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
Solution Title
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
Solution Title
Bio Bricks
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Keep Watching
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Restore Seaweed

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The Restore Seaweed solution is coming soon.
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Restore
Solution Title
Seaweed
Classification
Highly Recommended
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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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Deploy
Solution Title
Ocean Biomass Sinking
Classification
Not Recommended
Lawmakers and Policymakers
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Deploy Direct Air Capture

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An image of large fans used for direct air capture of carbon dioxide
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Summary

Direct air capture (DAC) is an industrial process that captures CO₂ from the air and then injects it deep underground for permanent, geologic storage. This process is energy-intensive. Therefore, DAC can only be effective for net carbon removal if it does not generate high levels of emissions during the process. This requires that DAC be powered by zero- or low-carbon energy sources and that the captured carbon is permanently stored rather than used for emission-generating applications. Unlike the situation for many other carbon removal methods, the amounts of CO₂ captured and stored using DAC can be reliably measured, which is an advantage in the carbon marketplace. However, the effectiveness of DAC has been extremely low so far. DAC is also expensive, up to US$1,000/t CO₂ removed and stored. Substantial funding to support DAC development has come from fossil-fuel interests or their government proxies, which view carbon capture as a strategy to extend society’s use of fossil fuels. Therefore, there is a risk that DAC could be used to delay or avoid emissions reductions and perpetuate or even expand fossil-fuel production and use. Based on this risk, as well as the functional and financial challenges for scaling this technology to remove globally meaningful amounts of CO₂, we conclude that DAC is “Not Recommended” as a climate solution.

Description for Social and Search
Direct air capture (DAC) is an industrial process that captures CO2 from the air and then injects it deep underground for permanent, geologic storage.
Overview

What is our assessment?

Based on the difficulty of capturing low concentrations of CO₂ from the air and the associated technological, energy consumption, and financial challenges facing DAC, it is unlikely that this climate technology can be scaled up to remove globally meaningful amounts of CO₂. Furthermore, based on the current financial and policy support for DAC from fossil-fuel interests, there is a clear risk that the technology will be used to enable and perpetuate the production and use of fossil fuels, which is antithetical to solving the climate crisis. Therefore, we conclude that deployment of DAC is “Not Recommended” as a climate solution.

Plausible Could it work? Yes
Ready Is it ready? No
Evidence Are there data to evaluate it? Yes
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? No

What is it? 

DAC is a suite of engineered technologies that remove CO₂ directly from the atmosphere, concentrate it, and then inject it underground for permanent storage. CO₂ is captured from the atmosphere by moving large volumes of air, usually with large fans, past a reactive material that selectively binds CO₂, either a solid sorbent (referred to as solid-DAC or S-DAC) or a liquid solvent (referred to as liquid-DAC or L-DAC). The captured CO₂ is recovered from the reactive material by applying heat, pressure, or chemical reactions, and collected and compressed for transportation and storage. The concentrated CO₂ is then injected deep underground into geological formations, such as saline aquifers or basalt formations, where it can be permanently stored. 

Does it work?

The technology and chemistry for the selective capture of CO₂ from air are effective, although the CO₂ capture efficiency varies with the reactive material and other factors. A variety of solid and liquid reactive materials have been developed, along with material-specific processes for recovering captured CO₂ and regenerating the sorbents. This process is very energy-intensive and, for liquid-DAC, water-intensive. To capture and recover 1 t CO₂, solid-DAC uses about 1,100 kWh, while liquid-DAC uses about 2,500 kWh and consumes as much as 7 t of water. Most of the energy for DAC (70–90%) is used to generate heat for recovery of the captured CO₂ and regeneration of the sorbent material. Liquid-DAC requires temperatures up to about 900 °C (1,652 °F), while solid-DAC requires temperatures of only about 100 °C (212 °F). Because the process is so energy intensive, DAC achieves net carbon removal – capturing and sequestering more CO₂ than it emits – only if it is powered by zero or low-carbon energy sources and/or uses waste heat. For example, recent reporting showed that the amount of CO₂ captured and stored by Climeworks, the largest commercial DAC company currently in operation, was insufficient to offset the facility’s operational GHG emissions. CO₂ captured by a DAC facility can also be used for other purposes, such as enhanced oil recovery or production of algae biofuels. However, life cycle analyses conducted by the National Energy Technology Laboratory show that these pathways do not result in net carbon removal due to the emissions from production and/or use of these other products. Therefore, in addition to its requirements for zero or low-carbon energy, DAC can only be an effective method for net carbon removal if the CO₂ it captures is permanently stored deep underground. With appropriate pre-injection site selection, geologic testing, and post-injection monitoring, underground storage of CO₂ is safe and effectively permanent.

