Removing carbon with the ocean

How far are we from removing carbon from the air with the ocean at scale?

Ocean alkalinity enhancement adds alkaline material to seawater so that the sea takes up more CO2 from the air. A small trial in Halifax, Canada, reported higher pH in the nearby water, and one registry has issued credits for that project. The removal figures in those credits are modelled, not observed, according to an independent review. Researchers say that measuring the carbon directly in open water is very hard. Sinking seaweed has physical evidence of transport to the deep sea, but how much carbon it stores is still unknown. No ocean method is ready for deployment at scale.

AI illustration · Illustrative scene, not a photograph of the specific device or experiment.
WHERE WE AREPilots

Close. Early pilots are running in the real world.

Field trials have added alkalinity to real coastal water, and one registry has issued credits. The removal in those credits is calculated with ocean models rather than measured, and the reviewer of the first credits says that verification confirmed the modelling followed the protocol, not that removal was observed. Sinking seaweed has field evidence of physical transport to depth, but no measured carbon storage. Because real water has been treated in pilot trials but net removal has not been measured and no method is deployed commercially, the stage is pilot.

The tech tree

Everything recorded on Vrulum that this future needs, oldest foundations at the top and the goal at the bottom. Lines show what each step requires.

What it takes: Removing carbon with the ocean Unlocked Being researched Locked
  1. 2016Direct air capture of CO2Pilot and early commercial Unlocked.
  2. 2022Seawater electrolysisLab demonstration Being researched.
  3. 2023Ocean alkalinity enhancementPilot and early commercial Unlocked.
  1. 2022National research strategy for ocean carbon removalDone Unlocked.
  2. 2023Small coastal alkalinity trialsDone Unlocked.
  3. 2023Electrochemical ocean CO2 removal at demonstration scaleIn progress Being researched.
  4. 2023Measurement and verification guide for alkalinity researchDone Unlocked.
  5. 2025First verified and credited alkalinity removalDone Unlocked.
  6. 2026Evidence of carbon storage from sinking seaweedIn progress Being researched.
  7. Date not recordedSinking seaweed and wood chips sold as creditsDone Unlocked.
  8. Date not recordedDirect measurement of CO2 removal in open waterNot started Locked.
  9. Date not recordedCommercial deployment at scaleNot started Locked.
  1. GoalRemoving carbon with the oceanGoalRequires: Ocean alkalinity enhancement, Direct air capture of CO2, Seawater electrolysis, National research strategy for ocean carbon removal, Measurement and verification guide for alkalinity research, Small coastal alkalinity trials, Sinking seaweed and wood chips sold as credits, First verified and credited alkalinity removal, Electrochemical ocean CO2 removal at demonstration scale, Direct measurement of CO2 removal in open water, Evidence of carbon storage from sinking seaweed, Commercial deployment at scale. Locked.

The road so far

5 of 9 steps done, 2 in progress.

  1. Done · 2022National research strategy for ocean carbon removal

    The National Academies published a research strategy for ocean-based carbon dioxide removal in 2022. Its technical chapters cover six approaches, including ocean alkalinity enhancement, seaweed cultivation and electrochemical engineering approaches, and it sets out the research needed to fill knowledge gaps. Its committee states that it is not advocating either for or against possible future ocean deployments. [1]

  2. Done · 27 Nov 2023Measurement and verification guide for alkalinity research

    A best-practice chapter on monitoring, reporting and verification for ocean alkalinity enhancement sets out what field trials should measure. It concludes that observations alone are unlikely to provide a sufficient basis for quantifying net carbon removal, because the added signal disperses and air-sea CO2 exchange is slow. The authors say that numerical models are required, and that current models have not yet been robustly validated for this purpose. [2]

  3. Done · 2023Small coastal alkalinity trials

    Planetary ran a two-month alkalinity addition at Tufts Cove, Halifax, in 2023. The company reports a maximum rate of nearly 12,000 tonnes of gross CO2 removal a year. That is a rate, not a measured total, and the company gives no net removal figure. It reports that the water's pH increased, and that preliminary data suggest no unanticipated change to the ecosystem. It describes these findings as preliminary, and an independent analysis by Dalhousie University was still under way when the company published them. [3]

  4. DoneSinking seaweed and wood chips sold as credits

    Running Tide sank wood chips coated with limestone or lime-kiln dust, and sold carbon credits. The company claimed just over 25,000 tons of CO2 removed from its 2023 deployments. These claims rely on the company's internal report, and the article names no third-party verifier. Running Tide announced its closure on 14 June 2024, citing weak demand in the voluntary carbon market. Canary Media reported in September 2024 that the company had not run experiments to determine the ecological impacts of its 2023 deployments. [7][8]

  5. Done · 16 Jun 2025First verified and credited alkalinity removal

    Isometric issued the first ocean alkalinity enhancement credits to Planetary on 16 June 2025, for its Tufts Cove project in Halifax. Planetary reports 625.6 tonnes of CO2 and says the credits were bought by Stripe, Shopify and British Airways. An independent review by the Carbon to Sea Initiative says the protocol was followed, but that the figure is modelled: a gross modelled removal of 881.8 tonnes, reduced to 730.4 tonnes after project emissions, and then to 625.6 tonnes after a 14.35 per cent uncertainty discount. The review says 'verified' in this context does not mean direct observation of CO2 removal. [4][5]

