Carbon Removal from the Ocean, Explained

The science is clear: We are not on track to limit temperature rise to 1.5 degrees C (2.7 degrees F) to prevent the most catastrophic impacts of climate change. Deeper, faster cuts to emissions are urgently needed across all sectors, and must be a central focus of climate policy. However, we have already added so much greenhouse gas (GHG) to the atmosphere that even far steeper cuts in emissions will not be enough to meet global climate goals; we also need to actively pull carbon dioxide (CO2) back out of the air.

So far, efforts to remove excess CO2 from the air have largely focused on what can be done on land, such as growing trees or building direct air capture plants. But now a growing number of researchers, companies and even national governments have begun to look at the ocean as a potential location for carbon dioxide removal.

These approaches aim to leverage the ocean’s natural chemical and biological processes to absorb and store more carbon from the atmosphere. While these approaches may have important potential as part of a climate solution, we know surprisingly little about how effective they are at sequestering carbon, and what kind of other impacts they would have on the environment and communities if they are pursued at scale.

A new WRI report synthesizes the latest science and research on ocean-based carbon removal and examines what’s needed to ensure that decisions around its development and deployment are responsible.

Here, we answer several key questions about ocean-based carbon removal:

1) How Can Oceans Remove More Carbon from the Atmosphere?

Ocean carbon removal approaches aim to enhance or accelerate natural biological or chemical processes that sequester carbon in the ocean. In a few cases, the approaches extract carbon dioxide dissolved in seawater for storage on land.

We know the ocean is good at sequestering carbon because it has already absorbed 30% of the CO₂ —  and 90% of excess heat — caused by human activities, significantly dampening the impacts of climate change. In total, the ocean holds around 42 times more carbon than the atmosphere.

At the same time, what the ocean has done to buffer the impacts of climate change has come at a high cost. Absorbing excess CO₂ and heat is causing ocean warming, acidification and oxygen loss; changing currents and nutrient cycling; and imperiling plants and animals essential to marine ecosystems. These changes are compromising the ocean’s ability to provide food, support livelihoods and insulate us from the worst effects of climate change. 

2) What Are the Different Ocean-based Approaches for Carbon Removal?

A range of ocean carbon removal approaches have been discussed in the scientific literature and a subset have been tested at sea. WRI’s new report covers the seven approaches most likely to attract investment and/or be deployable at scale.

One of these seven — coastal wetland restoration — has been practiced for decades, though mostly to support resilience and ecosystem benefits rather than to sequester carbon. The other six are at a far earlier stage of development.

Abiotic approaches harness the physical or chemical properties of the ocean to remove CO2 from the air. For instance, alkalinity enhancement involves adding certain minerals to seawater that enable more atmospheric CO2 to dissolve into the ocean and be stored. In some cases, this process can also reduce ocean acidity. Electrochemical techniques use electricity to mimic alkalinity enhancement, or directly extract CO₂ from seawater for storage on land. Artificial downwelling accelerates natural currents that carry carbon-rich surface water into the deep ocean in the Arctic and Antarctic. 

Biotic approaches leverage photosynthesizing organisms in seawater to take up carbon dioxide and store that carbon as biomass. Seaweed, for example, can be cultivated and then sunk to the deep ocean or the seafloor, storing a portion of the carbon-rich biomass. Nutrients like iron can be added to the ocean to spur phytoplankton growth, a process called ocean fertilization. Phytoplankton take up carbon dioxide, convert it to biomass, and then some will sink to the deep ocean, sequestering that carbon. This strategy relies on the ocean’s “biological carbon pump. Artificial upwelling aims for the same outcome as ocean fertilization, but does so by moving deeper, nutrient-rich water to the surface, rather than adding new nutrients.

3) What’s the Current State of Ocean-based Carbon Removal?

Most ocean carbon removal approaches are at an early stage of development, with little or no testing done in the field. It is not yet clear how effective these approaches would be at removing carbon, how that could change depending on a project’s scale or location, how long the carbon would remain sequestered, or what impact these approaches might have on ocean ecosystems or people whose livelihoods depend on them.   

Nevertheless, most governments include carbon dioxide removal in their long-term climate action strategies submitted to the United Nations Framework Convention on Climate Change (UNFCCC). Two countries — Japan and the United States — specifically mention ocean carbon removal.

Investments into companies actively developing pathways for ocean carbon removal are accelerating as well; for example, Stripe and Shopify have invested in ocean carbon removal companies including Vesta, Running Tide and Planetary Technologies. Interest and investment in ocean-based carbon dioxide removal is likely to surge as pressure to confront the climate crisis mounts. 

