Produce Bio Oils

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Peatland
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Produce Bio Oils is a "Keep Watching" Drawdown Explorer solution.
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Produce
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Bio Oils
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Produce Bio Bricks

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Produce Bio Bricks is a "Keep Watching" Drawdown Explorer solution.
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Produce
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Bio Bricks
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Deploy Ocean Biomass Sinking

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Summary

Ocean biomass sinking involves sinking terrestrial plant material and/or seaweed in the deep sea, where the carbon it has converted into biomass can be stored. Using terrestrial material diverts biomass that might otherwise break down on land and release CO₂, while using seaweed removes carbon by cultivating and sinking new biomass produced in the ocean. This practice might be able to remove over 0.1 Gt CO₂‑eq/yr, but estimates remain highly uncertain due to limited data, and the adoption levels needed to reach this threshold are probably impractical. Advantages include the use of terrestrial biomass that might otherwise degrade on land and emit CO₂, and the ability to reduce nutrient pollution in some ocean areas when cultivating marine biomass. Disadvantages include its unclear effectiveness and durability, potentially high environmental risks, limited feasibility to operate at scale (particularly for seaweed biomass), and complex monitoring and verification. We conclude that Deploy Ocean Biomass Sinking is “Not Recommended” as a climate solution.

Description for Social and Search
Ocean biomass sinking involves sinking terrestrial plant material and/or seaweed in the deep sea, where the carbon it has converted into biomass can be stored.
Overview

What is our assessment?

Our analysis finds that Deploy Ocean Biomass Sinking could have high potential environmental risks, including unknown impacts on marine ecosystems. It is also unclear how effective or durable carbon storage in the deep sea is from this approach. There are likely better alternative uses for terrestrial biomass, and cultivating seaweed at climate-relevant scales is probably not feasible. Even if it were, seaweed would probably provide greater value through other applications. Therefore, Deploy Ocean Biomass Sinking is currently “Not Recommended” as a climate solution.

Plausible Could it work? Yes
Ready Is it ready? No
Evidence Are there data to evaluate it? Limited
Effective Does it consistently work? No
Impact Is it big enough to matter? No
Risk Is it risky or harmful? Yes
Cost Is it cheap? ?

What is it?

Ocean biomass sinking relies on sinking terrestrial plant material and/or seaweed grown in the ocean to the deep sea or seafloor where it can be stored long-term. Cultivating and sinking seaweed removes carbon from the surface ocean, whereas sinking terrestrial biomass material can help reduce emissions that might otherwise occur if the material instead decomposed on land. While not a current practice, terrestrial biomass grown explicitly for sinking would also constitute a form of carbon removal. When biomass sinks naturally, most of it is degraded into CO₂ or other forms of carbon before reaching the deep sea. Deliberate sinking of biomass might avoid some of this degradation by expediting its delivery to the deep sea, depending on the method used. Once sunk, the biomass and any CO₂ or other forms of carbon produced from its degradation can be isolated from the atmosphere for decades to centuries due to the ocean’s slow circulation times at depth. Biomass sinking can be accomplished using active methods, like submersibles, or passive methods, like letting weighted bundles sink on their own. There has been a recent focus on sinking material in low-oxygen ocean basins (e.g., the Black Sea), which might help further minimize degradation, while improving the durability of sequestered carbon due to the long circulation time-scales typical of these regions.

Does it work?

Global estimates suggest that ~11% of carbon produced in natural seaweed ecosystems might be sequestered at depth, generally defined as below the mixed layer at around 1,000 m. However, very few studies have documented the export efficiency, or the fraction of carbon in surface waters that makes its way to the deep sea, of purposefully sunk terrestrial and seaweed biomass, as this practice is currently in the early stages of development and research. If biomass is quickly sunk, most carbon might make its way to the deep sea, while passive sinking techniques, if slower, could result in higher degradation rates. Sequestration also depends on the storage efficiency and durability of carbon once at depth. Some initial research suggests that biomass degradation may be slowed in low-oxygen basins, but this also remains poorly characterized in field studies. It is similarly unclear how durable the carbon stored below the mixed layer will be over climate-relevant timescales, both in the deep sea in general and in low-oxygen basins specifically.

Why are we excited?

