NOTICE: This opinion is subject to
formal revision before publication in the United States Reports.
Readers are requested to notify the Reporter of Decisions, Supreme
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															SUPREME COURT OF THE UNITED STATES

															_________________

															Nos. 23A349, 23A350, 23A351 and 23A384

															_________________

															OHIO, et al.

															23A349 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															KINDER MORGAN, INC., et al.

															23A350 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															AMERICAN FOREST & PAPER ASSOCIATION,
et al.

															23A351 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															UNITED STATES STEEL CORPORATION

															23A384 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															on applications for stay

															[June 27, 2024]

															Justice Gorsuch delivered the opinion of the
Court.

															The Clean Air Act envisions States and the
federal government working together to improve air quality. Under
that law’s terms, States bear “primary responsibility” for
developing plans to achieve air-quality goals. 42
U. S. C. §7401(a)(3). Should a State fail to prepare a
legally compliant plan, however, the federal government may
sometimes step in and assume that authority for itself.
§7410(c)(1). Here, the federal government announced its intention
to reject over 20 States’ plans for controlling ozone pollution. In
their place, the government sought to impose a single, uniform
federal plan. This litigation concerns whether, in adopting that
plan, the federal government complied with the terms of the
Act.

															I

															A

															“The Clean Air Act regulates air quality
through a federal-state collaboration.” EME Homer City
Generation, L.P. v. EPA , 795 F.3d 118, 124 (CADC 2015).
Periodically, the Environmental Protection Agency (EPA) sets
standards for common air pollutants, as necessary to “protect the
public health.” §§7409(a)(1), (b)(1). Once EPA sets a new standard,
the clock starts ticking: States have three years to design and
submit a plan—called a State Implementation Plan, or SIP—providing
for the “implementation, maintenance, and enforcement” of that
standard in their jurisdictions. §7410(a)(1); see EPA v. EME Homer City Generation, L. P. , 572
U.S. 489 , 498 (2014). Under the Act, States decide how to
measure ambient air quality. §7410(a)(2)(B). States pick “emission
limitations and other control measures.” §7410(a)(2)(A). And States
provide for the enforcement of their prescribed measures.
§7410(a)(2)(C).

															At the same time, States must design these plans
with their neighbors in mind. Because air currents can carry
pollution across state borders, emissions in upwind States
sometimes affect air quality in downwind States. See EME
Homer , 572 U. S., at 496. To address that externality
problem, under the Act’s “Good Neighbor Provision,” state plans
must prohibit emissions “in amounts which will . . .
contribute significantly to nonattainment in, or interfere with
maintenance by, any other State” of the relevant air-quality
standard. §7410(a)(2)(D)(i)(I).

															Because the States bear “primary responsibility”
for developing compliance plans, §7401(a)(3), EPA has “no authority
to question the wisdom of a State’s choices of emission
limitations.” Train v. Natural Resources Defense Council,
Inc. , 421 U.S.
60 , 79 (1975). So long as a SIP satisfies the “applicable
requirements” of the Act, including the Good Neighbor Provision,
EPA “shall approve” it within 18 months of its submission.
§7410(k)(3); see §§7410(k)(1)(B), (k)(2). If, however, a SIP falls
short, EPA “shall” issue a Federal Implementation Plan, or FIP, for
the noncompliant State—that is, “unless” the State corrects the
deficiencies in its SIP first. §7410(c)(1); EME Homer , 572
U. S., at 498. EPA must also ensure States meet the new
air-quality standard by a statutory deadline. See §7511.

															B

															A layer of ozone in the atmosphere shields the
world from the sun’s radiation. See National Resources Defense
Council v. EPA , 464 F.3d 1 , 3 (CADC 2006). But closer to earth, ozone can hurt
more than it helps. Forming when sunlight interacts with a wide
range of precursor pollutants, ground-level ozone can trigger and
exacerbate health problems and damage vegetation. 80 Fed. Reg.
65299, 65302, 65370 (2015).

															To mitigate those and other problems, in 2015
EPA revised its air-quality standards for ozone from 75 to 70 parts
per billion. Id ., at 65293–65294. That change triggered a
requirement for States to submit new SIPs. Id ., at 65437.
Along the way, EPA issued a guidance document advising States that
they had “flexibility” in choosing how to address their Good
Neighbor obligations. See EPA, Memorandum, Information on the
Interstate Transport State Implementation Plan Submissions for the
2015 Ozone National Ambient Air Quality Standards 3 (Mar. 27,
2018). With that and other guidance in hand, many (though not all)
States submitted SIPs. See 84 Fed. Reg. 66612 (2019). And many of
the States that did submit SIPs said that they need not adopt
emissions-control measures to comply with the Good Neighbor
Provision because, among other things, they were not linked to
downwind air-quality problems or they could identify no additional
cost-effective methods of controlling the emissions beyond those
they were currently employing. See, e . g ., 87 Fed.
Reg. 9798, 9810 (2022); 87 Fed. Reg. 9545, 9552 (2022); see
generally 88 Fed. Reg. 9336, 9354–9361 (2023).

															For over two years, EPA did not act on the SIPs
it received. See, e . g ., 87 Fed. Reg. 9838, 9845–9851
(2022). Then, in February 2022, the agency announced its intention
to disapprove 19 of them on the ground that the States submitting
them had failed to address adequately their obligations under the
Good Neighbor Provision.[ 1 ] A
few months later, the agency proposed disapproving four more
SIPs.[ 2 ] Pursuant to the Act,
the agency issued its proposed SIP disapprovals for public comment
before finalizing them. See §7607(d)(3).

															C

															During that public comment period, the agency
proposed a single FIP to bind all 23 States.[ 3 ] 87 Fed. Reg. 20036, 20038 (2022). Rather than
continue to encourage “ ‘flexibilit[y]’ ” and different
state approaches, EPA now apparently took the view that
“[e]ffective policy solutions to the problem of interstate ozone
transport” demanded that kind of “uniform framework” and
“[n]ationwide consistency.” 87 Fed. Reg. 9841; see 87 Fed. Reg.
20073. The FIP the agency proposed set as its target the reduction
of the emissions of one ozone precursor in particular: nitrous
oxide. See id. , at 20038. And it sought to impose nitrous
oxide emissions-control measures that “maximized
cost-effectiveness” in achieving “downwind ozone air quality
improvements.” Id ., at 20055; see also id ., at
20043.

															In broad strokes, here is how EPA’s proposed
rule worked to eliminate a State’s “significant contribution” to
downwind ozone problems. First, the agency identified various
emissions-control measures and, using nationwide data, calculated
how much each typically costs to reduce a ton of nitrous-oxide
emissions. Id ., at 20076; see, e . g ., id ., at 20077–20081. Next, the agency sought to predict how
much each upwind State’s nitrous-oxide emissions would fall if
emissions-producing facilities in the State adopted each measure. Id ., at 20076; see, e.g. , id. , at 20088–20089;
EPA, Ozone Transport Policy Analysis Proposed Rule TSD 22–23
(EPA–HQ–OAR–2021–0668, 2022) (Proposed Ozone Analysis). In making
those predictions, EPA often considered data specific to the
emissions-producing facilities in the State, and fed “unit-level
and state-level” values into its calculations. See id ., at
9–10, 13. Then, the agency estimated how much, on average, ozone
levels would fall in downwind States with the adoption of each
measure. 87 Fed. Reg . 20076; see, e.g. , id. ,
at 20092–20093, 20096–20097; Proposed Ozone Analysis 51–52. In
making those estimations, too, EPA calibrated its modeling to each
State’s features, “determin[ing] the relationship between changes
in emissions and changes in ozone contributions on a state-by-state
. . . basis.” Id ., at 33; see also id ., at
40, 42.

															To pick which measures would “maximiz[e] cost-
effectiveness” in achieving “downwind ozone air quality
improvements,” 87 Fed. Reg. 20055, EPA focused on what it called
the “ ‘knee in the curve,’ ” or the point at which more
expenditures in the upwind States were likely to produce “very
little” in the way of “additional emissions reductions and air
quality improvement” downwind, id. , at 20095 (hyphenation
omitted). EPA used this point to select a “uniform level” of cost,
and so a uniform package of emissions-reduction tools, for upwind
States to adopt. Id ., at 20076. And EPA performed this
analysis on two “parallel tracks”—one for power plants, one for
other industries. Ibid . Pursuant to the Clean Air Act,
§§7607(d)(1)(B), (d)(3)–(6), the agency published its proposed FIP
for notice and comment in April 2022, 87 Fed. Reg. 20036.

															Immediately, commenters warned of a potential
pitfall in the agency’s approach. EPA had determined which
emissions-control measures were cost effective at addressing
downwind ozone levels based on an assumption that the FIP would
apply to all covered States. But what happens if some or many of
those States are not covered? As the commenters portrayed the SIPs,
this was not an entirely speculative possibility. Many believed
EPA’s disapprovals of the SIPs were legally flawed. See, e.g. , Comments of Missouri Dept. of Natural Resources 3
(June 17, 2022) (referencing “all the technical, legal, and
procedural issues” with the proposed SIP disapproval); see also, e . g ., Comments of Louisiana Dept. of Environmental
Quality 1–3 (June 21, 2022); Comments of Texas Comm’n on
Environmental Quality 2–4 (June 21, 2022); EPA, Response to Public
Comments on Proposed Rule 9–11 (EPA–HQ–OAR–2021–0668). They added
that EPA’s FIP was “inextricably linked” to the SIP disapprovals. E . g ., Comments of Missouri Dept. of Natural
Resources, at 4. Without a SIP disapproval or missing SIP, after
all, EPA could not include a State in its FIP. See, e . g ., id. , at 3; supra , at 3.

