`(Slip Opinion)
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` OCTOBER TERM, 2021
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`Syllabus
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`1
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` NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
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` being done in connection with this case, at the time the opinion is issued.
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` The syllabus constitutes no part of the opinion of the Court but has been
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` prepared by the Reporter of Decisions for the convenience of the reader.
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` See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.
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`SUPREME COURT OF THE UNITED STATES
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` Syllabus
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`WEST VIRGINIA ET AL. v. ENVIRONMENTAL
`PROTECTION AGENCY ET AL.
`CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR
`
`THE DISTRICT OF COLUMBIA CIRCUIT
` No. 20–1530. Argued February 28, 2022—Decided June 30, 2022*
`
`In 2015, the Environmental Protection Agency (EPA) promulgated the
`Clean Power Plan rule, which addressed carbon dioxide emissions
`from existing coal- and natural-gas-fired power plants. For authority,
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`the Agency cited Section 111 of the Clean Air Act, which, although
`known as the New Source Performance Standards program, also au-
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`thorizes regulation of certain pollutants from existing sources under
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`Section 111(d). 42 U. S. C. §7411(d). Prior to the Clean Power Plan,
`EPA had used Section 111(d) only a handful of times since its enact-
`ment in 1970. Under that provision, although the States set the actual
`enforceable rules governing existing sources (such as power plants),
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`EPA determines the emissions limit with which they will have to com-
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`ply. The Agency derives that limit by determining the “best system of
`emission reduction . . . that has been adequately demonstrated,” or the
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`BSER, for the kind of existing source at issue. §7411(a)(1). The limit
`then reflects the amount of pollution reduction “achievable through the
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`application of” that system. Ibid.
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`In the Clean Power Plan, EPA determined that the BSER for exist-
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`ing coal and natural gas plants included three types of measures,
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`which the Agency called “building blocks.” 80 Fed. Reg. 64667. The
`first building block was “heat rate improvements” at coal-fired
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`plants—essentially practices such plants could undertake to burn coal
`——————
`*Together with No. 20–1531, North American Coal Corp. v. Environ-
`mental Protection Agency et al., No. 20–1778, Westmoreland Mining
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`Holdings LLC v. Environmental Protection Agency et al., and No. 20–
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`1780, North Dakota v. Environmental Protection Agency et al., also on
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`certiorari to the same court.
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`WEST VIRGINIA v. EPA
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`Syllabus
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`more cleanly. Id., at 64727. This sort of source-specific, efficiency-
`improving measure was similar in kind to those that EPA had previ-
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`ously identified as the BSER in other Section 111 rules.
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`Building blocks two and three were quite different, as both involved
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`what EPA called “generation shifting” at the grid level—i.e., a shift in
`electricity production from higher-emitting to lower-emitting produc-
`ers. Building block two was a shift in generation from existing coal-
`fired power plants, which would make less power, to natural-gas-fired
`plants, which would make more. Ibid. This would reduce carbon di-
`oxide emissions because natural gas plants produce less carbon dioxide
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`per unit of electricity generated than coal plants. Building block three
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`worked like building block two, except that the shift was from both coal
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`and gas plants to renewables, mostly wind and solar. Id., at 64729,
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`64748. The Agency explained that, to implement the needed shift in
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`generation to cleaner sources, an operator could reduce the regulated
`plant’s own production of electricity, build or invest in a new or exist-
`ing natural gas plant, wind farm, or solar installation, or purchase
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`emission allowances or credits as part of a cap-and-trade regime. Id.,
`at 64731–64732. Taking any of these steps would implement a sector-
`wide shift in electricity production from coal to natural gas and renew-
`ables. Id., at 64731.
