Blue carbon sequestration

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Definition of Blue carbon:
The term ‘Blue Carbon’ refers to the carbon sequestered by the world’s ocean or coastal vegetated ecosystems.
This is the common definition for Blue carbon, other definitions can be discussed in the article


Like terrestrial ecosystems, marine ecosystems are capable of capturing CO2 from the atmosphere and storing it in living organisms (especially vegetation) and in the soil. Coastal ecosystems such as salt marshes, mangroves, seagrass and seaweed (macroalgae) are exceptionally efficient in this regard. Although they occupy a small part of the global ocean (7.6%), coastal seas provide up to 30% of global marine primary production and about 50% of the organic carbon supplied to the deep ocean[1]. These ecosystems not only capture CO2, but also help protect the coast against erosion and flooding[2]. They strengthen the cohesion of seafloor sediments so that they are less easily washed away and possess self-sustaining capacity by promoting sedimentation, see Climate adaptation measures for the coastal zone. They also contribute to the goals of the Sendai Framework for Disaster Risk Reduction[3][4]. However, nearly 50% of the pre-industrial, natural extent of global coastal wetlands have been lost since the 19th century[5].

Nature-based coastal protection makes use of the protective functions of coastal wetlands by restoring wetlands that have forgone in the past, by protecting and maintaining them and by promoting the development of new coastal protection ecosystems in appropriate places. In addition to capturing CO2, nature-based coastal protection also provides a large number of ecosystem services, such as breaking down polluting and eutrophicating substances, food production, grounds for spawning, breeding and nursery for a multitude of organisms (e.g. fish, birds), and scenic beauty. This article examines the role of CO2 sequestration by different coastal ecosystems. In the context of ETS, the trading of CO2 emission rights, this role can be used to generate finances that contribute to the realization of nature-based coastal protection and other ecosystem services delivered by coastal wetlands.


Blue carbon ecosystems

To be considered blue carbon, an ecosystem should[6]:

  1. have a significant scale of greenhouse gas emissions or removals,
  2. support long term (>centuries) storage of carbon, and
  3. be amenable to management actions that enhance carbon storage or avoid greenhouse gas emissions.

The vegetated coastal ecosystems usually considered blue carbon sinks are salt marshes, mangroves and seagrass meadows.

Blue carbon did not originally include macroalgal ecosystems; however evidence is mounting that macroalgal ecosystems (seaweed) 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 carbon (DIC), dissolved organic carbon (DOC) 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[7]. Macroalgal beds therefore have the potential to regulate carbon dynamics in coastal ecosystems[8].

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. The process of calcification releases CO2 and thus these ecosystems are likely to be net CO2 sources rather than sinks[6] (see Ocean acidification for an explanation).

Blue carbon concept

The concept of blue carbon was introduced in 2009 in a United Nations assessment report[9], with the idea that the role of coastal ecosystems such as salt marshes, mangroves and seagrass meadows in absorbing carbon (C) to reduce net carbon dioxide 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.

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[10].

Current estimates of blue carbon sequestration in coastal wetlands (mangroves, salt marshes and seagrass meadows) worldwide are in the order of 80 Tg C / year (1 Tg = 1012 g). Restoration of coastal wetlands lost over the past century could increase this amount by a factor 2-4 (Macreadie et al., 2021[11]). This amounts to a few percent of global emissions in 2022 (10 Pg C /year, 1 Pg = 1015 g).

Not all the carbon buried in the soil is definitively sequestered. The decay rate in the top soil is much faster than in the deeper soil layers. Rough estimates of the carbon residence times vary between decades for the top soil and millennia for the deep soil (around 1 m or more)[12].

Mangroves

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 are close to 150,000 km2 [13]. 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[14] are of the order of 160-210 g C m-2yr-1 [15][16], 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 [17][15][16][18][19][20]. 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[16]), 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 5-12 Pg C [15][17][21][20]. Annual losses of mangrove systems have declined from 1-3% in the late 20th century to 0.3-0.6% in the early 21st century[22]. Alongi (2020[15]) estimated that the complete destruction of these mangroves for conversion to aquaculture or agriculture has yielded an increase of CO2 emissions of 0.2 %.

