On the assumption that a trading system can constitute a performance standard for GHGs under the CAA, the question arises whether “offsets” should be creditable for compliance. Crediting reduces compliance costs. For environmental integrity, however, creditable offsets should satisfy accepted offsets criteria, including additionality.  Incorporating those criteria into a performance standard may be impractical, achievable only incrementally, and entail legal risk. A probably viable alternative would define a super-category based solely on emission of GHGs, within which trading would be allowed.