Page:Michigan v. EPA.pdf/41

 Cite as: 576 U. S. ____ (2015)

19

KAGAN, J., dissenting

path, instructing the Agency, as a preliminary matter, to complete and consider a study about the residual harms to public health arising from those emissions. See ante, at 2– 3; supra, at 5. But once EPA found in its study that the acid rain provisions would not significantly affect power plants’ emissions of hazardous pollutants, any rationale for treating power plants differently from other sources discharging the same substances went up in smoke. See 65 Fed. Reg. 79830. At that point, the Agency would have had far more explaining to do if, rather than following a well-tested model, it had devised a new scheme of regulation for power plants only. Still more, EPA could not have accurately assessed costs at the time of its “appropriate and necessary” finding. See 8 Mercury Study, at 6–2 (noting the “many uncertainties” in any early-stage analysis of pollution control costs). Under the statutory scheme, that finding comes before— years before—the Agency designs emissions standards. And until EPA knows what standards it will establish, it cannot know what costs they will impose. Nor can those standards even be reasonably guesstimated at such an early stage. Consider what it takes to set floor standards alone. First, EPA must divide power plants into categories and subcategories; as explained earlier, those classification decisions significantly affect what floors are established. See supra, at 4, and n. 1, 11–12. And then, EPA must figure out the average emissions level already achieved by the top 12% in each class so as to set the new standards. None of that can realistically be accomplished in advance of the Agency’s regulatory process: Indeed, those steps are the very stuff of the rulemaking. Similarly, until EPA knows what “compliance options” it will develop, it cannot know how they will mitigate the costs plants must incur to meet the floor standards. See supra, at 13–14. And again, deciding on those options takes substantial time. So there is good reason for different