Marine Carbon Dioxide Removal (mCDR): Advancements in Technology, Commercialization and Funding Opportunities

Christina Janulis and Morgan Bennett-Smith Energy Solutions

Marine Carbon Dioxide Removal (mCDR): Advancements in Technology, Commercialization and Funding Opportunities
Morgan Bennett-Smith


- Marine carbon dioxide removal (mCDR) approaches leverage biological and chemical processes in the ocean to increase atmospheric CO₂ removal and storage.

- mCDR approaches can be biotic (e.g. coastal wetland restoration) or abiotic (e.g. electrochemical CO₂ removal).

- 400+ (and counting) scientists worldwide have signed an open letter advocating for responsible mCDR research and development.

- A research group at Oxford concluded that a diverse portfolio of CDR solutions, as well as significant capital and policy innovation, are needed to encourage technological advancement and resolve uncertainties.

- Responsible mCDR development will require further research and planning to determine technology scalability, multi-level governance and regulatory frameworks.

What is marine carbon dioxide removal (mCDR)?

Marine (ocean-based) carbon dioxide removal (mCDR) aims to enhance natural biological and chemical processes in the ocean to absorb and store more carbon from the atmosphere. There are both biotic and abiotic approaches to CDR; some strategies rely on the direct enhancement or cultivation of carbon-sequestering biomass (i.e. restoring and enhancing ecosystems abundant in blue carbon such as mangroves, salt marshes, seagrasses and kelp forests), while others displace CO₂ from the air by harnessing physical or chemical properties of the ocean. Ocean alkalinity enhancement, for example, entails adding alkaline minerals to seawater to speed up the ocean carbon cycle, enabling permanent storage and more CO₂ drawdown from the atmosphere.

Source: World Resources Institute

Why do we need mCDR?

The push for mCDR research and development (R&D) in recent years comes at a critical time for our planet, which is now experiencing a greater concentration of greenhouse gas emissions than at any other time in the last 800,000 years. Despite the ocean’s absorption of more than 90% of excess heat trapped in the atmosphere and 26% of excess carbon pollution, the planet has still warmed by 1.1° Celsius (1.9° Fahrenheit) on average since the early 20th century.

As a major buffer to the worst impacts of climate change, the ocean has experienced unprecedented changes including warming, acidification and deoxygenation. In August, we saw the hottest average global sea surface temperature since 2016, according to the EU’s Copernicus Climate Change Service. These changes have the potential to impact critical ocean circulation patterns that drive weather systems, nutrient cycling and heat distribution around the globe.

Source: Copernicus Climate Change Service

With atmospheric CO2 at an all-time high, deep cuts to emissions alone will not be enough to meet international temperature targets. Many countries have made net-zero pledges, which aim to supplement greenhouse gas (GHG) emissions reductions with CO2 removal. CDR technologies and sequestering carbon in forests and soils are among the five major categories of action outlined in the White House’s Pathways to Net-Zero Greenhouse Gas Emissions by 2050.

The first global, independent scientific assessment of CDR in January 2023 found that CO2 removal is essential in limiting global warming to 2° Celsius or lower. The comprehensive 120-page report, authored by 20 global CDR experts and led by Oxford’s Dr. Steve Smith, warns of the drastically underestimated role of CDR in achieving global net zero. Dr. Smith, Executive Director of Oxford Net Zero and CO₂RE, states that we need to both accelerate emissions reductions and increase carbon removal.

“CDR is not something we could do, but something we absolutely have to do to reach the Paris Agreement temperature goal.”

- Dr. Oliver Geden, Intergovernmental Panel on Climate Change lead author: Vice-Chair, Working Group III: Mitigation of Climate Change

What are the most promising approaches?

The mCDR approaches receiving the most attention for potential research and investment are blue carbon, macroalgae cultivation and sinking, microalgae cultivation, alkalinity enhancement, electrochemical techniques, deep sea storage and artificial upwelling & downwelling. The following section includes short summaries and diagrams of each mCDR approach, as well as linked fact sheets from Ocean Visions, a non-profit organization advancing innovative and durable solutions for ocean-climate restoration.

