Marine carbon dioxide removal: a dive in our largest carbon sink

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• The ocean has a vast capacity for carbon storage, and mCDR technologies like Ocean Alkalinity Enhancement (OAE) and Direct Ocean Capture (DOC) are emerging as scalable solutions for enhancing carbon sequestration.

• Effective carbon credit integrity requires rigorous monitoring, reporting, and verification (MRV) frameworks. Ocean modelling, combined with direct measurements, is crucial for quantifying the impact of mCDR technologies.

• Advancements in computational modelling, sensor technology, and environmental impact assessment frameworks are needed to scale mCDR responsibly. Increased investment from both private and public sectors is driving further innovation.

Storing CO2: a different view on ocean's potential

To mitigate the worst effects of climate change and have a chance at staying below 2oC of warming, science tells us that we need rapid emissions reductions and to develop technologies that can remove billions of tonnes of carbon dioxide from the atmosphere each year. 

The ocean stores 40 times more carbon than the atmosphere, and already absorbs a quarter of human-made carbon dioxide emissions. By leveraging the ocean’s vast surface area and its carbon storage capacity, mCDR (marine Carbon Dioxide Removal) technologies enhance the ocean’s ability to store carbon dioxide. Many of these technologies show extremely high scaling potential.

Indeed, the market for marine carbon dioxide removal is nascent, but rapidly growing. There are an increasing number of companies at various stages of research and development on a range of technologies including Ocean Alkalinity Enhancement (OAE), Direct Ocean Capture (DOC) and macroalgae sinking.

Public and private investment into these mCDR suppliers is increasing. On the private side, suppliers have received financial backing from buyers groups—like Frontier—the first round of carbon removal purchases from the US Department of Energy and the Carbon Removal XPRIZE. On the public sphere side, groups like the Carbon to Sea Initiative have received significant philanthropic and public funds to support the emerging ecosystem with targeted research, scientific resources and funding for field trials. 

Marine carbon dioxide removal explained

On geological timescales, the ocean responds to rises in the Earth’s temperatures by absorbing more carbon dioxide. The oceans capture carbon dioxide from the atmosphere where it is converted into bicarbonate ions which are stored for more than 10,000 years, durably removing the carbon dioxide from the atmosphere. mCDR technologies accelerate this natural process by reducing the amount of carbon dioxide dissolved in the surface of the ocean (known as the partial pressure). This in turn prompts the ocean to rebalance with the atmosphere and absorb more carbon dioxide from the air. 

The mCDR technologies that do this can be broadly categorized into:

• Abiotic mCDR approaches—which include OAE and DOC—reduce the amount of carbon dioxide in the ocean by leveraging the ocean’s natural chemistry or by directly extracting carbon dioxide from seawater. 

• Biotic mCDR approaches increase the growth of algae—which absorbs carbon dioxide from the ocean via photosynthesis—and enhance the storage of this organic carbon by sinking it into the deep ocean.   

Introducing integrity on carbon credits

To ensure this emerging industry scales responsibly, there is a need for scientific rigor and transparency on what constitutes a high quality mCDR carbon credit. This is the role of a carbon removal registry, like Isometric, whose task it is to develop rigorous monitoring, reporting and verification (MRV) protocols. In essence these protocols are rulebooks that carbon removal suppliers must follow when conducting CDR operations and quantifying carbon removals. Protocols ensure that the resulting credits meet the highest possible scientific standards. 

While scientific rigor ensures integrity, trust is built through transparency. Carbon markets, historically plagued by greenwashing and overcrediting, have long lacked transparency and accountability. This means it is essential that any carbon removal credit that enters the market must have its underlying data, calculations and third-party verification information published publicly. A solid foundation for scaling mCDR can only be built once there is confidence in the quantification. 

The role of ocean measurements and modeling

As with every CDR technology, MRV is crucial for mCDR as it enables us to quantify how much carbon dioxide has been durably stored in the ocean. At Isometric, we’re excited that we’ll be issuing the world’s first independently verified mCDR credits later this year.

