> Section 111(d) thus ensures that EPA regulates existing power plants’ emissions of all pollutants. When the pollutant at issue falls within the NAAQS or HAP programs, EPA need do no more. But when the pollutant falls outside those programs, Section 111(d) requires EPA to set an emissions level for currently operating power plants (and other stationary sources). That means no pollutant from such a source can go unregulated: As the Senate Report explained, Section 111(d) guarantees that “there should be no gaps in control activities pertaining to stationary source emissions that pose any significant danger to public health or welfare.” S. Rep. No. 91–1196, p. 20 (1970). Reflecting that language, the majority calls Section 111(d) a “gap-filler.” Ante, at 5. It might also be thought of as a backstop or catch-all provision, protecting against pollutants that the NAAQS and HAP programs let go by. But the section is not, as the majority further claims, an “ancillary provision” or a statutory “backwater.” Ante, at 20, 26. That characterization is a non-sequitur. That something is a backstop does not make it a backwater. Even if they are needed only infrequently, see ante, at 6, 20, backstops can perform a critical function—and this one surely does. Again, Section 111(d) tells EPA that when a pollutant—like carbon dioxide—is not regulated through other programs, EPA must undertake a further regulatory effort to control that substance’s emission from existing stationary sources. In that way, Section 111(d) operates to ensure that the Act achieves comprehensive pollution control.

That's pretty damn explicit.