`
`Having decided that the BSER was one that would reduce carbon
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`pollution mostly by moving production to cleaner sources, EPA then
`set about determining “the degree of emission limitation achievable
`through the application” of that system. §7411(a)(1). The Agency rec-
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`
`ognized that, in translating the BSER into an operational emissions
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`limit, it could choose whether to require anything from a little genera-
`tion shifting to a great deal. It settled on what it regarded as a “rea-
`sonable” amount of shift, which it based on modeling how much more
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`electricity both natural gas and renewable sources could supply with-
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`out causing undue cost increases or reducing the overall power supply.
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`Id., at 64797–64811. The Agency ultimately projected, for instance,
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`that it would be feasible to have coal provide 27% of national electricity
`generation by 2030, down from 38% in 2014. From these projected
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`changes, EPA determined the applicable emissions performance rates,
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`which were so strict that no existing coal plant would have been able
`to achieve them without engaging in one of the three means of gener-
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`ation shifting. The Government projected that the rule would impose
`billions in compliance costs, raise retail electricity prices, require the
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`retirement of dozens of coal plants, and eliminate tens of thousands of
`jobs.
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`This Court stayed the Clean Power Plan in 2016, preventing the rule
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`from taking effect. It was later repealed after a change in Presidential
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`administrations. Specifically, in 2019, EPA found that the Clean
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`3
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`Cite as: 597 U. S. ____ (2022)
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`Syllabus
`Power Plan had exceeded the Agency’s statutory authority under Sec-
`
`tion 111(d), which it interpreted to “limit[ ] the BSER to those systems
`that can be put into operation at a building, structure, facility, or in-
`stallation.” 84 Fed. Reg. 32524. EPA explained that the Clean Power
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`Plan, rather than setting the standard “based on the application of
`equipment and practices at the level of an individual facility,” had in-
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`stead based it on “a shift in the energy generation mix at the grid
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`level,” id., at 32523. The Agency determined that the interpretive
`question raised by the Clean Power Plan fell under the major questions
`doctrine. Under that doctrine, it determined, a clear statement is nec-
`essary for a court to conclude that Congress intended to delegate au-
`thority “of this breadth to regulate a fundamental sector of the econ-
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`omy.” Id., at 32529. It found none. The Agency replaced the Clean
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`Power Plan by promulgating a different Section 111(d) regulation,
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`known as the Affordable Clean Energy (ACE) rule. Id., at 32532. In
`that rule, EPA determined that the BSER would be akin to building
`block one of the Clean Power Plan: a combination of equipment up-
`grades and operating practices that would improve facilities’ heat
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`rates. Id., at 32522, 32537.
`
`A number of States and private parties filed petitions for review in
`the D. C. Circuit, challenging EPA’s repeal of the Clean Power Plan
`and its enactment of the replacement ACE rule. The Court of Appeals
`consolidated the cases and held that EPA’s “repeal of the Clean Power
`Plan rested critically on a mistaken reading of the Clean Air Act”—
`namely, that generation shifting cannot be a “system of emission re-
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`duction” under Section 111. 985 F. 3d 914, 995. The court vacated the
`Agency’s repeal of the Clean Power Plan and remanded to the Agency
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`for further consideration. It also vacated and remanded the ACE rule
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`for the same reason. The court’s decision was followed by another
`change in Presidential administrations, and EPA moved the court to
`partially stay its mandate as to the Clean Power Plan while the Agency
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`considered whether to promulgate a new Section 111(d) rule. No party
`opposed the motion, and the Court of Appeals agreed to stay its vaca-
`tur of the Agency’s repeal of the Clean Power Plan.
