As global carbon dioxide (CO2) concentrations continue to rise, policymakers and researchers are urgently seeking scalable, permanent carbon removal strategies beyond traditional land-based approaches. The ocean, which already absorbs roughly one-quarter of anthropogenic CO2 emissions, presents an immense natural reservoir for carbon storage. Ocean-based carbon capture and storage (CCS) encompasses a suite of technologies and natural processes designed to accelerate this uptake and lock carbon away for centuries to millennia. This article explores the scientific foundations, technological pathways, environmental trade-offs, and future prospects of deploying marine carbon sinks at scale.

The Mechanics of Ocean-Based Carbon Capture and Storage

Ocean-based CCS differs fundamentally from terrestrial geological storage in both the medium and the mechanisms involved. While land-based CCS typically injects CO2 into deep saline aquifers or depleted oil reservoirs, ocean-based approaches leverage the vast physical and biogeochemical capacity of seawater and seafloor sediments. The principal strategies fall into three categories: direct injection, mineralization enhancement, and biological pump amplification.

Deep-Sea CO2 Injection

Deep-sea injection involves capturing CO2 from point sources—such as power plants or cement factories—compressing it into a liquid or supercritical fluid, and then transporting it via pipeline or ship to a designated offshore location. The CO2 is released at depths greater than 1,000 meters, where hydrostatic pressure and low temperatures cause it to remain in a dense, liquid state or form a hydrate that slowly dissolves into the surrounding water. Plumes of CO2 droplets or hydrate particles will descend and eventually become geologically trapped in deep ocean basins or sub-seafloor sediments. Early field experiments, such as the 2010 CO2 Sequestration off the coast of Norway, demonstrated that injected CO2 could be contained in sub-seafloor reservoirs with minimal leakage when monitored properly. However, the long-term stability of these reservoirs depends on geological sealing and the prevention of upward migration through faults or fractures.

Ocean Alkalinity Enhancement (OAE)

Ocean alkalinity enhancement mimics natural rock weathering processes but accelerates them. Finely ground minerals—typically olivine, basalt, or limestone—are dispersed onto coastal areas, shelf waters, or directly into the ocean. These minerals dissolve over time, releasing bicarbonate ions that react with dissolved CO2 to form stable bicarbonate and carbonate species. The net effect shifts the ocean’s carbonate chemistry toward greater alkalinity, thereby increasing the total amount of inorganic carbon the water can hold as dissolved bicarbonate rather than as CO2 gas. This approach not only sequesters carbon but also counteracts ocean acidification, a critical co-benefit. Pilot studies, including those by the Ocean Alk-Removement initiative and Project Vesta, have shown that olivine weathering can sequester several tens of tons of CO2 per square kilometer per year, though the rate is highly dependent on particle size, water temperature, and mixing. Large-scale deployment would require massive mining and distribution operations, raising energy and environmental footprint concerns.

Artificial Upwelling and Downwelling

Artificial upwelling systems bring nutrient-rich deep waters to the sunlit surface, stimulating phytoplankton blooms that photosynthetically fix CO2 from the atmosphere. Some of this organic carbon sinks to the deep ocean when the plankton die, effectively exporting carbon from the surface. Conversely, downwelling systems push surface waters—already equilibrated with atmospheric CO2—down to depth, where the carbon can be stored for centuries due to slow mixing rates. These techniques are often grouped under “biological carbon pump” enhancement. Early trials, such as the German LOHAFEX experiment in 2009, demonstrated that iron fertilization (a form of upwelling enhancement) could create measurable carbon export, but the efficiency and permanence varied widely. Most experts now agree that engineered upwelling/downwelling requires stringent environmental safeguards to avoid unintended disruption of marine food webs, oxygen minimum zones, or harmful algal blooms.

Ocean Fertilization: Macro- and Micronutrient Approaches

Ocean fertilization adds limiting nutrients—most commonly iron, but also phosphorus or nitrogen—to nutrient-depleted surface waters to boost primary productivity. The best-studied pathway is iron fertilization in high-nutrient, low-chlorophyll (HNLC) regions of the Southern Ocean, the equatorial Pacific, and the subarctic Pacific. Small-scale experiments like SOIREE and EisenEx confirmed that iron additions quickly trigger diatom blooms that can sequester carbon at depth. However, the efficiency of carbon transfer to the deep ocean is often low—typically less than 10% of the fixed carbon sinks below the seasonal thermocline—and the ecological impacts on native plankton communities remain poorly understood. In 2013, the London Protocol Parties effectively banned commercial ocean fertilization except for legitimate scientific research. Since then, research has shifted toward more targeted, controllable approaches, such as combining iron fertilization with artificial downwelling to improve carbon export ratios.

