Skip to content
ContentLora

    Tip: press / anywhere to search.

    technology

    Ocean-based carbon dioxide removal

    Also known as ocean CDR, marine carbon dioxide removal, mCDR, ocean alkalinity enhancement, OAE

    The ocean has absorbed nearly a third of fossil-fuel CO2, and ocean-based carbon dioxide removal tries to increase that uptake through chemistry, such as adding alkalinity, or biology, such as growing and sinking seaweed.[1][2] The first EPA-permitted alkalinity field trial in 2025 found no measurable harm to marine life, but verifying how much CO2 the ocean actually takes up remains the central challenge.[3][4]

    Editor reviewedUpdated WikiClimate science and carbon removal, Energy and climate, Science
    Key facts

    Why the ocean

    Over the last century the ocean has absorbed nearly a third of the CO2 released by burning fossil fuels.[1] Ocean-based removal aims to enlarge that sink. Woods Hole Oceanographic Institution (WHOI) scientists study alkalinity enhancement, coastal “blue carbon” ecosystems, iron fertilization and seaweed cultivation.[2]

    Main approaches

    • Ocean alkalinity enhancement (OAE). Alkaline material is added to seawater, and the resulting uptake of atmospheric CO2 happens gradually over broad areas of ocean surface, so quantification relies mainly on ocean modelling.[4] Its global potential is poorly constrained, from under 1 to 100 billion tonnes a year, and costs are estimated at $100–150 per tonne, rising above $600 for first-of-a-kind projects.[5]
    • Direct ocean capture. Seawater is pumped through electrochemical systems that extract dissolved CO2, letting the depleted water take up more from the air; the technology is at lab-to-pilot scale with system costs estimated at $1,000–2,000 per tonne.[6]
    • Biomass sinking. Farmed seaweed could remove an estimated 0.1–1 billion tonnes a year, but likely costs exceed $1,000 per tonne and storage durability and ecosystem effects are uncertain.[7]

    Field trials

    WHOI’s LOC-NESS project ran the first US EPA-permitted OAE field trial in August 2025 in the Gulf of Maine’s Wilkinson Basin, adding purified sodium hydroxide with a tracer dye to surface water; the permit was approved in April 2025.[8] Preliminary results released in February 2026 showed the surface ocean taking up carbon and no measurable impact on monitored ocean life.[3] Frontier cites the Gulf of Maine trial alongside Ebb Carbon’s trial in Sequim Bay. It says the two showed the ocean taking up CO2 in response to added alkalinity, in a way that can be controlled and measured.[9]

    Commercial activity

    In August 2025 buyers in frontier-carbon-removal signed a $31.3 million offtake with Planetary for more than 115,000 tonnes of removal through alkalinity enhancement between 2026 and 2030.[10] Planetary says it delivered the first verified OAE credits, verified by Isometric.[11] Combined alkalinity enhancement and direct ocean capture removals were still very small in 2025, at about 0.004 million tonnes together.[12]

    A related approach treats wastewater rather than the open sea. In August 2026 Microsoft agreed to buy up to 23,602 removal units from Crew Carbon, which doses alkaline minerals such as calcium carbonate into wastewater treatment plants; the company says the process turns dissolved CO2 into bicarbonate that is then discharged to oceans, rivers or aquifers.[13][14] On 3 September 2026 Frontier awarded $2.1 million in grants to the University of St Andrews, the University of Cambridge, [C]Worthy and Submarine Scientific to work on problems in scaling alkalinity enhancement.[15] Frontier’s own estimate is that the method could exceed 10 billion tonnes a year at $70–200 per tonne using purpose-made alkalinity from limestone calcination.[16]

    Open questions

    The State of CDR report lists measurement of ocean alkalinity enhancement among the main unresolved uncertainties for novel removal, and says the ocean processes that govern CO2 uptake are still under study.[17][4] Lower-tech options such as placing minerals on the seabed are cheaper but face larger scientific and verification hurdles.[5]

    Frontier’s September 2026 grant call sets out three practical bottlenecks. The first is finding enough low-emission alkaline feedstock. The second is adding alkalinity without harming ecosystems or losing some of it to mineral precipitation, a risk that rises with higher dosing. The third is replacing the intensive field sampling and costly models used in early projects with cheaper verification.[18] Frontier also says scaling will need demonstration of minimal ecosystem risks and ocean models that can be trusted to quantify removal.[16]

    Next in the crash course

    More in Climate & carbon

    Developing in Science & Energy

    Questions readers ask

    What is ocean alkalinity enhancement?

