Ocean alkalinity enhancement

Also known as OAE

Ocean alkalinity enhancement adds alkaline material to seawater so that the sea takes up more carbon dioxide from the air. Small field trials and early carbon credits exist, but the removal in those credits was calculated with models rather than measured, ecological effects depend on the material used, and the legal framework is still being developed.

Pilot and early commercialReported, unverifiedReviewed 21 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

Ocean alkalinity enhancement, often shortened to OAE, adds alkaline material to seawater. Seawater is already slightly alkaline, and making it more alkaline lets it hold less dissolved carbon dioxide at the surface. The sea then draws more CO2 down from the air to rebalance, and some of that carbon is stored in the water as dissolved forms that do not return to the air quickly.

The difficulty is proving how much extra CO2 was removed. The added alkalinity spreads with currents and is diluted, and the sea exchanges CO2 with the air slowly, so a trial in open water does not easily show the difference. Early trials have added alkalinity to real water. In 2025 the Woods Hole Oceanographic Institution released a caustic sodium hydroxide solution into surface waters about 40 miles northeast of Cape Cod, in its LOC-NESS trial. Planetary Technologies has run trials and received the first OAE credits issued, for a project in Halifax Harbour, Canada. The credits were calculated with ocean models rather than observed directly, and researchers still need better ways to measure the effect and to test whether the material harms marine life.

Technical explanation

Seawater carbonate chemistry links total alkalinity, dissolved inorganic carbon and the partial pressure of CO2 (pCO2). At a fixed level of dissolved inorganic carbon, adding alkalinity lowers pCO2, so the water can take up more CO2 from the atmosphere. The Copernicus guide to best practice gives one worked example: under stated conditions (20 degrees Celsius, salinity 35, total alkalinity 2200 micromoles per kilogram and dissolved inorganic carbon 1965 micromoles per kilogram), an increase of 10 micromoles per kilogram lowers pCO2 by about 20 microatmospheres. Materials studied include sodium hydroxide and olivine. Electrochemical methods pass seawater through membranes that split it into acidic and basic streams, and the basic stream can be returned to the sea. Ebb Carbon deployed one such system at the Pacific Northwest National Laboratory's Sequim facility in November 2023, with a design target of 100 tons of CO2 a year. The U.S. Department of Energy describes that figure as a design goal, and the basic stream is held in large tanks for further experiments rather than returned to the sea.

Measurement is the central problem. The Copernicus guide concludes that observations alone are unlikely to quantify net removal, because the signal disperses, air-sea CO2 exchange is slow and the natural variability is large: a coastal seasonal pCO2 cycle of 210 plus or minus 76 microatmospheres, compared with 49 plus or minus 23 for the open ocean. It states that numerical models are needed, and that current models have not been robustly validated for this purpose. The independent review of Planetary's first credits describes a chain of modelled numbers: 881.8 tonnes of gross modelled removal, reduced to 730.4 tonnes after project emissions and to 625.6 tonnes after a 14.35 per cent uncertainty discount. The review says that 'verified' in this context does not mean direct observation of CO2 removal.

Ecological effects depend on the alkalinity source. In a 2024 mesocosm experiment with oysters, the authors compared olivine with dissolved sodium hydroxide. Olivine shifted bacterial communities in the water and, at the higher concentration, favoured Vibrio in oyster gills. Oyster mortality rose with alkalinity for both materials, from 20 per cent in the controls to between 23 and 38 per cent across treatments, and the authors suggest summer temperatures may also have contributed. The authors conclude that olivine poses greater ecotoxicological risks than sodium hydroxide, which caused minimal changes to water bacteria compared with the controls. They describe the study as a first step towards future field trials, with two mesocosms per treatment.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

Under stated seawater conditions (20 degrees Celsius, salinity 35, total alkalinity 2200 micromoles per kilogram, dissolved inorganic carbon 1965 micromoles per kilogram), an increase of 10 micromoles per kilogram in alkalinity lowers pCO2 by about 20 microatmospheres.

Sources: [1] · Carbonate chemistry section

The Copernicus guide concludes that observations alone are unlikely to provide a sufficient basis for quantifying OAE net carbon removal, that numerical models are needed, and that current models have not been robustly validated for this purpose.

Sources: [1] · Core conclusion

Planetary's first OAE credits, issued in June 2025 for a Halifax Harbour project, were based on 881.8 tonnes of gross modelled removal, reduced to 730.4 tonnes after project emissions and to 625.6 tonnes after a 14.35 per cent uncertainty discount. The independent reviewer says that 'verified' in this context does not mean direct observation of CO2 removal.

Sources: [5] · Credit figures and What verified means

In a 2024 mesocosm experiment with oysters, the authors concluded that olivine-based alkalinity enhancement poses greater ecotoxicological risks than dissolved sodium hydroxide.

Sources: [6] · Main findings and conclusion · medium confidence

The authors describe their 2024 mesocosm study as a first step towards future field trials and applications. The study used two mesocosms per treatment.

Sources: [6] · Conclusion and methods

The 2013 London Protocol amendment has not entered into force and does not yet cover ocean alkalinity enhancement. Parties adopted an OAE assessment framework at their October 2026 meeting, announced on 9 October 2026. Many delegations view the frameworks as best-practice guidance, and a number of delegations said they do not create new legally binding obligations.

Sources: [3] · medium confidence

Reported, unverified Stated by a company or team but not independently confirmed.

In the LOC-NESS trial, WHOI released sodium hydroxide into surface waters about 40 miles northeast of Cape Cod, and researchers tracked the alkalinity patch for several days using autonomous gliders and satellite imagery.

