Evaluates the degree to which an intervention (e.g., a CDR project) causes a climate benefit above and beyond what would have happened in a no-intervention baseline scenario. By definition, this counterfactual baseline scenario cannot be directly observed (because it did not happen), so can only be estimated or inferred based on contextual information. Additionality can be assessed at the level of individual projects or protocols that define categories of projects. In policy regimes such as cap-and-trade programs (or offset programs), where emissions are permitted in exchange for reduction or storage elsewhere, failures of additionality result in increased emissions.

source: J Wilcox, B Kolosz, & J Freeman (2021) CDR Primer; Concepts

Another way of describing additionality issues, particularly in carbon offset programs, is “leakage”. Leakage is when emissions are offset by removals elsewhere, but achieving those removals leads to some unmeasured additional emissions. Leakage is negative additionality. For example, substituting ethanol for petroleum was intended to reduce petroleum use, but leakage effects have caused the substitution to have the opposite effect. U.S. corn-based ethanol worse for the climate than gasoline, study finds | Reuters