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Ocean-Based Carbon Removal: The Science and the Challenges

Earthmover Science Team
Ocean carbon sequestration kelp forest

The ocean is the world's largest carbon reservoir, holding approximately 50 times more carbon than the atmosphere and absorbing about 25 to 30 percent of the CO2 emitted by human activities each year. This enormous natural carbon sink is the result of physical, chemical, and biological processes that have been operating over geological timescales — the solubility pump, the biological carbon pump driven by phytoplankton, and the chemical weathering of carbonate minerals. The question driving a growing field of ocean-based carbon removal research is whether and how human activities can intentionally enhance these natural processes to remove additional CO2 from the atmosphere at meaningful scales.

Ocean-based carbon dioxide removal (CDR) encompasses a diverse range of approaches, from cultivating and sinking macroalgae to adding alkaline minerals to seawater to stimulating phytoplankton growth through iron fertilization. Each approach has its own potential carbon removal mechanism, its own set of scientific uncertainties, its own ecological risks, and its own governance and measurement challenges. This article surveys the major ocean CDR approaches, evaluates the state of the science for each, and considers the path from research and pilot-scale testing to commercial deployment.

Ocean Alkalinity Enhancement: The Most Promising Near-Term Approach

Ocean alkalinity enhancement (OAE) is widely regarded by researchers as one of the most promising and scalable ocean-based CDR approaches. The concept draws on natural processes: when silicate or carbonate rocks weather at the Earth's surface, the dissolved ions they release — primarily calcium, magnesium, bicarbonate, and carbonate — flow to the ocean and increase its alkalinity. Higher ocean alkalinity means the ocean can absorb more atmospheric CO2 without becoming more acidic — in fact, adding alkalinity reverses ocean acidification while drawing down CO2.

Human-accelerated OAE involves adding alkaline minerals — typically calcium or magnesium hydroxides, olivine (magnesium silicate), or other silicate rocks — to coastal or open ocean waters. As these minerals dissolve, they increase alkalinity and shift the ocean's carbonate chemistry toward greater CO2 absorption. The carbon removal effect is real and well-understood thermodynamically, but quantifying it in practice requires measuring changes in seawater alkalinity and dissolved inorganic carbon (DIC) over large areas, accounting for background variability, and attributing the signal to the OAE intervention rather than natural processes. This measurement challenge is the primary bottleneck for OAE project development today.

Frontier, the advance market commitment for carbon removal, has committed funding to several OAE pilot projects, and companies including Planetary Technologies, Ebb Carbon, and Planetary Hydrogen are actively developing OAE commercial offerings. The MRV challenge for OAE is significant — it requires a network of ocean sensors, ships, and data integration infrastructure that does not yet exist at the required density and quality. Earthmover's ocean monitoring capabilities are in active development, and we expect to be able to support OAE project verification within the next 18 to 24 months.

Kelp and Macroalgae Sinking: Potential and Uncertainties

Macroalgae — particularly large kelp species like Macrocystis pyrifera — grow extremely rapidly, fixing CO2 from seawater and the atmosphere in their biomass. If harvested kelp or cultivated macroalgae could be reliably transported to the deep ocean and sunk below the "biological pump" depth (roughly 1,000 meters), the carbon in their biomass could be isolated from the atmosphere for timescales of centuries to millennia. This is the basic premise of macroalgae sinking as a carbon removal approach — and on paper, it is attractive because seaweed growth requires no land, no freshwater, and no synthetic fertilizer inputs.

In practice, however, macroalgae sinking faces significant scientific uncertainties. The fraction of sunk biomass that actually reaches the deep ocean floor and remains there, versus decomposing in the water column and releasing CO2 before it reaches sequestration depth, is poorly constrained. Estimates range from 20% to 80% efficiency, depending on water column oxygen levels, temperature, and decomposition rates. Accurately quantifying the carbon removal benefit of macroalgae sinking therefore requires oceanographic instrumentation — sediment traps, autonomous underwater vehicles, acoustic Doppler profilers — that is both expensive and logistically challenging to deploy at scale. Until these measurement challenges are resolved, macroalgae sinking carbon credits will face skepticism from demanding buyers.

Iron Fertilization: A Cautionary Tale

Ocean iron fertilization (OIF) was one of the earliest ocean CDR concepts to attract serious scientific and commercial attention. The idea is simple: phytoplankton growth in large areas of the Southern Ocean, equatorial Pacific, and subarctic Pacific is limited by iron availability rather than nitrogen, phosphorus, or light. Adding iron to these "high-nutrient, low-chlorophyll" (HNLC) zones should stimulate phytoplankton blooms, which would draw down CO2 from the atmosphere through photosynthesis. When the phytoplankton die, some fraction of their carbon sinks to the deep ocean, providing carbon storage.

