Let Earth Move the Molecules: The Power of Open-System CDR and Implications for MRV

Introducing the MRV Blog: The Carbon to Sea Initiative is launching a series of blog posts on monitoring, reporting, and verification (MRV) to offer up-to-date insights, reflections, and emerging perspectives into research, market trends, and current best practices. I’m Anna Madlener, MRV Manager at Carbon to Sea—in other words, I get to focus on MRV for ocean-based carbon dioxide removal (CDR) every day.

This post introduces a framework for closed- and open-system CDR, its implication for MRV, and current state for Ocean Alkalinity Enhancement (OAE).

Leveraging Earth’s natural systems: open-systems’ role in a global CDR portfolio

To mitigate the worst impacts of climate change, large-scale carbon dioxide removal (CDR) must complement urgent and substantial emissions reductions, according to the Intergovernmental Panel on Climate Change (IPCC). To limit global warming to 1.5°C, at least 100–1000 gigatonnes of CO₂ must be removed throughout the 21st century (IPCC 1.5°C Special Report). To meet this need, a diverse portfolio of CDR approaches is emerging, often categorized into closed- and open-system pathways.

Closed-system pathways, like direct air capture (DAC), rely on fully engineered processes to turn atmospheric CO₂ into a directly measurable stream of CO₂. The extracted CO₂ must be utilized in long-lived products (i.e. not fuel) or durably stored and monitored to qualify as carbon removal for crediting (European Scientific Advisory Board on Climate Change, 2025). Because many of the steps in a closed-system pathway are directly controlled or engineered, they are easier to monitor and more straightforward to quantify, but demand energy input for every moving molecule (Ellis, 2023). Importantly, as closed-system CDR methods are more mature, they have had an outsized influence on setting the standards for MRV.   

In contrast, open-system CDR, such as reforestation, ocean alkalinity enhancement (OAE), and enhanced rock weathering (ERW), harness natural processes happening in diffuse, open environments. These systems of CDR aim to accelerate or enhance different ways the Earth manages carbon, relying on nature to complete the removal process, but are more challenging to observe directly. 

OAE proposals have emerged to mimic the ocean’s powerful natural carbon management mechanism. Whenever CO₂ in the atmosphere is higher than in seawater, seawater absorbs atmospheric CO₂ until in equilibrium, through a process known as air-sea gas equilibration. Since the industrial revolution, the ocean has naturally absorbed about 30% of carbon emissions (Friedlingstein et al., 2023), but at a cost: ocean acidity has increased by roughly 30% (Doney et al., 2009; Gattuso et al., 2015). Alkalinity helps to buffer this acidity. For millions of years, alkaline rock weathered by rain, rivers, and other processes has washed into the ocean, helping to balance out this acidity and removing approximately 1 billion tons of CO₂ each year (Larkin et al., 2022).

According to the National Academies, OAE can theoretically scale to gigatonne-level CO₂ removal per year (NASEM, 2022). As such, OAE could become a powerful part of the CDR portfolio, alongside closed-system and other open-system pathways—if expanded research efforts continue to demonstrate efficacy and environmental safety. Since this potential scalability comes with a tradeoff on direct observation of CDR, it is necessary to explore fit-for-purpose approaches to measure and model the carbon removal.

Conversion, removal, storage: understanding open-system CDR and OAE as a spectrum

Comparing open- versus closed-system CDR is not a binary choice—any CDR process is, in theory, an open system in the sense that it interacts with the carbon cycle and removes CO₂ from the atmosphere (Keller et al., 2018). To understand MRV for open-system CDR, I find the following framework useful, suggested by Anu Khan, Founder & Director of the Carbon Removal Standards Initiative. 

To demonstrate this difference, direct air capture as a fully closed system and ocean alkalinity enhancement in its fully open-system form are compared below. 

Some novel CDR solutions approach open and closed systems from a hybrid perspective, for example by leveraging open-system removal and storage potential, while relying on a closed system to achieve the conversion, thereby facilitating the ability to directly quantify the first step. In the case of Direct Ocean Capture (DOC), for example, CO₂ is extracted from seawater in a closed system, resulting in a directly measurable stream of CO₂ and low-pCO₂ water that is added back to the oceans. Assuming safe and permanent underground storage of the extracted CO₂ stream, the eventual CDR then relies on additional atmospheric CO₂ being removed through air-sea gas equilibration in the ocean, leveraging open-system processes for removal and storage.

DAC, DOC, OAE, ERW, and Wastewater Alkalinity Enhancement (WAE) are shown on the spectrum of closed- vs. open-system CDR.

Quantifying OAE efficiency: measuring and modeling conversion, removal, and storage

The underlying science of OAE and its potential for carbon removal is well understood (Renforth and Henderson, 2017; Campbell et al., 2022). As a rule of thumb, under typical ocean surface conditions, one mole of alkalinity increase translates into ~0.8 moles of CO₂, transferred mostly to bicarbonate with a smaller portion stored as carbonate (He and Tyka, 2023). This is called the stoichiometric (or “theoretical”) efficiency.

However, several factors such as feedstock type, deployment mechanism, or conditions of receiving waters can reduce this theoretical efficiency, known as potential efficiency losses (Renforth and Henderson, 2017; He and Tyka, 2023; Bach, 2024).

