Document ID: ./input/supremecourt_opinions/opinions/06pdf/05-1120.pdf
Page Number: 36.0

30 

MASSACHUSETTS v. EPA 

Opinion of the Court 

the Clean Air Act’s capacious definition of “air pollutant,”
we hold that EPA has the statutory authority to regulate
the emission of such gases from new motor vehicles. 

VII 
The alternative basis for EPA’s decision—that even if it 
does  have  statutory  authority  to  regulate  greenhouse
gases,  it  would  be  unwise  to  do  so  at  this  time—rests  on 
reasoning  divorced  from  the  statutory  text.
  While  the 
statute  does  condition  the  exercise  of  EPA’s  authority  on
its formation of a “judgment,” 42 U. S. C. §7521(a)(1), that 
judgment  must  relate  to  whether  an  air  pollutant
“cause[s],  or  contribute[s]  to,  air  pollution  which  may
reasonably  be  anticipated  to  endanger  public  health  or 
welfare,” ibid.  Put another way, the use of the word “judg-
ment”  is  not  a  roving  license  to  ignore  the  statutory  text.
It  is  but  a  direction  to  exercise  discretion  within  defined 
statutory limits. 

If EPA makes a finding of endangerment, the Clean Air 
Act requires the agency to regulate emissions of the dele-
terious  pollutant  from  new  motor  vehicles.    Ibid.  (stating
that  “[EPA]  shall  by  regulation  prescribe  . . .  standards
applicable  to  the  emission  of  any  air  pollutant  from  any 
class  of  new  motor  vehicles”).    EPA  no  doubt  has  signifi-
cant latitude as to the manner, timing, content, and coor-
dination  of  its  regulations  with  those  of  other  agencies.
But once EPA has responded to a petition for rulemaking,
its  reasons  for  action  or  inaction  must  conform  to  the 
authorizing  statute.  Under  the  clear  terms  of  the  Clean 
Air  Act,  EPA  can  avoid  taking  further  action  only  if  it
determines  that  greenhouse  gases  do  not  contribute  to
climate change or if it  provides some reasonable explana-
tion  as  to why  it  cannot  or  will not  exercise  its  discretion
to  determine  whether  they  do.  Ibid.    To  the  extent  that 
this constrains agency discretion to pursue other priorities 
of the Administrator or the President, this is the congres-
sional design.