Multiple small-scale iron fertilization experiments confirmed that iron addition does indeed stimulate phytoplankton growth and short-term CO2 drawdown. However, the experiments also revealed several complications. First, the efficiency of carbon export to the deep ocean — the fraction of the stimulated phytoplankton biomass that actually sinks and stays there — appears to be much lower than early models predicted, partly because zooplankton graze the phytoplankton and return CO2 to the surface through respiration. Second, large-scale iron fertilization could alter ocean nutrient chemistry in ways that reduce productivity in downstream ecosystems. Third, the governance and legal framework for large-scale ocean iron fertilization is contested — the London Protocol on ocean dumping effectively prohibits commercial-scale OIF experiments. For these reasons, OIF has largely been sidelined as a commercial carbon removal pathway, though it continues as a subject of academic research.

Blue Carbon Ecosystems: Mangroves, Seagrasses, and Saltmarshes

Blue carbon refers to the carbon stored and sequestered in coastal marine ecosystems — primarily mangrove forests, seagrass meadows, and saltmarshes. These ecosystems are highly productive biologically, and the anaerobic conditions in their waterlogged soils dramatically slow the decomposition of organic matter, leading to the accumulation of deep, carbon-rich sediments that can store carbon for millennia. On a per-area basis, blue carbon ecosystems can sequester 3 to 5 times more carbon annually than tropical forests — making their conservation and restoration a high-priority carbon removal opportunity.

Blue carbon has been integrated into voluntary carbon markets through a growing number of projects that protect or restore mangroves, seagrasses, and saltmarshes. The Verra-approved VM0033 methodology and the Gold Standard's Blue Carbon Methodology provide frameworks for quantifying carbon stocks and estimating emission reductions from avoided ecosystem conversion. Compared to other ocean-based approaches, blue carbon has relatively well-developed measurement protocols and a track record of project development. However, concerns have been raised about the accuracy of some blue carbon project estimates, the permanence of carbon stocks in ecosystems subject to sea level rise, and the degree of community benefit sharing in coastal conservation projects in developing countries.

Measurement Challenges in Ocean CDR

Across all ocean-based carbon removal approaches, the measurement challenge is fundamentally harder than for terrestrial carbon removal. The ocean is vast, dynamic, and difficult to instrument at scale. Background variability in ocean chemistry — driven by currents, upwelling, biological productivity, and temperature — creates a high "noise floor" against which relatively small CDR signals must be detected. Attribution of measured changes to a specific intervention, versus natural variability, requires careful experimental design and statistical analysis. The lack of standardized ocean monitoring infrastructure means that each project must invest substantially in measurement equipment and expertise.

Several research initiatives — including the NOAA Ocean Acidification Program, the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program, and the European Integrated Carbon Observation System — are building the ocean monitoring networks that could eventually serve as a substrate for CDR verification. Emerging technologies like biogeochemical Argo floats (which can autonomously measure ocean chemistry at depth over years), autonomous surface vehicles, and satellite ocean color sensors are expanding monitoring capabilities. Earthmover is actively engaged with the ocean monitoring community to define the measurement standards and data integration approaches that will enable high-quality ocean CDR verification at scale.

Key Takeaways

  • Ocean CDR encompasses OAE, macroalgae sinking, iron fertilization, and blue carbon — each with different maturity, efficiency, and measurement challenges.
  • Ocean alkalinity enhancement is the most promising scalable approach, with clear thermodynamic basis but significant MRV challenges in real-world implementation.
  • Iron fertilization has largely been sidelined due to low deep-export efficiency, ecosystem risks, and legal constraints under the London Protocol.
  • Blue carbon (mangroves, seagrasses, saltmarshes) has the most developed market infrastructure and produces 3–5x more carbon per area than tropical forests.
  • Ocean CDR MRV is harder than terrestrial CDR due to vast spatial scale, high background variability, and limited monitoring infrastructure.
  • Biogeochemical Argo floats, autonomous surface vehicles, and satellite ocean color sensors are building the monitoring base needed for ocean CDR verification.

Conclusion

The ocean is an enormous and underutilized potential contributor to global carbon removal, but realizing that potential will require sustained investment in the science, measurement infrastructure, and governance frameworks that ocean CDR demands. The approaches that are closest to commercial readiness — OAE and blue carbon — still require significant measurement and verification development before they can generate high-quality credits at scale. Those that are most speculative — macroalgae sinking, iron fertilization — face both scientific and regulatory obstacles that will take years to resolve. At Earthmover, we are investing in the ocean monitoring capabilities that will be needed as this field matures, and we look forward to playing a role in ensuring that ocean CDR delivers genuine, measurable climate benefits when it does reach commercial scale.