GHG removals

Appropriate causal, spatial, and temporal system boundaries are required for credible CDR evaluation. Boundaries must include all direct and indirect emissions and removals caused by the deployment of the CDR. Direct emissions and removals occur in the stages in which CO₂ emissions from the atmosphere are captured, the stages in which captured CO₂ is stored, and all processes connecting the capture and storage of CO₂. 

Indirect emissions and removals could be market-mediated material or energy displacements (e.g. displacement of renewable energy from heat decarbonisation to removals by DACCS potentially causing greater GHG emissions overall than removals by DACCS). Indirect impacts also include effects induced by co-production of other useful products alongside GHG removal (e.g. energy generation by BECCS projects).   

It is rare that one project or company controls the full supply chain of a CDR. However, regardless of ownership, the full supply chain of each CDR must be considered to enable consistent assessment of all CDRs on the same basis.   

A coherent CDR evaluation needs to consider all changes caused by the deployment of the CDR within a relevant timeframe to climate change. The temporal boundaries need to be set to a minimum of 100 years for land-based removals utilised to offset land-based GHG emissions, and to a minimum of 1,000 years for offsetting fossil GHG emissions.   

Evaluation of robustness of boundaries definition 

To evaluate the completeness of the boundary definition reported by each CDR project, we propose using a scale 1 to 5, with 1 representing incomplete or partial boundaries, and 5 representing complete representation of the CDR option, including its interactions with the wider system and specification of the assessment period. A higher score indicates a more credible removal estimate. 

Assessing the removal provided by a CDR as a simple balance between emissions and sinks across its supply chain offers a partial view of the contribution of the CDR to removing atmospheric GHG. Besides net negative flux of GHG over the full life cycle, genuine removal requires that the CDR meets “additionality” requirements. This means that a given CDR must yield greater removal than would have otherwise occurred in its absence, i.e. counterfactual scenario. For instance, a BECCS supply chain using forestry residues from an intensively harvested forest may deliver removals overall. However, if leaving the forest unharvested would have fixed more CO₂ over the timeframe of analysis, e.g. 100 years, then the BECCS supply chain would in effect contribute to increasing GHG emissions over that time.  

To estimate additionality, we compare the quantity of CO₂ removed by the CDR to the quantity of CO₂ removed in the relevant counterfactual (see below).  

A counterfactual captures what would occur in the absence of a CDR project. The counterfactual scenario should be assessed on the same system boundaries and with the same accounting rules as the CDR deployment scenario. The counterfactual should also consider the same key emission and removal drivers over the assessment period as the CDR deployment scenario. This means that every emission sink and source category is included in the two scenarios, albeit under different circumstances, i.e. with and without deployment of the CDR option. The drivers should include both policy drivers, such as implemented or adopted policies, as well as non-policy drivers, such as economic conditions, energy prices, and technological development. If more than one set of drivers can be demonstrated as plausible, both should be considered, resulting in two or more counterfactuals.    

Note that if the CDR project delivers co-products, the delivery of these in the same amount should be also included in the counterfactual, although delivered by alternative processes. Continuing with the BECCS example in ‘Additionality’, if the BECCS project produces energy as well as it delivers removals, then the counterfactual should include the production of this energy by an alternative source.   

Evaluation of robustness of counterfactual(s) definition 

We propose assessing the accuracy of the counterfactual definition on a five-point scale, shown below, with 1 representing no definition of the counterfactual and 5 representing a well-evidenced counterfactual, covering the complete system boundaries, accounting methods, and key drivers.  

A high score in this case means that the counterfactual is robustly defined, hence the CDR case can be compared like-to-like against the counterfactual, to inform the real benefits and impacts from deploying the CDR option. For instance, the difference between the removals in the counterfactuals vs the CDR deployment scenarios will better characterise the uncertainty range around the size of removal delivered by a specific CDR option.   

Once a score is estimated, users should strive for improving it as much as possible by using the guidelines set by the GHG Protocol Policy and Action Standard.² 

Removed carbon must be durably stored out of the atmosphere and for a sufficient length of time to determine its contribution to climate mitigation.  

A commonly used minimum duration for carbon storage is 100 years. However, we suggest that the 100-year timeframe is only adequate if the CDR is used to offset land-based GHG emissions (comprising shorter-lived gases than CO₂, and/or land-use related CO₂ that would have the capacity to be re-captured within this timeframe through ecosystem processes). If the removals delivered by the CDR are to offset fossil GHG emissions, then the carbon store needs to be more permanent (1,000+ years), for instance in geological storage. This greater permanence reflects the time over which fossil emissions will continue impact the climate, lacking the potential re-capturability inherent to land-related CO₂ emissions.