Why are we excited about it?

Unlike some other carbon removal technologies and practices, a DAC facility has a relatively small footprint and can be located anywhere there is sufficient low-carbon energy and infrastructure and capacity to transport or store captured CO₂. In addition, the amount of CO₂ removed from the atmosphere can be directly measured by monitoring the flow and concentration of captured CO₂ at the point of storage. Compared to many other carbon removal approaches, this method provides a higher level of confidence in the amount of CO₂ being removed for investors and carbon credit purchasers. The geological sequestration of captured CO₂ has high permanence, effectively removing CO₂ from the atmosphere for thousands of years with a low risk of reversal. There are numerous research and pilot projects underway to improve CO₂ capture efficiency, reduce energy use, and reduce costs, which may improve the effectiveness and cost of this technology. 

Why are we concerned?

The concentration of CO₂ in the atmosphere is small, currently about 420 parts per million, or about 0.04%. This means that a DAC facility must process huge amounts of air – more than 1,600 t by one estimate – and consume more energy than a typical U.S. household uses in a month to capture 1 t CO₂. Scaled up to remove a globally meaningful amount of CO₂ (>0.1 Gt CO₂ /yr), DAC would consume more energy than the annual energy consumption of 10 million U.S. households. In addition, removing and storing CO₂ using DAC is very expensive, costing up to US$1,000/t CO₂ stored. This is more than twice the cost per t for all other commercially available carbon removal technologies and practices. 

For these reasons, the technical and financial feasibility of scaling DAC to remove globally meaningful amounts of CO₂ from the atmosphere is low. Despite these challenges, as of September 2025, more than 30 companies have sold more than 2.4 million t of future carbon removal credits. However, less than 1,300 t CO₂ has actually been removed so far – or only 0.05% of these promised credits. To put this in perspective, despite spending billions of dollars, DAC has removed about as much CO₂ as would be saved by keeping 250-300 cars off the road for a single year.

There is also an opportunity cost for DAC. Even if a DAC facility is powered by solar, wind, geothermal, or nuclear energy, that carbon-free energy could have been used to displace coal- and gas-powered electricity instead, reducing emissions by far more than a DAC facility can capture and store. Similarly, the large amounts of public and private sector funding going to DAC could be more cost-effective and carbon-effective if used for other, more effective actions to cut emissions or remove CO₂. There is also the risk that DAC will be used to delay or avoid emissions reduction actions or for greenwashing by fossil fuel companies and other emitters. Substantial amounts of the funding supporting the development of DAC are coming from fossil fuel companies, which have publicly stated that they view carbon capture as a strategy to extend society’s use of fossil fuels. Finally, unlike most other emissions reduction or carbon removal actions, DAC provides no obvious other benefits to nature or human well-being.

Solution in Action

Alexandersson, B. O. P and Grettisson, V. (2025) Climeworks’ capture fails to cover its own emissions. Heimildin. Link to source: https://heimildin.is/grein/24581/

Bashir, A., Ali, M., Patil, S., Aljawad, M. S., Mahmoud, M., Al-Shehri, D., Hoteit, H., & Kamal, M. S. (2024). Comprehensive review of CO2 geological storage: Exploring principles, mechanisms, and prospects. Earth-Science Reviews249, 104672. Link to source: https://www.sciencedirect.com/science/article/pii/S0012825223003616

Bindl, M., Edwards, M. R., & Cui, R. Y. (2025). Risks of relying on uncertain carbon dioxide removal in climate policy. Nature Communications16(1), 5958. Link to source: https://www.nature.com/articles/s41467-025-61106-4