`Held:
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`1. This case remains justiciable notwithstanding the Government’s
`contention that no petitioner has Article III standing, given EPA’s
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`stated intention not to enforce the Clean Power Plan and to instead
`engage in new rulemaking. In considering standing to appeal, the
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`question is whether the appellant has experienced an injury “fairly
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`traceable to the judgment below.” Food Marketing Institute v. Argus
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`Leader Media, 588 U. S. ___, ___. If so, and a “favorable ruling” from
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`the appellate court “would redress [that] injury,” then the appellant
`has a cognizable Article III stake. Ibid. Here, the judgment below
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`4
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`WEST VIRGINIA v. EPA
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`Syllabus
`vacated the ACE rule and its embedded repeal of the Clean Power
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`Plan, and accordingly purports to bring the Clean Power Plan back
`into legal effect. There is little question that the petitioner States are
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`injured, since the rule requires them to more stringently regulate
`power plant emissions within their borders. The Government counters
`that EPA’s current posture has mooted the prior dispute. The distinc-
`tion between mootness and standing matters, however, because the
`Government bears the burden to establish that a once-live case has
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`become moot. The Government’s argument in this case boils down to
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`its representation that EPA does not intend to enforce the Clean Power
`Plan prior to promulgating a new Section 111(d) rule. But “voluntary
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`cessation does not moot a case” unless it is “absolutely clear that the
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`allegedly wrongful behavior could not reasonably be expected to recur.”
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`Parents Involved in Community Schools v. Seattle School Dist. No. 1,
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`551 U. S. 701, 719. Here, the Government “nowhere suggests that if
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`this litigation is resolved in its favor it will not” reimpose emissions
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`limits predicated on generation shifting. Ibid. Pp. 14–16.
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`2. Congress did not grant EPA in Section 111(d) of the Clean Air Act
`the authority to devise emissions caps based on the generation shifting
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`approach the Agency took in the Clean Power Plan. Pp. 16–31.
`
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`(a) In devising emissions limits for power plants, EPA “deter-
`mines” the BSER that—taking into account cost, health, and other fac-
`tors—it finds “has been adequately demonstrated,” and then quanti-
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`fies “the degree of emission limitation achievable” if that best system
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`were applied to the covered source. §7411(a)(1). The issue here is
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`whether restructuring the Nation’s overall mix of electricity genera-
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`tion, to transition from 38% to 27% coal by 2030, can be the BSER
`within the meaning of Section 111.
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`Precedent teaches that there are “extraordinary cases” in which the
`“history and the breadth of the authority that [the agency] has as-
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`serted,” and the “economic and political significance” of that assertion,
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`provide a “reason to hesitate before concluding that Congress” meant
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`to confer such authority. FDA v. Brown & Williamson Tobacco Corp.,
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`529 U. S. 120, 159–160. See, e.g., Alabama Assn. of Realtors v. Depart-
`ment of Health and Human Servs., 594 U. S. ___, ___; Utility Air Reg-
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`ulatory Group v. EPA, 573 U. S. 302, 324; Gonzales v. Oregon, 546
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`U. S. 243, 267; National Federation of Independent Business v. OSHA,
`595 U. S. ___, ___. Under this body of law, known as the major ques-
`tions doctrine, given both separation of powers principles and a prac-
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`tical understanding of legislative intent, the agency must point to
`“clear congressional authorization” for the authority it claims. Utility
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`Air, 573 U. S., at 324. Pp. 16–20.
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`(b) This is a major questions case. EPA claimed to discover an
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`5
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`Cite as: 597 U. S. ____ (2022)
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`Syllabus
`unheralded power representing a transformative expansion of its reg-
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`ulatory authority in the vague language of a long-extant, but rarely
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`used, statute designed as a gap filler. That discovery allowed it to
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`adopt a regulatory program that Congress had conspicuously declined
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`to enact itself. Given these circumstances, there is every reason to
`“hesitate before concluding that Congress” meant to confer on EPA the
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`authority it claims under Section 111(d). Brown & Williamson, 529
`U. S., at 160.
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`Prior to 2015, EPA had always set Section 111 emissions limits
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`based on the application of measures that would reduce pollution by
`causing the regulated source to operate more cleanly, see, e.g., 41 Fed.
