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  • Edward Sabey

Riding the Wave of Ocean Carbon Dioxide Removal

While mitigating greenhouse gas emissions is critical, reducing and avoiding emissions alone will not be sufficient to limit global warming to 1.5°C.  The world needs to remove CO2 from the atmosphere immediately, and at scale. The IPCC estimates in a 1.5°C scenario that 6 gigatonnes of CO2 must be removed from the atmosphere every year, a value greater than the yearly emissions of the United States in 2022. The most common methods of Carbon Dioxide Removal (CDR) are nature-based (e.g. reforestation) but have been plagued with issues around additionality, permanence and land usage.

Technology-based removal solutions such as Direct Air Capture (DAC) are becoming well researched and are gaining in popularity. With reliable Measurement, Reporting and Validation (MRV), they can be high integrity forms of removing carbon from our atmosphere. This has been reflected by increased investment from the private sector such as Blackrock’s $550m USD investment in 1PointFive and favourable government incentives (e.g. Section 45Q credits as part of the Inflation Reduction Act).   

But Ocean CDR has not kept pace with the investment and commercialisation of DAC, despite the ocean being the Earth's greatest carbon sink. 31% of all anthropogenic CO2 emissions are stored in the top layer of our ocean, increasing water acidity which in turn has numerous negative impacts on the aquatic ecosystem. Australia is already witnessing the impacts of acidification, with 91% of the Great Barrier Reef affected by coral bleaching as a result of enhanced carbon dioxide levels in seawater.  Companies and research groups alike are now seeking to leverage the dynamic and vast nature of the ocean to help alleviate the increase in atmospheric emissions.  

Ocean CDR Techniques 

There are broadly two categories in which Ocean CDR methods can be classified: Biotic, which involves leveraging photosynthesising organisms and Abiotic, harnessing the physical and chemical properties of the Ocean. These techniques aim to mitigate the effects and reduce concentrations of atmospheric CO2 by increasing the capacity of the oceans to absorb and store carbon dioxide. 

Abiotic approaches

Biotic approaches

Alkalinity Enhancement

Coastal Blue Carbon

Electrochemical Techniques

Macroalgae (seaweed) Cultivation

Artificial Downwelling and Upwelling

Ocean Fertilisation

Of the six methods shown above, the three techniques which are considered the most practically viable, and hence the most ardently researched are: Alkalinity Enhancement, Electrochemical Techniques and Macroalgae Cultivation.  

Alkalinity Enhancement  

To combat excess CO2 and acidity present in the Earth's Ocean systems, safe quantities of alkaline material such as magnesium hydroxide or calcium carbonate can be added to bodies of water with elevated CO2. A neutralisation reaction then occurs, converting CO2 into its more stable forms, able to exist harmlessly for thousands of years. This acts as permanent carbon storage. These stable aqueous ions are predominantly; bicarbonate (HCO3-) and carbonate (CO3^2-), molecules that are in fact essential for the shell growth of molluscs and the regulation of acidity in the water. 

The Neutralisation reaction with calcium carbonate acting as the alkalinity source: 


CO2 (aq) + H2O + CaCO3 (or another alkalinity) → 2HCO3- (aq) + Ca2+ (aq) 

Neutralising CO2 in the ocean results in an imbalance between the CO2 in the atmosphere and the sea locally. Therefore, to ensure an equilibrium is maintained, CO2 from the atmosphere is absorbed into the ocean. This results in a net reduction in CO2 in the atmosphere. 

Ocean Alkalinity Enhancement (OAE) is increasingly being viewed as scalable due to decreasing costs, increasing discovery of the abundance of the required alkalinity and the simplicity of the infrastructure required for large scale removal. Some concerns still exist regarding the addition of alkalinity in gigatonne quantities. However, research suggests that if the alkalinity is dosed to the system in the correct concentrations and ample mixing is adhered to, adverse impacts will be negligible, with the local ecosystem benefiting from reductions in localised acidity. Despite these findings, the broader marine system impacts of OAE techniques are being monitored closely as the technology scales.

Macroalgae Cultivation  

One of the first and most simple hypothesised methods to remove CO2 from the ocean, and consequently the atmosphere, is the cultivation and processing of macroalgae. Seaweed is the most common macroalgae being utilised. While cultivating seaweed can have significant benefits for the ocean, the use of seaweed in products such as in food and cosmetics isn't considered permanent carbon removal as secure sequestration can't be proven. Permanence of at least 100 years is one of the essential components for CDR.