Bisotti, F., Hoff, K. A., Mathisen, A., & Hovland, J. (2023). Direct air capture (DAC) deployment: National context cannot be neglected. A case study applied to Norway. Chemical Engineering Science282, 119313. Link to source: https://www.sciencedirect.com/science/article/pii/S0009250923008692

Calma, J. (2023) To capture CO2 in the US, climate tech startups partner with oil and gas. The Verge. Link to source: https://www.theverge.com/2023/4/21/23690040/climeworks-direct-air-carbon-capture-oil-gas

CDR.fyi. (2025) Keep Calm and Remove On - CDR.fyi 2024 Year in Review Link to source: https://www.cdr.fyi/blog/2024-year-in-review

Chatterjee, S., & Huang, K. W. (2019). Unrealistic energy and materials requirement for direct air capture in deep mitigation pathways. Nat. Commun. 11, 3287. Link to source: https://www.nature.com/articles/s41467-020-17203-7

Chen, S. (2025) Energy and water use for DAC. Carbon180. Link to source: https://carbon180.org/blog/energy-and-water-use-for-dac/#:~:text=To%20estimate%20the%20amount%20of%20energy%20consumed%20by,%3D%20%28Energy%20per%20tCO2%29%20%2A%20%28Total%20DAC%20capacity%29

Eke, V., Sahu, T., Ghuman, K. K., Freire-Gormaly, M., & O'Brien, P. G. (2025). A comprehensive review of life cycle assessments of direct air capture and carbon dioxide storage. Sustainable Production and Consumption. Link to source: https://www.sciencedirect.com/science/article/pii/S2352550925000399

Gulden, L. E., & Harvey, C. (2025). Tracing sources of funds used to lobby the US government about carbon capture, use, and storage. Environmental Science & Policy, 171, 104171. Link to source: https://www.sciencedirect.com/science/article/pii/S146290112500187X

Hager, B. & MIT Climate Portal Writing Team (2024) What is the risk that CO2 stored underground after carbon capture will escape again? MIT Climate Portal. Link to source: https://climate.mit.edu/ask-mit/what-risk-co2-stored-underground-after-carbon-capture-will-escape-again

Hiar, C. (2023) Oil companies want to remove carbon from the air — using taxpayer dollars. Climatewire, E&E News. Link to source: https://www.eenews.net/articles/oil-companies-want-to-remove-carbon-from-the-air-using-taxpayer-dollars/

International Energy Agency (no date) Direct Air Capture. Website. Link to source: https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage/direct-air-capture

Isometric (2025) Direct Air Capture explained: Understanding the process, benefits and cost of DAC. Link to source: https://isometric.com/writing-articles/direct-air-capture-explained

Jacobson, M. Z. (2019). The health and climate impacts of carbon capture and direct air capture. Energy & Environmental Science12(12), 3567-3574. Link to source: https://web.stanford.edu/group/efmh/jacobson/Articles/Others/19-CCS-DAC.pdf

Jacobson, M. Z., Fu, D., Sambor, D. J., & Muhlbauer, A. (2025). Energy, health, and climate costs of carbon-capture and direct-air-capture versus 100%-wind-water-solar climate policies in 149 countries. Environmental Science & Technology59(6), 3034-3045. https://pubs.acs.org/doi/10.1021/acs.est.4c10686?ref=pdf

Lebling, K., Leslie-Bole, H., Byrum, Z., Wilcox, J. & Riedl, D. (2025) 6 Things to Know About Direct Air Capture. World Resources Institute. Link to source: https://www.wri.org/insights/direct-air-capture-resource-considerations-and-costs-carbon-removal

Mackler, S., Fishman, X., & Broberg, D. (2021). A policy agenda for gigaton-scale carbon management. The Electricity Journal34(7), 106999. Link to source: https://www.sciencedirect.com/science/article/pii/S1040619021000907

Maloney, C. B. and Khanna, R. (2022). Memorandum: Investigation of Fossil Fuel Industry Disinformation. U.S. House of Representatives, Committee on Oversight and Reform. Link to source: https://oversightdemocrats.house.gov/sites/evo-subsites/democrats-oversight.house.gov/files/2022.09.14%20FINAL%20COR%20Supplemental%20Memo.pdf

