As the Intergovernmental Panel on Climate Change (IPCC) has written, CO2 removal (CDR) remains an essential part of any transition to net zero, alongside eliminating emissions by decarbonizing all economic sectors.1 McKinsey has previously written about the need for action to reduce costs and increase scale of CDR delivery,2 and the continued flatlining of global emissions per capita3 makes the role of CDR as important as ever. Various CDR solutions are being researched and developed, and no single solution can provide the amount of removals that most emission pathways call for.4 This article highlights the role of ocean CDR (also referred to as marine CDR) in enhancing the ocean’s vital place in climate management while contributing to CDR at gigaton scale. It presents a synthesis of our current understanding of the technological and commercial maturity of ocean CDR solutions, highlighting opportunities and considerations for buyers, investors, project developers, and others.
The ocean covers around 70 percent of the Earth’s surface,5 providing a critical balance between atmospheric heat and emissions. It absorbs 90 percent of excess heat6 and holds over ten times more carbon than the atmosphere and all plants and soil combined.7 The ocean is not only critical to the global ecosystem but also vital to the global economy—for example, for food, logistics, and tourism.
The ocean captures carbon naturally in various ways. For instance, atmospheric CO2 dissolves in seawater and is carried into the deep ocean by currents, and tiny ocean plants called phytoplankton use sunlight to turn CO2 into organic matter through photosynthesis. When these plants—and the animals that eat them—die, some of their carbon sinks to the ocean floor, keeping it out of the atmosphere for long periods of time, especially in areas with low oxygen concentration.8 Coastal ecosystems, such as salt marshes and mangroves, also trap and store carbon efficiently in soil and plants.9 Ocean CDR solutions can accelerate these natural processes and can complement methods of minimizing carbon releases from human-caused disturbances (such as ocean trawling) in oceanic carbon sinks.10
This article discusses five major approaches to ocean CDR: coastal ecosystem restoration, direct ocean capture, ocean alkalinity enhancement (OAE), algae cultivation and harvesting, and open-ocean microalgae fertilization. Ocean CDR could become a central component of long-term CDR strategies. The following discussion outlines the state of the science, assesses challenges and opportunities ocean CDR solutions, and describes how interested stakeholders could seize new opportunities while lowering barriers to scale.
Working with the ocean’s natural processes and vast scale offers the potential for significant efficiency and scalability. But the ocean’s scale also creates complexity: Intricate, complicated biophysical and chemical mechanisms make it difficult to accurately quantify impacts, and unforeseen risks due to uncertain causal pathways are possible. Most ocean CDR solutions require a great deal of development before they can be operationalized at scale: Fundamental scientific questions must be answered, new regulatory frameworks need to be established, and broad community support will have to be garnered.
Given the complexity and development involved, building a foundational ocean CDR ecosystem over the next decade requires urgent and concerted action by funders, regulators, researchers, philanthropic communities, and industry leaders to derisk and accelerate innovation. Unless multiple stakeholders take action today, prohibitive costs and false starts could derail one of the largest-ever climate restoration opportunities.
Why ocean CDR could be central to climate restoration efforts
Multiple ocean CDR solutions have the potential to play a central role in a balanced CDR strategy. But CDR solutions—ocean-based and otherwise—vary greatly in terms of their scientific validation and development, scalability, and regional suitability. Because of this, a mixed portfolio of CDR solutions is likely the best way to derisk investments and achieve gigaton-scale removals in the long term. Research on ocean CDR has expanded across academic, public, and private sectors, but some ocean CDR solutions are still in early stages of development. Additionally, most proposed approaches lack consensus on methodologies, and some lack extensive scientific validation.11 Future R&D focused on the efficacy, efficiency, and environmental safety of ocean CDR solutions is needed.
Scientific consensus has not yet been reached on the effectiveness and safe deployment of some nascent ocean CDR solutions that use biological systems.12 In 2023, for example, the Scientific Groups of the London Convention and London Protocol determined that several ocean CDR solutions, including OAE and ocean biomass cultivation, could engender “deleterious effects that are widespread, long-lasting, or severe”; the groups commissioned further research into how these solutions could be implemented safely.13 While co-benefits (environmental or social benefits alongside CO2 removal) are likely for some solutions, such as ocean de-acidification, environmental risks, such as marine ecosystem disruption, could emerge. For instance, an intervention that triggers algae growth to remove carbon in one part of the ocean could deplete naturally occurring nutrients. Without the intervention, those nutrients may have been swept away by currents to support algae growth elsewhere, negating any net impact from the intervention. Further research into open-system mechanisms may help untangle these complex networks and improve confidence in the additionality of ocean CDR solutions.