															Commenters added that failing to include a State
could have consequences for the proposed FIP. If the FIP did not
wind up applying to all 23 States as EPA envisioned, commenters
argued, the agency would need “to conduct a new assessment and
modeling of contribution and subject those findings to public
comment.” E . g ., Comments of Air Stewardship Coalition
13–14 (June 21, 2022); Comments of Portland Cement Association 7
(June 21, 2022). Why? As noted above, EPA assessed “significant
contribution” by determining what measures in upwind States would
maximize cost-effective ozone-level improvements in the States
downwind of them. Supra , at 5–6. And a different set of
States might mean that the “knee in the curve” would shift. After
all, each State differs in its mix of industries, in its pre-
existing emissions-control measures, and in the impact those
measures may have on emissions and downwind air quality. See 87
Fed. Reg. 20052, 20060, 20071–20073; EPA, Technical Memorandum,
Screening Assessment of Potential Emissions Reductions, Air Quality
Impacts, and Costs from Non-EGU Emissions Units for 2026, pp. 12–13
(2022).[ 4 ]

															As it happened, ongoing litigation over the SIP
disap-

															provals soon seemed to vindicate at least some
of the commenters’ concerns. Two circuits issued stays of EPA’s SIP
denials for four States. See Order in No. 23–60069 (CA5, May 1,
2023) (Texas and Louisiana); Order in No. 23–1320 (CA8, May 25,
2023) (Arkansas); Order in No. 23–1719 (CA8, May 26, 2023)
(Missouri).

															Despite those comments and developments, the
agency proceeded to issue its final FIP. 88 Fed. Reg. 36654
(2023).[ 5 ] In response to the
problem commenters raised, EPA adopted a severability provision
stating that, should any jurisdiction drop out, its rule would
“continue to be implemented as to any remaining jurisdictions.” Id ., at 36693. But in doing so, EPA did not address whether
or why the same emissions-control measures it mandated would
continue to further the FIP’s stated purpose of maximizing
cost-effective air-quality improvement if fewer States remained in
the plan.

															D

															After EPA issued its final FIP, litigation
over the agency’s SIP disapprovals continued. One court after
another issued one stay after another.[ 6 ] Each new stay meant another

															State to which EPA could not apply its FIP.
See §7410(c)(1). Ultimately, EPA recognized that it could not apply
its FIP to 12 of the 23 original States.[ 7 ] Together, these 12 States accounted for over 70
percent of the emissions EPA had planned to address through its
FIP. See Application for Ohio et al. in No. 23A349, p. 1
(States’ Application); see also 88 Fed. Reg. 36738–36739.[ 8 ]

															A number of the remaining States and industry
groups challenged the remnants of the FIP in the D. C.
Circuit. They pointed to the Act’s provisions authorizing a court
to “reverse any . . . action” taken in connection with a
FIP that is “arbitrary” or “capricious.” §7607(d)(9)(A). And they
argued that EPA’s decision to apply the FIP to them even after so
many other States had dropped out met that standard. As part of
their challenge, they asked that court to stay any effort to
enforce the FIP against them while their appeal unfolded. After
that court denied relief, the applicants renewed their request
here. The Court has received and reviewed over 400 pages of
briefing and a voluminous record, held over an hour of oral
argument on the applications, and engaged in months of postargument
deliberations as we often do for the cases we hear.

															II

															A

															Stay applications are nothing new. They seek a
form of interim relief perhaps “as old as the judicial system of
the nation.” Scripps-Howard Radio, Inc. v. FCC , 316 U.S.
4 , 17 (1942). Like any other federal court faced with a stay
request, we must provide the applicants with an answer—“grant or
deny.” Labrador v. Poe , 601 U. S. ___, ___
(2024) (Kavanaugh, J., concurring in grant of stay) (slip op., at
2).

															In deciding whether to issue a stay, we apply
the same “sound . . . principles” as other federal
courts. Nken v. Holder , 556 U.S.
418 , 434 (2009) (internal quotation marks omitted).
Specifically, in this litigation, we ask (1) whether the applicant
is likely to succeed on the merits, (2) whether it will suffer
irreparable injury without a stay, (3) whether the stay will
substantially injure the other parties interested in the
proceedings, and (4) where the public interest lies. Ibid. ;
States’ Application 13; Response in Opposition for Respondent EPA
in No. 23A349 etc., p. 16 (EPA Response).[ 9 ]

															When States and other parties seek to stay the
enforcement of a federal regulation against them, often “the harms
and equities [will be] very weighty on both sides.” Labrador , 601 U. S., at ___ (opinion of Kavanaugh, J.)
(slip op., at 3). That is certainly the case here, for both sides
have strong arguments with respect to the latter three Nken factors. On one side of the ledger, the federal government points
to the air-quality benefits its FIP offers downwind States. EPA
Response 48–50. On the other side, the States observe that a FIP
issued unlawfully (as they contend this one was) necessarily
impairs their sovereign interests in regulating their own
industries and citizens—interests the Act expressly recognizes. See
Part I–A, supra ; States’ Application 24–26; Maryland v. King , 567 U.S. 1301, 1303 (2012) (Roberts, C. J., in
chambers). The States observe, too, that having to comply with the
FIP during the pendency of this litigation risks placing them at a
“competitive disadvantage” to their exempt peers. States’
Application 21. The States and the private applicants also stress
that complying with the FIP during the pendency of this litigation
would require them to incur “hundreds of millions[,] if not
billions of dollars.” Tr. of Oral Arg. 96. Those costs, the
applicants note, are “nonrecoverable.” Thunder Basin Coal
Co. v. Reich , 510 U.S.
200 , 220–221 (1994) (Scalia, J., concurring in part and
concurring in judgment); see, e . g ., States’
Application 24; Application for American Forest & Paper
Association et al. 25; see also Alabama Assn. of
Realtors v. Department of Health and Human Servs. , 594
U.S. 758, 765 (2021) ( per curiam ).

															Because each side has strong arguments about the
harms they face and equities involved, our resolution of these stay
requests ultimately turns on the merits and the question who is
likely to prevail at the end of this litigation. See Nken ,
556 U. S., at 434; Labrador , 601 U. S., at ___
(opinion of Kavanaugh, J.) (slip op., at 4).

															B

															When it comes to that question, the parties
agree on the rules that guide our analysis. The applicants argue
that a court is likely to hold EPA’s final FIP “arbitrary” or
“capricious” within the meaning of the Act and thus enjoin its
enforcement against them. 42 U. S. C. §7607(d)(9)(A);
see, e . g ., States’ Application 15–16; Application for
American Forest & Paper Association et al. 14; see also 5
U. S. C. §706(2)(A). An agency action qualifies as
“arbitrary” or “capricious” if it is not “reasonable and reasonably
explained.” FCC v. Prometheus Radio Project , 592 U.S.
414, 423 (2021). In reviewing an agency’s action under that
standard, a court may not “ ‘substitute its judgment for that
of the agency.’ ” FCC v. Fox Television Stations,
Inc. , 556 U.S.
502 , 513 (2009). But it must ensure, among other things, that
the agency has offered “a satisfactory explanation for its
action[,] including a rational connection between the facts found
and the choice made.” Motor Vehicle Mfrs. Assn. of United
States, Inc. v. State Farm Mut. Automobile Ins. Co ., 463 U.S.
29 , 43 (1983) (internal quotation marks omitted). Accordingly,
an agency cannot simply ignore “an important aspect of the
problem.” Ibid .

															We agree with the applicants that EPA’s final
FIP likely runs afoul of these long-settled standards. The problem
stems from the way EPA chose to determine which emissions
“contribute[d] significantly” to downwind States’ difficulty
meeting national ozone standards. 42 U. S. C.
§7410(a)(2)(D)(i)(I). Recall that EPA’s plan rested on an
assumption that all 23 upwind States would adopt
emissions-reduction tools up to a “uniform” level of “costs” to the
point of diminishing returns. 87 Fed. Reg. 20076, 20095; 88 Fed.
Reg. 36661, 36683–36684, 36719; see Part I–C, supra . But as
the applicants ask: What happens—as in fact did happen—when many of
the upwind States fall out of the planned FIP and it may now cover
only a fraction of the States and emissions EPA anticipated? See, e . g ., States’ Application 16–21; Application for
American Forest & Paper Association et al. 14–15, 19–20.
Does that affect the “knee in the curve,” or the point at which the
remaining States might still “maximiz[e] cost-effectiv[e]” downwind
ozone-level improvements? 87 Fed. Reg. 20055. As “the mix of states
changes, . . . and their particular technologies and
industries drop out with them,” might the point at which
emissions-control measures maximize cost-effective downwind
air-quality improvements also shift? Tr. of Oral Arg. 6.

															Although commenters posed this concern to EPA
during the notice and comment period, see Part I–C, supra ,
EPA offered no reasoned response. Indeed, at argument the
government acknowledged that it could not represent with certainty
whether the cost-effectiveness analysis it performed collectively
for 23 States would yield the same results and command the same
emissions-control measures if conducted for, say, just one State.
Tr. of Oral Arg. 58–59. Perhaps there is some explanation why the
number and identity of participating States does not affect what
measures maximize cost-effective downwind air-quality improvements.
But if there is an explanation, it does not appear in the final
rule. As a result, the applicants are likely to prevail on their
argument that EPA’s final rule was not “reasonably explained,” Prometheus Radio Project , 592 U. S., at 423, that the
agency failed to supply “a satisfactory explanation for its
action[,]” State Farm Mut. Automobile Ins. Co ., 463
U. S., at 43, and that it instead ignored “an important aspect
of the problem” before it, ibid . The applicants are
therefore likely to be entitled to “revers[al]” of the FIP’s
mandates on them. §7607(d)(9).[ 10 ]

															III

															A

															Resisting this conclusion, EPA advances three
alternative arguments. First , the government insists, the agency
did offer a reasoned response to the applicants’ concern, just not
the one they hoped. When finalizing its rule in response to public
comments, the government represents, “the agency did consider whether the [FIP] could cogently be applied to a subset of
the 23 covered States.” EPA Response 27; see also post , at
17–18 (Barrett, J., dissenting). And that consideration, the
government stresses, led EPA to add a “severability” provision to
its final rule in which the agency announced that the FIP would
“ ‘continue to be implemented’ ” without regard to the
number of States remaining, even if just one State remained subject
to its terms. EPA Response 27 (quoting 88 Fed. Reg. 36693). In
support of its severability provision, EPA cited, among other
things, its intent to address “ ‘important public health and
environmental benefits” and encourage reliance by others “on th[e]
final rule in their planning.’ ” Ibid. None of this, however, solves the agency’s
problem. True, the severability provision highlights that EPA was
aware of the applicants’ concern. But awareness is not itself an
explanation. The severability provision highlights, too, the
agency’s desire to apply its rule expeditiously and “ ‘to the
greatest extent possible,’ ” no matter how many States it
could cover. Ibid. But none of that, nor anything else EPA
said in support of its severability provision, addresses whether
and how measures found to maximize cost effectiveness in achieving
downwind ozone air-quality improvements with the participation of
23 States remain so when many fewer States, responsible for a much
smaller amount of the originally targeted emissions, might be
subject to the agency’s plan. Put simply, EPA’s response did not
address the applicants’ concern so much as sidestep it.[ 11 ] Second , the government pivots in nearly
the opposite direction. Now, it says, if its final rule lacks a
reasoned response to the applicants’ concern, it is because no one
raised that concern during the public comment period. And, the
agency stresses, a litigant may pursue in court only claims
premised on objections first “ ‘raised with reasonable
specificity’ ” before the agency during the public comment
period. Id ., at 19–20 (quoting §7607(d)(7)(B)); see also post , at 8–11.

