Document ID: ./input/supremecourt_opinions/opinions/06pdf/05-1120.pdf
Page Number: 9.0

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

3 

Opinion of the Court 

to endanger public health or welfare . . . .”7 

The  Act  defines  “air  pollutant”  to  include  “any  air  pollu-
tion  agent  or  combination  of  such  agents,  including  any
physical, chemical, biological, radioactive . . . substance or 
matter  which  is  emitted  into  or  otherwise  enters  the  am-
bient  air.”  §7602(g).    “Welfare”  is  also  defined  broadly: 
among other things, it includes “effects on . . . weather . . .
and climate.”  §7602(h).

When  Congress  enacted  these  provisions,  the  study  of
climate change was in its infancy.8  In 1959, shortly after 
the  U. S.  Weather  Bureau  began  monitoring  atmospheric 
carbon  dioxide  levels,  an  observatory  in  Mauna  Loa,  Ha-
waii, recorded a mean level of 316 parts per million.  This 
was well above the highest carbon dioxide concentration—
no  more  than  300  parts  per  million—revealed  in  the 
420,000-year-old  ice-core  record.9    By  the  time  Congress 

—————— 

7 The  1970  version  of  §202(a)(1)  used  the  phrase  “which  endangers
the  public  health  or  welfare”  rather  than  the  more-protective  “which
may  reasonably  be  anticipated  to  endanger  public  health  or  welfare.” 
See  §6(a)  of  the  Clean  Air  Amendments  of  1970,  84  Stat.  1690.    Con-
gress amended §202(a)(1) in 1977 to give its approval to the decision in 
Ethyl Corp. v. EPA, 541 F. 2d 1, 25 (CADC 1976) (en banc), which held
that  the  Clean  Air  Act  “and  common  sense  . . .  demand  regulatory
action  to  prevent  harm,  even  if  the  regulator  is  less  than  certain  that
harm  is  otherwise  inevitable.”    See  §401(d)(1)  of  the  Clean  Air  Act
Amendments  of  1977,  91  Stat.  791;  see  also  H. R.  Rep.  No.  95–294, 
p. 49 (1977). 

8 The  Council  on  Environmental  Quality  had  issued  a  report  in  1970
concluding  that  “[m]an  may  be  changing  his  weather.”    Environmental 
Quality: The First Annual Report 93.  Considerable uncertainty remained
in  those  early  years,  and  the  issue  went  largely  unmentioned  in  the 
congressional debate over the enactment of the Clean Air Act.  But see 
116  Cong.  Rec.  32914  (1970)  (statement  of  Sen.  Boggs  referring  to
Council’s  conclusion  that  “[a]ir  pollution  alters  the  climate  and  may
produce global changes in temperature”). 

9 See  Intergovernmental  Panel  on  Climate  Change,  Climate  Change
2001: Synthesis Report, pp. 202–203  (2001).  By drilling through thick 
Antarctic  ice  sheets  and  extracting  “cores,”  scientists  can  examine  ice