The need for further R&D notwithstanding, several aspects of ocean CDR solutions show great potential to achieve meaningful decarbonization.
- Gigaton-scale potential. Nearly all ocean CDR solutions have the potential to reach gigaton scale, and the total scale of annual CO2 removals supported by the ocean could be more than ten gigatons.14 The ocean already naturally absorbs 30 percent of global CO2 emissions; it is a natural carbon reservoir that eclipses the size of other reservoirs, such as the atmosphere, forests, and soil.15 Even a proportionally small increase in the ocean’s carbon storage capacity could thus reduce atmospheric carbon concentrations substantially.
- High efficiency. Ocean CDR solutions are potentially very highly efficient in terms of cost, energy, material, and land use; this could reduce the resources needed to remove CO2 on a globally significant scale. And while ocean CDR solutions are currently expensive, costs may drop with economies of scale and technological advances for some of the paths with the highest potential. Ocean CDR solutions could also be less expensive than non-ocean CDR solutions, because they simply enhance naturally occurring biological, chemical, and physical processes for capturing and storing carbon.
- High compatibility. Most ocean-based solutions do not compete with other uses for scarce land, such as agriculture, and are typically compatible with other ocean uses, such as fisheries. In fact, many ocean CDR approaches have the potential to improve aquaculture productivity.
- Restoration potential. Some ocean CDR solutions may have restorative co-benefits that could support biodiversity and coastal communities, such as by reversing ocean acidification or remediating coastal habitats.
- Climate adaptation benefits. Ocean CDR approaches can help boost resilience to the effects of climate change. For example, coastal ecosystem improvements such as mangrove restoration can provide flood and storm protection for coastal communities.
Many promising aspects of ocean CDR also underscore why decarbonization is imperative: Unabated CO2 emissions exacerbate seawater acidification, warming, stratification, and deoxygenation, all of which endanger ecosystems that sustain marine life and coastal economies. This damage to the ocean has been compounded by unsustainable extractive and other human practices; if the ocean-atmosphere climate system passes tipping points and enters a new state, systemic changes in behavior and impacts are likely.16 Some of these changes, such as the loss of summer arctic sea ice or dramatic shifts in ocean currents that form part of the ocean’s carbon- and heat-transporting conveyor belt, could be irreversible.
Scaling ocean CDR: Research is vital to assess and develop solutions
Long-term success in scaling ocean CDR solutions to meet climate needs depends on safe, effective, and well-researched deployment of technologies in the near term. Those working in the ocean CDR field are still evaluating which approaches, methods, and combinations of technologies and value chains yield the most cost-effective removals. They are considering energy, mineral feedstocks, and other resource inputs, as well as emissions across the full supply chain and life cycle of ocean CDR solutions. In addition, CDR solutions will need robust methodologies for rebalancing CO2 between the ocean and the atmosphere once solutions have successfully removed CO2 from the ocean.
Assessing the environmental safety of different ocean CDR solutions is a complicated endeavor. To assess risks properly, specificity is key: Each solution could have different impacts depending on water quality (including salinity, acidity, nutrient profile, mineral profile, currents, or average temperature) and the makeup of the coastal ecosystem. Shared research results, such as the ocean alkalinity efficiency map published by CarbonPlan and [C]Worthy,17 can help, because research transparency is essential to build trust in CDR outcomes. Further research could also incorporate the views, insights, and needs of coastal Indigenous peoples and members of other communities with deep ties to, and dependence on, ocean ecosystems—for example, through fisheries or ocean ecotourism.
No single solution: An overview of ocean CDR approaches
Ocean CDR encompasses multiple approaches, each of which enhances specific aspects of the ocean’s natural carbon cycle to capture CO2 and store carbon. But in general, there are two broad categories of ocean CDR approaches (abiotic and biotic) and six major ocean CDR solutions in various stages of research and development. We consider five of these solutions to be approaching commercial viability (Exhibit 1).
The abiotic and biotic approaches detailed below use the ocean’s chemical and physical properties and processes that help capture or store CO2; biotic approaches also incorporate the ocean’s living ecosystems, such as algae.
