Today was Clean Air Act day at the Supreme Court, as the justices issued two long-awaited opinions in cases involving the scope of the EPA's power under the Clean Air Act.

The marquee case was Massachusetts v. EPA, in which the Court held, 5-4, that (a) states have standing to sue the EPA alleging injuries from climate change, (b) the EPA has the authority to regulate greenhouse gases as "pollutants" under the Clean Air Act, and (c) the EPA did not adequately justify its decision not to regulate greenhouse gas emissions from motor vehicles under the Act. On remand, the EPA must ground its decision whether to regulate greenhouse gas emissions on the nature and magnitude of the risk posed by climate change, as the CAA requires.

As longtime readers know, I disagree with the Court's first two conclusions. I do not believe that the petitioners had standing (as argued by Chief Justice Roberts in his dissent), nor do I believe that Congress delegated the EPA authority to regulate greenhouse gases. Given that the Court concluded otehrwise on these two points, however, I think it is very difficult to argue, as the EPA did, that the EPA declined to exercise its regulatory authority on statutorily permissible grounds. I will have more to say about the case once I've had a chance to read all of the opinions.

The Supreme Court also issued its opinion in Environmental Defense v. Duke Energy unanimously siding with the EPA's interpretation of the New Source Review provisions of the Clean Air Act. I blogged about the case here, and I hope to have more to say about this case later today as well.