    Alkalinity is added to seawater so the surface ocean absorbs more CO2 from the air; the uptake happens gradually over wide areas, so it is quantified mainly with ocean models.[4]

    Is it safe for marine life?

    The first EPA-permitted field trial, in the Gulf of Maine in August 2025, reported no measurable impact on monitored ocean life in preliminary results.[3][8]

    Is anyone paying for ocean carbon removal?

    Yes. In August 2025 Frontier buyers agreed to pay Planetary $31.3 million for more than 115,000 tonnes of removal through alkalinity enhancement between 2026 and 2030.[10]

    Is growing and sinking seaweed a good option?

    Estimated potential is 0.1–1 billion tonnes a year, but likely costs exceed $1,000 per tonne and storage durability and ecosystem effects are uncertain.[7]

    What is holding ocean alkalinity enhancement back?

    Frontier lists three bottlenecks - finding scalable low-emission alkaline feedstocks, adding alkalinity without ecosystem hazards or losses to mineral precipitation, and cheaper ways to measure and verify removal.[18]

    Sources

    · 20 claims from 5 publishers

    Each numbered claim is a statement we checked against the sources listed with it. Tap a number in the text to see its sources in place.

    1. [1]

      Over the last century the ocean has absorbed nearly a third of the CO2 emitted by burning fossil fuels. confirmedas of 2026-10-10

    2. [2]

      WHOI scientists study ocean alkalinity enhancement, coastal blue-carbon ecosystems such as seagrass and mangroves, iron fertilization and seaweed cultivation as ocean-based climate solutions. confirmedas of 2026-10-10

    3. [3]

      Preliminary LOC-NESS results released in February 2026 showed the surface ocean taking up carbon from the atmosphere and no measurable impact on monitored ocean life. WHOI says part of the CO2 uptake will be quantified in upcoming peer-reviewed publications. confirmedas of 2026-02-25

    4. [4]

      In ocean alkalinity enhancement, alkalinity is added to seawater and the resulting uptake of atmospheric CO2 happens gradually over broad areas of ocean surface, so quantification relies mainly on ocean modelling. confirmedas of 2026-06-01

    5. [5]

      The global technical potential of alkalinity enhancement is poorly constrained (under 1 to 100 billion tonnes a year) and costs are estimated at $100–150 per tonne, rising above $300 for some technologies and $600 for first-of-a-kind deployments; lower-tech options such as placing minerals on the seabed are cheaper but face larger scientific and MRV hurdles. confirmedas of 2026-06-01

    6. [6]

      Direct ocean capture pumps seawater through electrochemical systems to extract dissolved CO2, after which the CO2-depleted seawater gradually takes up atmospheric CO2; it is at lab-to-pilot scale (TRL 5) with system-level costs estimated at $1,000–2,000 per tonne. confirmedas of 2026-06-01

    7. [7]

      Farmed macroalgae sinking has an estimated potential of 0.1–1 billion tonnes of CO2 a year, but likely removal costs exceed $1,000 per tonne and the durability of storage and ecosystem impacts are uncertain. confirmedas of 2026-06-01

    8. [8]

      WHOI's LOC-NESS project ran the first US EPA-permitted ocean alkalinity enhancement field trial in August 2025 in the Gulf of Maine's Wilkinson Basin, adding purified sodium hydroxide and a tracer dye to surface waters; the permit was approved in April 2025. confirmedas of 2026-02-25

    9. [9]