Direct Ocean Capture

Direct ocean capture (DOC) is a technological approach that extracts CO2 directly from seawater rather than from the atmosphere. The process uses electrochemical cells or pH-swing methods to shift the carbonate equilibrium, causing dissolved CO2 to be released as a gas that can be collected and stored. The CO2-depleted seawater is then returned to the ocean, where it reabsorbs atmospheric CO2 to re-equilibrate, effectively enabling multi-pass capture. Companies like Ebb Carbon and Captura are piloting DOC systems that leverage existing desalination or industrial infrastructure. While the energy intensity of DOC is similar to direct air capture (DAC)—around 1–2 MWh per ton of CO2—the higher concentration of carbon in seawater (approximately 100 times that of air in terms of dissolved inorganic carbon) may reduce the theoretical thermodynamic cost. Nevertheless, scaling DOC to millions of tons per year remains a formidable engineering and cost challenge.

Advantages of Ocean-Based CCS Over Terrestrial Alternatives

Proponents argue that ocean-based approaches offer several unique benefits that complement purely land-based storage:

  • Immense storage capacity: The ocean’s volume is enormous; the deep ocean below 1,000 m already holds roughly 38,000 gigatons of carbon, and by enhancing alkalinity or injecting CO2 into deep basins, we could theoretically sequester hundreds to thousands of additional gigatons—far exceeding the storage potential of all known deep saline aquifers on land.
  • Durability of storage: CO2 dissolved in deep ocean waters or converted to bicarbonate remains sequestered for centuries to millennia due to the slow overturning circulation of the deep sea. In sub-seafloor reservoirs, geological trapping can lock carbon away for geological timescales.
  • Lower land-use conflict: Ocean-based storage avoids competing for scarce agricultural, residential, or conservation land, which is a growing issue for terrestrial CCS and DAC facilities.
  • Potential to reverse ocean acidification: Technologies like ocean alkalinity enhancement actively counteract the pH decrease caused by CO2 absorption, protecting shell-forming organisms and coral reefs. This co-benefit is not available with any land-based approach.
  • Distributed deployment: Coastal nations with large exclusive economic zones can deploy ocean-based projects without relying on cross-border pipeline networks, reducing infrastructure complexity and geopolitical risks.

Environmental and Technical Challenges

Despite its promise, ocean-based CCS faces significant obstacles that must be resolved before large-scale deployment can be considered safe and effective.

Ecological Impacts

The injection of CO2 into deep waters raises local pH levels, creating acidic plumes that can harm midwater and benthic organisms, including jellyfish, zooplankton, and deep-sea corals. Even small decreases in pH can impair larval development and metabolic functions. Similarly, ocean alkalinity enhancement involves spreading mineral dust that may smother seafloor habitats or release heavy metals (e.g., nickel, chromium) bound in olivine or basalt. Artificial upwelling can entrain anoxic deep water rich in hydrogen sulfide or ammonium, causing toxicity in surface waters if not carefully managed. Ecological impact assessments for each method remain incomplete, and regulators have called for comprehensive environmental monitoring before permitting commercial-scale operations.

Leakage and Permanence

CO2 injected into the water column remains mobile; unlike geological storage where caprock provides a physical barrier, dissolved CO2 can be transported by currents and eventually re-emerge at the surface through upwelling regions. The residence time of injected CO2 depends on injection depth—deeper water has slower mixing rates—but even at 3,000 m, some fraction will reach the surface within a few hundred years. This leakage risk undermines the “permanent” classification required by carbon accounting frameworks like the Science Based Targets initiative or the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). For sub-seafloor injection, similar to geological CCS, careful site selection, caprock integrity, and monitoring are required to avoid seepage through fractures or abandoned wells.

Technical and Logistical Hurdles

Transporting captured CO2 to offshore injection sites entails building extensive pipeline networks or deploying specialized CO2 carrier ships. The energy required for compression, pumping, and operation of offshore platforms adds to the overall carbon footprint of the process, which must be net negative for climate benefit. Alkalinity enhancement requires mining and grinding billions of tons of rock each year to achieve a meaningful impact on global CO2 levels—the required scale dwarfs current mining output. The cost of ocean alkalinity enhancement is estimated at $20–$100 per tonne of CO2 at best, while direct injection costs range from $30 to $100 per tonne, depending on the source and transport distance. These figures are competitive with DAC but still several times the current price of carbon in most regulated markets.