															We cannot agree. The Act’s “reasonable
specificity” requirement does not call for “a hair-splitting
approach.” Appalachian Power Co. v. EPA , 135 F.3d 791 , 817 (CADC 1998). A party need not “rehears[e]”
the identical argument made before the agency; it need only confirm
that the government had “notice of [the] challenge” during the
public comment period and a chance to consider “in substance, if
not in form, the same objection now raised” in court. Id. ,
at 818; see also, e.g. , Bahr v. Regan , 6
F. 4th 1059, 1070 (CA9 2021).

															Here, EPA had notice of the objection the
applicants seek to press in court. Commenters alerted the agency
that, should some States no longer participate in the plan, the
agency would need to return to the drawing board and “conduct a new
assessment and modeling of contribution” to determine what
emissions-control measures maximized cost effectiveness in securing
downwind ozone air-quality improvements. Comments of Air
Stewardship Coalition, at 13–14; see also Part I–C, supra (noting examples of other comments). And, as we have just seen,
EPA’s own statements and actions confirm the agency appreciated
that concern. In preparing the final rule in response to public
comments, the agency emphatically insists, it “ did consider
whether the [r]ule could cogently be applied to a subset of the 23
covered States.” EPA Response 27. And as a result of that
consideration, the agency observes, it opted to add a severability
provision to its final rule. Ibid. By its own words and
actions, then, the agency demonstrated that it was on notice of the
applicants’ concern. Yet, as we have seen, it failed to address the
concern adequately.[ 12 ] Third , the government pursues one more
argument in the alternative. As the agency sees it, the applicants
must return to EPA and file a motion asking it to reconsider its
final rule before presenting their objection in court. They must,
the agency says, because the “grounds for [their] objection arose
after the period for public comment.” §7607(d)(7)(B); see EPA
Response 20–21. As just discussed, however, EPA had the basis of
the applicants’ objection before it during the comment period. It
chose to respond with a severability provision that in no way
grappled with their concern. Nothing requires the applicants to
return to EPA to raise (again) a concern EPA already had a chance
to address.

															Taking the government’s argument (much) further,
the dissent posits that every “objection that [a] final rule was
not reasonably explained” must be raised in a motion for
reconsideration. Post , at 7 (internal quotation marks
omitted; emphasis deleted). But there is a reason why the
government does not go so far. The Clean Air Act opens the
courthouse doors to those with objections the agency already
ignored. If an “objection [is] raised with reasonable specificity
during the period for public comment” but not reasonably addressed
in the final rule, the Act permits an immediate challenge.
§7607(d)(7)(B). A person need not go back to the agency and insist
on an explanation a second time. Tellingly, the case on which the
dissent relies involves an entirely different situation: a
“ ‘logical outgrowth’ challeng[e].” Post , at 7. There,
the objection was that EPA had supposedly “ ‘significantly
amend[ed] the [r]ule between the proposed and final
versions,’ ” making it impossible for people to comment on the
rule during the comment period. Ibid. (quoting EME
Homer , 795 F. 3d, at 137). That is nothing like the
challenge here, where EPA failed to address an important problem
the public could and did raise during the comment period.

															B

															With the government’s theories unavailing, the
dissent advances others of its own. It begins by suggesting that
the problem the applicants raise was not
“ ‘ important ’ ” enough to warrant a reasoned reply
from the agency because the methodology EPA employed in its FIP
“appear[s] not to depend on the number of covered States.” Post , at 12–17, 18–19. Then, coming at the same point from
another direction, the dissent seeks to excuse the agency’s lack of
a reasoned reply as “harmless” given, again, “the apparent lack of
connection between the number of States covered and the FIP’s
methodology.” Post , at 20.

															The trouble is, if the government had arguments
along these lines, it did not make them. It did not despite its
ample resources and voluminous briefing. See supra , at 9.
This Court “normally decline[s] to entertain” arguments “forfeited”
by the parties. Kingdomware Technologies, Inc. v. United
States , 579 U.S. 162, 173 (2016). And we see no persuasive
reason to depart from that rule here.

															If anything, we see one reason for caution after
another. Start with the fact the dissent itself expresses little
confidence in its own theories, contending no more than it
“ appear [ s ]” EPA’s methodology did not depend on the
number of covered States. Post , at 14 (emphasis added). Add
to that the fact that, at oral argument, even the government
refused to say with certainty that EPA would have reached the same
conclusions regardless of which States were included in the FIP.
See Tr. of Oral Arg. 59. Combine all that with the further fact
that, in developing the FIP, EPA said it used the “same regulatory
framework” this Court described in EME Homer City Generation, L.
P. v. EPA , 572 U.S.
489 . E . g ., EPA Response 7–8. And, at least as the
Court described that framework, state-level analyses play a
significant role in EPA’s work.[ 13 ] Finally, observe that, while the Act seems to
anticipate, as the dissent suggests, that the agency’s “procedural
determinations” may be subject to harmless-error review,
§7607(d)(8), the Act also seems to treat separately challenges to
agency “actions” like the FIP before us, authorizing courts to
“reverse any . . . action,” found to be “arbitrary” or
“capricious,” §7607(d)(9)(A) (emphasis added). With so many reasons
for caution, we think sticking to our normal course of declining to
consider forfeited arguments the right course here.[ 14 ]

															*

															The applications for a stay in Nos. 23A349,
23A350, 23A351, and 23A384 are granted. Enforcement of EPA’s rule
against the applicants shall be stayed pending the disposition of
the applicants’ petitions for review in the United States Court of
Appeals for the D. C. Circuit and any petition for writ of
certiorari, if such writ is timely sought. Should the petition for
certiorari be denied, this order will terminate automatically. If
the petition is granted, this order shall terminate upon the
sending down of the judgment of this Court.