Above, the four groups of potential efficiency losses, as identified by Dr. Abby Lunstrum during her 2025 Carbon to Sea Initiative Research Fellowship.

Feedstock dissolution and solution stability is most relevant at the conversion stage (Hartmann et al., 2023). Subduction can affect the eventual removal, even if feedstock was fully dissolved and remained stable, while additionality is largely an efficiency loss in the context of net-accountable carbon removal (i.e., an offset), but does not affect the potential to remove CO₂ in the first place. Current research aims to generate new knowledge of how these factors interact and under which conditions they tend to occur, thereby informing how OAE may be applied in practice and how measurements and models can best address conversion, removal, and storage holistically.

Measuring Conversion

The first step of quantification is demonstrating the successful shift of DIC components (i.e., more bicarbonate and carbonate, less CO₂) through the addition of alkaline minerals or solutions. To the extent that this step takes place in the vicinity of where alkalinity was added, in-situ measurements and discrete bottle sample analysis of the carbonate system (pH, DIC, pCO₂, and TA) help verify whether the conditions of theoretical efficiency hold true. Additional sensing methods, such as sediment biogeochemical flux analyses or particle monitoring, can identify and quantify efficiency losses if they arise (Bach, 2024; Hartmann et al., 2023).

Laboratory and mesocosm studies on potential efficiency losses at the conversion stage have helped understand feedstock dissolution, solution stability, and some additionality processes. This research is already informing the design of first-of-their-kind OAE applications in practice, minimizing the risk of such losses in the first place. While ideal conditions can be calculated and modeled, real-world validation remains essential to build confidence in optimal OAE deployment.

Both the conversion and removal steps must account for natural variability in the carbonate system, including daily and seasonal alkalinity fluctuations, which often exceed OAE-induced changes such that signals from OAE activities may not be directly distinguishable (Ho et al., 2023). In this early phase of small-scale field research, this complicates demonstrating additionality for carbon crediting, as the background baseline constantly shifts. The optimal duration and quantity of baseline data that is sufficient to address these complications is an open area of research. Though at a minimum, one full seasonal cycle will help understand seasonal variation.

Modeling Removal and Storage 

Given successful feedstock dissolution and solution stability at the conversion stage, subduction becomes the primary potential efficiency loss at the removal stage. Physical transport mechanisms disperse alkalized, pCO₂-depleted waters and dissolved inorganic carbon, diluting signals and complicating measurement efforts further. As such, CO₂ removal and storage can potentially occur far from the point of alkalinity addition and over varying timescales of weeks to years and beyond. Consequently, measurement-informed models at different scales are necessary to form a comprehensive picture of the fate of alkalinity and carbon over time and space. 

High-resolution, near-field models simulate the hydrodynamics of alkalinity mixing immediately after addition. These are essential tools to evaluate how alkalized water, exhibiting higher pH and Total Alkalinity values, interacts with background values as it spreads within this immediate near field zone. Efforts are also beginning to robustly assess feedstock dissolution and solution stability at this scale. Regional models, ranging from sub-100 m to ~50 km resolution, track biogeochemical processes to estimate removal efficiency and potential subduction over time. Global ocean circulation models are required to capture the long-term storage and fate of DIC within global ocean cycles. 

Eventually, models will also need to be able to account for efficiency processes that affect the additionality of OAE in the context of net-creditable removal. As of today, most models do not fully reflect the complexity of these processes. 

Each model scale relies on ocean physics and atmospheric data, for example temperature, salinity, currents, or surface wind, and is validated using biogeochemical measurements. For calibration and validation, near-field and regional models can leverage project-specific assets like moorings and ship-based or autonomous measurements, while global models depend on long-term, internationally-coordinated observational networks such as GLODAP, ARGO, and SOCCOM. Finally, Earth system models can simulate the response of the climate and carbon cycle as a whole, not only in the oceans.

Expertise and demonstration of individual solutions of these modeling approaches in non-CDR contexts already exists. Developing individual models for specific sites and OAE applications, and validating them, currently takes time and is costly, but is ultimately a solvable challenge. A core focus now is demonstrating their skill and viability in MRV contexts and integrating models across different scales.

What’s next?

MRV for open-system CDR will rely on measurement-informed models to address complexities that measurements alone cannot capture. Real-world prototypes of OAE projects are making significant progress to understand both the potential and limitations of models and measurements. The future goal is improving how we quantify their joint reliability by reducing uncertainty and defining error margins. The challenge of open-system MRV isn’t necessarily achieving perfect accuracy, but understanding how far estimates may deviate from reality. Clear error quantification is essential for building trust in OAE’s long-term CO₂ removal impact. As measurement quality and data availability increase, so will operational precision in the conversion phase, leading to greater confidence in CO₂ removal and storage in the future.

In the next blog post (please subscribe to our email newsletter for updates), we will provide a comprehensive overview of current MRV practices in both company and academic settings, while also highlighting opportunity areas for R&D and technological innovation in MRV.

Editor’s note: The graph depicting open and closed systems across various CDR approaches was updated on March 7 to reflect both open- and closed-system storage for Direct Ocean Capture. While the definition of storage in this article refers to the storage of carbon removed from the atmosphere, the graph now reflects the need in DOC for closed-system storage of the extracted CO₂ stream from the water.