Blue carbon sequestration
The term ‘Blue Carbon’ refers to the carbon captured by the world’s ocean or coastal vegetated ecosystems.
Blue carbon ecosystems
To be considered blue carbon, an ecosystem should:
- have a significant scale of greenhouse gas emissions or removals,
- support long term (>centuries) storage of carbon, and
- be amenable to management actions that enhance carbon storage or avoid greenhouse gas emissions.
Blue carbon did not originally include macroalgal ecosystems; however evidence is mounting that macroalgal ecosystems contribute substantially to marine carbon sequestration. Macroalgal beds are the most extensive vegetated coastal habitats in the global ocean, and their global net primary production is larger than that of all other vegetated coastal habitats. Existing datasets imply that carbon outwelling (i.e., lateral fluxes or horizontal exports of dissolved inorganic (DIC), organic (DOC) carbon and particulate organic carbon (POC)) from coastal habitats followed by ocean storage is relevant and may exceed local sediment burial as a long-term (>centuries) blue carbon sequestration mechanism. Macroalgal beds therefore have the potential to regulate carbon dynamics in coastal ecosystems.
Habitats dominated by calcifying organisms (e.g. coral reefs, oyster reefs) contribute to climate change adaptation through energy dissipation and contribution to sediments, but not through greenhouse gas mitigation, as the process of calcification releases CO2 and thus these ecosystems are likely to be net CO2 sources rather than sinks.
Blue carbon concept
The concept of blue carbon was introduced in 2009 in a United Nations assessment report, with the idea that the role of coastal ecosystems such as salt marshes, mangroves and seagrass meadows in absorbing carbon (C) to reduce emissions is of global significance. These vegetated ocean ecosystems should therefore be protected and, if necessary, restored in order to maintain and expand their ability as critical C sinks.
The major global C pools include the atmosphere, oceans, fossil fuels, and – collectively – vegetation, soils, and detritus. The oceans are the largest C pool, encompassing an estimated 38 000 petagrams of C (petagram C = Pg C = 1015 g C). The geological C pool, composed primarily of fossil fuels, is the next largest pool, estimated at 2000-4000 Pg C. Vegetation (mostly terrestrial, above and below ground) and detritus hold around 2000 Pg C, followed by the atmosphere, which holds about 800 Pg C. The oceans’ role as a sink for CO2 is driven by two processes: the solubility pump and the biological pump. The solubility pump is a function of CO2 solubility in seawater and the thermal stratification of the ocean. Cold, deep waters are generally rich in dissolved inorganic C because the solubility of CO2 increases in cold water. When deep water upwells into warmer equatorial regions, there is extensive outgassing of CO2 to the atmosphere resulting from the reduced solubility of the gas. The biological C pump refers to the uptake of CO2 by marine plankton from the surface waters through photosynthesis; as a result of this process, a small fraction of the biomass produced is transferred to the deep ocean and buried in sediments. Unlike terrestrial soils, the sediments in which mangroves, salt marshes, and seagrass meadows grow do not become saturated with C because sediments accrete vertically in response to rising sea level, assuming ecosystem health is maintained. The rate of blue carbon sequestration in coastal sediments and the size of the corresponding sink may therefore continue to increase over time.
Ocean CO2 uptake
Based on the 2019 assessment of the Global Carbon Project, the ocean took up on average about 2.5 Pg C/yr. This sink estimate is based on simulation results from global ocean biogeochemical models (GOBMs) and is compared to data-products based on observations of surface ocean pCO2 (partial pressure of CO2) accounting for the outgassing of river-derived CO2. There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening of the sink in the 2000s. Explanations for this multi-year variability range from the ocean’s response to changes in atmospheric circulation (especially the variations in the upper ocean overturning) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO2 growth rate. Fossil fuel CO2 emissions reached 10 Pg C /yr in 2018, but the fraction of the CO2 remaining in the atmosphere has been fairly stable at 45% on average since 1958. The ocean has sequestered about 25% of cumulative CO2 emissions in the period 2010-2019. The land has sequestered 30% of cumulative emissions over the same period, but has also released a substantial amount (order 50%) of CO2 by land-use change emissions.
Mangroves are salt-tolerant forested wetlands at the interface between the terrestrial and marine environment in tropical and subtropical regions. The dominant vegetation are several species of woody trees and shrubs with a thick, partially exposed network of roots that grow down from the branches into the water and sediment. They settle where the average monthly temperature is higher than 20°C, where the substrate is fine-grained, and where sediments are deposited by small-moderate tides and waves.
Estimates of the area covered by mangroves worldwide range between 84,000 and 136,000 km2 . The most highly developed and most species-rich mangals are found in Indonesia, Australia and Malaysia.
Average rates of carbon sequestration, according to methods described by the International Blue Carbon Initiative are of the order of 160-210 g C m-2yr-1 , but varying between regions by a factor 10-100.
Estimates of global C sequestration by mangroves fall in the range 10-40 Tg C yr-1  (1 Tg = 1012 g). The spread in the results is mainly related to global scale extrapolation of C sequestration rates from different regions. According to Wang et al. (2021), the greatest C sequestration per country takes place in Indonesia with 14.7 Tg C yr-1, and the second greatest in Australia with 6.86 Tg C yr-1.
Reported estimates of the total global stock of sequestered carbon are in the range 4-20 Pg C . Losses of mangrove systems have declined from 1 – 3% in the late 20th century to 0.3 – 0.6% in the early 21st century. Alongi (2020) estimated that complete destruction of these mangroves for conversion to aquaculture or agriculture yields an increase of CO2 emissions of 0.2 %.