															It is so ordered . Notes 1 See 87 Fed. Reg. 9463
(2022) (Maryland); 87 Fed. Reg. 9484 (2022) (New York, New Jersey);
87 Fed. Reg. 9498 (2022) (Kentucky); 87 Fed. Reg. 9516 (2022) (West
Virginia); 87 Fed. Reg. 9533 (2022) (Missouri); 87 Fed. Reg. 9545
(2022) (Alabama, Mississippi, Tennessee); 87 Fed. Reg. 9838 (2022)
(Illinois, Indiana, Michigan, Minnesota, Ohio, Wisconsin); 87 Fed.
Reg. 9878 (2022) (Arkansas, Louisiana, Oklahoma,
Texas). 2 See 87 Fed. Reg. 31443
(California); 87 Fed. Reg. 31470 (2022) (Utah); 87 Fed. Reg. 31485
(2022) (Nevada); 87 Fed. Reg. 31495 (2022) (Wyoming). 3 EPA also added three more
States: Pennsylvania and Virginia, which had not submitted SIPs,
and Delaware, whose SIP, EPA said, it had approved in “error.” 87
Fed. Reg. 20036, 20038 (2022). 4 Commenters pointed out
the variance among emissions-producing facilities too. See, e . g ., Comments of Indiana Municipal Power Agency 9
(June 20, 2022) (the “cost effectiveness” of one tool “will be
highly variable” across different power plants); Comments of Lower
Colorado River Authority 21 (June 21, 2022) (power plants that
“have already invested” in one emissions-control tool “have already
undertaken significant costs to achieve [nitrous oxide] reductions
and have less to gain from additional control installation”);
Comments of Air Stewardship Coalition 27 (June 21, 2022) (noting
that the knee in the curve appeared to be at a different cost
depending on which mix of industries were considered); Comments of
Wisconsin Paper Council 2 (June 21, 2022) (the air-quality benefits
from controlling one industry—pulp and paper mills—had a “maximum
estimated improvement” in ozone levels in downwind States of just
0.0117 parts per billion). 5 The final FIP covered 23
States. 88 Fed. Reg. 36654, 36656 (2023). That plan included
Pennsylvania and Virginia, but EPA declined to cover Tennessee or
Wyoming at the time, even though it had announced its intention to
disapprove those States’ SIPs. Ibid. ; see also supra ,
at 4, and nn. 1–2. EPA has since proposed a plan for Tennessee and
several other States. 89 Fed. Reg. 12666 (2024). 6 See, e.g. , Order
in No. 23–60069 (CA5, June 8, 2023) (Mississippi); Order in No.
23–682 (CA9, July 3, 2023) (Nevada); Order in No. 23–1776 (CA8,
July 5, 2023) (Minnesota); Order in No. 23–3216 (CA6, July 25,
2023) (Kentucky); Order in No. 23–9520 etc. (CA10, July 27, 2023)
(Utah and Oklahoma); Order in No. 23–11173 (CA11, Aug. 17, 2023)
(Alabama); see also Order in No. 23–1418 (CA4, Aug. 10, 2023) (West
Virginia, pending oral argument on preliminary motions to stay and
to transfer); Order in No. 23–1418 (CA4, Jan. 10, 2024) (West
Virginia, after oral argument and pending merits review of
petition). 7 See 88 Fed. Reg. 49295
(2023) (Arkansas, Kentucky, Louisiana, Mississippi, Missouri, and
Texas); 88 Fed. Reg. 67102 (2023) (Alabama, Minnesota, Nevada,
Oklahoma, Utah, and West Virginia). EPA has since proposed settling
the litigation over the Nevada SIP disapproval. 89 Fed. Reg. 35091
(2024). 8 Of course, this could
change again as litigation over the SIP denials progresses past
preliminary stay litigation and toward final decisions on the
merits. 9 Approaching the
applications before us like any other stay request both accords
with the Clean Air Act’s text, see 42 U. S. C. §7607(d),
and usual practice in this field, see, e . g ., Texas v. EPA , 829 F.3d 405, 424 (CA5 2016); West
Virginia v. EPA , 90 F. 4th 323, 331 (CA4 2024); In re Murray Energy Corp. , 788 F.3d 330, 335 (CADC
2015). 10 Various applicants offer various
other reasons why they believe they are likely to succeed in
challenging EPA’s FIP. Having found that they are likely to succeed
on the basis discussed above, however, we have no occasion to
address those other arguments. 11 As
the applicants conceded at oral argument, see Tr. of Oral Arg.
25–26, EPA did not need to address every possible permutation when
it sought to adopt a multi-State FIP. Our conclusion is narrower:
When faced with comments like the ones it received, EPA needed to
explain why it believed its rule would continue to offer
cost-effective improvements in downwind air quality with only a
subset of the States it originally intended to cover. To be sure,
after this Court heard argument, EPA issued a document in which it
sought to provide further explanations for the course it pursued.
See 89 Fed. Reg. 23526 (2024). But the government has not suggested
that we should consult this analysis in assessing the validity of
the final rule. See Letter from E. Prelogar, Solicitor General, to
S. Harris, Clerk of Court 1 (Mar. 28, 2024). Nor could it, since
the Clean Air Act prevents us (and courts that may in the future
assess the FIP’s merits) from consulting explanations and
information offered after the rule’s promulgation. See 42
U. S. C. §§7607(d)(6)(C) (“The promulgated rule may not
be based (in part or whole) on any information or data which has
not been placed in the docket as of the date of such
promulgation”), 7607(d)(7)(A) (restricting the “record for judicial
review”). We therefore look to only “the grounds that the agency
invoked when it” promulgated the FIP. Michigan v. EPA , 576 U.S.
743 , 758 (2015). Should the applicants show the FIP was
arbitrary or capricious on the existing record, as we have
concluded is likely, the Clean Air Act entitles them to
“revers[al]” of that rule’s mandates on them.
§7607(d)(9)(A). 12 The
dissent resorts to a “hair-splitting approach” to the public
comments. Post , at 8–11. It stresses, for example, that some
comments highlighted variances among specific emissions-producing
facilities and industries, “not States.” Post , at 8
(emphasis deleted). But the dissent fails to acknowledge that, for
purposes of the FIP, States are a sum of their emissions-producing
facilities. See, e.g. , Ozone Transport Policy Analysis Final
Rule TSD 12 (EPA–HQ–OAR–2021–0668, 2023) (Final Ozone Analysis).
Similarly, the dissent characterizes the comment indicating EPA
would need to “conduct a new assessment and modeling” if States
dropped out of the FIP as a complaint about the “sequencing” of the
proposed SIP disapprovals and the FIP. Post , at 10. But why
would the sequencing matter? Because the FIP cannot apply to a
State if its SIP is not disapproved. See Part I–A, supra .
And why would EPA need to perform a “new assessment and modeling of
contribution”? Because it may be that “the math . . .
wouldn’t necessarily turn out the same” if some States were not
covered by the FIP. Tr. of Oral Arg. 59. Fairly on notice of the
concern, EPA needed to, and by its own admission sought to,
“consider” whether its FIP could apply to a subset of States. EPA
Response 27. 13 The
agency, we said, “first calculated, for each upwind State, the
quantity of emissions the State could eliminate at each of several
cost [levels]”; next, it “conducted complex modeling to establish
the combined effect of the upwind reductions projected at each cost
[level] would have on air quality in downwind States”; and only
after all that did the agency “then identif[y] significant cost
[levels]” to use in setting its emissions budgets. EME Homer
City , 572 U. S., at 501–502. 14 Admittedly, the dissent points to
some statements in the FIP suggesting EPA considered nationwide
data in parts of its analysis. See, e . g ., post , at 14; see also, e . g ., 88 Fed. Reg.
36721, 36727. But other statements in that rule and supporting
documents also seem to suggest EPA considered state-specific
information. See Part I–C, supra . If, as the dissent posits,
only nationwide data informed EPA’s analysis, why would EPA say
that, “for purposes of identifying the appropriate level of
control,” it focused on “the 23 upwind states that were linked” to
the downwind States, rather than, say, “all states in the
contiguous U.S.”? Final Ozone Analysis 3 (footnote omitted). Why
would EPA explain that its “findings regarding air quality
improvement” downwind were a “central component” of picking the
appropriate cost levels and so defining a State’s significant
contribution? 88 Fed. Reg. 36741. And why would EPA bother to
“determine the relationship between changes in emissions and
changes in ozone contributions on a state-by-state . . .
basis” and “calibrat[e]” that relationship “based on state-specific
source apportionment”? Final Ozone Analysis 43. In asking these
questions, we do not profess answers; we simply highlight further
reasons for caution. SUPREME COURT OF THE UNITED STATES

															_________________

															Nos. 23A349, 23A350, 23A351 and 23A384

															_________________

															OHIO, et al.

															23A349 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															KINDER MORGAN, INC., et al.

															23A350 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															AMERICAN FOREST & PAPER ASSOCIATION,
et al.

															23A351 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															UNITED STATES STEEL CORPORATION

															23A384 v. ENVIRONMENTAL PROTECTION AGENCY,
et al.

															on applications for stay

															[June 27, 2024]

															Justice Barrett, with whom Justice Sotomayor,
Justice Kagan, and Justice Jackson join, dissenting.

															The Court today enjoins the enforcement of a
major Environmental Protection Agency rule based on an
underdeveloped theory that is unlikely to succeed on the merits. In
so doing, the Court grants emergency relief in a fact- intensive
and highly technical case without fully engaging with both the
relevant law and the voluminous record. While the Court suggests
that the EPA failed to explain itself sufficiently in response to
comments, this theory must surmount sizable procedural obstacles
and contrary record evidence. Applicants therefore cannot satisfy
the stringent conditions for relief in this posture.

															I

															I will start by setting the record straight
with respect to some important background. First , the Court downplays EPA’s
statutory role in ensuring that States meet air-quality standards. Ante , at 2–3. The Clean Air Act directs EPA to “establish
national ambient air quality standards (NAAQS) for pollutants at
levels that will protect public health.” EPA v. EME Homer
City Generation, L. P. , 572 U.S.
489 , 498 (2014); see 42 U. S. C. §§7408, 7409. States
must create State Implementation Plans (SIPs) to ensure that their
air meets these standards. §7410(a)(1). But States also face an
externality problem: “Pollutants generated by upwind sources are
often transported by air currents . . . to downwind
States,” relieving upwind States “of the associated costs” and
making it difficult for downwind States to “maintain satisfactory
air quality.” EME , 572 U. S., at 496. So the Act’s Good
Neighbor Provision requires SIPs to “prohibi[t]” the State’s
emissions sources from “emitting any air pollutant in amounts which
will . . . contribute significantly to nonattainment in,
or interfere with maintenance by, any other State with respect to
any [NAAQS].” §7410(a)(2)(D)(i)(I).

