States, firms, and individuals can “offset” their greenhouse gas emissions by buying offset certificates, paying for the assurance that a quantified number of emissions will be avoided on their behalf in a suitable emission mitigation project. Meaningful offsetting can only be achieved when the mitigation project is the difference-making cause for the emission reduction. Difference-making has two dimensions: we expect the project to reduce greenhouse gas emissions when it is present (positive tracking), but we also expect that without the project no reduction takes place (negative tracking). Apart from this notion of causal additionality, however, there is a second issue that has attracted less attention: to endorse the practice of offsetting from a normative perspective, we should not only demand that the offsetting project is causally additional, we should also demand that it is morally additional. Moral additionality requires that there is no relevant agent under a moral obligation to perform the project anyhow. Moral additionality is violated, for instance, if offsetting projects are offered by certificate sellers who are both able and morally required to bring about emission reductions, but instead choose to sell these emission reductions as carbon offsets to others. Thus, even if an offset is a causal difference maker---assuming that the seller would reliably fail to perform his duties in the absence of the offset---moral additionality remains a concern. In such cases, the buyer of the offset discharges his obligation to mitigate by paying for an activity that the seller was already morally obliged to perform regardless of the offset, which makes the whole arrangement normative questionable. The issue of moral additionality has wider implications, as it can be applied, more generally, to partial compliance settings whenever a principal instructs an agent to perform morally required obligations on his behalf.