Sources: [4] · medium confidence

The LOC-NESS project lead, Adam Subhas, estimated that the trial sequestered about 2 to 10 tons of CO2 over four days, with a best case of up to about 50 tons. The article presents these as estimates.

Sources: [4] · low confidence

Proposed A design or plan that has not yet been built or tested.

The authors say that placing alkaline minerals in the sea may, at least in some situations, be classified as dumping under international law. They note that the 2013 London Protocol amendment would require a permit for listed marine geoengineering activities, issued only where pollution is, as far as practicable, prevented or reduced to a minimum. OAE is not yet listed.

Sources: [2] · Conclusion 9 and the permit discussion · medium confidence

Limitations and open problems

  • Net CO2 removal is unlikely to be quantified from observations alone, because the alkalinity signal disperses, air-sea CO2 exchange is slow and natural variability is large (Copernicus guide, 2023).
  • Current credits rely on ocean models that are not robustly validated, and the first credits were based on modelled rather than observed removal (Copernicus guide, 2023; Carbon to Sea Initiative review, 2025).
  • Natural variability in coastal pCO2 can be larger than the expected alkalinity signal, so detection is hard, especially in the coastal areas where OAE is most likely to be used (Copernicus guide, 2023).
  • Ecological effects depend on the material used. A 2024 mesocosm study found olivine posed greater ecotoxicological risk than sodium hydroxide, but the study was small and its long-term and field effects are not known (Antoni and colleagues, 2025).
  • The 2013 London Protocol amendment has not entered into force and does not yet cover OAE. Many delegations describe the October 2026 assessment framework as best-practice guidance, and a number of delegations say it does not create new legally binding obligations (Columbia Climate Law Blog, 2026).
  • The trials reported so far are small. The LOC-NESS biological results are preliminary, with no significant difference in bacteria, phytoplankton or lobster larvae inside and outside the patch, and the peer-reviewed analysis was still under way when the article was published (Boston Globe, 2026).

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Adding alkalinity at existing coastal industrial outfalls, if monitoring and permits can show a measurable and safe removal.
  • Pairing electrochemical plants with ocean discharge, so that alkaline streams are returned to the sea at scale rather than stored.
  • Raising local pH at shellfish farms affected by acidification.

Timeline

  1. Nov 2023Ebb Carbon deploys an electrochemical ocean CO2 removal system at the PNNL Sequim facility in Washington State; the design target of 100 tons of CO2 a year is described as a design goal, not an achieved result. Sources: [7]
  2. Nov 2023Copernicus publishes a best-practice guide chapter on monitoring, reporting and verification for OAE, concluding that models are required to quantify net removal. Sources: [1]
  3. Jun 2025Planetary's first OAE credits are issued for a Halifax Harbour project, based on modelled removal; the independent review says the protocol was followed but that 'verified' does not mean direct observation. Sources: [5]
  4. Aug 2025WHOI's LOC-NESS trial releases sodium hydroxide into surface waters about 40 miles northeast of Cape Cod. The trial is reported in February 2026 as the first EPA-approved, small-scale environmental research trial of OAE. Sources: [4]
  5. Sept 2025Mesocosm study comparing olivine and sodium hydroxide with oysters is published in Frontiers in Microbiomes. Sources: [6]
  6. Oct 2026Parties to the London Convention and London Protocol adopt an OAE assessment framework at their October 2026 meeting, announced on 9 October 2026, while the 2013 amendment remains not in force. Sources: [3]

Futures this could help bring about

AI illustration
How far away?

Removing carbon with the ocean

Pilots · 5 of 9 steps done

Glossary

Total alkalinity
A measure of how well seawater resists changes in pH. Adding alkaline material raises it.
pCO2
The partial pressure of carbon dioxide in water or air. It sets how much CO2 moves between the sea and the atmosphere.
Mesocosm
A contained experimental system, larger than a laboratory tank, that holds real seawater and organisms for study.
Monitoring, reporting and verification (MRV)
The measurement, reporting and checking process used to support carbon removal claims.

Sources

  1. 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), Nov 2023. Accessed 21 Oct 2026.
  2. other
    Parties to the London Convention and Protocol Debate Marine Carbon Dioxide Removal Governance
    Korey Silverman-Roati, Romany Webb. Columbia Climate Law Blog, 7 Oct 2026. Accessed 21 Oct 2026.
  3. news
    Controversial geoengineering 'antacid' test in ocean attempts to reverse climate change
    Kate Selig. The Boston Globe, 26 Feb 2026. Accessed 21 Oct 2026.
  4. other
    The First OAE Credits: An Independent MRV Review
    Anna Madlener. Carbon to Sea Initiative, 25 Nov 2025. Accessed 21 Oct 2026.
  5. peer reviewed paper
    Olivine and dissolved alkalinity trigger different bacterial community shifts in water and oyster gills: insights from a mesocosm experiment
    Dominik Antoni, Marco Rump, Gunnar Gerdts. Frontiers in Microbiomes, 26 Sept 2025. DOI: 10.3389/frmbi.2025.1659695. Accessed 21 Oct 2026.
  6. government report
    First-of-its-Kind Marine Carbon Dioxide Removal Device Deployed in Washington State
    U.S. Department of Energy, 14 Mar 2024. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 21 Oct 2026
Published on owner instruction (content loop iteration 57)956256a
Sources and claims re-checked against originalsfbb434c
Initial profile drafted from primary sources7e66249