															Given the incentives of upwind States to
underregulate the pollution they send downwind, the Act requires
EPA to determine whether a State “has failed to submit an adequate
SIP.” EME , 572 U. S . , at 498; see §7410(c)(1).
If a SIP does not prevent the State’s polluters from significantly
contributing to nonattainment in downwind States, EPA “shall”
promulgate a Federal Implementation Plan (FIP) that does.
§7410(c)(1). And EPA must stop the State’s significant
contributions by the statutory deadline for the affected downwind
States to achieve compliance. See Wisconsin v. EPA ,
938 F.3d 303, 313–314 (CADC 2019) ( per curiam );
§7511. Second , the Court fails to recognize that
EPA’s SIP disapprovals may, in fact, be valid. EPA justified its
findings that 23 States had failed to submit adequate SIPs. It
found that these States all significantly contributed to ozone
pollution in downwind States. See 88 Fed. Reg. 36656 (2023). But 21
of these States, including applicants, proposed to do nothing to reduce their ozone-precursor ( i.e ., NOx)
emissions—arguing that they did not actually contribute to downwind
nonattainment or that there were no other cost-effective
emissions-reduction measures they could impose. See 88 Fed. Reg.
9354–9361 (2023). The other two States failed to submit a SIP at
all. See 84 Fed. Reg. 66614 (2019). While 12 of EPA’s SIP
disapprovals have been temporarily stayed, no court yet has
invalidated one. So EPA’s replacement FIP—the Good Neighbor
Plan—may yet apply to all 23 original States. Indeed, EPA and the
plaintiffs who challenged Nevada’s SIP disapproval have proposed a
settlement that would lift that stay. 89 Fed. Reg. 35091
(2024). Third , the Court claims that commenters
on the proposed FIP warned that its emissions limits might change
if it covered fewer States, but EPA failed to respond. Ante ,
at 6–8. Not exactly. As I will elaborate below, commenters merely
criticized EPA’s decision to propose a FIP before its SIP
disapprovals were final. EPA responded that this sequencing was
“consistent with [its] past practice in [its] efforts to timely
address good neighbor obligations”: Given the August 2024 deadline
for certain States to comply with the 2015 ozone NAAQS, EPA was
“obligated” to start the years-long process of promulgating a FIP
so that one could be effective in time. EPA, Response to Public
Comments on Proposed Rule 149–150, (EPA–HQ–OAR–2021–0668–1127, June
2023) (Response to Comments); see Wisconsin , 938 F. 3d,
at 313–314. Finally , the Court repeatedly
characterizes the FIP as relying on an “assumption that [it] would
apply to all covered States.” Ante , at 6; see ante ,
at 12. But try as it might, the Court identifies no evidence that
the FIP’s emissions limits would have been different for a
different set of States or that EPA’s consideration of
state-specific inputs was anything but confirmatory of the limits
it calculated based on nationwide data. See ante , at 5–6,
19, n. 14. The Court leans on the fact that EPA “considered data
specific to the emissions-producing facilities in [each] State” to
calculate “how much each upwind State’s [NOx] emissions would fall”
if the State’s emitters “adopted each [emissions-control] measure.” Ante , at 5 (citing EPA, Ozone Transport Policy Analysis
Proposed Rule TSD 9–10, 13, 22–23, (EPA–HQ–OAR–2021–0668–0133, Feb.
2022) (Proposed Ozone Analysis)). But the Proposed Ozone Analysis
makes clear that EPA did these state-specific calculations to
determine each State’s “emissions budget.” Proposed Ozone Analysis
7–13. A State’s budget consists of the “emissions that would
remain” after the State’s power plants meet the emissions
limits that EPA independently calculated. 88 Fed. Reg. 36762; see
Proposed Ozone Analysis 13 (“adjust[ed]” “unit-level emissions are
summed up to the state level”); n. 6, infra . Of course
each State’s emissions budget will depend on the emitters in that
State. What matters is whether the limits the FIP imposes on
each emitter depend on the number of States the FIP covers.
Tellingly, the Court does not identify any NOx limit for any
industry that relied on state-specific data.

															On the contrary, as I will explain in Part II–B,
the final rule and its supporting documents suggest that EPA’s
methodology for setting emissions limits did not depend on the
number of States in the plan, but on nationwide data for the
relevant industries—and the FIP contains many examples of emissions
limits that EPA created using nationwide inputs. Moreover, EPA has
now confirmed this interpretation. During this litigation, EPA
received petitions seeking reconsideration of the FIP on the ground
that it should not be implemented in just a subset of the original
States. EPA denied these petitions on April 4, 2024. 89 Fed. Reg.
23526. It thoroughly explained how its “methodology for defining”
each State’s emissions obligations is “independent of the number of
states included in the Plan” because it “relies on a determination
regarding what emissions reductions each type of regulated source
can cost-effectively achieve.” EPA, Basis for Partial Denial of
Petitions for Reconsideration on Scope 1,
(EPA–HQ–OAR–2021–0668–1255, Apr. 2024) (Denial). The “control
technologies and cost-effectiveness figures the EPA consider[ed]
. . . do not depend in any way on the number of states
included.” Id ., at 2. So “[s]ources in the remaining upwind
states currently regulated by the Plan . . . would bear
the same actual emission reduction obligations” regardless of the
number of covered States. Id. , at 3–4.

															II

															To obtain emergency relief, applicants must,
at a minimum, show that they are likely to succeed on the merits,
that they will be irreparably injured absent a stay, and that the
balance of the equities favors them. Nken v. Holder , 556 U.S.
418 , 425–426 (2009). Moreover, we should grant relief only if
we would be likely to grant certiorari were the applicants’ case to
come to us in the usual course. See Does 1–3 v. Mills , 595 U. S. ___, ___ (2021) (Barrett, J.,
concurring in denial of application for injunctive relief ); Hollingsworth v. Perry , 558 U.S.
183 , 190 (2010) ( per curiam ). In my view, the
applicants cannot satisfy the stay factors. Most significantly,
they have not shown a likelihood of success on the merits.

															The Court holds that applicants are likely to
succeed on a claim that the Good Neighbor Plan is “arbitrary” or
“capricious.” 42 U. S. C. §7607(d)(9). The
“arbitrary-and-capricious standard requires that agency action” be
both “[1] reasonable and [2] reasonably explained.” FCC v. Prometheus Radio Project , 592 U.S. 414, 423 (2021). The
Court’s theory is that EPA did not “ ‘reasonably explai[n] ’ ” “why the number and identity of
participating States does not affect what measures maximize
cost-effective downwind air-quality improvement.” Ante , at
13 (quoting only the second part of Prometheus Radio ’s
formulation (emphasis added)). So to be clear, the Court does not
conclude that EPA’s actions were substantively unreasonable— e . g ., that the FIP cannot rationally be
applied to fewer States because a change in the number of
participants would undermine its rationale or render it
ineffective. Nor could it, given the significant evidence in the
record (not to mention EPA’s denial of reconsideration) that the
covered States did not, in fact, affect the plan’s
emissions-reduction obligations. See Part II–B, infra . Thus,
the only basis for the Court’s decision is the argument that EPA
failed to provide “ ‘a satisfactory explanation for its
action’ ” and a “reasoned response” to comments. Ante ,
at 12–13 (quoting Motor Vehicle Mfrs. Assn. of United States,
Inc. v. State Farm Mut. Automobile Ins. Co. , 463 U.S.
29 , 43 (1983)). There are at least three major barriers to
success on such a claim.

															A

															The Clean Air Act imposes a procedural bar on
the challenges that a plaintiff can bring in court: Only objections
that were “raised with reasonable specificity during the period for
public comment . . . may be raised during judicial
review.” §7607(d)(7)(B). If it was “impracticable to raise such
objection within such time or if the grounds for such objection
arose after the period for public comment,” the challenger may
petition for reconsideration of the rule and can obtain judicial
review only if EPA refuses. Ibid. While EPA has now
separately denied petitions for reconsideration of the Good
Neighbor Plan, this case came to us directly; we are assessing
applicants’ likelihood of success in

															challenging the plan itself, not the denial of
reconsideration. So the procedural bar on objections not raised in
the comments presents a significant obstacle—in two ways. First , consider the Court’s basic theory:
that EPA offered “no reasoned response” to comments allegedly
questioning whether the plan’s emissions limits depend on the
States covered. Ante , at 12. That EPA failed to adequately
explain its final rule in response to comments is “an objection to
the notice and comment process itself,” which applicants “obviously
did not and could not have raised . . . during the period
for public comment.” EME Homer City Generation,
L. P. v. EPA , 795 F.3d 118, 137 (CADC 2015)
(Kavanaugh, J.). No one could have raised during the proposal’s
comment period the objection that the “ final rule was
not ‘reasonably explained.’ ” Ante , at 13 (emphasis
added).

															The D. C. Circuit, on remand in EME
Homer , considered a similar objection that EPA had “violated
the Clean Air Act’s notice and comment requirements”: EPA had
“significantly amend[ed] the Rule between the proposed and final
versions without providing additional opportunity for notice and
comment.” 795 F. 3d, at 137. But because this procedural
objection could not have been raised during the comment period,
“the only appropriate path for petitioners” under §7607(d)(7)(B)
was to raise it “through an initial petition for reconsideration to
EPA.” Ibid. So the D. C. Circuit lacked “authority at
th[at] time to reach this question.” Ibid. While such
“logical outgrowth” challenges typically are cognizable under the
Administrative Procedure Act, see Shell Oil Co. v. EPA , 950 F.2d 741, 747 (CADC 1991), the Clean Air Act
channels these challenges through reconsideration proceedings. This
Court’s failure-to-explain objection may face the same problem: It
is not judicially reviewable in its current posture.[ 1 ] Second , even putting aside this aspect of
§7607(d)(7)(B), it is not clear that any commenter raised with
“reasonable specificity” the underlying substantive issue: that the
exclusion of some States from the FIP would undermine EPA’s
cost-effectiveness analyses and resulting emissions controls.
§7607(d)(7)(B); see ante , at 13. The Court concludes
otherwise only by putting in the commenters’ mouths words they did
not say. It first cites a bevy of comments arguing that EPA’s
“disapprovals of the SIPs were legally flawed” and noting the
obvious point that EPA cannot “include a State in its FIP” unless
it validly disapproves the State’s SIP. Ante , at 6. These
comments do not address the continued efficacy of a FIP that
applies to a subset of the originally covered States.

															Another collection of the Court’s inapposite
comments relates to the inclusion of specific sources, emissions
controls, and industries in the proposed plan— not States.
See ante , at 7, n. 4. For example, one commenter argued
that the “cost effectiveness of the requirement to employ SNCR will
be highly variable, and is unlikely to meet EPA expectations in
even the most optimistic case.” Comments of Indiana Municipal Power
Agency 9 (June 20, 2022). That is a challenge to EPA’s endorsement
of a particular emissions-control technology; it says nothing about
the FIP’s dependence on a particular number of States. See also, e . g ., Comments of Lower Colorado River Authority
21–22 (June 21, 2022). Similarly, another commenter argued that
pulp and paper mills should not be included because the “maximum
estimated improvement” in ozone levels from controlling their
emissions would be “too small to even measure.” Comments of
Wisconsin Paper Council 2 (June 21, 2022).[ 2 ] An argument that the maximum benefits from
regulating an industry are too small is not an argument that those
benefits would become too small if fewer States were
covered.[ 3 ]

															The closest comment that the Court can
find—which it quotes repeatedly—is one sentence that obliquely
refers to some “new assessment and modeling of contribution” that
EPA might need to perform. Comments of Air Stewardship Coalition
13–14 (June 21, 2022). The Court dresses up this comment by
characterizing it as a warning about what might happen “[i]f the
FIP did not wind up applying to all 23 States” and responding to
the concern that a “different set of States might mean that the
‘knee in the curve’ might shift” and change the cost-effective
“emissions-control measures.” Ante , at 7. But those words
are the Court’s, not the commenter’s.

