Document ID: ./input/supremecourt_opinions/opinions/06pdf/05-1120.pdf
Page Number: 1.0

(Slip Opinion) 

OCTOBER  TERM,  2006 

1 

Syllabus 

NOTE:  Where  it  is  feasible,  a  syllabus  (headnote)  will  be  released,  as  is
being  done  in  connection  with  this  case,  at  the  time  the  opinion  is  issued.
The  syllabus  constitutes  no  part  of  the  opinion  of  the  Court  but  has  been
prepared  by  the  Reporter  of  Decisions  for  the  convenience  of  the  reader. 
See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337. 

SUPREME COURT OF THE UNITED STATES 

Syllabus 

MASSACHUSETTS ET AL. v. ENVIRONMENTAL PRO
-
TECTION AGENCY ET AL. 

CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR 
THE DISTRICT OF COLUMBIA CIRCUIT 

No. 05–1120.  Argued November 29, 2006—Decided April 2, 2007 

Based  on  respected  scientific  opinion  that  a  well-documented  rise  in
global temperatures and attendant climatological and environmental 
changes have resulted from a significant increase in the atmospheric 
concentration of “greenhouse gases,” a group of private organizations 
petitioned the Environmental Protection Agency (EPA) to begin regu-
lating the emissions of four such gases, including carbon dioxide, un-
der  §202(a)(1)  of  the  Clean  Air  Act,  which  requires  that  the  EPA
“shall  by  regulation  prescribe  . . .  standards  applicable  to  the  emis-
sion of any air pollutant from any class . . . of new motor vehicles . . . 
which  in  [the  EPA  Administrator’s]  judgment  cause[s],  or  contrib-
ute[s]  to,  air  pollution  . . .  reasonably  . . .  anticipated  to  endanger
public  health  or  welfare,”  42  U. S. C.  §7521(a)(1).    The  Act  defines 
“air pollutant” to include “any air pollution agent . . . , including any 
physical,  chemical  . . .  substance  . . .  emitted  into  . . .  the  ambient
air.”    §7602(g).    EPA  ultimately  denied  the  petition,  reasoning  that
(1)  the  Act  does  not  authorize  it  to  issue  mandatory  regulations  to
address global climate change, and (2) even if it had the authority to
set greenhouse gas emission standards, it would have been unwise to
do  so  at  that  time  because  a  causal  link  between  greenhouse  gases
and  the  increase  in  global  surface  air  temperatures  was  not  un-
equivocally  established.    The  agency  further  characterized  any  EPA
regulation of motor-vehicle emissions as a piecemeal approach to cli-
mate change that would conflict with the  President’s comprehensive 
approach  involving  additional  support  for  technological  innovation,
the  creation  of  nonregulatory  programs  to  encourage  voluntary  pri-
vate-sector  reductions  in  greenhouse  gas  emissions,  and  further  re-
search  on  climate  change,  and  might  hamper  the  President’s  ability