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In recent months, marine carbon dioxide removal (mCDR) has found itself at the centre of climate conversations. The vast capacity of our oceans offers a compelling – yet often misunderstood – frontier for climate action.
Far from being a singular solution, mCDR covers a range of both natural oceanic processes and technical interventions. And, perhaps unsurprisingly, each solution comes with its own unique features, challenges and, crucially, opportunities. In this article, we explore these distinctive features and also look to reframe some of the perceived hurdles that the pathway faces today.
The ocean: a natural scalability advantage
Given the vast expanse of the global ocean, one of the principal arguments in favour of mCDR lies in its inherent scalability. The ocean currently absorbs nearly 30% of the carbon dioxide (CO₂) emitted by human activities. Storing over 50 times more carbon than the atmosphere1, it represents not only a natural sink for CO₂, but also one of the most promising environments for its long-term removal and storage.
The ocean covers roughly 71% of the Earth’s surface2 and offers an immense and largely under-utilised reservoir for carbon sequestration. This contrasts sharply with land-based approaches. Unlike many terrestrial CDR solutions, which often face competition from urban development and human activities, most mCDR pathways avoid such land-use conflicts.
While considerations around environmental impacts and regulatory frameworks still very much apply, the fundamental constraint of physical space is significantly mitigated in the ocean context. Many mCDR technologies can be deployed within marine environments, and others that require more infrastructure are typically co-located with industry, decreasing the need for extensive land requirements
Beyond carbon removal
While the priority for mCDR projects is to remove CO₂, many of these innovative solutions also offer compelling co-benefits, addressing critical environmental and socio-economic challenges.
Perhaps most notably, many mCDR technologies can help alleviate the damaging effects of ocean acidification3 – at least in the short term. By lowering CO₂ concentrations in seawater, these approaches temporarily increase local pH levels, creating more favourable conditions for marine ecosystems. While the ocean tends to reabsorb CO₂ from the atmosphere to restore equilibrium, this buffering effect can still provide meaningful, if transient, relief for vulnerable marine species and habitats.
Among the organisms most affected by acidification are calcifying species such as corals, shellfish, and plankton, which rely on stable pH conditions to build their calcium carbonate shells and skeletons4. Supporting these foundational species has broader ecological and economic implications: healthier coral reefs and shellfish beds can lead to more resilient marine food webs and improved fish stocks. In turn, this benefits fisheries and aquaculture, contributing to a more productive and sustainable marine farming industry.
Other mCDR approaches also contribute to increased ocean biodiversity. Seaweed cultivation for carbon removal, for example, can create new habitats, fostering a richer ocean environment.
Given the diverse nature of each of the mCDR subpathways5, each naturally comes with its own unique co-benefits. See ‘Figure 16’ for some of the main co-benefits, as well as some of the risks for each.
However, it is certainly not all rainbows for mCDR. In the second half of this article, we take a look at some of the salient challenges that the pathway faces today, and how progress is being made to overcome them.
Each subpathway contains key known risks that need to be managed by individual projects and overarching governing bodies in order to ensure that mCDR can be undertaken with a “do no harm” approach. For example, some subpathways, such as electrochemical or OAE, typically release a stream of alkalinity back into the ocean which can significantly help with combatting ocean acidification. On the other hand, this alkalinity stream can also negatively impact marine organisms if there is a rapid shift in the pH or toxic elements – such as nickel7 – are introduced into the environment. Currently, there are ongoing studies to de-risk these issues – but the potential negative impacts should be highlighted nonetheless.
Additionally, while subpathways that enhance surface nutrient levels can effectively boost CO2 drawdown, they also carry the risk of unintended ecological consequences and disrupt local ecosystems. For instance, nutrient enrichment may trigger harmful algal blooms or disrupt macro-nutrient balances, potentially favouring the growth of certain algal species over others - in turn, alter the ecosystem and food web structure8. Understanding and mitigating these potential side-effects is a central focus of current research, with numerous projects actively investigating strategies to minimise and mitigate ecological disruption.
The evolution of measurement, reporting, and verification regulatory hurdles
Perhaps one of the most frequently cited challenges for mCDR is the difficulty that it faces around measurement, reporting, and verification (MRV). While quantifying carbon removal in controlled, closed systems is typically more manageable, doing so in dynamic ocean environments is significantly more complex.