															The commenter’s actual objection was to EPA’s
sequencing of its actions—proposing a FIP before it finalized its
SIP disapprovals. The commenter titled this section “EPA Step Two
Screening is Premised on the Premature Disapproval of 19 Upwind
States[’] Good Neighbor SIPs.” Air Stewardship Comments 13
(boldface omitted). And the relevant sentence reads in full:

															“The proposed FIP essentially prejudges
the outcome of those pending SIP actions and, in the event EPA
takes a different action on those SIPs than contemplated in this
proposal, it would be required to conduct a new assessment and
modeling of contribution and subject those findings to public
comment.” Id ., at 14.

															This sentence says nothing about what would be
required if after EPA finalizes its SIP disapprovals and
issues a final FIP, some States drop out of the plan. Nor does it
suggest that the plan’s cost-effectiveness thresholds or emissions
controls would change with a different number of States. Nor is it
clear what the comment means by its bare reference to a “new
assessment and modeling of contribution”: Would EPA be required to
perform a new evaluation of which upwind States cause pollution in
downwind States? A new analysis of how much pollution each source
must eliminate? A new assessment of the plan’s impact on downwind
States?

															It is therefore difficult to see how this
comment raised with “reasonable specificity” the objection that the
removal of some States from the final plan would invalidate EPA’s
cost-effectiveness thresholds and chosen emissions-control
measures.[ 4 ] That is not how
EPA understood it. EPA characterized this comment as arguing that
“by taking action before considering comments on the proposed
disapprovals, the EPA is presupposing the outcome of its proposed
rulemakings on the SIPs.” Response to Comments 147 (noting this
comment’s ID number, 0518). And EPA explained that it “disagree[d]”
with the argument that the “sequence” of its actions was “improper,
unreasonable, or bad policy”; EPA had a statutory obligation to
promulgate a FIP by the August 2024 NAAQS attainment deadline. Id ., at 150. If a commenter had said with reasonable
specificity what the Court says today—that “a different set
of States might mean that the ‘knee in the curve’ might shift,” ante , at 7—EPA could have responded with more explanation of
why its methodology did not depend on the number of covered
States—as it has recently explained. But EPA cannot be penalized if
it did not have reasonable notice of this objection.[ 5 ]

															In sum, §7607(d)(7)(B)’s procedural bar likely
forecloses both the failure-to-explain objection that the Court
credits and any substantive challenge to the reasonableness of
applying the FIP to a subset of the originally covered States.

															B

															Even if applicants clear §7607(d)(7)(B)’s
procedural bar, they face an uphill battle on the merits. To
prevail on the Court’s theory, applicants must show that EPA’s
actions were “arbitrary” or “capricious.” §§7607(d)(9)(A), (D).
“The scope of review under the ‘arbitrary and capricious’ standard
is narrow and a court is not to substitute its judgment for that of
the agency.” State Farm , 463 U. S., at 43. A rule is
arbitrary and capricious if the agency “ entirely failed to
consider an important aspect of the problem.” Ibid. (emphasis added). But we will “ ‘uphold a decision of less
than ideal clarity if the agency’s path may reasonably be
discerned.’ ” Ibid. (quoting Bowman Transp.,
Inc. v. Arkansas-Best Freight System, Inc. , 419 U.S.
281 , 286 (1974)). Given the explanations and state-agnostic
methodology apparent in the final rule and its supporting
documentation—and the paucity of comments specifically raising the
issue—EPA may well have done enough to justify its plan’s
severability.

															To begin, the rule and its supporting documents
arguably make clear that EPA’s methodology for calculating
cost-effectiveness thresholds and imposing emissions controls did
not depend on the number of covered States. The rule applied EPA’s
longstanding “4-step interstate transport framework” to create
emissions limits that will prevent NOx sources in upwind States
from significantly contributing to ozone pollution in downwind
States. 88 Fed. Reg. 36659; see 42 U. S. C.
§7410(a)(2)(D). Under that framework, EPA (1) identifies “downwind
receptors that are expected to have problems attaining or
maintaining the NAAQS”; (2) identifies which upwind States are
“ ‘link[ed]’ ” to those downwind receptors because they
contribute at least 1% of a receptor’s

															ozone; (3) determines which NOx sources in the
linked upwind States “significantly contribute” to downwind
nonattainment or interference; and (4) implements emissions limits
to stop those sources’ significant contributions. 88 Fed. Reg.
36659; see EME , 572 U. S., at 500–501 (describing
similar approach used in earlier FIP). The first two steps
determine which States the FIP must cover. The rubber meets the
road at steps 3 and 4: How much do sources in those States
“significantly contribute” to downwind pollution, and what must
they do about it?

															Here is how EPA explains that methodology. A
source “significantly contributes” to downwind pollution if there
are cost-effective measures it could implement to reduce its
emissions: It must halt those emissions that can be eliminated at a
cost “under the cost threshold set by the Agency” for sources in
that industry. EME , 572 U. S., at 518 (upholding this
approach). So the “ ‘amount’ of pollution” that sources must
eliminate is “that amount . . . in excess of the
emissions control strategies the EPA has deemed cost effective.” 88
Fed. Reg. 36676. EPA calculates for each type of source a “uniform
level of NOx emissions control stringency” expressed as a “cost per
ton of emissions reduction.” Id ., at 36719. This
cost-effectiveness threshold is based on the point “at which
further emissions mitigation strategies become excessively costly
on a per-ton basis while also delivering far fewer additional
emissions reductions.” Id ., at 36683 (describing this
“ ‘knee in the curve’ ” analysis). The plan requires
sources in each covered State to reduce their emissions
accordingly.[ 6 ]

															Crucially, the final rule suggests that EPA
calculated cost-effectiveness thresholds based on the likely cost
and impact of available emissions-reduction technology given national, industry-wide data . Contrary to the Court’s
speculations, ante , at 12, these thresholds and the FIP’s
resulting emissions limits appear not to depend on the number of
covered States. Consider the plan’s approach to power plants
(“electric generating units,” or EGUs). EPA assessed the cost and
impact of different NOx mitigation strategies that EGUs could
implement. One strategy was to fully operate “selective catalytic
reduction” (SCR) technology. 88 Fed. Reg. 36655; see id ., at
36720. EPA estimated that a “representative marginal cost” for this
strategy would be $1,600 per ton, and a “reasonable level of
performance” would be 0.08 lb/mmBtu—based on “nationwide” power
plant “emissions data.” Id ., at 36720–36721. EPA thus
determined that SCR optimization was a “viable mitigation strategy
for the 2023 ozone season” and built this assumption into the
plan’s emissions limits. Id ., at 36720. In other words, EPA
relied on nationwide industry data to select cost thresholds that
corresponded to how much it would cost to use particular
emissions-reduction technologies, and it applied that “uniform
control stringency to EGUs within the covered upwind states.” Id ., at 36680.[ 7 ]

															In fact, some commenters criticized EPA’s
reliance on a “nationwide data set” to calculate emissions limits,
arguing that EPA should “limit the dataset to . . . just
the covered states”—an approach that would have made the
cost- effectiveness thresholds depend on which States were covered. Id ., at 36723. But EPA expressly defended its approach based
on its “intention to identify a technology-specific representative emissions rate” and its interest in “the performance
potential of a technology ”—which were best served by the
“largest dataset possible ( i . e ., nationwide ).” Id ., at 36723–36724 (emphasis added). EPA explained that it
used the same approach it had successfully applied in previous
rulemakings: It “derive[d] technology performance averages” based
on nationwide data. Id ., at 36724. Then it applied the
relevant industry standard “on a uniform basis” to each emitter
across the covered States. Id ., at 36817.[ 8 ]

															The Court, perhaps recognizing the problem that
the FIP’s seemingly state-agnostic methodology poses for its
theory, throws at the wall a cherry-picked assortment of EPA
statements mentioning state data. See ante , at 5–6, 19, n.
14. None stick. The fundamental problem with the Court’s citations
is that they discuss analyses that EPA performed after it
chose cost thresholds and emissions limits based on nationwide
industry data. EPA did assess the impact on downwind States if
particular upwind States met the proposed emissions limits, and
that impact depended on the States included in the modeling. Ante , at 5, 19, n. 14. But EPA said that these
“ ‘findings regarding air quality improvement,’ ” ante , at 19, n. 14 (quoting 88 Fed. Reg. 36741), served
only to “ cement EPA’s identification of the selected
. . . mitigation measures as the appropriate control
stringency,” 88 Fed. Reg. 36741 (emphasis added); see Denial 18.
EPA explained that the statutory requirement to “eliminate
significant contribution” depends on the implementation of
cost-effective emissions controls at individual “industrial
sources,” not some overall impact on “downwind areas’
nonattainment and maintenance problems.” 88 Fed. Reg. 36741. EPA
assessed the FIP’s impact assuming the participation of particular
States primarily to ensure that its emissions limits did not result
in “overcontrol”— i . e ., more reductions than necessary
to help downwind States comply with the NAAQS. Ibid. ; see EME Homer City , 572 U. S., at 521. The technical
document that the Court cites, ante, at 5, makes this point
clear: “The downwind air quality impacts are used to inform EPA’s
assessment of potential overcontrol.” Proposed Ozone Analysis
31.

															EPA’s analysis confirmed that its chosen
emissions limits would not result in overcontrol if they were
implemented in the States originally covered by the FIP. 88 Fed.
Reg. 36741. Importantly, implementing the FIP “in fewer upwind
states does not (and cannot possibly) result in overcontrol” given
that “there was no overcontrol even when more states, making more
emission reductions, were included.” Denial 22. So the fact that
EPA used state-specific data in its overcontrol analysis does not
mean that the FIP’s emissions limits depended on the number of
States it covered. And the inclusion of fewer States in that
analysis logically could not have affected the results.