`Reg. 48706—never by looking to a “system” that would reduce pollu-
`tion simply by “shifting” polluting activity “from dirtier to cleaner
`sources.” 80 Fed. Reg. 64726. The Government quibbles with this his-
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`tory, pointing to the 2005 Mercury Rule as one Section 111 rule that it
`says relied upon a cap-and-trade mechanism to reduce emissions. See
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`70 Fed. Reg. 28616. But in that regulation, EPA set the emissions
`limit—the “cap”—based on the use of “technologies [that could be] in-
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`stalled and operational on a nationwide basis” in the relevant
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`timeframe. Id., at 28620–28621. By contrast, and by design, there are
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`no particular controls a coal plant operator can install and operate to
`attain the emissions limits established by the Clean Power Plan. In-
`deed, the Agency nodded to the novelty of its approach when it ex-
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`plained that it was pursuing a “broader, forward-thinking approach to
`the design” of Section 111 regulations that would “improve the overall
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`power system,” rather than the emissions performance of individual
`sources, by forcing a shift throughout the power grid from one type of
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`energy source to another. 80 Fed. Reg. 64703 (emphasis added). This
`view of EPA’s authority was not only unprecedented; it also effected a
`“fundamental revision of the statute, changing it from [one sort of]
`scheme of . . . regulation” into an entirely different kind. MCI Tele-
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`communications Corp. v. American Telephone & Telegraph Co., 512
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`U. S. 218, 231.
`
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`The Government attempts to downplay matters, noting that the
`Agency must limit the magnitude of generation shift it demands to a
`level that will not be “exorbitantly costly” or “threaten the reliability
`of the grid.” Brief for Federal Respondents 42. This argument does
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`not limit the breadth of EPA’s claimed authority so much as reveal it:
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`On EPA’s view of Section 111(d), Congress implicitly tasked it, and it
`alone, with balancing the many vital considerations of national policy
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`implicated in the basic regulation of how Americans get their energy.
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`There is little reason to think Congress did so. EPA has admitted that
`issues of electricity transmission, distribution, and storage are not
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`within its traditional expertise. And this Court doubts that “Congress
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`6
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`WEST VIRGINIA v. EPA
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`Syllabus
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`. . . intended to delegate . . . decision[s] of such economic and political
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`significance,” i.e., how much coal-based generation there should be
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`over the coming decades, to any administrative agency. Brown & Wil-
`liamson, 529 U. S., at 160. Nor can the Court ignore that the regula-
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`tory writ EPA newly uncovered in Section 111(d) conveniently enabled
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`it to enact a program, namely, cap-and-trade for carbon, that Congress
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`had already considered and rejected numerous times. The importance
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`of the policy issue and ongoing debate over its merits “makes the
`oblique form of the claimed delegation all the more suspect.” Gonzales,
`546 U. S., at 267–268. Pp. 20–28.
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`(c) Given that precedent counsels skepticism toward EPA’s claim
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`that Section 111 empowers it to devise carbon emissions caps based on
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`a generation shifting approach, the Government must point to “clear
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`congressional authorization” to regulate in that manner. Utility Air,
`573 U. S., at 324. The Government can offer only EPA’s authority to
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`establish emissions caps at a level reflecting “the application of the
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`best system of emission reduction . . . adequately demonstrated.”
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`§7411(a)(1). The word “system” shorn of all context, however, is an
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`empty vessel. Such a vague statutory grant is not close to the sort of
`clear authorization required. The Government points to other provi-
`sions of the Clean Air Act—specifically the Acid Rain and National
`Ambient Air Quality Standards (NAAQS) programs—that use the
`word “system” or “similar words” to describe sector-wide mechanisms
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`for reducing pollution. But just because a cap-and-trade “system” can
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`be used to reduce emissions does not mean that it is the kind of “system
`of emission reduction” referred to in Section 111.
