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Blue carbon is a concept within climate change mitigation that refers to "biologically driven carbon fluxes and storage in marine systems that are amenable to management". Most commonly, it refers to the role that , and Seagrass meadow can play in carbon sequestration. These ecosystems can play an important role for climate change mitigation and ecosystem-based adaptation. However, when blue carbon ecosystems are degraded or lost, they release carbon back to the atmosphere, thereby adding to greenhouse gas emissions.
The methods for blue carbon management fall into the category of "ocean-based biological carbon dioxide removal (CDR) methods".Canadell, J. G., P. M. S. Monteiro, M. H. Costa, L. Cotrim da Cunha, P. M. Cox, A. V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P. K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, and K. Zickfeld, 2021: Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Masson-Delmotte,. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 673–816, . They are a type of biological carbon fixation.
Scientists are looking for ways to further develop the blue carbon potential of ecosystems. However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution is under debate.
The term deep blue carbon is also in use and refers to storing carbon in the deep ocean waters.
Another definition states: "Blue carbon refers to organic carbon that is captured and stored by the oceans and coastal ecosystems, particularly by vegetated coastal ecosystems: Seagrass meadow, Tidal marsh, and Mangrove forest." Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
Coastal blue carbon focuses on "rooted vegetation in the coastal zone, such as , and ". Seagrass, salt marshes and mangroves are sometimes referred to as "blue forests" in contrast to land-based "green forests".
Deep blue carbon is located in the high seas beyond national jurisdictions. It includes carbon contained in "continental shelf waters, Deep sea waters and the sea floor beneath them" and makes up 90% of all ocean carbon. Deep blue carbon is generally seen as "less amenable to management" and challenging due to lack of data "relating to the permanence of their carbon stores".
The vegetated coastal ecosystems of tidal marshes, mangroves and seagrasses (which are grouped as "blue carbon") have high carbon burial rates. This is because they accumulate carbon in their soils and .
Such ecosystems can contribute to climate change mitigation and also to ecosystem-based adaptation. However, when coastal blue carbon ecosystems are degraded or lost they release carbon back to the atmosphere.
Mangroves, salt marshes and seagrasses can store carbon and are highly efficient . They capture from the atmosphere by sequestering the carbon in their underlying sediments, in underground and below-ground biomass, and in dead biomass.National Academies of Sciences, Engineering, and Medicine (2025). 9780309484527 . ISBN 9780309484527
Although vegetated coastal ecosystems cover less area and have less aboveground biomass than terrestrial plants they have the potential to impact long term C sequestration, particularly in sediment sinks.
One of the main concerns with blue carbon is that the rate of loss of these important marine ecosystems is much higher than any other ecosystem on the planet, even compared to . Current estimates suggest a loss of 2-7% per year, which is not only lost carbon sequestration, but also lost habitat that is important for managing climate, coastal protection, and health.
habitats that sequester carbon are altered and decreased, that stored amount of C is being released into the atmosphere, continuing the current accelerated rate of climate change. Impacts on these habitats globally will directly and indirectly release the previously stored carbon, which had been sequestered in the sediments of these habitats. Declines in vegetated coastal habitats are seen worldwide.
Quantifying rates of decrease are difficult to calculate, however measurements have been estimated by researchers indicating that if blue carbon ecosystems continue to decline, for any number of reasons, 30-40% of tidal marshes and seagrasses and approximately 100% of mangroves could be gone in the next century.
Reasons for the decline of mangroves, seagrass, and marshes include land use changes, climate and drought related effects, dams built in the watershed, convergence to aquaculture and agriculture, land development and sea-level rise due to climate change. Increases in these activities can lead to significant decreases in habitat availability and thus increases in released C from sediments.
As anthropogenic effects and climate change are heightened, the effectiveness of blue carbon sinks will diminish and CO2 emissions will be further increased. Data on the rates at which CO2 is being released into the atmosphere is not robust currently; however, research is being conducted to gather better information to analyze trends. Loss of underground biomass (roots and rhizomes) will allow for CO2 to be emitted changing these habitats into sources rather than carbon sinks.
Research done on mangrove soils from the Red Sea have shown that increases in nutrient loads to these soils do not increase carbon mineralization and subsequent CO2 release. This neutral effect of fertilization was not found to be true in all mangrove forest types. Carbon capture rates also increased in these forests due to increased growth rates of the mangroves. In forests with increases in respiration there were also increases in mangrove growth of up to six times the normal rate.