Open-system approaches often require extensive ongoing sampling and monitoring before models can reliably estimate long-term carbon sequestration, creating a high technical and financial barrier to entry. Moreover, understanding and tracking downstream effects, such as changes in biogeochemistry or ecosystem impacts, is critical for both open and closed systems and adds an additional layer of complexity to robust MRV efforts.
However, while this challenge undoubtedly remains front-and-centre for the pathway, progress is being made in the development of robust MRV methodologies. Companies are actively working on – and achieving – crucial milestones in protocol development in a bid to provide a clear pathway for credit issuance. Puro.earth, for example, has recently opened up a public consultation for its latest ocean-based carbon removal methodology: Microalgae Carbon Fixation and Sinking. Last month, Isometric also released its Direct Ocean Capture and Storage (DOCS) Protocol for public consultation.
In fact, mCDR is arguably following a well-trodden path. The enhanced rock weathering (ERW) pathway also faced initial difficulties around MRV, but has since seen methodologies mature and credit deliveries follow closely behind.
Nonetheless, it is clear that mCDR still has progress to make on this front. While significant efforts are underway, it may take time for methodologies and registries to fully catch-up with the evolving science. Encouragingly, a growing number of non-profit organisations – Carbon-to-Sea, SeaO2-CDR, and Cascade Climate, among others – are actively advancing the mCDR agenda, helping build the scientific, technical, and governance foundations needed for responsible scaling.
Navigating regulatory hurdles
Adding further complexity, the regulatory landscape around mCDR is highly complicated, and presents a number of unique challenges. Permitting processes vary widely across local, state, and national jurisdictions, often creating uncertainty and delays.
A major issue is that most permitting frameworks were not designed with mCDR technologies in mind. This leads to ambiguity about how existing laws apply, especially for novel techniques like OAE. Securing approvals often requires extensive case-by-case interpretation, consultation, and negotiation – adding friction to a space that urgently needs momentum.
As a result, the extra time needed to secure permits can significantly extend project timelines, in turn impacting scalability and investment. For early-stage mCDR companies, these delays can be particularly costly – prolonged timelines may outlast funding runways, stalling promising projects before they reach the water.
Operating in international waters introduces further complexity with multinational legal regimes, such as the London Protocol to be navigated.
Yet the presence of environmental regulation, albeit evolving, helps to provide safeguards for mCDR. Many countries already have robust regulatory frameworks in place to monitor ocean discharges, for example. While not yet tailored to carbon removal, these systems can be adapted to support early-stage mCDR deployment. The challenge now is to evolve permitting pathways that are both scientifically grounded and responsive enough to keep pace with technological innovation.
Co-locating projects with existing industries that already hold the relevant permits can further streamline the process. Captura, for example, deploys a high-performance bipolar membrane electrodialysis system, which is available for a broad range of industries including desalination, wastewater treatment, critical mineral extraction, new energy, and life sciences9.
While by no means perfect, there is at least a growing awareness of the regulatory gaps and the need for regulation to be developed to fit the future shape of a nascent mCDR industry.
mCDR: flipping the script
The nascent field of mCDR stands at a pivotal juncture. The opportunities outlined in this article point to a pathway that goes beyond climate mitigation; a pathway that also has the potential to actively contribute to restoring the health of one of the Earth’s most vital ecosystems: the ocean.
While challenges around MRV, regulation and broader ecosystem risks undoubtedly remain – and must be addressed before the pathway can scale to its full potential – it is important to recognise the meaningful progress being made. Continued research, rigorous environmental impact assessments, and independent community engagement will be critical components of responsible development. Encouragingly, the evolving sophistication of MRV methodologies, coupled with the acknowledgement of the need for regulation signals a trajectory towards future, scalable deployment.
For mCDR to advance at the pace required to meet the scale of carbon removal required by 2050, the focus must shift towards cultivating a strong demand signal. This will require more than scientific evidence alone. It calls for a regulatory response to establish the long-term demand signal required to incentivise investment and scale deployment.
In our next and final article in the series, working closely with both buyers and suppliers with hands-on experience in the market, we present real-world case studies to demonstrate how stakeholders can actively participate in shaping a more sustainable future – for both our oceans, and our planet.