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`Finally, the Court has no occasion to decide whether the statutory
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`phrase “system of emission reduction” refers exclusively to measures
`that improve the pollution performance of individual sources, such
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`that all other actions are ineligible to qualify as the BSER. It is perti-
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`nent to the Court’s analysis that EPA has acted consistent with such
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`a limitation for four decades. But the only question before the Court
`is more narrow: whether the “best system of emission reduction” iden-
`tified by EPA in the Clean Power Plan was within the authority
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`granted to the Agency in Section 111(d) of the Clean Air Act. For the
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`reasons given, the answer is no. Pp. 28–31.
`985 F. 3d 914, reversed and remanded.
`ROBERTS, C. J., delivered the opinion of the Court, in which THOMAS,
`ALITO, GORSUCH, KAVANAUGH, and BARRETT, JJ., joined. GORSUCH, J.,
`filed a concurring opinion, in which ALITO, J., joined. KAGAN, J., filed a
`dissenting opinion, in which BREYER and SOTOMAYOR, JJ., joined.
`
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`

`

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` Cite as: 597 U. S. ____ (2022)
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`Opinion of the Court
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`1
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` NOTICE: This opinion is subject to formal revision before publication in the
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` preliminary print of the United States Reports. Readers are requested to
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` notify the Reporter of Decisions, Supreme Court of the United States, Wash-
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` ington, D. C. 20543, of any typographical or other formal errors, in order that
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` corrections may be made before the preliminary print goes to press.
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`SUPREME COURT OF THE UNITED STATES
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`
`
`_________________
` Nos. 20–1530, 20–1531, 20–1778 and 20–1780
`_________________
`
` WEST VIRGINIA, ET AL., PETITIONERS
`
` v.
`20–1530
` ENVIRONMENTAL PROTECTION AGENCY, ET AL.
`
`
`
`
`
`THE NORTH AMERICAN COAL CORPORATION,
`PETITIONER
`20–1531
`
`v.
`ENVIRONMENTAL PROTECTION AGENCY, ET AL.
`
`
`
`
`WESTMORELAND MINING HOLDINGS LLC,
`PETITIONER
`
`20–1778
`v.
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`ENVIRONMENTAL PROTECTION AGENCY, ET AL.
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`NORTH DAKOTA, PETITIONER
`
`20–1780
`v.
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`ENVIRONMENTAL PROTECTION AGENCY, ET AL.
`ON WRITS OF CERTIORARI TO THE UNITED STATES COURT OF
`
`APPEALS FOR THE DISTRICT OF COLUMBIA CIRCUIT
`[June 30, 2022]
`CHIEF JUSTICE ROBERTS delivered the opinion of the
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`Court.
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`The Clean Air Act authorizes the Environmental Protec-
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`tion Agency to regulate power plants by setting a “standard
`of performance” for their emission of certain pollutants into
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`2
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`WEST VIRGINIA v. EPA
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`Opinion of the Court
`the air. 84 Stat. 1683, 42 U. S. C. §7411(a)(1). That stand-
`ard may be different for new and existing plants, but in
`each case it must reflect the “best system of emission reduc-
`tion” that the Agency has determined to be “adequately
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`demonstrated” for the particular category. §§7411(a)(1),
`(b)(1), (d). For existing plants, the States then implement
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`that requirement by issuing rules restricting emissions
`from sources within their borders.
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`Since passage of the Act 50 years ago, EPA has exercised
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`this authority by setting performance standards based on
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`measures that would reduce pollution by causing plants to
`operate more cleanly. In 2015, however, EPA issued a new
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`rule concluding that the “best system of emission reduction”
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`for existing coal-fired power plants included a requirement
`that such facilities reduce their own production of electric-
`ity, or subsidize increased generation by natural gas, wind,
`or solar sources.
`
`
`The question before us is whether this broader conception
`of EPA’s authority is within the power granted to it by the
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`Clean Air Act.
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`
`
`I
`A
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`The Clean Air Act establishes three main regulatory pro-
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`grams to control air pollution from stationary sources such
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`as power plants. Clean Air Amendments of 1970, 84 Stat.
