate change, numerous leaders and organisations are interested in collaborating with BC projects.

How do BC ecosystems absorb CO2?

Water bodies chemically dissolve CO2 at their surfaces because CO2 is soluble in water (H2O):

CO2 (aqueous) + H2O (liquid) —-> H2CO3 (aqueous).

Carbonic acid is the end product of this chemical process.

Although CO2 can remain dissolved in water, marine life usually uses it for other purposes. This is due to the fact that the carbon cycle includes processes like photosynthesis and cellular respiration, which are carried out by marine plants and animals to produce energy and food. For instance, marine plants like phytoplankton that float at the ocean’s surface carry out photosynthesis using light, water, and carbon dioxide. For phytoplankton, the CO2 becomes organic matter or food.

CO2 penetrates deeper into water bodies than the water’s surface, where it is used by organisms like oysters to build and maintain their shells. In particular, oysters take calcium and carbonate ions from the surrounding seawater through a process known as biomineralization, which then combines to form calcium carbonate, or the shell’s skeleton:

Ca + CO3 —-> CaCO3.

When these organisms pass away, the CO2 that was in their shells and other body components is released into the water as a result of the disintegration of their bodies. CO2 eventually descends to the ocean floor where it is trapped in stone that decaying shells or sediment cement into.

Blue Carbon Ecosystems

Coastal Ecosystems

In order to emphasise the disproportionately large contribution of coastal vegetated ecosystems to global carbon sequestration and the necessity to safeguard these resources, the phrase “blue carbon” was first used ten years ago. Growing in depositional soils, mangrove forests, saltmarshes, and occasionally seagrasses accumulate significant stores of organic carbon in the coastal zone. In comparison to most terrestrial ecosystems, marine angiosperm (higher plant) habitats’ soils can store up to 1,000 tC ha1 of carbon. Ocean acidification is a result of increasing atmospheric CO2 levels, although the increased CO2 in saltwater can encourage photosynthesis, removing carbon from the seawater. Consequently, one advantage of protecting habitats for submerged blue carbon is that they may ameliorate ocean acidification locally.

Open Ocean Ecosystems

Compared to worldwide terrestrial soils, the dissolved carbon stores in the ocean are at least an order of magnitude higher. This dissolved carbon is mostly bicarbonate, which has been present in the ocean for about 100,000 years. Oceanic dissolved organic carbon is around 200 times as abundant in carbon as living marine biomass and is nearly equal to atmospheric CO2. Marine phytoplankton fix dissolved inorganic carbon, which is subsequently primarily eaten and stored in the biomass of other species, and account for around 50% of the world’s primary production (about 50 Gt C/year). Regional and temporal variations in the amount of carbon fixed by phytoplankton and then stored rely on surface water productivity, grazing/microbial degradation, and physical processes such as turbulence.

Deep-Sea Ecosystems

The majority of the organic carbon in the ocean is found at depths more than 1,000 metres, where it is dissolved and remains thousands of years distant from the atmosphere. In order to maximise their own eating and escape predators, mesopelagic zooplankton and fish often travel great distances every day, and thus plays a significant role in moving carbon from surface waters down. The majority of this carbon is then transferred by deeper water fish into long-term storage below 1,000 m depth, where it is kept sequestered from the atmosphere for thousands of years. 

Issues and Conservation Initiatives

The ocean plays a critical role in global climate regulation through uptake and storage of heat and carbon dioxide. However, this regulating service causes warming, acidification and deoxygenation and leads to decreased food availability at the seafloor. These changes are likely to affect the productivity, biodiversity and distributions of deep-sea fauna, thereby compromising key ES.

Acidification is first caused by too much CO2 in the atmosphere, which then affects water bodies. Acid rain is created when CO2 is released into the atmosphere and combines with the water vapour in clouds. This rain eventually ends up in water bodies. Water bodies and CO2 can also interact more directly since seawater and CO2 can interact to form carbonic acid. The issue is that acidic, salty surroundings make it harder for marine life, such as oysters, corals, and starfish, to form their shells. Additionally, as oceans get warmer, the amount of oxygen in the water decreases, affecting all marine life.

Schematic illustration of key components and changes of the ocean and cryosphere, and their linkages in the Earth system through the movement of heat, water, and carbon. Source: IPCC SROCC report 2019.

The BC that these ecosystems have been storing for centuries will be released into the atmosphere if they are destroyed, which will be especially harmful and add to the disastrous effects of climate change. For instance, damaged wetlands emit more carbon dioxide (CO2) annually—450 million tons—than Australia did in 2016.

The International Blue Carbon Initiative

The International Blue Carbon Initiative — a partnership among Conservation International, IUCN and IOC-UNESCO — is an integrated program focused on mitigating climate change by conserving and restoring coastal marine ecosystems globally.

Formed in 2011, the Blue Carbon Initiative works with scientists, private organizations and government agencies to achieve its goal through:

  • coordinating the International Blue Carbon Scientific and Policy Working Groups
  • supporting the integration of blue carbon into international policy
  • working with national governments to develop national approaches to blue carbon including in Indonesia, the Philippines, Ecuador, and Costa Rica
  • identifying and promoting priority scientific research needed to describe and monitor carbon in coastal ecosystems
  • developing conservation and management tools to protect coastal systems for their carbon sequestration and storage capacity
  • supporting capacity building and the development of in-situ blue carbon demonstration projects.



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