Martin, P. (2023) Why Direct Air Capture Sucks (and not in a good way!). LinkedIn.Link to source: https://www.linkedin.com/pulse/why-direct-air-capture-sucks-good-way-paul-martin/

Milman, O. (2023) The world’s biggest carbon capture facility is being built in Texas. Will it work? The Guardian. Link to source: https://www.theguardian.com/environment/2023/sep/12/carbon-capture-texas-worlds-biggest-will-it-work

National Academies of Sciences, Medicine, Division on Earth, Life Studies, Ocean Studies Board, Board on Chemical Sciences, ... & Reliable Sequestration. (2019). Negative emissions technologies and reliable sequestration: A research agenda. Link to source: https://nap.nationalacademies.org/read/25259/chapter/7#203

OPIS and CDR.fyi. (2025) Bridging the Gap: Durable CDR Market Pricing Survey: Purchaser and Supplier Expectations in 2025 and 2030. Link to source: https://www.cdr.fyi/reports/pricing-survey-jan-2025.pdf

Ozkan, M. (2025). Atmospheric alchemy: The energy and cost dynamics of direct air carbon capture. MRS Energy & Sustainability, 12(1), 46-61. Link to source: https://link.springer.com/content/pdf/10.1557/s43581-024-00091-5.pdf

Pett-Ridge, J., Ammar, H., & Aui, A. (2023). Roads to Removal. Options for Carbon Dioxide Removal in the United States. Chapter 7. Direct Air Capture with Storage (DACS) and Renewable Energy. Link to source: https://roads2removal.org/wp-content/uploads/07_RtR_Direct-Air-Capture.pdf

Scott, M. and T. Slavin (2023) Fossil-fuel industry embrace raises alarm bells over direct air capture. Reuters. Link to source: https://www.reuters.com/sustainability/climate-energy/fossil-fuel-industry-embrace-raises-alarm-bells-over-direct-air-capture-2023-10-10/

Skone, T. J. (2021) Life Cycle Greenhouse Gas Analysis of Direct Air Capture Systems. National Energy Technology Laboratory. Link to source: https://netl.doe.gov/sites/default/files/netl-file/21DAC_Skone.pdf  

Terlouw, T., Treyer, K., Bauer, C., & Mazzotti, M. (2021). Life cycle assessment of direct air carbon capture and storage with low-carbon energy sources. Environmental science & technology55(16), 11397-11411. Link to source: https://pubs.acs.org/doi/10.1021/acs.est.1c03263

U. S. Department of Energy, Fossil Energy and Carbon Management (2024) Direct Air Capture Explained. https://www.energy.gov/sites/default/files/2024-08/Direct%20Air%20Capture%20Factsheet_August%202024.pdf

Wang, J., Li, S., Deng, S., Zeng, X., Li, K., Liu, J., ... & Lei, L. (2023). Energetic and life cycle assessment of direct air capture: a review. Sustainable Production and Consumption36, 1-16. Link to source: https://www.sciencedirect.com/science/article/abs/pii/S2352550922003384

World Resources Institute. (no date) U.S. Climate Policy Resource Center, Direct Air Capture. Link to source: https://www.wri.org/us-climate-policy-implementation/sectors/direct-air-capture

Young, J., McQueen, N., Charalambous, C., Foteinis, S., Hawrot, O., Ojeda, M., ... & Van Der Spek, M. (2023). The cost of direct air capture and storage can be reduced via strategic deployment but is unlikely to fall below stated cost targets. One Earth 6, 899–917Link to source: https://www.sciencedirect.com/science/article/pii/S2590332223003007?ref=pdf_download&fr=RR-2&rr=96c1a3aebb261758

Credits

Lead Fellows

  • Jonathan Foley, Ph.D.
  • Christina Swanson, Ph.D.