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`1676, 42 U. S. C. §7401 et seq. One program is the New
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`Source Performance Standards program of Section 111, at
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`issue here. The other two are the National Ambient Air
`Quality Standards (NAAQS) program, set out in Sections
`108 through 110 of the Act, 42 U. S. C. §§7408–7410, and
`the Hazardous Air Pollutants (HAP) program, set out in
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`Section 112, §7412. To understand the place and function
`of Section 111 in the statutory scheme, some background on
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`the other two programs is in order.
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`The NAAQS program addresses air pollutants that “may
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`Opinion of the Court
`reasonably be anticipated to endanger public health or wel-
`
`fare,” and “the presence of which in the ambient air results
`from numerous or diverse mobile or stationary sources.”
`§7408(a)(1). After identifying such pollutants, EPA estab-
`lishes a NAAQS for each. The NAAQS represents “the max-
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`imum airborne concentration of [the] pollutant that the
`public health can tolerate.” Whitman v. American Trucking
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`Assns., Inc., 531 U. S. 457, 465 (2001); see §7409(b). EPA,
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`though, does not choose which sources must reduce their
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`pollution and by how much to meet the ambient pollution
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`target. Instead, Section 110 of the Act leaves that task in
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`the first instance to the States, requiring each “to submit to
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`[EPA] a plan designed to implement and maintain such
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`standards within its boundaries.” Train v. Natural Re-
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`sources Defense Council, Inc., 421 U. S. 60, 65 (1975); §7410.
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`The second major program governing stationary sources
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`is the HAP program. The HAP program primarily targets
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`pollutants, other than those already covered by a NAAQS,
`that present “a threat of adverse human health effects,” in-
`cluding substances known or anticipated to be “carcino-
`genic, mutagenic, teratogenic, neurotoxic,” or otherwise
`“acutely or chronically toxic.” §7412(b)(2).
`
`EPA’s regulatory role with respect to these toxic pollu-
`tants is different in kind from its role in administering the
`NAAQS program. There, EPA is generally limited to deter-
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`mining the maximum safe amount of covered pollutants in
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`the air. As to each hazardous pollutant, by contrast, the
`Agency must promulgate emissions standards for both new
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`and existing major sources. §7412(d)(1). Those standards
`must “require the maximum degree of reduction in emis-
`sions . . . that the [EPA] Administrator, taking into consid-
`eration the cost of achieving such emission reduction, and
`any non-air quality health and environmental impacts and
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`energy requirements, determines is achievable . . . through
`application of measures, processes, methods, systems or
`techniques” of emission reduction. §7412(d)(2). In other
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`4
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`WEST VIRGINIA v. EPA
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`Opinion of the Court
`words, EPA must directly require all covered sources to re-
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`duce their emissions to a certain level. And it chooses that
`level by determining the “maximum degree of reduction” it
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`considers “achievable” in practice by using the best existing
`technologies and methods. §7412(d)(3).
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`Thus, in the parlance of environmental law, Section 112
`directs the Agency to impose “technology-based standard[s]
`for hazardous emissions,” Alaska Dept. of Environmental
`Conservation v. EPA, 540 U. S. 461, 485, n. 12 (2004) (em-
`phasis added). This sort of “‘technology-based’ approach fo-
`cuses upon the control technologies that are available to in-
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`dustrial entities and requires the agency to . . . ensur[e]
`that regulated firms adopt the appropriate cleanup technol-
`ogy.” T. McGarity, Media-Quality, Technology, and Cost-
`Benefit Balancing Strategies for Health and Environmen-
`tal Regulation, 46 Law & Contemp. Prob. 159, 160 (Summer
`
`1983) (McGarity). (Such “technologies” are not limited to
`literal technology, such as scrubbers; “changes in the design
`and operation” of the facility, or “in the way that employees
`perform their tasks,” are also available options. Id., at 163,
`n. 18.)