The advantages of ocean biomass sinking include its potential ability to use land-based biomass that might otherwise be degraded in landfills or incinerated, both of which lead to CO₂ emissions. In some regions, seaweed cultivation could help reduce nutrient pollution, provide habitat for marine organisms, and locally buffer against ocean acidification. Estimates of potential climate impacts suggest that ocean biomass sinking using biomass from seaweed farms could theoretically exceed 0.1 Gt CO₂‑eq/yr. Still, those estimates remain highly speculative and require more research. Costs are poorly quantified, but some estimates suggest they could be low to moderately expensive compared to other marine carbon dioxide removal approaches, close to US$100/t CO₂.

Why are we concerned?

Ocean biomass sinking has many environmental and social risks that, though not currently fully understood, could make it unfeasible to deploy the technology at scale. Deep-sea and seafloor ecosystems are highly understudied, and it's unclear how new biomass might alter these unique environments. Potential impacts include increased acidification, nutrient pollution, and oxygen depletion of the deep sea, which could affect diverse marine life. Large-scale seaweed cultivation could reduce phytoplankton abundance, disrupt food webs, and deplete nutrients needed by other ecosystems. Cultivation in open ocean areas might relieve demand for coastal space, but they are often nutrient-poor, and adding nutrients raises serious concerns (see Ocean Fertilization). Terrestrial biomass sources could introduce contaminants into the ocean due to inadvertent inclusion of plastics or other pollutants in sunken biomass. This practice also comes with social risks. Some countries might disproportionately bear negative impacts wherever biomass is cultivated and/or sunk, as it could alter marine food webs and livelihoods. There could also be issues with public perception due to historical injustices around ocean dumping, potentially impeding future projects without meaningful community engagement and transparency. 

Moreover, there are numerous technical challenges relating to the effectiveness and durability of carbon sequestration. Biomass sources differ in how easily they break down, affecting how much carbon is stored at depth. Sunk biomass could also potentially release other greenhouse gases, such as methane and nitrous oxide. The location where biomass is disposed of also matters, impacting how much carbon reaches and stays at depth. However, all of these factors remain poorly constrained. Operational and technical challenges are also significant. To remove at least 0.1 Gt CO₂‑eq/yr using marine biomass, nearly 7 million ha of ocean – over 60% of the global coastline – could be needed for seaweed cultivation, which is impractical. Measurement and verification pose additional hurdles. In the case of seaweed cultivation, tracking carbon removal requires monitoring both CO₂ uptake at the ocean’s surface and export as well as storage at depth across large spatial and temporal scales. In addition, the opportunity cost of sinking terrestrial biomass is high due to competing land-based uses, as waste biomass and crop residues are finite resources. Growing new biomass explicitly for ocean sinking would introduce new risks, given that land is also a finite resource. Similarly, seaweed probably has higher value and carbon benefits as food, fertilizer, and other products.

Solution in Action

Arzeno-Soltero, I. B., Saenz, B. T., Frieder, C. A., Long, M. C., DeAngelo, J., Davis, S. J., & Davis, K. A. (2023). Large global variations in the carbon dioxide removal potential of seaweed farming due to biophysical constraints. Communications Earth & Environment, 4(1), 185. Link to source: https://doi.org/10.1038/s43247-023-00833-2

Bach, L. T., Tamsitt, V., Gower, J., Hurd, C. L., Raven, J. A., & Boyd, P. W. (2021). Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt. Nature Communications, 12(1), 2556. Link to source: https://doi.org/10.1038/s41467-021-22837-2

Boettcher, M., Chai, F., Canothan, M., Cooley, S., Keller, D. P., Klinsky, S., ... & Webb, R. M. (2023). A code of conduct for marine carbon dioxide removal research. Link to source: https://www.aspeninstitute.org/publications/a-code-of-conduct-for-marine-carbon-dioxide-removal-research/

Chopin, T., Costa-Pierce, B. A., Troell, M., Hurd, C. L., Costello, M. J., Backman, S., ... & Yarish, C. (2024). Deep-ocean seaweed dumping for carbon sequestration: Questionable, risky, and not the best use of valuable biomass. One Earth, 7(3), 359-364. Link to source: https://doi.org/10.1016/j.oneear.2024.01.013

Duarte, C. M., Wu, J., Xiao, X., Bruhn, A., & Krause-Jensen, D. (2017). Can seaweed farming play a role in climate change mitigation and adaptation?. Frontiers in Marine Science, 4, 100. Link to source: https://doi.org/10.3389/fmars.2017.00100

Hurd, C. L., Gattuso, J. P., & Boyd, P. W. (2024). Air‐sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?. Journal of Phycology, 60(1), 4-14. Link to source: https://doi.org/10.1111/jpy.13405

