(case involves federal government's power to regulate "greenhouse gases" from fixed sources; Circuit Court substantially upheld EPA regulatory decisions and held that EPA did not need to establish minimum threshold of risk or harm before determining whether air pollutant endangers the climate; bulk of regulations did not injure any motor-vehicle-related petitioner, and no non-motor vehicle-related petitioner had shown injury in-fact; EPA lacked discretion to defer promulgation of Tailpipe Rule; U.S. Supreme Court granted review on sole issue of whether EPA permissibly determined that regulation of "greenhouse gas" emissions from new motor vehicles triggered Clean Air Act (CAA) permitting requirements under CAA fro stationary sources that emit "greenhouse gases").