`
`
`The third air pollution control scheme is the New Source
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`Performance Standards program of Section 111. §7411.
`
`That section directs EPA to list “categories of stationary
`sources” that it determines “cause[], or contribute[] signifi-
`cantly to, air pollution which may reasonably be anticipated
`to endanger public health or welfare.” §7411(b)(1)(A). Un-
`der Section 111(b), the Agency must then promulgate for
`
`
`each category “Federal standards of performance for new
`sources,” §7411(b)(1)(B). A “standard of performance” is
`one that
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`“reflects the degree of emission limitation achievable
`through the application of the best system of emission
`reduction which (taking into account the cost of achiev-
`ing such reduction and any nonair quality health and
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`5
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`Opinion of the Court
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` environmental impact and energy requirements) the
`[EPA] Administrator determines has been adequately
`demonstrated.” §7411(a)(1).
`
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`Thus, the statute directs EPA 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. Ibid.; see also 80 Fed. Reg. 64538 (2015).
`Generally speaking, a source may achieve that emissions
`
`cap any way it chooses; the key is that its pollution be no
`
`more than the amount “achievable through the application
`of the best system of emission reduction . . . adequately
`
`demonstrated,” or the BSER. §7411(a)(1); see §7411(b)(5).
`EPA undertakes this analysis on a pollutant-by-pollutant
`
`basis, establishing different standards of performance with
`respect to different pollutants emitted from the same source
`category. See, e.g., 73 Fed. Reg. 35838 (2008); 42 Fed. Reg.
`22510 (1977).
`
`
`Although the thrust of Section 111 focuses on emissions
`limits for new and modified sources—as its title indicates—
`the statute also authorizes regulation of certain pollutants
`from existing sources. Under Section 111(d), once EPA “has
`set new source standards addressing emissions of a partic-
`ular pollutant under . . . section 111(b),” 80 Fed. Reg. 64711,
`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. §7411(d)(1). Existing
`power plants, for example, emit many pollutants covered by
`a NAAQS or HAP standard. Section 111(d) thus “operates
`as a gap-filler,” empowering EPA to regulate harmful emis-
`sions not already controlled under the Agency’s other au-
`
`
`thorities. American Lung Assn. v. EPA, 985 F. 3d 914, 932
`(CADC 2021).
`
`

`

`6
`
`
`WEST VIRGINIA v. EPA
`
`Opinion of the Court
`Although the States set the actual rules governing exist-
`
`ing power plants, EPA itself still retains the primary regu-
`latory role in Section 111(d). The Agency, not the States,
`decides the amount of pollution reduction that must ulti-
`mately be achieved. It does so by again determining, as
`
`when setting the new source rules, “the best system of emis-
`sion reduction . . . that has been adequately demonstrated
`for [existing covered] facilities.” 40 CFR §60.22(b)(5) (2021);
`see also 80 Fed. Reg. 64664, and n. 1. The States then sub-
`
`
`mit plans containing the emissions restrictions that they
`intend to adopt and enforce in order not to exceed the per-
`missible level of pollution established by EPA. See §§60.23,
`
`60.24; 42 U. S. C. §7411(d)(1).
`Reflecting the ancillary nature of Section 111(d), EPA has
`
`
`used it only a handful of times since the enactment of the
`statute in 1970. See 80 Fed. Reg. 64703, and n. 275 (past
`regulations pertained to “four pollutants from five source
`categories”). For instance, the Agency has established
`emissions limits on acid mist from sulfuric acid production,
`41 Fed. Reg. 48706 (1976) (identifying “fiber mist elimina-
`tor” technology as BSER); sulfide gases released by kraft
`pulp mills, 44 Fed. Reg. 29829 (1979) (determining BSER
`
`to be a combination of scrubbers, incineration, filtration
`systems, and temperature control); and emissions of vari-
`ous harmful gases from municipal landfills, 61 Fed. Reg.
