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Page Number: 4

4 

MASSACHUSETTS v. EPA 

Syllabus 

first step might be tentative does not by itself negate federal-court ju-
risdiction.    And  reducing  domestic  automobile  emissions  is  hardly 
tentative.  Leaving aside the other greenhouse gases, the record indi-
cates that the U. S. transportation sector emits an enormous quantity
of carbon dioxide into the atmosphere.  Pp. 20–21.

(d) While  regulating  motor-vehicle  emissions  may  not  by  itself 
reverse global warming, it does not follow that the Court lacks juris-
diction to decide whether EPA has a duty to take steps to slow or re-
duce it.  See Larson v. Valente, 456 U. S. 228, 243, n. 15.  Because of 
the enormous potential consequences, the fact that a remedy’s effec-
tiveness  might  be  delayed  during  the  (relatively  short)  time  it  takes
for a new motor-vehicle fleet to replace an older one is essentially ir-
relevant.  Nor is it dispositive that developing countries are poised to
substantially  increase  greenhouse  gas  emissions:  A  reduction  in  do-
mestic  emissions  would  slow  the  pace  of  global  emissions  increases,
no matter what happens elsewhere.  The Court attaches considerable 
significance to EPA’s espoused belief that global climate change must 
be addressed.  Pp. 21–23. 

2. The scope of the Court’s review of the merits of the statutory is-
sues is narrow.  Although an agency’s refusal to initiate enforcement
proceedings  is  not  ordinarily  subject  to  judicial  review,  Heckler  v. 
Chaney,  470  U. S.  821,  there  are  key  differences  between  nonen-
forcement and denials of rulemaking petitions that are, as in the pre-
sent  circumstances,  expressly  authorized.    EPA  concluded  alterna-
tively in its petition denial that it lacked authority under §7521(a)(1)
to  regulate  new  vehicle  emissions  because  carbon  dioxide  is  not  an
“air pollutant” under §7602, and that, even if it possessed authority,
it would decline to exercise it because regulation would conflict with
other  administration  priorities.    Because  the  Act  expressly  permits 
review of such an action, §7607(b)(1), this Court “may reverse [it if it
finds  it  to  be]  arbitrary,  capricious,  an  abuse  of  discretion,  or  other-
wise not in accordance with law,” §7607(d)(9).  Pp. 24–25.

3. Because  greenhouse  gases  fit  well  within  the  Act’s  capacious 
definition of “air pollutant,” EPA has statutory authority to regulate 
emission  of  such  gases  from  new  motor  vehicles.    That  definition— 
which  includes  “any  air  pollution  agent  . . .  ,  including any  physical,
chemical,  . . .  substance  . . .  emitted  into  . . .  the  ambient  air  . . . ,” 
§7602(g)  (emphasis  added)—embraces  all  airborne  compounds  of 
whatever  stripe.    Moreover,  carbon  dioxide  and  other  greenhouse
gases  are  undoubtedly  “physical  [and]  chemical  . . .  substance[s].” 
Ibid.  EPA’s reliance on postenactment congressional actions and de-
liberations  it  views  as  tantamount  to  a  command  to  refrain  from 
regulating  greenhouse  gas  emissions  is  unavailing.  Even  if  pos-
tenactment legislative history could shed light on the meaning of an