The oceans, covering over 70% of our planet's surface, carry significant potential for carbon removal. Holding more than 37 trillion tonnes of dissolved inorganic carbon, they dwarf any terrestrial carbon sink by orders of magnitude. But as we look to the ocean for solutions to our climate crisis, we must also address the complexities and uncertainties inherent in such a vast and dynamic system.

The ocean’s potential is immense, yet uncertain

To put the ocean's capacity into perspective, removing 10 gigatonnes of CO₂ per year—meeting our total estimated 2050 carbon removal requirement—would alter the ocean's dissolved inorganic carbon budget by a mere 0.007%. This small change highlights the ocean's potential to absorb substantial amounts of CO₂ without significantly altering its chemistry, making it a tantalising prospect for scalable carbon removal. The goal of ocean CDR (marine CDR and ocean CDR are used interchangeably here) is to remove enough CO₂ to meaningfully contribute to global mitigation efforts, whilst remaining within safe limits for ocean health. There is already significant annual variability in ocean CO₂ fluxes as a result of seasonal changes in phytoplankton, temperature and mixing (eg. 1, 2).

Ocean carbon removal pathways are diverse and act on two main carbon pumping mechanisms: enhancing the solubility pump, often thought of as abiotic approaches, and enhancing the biological pump, or biotic approaches. The solubility pump encompasses physico-chemical processes such as temperature, alkalinity, and currents, which regulate the movement of CO₂ to and from the ocean. Meanwhile the biological pump transfers organic and inorganic carbon, fixed by photosynthetic organisms, from the surface to the seafloor through sinking particles.

While the potential of these pathways is vast, the efficacy, benefits and risks are highly uncertain. The IPCC’s range for ocean alkalinisation at 1-100Gt CO₂ underscores just how unsure we are. To date there have only been a handful of field trials monitoring the impacts of CDR in real-world conditions, and hence our understanding of the impacts of interventions are far from complete. Ocean Visions has produced a field trial database tracking the results to date. Recognising this shallow understanding, governments are supporting ocean CDR research specifically. The US committed over $60m to ocean CDR research in 2023, and the National Oceanic and Atmospheric Administration (NOAA) have laid out a strategy for the development and implementation of ocean CDR research that could receive up to $1 billion from the US Carbon Dioxide Removal Research and Development Act, 2023.

Four core challenges for marine CDR

Harnessing the carbon removal potential of the oceans is far from straightforward. They are a complex, open system with innumerable variables at play. Some of the challenges include:

We don’t know what we don’t know

Approaches like ocean alkalinity enhancement that adjust ocean pH levels may affect marine life, particularly species sensitive to pH changes, such as shell-forming organisms. Other approaches, like sinking large quantities of biomass could sequester import nutrients away from coastal ecosystems, and could impact the largely forgotten organisms living on the seabed.

(Although it is worth noting that with no action the ocean is becoming increasingly acidic, causing devastating consequences to marine ecosystems and impacting the efficiency of the biological pump)

Navigating global governance and red tape

The ocean is a shared resource governed by international law and currently lacks a governance framework for implementing ocean CDR. Many ocean CDR methods involve adding materials to the ocean (e.g., adding alkalinity for OAE, or nutrients for ocean fertilisation). Under current international law (such as the London Convention and Protocol), such activities could be classified as “dumping.” This classification brings stringent restrictions unless projects are deemed "legitimate scientific research," which must undergo rigorous assessment. Even the definition of legitimate scientific research is being debated… For example, academic research linked to a company is not counted due to possible misguided incentives. Romany Webb and colleagues are advocating for the development of legal frameworks for in-ocean CDR research.

Companies can avoid some of these issues by coupling with near ocean industries, such as desalination or wastewater, and therefore tapping into existing permitting protocols for effluent management.

Winning over public trust and approval

Perhaps the most famous example is from 2012 where the Haida Salmon Restoration Corporation (HSRC) conducted a small ocean iron fertilisation (OIF) experiment with the purpose of boosting salmon populations and sequestering CO₂ off the west coast of Canada. The experiment was met with widespread criticism for violating international protocols and potentially causing harmful ecosystem impacts, gaining the media label geo-vigilantism. This experiment, which was not coordinated by a team of scientists, has severely impeded the progress of the OIF field.

Even with well thought out and staged public engagement sessions, Planetary Tech faced protest from locals around their alkalinity enhancement trial in Cornwall, UK with concerns about potential harm to the marine ecosystem.

And last but certainly not least, MRV

One of the most significant barriers to ocean-based CDR is the challenge of developing measurement, reporting, and verification (MRV). Currently the clarity around the efficacy of mCDR approaches and their net climate benefit are highly uncertain. Some of the key MRV challenges include:

Air-Sea Equilibration The air and sea are in equilibrium, which means that a decrease in the concentration of CO₂ in the sea leads to a decrease in atmospheric CO₂ as it is re-absorbed by the oceans. (It works in reverse too, picture your sparkling water slowly becoming flat as it equilibrates with the air around it). The equilibration time can take months to years and the completeness of re-absorption is affected by the dispersal of CO₂-depleted water and its residence time at the surface. Determining how effectively CO₂ removal efforts translate into atmospheric CO₂ reductions will require modelling the dispersal of CO₂-depleted water in the ocean and inferring how seasonality, ocean currents, and global climate impact the ultimate air-sea equilibration.