As governments and the private sector explore ocean carbon removal, it will be important to examine both the potential and the risks of these new approaches. Developing ocean carbon removal responsibly will require looking beyond calculations of efficacy and cost. It will mean carefully selecting approaches that merit further development and ruling out those that could pose environmental or ecological risks that outweigh expected benefits.

Ultimately, both unabated climate change and ocean-based carbon dioxide removal involve risks, but these differ in scope and potential impact. Policymakers will need to weigh the tradeoffs carefully before moving forward and put guardrails in place to avoid causing irreparable harm.

4) How Is Ocean Carbon Removal Governed?

How an ocean carbon removal project is regulated depends on where it is. Projects deployed within 200 miles of a country’s coastline would fall under that country’s jurisdiction. Areas outside this zone are in the “high seas” and regulated purely by international frameworks created before ocean carbon removal existed. These frameworks are typically designed to address problems such as polluting, dumping or harming biodiversity in the marine environment. This existing governance system is ill-suited to the task of filling the gaps in scientific knowledge, ensuring robust stakeholder engagement, and supporting transparent and accountable research and deployment of ocean carbon removal. 

Even if countries conduct ocean carbon removal projects entirely within national waters, the impacts could still be far-reaching and transboundary. And in the long-run, reaching large-scale carbon removal in the ocean will require projects to be implemented in the high seas. Without clear rules regarding monitoring and transparency of data, impacts of ocean carbon removal could go undetected.  

Fragmented, improvised governance systems will make it hard to determine whether research is conducted responsibly, halt research if negative environmental or social impacts outweigh the benefits, or determine which regulators or stakeholders have the power to make these assessments. It in no way equips or prepares us to manage the potential environmental and social impacts of future large-scale deployment.

5) How Can We Ensure Ocean Carbon Removal Is Done Responsibly?

A comprehensive and proactive governance framework would need to consider the full lifecycle of ocean carbon removal research and projects. It would need to ensure that: 

• Countries that implement ocean carbon removal projects in national waters do so within strong national and local sustainable ocean management frameworks
• Appropriate codes of conduct and safeguards are put in place for at-sea research demonstrations and for commercial deployment
•  Broad and inclusive stakeholder engagement informs project development
• There are sound mechanisms for resolving who decides when, if, under what conditions – for example what level of scientific uncertainty – carbon dioxide removal approaches can move from research to deployment
• There is a scientifically rigorous and robust process for reaching consensus on the balance between the potential for harm to the ocean environment and benefits of deployment
• Equity issues around intellectual property, benefit sharing, and commercialization of deployment in the high seas are addressed

Establishing a code of conduct for research, including for commercial purposes, would be an important initial step. This could serve as a basis for developing a code of conduct that includes commercial deployment and could serve as a foundation for a new international framework on ocean carbon removal. To be fit for purpose, this framework must ensure that steps toward developing and deploying ocean carbon removal are responsible, sustainable, and equitable and do not jeopardize the health of the ocean ecosystems or those who depend on them. 

6) What Do We Need Next for Ocean Carbon Removal?

Moving forward responsibly will require an iterative and adaptive approach, according to our findings and those of the National Academies in their landmark 2021 report on a federal research agenda for ocean-based carbon dioxide removal. Governments and other stakeholders need to:

1. Support more research to understand which approaches are viable for large-scale deployment with minimal negative impact on ocean systems, ecosystems, and coastal communities. This must include increased levels of funding from governments, philanthropies, and the private sector for collaborative research, both in laboratories and, if necessary, at sea. Scientists will need to measure, report, understand, and verify environmental and social impacts. Establishing an independent, interdisciplinary committee, drawing on scientific experts from the major international scientific bodies, could help advance consensus on priorities for a global research agenda and what constitutes responsible field tests
2. Build governance frameworks, at the local, national, and international levels, that ensure research and small-scale pilots are conducted responsibly and all stakeholders are informed and included. Top priority within in this is to draw up an international code of conduct for at-sea research trials and make adhering to it a condition for receiving public and/or philanthropic funding or permits. It will also be important to convene a ministerial dialogue on ocean carbon removal – under the joint auspices of the UNFCCC, CBD, London Convention, and London Protocol – to respond to the recommendations from the scientific committee, create a forum and agenda for further discussions, and promote greater coherence across existing international frameworks.
3. Lastly, we must begin to lay the foundation for robust governance of large-scale deployment in the future, by initiating a process to explore a new international framework for ocean carbon removal.  

Companies and investors are moving ahead with the development of ocean carbon removal. This should be done in ways that minimize risks to the health of the ocean and the welfare of communities that depends on it. To fully understand what these interventions could do to safely meet global climate goals, and to ensure that decisions are not just driven by efficiency or profits, we will need far greater investment in research and robust governance. This will not be quick or easy, so there is every reason to begin now.