Reducing carbon dioxide (CO₂) emissions alone is no longer sufficient to limit global warming to 1.5°C. To effectively address climate change, scientific consensus highlights the need to remove carbon dioxide already present in the atmosphere. Marine carbon dioxide removal (mCDR), a form of ocean-based carbon removal, offers a scalable solution by leveraging the ocean’s natural ability to absorb and store carbon.

This guide explores the fundamentals of mCDR, including how it works, the technologies involved, and its role in global decarbonization.

What is marine carbon dioxide removal (mCDR)?

mCDR uses ocean-based processes to capture and store carbon dioxide from the atmosphere. mCDR techniques fall into two primary categories: 

• Biotic CDR, which includes removing and storing atmospheric CO₂ through biological processes such as harnessing photosynthetic fixation through micro- and macroalgae cultivation and terrestrial biomass sinking. 

• Abiotic CDR, which influences the chemistry of seawater to increase the net amount of atmospheric CO₂ absorbed by the ocean without increasing the acidity. There are two main abiotic marine carbon dioxide removal pathways: ocean alkalinity enhancement (OAE) and direct ocean removal (DOR). 

How marine carbon dioxide removal works

The ocean plays a crucial role in regulating atmospheric CO₂ by absorbing and releasing this greenhouse gas through interactions at its surface. This process, known as air-sea gas exchange, balances the CO₂ concentrations in the atmosphere and ocean. When atmospheric CO₂ levels rise, the ocean absorbs more CO₂, acting like a carbon sink and helping to mitigate climate change. Once CO₂ enters the ocean, it either remains dissolved or transforms into various carbonate species, primarily bicarbonate, where the relative concentrations of each species are in equilibrium. The concentrations of dissolved CO₂ and carbonate species influence the ocean's acidity or alkalinity (pH). Changes in ocean chemistry, especially pH, impacts the concentration of carbonate species and dissolved CO₂, ultimately impacting how much CO₂ the ocean can absorb or release.

CO₂ stored as bicarbonates in the ocean can remain sequestered for over 10,000 years, making the ocean a viable solution for long-term carbon dioxide removal and climate change mitigation. In response to rising atmospheric CO₂ levels, the oceans already absorb 10 gigatons of carbon dioxide (GtCO2) annually, accounting for about 25% of anthropogenic CO₂. This increase is disrupting the natural CO₂ balance and contributing to ocean acidification. 

Marine carbon dioxide removal methods, such as ocean alkalinity enhancement and direct ocean removal, are tapping into these mechanisms to safely increase the amount of CO₂ absorbed from the atmosphere while mitigating ocean acidification. The carbon dioxide removal potential of ocean alkalinity enhancement and direct ocean removal is estimated to be up to 15 GtCO₂ and 10 GtCO₂, respectively, per year.

The technical details: ocean alkalinity enhancement (OAE)

Ocean alkalinity enhancement (OAE) captures and stores atmospheric CO₂ as dissolved bicarbonate in the ocean by increasing the alkalinity of seawater. Increasing the alkalinity results in dissolved CO₂ transforming into bicarbonates, decreasing the concentration of dissolved CO₂ and increasing the flux of CO₂ from the atmosphere to the oceans. This process essentially increases the ocean's capacity to absorb atmospheric CO₂ while maintaining natural pH levels and even reducing acidification. There are two primary ocean alkalinity enhancement methods:

• Mineral-based ocean alkalinity enhancement: Adding alkaline minerals such as olivine or basalt to seawater to decrease the concentration of dissolved CO₂.

• Electrochemical-based ocean alkalinity enhancement: Electrochemically splitting seawater via electrolysis or electrodialysis into an acid and base. The base is used to increase the alkalinity of seawater. 

Mineral-based OAE vs. Electrochemical-based OAE

Mineral-based ocean alkalinity enhancement methods may be less energy-intensive but adding materials to the ocean carries potential risks and uncertainties, which necessitates additional MRV measures. Electrochemical-based ocean alkalinity enhancement methods may not require additional materials to the ocean, but the specialized equipment and large energy requirements to alter seawater's chemistry drive up costs. 

The technical details: direct ocean removal (DOR)

Direct ocean removal (DOR) shifts the carbonate equilibrium in seawater to enable CO₂ extraction from the seawater as gaseous CO₂ or mineral carbonates, lowering the concentration of CO2 in the seawater. The CO₂-depleted seawater can then absorb an equivalent amount of atmospheric CO₂. This step can be done in the open ocean or a closed system. There are two primary direct ocean removal methods:

• Extracting gaseous CO₂: Seawater is acidified via electrochemical methods such as electrolysis or electrodialysis. This shifts the carbonate equilibrium from dissolved bicarbonates to CO₂. The CO₂ is then extracted and stored, such as in direct air capture technologies.

• Extracting mineral carbonates: Seawater is basified via electrochemical methods or through the addition of minerals, shifting the equilibrium to carbonates that, in the presence of cations (positively charged ions) such as calcium, form solid calcium carbonate, which is extracted.

The advantages of direct ocean removal methods include precise monitoring of the quantity of CO₂ extracted and the elimination of the need to introduce externally sourced materials to the ocean; however, these methods tend to have a higher cost mainly due to the energy intensity of chemically influencing seawater to form acids and the CO₂ desorption step.