Based on Monday's oral arguments - from which one should never try to guess, let alone predict, an outcome - Richard Frank (UC, Davis) predicts an EPA loss (see here), but Megan Herzog (UCLA) predicts an EPA win (see here). Both saw the same oral arguments. Their fellow Legal Planet blogger Dan Farber agrees with Frank that a "narrow loss" by EPA seems most likely (here). At least Farber warned of the hazards of predicting outcomes from oral arguments.
Farber also pointed out quite rightly that the plain language of the statutory provision at issue in the could hardly be plainer. The only question before the Court is whether the EPA erred in interpreting the phrase "any air pollutant regulated under this chapter" from the PSD program includes carbon dioxide gases. Because carbon dioxide already is regulated under Title II of the Act covering mobile sources, it is a pollutant "regulated under this chapter."
Not only is the language of the statute plain, but EPA's interpretation of it has been consistent since the PSD program was first created in 1977. So, this case presents a real test for Justice Scalia, who describes himself as a textualist. Really, all he needs to do is pull out his trusty Webster's dictionary and look up the meaning of the word "any." Will he do that? I doubt it. In the past, I have found that Scalia is not the textualist and originalist he claims to be. Like most justices, he turns out to be a functionalist or instrumentalist - that is, an outcome-oriented justice who uses textualism and originalism only when they get him to the conclusions he prefers.
From a legal point of view, I consider this an "easy" case. Arguably, the Court should be especially reluctant to insert itself in this kind of case, where the statutory language is clear and Congress has full authority to overrule or alter EPA's regulations by amending the underlying statute. (The Clean Air Act may well be a bad fit for regulating greenhouse gases, but the Court already decided that issue in Massachusetts v. EPA.) Nevertheless, as Judge Posner has pointed out (see here), the US Supreme Court is not so much a court of law as a political court. I cannot possible predict what that political (and highly politicized) court will decide about politically controversial EPA climate regulations.