      Frontier says field trials by Ebb Carbon in Sequim Bay and WHOI in the Gulf of Maine showed the ocean taking up CO2 in response to added alkalinity, in a process that can be controlled and measured. confirmedas of 2026-09-03

    10. [10]

      In August 2025 Frontier buyers signed a $31.3 million offtake with Planetary to remove more than 115,000 tonnes of CO2 between 2026 and 2030 through ocean alkalinity enhancement, starting at Tufts Cove, Nova Scotia. confirmedas of 2025-08-26

    11. [11]

      Planetary says it delivered the world's first verified ocean alkalinity enhancement credits, verified by Isometric. reportedas of 2025-08-26

    12. [12]

      Alkalinity enhancement of water bodies and direct ocean capture were active in 2025 but at small scale, removing a combined 0.004 million tonnes of CO2. confirmedas of 2026-06-01

    13. [13]

      In August 2026 Microsoft signed a deal with Crew Carbon for up to 23,602 removal units from wastewater alkalinity enhancement, its third removal deal since reports that it was adjusting the pace of purchases, after BioCirc (650,000 tonnes) and enhanced weathering firm Alt Carbon (36,920 tonnes); DatacenterDynamics said its purchasing cadence had dropped significantly in 2026. reportedas of 2026-08-05

    14. [14]

      Crew Carbon doses alkaline minerals such as calcium carbonate into wastewater treatment plants and says this converts dissolved CO2 into bicarbonate that is then discharged to oceans, rivers or aquifers. reportedas of 2026-08-05

    15. [15]

      On 3 September 2026 Frontier awarded $2.1 million in grants to the University of St Andrews, the University of Cambridge, [C]Worthy and Submarine Scientific to tackle technical hurdles in scaling ocean alkalinity enhancement. confirmedas of 2026-09-03

    16. [16]

      Frontier estimates ocean alkalinity enhancement could exceed 10 billion tonnes a year at $70–200 per tonne using limestone calcination, but says scaling needs cheaper calcination, demonstration of minimal ecosystem risks and trustworthy ocean models to quantify removal. confirmedas of 2026-06-17· interpretation

    17. [17]

      The State of CDR report lists open problems for novel CDR quality, including sustainable biomass sourcing for BECCS and biochar, permanence of storage, measurement of enhanced weathering and ocean alkalinity enhancement, and lifecycle emissions of energy-intensive DACCS. confirmedas of 2026-06-01

    18. [18]

      Frontier identifies three bottlenecks for ocean alkalinity enhancement - finding scalable low-emission alkaline feedstocks, adding alkalinity without ecosystem hazards or precipitation losses, and cheaper measurement and verification - noting that precipitation risk rises with dosing rates. confirmedas of 2026-09-03

    19. [19]

      Enhanced weathering spreads crushed rock, usually silicate rock on farmland, to speed natural reactions that take up CO2; the reactions take months to centuries, and readiness estimates range from TRL 3–4 (conservative) to 8–9 (suppliers). Most applications use farmland, where acidic soils speed up reactions and existing supply chains can distribute the rock. confirmedas of 2026-06-01

    20. [20]

      Solid-sorbent and mineral-looping direct air capture are commercially deployed (TRL 9), liquid-solvent systems are at pilot scale (TRL 6–7), and electrochemical regeneration is earlier (TRL 4–6); all are energy intensive. confirmedas of 2026-06-01

    Revision history (2)
    1. Page created.
    2. Refresh: added Frontier's September 2026 research grants, the technical bottlenecks for alkalinity enhancement, its scale estimate and Microsoft's wastewater alkalinity deal.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "Ocean-based carbon dioxide removal." ContentLora, updated Oct 10, 2026. https://contentlora.com/wiki/ocean-carbon-dioxide-removal

    Spotted an error? Suggest a correction or emailcorrections@contentlora.com.

    Connections
    Enhanced rock weath… Land counterpartDirect air capture Air-based analogueFrontier Buyer
    ☰ Contents