Blue Carbon Restoration and Carbon Sequestration

Blue carbon refers to the carbon stored in roots and sediments of coastal and marine ecosystems such as mangroves, seagrass and salt marshes. These habitats are especially effective at taking up and holding carbon in their biomass; mangroves and salt marshes, for example, take up carbon 10x faster than tropical forests. Restoring and enhancing these habitats is one method of increasing storage potential in natural carbon sinks and preventing blue carbon from being released back into the atmosphere.

© Mesa Schumacher

Macroalgae Cultivation and Carbon Sequestration

Macroalgae (seaweeds) convert dissolved carbon in seawater into biomass and organic compounds via photosynthesis. As they absorb carbon and sink to the ocean floor (either naturally or intentionally), atmospheric CO2 is drawn down and sequestered in sediment, which eventually makes its way to permanent storage in the deep ocean. This mCDR approach entails large-scale macroalgae farming to increase blue carbon sequestration at the bottom of the ocean. Alternatively, the farmed seaweed can be harvested and processed for algal bioenergy or for long-lived products like bioplastics.

 © Mesa Schumacher

Microalgae Cultivation

Microalgae cultivation (also known as ocean fertilization) stimulates phytoplankton growth by adding nutrients such as iron, nitrogen and/or phosphorus to surface waters. Through photosynthesis, phytoplankton in the Southern Ocean alone absorb an estimated 40% of all carbon dioxide emissions and 60-90% of excess heat generated by humans. Augmenting primary productivity (photosynthesis) has the potential to remove large amounts of atmospheric CO₂ and transport it to the deep ocean in different biogenic forms created by phytoplankton.

© Mesa Schumacher

Ocean Alkalinity Enhancement

In addition to photosynthetic phytoplankton, the chemistry of seawater makes the ocean the world’s largest carbon sink. In response to the natural chemical reaction that forms bicarbonates from inorganic CO2 and alkaline molecules, the carbon cycle draws down more atmospheric carbon to rebalance the equation. Bicarbonates are stable forms of carbon that can less easily escape the ocean, remaining sequestered for thousands of years. This mCDR approach entails directly adding alkaline minerals like olivine, basalt and carbonate to seawater to enhance this natural process, creating a disequilibrium that draws atmospheric CO₂ into the ocean to rebalance carbon concentrations.

© Mesa Schumacher

Electrochemical Ocean Carbon Dioxide Removal

Electrochemical CO₂ removal harnesses electricity to rearrange water and salt molecules in seawater into acidic and basic solutions. Acidic solutions can be used to extract CO2 from seawater in its gaseous form to be sequestered underground for storage and other uses, or to weather alkaline rocks and thereby increase alkalinity. Basic solutions can be used to enhance ocean alkalinity (see above) or to precipitate carbonate directly out of the seawater, stabilizing carbon in a durable, solid form.

© Mesa Schumacher

Deep Sea Storage

Deep sea storage can sequester carbon from multiple sources near the ocean floor. Extreme temperature and pressure conditions at these depths (greater than 2700m) turn dissolved CO₂ to liquid CO₂, which is denser than seawater and thus remains in the deep sea for long-term storage. The carbon captured for storage can come from direct air capture facilities on land, agricultural production or seaweed farms. Seafloor rocks can also mineralize captured CO₂ upon reaction for permanent carbon storage.

© Mesa Schumacher

Artificial Upwelling and Downwelling

Artificial upwelling and downwelling approaches rely on harnessing and manipulating natural seawater flow processes in the ocean to support carbon drawdown and export to the deep ocean.

During upwelling, cold water from the deep rises towards the surface. These waters are typically nutrient rich and fertilize phytoplankton, supporting high primary productivity near the surface (see microalgae cultivation above). During downwelling, different water masses converge and push surface waters downward, sending carbon-rich biomass and organic carbon to the deep ocean.

Artificial mechanisms to stimulate upwelling and downwelling – such as mechanical pumps, artificial cooling and salinity manipulation – can significantly improve the rate of carbon storage in the ocean.