Hurd, C. L., Law, C. S., Bach, L. T., Britton, D., Hovenden, M., Paine, E. R., ... & Boyd, P. W. (2022). Forensic carbon accounting: Assessing the role of seaweeds for carbon sequestration. Journal of Phycology, 58(3), 347-363. Link to source: https://doi.org/10.1111/jpy.13249

Jones, D. C., Ito, T., Takano, Y., & Hsu, W. C. (2014). Spatial and seasonal variability of the air‐sea equilibration timescale of carbon dioxide. Global Biogeochemical Cycles, 28(11), 1163-1178. Link to source: https://doi.org/10.1002/2014GB004813

Keil, R. G., Nuwer, J. M., & Strand, S. E. (2010). Burial of agricultural byproducts in the deep sea as a form of carbon sequestration: A preliminary experiment. Marine Chemistry, 122(1-4), 91-95. Link to source: https://doi.org/10.1016/j.marchem.2010.07.007

National Academies of Sciences, Engineering, and Medicine. (2021). A research strategy for ocean-based carbon dioxide removal and sequestration. Link to source: https://www.nationalacademies.org/our-work/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration

Raven, M. R., Crotteau, M. A., Evans, N., Girard, Z. C., Martinez, A. M., Young, I., & Valentine, D. L. (2024). Biomass storage in anoxic marine basins: Initial estimates of geochemical impacts and CO2 sequestration capacity. AGU Advances, 5(1), e2023AV000950. Link to source: https://doi.org/10.1029/2023AV000950

Raven, M. R., Evans, N., Martinez, A. M., & Phillips, A. A. (2025). Big decisions from small experiments: observational strategies for biomass-based marine carbon storage. Environmental Research Letters, 20(5), 051001. Link to source: https://doi.org/10.1088/1748-9326/adc28d

Ricart, A. M., Krause-Jensen, D., Hancke, K., Price, N. N., Masqué, P., & Duarte, C. M. (2022). Sinking seaweed in the deep ocean for carbon neutrality is ahead of science and beyond the ethics. Environmental Research Letters, 17(8), 081003. Link to source: https://doi.org/10.1088/1748-9326/ac82ff

Ross, F. W., Boyd, P. W., Filbee-Dexter, K., Watanabe, K., Ortega, A., Krause-Jensen, D., ... & Macreadie, P. I. (2023). Potential role of seaweeds in climate change mitigation. Science of the Total Environment, 885, 163699. Link to source: https://doi.org/10.1016/j.scitotenv.2023.163699

Sheppard, E. J., Hurd, C. L., Britton, D. D., Reed, D. C., & Bach, L. T. (2023). Seaweed biogeochemistry: Global assessment of C: N and C: P ratios and implications for ocean afforestation. Journal of Phycology, 59(5), 879-892. Link to source: https://doi.org/10.1111/jpy.13381

Strand, S. E., & Benford, G. (2009). Ocean sequestration of crop residue carbon: recycling fossil fuel carbon back to deep sediments. Environmental Science and TechnologyLink to source: https://doi.org/10.1021/es8015556

Visions, O. (2022). Answering Critical Questions About Sinking Macroalgae for Carbon Dioxide Removal: A Research Framework to Investigate Sequestration Efficacy and Environmental Impacts. Link to source: https://oceanvisions.org/wp-content/uploads/2022/10/Ocean-Visions-Sinking-Seaweed-Report_FINAL.pdf

Wu, J., Keller, D. P., & Oschlies, A. (2023). Carbon dioxide removal via macroalgae open-ocean mariculture and sinking: an Earth system modeling study. Earth System Dynamics, 14(1), 185-221. Link to source: https://doi.org/10.5194/esd-14-185-2023

Xiao, X., Agusti, S., Lin, F., Li, K., Pan, Y., Yu, Y., ... & Duarte, C. M. (2017). Nutrient removal from Chinese coastal waters by large-scale seaweed aquaculture. Scientific Reports, 7(1), 46613. Link to source: https://doi.org/10.1038/srep46613

Xiao, X., Agustí, S., Yu, Y., Huang, Y., Chen, W., Hu, J., ... & Duarte, C. M. (2021). Seaweed farms provide refugia from ocean acidification. Science of the Total Environment, 776, 145192. Link to source: https://doi.org/10.1016/j.scitotenv.2021.145192

Credits

Lead Fellow

  • Christina Richardson, Ph.D.

Internal Reviewer

  • Christina Swanson, Ph.D.
Speed of Action
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Caveats
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Risks
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Consensus
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Action Word
Deploy
Solution Title
Ocean Biomass Sinking
Classification
Not Recommended
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