Analysis of the origin of mangrove soil organic matter suggests that external inputs from the sea and catchment provide a considerable contribution to the carbon stock of mangrove soils. Thus, it is likely that some mangrove ecosystems will not be in metabolic balance with some systems being net importers and others being net exporters of carbon[20].

Salt marshes

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 55 000 km2 [23][24] (other estimates are a few times larger[11]). Average rates of C sequestration by salt marshes are found in the range 170-240 g C m-2yr-1 [15][16]. However, when ageing, the carbon sequestration rate of salt marshes is reduced[25]. The global C sequestration by salt marshes is currently estimated in the range 11-13 Tg C yr-1 [15][16]. Alongi (2020[18]) estimated the global stock of sequestered C in salt marshes to be about 1.8 Pg C and Macreadie et al. (2021[11]) 0.9-1.4 Pg C.

Seagrass

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 seagrass genus.

Seagrasses cover globally an area of about 160 000 km2 [26]. Alongi (2020[15]) 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 35 Tg C yr-1 and the global stock of sequestered C about 2.6 Pg C. The estimate of Macreadie et al. (2021[11]) is 3.8-21 Pg C.

Clarification is needed of the role of calcifying organisms (i.e., calcareous macroalgae, coralline algal crusts, and epiphytes) in seagrass meadows. Seagrass meadows with high calcification rates can be net sources of CO2 to the atmosphere (as explained in the article Ocean acidification). In contrast, there is also evidence that some seagrass habitats cause dissolution of calcium carbonate sediments, raising seawater alkalinity that shifts the carbonate system of the seawater so that it becomes a net sink of CO2. [27].

The restoration of degraded seagrass meadows contributes after some time to carbon storage.[28] However, a field study in Western Australia suggests that it takes at least five years for a restored seagrass meadow to begin sequestering significant amounts of carbon.[29]

Seaweed (macroalgae)

Seaweed (macroalgae) are common on shores worldwide. While kelps (seaweed belonging to the order Laminariales) are most common in temperate climate zones, other brown macroalgae (e.g. Turbinaria spp. and Sargassum spp. ) abound along most tropical coasts. According to a recent compilation, seaweed ecosystems cover globally an area of about 6 million km2 [30], and the net primary production was estimated at about 1.3 Pg C yr-1. The high effectiveness of carbon fixation by macroalgae stems from their stoichiometric C:N ratio of approximately 400:20, which is much higher than the 106:16 ratio of the general phytoplankton. 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. The global C burial rate in shelf sediments was estimated at 6 -14 Tg C yr-1 [31][32]. Most of the organic C is produced by Kelp and is released to the aquatic environment in the form of particulate organic carbon (POC) and dissolved organic carbon (DOC). A significant part of this organic carbon is refractory (not easily mineralised) and exported to the deep sea[33]. 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[31][8]. However, these figures are criticized as not all contributions to the release of CO2 are taken into account[34].

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[35]. The climate mitigation value of seaweeds is not solely determined by the amount of carbon taken up and fixed in the algal biomass, but also by the rate of exudation of labile DOC, grazing, microbial activity, carbon transport to sediments or deep water, release of other greenhouse gasses such as methane, inputs of carbon to the system, and the balance between all of the heterotrophic and autotrophic processes within a seaweed system[36]. 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.

A specific topic of discussion is the concept of mitigating climate change on a global scale by using all ocean areas potentially suitable for seaweed farming to produce vast amounts of algal biomass that are then sunk into the deep ocean. A theoretical Earth System Model study of open-ocean macroalgae mariculture (not considering engineering constraints) estimates that about 2 Pg C/y can be removed from the atmosphere in this way[37]. This is to be compared with the 2019 total global emissions of about 10 Pg C /y. Carbon removal can be further boosted to about 3.5 Pg C /y by applying artificial upwelling (pumping nutrient-rich ocean waters to the surface). According to the theoretical model, most of the carbon reaching the deep ocean is sequestered for at least thousand years. The negative side effects of the concept are numerous, however. For example, phytoplankton net primary production can collapse due to the competition for nutrients with macroalgae and due to canopy shading. New oxygen minimum zones may be created on the seafloor by oxygen consumption from remineralization of sunken biomass. The injection of organic C into deep water can alter food webs and species composition. Marine surface ecology could be affected, as could ocean services such as food provision. The costs of growing and sinking seaweed biomass are also a possible bottleneck[22]. The concept requires further research with less idealized experimental settings to determine if its benefits for carbon dioxide removal outweigh the negative side effects[37].