Abiotic solutions
- Direct ocean capture (DOC) entails filtering seawater with electrochemical or electro-membrane processes to extract and capture dissolved CO2 from the ocean. The captured carbon is stored elsewhere, such as underground in geologic formations.
- OAE reverses ocean acidification by helping the ocean convert dissolved CO2 into bicarbonate (a safe, long-lasting form of carbon in seawater), increasing the ocean’s ability to absorb more CO2 from the atmosphere. Alkalinity can be enhanced by dissolving certain minerals, such as silicates, in seawater or by electrochemically removing acid from seawater.
Biotic solutions
- Coastal ecosystem restoration focuses on enhancing natural carbon uptake and storage in coastal ecosystems including mangroves, reefs, seagrass meadows, and tidal salt marshes.
- Algae cultivation and harvesting use algae photosynthesis to capture CO2, either by methods such as mariculture of kelp or harvesting of naturally occurring sargassum seaweed, or by cultivating tiny microalgae in contained ponds or the open ocean. To remove the carbon, the collected biomass may be sunk into the deep ocean, buried on land, used with biomass with carbon capture and storage (BECCS),18 or embedded into long-lived materials.
- Open-ocean microalgae fertilization aims to trigger algae growth in the open ocean to increase CO2 capture by photosynthesis and biomass sinking into the deep ocean. The main solution under focus entails fertilizing the ocean’s surface with iron or other scarce nutrients, though an alternative, theoretical approach entails pumping nutrient-rich deeper waters to the surface.
Each ocean CDR approach has specific advantages and challenges. And individual approaches and technologies are not necessarily mutually exclusive—a single project can incorporate multiple approaches, and ocean CDR innovators are combining individual technologies and steps in novel ways (see sidebar, “Ocean CDR in the real world: Solutions in action”).
There are additional ocean CDR solutions that we do not assess further, such as artificial upwelling and downwelling, because of their precommercial status. While this article focuses on carbon that is captured in the ocean, there may be additional opportunities to use the ocean’s storage capacity to effectively store carbon captured using air- and land-based CDR approaches.
Assessing ocean CDR’s potential: Benefits, risks, and challenges
To better understand the opportunity in the ocean, investors, regulators, innovators, and other stakeholders can approach their assessment of different CDR technologies along the dimensions noted below.
- Quality and integrity. Solutions should have all the necessary features to drive climate mitigation and restoration, including additionality, high permanence, low risk of reversals (the re-release of captured carbon), no net harm or ecological risks, and robust quantification and verifiability. Co-benefits are valuable.
- Technological maturity and commercial traction. Stakeholders must clearly understand a technology’s level of advancement and project developers’ ability to use existing infrastructure to implement the technology at scale.
- Cost profile, scale, and cost-down potential. To provide affordable removals, solutions must be competitive with other uses of raw materials and energy as well as with other cost drivers. Every CDR solution is ultimately aimed at removing CO2 at a scale and pace needed to hit emissions targets; thus, it is crucial to understand each solution’s potential ability to scale, cost competitiveness, and obstacles or limitations to scaling, such as resource scarcity or resource competition with other economic activities.
These factors are integral when considering a portfolio of CDR solutions, and they form the basis of the analysis provided throughout this article (Exhibit 2).
Ocean-based solutions stand out among all other CDR options because of their potential for high efficiency (and thus potentially lower cost at scale) and high scaling potential. Nonetheless, with these inherent advantages come inherent challenges—notably, the relative uncertainty and risk that come from working in complex oceanic systems, often with incomplete scientific understanding.
Ocean CDR has the potential to deliver cost-competitive CDR credits (especially when compared to high-durability land-based CDR solutions) at gigaton-scale sequestration. But to incorporate it safely and effectively into the global strategy for achieving net-zero emissions, research and development are needed now.
At a systems level, a diversified portfolio approach that integrates ocean-based and other CDR solutions is essential to accelerate learning, drive competition, and determine which solutions can most safely deliver the greatest benefits at the lowest costs. The engagement and commitment of industry and public sector leaders are paramount to ensure the timely innovation, validation, and deployment of these unique and potentially transformative technologies.
Download the full article here to learn more about ocean CDR solutions and how various stakeholders could help research, develop, regulate, and scale these unique climate technologies.
