A “cap-without-trade program” to regulate greenhouse gas emissions under §111 of the CAA — a program that applies a mass-based cap to each regulated source — provides compliance flexibility, making it feasible to rely on end-use energy efficiency as a compliance technique, without running the legal risks involved in trying to graft an emissions trading program on the technology-based structure written into §111. These risks are substantial but vary with the scope of the trading program. A “cap-without-trade” approach also provides pollution sources with continuing incentives to favor comprehensive legislation addressing climate disruption and may facilitate a negotiated solution to §111 controversies.