COP30, held in Brazil in November 2025, served as a reminder that the world remains far off track from meeting its climate goals. Worryingly, delegates at the meeting failed to reach an agreement on cutting fossil-fuel production. Reducing emissions is essential to meeting international targets, but the world has already released so much greenhouse gas that deep cuts alone will not be enough. To meet “net zero” and stabilise the climate, global efforts must also seek to actively remove carbon dioxide from the atmosphere.

The ocean already plays an outsized role in this process: it has absorbed about 30% of all anthropogenic carbon dioxide. Ocean-based carbon-dioxide removal (CDR) approaches aim to enhance or accelerate these natural pathways including by extracting carbon directly from seawater and enhancing the ocean’s natural capacity to store carbon.

Excessive carbon dioxide in the atmosphere is already harming the ocean by creating dead zones, warming temperatures and disrupting currents and nutrient cycles. The natural absorption of carbon dioxide into the ocean is changing seawater chemistry and causing acidification. These shifts threaten both marine ecosystems and the human communities that rely on them. Any intervention into the carbon cycle must therefore be done with great care and be supported by rigorous research. “The ocean is viewed as more natural than the land. People have strong emotional connections to marine systems, for good reasons,” says Paul Halloran, a professor in ocean and climate science at the University of Exeter. “That means more technical challenges, higher costs for early studies, permitting difficulties and greater scrutiny around social license.”

The SeaCURE project, funded by the British government and led by Mr Halloran, was created to pilot CDR schemes and study the process. “We proposed to the government that removing carbon from seawater might make more sense than removing it from air. And our initial nine-month project showed strong results,” says Professor Halloran. The team built a plant that acidified seawater to release dissolved carbon dioxide, captured the gas, and returned the treated, low-carbon water to the ocean.

Chemical capabilities

Commercial companies, including Captura, from California, and SeaO₂, from the Netherlands, are also now showing how the ocean’s carbon-removal potential can be harnessed without compromising marine health. These two companies take an approach known as direct ocean capture (DOC), which relies on the equilibrium between atmospheric and dissolved carbon dioxide in seawater. As atmospheric carbon dioxide rises, the ocean absorbs more; similarly, when carbon dioxide is removed from seawater, the atmosphere replenishes it. “The ocean plays a central role in Earth’s climate system, but it doesn’t receive the recognition it deserves,” says Steve Oldham, the chief executive of Captura.

Their method uses seawater’s own chemistry. Seawater is primarily composed of sodium chloride , hydrogen and oxygen. Electrodialysis rearranges these components into hydrochloric acid and sodium hydroxide . The hydrochloric acid is used to acidify seawater in a controlled environment, causing the dissolved carbon to convert into carbon dioxide gas, which is captured in a tank. The sodium hydroxide then neutralises the hydrochloric acid, enabling the water to be restored to a safe pH so it can be returned to the ocean. The result is the same seawater, simply without the dissolved carbon dioxide, ready to absorb more from the atmosphere. This system requires no added materials and generates no waste products. “We don’t have to build new infrastructure,” says Mr Oldham. “At the scale required, potentially ten billion tonnes of carbon-dioxide removal per year, this matters enormously. If you generate a tonne of waste per tonne removed, that’s an unmanageable amount of waste. Our system avoids all of that.”

Robust research is essential to ensure that DOC could be deployed safely. A major focus of the SeaCURE project, therefore, was generating the data needed to determine safe methods across different marine environments. “The basic chemistry is well understood, and adapting the principles of direct air capture to the ocean is relatively straightforward, but it comes with its own challenges,” explains Professor Halloran. “The real question is what happens in the vicinity of the discharge point, where you’re going to be releasing it with a significant change in chemistry compared to normal seawater, so it needs to be rebalanced.” Establishing an evidence base is crucial. “It’s important for us to prove that our approach is safe for the ocean, and won’t cause additional harm, which requires lots of modelling and monitoring,” says Ruben Brands, founder and chief executive of SeaO₂. The technology is complex, but understanding the fundamentals helps build confidence.

Building support

To scale up DOC, companies must not only build the technology but also build public understanding and trust. SeaO₂’s experience shows that communication becomes critical as projects move to real-world deployment. “We have to communicate very well to get the public support that is needed for a technique like this,” says Mr Brands. Their first prototype was situated on a remote stretch of coastline with no nearby residents or businesses, an ideal setting for technical development which required minimal community engagement. As the company prepares to move its pilot plant to a location near The Hague, a city in the Netherlands, their communication strategy is shifting accordingly. In partnership with the harbour authority, they are organising sessions to gather input and answer the public’s questions. “Several initiatives in this field have failed because they neglected public engagement,” notes the company’s chief scientific officer, Mattheus Meijssen. “We’re very aware of that history, and we recognise that transparency and open communication are essential for earning trust.”

Professor Halloran echoes this. “We’re keen to use our pilot to stimulate conversations,” he says. “It’s not just about regulation or social licence; it’s about building awareness and understanding what people are happy with.”

Captura recognises the importance of identifying the benefits of their technology. Its 1,000-tonne-per-year pilot plant is already operating and the company is evaluating locations for its first commercial-scale facility while exploring end uses for the captured carbon dioxide which will support other businesses and industries. “Our approach is non-disruptive and cost-effective,” says Mr Oldham. “Once we prove that on a larger scale, we hope the public and global policymakers will take decarbonisation more seriously.” Beyond carbon removal, Captura’s electrodialysis technology offers additional applications, from desalination to lithium extraction, creating multiple trajectories for scale and commercialisation.

Carbon credits as a pathway

SeaO₂ is working to establish carbon credits to support their business model. They collaborated with Isometric, a company which issues carbon removal certifications, to establish a credit protocol that uses an accurate calculation of the amount of carbon removed, including all operational emissions. Captura also worked closely with Isometric on this protocol, as well as developing one in-house; in 2023, they became the first DOC company to have published their own protocol, and have since released an update.

Once verified, the captured carbon dioxide is stored through partners such as Paebbl, which mineralises carbon dioxide into concrete, or through long-term geological storage hubs like the Northern Lights project on the Norwegian seafloor. These removal pathways enable the creation of high-quality, trustworthy carbon-removal credits for companies that must compensate for hard-to-abate sectors.

In the Netherlands, the Sustainable Energy Production and Climate Transition Incentive Scheme (SDE++) already provides a pathway to scale carbon credits, with the government supplying funding to prevent losses and enable technologies to scale until costs fall closer to market levels. Amid a slowdown in the voluntary carbon market, SeaO₂ is working to prove that they could make a meaningful difference on the EU level. “In the future, integration of high quality CDR into the European Union’s Emissions Trading Scheme would make sense, and would make a big difference for a lot of companies,” says Mr Brands. “We are advocating for that, as well as dedicated funding calls for CDR technologies.” These will better help early-stage technologies reach the scale needed to meaningfully impact climate targets: the UK, for instance, has set ambitious CDR targets for 2030. Remaining committed to these targets is a crucial part of a holistic climate agenda, and will complement efforts to reduce emissions by addressing the carbon dioxide that is already in the atmosphere.

Harnessing the ocean for carbon capture