															Thus, EPA generally characterized the FIP’s
emissions limits as dependent on nationwide data, not on any
particular set of States.[ 9 ]
Confirming this interpretation, the final rule contemplates its
application to a different number of States. It recognizes that
“states may replace FIPs with SIPs if EPA approves them,” and
several sections explain how States may exit this FIP. 88
Fed. Reg. 36753, 36838–36843. And the rule’s severability provision
explains that EPA views the plan as “severable along
. . . state and/or tribal jurisdictional lines.” Id ., at 36693.

															Moreover, EPA justified the FIP’s severability:
EPA “must address good neighbor obligations as expeditiously as
practicable and by no later than the next applicable attainment
date”; severability serves “important public health and
environmental benefits” and ensures that stakeholders can “rely on
this final rule in their planning.” Ibid . These rationales
align with EPA’s response to critics of its decision to propose a
FIP before finalizing its SIP disapprovals: Quickly proposing a
FIP—just like keeping the FIP in place even if some States drop
out—“is a reasonable and prudent means of assuring that [EPA’s]
statutory obligation to reduce air pollution affecting the health
and welfare of people in downwind states is implemented without
delay.” Response to Comments 151.

															Given these justifications and the
state-agnostic methodology apparent in the final rule, EPA’s
“ ‘path may reasonably be discerned.’ ” State
Farm , 463 U. S., at 43. The FIP’s cost thresholds and
emissions limits did not depend in any significant way on the
number of States included, so the drawbacks of severability were
minimal. On the other hand, severability was necessary so that EPA
could fulfill, to the greatest extent possible, its statutory
obligation to eliminate the significant ozone contributions of
upwind States and reduce harmful pollution in downwind States in
time to meet the attainment deadlines. See Response to Comments 150
(noting the August 2024 ozone-NAAQS attainment deadline). If the
FIP were not severable, EPA would have to go back to the drawing
board for all States whenever a single State is
removed—thwarting its mission for little reason.[ 10 ]

															Finally, it is unlikely that EPA’s response to
comments evinces a “fail[ure] to consider an important aspect of the problem.” State Farm , 463 U. S., at 43
(emphasis added). An agency must respond to
“ ‘ relevant ’ and ‘ significant ’ public comments,”
and that requirement is not “particularly demanding”; the “agency
need not respond at all to comments that are ‘purely speculative
and do not disclose the factual or policy basis on which they
rest.’ ” Public Citizen, Inc. v. FAA , 988 F.2d
186, 197 (CADC 1993) (quoting Home Box Office, Inc. v. FCC , 567 F.2d 9, 35, and n. 58 (CADC 1977); emphasis
added); see §7607(d)(6)(B) (EPA must respond to “significant”
comments). EPA received hundreds of comments, and its response
numbered nearly 1,100 pages. Given the likelihood that the FIP’s
emissions limits did not depend on the covered States, the risk of
it applying to fewer States may not be “important,” and comments
purportedly raising that possibility might not be “relevant” and
“significant.” Moreover, the one comment that vaguely referred to a
need for a “new assessment and modeling,” Air Stewardship Comments
14, was “purely speculative” and “disclose[d]” no “factual or
policy basis”; it likely merited no response, Home Box
Office , 567 F. 2d, at 35, n. 58. Requiring more from
EPA risks the “sort of unwarranted judicial examination of
perceived procedural shortcomings” that might “seriously interfere
with that process prescribed by Congress.” Vermont Yankee
Nuclear Power Corp. v. Natural Resources Defense Council,
Inc. , 435 U.S.
519 , 548 (1978).[ 11 ]

															C

															Applicants face one more impediment: the Clean
Air Act’s stringent harmless-error rule. A court “reviewing alleged
procedural errors . . . may invalidate [an EPA] rule only
if the errors were so serious and related to matters of such central relevance to the rule that there is a substantial likelihood that the rule would have been significantly changed if such errors had not been made.”
§7607(d)(8) (emphasis added). This provision appears “tailor-made
to undo” any “rigid presumption of vacatur” that might apply in
other contexts. N. Bagley, Remedial Restraint in Administrative
Law, 117 Colum. L. Rev. 253, 291 (2017).

															The alleged error here plausibly is subject to
§7607(d)(8)’s harmless-error rule. As explained above, the Court
does not suggest that it is substantively “[un]reasonable”
to apply the FIP to fewer States, only that EPA did not “reasonably
explai[n]” the FIP’s severability in response to comments. Prometheus , 592 U. S., at 423. That is arguably an
“alleged procedural error” within the meaning of §7607(d)(8). In
fact, the Act contemplates that at least some “arbitrary or
capricious” challenges allege failures to “observ[e]
. . . procedure required by law,” and such challenges may
only succeed if §7607(d)(8)’s “condition is . . . met.”
§7607(d)(9)(D).

															If the Act’s harmless-error rule applies,
applicants are unlikely to prevail. Given the apparent lack of
connection between the number of States covered and the FIP’s
methodology for determining cost thresholds and emissions limits,
it is difficult to imagine a “substantial” likelihood that the rule
would have been “significantly” different had EPA just responded
more thoroughly. In fact, applicants seem to have conceded as much.
See Tr. of Oral Arg. 6 (“[W]ith full candor to the Court, [the cost
threshold] could be the same or even be more expensive”); id ., at 9 (“I can’t tell you what that looks like, whether
there is a difference in the obligations or not”). And EPA, the
Court says, had “notice” of the alleged concern that the cost
thresholds might change with different States. Ante , at 15.
Yet EPA still chose to make the FIP severable because of its
statutory obligation to reduce downwind pollution—an obligation it
repeatedly referenced. See, e . g ., 88 Fed. Reg. 36693;
Response to Comments 149–151. Would that same EPA have
“significantly changed” the FIP had it just explained more
thoroughly why the plan did not depend on the States
covered?[ 12 ] And on top of
all this, EPA has in fact refused to reconsider the FIP now that it
applies to fewer States, explaining in detail why its methodology
was unaffected by the States it covered.[ 13 ]

															*  *  *

															With little to say in response to the FIP’s
apparent state-agnostic methodology for setting emissions limits
and the Clean Air Act’s stringent harmless-error rule, the Court
resorts to raising forfeiture. Ante , at 17–19. But it is the
Court that goes out of its way to develop a failure-to-explain
theory largely absent from applicants’ briefs. One can search
diligently in the hundreds of pages of applicants’ opening briefs
for the Court’s theory—that EPA failed to explain in its final rule
why the FIP’s cost-effectiveness thresholds for imposing emissions
limits do not shift with a different mix of States—and be left
wondering where the Court found it. That theory appears not to have
crystallized until oral argument, during which counsel for the
state applicants struggled to locate it in the States’ brief. Tr.
of Oral Arg. 11–12. Consider just one illustrative example. Given
the importance to the Court’s theory of how the “knee in the curve”
might change with different States, see ante , at 6, 7, and
n. 4, 12, one might expect to find some mention of that idea
in applicants’ briefs. One would be wrong.

															Given that applicants’ theory has evolved
throughout the course of this litigation, we can hardly fault EPA
for failing to raise every potentially meritorious defense in its
response brief. That is particularly true given the compressed
briefing schedule in this litigation’s emergency posture: The Court
gave EPA less than two weeks to respond to multiple applications
raising a host of general and industry-specific technical
challenges, filed less than a week earlier. Even still, EPA raised
§7607(d)(7)(B)’s procedural bar. Brief for Respondents 19. And on
the merits, EPA expressly argued that the FIP’s “viability and
validity do not depend on the number of jurisdictions it covers”;
the “Rule need not apply to any minimum number of States in order
to operate coherently.” Id ., at 24. EPA could also have
demonstrated how the FIP’s state-agnostic methodology for selecting
cost thresholds was apparent in the final rule. But EPA cannot have
forfeited that more specific point because applicants did not raise
it to begin with.

															Because EPA did not forfeit these responses to
the merits of applicants’ arbitrary-or-capricious challenge, there
is no need to consider whether a departure from our typical
approach to forfeited arguments is justified. See ante , at
18. It remains applicants’ burden to show that the FIP’s
alleged dependence on the covered States likely was an “important”
problem that EPA “entirely failed to consider.” State Farm ,
463 U. S., at 43. And that is on top of their burden to
overcome §7607(d)(7)(B)’s procedural bar and the lack of
“significant,” specific comments raising this issue.
§7607(d)(6)(B).

															Finally, I would exercise our discretion to
consider §7607(d)(8)’s harmless-error rule. Even putting aside the
expedited briefing schedule and the limited discussion of the
Court’s theory in applicants’ briefs, applicants bear the burden in
seeking emergency relief to show a likelihood of success on
the merits. In other words, we must predict whether
applicants will overcome every barrier to relief at the end of the
day, after full merits briefing and argument in the lower courts
and, potentially, again in this Court. Section 7607(d)(8)’s
harmless-error rule is one such important obstacle, and EPA has
already signaled that it will raise it as litigation progresses.
See Denial 35, n. 38 (arguing that any failure to more fully
explain “how the Rule is not interdependent” is harmless error
under §7607(d)(8)). I see no reason not to consider it now.

															III

															Given the emergency posture of this
litigation, my views on the merits of the failure-to-explain
objection and the application of the Clean Air Act’s procedural bar
and harmless-error rule are tentative. But even a tentative adverse
conclusion can undermine applicants’ likelihood of success. And
applicants, to prevail, must run the table; they face the daunting
task of surmounting all of these significant obstacles. They
are unlikely to succeed.

															The Court, seizing on a barely briefed
failure-to-explain theory, grants relief anyway. It enjoins the
Good Neighbor Plan’s enforcement against any state or industry
applicant pending review in the D. C. Circuit and any petition
for certiorari. Ante , at 19. Given the number of companies
included and the timelines for review, the Court’s injunction
leaves large swaths of upwind States free to keep contributing
significantly to their downwind neighbors’ ozone problems for the
next several years—even though the temporarily stayed SIP
disapprovals may all be upheld and the FIP may yet cover all the
original States. The Court justifies this decision based on an
alleged procedural error that likely had no impact on the plan. So
its theory would require EPA only to confirm what we already know:
EPA would have promulgated the same plan even if fewer States were
covered. Rather than require this years-long exercise in futility,
the equities counsel restraint.