For abiotic mCDR technologies—like OAE—the primary way to quantify net carbon removal is by using high quality models that have been extensively validated with ocean measurements. Models simulate carbon removal above the baseline and are necessary because natural systems in baseline scenarios are highly variable. Currently, models are the only way to fully capture that variability.

Anyone who has conducted direct measurement of abiotic mCDR in the field knows that this is no easy task. For example, a technology like OAE takes place across a vast area of ocean over a long period of time, making it difficult to take a large number of direct measurements. To manage this, Isometric’s OAE Protocol currently focuses on coastal outfalls—which are easier to directly measure because alkalinity addition is occurring from a point source rather than a distributed release. 

Nevertheless, to quantify the climate benefit of technologies like OAE, robust ocean models—supplemented by direct measurements—are essential. Ocean modeling will continue to develop over time, but for now the combination of measurements and models is important and a strong example of using the right tools for the job.

Schematic of the four spatio-temporal regimes that need to be characterized for the calculation of the gross CO₂ removal, from Isometric’s Ocean Alkalinity Enhancement from Coastal Outfalls protocol.

The other side of MRV is monitoring for potential environmental impacts. There is potential for some mCDR technologies to have significant co-benefits in the local environment—such as locally reversing ocean acidification. More research is needed on these impacts, but early work shows some potential benefits to local ecosystems from mCDR technologies.

Cutting-Edge solutions shaping the path forward

A huge amount of progress on mCDR has been made in recent years, but much more is needed to scale these technologies—responsibly and fast—for meaningful climate impact. MRV for mCDR is highly complex due to the vast area of the ocean and long timeframes its processes operate on. There are three key challenges that we must address to set us on the right path: 

Improving computational tools

The quantification methods used in Isometric’s mCDR protocols so far have been informed by our research partnership with [C]Worthy—a leading Focused Research Organization that is building open-source software tools to support MRV of mCDR projects. The aim of this partnership is to use [C]worthy’s open-source modeling tools—known as C-Star—to better quantify uncertainty associated with ocean models used to calculate the net carbon dioxide removal of mCDR projects. 

We are excited to continue this partnership and use other groundbreaking scientific research to inform future iterations of our protocols. This is crucial to further improve confidence in quantification of mCDR for buyers. 

Developing sensors for carbonate chemistry 

Today, the lack of sensors makes taking direct measurements for mCDR difficult. For example, sensors that measure Total Alkalinity in local environments are limited. As a result, carbon dioxide and pH sensors, along with bottle samples, are used to measure Total Alkalinity and Dissolved Inorganic Carbon. 

The development of better quality, portable, in-situ sensors for these complex parameters will accelerate the oceanographic community’s ability to directly measure what is happening and reduce uncertainties in those measurements. For example, Aquatic Labs are developing the first commercial sensor that can directly measure Total Alkalinity in seawater—in real time—with greater precision than sampling in the field. 

Assessing environmental impacts

Currently, there is no globally applicable, standardized framework for assessing the environmental impacts of mCDR deployments. This is often considered difficult to develop due to regional specificity—i.e., which species are likely to be impacted by mCDR deployments. Additionally the current mental model of many in the mCDR community is that local permitting authorities are best set up to be the decision-makers for how an environmental impact assessment should be carried out for mCDR.

However, last year, Ocean Visions announced the development of an Environmental Impact Assessment Framework (EIAF). The outcomes of this process will help build consensus on what a global set of environmental assessment criteria should look like for mCDR. 

Solving these challenges using the best available science will require close collaboration between academia, non-profits and industry, as well as the development of more ocean sensors, measurements, models and data analysis tools. Above all, it is critical that we continue to learn from early mCDR deployments, iterate lessons learnt and transparently report on findings so we can continue building confidence in this important set of technologies.