An alternative 'permanent' method of macroalgae cultivation is harvesting and sinking seaweed to the ocean floor. However, research has revealed that this process may release methane as it decomposes, a greenhouse gas almost 28x more potent than CO2, thus reversing the initial benefits from cultivation.

One of Twynam's portfolio companies Ocean Rainforest, have demonstrated the full process of taking seaweed from nurture and cultivation through to consumer products. A scalability limitation still exists in the labour required to harvesting and process seaweed, however if these processes can be automated and the operating costs reduced, we will see net negative CO2 products on the market.  

Electrochemical Techniques 

A more novel method being developed encases electrochemistry, utilising electricity to split seawater into its constituent base and acid ions. The acidic stream is usually degassed using a membrane, removing CO2 which can then be sequestered or used as feedstock for commercial products. The basic stream, containing hydroxide ions is then returned to the sea to neutralise acidic waters, similarly to the OAE process. Although there are many variations to electrochemical methods, the description below gives a broad overview of the process.  

California-based company, Equatic is demonstrating the potential for large scale deployment of Ocean CDR facilities using electrochemical methods. Their process results in the removal of CO2 via bubbling atmospheric air through an alkaline solution formed from their novel electrolyser configuration. This method can produce green hydrogen as a by-product, providing an additional source of revenue and energy. They have signed a US50m+ deal with Boeing to supply both carbon credits and green hydrogen over the next 5 years.  

Measurement, Reporting and Verification (MRV) 

The biggest hurdle facing the Ocean CDR industry is the difficulty in quantifying the CO2 removed as a result of the removal technique implemented. If MRV methods cannot be reliably implemented to accurately quantify the CO2 removed or converted, the generation of carbon credits is prevented. These credits are the most ubiquitous revenue stream for CDR companies.  

As the carbon credit market grows, CDR regulation is becoming much more stringent as companies look to offset their emission base. This given, companies are investing heavily to ensure an adequate supply of high quality carbon credits. 

MRV is difficult to implement for Ocean CDR due to the dynamism of the sea and the vast number of variables affecting the amount of CO2 in the water. Temperature, pH, currents, varying chemical compounds and sunlight, just to name a few, have a bearing on the amount of CO2 present in any set volume of seawater. Building a model which encompasses these variables has significant limitations and with that, a high degree of uncertainty.  

Carbon Credit Market  

The voluntary offset market alone is projected to expand from $2 billion USD in 2020 to an estimated $250 billion USD in 2050. However, much of this market is dominated by low cost and low integrity avoidance-based credits. Essentially, a ton of carbon avoided is currently treated the same as a ton of carbon removed, incentivising corporations to opt for low-cost options. However, we are starting to see first movers in the CDR offset space with companies such as Stripe willing to pay >$1,000/t CO2 to accelerate CDR technologies down the cost curve. 

Given the need and urgency for multi-gigatonne CDR, a well-functioning carbon credit market which caters specifically for large scale CDR will need to be developed. Assuming accurate MRV can be achieved, Ocean CDR may be able to alleviate the burden and even overtake DAC and Enhanced Weathering as a source of high-quality removal credits, due to the capacity of the Ocean to safely store carbon at scale.  

Impact on Aquatic Ecosystems  

All three Ocean CDR methods proposed take measures to minimise disruption to the aquatic environment. However, it is not well understood how implementing these techniques at gigatonne scale may impact the health and balance of marine ecosystems. Take macroalgae cultivation for instance, although adding large amounts of seaweed to an undernourished environment may have CO2 capture benefits, there is also potential to disturb light penetration which has potential for large, unforeseen ramifications. MRV will again be the centre piece in combating this, leveraging data to ensure that these systems are maintaining a healthy aquatic equilibrium.

Twynam’s Closing Thoughts on the Ocean CDR Space


A series of barriers remain to achieve substantial carbon removal. Excitingly, we are seeing a growing number of companies developing innovative counter measures.


It's evident, CDR will play a pivotal role in achieving Net Zero by 2050. Right now, CDR technologies like DAC face high costs and increasingly, Ocean CDR is emerging as a logical option due to its simplicity which limits both the CAPEX and OPEX of operation. Therefore, it is vitally important that we fund and scale companies capable of delivering meaningful CDR at reasonable prices. 


Twynam Funds Management Pty Ltd ACN 665 482 119 (Twynam Funds Management) is a corporate authorised representative (CAR) (CAR Number 1302016) of Boutique Capital Pty Ltd ACN 621 697 621 (Boutique Capital) AFSL 508011.

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