This morning, the U.S. Supreme Court issued a surprising decision in Utility Air Regulatory Group v. Environmental Protection Agency on the U.S. Environmental Protection Agency’s (EPA) authority to regulate greenhouse gases under the Clean Air Act (CAA).  The Supreme Court has held that EPA cannot regulate stationary sources under the CAA Prevention of Significant Deterioration (PSD) or Title V permitting programs based exclusively on a source’s greenhouse gas (GHG) emissions, where a sources does not otherwise trigger regulatory thresholds.  On the other hand, the Supreme Court endorsed the idea that if a source triggers PSD permitting for another pollutant, EPA can require the source to undergo a Best Available Control Technology determination for GHGs.  This decision will undoubtedly have a significant impact on industrial source permitting in California and across the U.S.  For more details on the decision and its implications, read our alert here.