Document ID: ./input/supremecourt_opinions/opinions/23pdf/23a349_0813.pdf
Page Number: 35.0

Cite as:  603 U. S. ____ (2024) 

13 

BARRETT, J., dissenting 

ozone; (3) determines which NOx sources in the linked up-
wind States “significantly contribute” to downwind nonat-
tainment  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 (describ-
ing  similar  approach  used  in  earlier  FIP).    The  first  two 
steps determine which States the FIP must cover.  The rub-
ber meets the road at steps 3 and 4: How much do sources 
in those States “significantly contribute” to downwind pol-
lution, 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 elimi-
nated at a cost “under the cost threshold set by the Agency” 
for sources  in that  industry.   EME,  572  U. S.,  at  518 (up-
holding 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 effec-
tive.”  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 be-
come excessively costly on a per-ton basis while also deliv-
ering  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 

—————— 

6 For power plants, EPA implements these requirements by allocating 
each State an “ ‘emissions budget’ . . . representing the EPA’s quantifica-
tion  of  the emissions  that  would  remain” if  plants  in  that  State  elimi-
nated all the emissions that EPA determines can be eliminated for less 
than the cost threshold.  88 Fed. Reg. 36762.  EPA then allocates trade-
able “ ‘allowances’ ” proportionally among the State’s sources, creating a