© Mesa Schumacher

You can find estimates of the amount of carbon stored via these approaches in the Natural Resource Defence Council’s Ocean Carbon Dioxide Removal Methods (2022).

What are scientists saying?

Since September 2023, more than 400 scientists (and counting) have signed an open letter advocating for responsible mCDR research and development. In recognition of the current levels of atmospheric carbon dioxide, these oceanographers, environmental scientists and climate experts stress the urgency for global climate action. Specifically, they highlight the need for accelerated mCDR engineering and controlled field trials with monitoring and evaluation frameworks, third-party independent review of results, consequence safeguards and inclusive governance structures.

“Society must advance responsible research, development, and field testing of ocean-based carbon dioxide removal techniques to determine their potential to help restore the climate and the ocean.”

- Ocean CDR Science Letter

The call for further research into mCDR techniques has reverberated throughout the scientific community. A week after the open letter was published, the National Science Foundation (NSF) published a Dear Colleague Letter to show joint support for advancing research to increase understanding of CDR and Solar Radiation Modification (SRM) science, governance and consequences. While reducing anthropogenic emissions remains a top priority, the NSF is encouraging submission of proposals for research projects on CDR and SRM that have integrative engagement with ethical frameworks, governance structures, and/or environmental justice issues to help guide research on scaling and deployment.

The letter was organized by Ocean Visions, who recently shared a high level roadmap of priorities for working toward proving or disproving different mCDR technologies by 2030. The three-pillar program is a living document, currently centered around:

  1. Science and engineering at the appropriate scales to answer important outstanding questions about mCDR technologies;
  2. Development of enabling policy for advancing mCDR technologies through political and societal support, robust regulatory frameworks and inclusive governance structures;
  3. Improvement and optimization of mCDR technologies to increase their potential to achieve climate-relevant scale and impact.

Ocean Visions is on a mission to help tackle these challenges by facilitating multi-sector collaborations within its extensive network and beyond, bringing together leading research institutions, public-interest organizations and the private sector. The principles guiding their work on mCDR evince their thoughtful approach to exploring these technologies urgently and responsibly.

What are obstacles to large-scale mCDR deployment?

We have insufficient evidence to determine if most proposed mCDR techniques are efficient, socially responsible, equitable or lower risk for people and nature than direct air capture. Most efforts to remove excess carbon dioxide have been land-based or mechanically engineered, namely growing trees and developing direct air capture plants. While we know that the ocean holds somewhere around 42 times more carbon than the atmosphere, we know relatively little about the effectiveness of applying marine sequestration methods at scale.

Despite its high potential, climate engineering (i.e. geoengineering) has also been accompanied by considerable controversy in recent years. Some environmental advocates maintain that mCDR distracts from efforts to cut emissions, and question the permanence of carbon storage as well as the controllability, environmental impact, containment and risk to local populations. This 2023 publication in Annual Review of Marine Science outlines the state of technical assessments of mCDR approaches as well as associated financial, social and ethical concerns through a sociotechnical system lens.

Intuitively, mCDR approaches perceived as “more natural” (e.g. coastal ecosystem restoration) are generally favored by the public over those perceived as synthetic or engineered (e.g. electrochemical mCDR). There is inherent risk in augmenting or altering biological and chemical processes in an ecosystem as central and complex as the ocean, but we will not know the carbon sequestration potential of any one mCDR pathway until we thoroughly and responsibly test them all.

How can we commercialize mCDR?

A 2022 Oxford review article on commercialization mechanisms for CDR found that significant funding is needed to scale deployment and achieve net zero targets. The researchers surveyed current policy mechanisms that incentivize CDR, cost estimates for common CDR approaches and how those costs are distributed. Acknowledging the need for a diverse portfolio of CDR solutions, they determined that policy innovation is equally if not more important than technology development. Resolving key scientific and technological uncertainties is only possible with the support of suitable policies and markets.