Internal Reviewer

  • Sarah Gleeson, Ph.D.
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Action Word
Deploy
Solution Title
Direct Air Capture
Classification
Not Recommended
Updated Date

Advance Artificial Upwelling

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Peatland
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The Advance Artificial Upwelling solution is coming soon.
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Advance
Solution Title
Artificial Upwelling
Classification
Not Recommended
Updated Date

Deploy Ocean Fertilization

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Summary

Ocean fertilization uses nutrients to enhance photosynthesis by marine phytoplankton, which remove CO₂ and convert it into biomass that can sink to the deep ocean. This practice is a carbon removal technology that could achieve Gt-scale CO₂ removal annually. Potential advantages of ocean fertilization include localized reduction of ocean acidification and low costs. Disadvantages include high and uncertain risks of altering ecosystems both near dispersal sites and farther away, unclear but probably low effectiveness, potentially difficult operational upscaling, and challenges with monitoring and verification. We conclude that Deploy Ocean Fertilization is “Not Recommended” as a climate solution given its likely low effectiveness, technical challenges, and high environmental risks.

Description for Social and Search
Ocean fertilization uses nutrients to enhance photosynthesis by marine phytoplankton, which remove CO₂ and convert it into biomass that can sink to the deep ocean. We conclude that Deploy Ocean Fertilization is “Not Recommended” as a climate solution given its likely low effectiveness, technical challenges, and high environmental risks.
Overview

What is our assessment?

Based on the scientific uncertainties regarding its effectiveness and the potential serious environmental and social risks, we conclude that Ocean Fertilization is “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? Yes
Risk Is it risky or harmful? Yes
Cost Is it cheap? ?

What is it?

Ocean fertilization involves adding nutrients, such as iron, to seawater to promote photosynthesis in the surface ocean. As phytoplankton draw in seawater CO₂ and convert it into biomass, the ocean can absorb more CO₂ from the atmosphere. Some of the carbon eventually sinks or is transported to the deep sea or seafloor, where it can be stored for decades or centuries. Most ocean fertilization efforts are focused on iron because it is a micronutrient already required in small amounts for photosynthesis and because iron limitation is common in many global ocean regions. The Southern Ocean, in particular, has been highlighted as a potential target due to its widespread iron limitation.

Does it work?

As a carbon removal technique, ocean fertilization requires that the nutrient addition enhances phytoplankton uptake of seawater CO₂ and subsequent absorption of additional CO₂ from the atmosphere, and that the carbon is transported and durably stored in the deep sea. Research since the 1990s has shown that ocean iron fertilization does lead to increased seawater CO₂ uptake due to enhanced photosynthesis. However, the ultimate fate and durability of that carbon are less well understood. To be sequestered, carbon must be transported below water depths where annual mixing occurs, often considered to be ~1,000 m, but research suggests that, on average, 66% of carbon at these depths can be re-exposed to the atmosphere in less than 40 years. Ocean fertilization might also increase production of GHGs, such as nitrous oxide and methane, which could impact the effectiveness of this practice, although these effects remain understudied. In places like the Southern Ocean, sunlight and changes in the availability of other nutrients, such as silicate, can also limit the effects of iron addition. Additionally, nutrients such as iron can have high loss rates, up to 75%, after dispersal into seawater due to conversion into forms inaccessible to phytoplankton, potentially further reducing the effectiveness of nutrient addition.

Why are we excited?

If ocean fertilization were broadly deployed and functioned as intended, its global climate impact could reach 0.1–1.0 Gt CO₂ /yr. Ocean fertilization is expected to increase surface water pH, which could help temporarily reduce ocean acidification locally. However, some studies suggest this benefit will come at the cost of increased acidification of deeper ocean regions. While costs remain highly uncertain, estimates of ocean fertilization range between US$80/t CO₂ and US$457/t CO₂, suggesting this practice might also be relatively inexpensive compared to other marine CO₂ removal practices.

Why are we concerned?

Ocean fertilization poses several technical challenges, along with significant environmental and social risks. Tracking the amount of carbon sequestered from ocean fertilization is difficult because carbon export efficiencies – the amount of carbon produced in surface waters that makes its way to the deep sea – can be low and highly variable in time and space. Addressing this will require both field studies and models capable of capturing global and multi-decadal changes in carbon cycling due to fertilization, given the long time scales and large spatial areas involved. Implementing ocean fertilization at globally meaningful carbon removal levels could raise additional feasibility concerns, given the potential difficulty of dispersing sufficiently large quantities of nutrients across vast areas and the need for fertilization to be done continuously to minimize carbon returning to the atmosphere. 

