The U.S. Supreme Court has delivered a stunner with its decision this morning in Utility Air Regulatory Group v. Environmental Protection Agency. The Supreme Court has curtailed the U.S. Environmental Protection Agency’s (EPA) regulation of stationary source greenhouse gas (GHG) emissions under two Clean Air Act permitting programs - New Source Review Prevention of Significant Deterioration (PSD) and Title V. EPA can no longer require PSD or Title V permits for stationary sources based on a source’s GHG emissions, unless a source is already subject to the permitting programs. However, if a source triggers PSD permitting for another pollutant, the Court has left the door open for EPA to require the source to undergo a Best Available Control Technology determination for GHGs. Today’s decision in Utility Air Regulatory Group has significant ramifications for industrial source permitting.