Ocean observation measures To understand how much inorganic carbon (CO₂, bicarbonate, carbonate) exists in our oceans, we need four crucial measurements: pH, partial pressure of CO₂ (pCO₂), total alkalinity (TA), and dissolved inorganic carbon (DIC). While we have autonomous sensors for pCO₂ and pH, these alone aren’t ideal for accurately defining the carbonate system. This is because these measurements tend to vary together, leading to wide error margins when inferring total inorganic carbon. TA and DIC are more reliable indicators, especially for capturing changes driven by calcification, yet we lack sensors for these parameters. Ultimately, measuring at least three parameters is key to fully understanding the ocean’s carbonate chemistry.

Dissolved Organic Carbon Understanding how interventions affect organic carbon forms and their roles in the carbon cycle is another area of growing research that could both positively and negatively impact the net amount of carbon removed.

While sensor technology is advancing, measurements alone are insufficient. They must be complemented by robust models that assess the impact of space and time. [C]Worthy and CarbonPlan are leading efforts to model and assess the carbon removal efficiency of ocean alkalinity enhancement. Their new tool demonstrates how measurements will only take you so far when measuring interventions like adding alkalinity, which disperses across the ocean and the carbon removal (or lack of) may happen far from where the alkalinity was introduced.

Advancements in AI and machine learning have exciting implications for ocean modelling. Where satellite data, coupled with sensor data from robotic fleets, drones, and ships will help predict the impacts and feedbacks of ocean CDR inventions.

Learnings from commercial closures

The recent closure of Running Tide exemplifies how challenging the landscape is for marine CDR companies. Despite receiving $70m in funding and developing a state-of-the-art ocean monitoring platform, the lack of an existing ecosystem of support infrastructure meant Running Tide was having to tackle it all, from R&D, project design and delivery, establishing MRV protocols, and overseeing challenging logistics unique to marine settings. It also raises the question of whether venture-funded startups are well positioned to prove out open system solutions when fundamental scientific questions remain unanswered. Finally it underscores the importance of systems thinking when investing (we have written about this previously), where companies will often struggle to succeed on their own.

How we are approaching investment:

As investors committed to catalysing the carbon removal industry, we must tread carefully in the waters of ocean-based carbon removal. It is a delicate balance between our role as risk-takers at the forefront of innovation in CDR and our duty of care to the responsible scale up of the industry. Here's how we're approaching investments in marine CDR companies today:

Focusing on manageable MRV

We see promise in systems with clearer route to reliable and robust MRV:

• Closed systems solutions in the near term that operate in controlled environments simplify measurement.

• “Constrained” open systems like rivers or water treatment plants that have one directional flow, a clearly defined area, and fewer variables affecting their chemistry offer a more manageable and practical environment for measuring the impacts of alkalinity enhancement.

CarbonRun was a clear standout for us, harnessing open systems and the power of rivers to accelerate the mineral weathering process, whilst establishing a clear path to MRV based on decades of research. Solutions like these can serve as commercial forerunners, addressing MRV challenges incrementally and paving the way for broader ocean alkalinity enhancement efforts.

Prioritising socio-economic benefits

Gaining community buy-in is crucial. The ocean is integral to the livelihoods of countless coastal communities. Solutions that create jobs, support local industries or provide environmental benefits such as improving water quality, enhancing fisheries, or restoring habitats are more likely to have project acceptance and long-term success. And it's not just being good, but also putting the work in and demonstrating that to and with the support of local communities.

Leveraging and adding value to existing industry

Solutions that can capitalise off existing operations like shipping routes, desalination, or offshore installations will have an easier route to scale. However solutions that go beyond mere co-location and add value of their own are more likely to stick. These strategies can reduce costs, simplify logistics, and accelerate projects. This was central to our investment in CREW Carbon, which leverages the trillions of dollars of existing wastewater infrastructure for their geochemical carbon removal solution. Their solution simultaneously optimises treatment by providing alkalinity and pH regulation and supports compliance through N2O reduction.

Developing MRV technologies with broader applications

Sensor technologies for key MRV parameters are a key missing element. However the markets to test and deploy these sensors are limited to scientific research and aquaculture today. Finding applicable markets to develop sensor IP will enable companies to build viable businesses whilst contributing to the data collection needed for effect ocean CDR. Aquatic Labs are intriguing who, building on Scripps Institution of Oceanography’s work on DIC sensors, are developing next generation solid state sensors for multiple markets, including ocean CDR.

So that’s how we’re approaching it for now.

As we continue to explore the depths of ocean-based carbon removal, collaboration between scientists, entrepreneurs, policymakers, and investors will be key. We are grateful for organisations like Ocean Visions, [C]Worthy, Carbon to Sea, Cascade, Hourglass and others who are helping build consensus around some of the toughest questions in ocean CDR and laying the foundations for a carbon removal industry that is able to scale responsibly.

We've mapped out some of the companies working across ocean CDR today.

Key resources:

National Academy of Sciences, A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, 2022

NOAA CDR Resources

Romany Webb International Governance of Ocean-Based Carbon Dioxide, 2024

Ocean Visions Ocean-Based Carbon Dioxide Removal: Road Maps

CarbonPlan Mapping the efficiency of ocean alkalinity enhancement, 2024