Carbon outwelling

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[7]. 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 [18]), while seagrass releases a large amount of mobile carbon as POC and DOC (estimated in the order of 87 Tg C yr-1 [38]). 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. Ortega et al. (2019[39]) found ubiquitous presence of macroalgal DNA in the ocean up to a depth of 4000 km and 4860 km away from the nearest coastline, suggesting widespread carbon export to the open ocean. In contrast, direct measurements of carbon outwelling by a seagrass meadow in the Mediterranean revealed CO2 to be a dominant component (due to heterotrophy and aerobic metabolism) that eventually escapes to the atmosphere. It appeared that CO2 emissions following offshore outwelling largely exceeded long term sequestration in local soils during the study period[40].

Maintaining or creating suitable habitats for macroalgae or seaweed contributes to limiting greenhouse gas (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 sequestration rates are not yet available[35], see also Ocean carbon sink. While the local carbon sequestration of mangroves, salt marshes, eelgrass and seaweed is only on the order of 1% of current anthropogenic greenhouse gas emissions, the inclusion of carbon outflow could potentially increase the blue carbon contribution to about 3% [41].

Emission of greenhouse gases from coastal wetlands

Emissions of other greenhouse gases, especially methane CH4, can counteract the carbon accumulation benefits in these systems[42]. The global warming potential of methane is many times that of carbon dioxide. The emissions of an even stronger greenhouse gas, nitrous oxide N2O, in coastal ecosystems are generally small. Mangrove and saltmarsh habitats show a similar range of methane fluxes, whereas seagrass CH4 emissions are significantly lower. Although CH4 decrease with increasing salinity, even in high salinity systems, mean CH4 emissions can be greater than 5% of the median soil carbon sequestration, which is significant for accounting[43]. Understanding how best to account for methane accurately in tidal marsh systems is one of the largest remaining hurdles to leveraging climate mitigation opportunities[27].

Climate change impact on blue carbon

Wang et al. 2021[16] compared blue carbon sequestration for a large number of tidal wetlands (saltmarshes and mangroves) 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. Under the high emission scenario the simulated C sequestration increased by more than 35% in 2100 and under the medium scenario by 10-34%. Even considering a decrease of the total wetland area by 30% as a result of limited accommodation space[44], the global tidal wetland C accumulation will likely increase under both a moderate and a high-emission scenario[16]. Analysis of the carbon storage history in a salt march in northern England over a period of more than hundred years also provides evidence of a positive correlation between the rate of carbon storage with the rate of sea level rise[45]. 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.

Seagrass growth depends on temperature. For each seagrass species, optimum growth occurs at a specific temperature; growth declines at higher and lower temperatures. The optimum temperature is higher for tropical species than for subtropical and temperate species. As the current seagrass distribution is tuned to present local temperature regimes, global warming will alter existing seagrass community structures. Severe habitat loss is projected for endemic species such as Posidonia oceanica in the Mediterranean. Ocean acidification meanwhile increases seagrass productivity and could promote the survival of existing seagrass meadows at higher temperatures[46].

Wild kelp forests that cover about 25% of the world’s coastline, mostly in temperate and polar regions, are intolerant to temperatures exceeding 20oC. Whereas ocean acidification has little effect on kelp, rising temperature is a major contributor to the decline of kelp ecosystems. The loss of kelp forests on rocky substrates is followed by the colonization of algal turfs, leading to a reduction in habitat complexity, carbon storage and diversity[46][1].

Relevance of blue carbon for co-financing nature-based coastal protection measures

In most countries, coastal protection is considered a non-marketable public service. This means that coastal protection measures are financed with public funds collected through taxes. However, public funding is not always sufficient to ensure adequate coastal protection that avoids the much higher cost of storm surge damage risk. Additional funding can be raised for nature-based coastal protection projects that generate new carbon sinks.

