What “Ocean CDR” actually means

Ocean carbon dioxide removal is a family of techniques that use the sea — the planet’s largest active carbon reservoir, holding roughly 50 times more carbon than the atmosphere — to draw down atmospheric CO₂ and keep it down. The ocean already absorbs about a quarter of annual anthropogenic emissions through air-sea gas exchange (Friedlingstein et al., 2023). Ocean CDR approaches either accelerate that natural uptake (by shifting seawater chemistry) or use biology to fix carbon and export it below the mixed layer, where it stays out of atmospheric contact for centuries to millennia. The appeal for buyers chasing durable removal is straightforward: the storage reservoir is enormous, and the residence times — particularly for bicarbonate ion in the deep ocean — are on the order of 10,000 years (Siegel et al., 2021).

The category is also unusually broad. The 2021 National Academies research strategy groups it into six families: ocean alkalinity enhancement, electrochemical methods, nutrient fertilisation, artificial upwelling/downwelling, seaweed cultivation, and ecosystem recovery. Most commercial activity today clusters in the first three.

How it works

Ocean alkalinity enhancement (OAE) adds base — typically ground silicate or carbonate minerals, or dissolved hydroxides — to surface seawater. The added alkalinity shifts the carbonate equilibrium, converting dissolved CO₂ into bicarbonate (HCO₃⁻) and pulling more CO₂ from the atmosphere to restore partial-pressure equilibrium. The chemistry is well-understood but operationally fragile: dose too aggressively and you trigger runaway CaCO₃ precipitation, which releases CO₂ and defeats the purpose (Hartmann et al., 2022). Field deployments therefore aim for dilute, distributed dosing.

Electrochemical methods — direct ocean capture (DOC) and electrochemical OAE — use bipolar membrane electrodialysis or related cells to split seawater into acidic and alkaline streams. The acid strips CO₂ from seawater (which can then be compressed and stored geologically); the alkaline stream is returned to drive further uptake. Energy intensity is the binding constraint, currently in the 1–2.5 MWh/tCO₂ range depending on configuration.

Biological pathways — seaweed sinking, microalgae stimulation, biomass burial — rely on the biological pump to transport organic carbon below the thermocline. These approaches have the lowest measurement confidence: carbon-accounting work on seaweed in particular has shown that a meaningful fraction of fixed carbon respires before reaching depth (Hurd et al., 2022).

Coastal margins complicate everything. They are the highest-flux interface in the ocean carbon cycle and the most biogeochemically variable region to measure against (Dai et al., 2022) — which matters because most commercial deployments are coastal.

Who’s building it

A non-exhaustive snapshot of operating approaches:

  • Planetary (Halifax) doses magnesium hydroxide through existing power-plant outfalls and delivered the first independently verified OAE credits — 625.6 tonnes — to Stripe.
  • Ebb Carbon (South San Francisco) bolts bipolar-membrane electrodialysis onto desalination brine streams, returning an alkaline stream to the sea.
  • Captura (Pasadena) runs a 1,000 t/yr direct ocean capture pilot in Kona, Hawaii, opened February 2025, that extracts gaseous CO₂ from seawater for geological storage.
  • Equatic (Los Angeles) couples seawater electrolysis with green hydrogen co-production, generating both dissolved bicarbonate and solid carbonate minerals.
  • Limenet (Augusta, Sicily) reacts biogas CO₂ with hydrated lime in a reactor before discharging an equilibrated calcium-bicarbonate solution — a closed-loop variant designed to sidestep runaway precipitation risks.
  • Vesta (San Francisco) deploys olivine sand in nearshore waters for enhanced weathering — the slowest-acting but lowest-energy OAE approach.
  • Gigablue (US/Israel) deploys engineered substrates to stimulate phytoplankton growth and marine-snow export — a biological pathway that drew scientific criticism in 2025 over its quantification methods.
  • Banyu Carbon (Seattle), a University of Washington spinout, uses sunlight-activated reversible photoacids to release CO₂ from seawater at lower energy cost than electrodialysis.

Beyond these, the directory includes seaweed-sinking plays (Seafields, SeaGen, Fiora Mara), biomass-burial approaches (Carboniferous in the anoxic Orca Basin), and earlier-stage OAE work in Europe (Pronoe, Planeteers, Carbon Time).

The durability and MRV problem

Permanence varies enormously across these methods. Bicarbonate produced by OAE has an effective storage timescale of millennia, provided the alkalinity actually equilibrates with the atmosphere before being subducted (Siegel et al., 2021). Direct ocean capture with geological injection inherits the durability of the geological store (>1,000 years). Seaweed sinking sits at the other end: storage durability depends on sinking depth, sediment oxygen, and respiration losses, and credible estimates range from decades to centuries with wide error bars.

Measurement, reporting and verification (MRV) is the binding constraint on the entire category. CO₂ uptake from OAE happens over weeks to months across a parcel of water that has drifted away from the dosing site. No one can measure it directly at scale; everyone is using a combination of in-situ alkalinity and dissolved-inorganic-carbon sensors, dye tracers, and reactive-transport models calibrated to the deployment. Mesocosm work has begun probing ecological side-effects — alkalinity loading can shift phytoplankton community composition even where total biomass is unchanged (Ferderer et al., 2022). The registries (Isometric, Puro.earth) have published OAE and DOC methodologies, but