  6. In progress · Nov 2023Electrochemical ocean CO2 removal at demonstration scale

    Ebb Carbon deployed an electrochemical system at PNNL's Sequim facility in Washington State in November 2023, with NOAA's Pacific Marine Environmental Laboratory and the University of Washington. The design sequestration target is 100 tons of CO2 a year. This is a target, not a reported achieved figure. The basic stream is held in large tanks for experiments and is not currently released back to the bay. The experiments include oysters and seagrass, which the Department of Energy article notes are vulnerable to acidification. [9][10]

  7. Not startedDirect measurement of CO2 removal in open water

    No source reviewed shows a method that measures, in open water, the extra CO2 drawn down by an alkalinity addition. Mongabay reports that accurate monitoring needs better CO2 drawdown sensors than are currently available. A Carbon to Sea database of 250 or more measurements across five field projects does not assess whether any project's MRV is adequate, and does not validate removal estimates. [10][6]

  8. In progress · 23 Jan 2026Evidence of carbon storage from sinking seaweed

    A 2026 study of Greenland's coastal seaweed used satellite images, drifter tracking and modelling. It found floating mats that can travel hundreds of kilometres offshore, and its models suggest that winter mixing can submerge them. The authors present this as evidence of a pathway, not a measurement of stored carbon. A 2025 systematic review found that the rate and magnitude of macroalgal carbon sequestration are poorly resolved. It reports a mean half-life of about 51 days for macroalgal biomaterial, not carbon alone, and most of the studies it reviewed were short, with a median duration of 35 days. [11][12]

  9. Not startedCommercial deployment at scale

    No ocean carbon removal method is ready for deployment, according to a University of Virginia researcher quoted in a December 2024 Mongabay article, who said that scaling will take a long time. Plymouth Marine Laboratory's Helen Findlay, quoted in the same article, said that reaching gigatonne scale would mean processing about 1 per cent of the ocean's surface water each year. [10]

Why it matters

If ocean methods work and can be measured, they could add to carbon removal options that do not need land. Credits sold on these methods will set expectations for the wider carbon removal market. If the measurement is weak, buyers may pay for removal that does not happen, and that would damage trust in the whole sector.

What stands in the way

  • Removal cannot yet be measured directly in open water, and the sensors for CO2 drawdown are not good enough for verification.
  • Net removal must be calculated with ocean models, and those models are not yet robustly validated against observations.
  • Added alkalinity disperses and air-sea CO2 exchange is slow, so the signal may be hard to detect against the background.
  • The sources reviewed do not show a shared regulatory standard for monitoring alkalinity enhancement.
  • The fate of sinking seaweed carbon is poorly resolved, and the experiments done so far are short.
  • Ecological effects of large-scale additions have not been tested. The Halifax trial was small in scale, and Running Tide ran no ecological impact experiments before its 2023 deployments.
  • Voluntary carbon demand fell sharply, which has ended at least one company's operations.

Signals to watch

  • Publication of Dalhousie's independent analysis of the Halifax trials, including any measured net removal figure.
  • A shared regulatory or registry standard for monitoring alkalinity enhancement.
  • An open-water measurement of CO2 drawdown that is checked against model predictions.
  • Results from the Sequim Bay experiments and any decision to return the basic stream to the bay.
  • Field studies of sinking seaweed that measure export to the seabed, not only transport.
  • Further credit issuance, and whether reviewers can compare modelled removal with in-trial observations.

Sources

  1. government report
    A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration
    National Academies of Sciences, Engineering, and Medicine (National Academies Press), 2022.
  2. review article
    Monitoring, reporting, and verification for ocean alkalinity enhancement
    David T. Ho, Laurent Bopp, Jaime B. Palter et al.. Copernicus Publications, State of the Planet (Guide to Best Practices in Ocean Alkalinity Enhancement Research, Chapter 12), 27 Nov 2023.
  3. company announcement
    Preliminary Findings from Halifax 2023
    Planetary Technologies, 29 Feb 2024.
  4. company announcement
    The World's First Verified OAE Credits
    Planetary Technologies, 16 Jun 2025.
  5. other
    The First OAE Credits: An Independent MRV Review
    Anna Madlener. Carbon to Sea Initiative, 25 Nov 2025.
  6. other
    Launching the Carbon to Sea MRV Database: A First-of-Its-Kind Snapshot of Five Field Projects
    Carbon to Sea Initiative, 26 Feb 2026.
  7. news
    Portland startup that became global leader in capturing carbon shuts down, lays off all staff
    Portland Press Herald, 15 Jun 2024.
  8. news
    Under the sea: Running Tide's ill-fated adventure in ocean carbon removal
    Amanda Simson, Abby Rabinowitz. Canary Media, 19 Sept 2024.
  9. other
    First-of-its-Kind Marine Carbon Dioxide Removal Device Deployed in Washington State
    US Department of Energy, 14 Mar 2024.
  10. news
    Electrochemical removal of ocean CO2 offers potential and concerns
    Sean Mowbray. Mongabay, 20 Dec 2024.
  11. university press release
    New evidence reveals how Greenland's seaweed locks away carbon in the deep ocean
    Plymouth Marine Laboratory, 23 Jan 2026.
  12. review article
    A systematic review of marine macroalgal degradation: Toward a better understanding of macroalgal carbon sequestration potential
    Jessica R. Kennedy, Caitlin O. Blain. Journal of Phycology, 2025.

Last reviewed 21 Oct 2026.