Marshes have high productivity, with a large portion of primary production in belowground biomass. This belowground biomass can form deposits up to 8m deep. Marshes provide valuable habitat for plants, birds, and juvenile fish, protect coastal habitat from storm surge and flooding, and can reduce Eutrophication to coastal waters. Similarly to mangrove and seagrass habitats, marshes also serve as important . Marshes sequester C in underground biomass due to high rates of organic sedimentation and anaerobic-dominated decomposition. Salt marshes cover approximately 22,000 to 400,000 km2 globally, with an estimated carbon burial rate of 210 g C m−2 yr−1.
Salt marshes may not be expansive worldwide in relation to forests, but they have a C burial rate that is over 50 times faster than tropical rainforests. Rates of burial have been estimated at up to 87.2 ± 9.6 Tg C yr−1 which is greater than that of tropical rainforests, 53 ± 9.6 Tg C yr−1. Since the 1800s salt marshes have been disturbed due to development and a lack of understanding of their importance. The 25% decline since that time has led to a decrease in potential C sink area coupled with the release of once buried C. Consequences of increasingly degraded marsh habitat are a decrease in C stock in sediments, a decrease in plant biomass and thus a decrease in photosynthesis reducing the amount of CO2 taken up by the plants, failure of C in plant blades to be transferred into the sediment, possible acceleration of erosive processes due to the lack of plant biomass, and acceleration of buried C release to the atmosphere.
Tidal marshes have been impacted by humans for centuries, including modification for grazing, haymaking, reclamation for agriculture, development and ports, evaporation ponds for salt production, modification for aquaculture, insect control, tidal power and flood protection. Marshes are also susceptible to pollution from oil, industrial chemicals, and most commonly, eutrophication. Introduced species, sea-level rise, river damming and decreased sedimentation are additional longterm changes that affect marsh habitat, and in turn, may affect carbon sequestration potential.
Global mangrove canopy cover is estimated as between 83,495 km2 and 167,387 km2 in 2012 with Indonesia containing approximately 30% of the entire global mangrove forest area. Mangrove forests are responsible for approximately 10% of global carbon burial, with an estimated carbon burial rate of 174 g C m−2 yr−1.
Mangroves, like seagrasses, have potential for high levels of carbon sequestration. They account for 3% of the global carbon sequestration by tropical forests and 14% of the global coastal ocean's carbon burial.
Mangroves are naturally disturbed by floods, , coastal storms like and hurricanes, lightning, disease and pests, and changes in water quality or temperature. Although they are resilient to many of these natural disturbances, they are highly susceptible to human impacts including urban development, aquaculture, mining, and overexploitation of shellfish, crustaceans, fish and timber. Mangroves provide globally important ecosystem services and carbon sequestration and are thus an important habitat to conserve and repair when possible.
Dams threaten habitats by slowing the amount of freshwater reaching mangroves. Coral reef destruction also plays a role in mangrove habitat health as reefs slow wave energy to a level that mangroves are more tolerant of.
Carbon primarily accumulates in Pelagic sediment, which are Anoxic waters and thus continually preserve organic carbon from decadal-millennial time scales. High accumulation rates, low oxygen, low sediment conductivity and slower microbial decomposition rates all encourage carbon burial and carbon accumulation in these coastal sediments.
Compared to terrestrial habitats that lose carbon stocks as CO2 during decomposition or by disturbances like fires or deforestation, marine carbon sinks can retain C for much longer time periods. Carbon sequestration rates in seagrass meadows vary depending on the species, characteristics of the sediment, and depth of the habitats, but on average the carbon burial rate is approximately 138 g C m−2 yr−1.
Seagrass habitats are threatened by coastal eutrophication, increased seawater temperatures, increased sedimentation and coastal development, and sea level rise which may decrease light availability for photosynthesis. Seagrass loss has accelerated over the past few decades, from 0.9% per year prior to 1940 to 7% per year in 1990, with about 1/3 of global loss since WWII. The decline in seagrasses is due to a number of factors including drought, water quality issues, agricultural practices, invasive species, pathogens, fishing and climate change.
Scientists encourage the protection and continued research of these ecosystems for organic carbon storage, valuable habitat and other ecosystem services.
Restored seagrass meadows were found to start sequestering carbon in sediment within about four years. This was the time needed for the meadow to reach sufficient shoot density to cause sediment deposition.
The deep blue carbon terminology has been used in passing as early as 2017. The Ocean Frontier Institute has made it a centrepiece of their participation at COP27. It is investing significant resources into deep blue carbon research. In terms of net-new-carbon sequestration deep blue carbon offers an estimated 10-20 times higher potential than coastal blue carbon to achieve net-zero goals. There is still a lack of data in this area along with financial, ecological and environmental concerns. Advancements in research and technical capabilities are raising international interest in this kind of storage.
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