Salt marshes are terrestrial halophytic ecosystems at the land-sea interface. They are covered by salty or brackish water for at least part of the time. Salt marshes are ubiquitous in deltas and estuaries in temperate zones all over the world, but seldom occur on open coasts, because the development is inhibited by wave action. The dominant flora is composed of halophytic plants such as grasses, shrubs and herbs.
Salt marshes cover globally an area of about 55 109 m2 . Average rates of C sequestration by salt marshes are found in the range 170-240 g C m-2yr-1 . The global C sequestration by salt marshes is currently estimated in the range 11-13 Tg C yr-1 . Alongi (2020) estimated the global stock of sequestered C in salt marshes to be about 1.8 Pg C.
Seagrasses generally inhabit the protected shallow waters of temperate and tropical coastal areas. Seagrass meadows can be patchy, but more often they form large swaths of vegetation, sometimes over 10,000 km2 in size. The most extensive areas are found in the tropics, where Thalassia is the dominant primary producer.
Seagrasses cover globally an area of about 314 109 m2 . Alongi (2020) estimated the average rate of C sequestration by seagrasses to be about 221 g C m-2yr-1 and the average sequestered stock to be 16.3 kg C m-2. According to these figures, the global C sequestration by seagrasses is about 70 Tg C yr-1 and the global stock of sequestered C about 5 Pg C.
Macroalgae are common on shores worldwide, covering an area of about 3.5 million km2. Kelps are most common in temperate climate zones, while large brown macroalgae (e.g. Turbinaria spp. and Sargassum spp.) abound along most tropical coasts. Macroalgae preferably grow on rocky shores where sediment accretion does hardly occur. Only a modest part of the net production of organic material is buried and sequestered in the soil. A rough estimate of the global C burial rate is 6 -14 Tg C yr-1 . A much larger part of the organic C (order of 600 – 700 Tg C yr-1) is released to the aquatic environment in the form of particulate organic carbon (POC) and dissolved organic carbon (DOC). Part of this organic carbon is refractory (not easily mineralised) and exported to the deep sea. The amount of POC and DOC originating from macroalgae that is sequestered globally in the deep sea for substantial periods (> centuries) is estimated in the range 50–250 Tg C yr-1, on average about 165 Tg C yr-1.
For some time already a debate is ongoing whether sequestered C originating from macroalgae should be considered Blue Carbon and whether it can be included in national Blue Carbon Accounting. The Verified Carbon Standard (VCS), which is the most commonly used verification standard for greenhouse gas (GHG) accounting, requires that GHG emissions reduction or removal must be ‘real’, ‘measurable’, ‘permanent’, ‘unique’ and ‘additional’. Currently, C sequestration beyond the habitat where the conservation or habitat creation takes place is excluded by the VCS.
Release of mobile C (inorganic, organic, labile and refractory) from vegetated coastal and marine habitats is called outwelling. Outwelling is not only an important C export mechanism for macroalgae, but for coastal vegetated habitats in general. Mangroves and salt marshes release mobile carbon mostly in the form of dissolved inorganic carbon (DIC) (about 124 and 29 Tg C yr-1, respectively ), while the mobile carbon released by seagrass is mainly POC (globally about 87 Tg C yr-1 ). If part of the outwelling carbon from macroalgae, mangroves, salt marshes and seagrass is stored in the deep ocean, its contribution to C sequestration can be much larger than the blue carbon contribution related to local sediment burial.
Maintaining or creating suitable habitats for macroalgae or seaweed contributes to limiting GHG emission. However, for outwelling carbon to be accountable, the locations of origin and sequestration must take place in an area that is owned by the relevant jurisdiction. This may be relatively straightforward for nations with large exclusive economic zones (EEZ), but problematic otherwise. Moreover, while there is ample evidence that macroalgal C is sequestered in oceanic sinks beyond the macroalgal habitat, direct estimates of macroalgal C burial rates are not yet available.
Wang et al. 2021 compared blue carbon sequestration for a large number of tidal wetlands with contrasting annual temperature, mean annual precipitation, tidal range, elevation, relative sea level rise, total suspended matters and tropical cyclone frequency. By establishing linear relationships between these environmental factors and measured soil C accumulation and by assuming their general validity under projected emissions scenarios, they estimated the possible effect of climate change on blue carbon sequestration in tidal wetlands. The results of this study suggest that global tidal wetland C accumulation will increase under both a moderate and a high-emission scenario, even considering a decrease of the total wetland area by 30% as a result of limited accommodation space. Under the high emission scenario the C sequestration increased by more than 35% in 2100 and under the medium scenario by 10-34%. These results demonstrate that preserving and rehabilitating mangroves and salt marshes will remain an effective approach to tackling global climate change with significant regional benefits in tidal wetland-rich countries.
Projects that aim at enhancing blue carbon storage by maintaining and restoring mangrove forests, salt marshes and seagrass meadows deliver also many other ecosystem services. They contribute to the goals of the Sendai Framework for Disaster Risk Reduction. They also contribute to protecting vulnerable coastal zones against the impacts of climate change and sea level rise, see Climate adaptation measures for the coastal zone.
Financing blue carbon sequestration projects
Carbon sequestration through wetland maintenance and restoration projects contributes to mitigating climate change and contributes to meeting the Paris Climate Agreement signatories' commitments to reduce national emissions. Such projects could therefore be eligible for financing through the CO2 emissions trading system (ETS). Mangrove forests have the greatest storage potential and large parts of the existing mangrove forests worldwide could be protected with resources from the carbon ETS. Reliable estimates of commitments are required as projects must meet the 'additionality criterion' for certifiable carbon credits under the United Nations Framework Convention on Climate Change rules. Methods and protocols need to be developed and should prevent the displacement of deforestation areas. Including blue carbon sequestration in the carbon ETS should further address economic barriers related to upfront costs, profit sharing and long-term lock-in contracts.
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