The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal (CDR)^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. http://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal (CDR)^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. http://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal (CDR)^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. http://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal (CDR)^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

The term “blue carbon” has historically referred to three (vascularly) vegetated coastal ecosystems: mangrove forests, salt marshes, and seagrass beds^[1]Lovelock CE, Duarte CM. 2019 Dimensions of Blue Carbon and emerging perspectives. Biol. Lett. 15: 20180781. http://dx.doi.org/10.1098/rsbl.2018.0781 . These ecosystems are among the most productive in the world^[2]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259.  and sequester carbon in their underlying substrates (e.g., soil or sediment). Estimates of their global distribution reach ~36-185 million hectares, storing as much as 33 billion tons of carbon^[3]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 , however there remain large knowledge gaps around spatial extent and storage capacity, owing to the high variability across regions and ecosystems. The carbon in the substrate can be stored as organic or inorganic carbon and can remain sequestered for millennia^[4]Macreadie, P. I., Costa, M. D. P., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., ... Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment. doi:10.1038/s43017-021-00224-1 ,^[5]Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. & Marba, N. 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nat. Clim. Chang. 3:961–8. , positioning these ecosystems as important parts of the global carbon cycle. Situated between land and water, these ecosystems also play critical roles in human society and provide many services including coastal buffering from storms, surges and erosion control generally, nursery grounds for fish, culturally significant sites, and sources of income from fishing and tourism. Blue carbon ecosystems have gained attention as levers for climate change mitigation, including carbon sequestration^[6]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf . 

Some of this attention is related to the major uptick in interest and attention around carbon dioxide removal^[7]“Anthropogenic activities removing CO₂ from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities.” https://www.ipcc.ch/sr15/chapter/glossary/ , which has again raised questions about the capacity of blue carbon ecosystems to sequester carbon and combat climate change^[8]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[9]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278. ,^[10]Cross, J.N., Sweeney, C., Jewett, E.B., Feely, R.A., McElhany, P., Carter, B., Stein, T., Kitch, G.D., and Gledhill, D.K., 2023. Strategy for NOAA Carbon Dioxide Removal Research: A white paper documenting a potential NOAA CDR Science Strategy as an element of NOAA’s Climate Interventions Portfolio. NOAA Special Report. NOAA, Washington DC. DOI: 10.25923/gzke-8730 ,^[11]National Academies of Sciences, Engineering, and Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. https://doi.org/10.17226/25259. . This discussion has also broadened the conversation around the definition of blue carbon, with the IPCC (2019)^[12]IPCC, 2019: Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 677–702. https://doi.org/10.1017/9781009157964.010.  defining blue carbon as ‘all biologically driven carbon fluxes and storage in marine systems that are amenable to management’. This definition can and has been interpreted to encompass not only coastal vegetated ecosystems that store carbon in their structure and sediments, but also other marine life such as fish and mammals, as well as natural seaweed stands and seaweed farms - all systems that not only store and recycle carbon through their living tissues, but also respond to anthropogenic manipulation and management. Two notable examples of this broadened definition include the National Academies of Sciences, Engineering, and Medicine’s (NASEM) 2022 report on ocean carbon dioxide removal in which they include a chapter on “Recovery of Marine Ecosystems” for CDR (including benthic, pelagic, and offshore ocean systems along with mangrove forests, salt marshes, and seagrass beds) and the National Oceanographic and Atmospheric Administration’s (NOAA) 2023 Strategy for Carbon Dioxide Removal Research which includes coastal blue carbon (defined as mangrove forests, salt marshes, and seagrass beds) as well as ‘Marine Ecosystem Recovery’ (fishes and marine mammals). (Also note NOAA, 2021.).

Following these recent advances, this road map will consider carbon stored in natural seaweed stands and in marine animals (e.g., fishes and mammals) in the definition of blue carbon, alongside mangrove forests, salt marshes, and seagrass beds. Natural seaweed stands and marine animals have received substantially less attention as potential pathways for additional carbon dioxide sequestration and will require accelerated research to determine their potential. The nascent nature of these pathways makes them particularly well suited to be assessed in this road map.