Policy support will vary based on each mCDR approach’s business model, maturity and cost. In September 2023, the National Oceanic and Atmospheric Administration (NOAA) Ocean Acidification Program announced a $24.3 million investment in advancing mCDR research with the goal of uniting academic researchers, government scientists and major industry players for the cause.

Ebb Carbon is among the startups receiving funding from the program. Their new shipping-container-sized CDR system currently passes seawater in Sequim Bay, WA through a series of membranes to remove acid from seawater. This electrochemical process increases the water’s ability to absorb and store additional atmospheric CO₂ by adding the alkaline solution produced in the process back into the Bay, where it transforms into a bicarbonate. Ebb Carbon also received funding from private partners, academic partners and the philanthropic Climateworks Foundation.

Beyond subsidies for early-stage R&D pilot projects, government incentives for mCDR are limited. Addressing this gap and other barriers, including skill development and CO₂ transport and storage, will require financial interventions and direct financial incentives. There is also a particular need for results-based long term policy instruments. While there are operational market-based mechanisms, public procurement type mechanisms and direct fiscal incentives, few of them explicitly incentivize CDR solutions. 














Coastal wetland restoration is much more developed than other mCDR approaches because it has been practiced for decades to support other ecosystem services, including coastal resilience, water quality improvement and commercial fisheries. Most mCDR technologies have not been similarly tested or deployed at scale. Investment in a wide range of approaches, along with policy innovation, is vital in our efforts to remove excess carbon from the atmosphere to ameliorate the worst effects of climate change.

How can we responsibly move forward?

Support of mCDR research and development efforts is growing: The Ocean Conservancy released The Ocean Carbon Dioxide Removal Decision-Making Landscape in February 2023 in anticipation of the wide range of information the diverse stakeholders will need in the rapidly advancing field; NOAA released a Carbon Dioxide Removal Strategy in June 2023; and the White House just announced a Fast-Track Action Committee on Marine Carbon Dioxide Removal in October 2023.

Similar to Ocean Visions, the World Resources Institute’s (WRI) 2022 report, Toward Responsible and Informed Ocean-Based Carbon Dioxide Removal, recommends a three-pronged approach:

  1. Resolve uncertainties to determine best ocean-based CDR approaches for large-scale, low-impact deployment (e.g. increase public, private and philanthropic funding for collaborative research that prioritizes capacity development for early-career researchers in climate-vulnerable communities, underrepresented groups, Indigenous Peoples, and the Global South).
  2. Improve local, national, and international governance frameworks to support responsible research and small-scale pilots than include all stakeholders (e.g. convene a ministerial dialogue on ocean CDR under the joint auspices of the United Nations Framework Convention on Climate Change (UNFCCC), the Convention on Biological Diversity (CBD), Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Convention), and the Protocol to the London Convention (London Protocol).
  3. Lay the foundation for robust governance of large-scale deployment in the future (e.g. explore a new agreement or framework to proactively govern ocean CDR that identifies a mandating institution and requires independent and peer-review assessments of impacts).

What’s next?

Top research institutions have published exciting research in mCDR technology this year, including an asymmetric chloride-mediated electrochemical process from MIT and hollow fiber membrane contactors and encapsulated solvents from the University of Pittsburgh, with more on the horizon, such as University of Hawaii-led research on the impact of alkalinity enhancement technologies.

New sources of funding are becoming available for ocean-based science and technology solutions, which could include mCDR technologies. For example, climate tech VC Propeller recently partnered with Woods Hole Oceanographic Institution to launch a $100 million fund to invest in a portfolio of ocean tech startups, which includes, among other things, marine and genetically engineered carbon capture.

While ocean-based solutions currently represent only a fraction of VC investment in climate tech, that pool of money is growing, along with public funding. For example, the U.S. Department of Energy recently announced $36 million for 11 projects across 8 states to develop mCDR technologies. Continuing to grow these sources of financing will play a critical role in enabling the incorporation of these technologies into the $2.5 trillion blue economy. Both public and private markets will be necessary to accelerate these potentially planet-saving solutions derived from our ocean.

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