Massachusetts v. EPA was not the only Clean Air Act decision handed down yesterday. In a second decision, Environmental Defense v. Duke Energy, the Supreme Court waded into the ongoing controversy over "New Source Review," specifically when modifications to existing power plants and industrial facilities trigger costly upgrades in emission control technology.

Some hoped or expected the Supreme Court would put an end to NSR litigation, clarifying once and for all the scope of the Clean Air Act's relevant provisions, but this was not to be. Duke Energy is an extremely narrow opinion. A unanimous Court overturned the Fourth Circuit's opinion, rejecting its the argument that the EPA must define what constitutes a plant "modification" consistently throughout all relevant regulatory programs, but it did little else. (Justice Thomas wrote a separate opinion concurring in part rejecting a portion of the Court's reasoning, but not disputing the end result.) The court explicitly left untouched Duke Energy's other arguments against the EPA's enforcement actions, and it failed to address an interesting jurisdictional issue that some thought prompted the Court to hear the case in the first place.

What this means is that NSR litigation will continue, at least until the remaining cases are settled or Congress finally reforms the program. There is little dispute that certain portions of Clean Air Act are woefully out-of-date and effectively lock-in older, more polluting facilities. Yet the EPA lacks the statutory authority to rewrite the relevant NSR provisions unilaterally (though the Bush Administration has tried), and there is no consensus in Congress, as of yet, about how to proceed. So don't be surprised if the Supreme Court is once again petitioned to consider NSR in the years ahead.

David Rivkin, who authored the law professors' amicus brief in which I participated, has more thoughts on SCOTUSBlog here. I previewed the case here.