The Clean Air Act establishes the New Source Performance Standards program of Section 111. The National Ambient Air Quality Standards (NAAQS) program is set out in Sections 108 through 110 of the Act. The Hazardous Air Pollutants (HAP) program, is set out in Section 112. Section 111 directs D to list “categories of stationary sources” that it determines “cause[ ], or contribute[ ] significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare.” The statute directs D to (1) “determine[ ],” taking into account various factors, the “best system of emission reduction which . . . has been adequately demonstrated,” (2) ascertain the “degree of emission limitation achievable through the application” of that system, and (3) impose an emissions limit on new stationary sources that “reflects” that amount. A source may achieve that emissions cap any way it chooses. Its pollution must be no more than the amount “achievable through the application of the best system of emission reduction . . . adequately demonstrated,” or the BSER. The statute also authorizes the regulation of certain pollutants from existing sources. Under Section 111(d), once D “has set new source standards addressing emissions of a particular pollutant under . . . section 111(b),” it must then address emissions of that same pollutant by existing sources-but only if they are not already regulated under the NAAQS or HAP programs. Section 111(d) “operates as a gap-filler,” empowering EPA to regulate harmful emissions not already controlled under the Agency’s other authorities. Under Section 111(d), D, not the States, decides the amount of pollution reduction that must ultimately be achieved. It does so by again determining “the best system of emission reduction . . . that has been adequately demonstrated for [existing covered] facilities.” The States then submit plans containing the emissions restrictions that they intend to adopt and enforce in order not to exceed the permissible level of pollution established by D. D has used Section 111(d) only a handful of times since the enactment of the statute in 1970. In October 2015, D promulgated two rules addressing carbon dioxide pollution from power plants-one for new plants under Section 111(b), and the other for existing plants under Section 111(d). Both were premised on the Agency’s earlier finding that carbon dioxide is an “air pollutant” that “may reasonably be anticipated to endanger public health or welfare” by causing climate change. Carbon dioxide is not subject to a NAAQS and has not been listed as a toxic pollutant. The rule distinguished coal-fired and natural-gas-fired plants. D began regulating carbon dioxide from new coal and gas plants. D determined “much larger emission reductions [were] needed from [coal-fired plants] to address climate change.” D mandated a shift in electricity production from existing coal-fired power plants to natural-gas-fired plants. Or a shift from both coal- and gas-fired plants to “new low- or zero-carbon generating capacity,” mainly wind and solar. D projected that by 2030, it would be feasible to have coal provide 27% of national electricity generation, down from 38% in 2014. D then developed a series of complex equations to “determine the emission performance rates” that States would be required to implement. The calculations resulted in numerical emissions ceilings so strict that no existing coal plant would have been able to achieve them without engaging in one of the three means of shifting generation. D's new emissions limit the Clean Power Plan established for existing power plants is stricter than the cap imposed by the simultaneously published standards for new plants. D’s modeling and that of others showed that the rule would entail billions of dollars in compliance costs (to be paid in the form of higher energy prices), require the retirement of dozens of coal-fired plants, and eliminate tens of thousands of jobs across various sectors. The Energy Information Administration projected that the rule would cause retail electricity prices to remain persistently 10% higher in many States, and would reduce GDP by at least a trillion 2009 dollars by 2040. On the same day that D promulgated the rule, dozens of parties (including 27 States) petitioned for review. D eventually repealed the rule in 2019, concluding that the Clean Power Plan had been “in excess of its statutory authority” under Section 111(d). D determined that “the interpretative question raised” by the Clean Power Plan-“i.e., whether a ‘system of emission reduction’ can consist of generation-shifting measures”-fell under the “major question doctrine.” D argued that under the major questions doctrine, a clear statement was necessary to conclude that Congress intended to delegate authority “of this breadth to regulate a fundamental sector of the economy.” It found none. D then came up with the Affordable Clean Energy (ACE) Rule. D determined that the application of its BSER measures would result in only small reductions in carbon dioxide emissions. A number of States and private parties immediately filed petitions. The Court of Appeals concluded the statute could reasonably be read to encompass generation shifting. It concluded that the major questions doctrine did not apply, and thus rejected the need for a clear statement of congressional intent to delegate such power to D. The Court vacated D’s repeal of the Clean Power Plan and remanded it for further consideration. It also vacated and remanded the ACE Rule, for the same reason. The Supreme Court granted certiorari.