The mass of the biomass feedstock can be directly measured and converted to CO₂e using a default factor or direct measurements of biomass carbon and moisture content. The feedstock quantity can be checked for consistency against operational records, and will form the basis for estimating feedstock-related upstream emissions (see Carbon storage counterfactual, Feedstock use counterfactual, and Market effects) as well as understanding the removal efficiency of the process. In addition to characterizing carbon content, a feedstock sampling protocol should characterize embodied nutrients such as nitrogen, phosphorus, and potassium. This enables an assessment of potential nutrient depletion and may inform the estimation of upstream emissions (see Feedstock use counterfactual). Any emissions associated with the process of biomass cultivation or procurement must be included in the project’s lifecycle assessment (see Materials, Energy, and Non-CO₂ biomass emissions). The chemical and physical characteristics of the feedstock will also inform estimates of feedstock decomposition and outgassing (see Outgassing).

The mass of sunk biomass carbon can be measured directly and converted to CO₂e using a default factor or direct measurements. This quantity can be checked for consistency against operational records around biomass sourcing and transportation (see Biomass procurement). The physical configuration in which biomass is sunk should be recorded to inform estimates of feedstock decomposition (see Solid phase deepwater storage).

Some fraction of the deployed biomass may not successfully sink to a depth for long-term storage. This could be a result of biomass being shed or consumed during the sinking process, which is a function of biomass type, sinking configuration, and sinking velocity. Physical biomass loss during sinking should therefore be quantified for the specific biomass type and sinking configuration utilized based on field observations. For biomass that sinks after some period of floating in the open ocean (e.g. biomass-based buoys), the possibility that biomass may end up being beached or sinking in areas that do not result in permanent storage (e.g. shallow waters or areas of upwelling) must also be considered. Modeling this behavior is likely to be challenging due to the difficulty of mimicking the free-floating behavior of biomass with monitoring equipment. Negligible uncertainty is likely only achieved in cases where biomass sinking is active and rapid.

Sunk biomass will likely decompose, and the resulting dissolved products will mix in the water reservoir. When the mixing dynamics of the reservoir bring decomposition products back to the surface, they will equilibrate with the atmosphere, which could result in the outgassing of CO₂ or other greenhouse gases. The estimated timescale of this outgassing is dependent on the location of sinking, the storage depth achieved, and local stratification and circulation dynamics. Any claim about the volume of total carbon removal from biomass sinking should therefore be accompanied by a clear claim about the portion of biomass that is assumed to remain in solid-phase storage (see Solid Phase Deepwater Storage) and a clear disclosure of the time horizon over which outgassing dynamics are considered. In the absence of clear evidence that the sunk biomass resists decomposition, it is conservative to assume that all sunk biomass is remineralized at the bottom of the reservoir in which it is sunk. Our evaluation reflects the uncertainties associated with estimating carbon stored for 1000+ years in a diversity of potential reservoirs, including the deep ocean or anoxic basins like the Black Sea. Characterizing the durability of carbon storage in these reservoirs is an area of active research, and those interested in particular basins or on longer storage timescales may find it appropriate to adjust these uncertainties. The outgassing of such non-CO₂ gasses such as methane, nitrous oxide, and sulfide should be quantified, and any GWP assumptions used to incorporate non-CO₂ outgassing into the calculation of net carbon removal should be transparently disclosed.

While nearly all biomass carbon eventually makes its way back to the atmosphere, the effective drawdown from more durably storing biomass carbon occurs when the biomass feedstock would otherwise have released CO₂ into the atmosphere. If the counterfactual fate of the biomass feedstock would have resulted in little to no carbon storage — for example, burning or rapid decomposition — any durable storage achieved represents additional carbon removal. If, however, the counterfactual fate of the biomass feedstock would have resulted in medium-to-long term carbon storage — for example in soils, slow-degradation environments like some landfills, or living ecosystems — the mass of stored carbon in the counterfactual should not count as additional carbon removal unless and until counterfactual emissions would have occurred. This component may not apply in cases where carbon is purpose grown.

If a biomass feedstock currently serves a function that will need to be replaced if the feedstock is used for CDR, any emissions associated with the replacement must be considered. For example, if agricultural waste is currently used as animal feed or left on the fields to contribute to nitrogen, phosphorus, and potassium in soils, using that agricultural waste for CDR could result in new demand for feed or fertilizer, respectively. Current feedstock uses can be evaluated on a project-by-project basis, and the carbon impact of replacements can be estimated via lifecycle assessment. We recommend accounting for feedstock replacement emissions that involve existing feedstock uses rather than potential future uses. For example, agricultural feedstocks used for CDR should be evaluated based on their current uses (e.g. for animal feed or soil nutrients) rather than potential future uses that are not practiced today. The counterfactual for existing utilization should be flexible, and re-evaluated in the future if prevailing practices change. This component may not apply in cases where carbon is purpose grown.

Any emissions associated with market effects as a result of feedstock demand must be considered. For example, paying farmers for agricultural waste will likely increase the profitability of their operations and could result in an increase in acres planted. Directly growing crops or creating plantations could displace food production onto other lands and contribute to deforestation. Estimating the system emissions resulting from new feedstock demand will likely be difficult in practice and require careful economic modeling that reflects the overall scale of new biomass demand.

Using a biomass feedstock for CDR could potentially avoid non-CO₂ emissions such as methane or nitrous oxide that would have occurred in the counterfactual, for example during biomass decay. We recommend that these avoided emissions be considered a co-benefit of the project, and not be included in the accounting of net carbon removal. GWP assumptions used to characterize any avoided non-CO₂ emissions should be transparently disclosed.

The embodied emissions of all non-consumable and consumable materials that enable biomass sinking — including biomass feedstocks and any materials used to deploy the biomass — should be estimated using a cradle-to-grave lifecycle assessment (LCA). If biomass is purpose-grown, emissions from biomass cultivation and direct land use change must be taken into account. 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 transporting biomass feedstock, combustion emissions associated with fuel use during the project, and energy use associated with monitoring and maintenance.

If a project’s management of the biomass feedstock before long-term storage leads to non-CO₂ emissions, these must be accounted for. Non-CO₂ emissions such as methane or nitrous oxide could be produced via processes like anaerobic decomposition during feedstock storage and transport. These emissions are considered separately from any non-CO₂ gasses that are emitted after biomass storage (see Leakage). GWP assumptions used to incorporate these emissions into the calculation of net carbon removal should be transparently disclosed.

The extent to which sunk biomass remains stored in solid form and is incorporated into sediments is a function of biomass type, sinking configuration, and the environmental conditions of the sinking location. Characterizing these dynamics is an area of active research. In the absence of clear evidence of solid phase storage, it is conservative to assume that all sunk biomass is remineralized (see Dissolved phase deepwater storage) and that water circulation will eventually return the CO₂ back to the surface, where it will be brought into equilibrium with the atmosphere (see Outgassing). This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Outgassing component.

From a carbon management perspective, it is conservative to assume that a significant portion of the sunk biomass is remineralized and that water circulation and diffusion will distribute the dissolved carbon in the water reservoir. Depending on the circulation dynamics of the reservoir, some dissolved carbon may persist in stagnant deep water or stratified circulation. Any dissolved carbon that reaches the surface water will be brought into equilibrium with the atmosphere. The estimated timescale of this ventilation is dependent on the specific sinking location, which must be characterized at the project level but can be informed by models of the specific storage reservoir. The potential climate sensitivity of circulation dynamics is reservoir-specific and an outstanding area of uncertainty. This component is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Outgassing component.