The mass of CO₂ injected for geologic storage, or used as an input for another storage process (e.g. mineralization in concrete), can be measured directly as a metered output from the DAC system and a metered input to the storage system. It can also be checked for consistency against operational data from the CO₂ capture system. We assume here that injected CO₂ is not used for enhanced oil recovery (EOR). In the case of EOR, any GHG emissions associated with enhanced oil recovery and fuel use would have to be accounted for, as well as additionality questions about the appropriate baseline against which to credit the project.

The mass of CO₂ that escapes from the storage system must include a consideration of both observed leakage and potential future leakage. For terrestrial geologic storage, fugitive emissions can be directly monitored during and after the injection period, though this is more challenging for subseafloor reservoirs. If geologic storage results in 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, however, the integrity of geologic 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 fugitive emissions from the storage system.

The embodied emissions of any materials consumed during operation, like mineral feedstocks or chemical solvents, can be estimated based on a cradle-to-grave lifecycle assessment of the material input. The embodied emissions of non-consumed project equipment and infrastructure must also be considered, amortized over the expected lifetime of operation. 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 these boundary assumptions, as well as the data sources and associated uncertainties across the board, is necessary for LCA consistency and comparability.

The emissions associated with energy use for the process. This component should be estimated based on an assessment of lifecycle emissions for the specific electricity or energy sources consumed on-site.

The emissions associated with the system response to the energy demand of CDR. This component is less of a concern if a CDR project builds its own renewables, or if it is dispatchable in a way that supports grid deployment of renewables. In the case that the 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 permanence. If, however, the integrity of storage requires ongoing monitoring and maintenance — for example with the injection of supercritical CO₂ — an evaluation of permanence 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.

Any rigorous evaluation of what a carbon removal project has achieved requires clarity around a project's operations. This includes (1) transparent operations which allow external parties to observe the CDR process, (2) auditable operational records that can be cross-checked against carbon removal claims, (3) transparent protocols around how outcomes are quantified (if such claims are being made by the project), and (4) an auditable — and preferably public — registry that ensures against double-counting of carbon removal benefits.