Note: While the blue carbon definition above could be interpreted to include microalgae cultivation, we have excluded it from consideration in this road map because we have already released a road map for Microalgae Cultivation and Carbon Sequestration. In addition, we limit discussion of seaweed (macroalgae) farms to coastal farms which passively sequester carbon through biomass export to underlying sediments or offshore to deeper waters. We exclude discussion of seaweed farming coupled to active sequestration (such as sinking or harvesting to produce biochar) because this topic is covered in a different road map, Macroalgae Cultivation and Carbon Sequestration.

This road map focuses on assessing the potential for blue carbon to contribute to carbon dioxide removal (CDR), or the additional sequestration of carbon, and what is needed to achieve that potential. In many cases, this will look like the large-scale restoration of ecosystems or rebuilding of animal populations (and, in some cases, expansion of ecosystems or animal populations beyond past limits). Recent work by the Environmental Defense Fund and the High Level Panel for a Sustainable Ocean Economy suggests that even the most rigorous and optimistic future estimates of blue carbon restoration may yield small gains in additional carbon sequestration when compared with the scale of anthropogenic emissions and other possible CDR pathways^[13]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf ,^[14]Collins, J. R., Boenish, R. E., Kleisner, K. M., Rader, D. N., Fujita, R. M., and Moritsch M. M. 2022. Coastal natural climate solutions: an assessment of scientific knowledge surrounding pathways for carbon dioxide removal and avoided emissions in nearshore blue carbon ecosystems. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/files/2022-10/Coastal%20Natural%20Climate%20Solutions.pdf ,^[15]Collins, J. R., Boenish, R. E., Fujita, R. M., Rader, D. N., and Moore, L. A. 2022. Natural climate solutions in the open ocean: scientific knowledge and opportunities surrounding four potential pathways for carbon dioxide removal or avoided emissions. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Natural%20Climate%20Solutions%20in%20the%20Open%20Ocean.pdf ,^[16]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change . One reason for this is the spatial limitation presented by the restoration of coastal vegetated ecosystems (i.e., mangrove forests, seagrass beds, and salt marshes). These ecosystems have spatial limits on their potential for restoration, and thus, their potential for carbon removal, however other sources of blue carbon, such as animal biomass and seaweed farms, are far less limited by spatial constraints. Moreover, blue carbon restoration offers low or no risk pathways, which sets it apart from other proposed CDR solutions. Not only do restoration efforts of blue carbon pose fewer risks to the environment and society than other proposed CDR solutions, but they also offer an enormous capacity for other benefits to both ecosystems and communities. In this way, carbon removal may in fact be best thought of as a “co-benefit” to blue carbon restoration and efforts^[17]Williamson P and Gattuso J-P (2022) Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness. Front. Clim. 4:853666. doi: 10.3389/fclim.2022.853666 . Regardless, there is still critical research that would increase our understanding of the role blue carbon plays in the carbon cycle and inform responsible decision making at the ocean-climate nexus.

In light of current and future climate disruption and risk, there is a growing need for solutions that will help to remove and durably store legacy greenhouse gas emissions. However, there remain a substantial set of unanswered questions around the capacity and efficacy of carbon storage in blue carbon systems. As scientists, practitioners, and policy makers continue investigating blue carbon as a climate solution, it is critical to be clear about what is known, what remains unknown, and identify a pathway by which to answer the most important outlying questions. This road map lays out the current state of knowledge around carbon sequestration and storage in blue carbon systems, details the most pressing unanswered questions to understand the potential for blue carbon sequestration, and proposes ways in which to accelerate the development and testing of these potential solution pathways.

A Note on Restoration vs. Protection

Protection of existing blue carbon ecosystems and their embedded carbon avoid emissions generated as these systems degrade. This is complementary and distinct from restoration of blue carbon systems, which is regrowth of these ecosystems. This regrowth is where additional carbon can be sequestered. Because the focus of this road map is the potential for blue carbon systems to contribute to carbon dioxide removal, this map will only focus on restoration efforts, their state of progress, challenges, knowledge gaps, and opportunities to drive forward progress.

A Note on Units of Measurement

This road map differs from the other Ocean Visions’ Ocean CDR Road Maps in its use of ‘Gigaton CO2 equivalent’ (Gt CO2e) in order to account for the important role played by non-CO2 greenhouse gases (e.g., methane, CH4, and nitrous oxide, N2O) within blue carbon systems. Although petagrams (Pg) is a unit commonly used in blue carbon literature, we use the equivalent gigatons (or gigatonnes; Gt) in this road map.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea .

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea .

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.