`9907 (1996) (setting BSER as use of a flare to combust the
`gases). It was thus only a slight overstatement for one of
`
`the architects of the 1990 amendments to the Clean Air Act
`to refer to Section 111(d) as an “obscure, never-used section
`of the law.” Hearings on S. 300 et al. before the Subcom-
`mittee on Environmental Protection of the Senate Commit-
`tee on Environment and Public Works, 100th Cong., 1st
`Sess., 13 (1987) (remarks of Sen. Durenberger).
`B
`Things changed in October 2015, when EPA promulgated
`
`
`
`
`
`

`

`7
`
`
`
`
`
`
`
` Cite as: 597 U. S. ____ (2022)
`
`Opinion of the Court
`two rules addressing carbon dioxide pollution from power
`plants—one for new plants under Section 111(b), the other
`for existing plants under Section 111(d). Both were prem-
`ised 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. 80 Fed. Reg. 64530. Carbon dioxide is not subject
`to a NAAQS and has not been listed as a toxic pollutant.
`The first rule announced by EPA established federal car-
`
`bon emissions limits for new power plants of two varieties:
`fossil-fuel-fired electric steam generating units (mostly coal
`fired) and natural-gas-fired stationary combustion tur-
`bines. Id., at 64512. Following the statutory process set
`out above, the Agency determined the BSER for the two cat-
`egories of sources. For steam generating units, for instance,
`EPA determined that the BSER was a combination of high-
`efficiency production processes and carbon capture technol-
`ogy. See 80 Fed. Reg. 64512. EPA then set the emissions
`
`limit based on the amount of carbon dioxide that a plant
`would emit with these technologies in place. Id., at 64513.
`The second rule was triggered by the first: Because EPA
`
`
`was now regulating carbon dioxide from new coal and gas
`
`plants, Section 111(d) required EPA to also address carbon
`
`See
`emissions from existing coal and gas plants.
`§7411(d)(1). It did so through what it called the Clean
`
`Power Plan rule.
`In that rule, EPA established “final emission guidelines
`
`
`for states to follow in developing plans” to regulate existing
`
`power plants within their borders. Id., at 64662. To arrive
`at the guideline limits, EPA did the same thing it does when
`imposing federal regulations on new sources: It identified
`
`the BSER.
`
`The BSER that the Agency selected for existing coal-fired
`
`power plants, however, was quite different from the BSER
`
`it had chosen for new sources. The BSER for existing plants
`included three types of measures, which the Agency called
`
`

`

`8
`
`
`WEST VIRGINIA v. EPA
`
`Opinion of the Court
`“building blocks.” Id., at 64667. The first building block
`was “heat rate improvements” at coal-fired plants—essen-
`tially practices such plants could undertake to burn coal
`more efficiently. Id., at 64727. But such improvements,
`
`EPA stated, would “lead to only small emission reductions,”
`because coal-fired power plants were already operating
`near optimum efficiency. Ibid. On the Agency’s view,
`
`“much larger emission reductions [were] needed from [coal-
`fired plants] to address climate change.” Ibid.
`So the Agency included two additional building blocks in
`
`
`its BSER, both of which involve what it called “generation
`shifting from higher-emitting to lower-emitting” producers
`of electricity. Id., at 64728. Building block two was a shift
`in electricity production from existing coal-fired power
`
`plants to natural-gas-fired plants. Ibid. Because natural
`
`gas plants produce “typically less than half as much” carbon
`
`dioxide per unit of electricity created as coal-fired plants,
`
`the Agency explained, “this generation shift [would] re-
`duce[ ] CO2 emissions.” Ibid. Building block three worked
`
`the same way, except that the shift was from both coal- and
`gas-fired plants to “new low- or zero-carbon generating ca-
`pacity,” mainly wind and solar. Id., at 647