Beyond these technical challenges, ocean fertilization also poses several potentially severe, environmental risks. Enhancing primary production could disrupt existing nutrient pools in the ocean, reducing the nutrients available for ecosystems far from dispersal sites. Another consequence of ocean fertilization is that increased organic carbon supply can enhance microbial processes that consume dissolved oxygen, potentially impairing respiration in marine organisms and leading to mortality. Other unintended consequences of nutrient fertilization include promoting harmful algal blooms that can release toxins that negatively impact a wide array of life, from shellfish to marine mammals to humans. Ocean fertilization also carries significant social risks, as global-scale modification of marine ecosystems is likely to create inequities in environmental and economic impacts.

Solution in Action

Aumont, O., & Bopp, L. (2006). Globalizing results from ocean in situ iron fertilization studies. Global Biogeochemical Cycles20(2). Link to source: https://doi.org/10.1029/2005GB002591

Bakker, D. C. (2004). Storage of carbon dioxide by greening of oceans. In C. B. Field & M. R. Raupach (Eds.), The global carbon cycle: Integrating humans, climate, and the natural world (pp. 453–469). Island Press.

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. Aspen Institute. Link to source: https://www.aspeninstitute.org/publications/a-code-of-conduct-for-marine-carbon-dioxide-removal-research/

Boyd, P. W. (2008). Implications of large-scale iron fertilization of the oceans. Marine Ecology Progress Series, 364, 213–218. Link to source: https://www.int-res.com/articles/theme/m364p213.pdf

Boyd, P. W., Jickells, T., Law, C. S., Blain, S., Boyle, E. A., Buesseler, K. O., ... & Watson, A. J. (2007). Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions. Science, 315(5812), 612–617. Link to source: https://doi.org/10.1126/science.1131669

Buesseler, K. O., & Boyd, P. W. (2003). Will ocean fertilization work?. Science, 300(5616), 67–68. Link to source: https://doi.org/10.1126/science.1082959

Cao, L., & Caldeira, K. (2010). Can ocean iron fertilization mitigate ocean acidification? A letter. Climatic Change99(1), 303–311. Link to source: https://link.springer.com/article/10.1007/s10584-010-9799-4

Emerson, D., Sofen, L. E., Michaud, A. B., Archer, S. D., & Twining, B. S. (2024). A cost model for ocean iron fertilization as a means of carbon dioxide removal that compares ship‐and aerial‐based delivery, and estimates verification costs. Earth's Future, 12(4), e2023EF003732. Link to source: https://doi.org/10.1029/2023EF003732

Gattuso, J. P., Williamson, P., Duarte, C. M., & Magnan, A. K. (2021). The potential for ocean-based climate action: negative emissions technologies and beyond. Frontiers in Climate, 2, 575716. Link to source: https://doi.org/10.3389/fclim.2020.575716

Harrison, D. P. (2013). A method for estimating the cost to sequester carbon dioxide by delivering iron to the ocean. International Journal of Global Warming, 5(3), 231–254. Link to source: https://doi.org/10.1504/IJGW.2013.055360

Harvey, J. (2020). 30 years: The iron hypothesis is no more. Moss Landing Marine Laboratories BlogLink to source: https://mlml.sjsu.edu/2020/06/18/30-years-the-iron-hypothesis-is-no-more/

Jin, X., & Gruber, N. (2003). Offsetting the radiative benefit of ocean iron fertilization by enhancing N2O emissions. Geophysical Research Letters, 30(24). Link to source: https://doi.org/10.1029/2003GL018458

Marinov, I., Gnanadesikan, A., Toggweiler, J. R., & Sarmiento, J. L. (2006). The southern ocean biogeochemical divide. Nature, 441(7096), 964–967. Link to source: https://doi.org/10.1038/nature04883

Martin, J. H., Gordon, M., & Fitzwater, S. E. (1991). The case for iron. Limnology and Oceanography, 36(8), 1793–1802. Link to source: https://doi.org/10.4319/lo.1991.36.8.1793

