Document ID: ./input/supremecourt_opinions/opinions/21pdf/20-1530_n758.pdf
Page Number: 13

Cite as:  597 U. S. ____ (2022) 

7 

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
emissions  from  existing  coal  and  gas  plants. 
See 
§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