Document ID: ./input/supremecourt_opinions/opinions/06pdf/05-1120.pdf
Page Number: 7

Cite as:  549 U. S. ____ (2007) 

1 

Opinion of the Court 

NOTICE:  This opinion is subject to formal revision before publication in the
preliminary  print  of  the  United  States  Reports.  Readers  are  requested  to
notify the Reporter of Decisions, Supreme Court of the United States, Wash-
ington,  D. C.  20543,  of  any  typographical  or  other  formal  errors,  in  order
that corrections may be made before the preliminary print goes to press. 

SUPREME COURT OF THE UNITED STATES 

_________________ 

No. 05–1120 
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MASSACHUSETTS, ET AL., PETITIONERS v. ENVIRON-
MENTAL PROTECTION AGENCY ET AL. 

ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF

APPEALS FOR THE DISTRICT OF COLUMBIA CIRCUIT

[April 2, 2007]

 JUSTICE STEVENS delivered the opinion of the Court. 
A  well-documented  rise  in  global  temperatures  has
coincided  with  a  significant  increase  in  the  concentration
of carbon dioxide in the atmosphere.  Respected scientists
believe the two trends are related.  For when carbon diox-
ide is released into the atmosphere, it acts like the ceiling 
of  a  greenhouse,  trapping  solar  energy  and  retarding  the
escape  of  reflected  heat. 
It  is  therefore  a  species—the
most important species—of a “greenhouse gas.” 

Calling  global  warming  “the  most  pressing  environ-
mental  challenge  of  our  time,”1  a  group  of  States,2  local 
governments,3  and  private  organizations,4  alleged  in  a 

—————— 

1 Pet. for Cert. 22. 
2 California,  Connecticut,  Illinois,  Maine,  Massachusetts,  New  Jersey, 

New Mexico, New York, Oregon, Rhode Island, Vermont, and Washington. 
3 District of Columbia, American Samoa, New York City, and Baltimore. 
4 Center  for  Biological  Diversity,  Center  for  Food  Safety,  Conserva-
tion  Law  Foundation,  Environmental  Advocates,  Environmental 
Defense,  Friends  of  the  Earth,  Greenpeace,  International  Center  for 
Technology  Assessment,  National  Environmental  Trust,  Natural 
Resources Defense Council, Sierra Club, Union of Concerned Scientists, 
and U. S. Public Interest Research Group.