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

Ocean Visions. (2023). Microalgae cultivation. Link to source: https://oceanvisions.org/microalgae-cultivation/

Oschlies, A., Koeve, W., Rickels, W., & Rehdanz, K. (2010). Side effects and accounting aspects of hypothetical large-scale Southern Ocean iron fertilization. Biogeosciences7(12), 4017–4035. Link to source: https://bg.copernicus.org/articles/7/4017/2010/bg-7-4017-2010.pdf

Oschlies, A., Slomp, C., Altieri, A. H., Gallo, N. D., Grégoire, M., Isensee, K., Levin, L. A., & Wu, J. (2025). Potential impacts of marine carbon dioxide removal on ocean oxygen. Environmental Research Letters, 20(1), 011001. Link to source: https://doi.org/10.1088/1748-9326/ade0d4

Robinson, J., Popova, E. E., Yool, A., Srokosz, M., Lampitt, R. S., & Blundell, J. R. (2014). How deep is deep enough? Ocean iron fertilization and carbon sequestration in the Southern Ocean. Geophysical Research Letters, 41(7), 2489–2495. Link to source: https://doi.org/10.1002/2013GL058799

Sarmiento, J. L., Gruber, N., Brzezinski, M. A., & Dunne, J. P. (2004). High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature, 427(6969), 56–60. Link to source: https://doi.org/10.1038/nature02127

Shepherd, J. G. (2009). Geoengineering the climate: science, governance and uncertainty. The Royal Society. Link to source: https://royalsociety.org/-/media/policy/publications/2009/8693.pdf

Strong, A., Chisholm, S., Miller, C., & Cullen, J. (2009). Ocean fertilization: time to move on. Nature461(7262), 347–348. Link to source: https://doi.org/10.1038/461347a

Tagliabue, A., Aumont, O., DeAth, R., Dunne, J. P., Dutkiewicz, S., Galbraith, E., Misumi, K., Moore, J. K., Ridgwell, A., Sherman, E., Stock, C., Vichi, M., Völker, C., & Yool, A. (2016). How well do global ocean biogeochemistry models simulate dissolved iron distributions?. Global Biogeochemical Cycles, 30(2), 149–174. Link to source: https://doi.org/10.1002/2015GB005289

Tagliabue, A., Twining, B. S., Barrier, N., Maury, O., Berger, M., & Bopp, L. (2023). Ocean iron fertilization may amplify climate change pressures on marine animal biomass for limited climate benefit. Global Change Biology, 29(18), 5250–5260. Link to source: https://doi.org/10.1111/gcb.16854

Trick, C. G., Bill, B. D., Cochlan, W. P., Wells, M. L., Trainer, V. L., & Pickell, L. D. (2010). Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas. Proceedings of the National Academy of Sciences, 107(13), 5887–5892. Link to source: https://doi.org/10.1073/pnas.0910579107

Yoon, J. E., Yoo, K. C., Macdonald, A. M., Yoon, H. I., Park, K. T., Yang, E. J., Kim, H. C., Lee, J. I., Lee, M. K., Jung, J., Park, J., Lee, J., Kim, S., Kim, S. S., Kim, K., & Kim, I. N. (2018). Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project. Biogeosciences, 15(19), 5847-5889. Link to source: https://doi.org/10.5194/bg-15-5847-2018

Credits

Lead Fellow

  • Christina Richardson, Ph.D.

Internal Reviewer

  • Christina Swanson, Ph.D.
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Deploy
Solution Title
Ocean Fertilization
Classification
Not Recommended
Updated Date

Boost Whale Restoration

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Peatland
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Description for Social and Search
The Boost Whale Restoration solution is coming soon.
Solution in Action
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Boost
Solution Title
Whale Restoration
Classification
Worthwhile
Updated Date

Boost Large Herbivore Restoration

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Peatland
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The Boost Large Herbivore Restoration solution is coming soon.
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Boost
Solution Title
Large Herbivore Restoration
Classification
Worthwhile
Updated Date

Explore Ocean Electrochemistry

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The Explore Ocean Electrochemistry solution is coming soon.
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Explore
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Ocean Electrochemistry
Classification
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