Carbon sequestration through the creation, maintenance and restoration of shore protecting coastal wetlands contributes to mitigating climate change and contributes to meeting the Paris Climate Agreement signatories' commitments to reduce national emissions. Such projects may therefore qualify for financing through the CO2 emissions trading system (ETS). For instance, large parts of the existing mangrove forests worldwide could be protected with resources from the carbon ETS[47]. Mangrove forests are not only highly efficient for wave attenuation[48] and storm surge damping[49], but also have great potential for carbon storage. Reliable estimates of carbon sequestration 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 for allocating carbon credits to nature-based coastal protection projects are still under development[50]. 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[47].

Credits for blue carbon sequestration currently (2024) depend on the Voluntary Carbon Market (VCM). This is a market of voluntary carbon buyers who whish to achieve net carbon neutrality by offsetting their greenhouse gas emissions. These buyers are not required to reduce emissions and may only do so to a limited extent. The price of carbon credits in the VCM are currently in the range 1-10 US$/ton CO2 equivalent for Verra-certified nature-based carbon sequestration projects. This is in stark contrast to the price of 50-75 euros on the European compliance market, where a limited stock of carbon credits are traded to meet legally binding emissions reduction targets. In most countries, nature-based carbon sequestration projects are not economically viable if they are solely based on current VCM prices (i.e. without taking into account the net added economic value provided by other ecosystem services).[51] See also Governance policies for a blue bio-economy.

Appendix: Example of carbon credits for mangrove restoration

Fig. 1. Mangrove restoration in Java (Indonesia) by creating appropriate conditions for sediment deposition with permeable bamboo dams[52]. Photo credit Indonesian Ministry of Marine Affairs and Fisheries (MMAF).

Here is a fictitious example of carbon credits that can potentially be earned from mangrove restoration. The project is located in a muddy coastal zone with low wave exposure. We assume that a 4 km long 'soft' coast-parallel breakwater at 250 m off the shoreline provides sufficient wave damping to allow the natural recovery of mangroves landward from the breakwater. According to the earlier given estimate, the resulting 100 ha mangrove forest can store about 160-210 tons C /yr or 600-770 tons CO2 /yr. The carbon credits would be worth approximately 70 euros per ton CO2 if they could be traded on the European Compliance Market. The blue carbon yields of the project would in that case be in the order of 50,000 euros per year. However, the price of carbon credits on the voluntary market is lower by a factor of 10-100. Taking the example of the Mekong Delta, the construction costs of a bamboo breakwater are in the range of 50-140 €/m [53]. The cost of the breakwater of 4 km is then between 200,000 and 560,000 €. We assume that a 10-year lifespan of the bamboo breakwater is sufficient for natural recovery of the mangrove forest, that the mature mangrove belt is self-sustaining, and that carbon sequestration is well-validated. Construction costs will then be fully covered with the EU-compliance carbon market price, but only a few percent of the costs will be covered with the current price on the voluntary market. In this example we only have considered the blue carbon benefits of mangrove restoration. It should be noted that the monetary value of other benefits is much higher, especially benefits related to coastal protection (avoided storm surge and flood damage)[54].

The creation, maintenance, and management of eelgrass and macroalgal beds in Hakata Bay (Fukuoka City, Japan) was co-financed with carbon credits based on an estimation of the amount of sequestered carbon. A description of the implementation and lessons learned of this innovative financing mechanism is given by Kuwae et al. (2022)[55]. Increasing the viability of nature-based carbon sequestration projects requires broadening the market for blue carbon projects and valuing the economic co-benefits. Key issues include monitoring and validating the amounts of carbon captured and the inclusion of blue carbon in national carbon accounting systems.


Related articles

Mangroves
Salt marshes
Seagrass meadows
Seaweed (macro-algae) ecosystem services
Nature-based shore protection
Ocean carbon sink
Governance policies for a blue bio-economy


External sources

  • https://www.thebluecarboninitiative.org/ The Blue Carbon Scientific Working Group provides the scientific foundation for the Blue Carbon Initiative by synthesizing current and emerging science on blue carbon and by providing a robust scientific basis for coastal carbon conservation, management and assessment.
  • https://verra.org/project/vcs-program/ Verra develops standards for certifying carbon credits generated by projects that are reducing greenhouse gas emissions elsewhere, administers the implementation and keeps the carbon credit registry system.


References

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