															Our emergency docket requires us to evaluate
quickly the merits of applications without the benefit of full
briefing and reasoned lower court opinions. See Does 1–3 ,
595 U. S., at ___ (opinion of Barrett, J.). Given those
limitations, we should proceed all the more cautiously in cases
like this one with voluminous, technical records and thorny legal
questions. I respectfully dissent. Notes 1 The Court offers a feeble
response to this application of §7607(d)(7)(B)’s procedural bar. Ante , at 17. It simply quotes §7607(d)(7)(B) and asserts
without support that it means that a plaintiff “need not go back to
the agency and insist on an explanation a second time.” Ibid . The Court fails to engage with the logic of this
argument: The objection that the final rule did not contain
sufficient explanation was not and could not have been raised
during the comment period, so it must be raised in a petition for
reconsideration. EME Homer , 795 F. 3d, at 137. The
Court claims that its theory is different from the
logical-outgrowth challenge the D. C. Circuit considered in EME Homer . Ante , at 17. But the Court ignores the
fact that its failure-to-explain challenge and logical-outgrowth
challenges are both “objection[s] to the notice and comment process
itself ” that depend on the content of the final rule. EME
Homer , 795 F. 3d, at 137. Even if the public raised an
“important problem . . . during the comment period,” ante , at 17, the Court’s basis for enjoining the FIP’s
enforcement is not that the alleged problem is real, but that the
final rule did not address it. 2 The Court claims that in
distinguishing comments about particular industries from comments
that question whether the plan depends on a number of States, I
“fai[l] to acknowledge” that the FIP treats States as the “sum of
their emissions-producing facilities.” Ante , at 16,
n. 12. But in reality, it is the Court that ignores how the
FIP works. The FIP determines emissions limits for particular sources based on their industries ; the total NOx
emissions limit for each State is simply the sum of the limits the
plan imposes on each of the State’s sources. See, e . g ., 88 Fed. Reg. 36678, 36762; Part II–B, infra . So comments critiquing a particular industry-specific
emissions limit or technology assumption say nothing about the
FIP’s dependence on a certain number of States. 3 Nor did the Air
Stewardship Coalition’s comment about the “knee in the curve” raise
concerns about which States are included. See ante , at 7,
n. 4. Rather, this comment questioned EPA’s proposed average
cost-effectiveness threshold of $7,500 per ton for non-power-plant
sources; it argued that EPA should use different thresholds for
different industries. Comments of Air Stewardship Coalition 27
(June 21, 2022) (Air Stewardship Comments). It did not link its
concern about cost thresholds to the States covered by the
plan. 4 So too with Portland
Cement’s comment. See ante , at 7. That comment simply echoes
the quoted sentence from the Air Stewardship Coalition almost word
for word, also in the context of objecting to EPA’s decision to
propose a FIP before finalizing its SIP disapprovals. Comments of
Portland Cement Association 7 (June 21, 2022). 5 The Court concludes to
the contrary only by building out the comment’s bare reference to a
“new assessment and modeling” with its own inferences about the
possible effect of different numbers of States on “the math.” Ante , at 16, n. 12 (internal quotation marks omitted).
But as explained above, the comment itself said nothing about
States dropping out of the final plan or the possible impact of
different numbers of States on the FIP’s cost thresholds or
emissions limits. It is hard to believe that a single sentence with
no elaboration or explanation of the potential issue—in a sea of
thousands of pages of comments—gave EPA reasonable notice that it
should have included in its final rule a detailed explanation of
why the FIP’s emissions limits did not depend on the number of
States. Cf. Public Citizen, Inc. v. FAA , 988 F.2d
186, 197 (CADC 1993) (An “agency need not respond at all to
comments that are ‘purely speculative and do not disclose the
factual or policy basis on which they rest’ ”). 6 For power plants, EPA
implements these requirements by allocating each State an
“ ‘emissions budget’ . . . representing the EPA’s
quantification of the emissions that would remain” if plants in
that State eliminated all the emissions that EPA determines can be
eliminated for less than the cost threshold. 88 Fed. Reg. 36762.
EPA then allocates tradeable “ ‘allowances’ ”
proportionally among the State’s sources, creating a marketplace
for emissions. Ibid . With respect to other sources, EPA
determined that nine industries in the covered States produced the
most significant emissions. Id ., at 36817. The rule requires
sources in each of those industries to meet specific emissions
limits that were calculated based on the reductions that they can
cost-effectively achieve. Ibid. 7 EPA ultimately selected
(based on nationwide data) a cost threshold of $1,800 per ton of
NOx reduction that would apply in earlier years, and a cost
threshold of $11,000 per ton that would apply in later years. 88
Fed. Reg. 36749, 36846. These cost thresholds corresponded to the
cost of different emissions-control measures: For example, EGUs can
“retrofi[t ] state-of-the-art combustion controls” and
“[o]ptimiz[e] idled SCRs” for less than $1,800 per ton, and they
can “instal[l  new] SCRs” for less than $11,000 per ton. Ozone
Transport Policy Analysis Final Rule TSD 5
(EPA–HQ–OAR–2021–0668–1080, Mar. 2023) (Final Ozone Analysis). EPA
then calculated each State’s emissions budget based on the
assumption that the State’s EGUs would implement the
emissions-reductions strategies that cost less than the chosen
thresholds. See n. 6, supra ; 88 Fed. Reg. 36762; Final
Ozone Analysis 6, 9. Given the likelihood that EPA selected cost
thresholds based on nationwide data, each State’s budget would be
the same even if the FIP covered different States. 8 While EPA’s methodology
with respect to other industrial sources (non-EGUs) was more
complicated, it also seems to have relied on nationwide data. EPA
chose a “$7,500 marginal cost-per-ton threshold,” 88 Fed. Reg.
36740, which corresponded to the point of diminishing returns (the
“knee in the curve”) when EPA assessed the impact of emissions
controls in the highest impact industries and in “all industries”
on the total “ozone season NOx reduction potential,” Technical
Memorandum, Screening Assessment of Potential Emissions Reductions,
Air Quality Impacts, and Costs From Non-EGU Emissions Units for
2026, p. 4, (EPA–HQ–OAR–2021–0668–150, Feb. 2022) (boldface
omitted). This figure thus appears to have been determined based on
industry-wide cost and emissions data rather than state-specific
calculations. So too with the specific emissions limits EPA decided
could be implemented for less than that cost. See, e.g. , 88
Fed. Reg. 36825 (“EPA based the proposed emissions limits for
cement kilns on the types of limits being met across the
nation”). 9 The Court argues that EPA
equated the framework it used here with the one that we described
in EME Homer City . Ante , at 18, and n. 13. But
even if EME described an approach that selected cost
thresholds “only after” conducting downwind air-quality
assessments, ibid. , it is not clear that the Good Neighbor
Plan adopted this aspect of the EME framework. In
fact, there are other key similarities between this FIP and EME ’s approach. For example, the final FIP refers to EME in order to note that EPA’s “uniform framework of policy
judgments”— i . e ., applying the same cost thresholds
with “[n]ationwide consistency”—was upheld in that case. 88 Fed.
Reg. 36673. And the final FIP identifies cost thresholds that EPA
chose based on nationwide data—like $1,800 based on the cost of
EGUs “optimiz[ing] . . . existing SCRs and SNCRs.” Id ., at
36749. 10 The
Court claims that the severability clause is evidence that EPA “had
notice of the objection the applicants seek to press in court,” yet
EPA’s justifications for it did not address (alleged) concerns
about how the cost-effectiveness thresholds would change with fewer
States. Ante , at 15–16. But as explained above, commenters
did not raise that issue with specificity; they simply pointed out
that some SIP disapprovals might be invalid. The severability
clause is evidence that EPA was aware of that possibility,
and the clause was EPA’s response to it. 11 Despite the Court’s suggestion of
forfeiture, ante , at 17, EPA could not have forfeited the
argument that the comments the Court cites were too insubstantial
to merit a response. The Court relies on comments that were not
raised until the applicants’ reply briefs or that were uncovered
later by the Court itself. See, e . g ., Reply Brief in
No. 23A351, p. 11 (raising the Air Stewardship Coalition “modeling”
comment for the first time). 12 The
Court faults EPA for “refus[ing] to say with certainty” at oral
argument that it would have reached the same conclusions if
different States were included in the FIP. Ante , at 18. But
§7607(d)(8) does not require the Government to show that the rule
would be the same; it is most naturally read to require the
challenger to demonstrate a “substantial likelihood that the rule would have been significantly changed if such errors had not
been made.” (Emphasis added.) And though §7607(d)(9)(A) appears to
allow reversal of “ ‘any’ ” arbitrary or capricious
“ ‘action,’ ” ante , at 19, §7607(d)(8)’s more
specific harmless-error rule and §7607(d)(9)(D)’s more specific
requirements for reversal based on arbitrary or capricious
procedural errors would seem to control. 13 The
Court claims that the Clean Air Act prevents us from considering
EPA’s denial of reconsideration. Ante , at 14–15, n. 11.
But it is not obvious that the relevant provision of the
Act—§7607(d)(7)(A)’s definition of the “record for judicial
review”—bars consideration of later developments for purposes of
the Act’s stringent harmless-error rule, §7607(d)(8). Even assuming
that the denial of reconsideration itself cannot count as
evidence of harmlessness, we are judging applicants’ likelihood of success on the merits. On the merits, we can
expect EPA to make just the sort of arguments it made in its
denial: EPA likely will explain why the covered States did not
matter by citing and interpreting material in the record. See, e . g ., Denial 11 (“Record Basis Establishing Why the
Plan Functions Independently by State”); id ., at 15, and
nn. 16–18 (citing the final rule and technical support
documents on the rulemaking docket).