The mass of CO₂ removed from seawater through electrochemical separation — or other approaches — can be measured directly as a metered output from the direct ocean removal (DOR) system. It can also be checked for consistency against operational data from the DOR system and characterization of the seawater effluent that is depleted of dissolved inorganic carbon (DIC). The quantity of CO₂ directly removed from seawater may not be equal to the quantity of CO₂ stored (see Leakage) or the quantity of CO₂ indirectly removed by atmospheric drawdown (see Air sea gas exchange). Direct removal of CO₂ from seawater produces DIC-depleted seawater and results in subsequent atmospheric carbon removal by inducing air-sea gas exchange. Thus, the quantity of CO₂ removed directly from seawater must be clearly quantified and validated as a separate measurement from CO₂ storage or atmospheric CO₂ drawdown.

Any difference between the CO₂ removed from seawater and the CO₂ that is measured as a metered input to the storage system must be accounted for as leakage. For terrestrial geologic storage, post-storage leakage can be directly monitored during and after the injection period, though this is more challenging for subseafloor reservoirs. If geological storage results in a functionally stable form on a short timescale — for example, via subsurface mineralization — fugitive emissions associated with the full lifetime of storage may be estimated based on direct observations of the storage reservoir. If instead the integrity of geological storage requires ongoing monitoring and maintenance — for example with the injection of supercritical CO₂ — the potential for future fugitive emissions must be modeled. For alternative storage systems, like mineralization in concrete, it is possible to directly measure the conversion of input CO₂ into a functionally stable form and therefore the total leakage from the storage system.

Removing CO₂ from seawater shifts the pH and carbonate ion concentration, and could cause abiotic precipitation of calcium carbonate (CaCO₃) or brucite (Mg(OH)₂). This is important to characterize as this precipitation results in CO₂ outgassing. It remains to be determined what specific combination of suitable nucleation conditions and supersaturation of CaCO₃ is needed to cause significant precipitation. Abiotic precipitation can be minimized by controlling the dilution and dispersal of DIC-depleted water and ensuring pH shifts stay below precipitation thresholds.

Since calcification releases CO₂, any changes to the rate of biotic calcification in response to shifts in pH and dissolved inorganic carbon (DIC) concentrations driven by the direct ocean removal process must be considered. Changes in biotic calcification rates could occur in both coastal and open ocean waters, and at the level of individual calcifiers or calcifier populations. In practice, quantifying the biotic calcification response to carbon-depleted water may pose significant spatial, temporal, and signal-to-noise challenges.

Atmospheric uptake is reduced if dissolved inorganic carbon (DIC) depleted water is transferred from the surface ocean before either increasing the uptake of atmospheric CO₂ or decreasing the outgassing of CO₂ to the atmosphere. The precise dispersal of DIC-depleted water, its residence time at the surface, and hence the associated completeness of air-sea gas equilibration is dependent on the location and timing of the dispersal of DIC-depleted water, and is hard to directly observe. In-situ measurements can potentially track post-deployment changes in pCO₂, pH, and DIC and deploy water mass tracers to help validate models of plume dispersal and atmospheric CO₂ drawdown, though the sensitivity and cost of sensors may be a challenge. Further, oceanographic models can be used to develop probabilistic estimates of equilibration efficiencies for particular locations, contingent on seasonal or interannual variations in forcing regimes. Typical gas exchange timescales of equilibration for the surface ocean and atmosphere are on the order of months to years.

Depletion of dissolved inorganic carbon (DIC) could lead to less particulate organic carbon (POC) formation in the surface ocean. This effect could be small if plankton communities are primarily limited by nutrient availability, rather than DIC availability. POC export mediated by calcifying communities could also change in response to changes in biogenic calcification (see Biotic calcification response). Data on potential biological responses to removing CO₂ from the surface waters is still lacking.

Eventually, the formation and burial of carbonate minerals will release CO₂ stored. The carbonate precipitation process occurs slowly and is unlikely to have a significant impact on estimates of carbon storage on decadal to millennial timescales. However on geologic timescales, it is an important process to characterize.

The embodied emissions of any materials consumed during operation can be estimated based on a cradle-to-grave lifecycle assessment (LCA) of the material input. Emissions associated with any built infrastructure, including monitoring and maintenance equipment, should include both construction emissions and material embodied emissions. There are not yet consistent best practices around whether or how to account for the embodied emissions of equipment or infrastructure that is used but not owned by the project. Transparency around boundary assumptions, data sources, and uncertainties is critical for LCA consistency and comparability.

The emissions associated with energy use for project activities should be estimated using an assessment of lifecycle emissions for the specific electricity or other energy sources required by the project. This should include the energy use associated with monitoring activities.

The emissions associated with the system response to CDR energy demand must be considered for energy-intensive approaches. This component is less of a concern if a CDR project builds its own renewables, or if project energy demand is dispatchable in a way that supports grid deployment of renewables. If a CDR project introduces a new base load or is otherwise connected to the grid in a manner that could displace demand for renewables, any emissions associated with the energy system response must be considered. There is not yet a clear best practice for how to account for this system interaction.

If storage results in a functionally stable form of CO₂ on a short timescale — for example, via subsurface mineralization or mineralization in concrete — demonstration that the stable form has been achieved is enough to establish durability. However, if the integrity of storage requires ongoing monitoring and maintenance — for example, with the injection of supercritical CO₂ — an evaluation of durability claims must consider the monitoring and maintenance plan, as well as any applicable regulatory structure that assigns ongoing liability for storage integrity. One important consideration is whether or not there is a track record of sufficient administrative capacity to guarantee execution of monitoring, maintenance, and liability arrangements. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Leakage component.

The concept of ocean dissolved inorganic carbon (DIC) lifetime is a well-established theory and is accepted within the community to lie somewhere between 10,000 and 100,000 years. Since capturing atmospheric CO₂ through direct ocean removal results in restoring seawater to its original state, this intervention is not expected to change the long-term patterns of abiotic precipitation and dissolution of carbonate minerals — a central control on DIC residence time — compared to the counterfactual. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Marine carbonate precipitation component.

The precipitation and dissolution of carbonate minerals modifies ocean DIC (see DIC residence time). Carbonates preserved on the seafloor are likely to store carbon on geologic timescales. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Marine carbonate precipitation component.