Regulatory and Governance Frameworks

Ocean-based CCS is governed by a patchwork of international treaties, national laws, and regional agreements. The London Protocol and its 2009 amendment (not yet in force) aim to regulate ocean fertilization and other marine geoengineering activities. The United Nations Convention on the Law of the Sea (UNCLOS) establishes coastal state jurisdiction over the seabed and sub-seafloor resources, which includes deep-sea reservoirs. Additionally, the Intergovernmental Panel on Climate Change (IPCC) has published Special Reports that outline best practice guidelines for monitoring, reporting, and verification (MRV) of ocean-based carbon removal. A key challenge is the lack of a unified carbon accounting methodology for ocean-based storage—unlike terrestrial geological storage, which benefits from decades of oil and gas industry experience, marine systems are more complex to monitor and verify. Several pilot projects, including the Norwegian Northern Lights project and the UK’s Acorn CCS initiative, are exploring offshore storage with rigorous MRV frameworks that could serve as blueprints for broader deployment.

Current Research and Pilot Projects

Several major initiatives are advancing ocean-based CCS from theory to practice:

  • Project Vesta (Caribbean, Atlantic) – Focuses on olivine weathering on tropical beaches to enhance coastal alkalinity and protect coral reefs from acidification. Early results suggest carbon sequestration rates of up to 30 tonnes of CO2 per hectare per year.
  • Ebb Carbon’s Direct Ocean Capture (USA) – Uses an electrochemical seawater‑based system that captures 200 tonnes of CO2 per year from a pilot plant and returns alkalinity to the ocean.
  • Ocean NETs (European Union) – A Horizon 2020 project investigating multiple ocean-based negative emission technologies, including iron fertilization, artificial upwelling, and ocean alkalinity enhancement, with integrated life-cycle assessment and governance analysis.
  • CO2-DISSOLVE (Japan) – A decade-long field experiment near the Okinawa Trough that injected 5,000 tonnes of liquid CO2 into a deep sub-seafloor reservoir and has been monitoring plume migration, chemical changes, and biological effects.
  • Ocean Carbon & Biogeochemistry (OCB) Program (USA) – A scientific coordinating body that publishes best practice guidelines for measuring carbon uptake and export in marine carbon dioxide removal (mCDR) experiments.

Future Outlook and Roadmap

The IPCC’s 2022 Working Group III report concluded that most modelled pathways limiting warming to 1.5 °C rely on some form of carbon dioxide removal, including ocean-based options. However, the same report stressed that “the potential and risks of ocean-based CDR remain poorly constrained.” To bridge this gap, a stepped approach is needed:

  1. R&D scaling: Increase funding for pilot projects from current ~$100 million per year to several billion dollars per year, focusing on environmental impact assessment, engineering optimization, and MRV technologies.
  2. Field trial networks: Establish a global network of pre‑commercial test sites in different ocean basins (coastal, open ocean, and deep sea) to assess site‑specific feasibility.
  3. Regulatory clarity: Develop standardized carbon accounting protocols under the Paris Agreement’s Article 6.4 mechanism to allow ocean-based carbon credits to be traded in voluntary and compliance markets.
  4. Public engagement: Engage coastal communities, Indigenous groups, and environmental NGOs early to build trust and address concerns about potential ecological disruptions.
  5. Integration with other CDR: Combine ocean-based CCS with land-based afforestation, bioenergy with carbon capture and storage (BECCS), and direct air capture to create a diversified portfolio that manages risk and cost.

Conclusion

Ocean-based carbon capture and storage offers a scientifically credible, high‑capacity strategy for removing CO2 from the atmosphere and countering ocean acidification. Yet the technology is not a silver bullet. Deep-sea injection, alkalinity enhancement, and biological pump interventions all carry ecological uncertainties that demand rigorous, transparent research and adaptive management. The coming decade will be critical: without a massive, organized effort to field‑test and scale ocean‑based CCS, the world risks locking itself into a future where emissions overshoot is irreversible. With proper governance, international collaboration, and sustained investment, the oceans could become an indispensable front in the fight against climate change—not just a passive victim of rising CO2, but an active partner in restoring planetary balance.

Further reading: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate | NOAA Ocean Acidification Program | Woods Hole Oceanographic Institution – Ocean Carbon Removal