Friday, 

April 24, 2009 

Part III 

Environmental 

Protection Agency 

40 CFR Chapter 1 

Proposed Endangerment and Cause or 

Contribute Findings for Greenhouse 

Gases Under Section 202(a) of the Clean 

Air Act; Proposed Rule 


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18886 Federal Register / Vol. 74, No. 78 / Friday, April 24, 2009 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Chapter 1 

[EPA–HQ–OAR–2009–0171; FRL–8895–5] 

RIN 2060–ZA14 

Proposed Endangerment and Cause or 

Contribute Findings for Greenhouse 

Gases Under Section 202(a) of the 

Clean Air Act 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Proposed rule. 

SUMMARY: Today the Administrator is 

proposing to find that greenhouse gases 

in the atmosphere endanger the public 

health and welfare of current and future 

generations. Concentrations of 

greenhouse gases are at unprecedented 

levels compared to the recent and 

distant past. These high atmospheric 

levels are the unambiguous result of 

human emissions, and are very likely 

the cause of the observed increase in 

average temperatures and other climatic 

changes. The effects of climate change 

observed to date and projected to occur 

in the future—including but not limited 

to the increased likelihood of more 

frequent and intense heat waves, more 

wildfires, degraded air quality, more 

heavy downpours and flooding, 

increased drought, greater sea level rise, 

more intense storms, harm to water 

resources, harm to agriculture, and harm 

to wildlife and ecosystems—are effects 

on public health and welfare within the 

meaning of the Clean Air Act. In light 

of the likelihood that greenhouse gases 

cause these effects, and the magnitude 

of the effects that are occurring and are 

very likely to occur in the future, the 

Administrator proposes to find that 

atmospheric concentrations of 

greenhouse gases endanger public 

health and welfare within the meaning 

of Section 202(a) of the Clean Air Act. 

She proposes to make this finding 

specifically with respect to six 

greenhouse gases that together 

constitute the root of the climate change 

problem: carbon dioxide, methane, 

nitrous oxide, hydrofluorocarbons, 

perfluorocarbons, and sulfur 

hexafluoride. 

The Administrator is also proposing 

to find that the combined emissions of 

carbon dioxide, methane, nitrous oxide, 

and hydrofluorocarbons from new 

motor vehicles and new motor vehicle 

engines are contributing to this mix of 

greenhouse gases in the atmosphere. 

Thus, she proposes to find that the 

emissions of these substances from new 

motor vehicles and new motor vehicle 

engines are contributing to air pollution 

which is endangering public health and 

welfare under section 202(a) of the 

Clean Air Act. 

DATES: Comments on this proposed 

action must be received on or before 

June 23, 2009. If you submitted 

comments on the issues raised by this 

proposal in dockets for other Agency 

efforts (e.g., the Advance Notice of 

Proposed Rulemaking on Regulating 

Greenhouse Gases under the Clean Air 

Act), you must still submit your 

comments to the docket for this action 

(EPA–HQ–OAR–2009–0171) by the 

deadline if you want them to be 

considered. 

There will be two public hearings. 

One hearing will be held on May 18, 

2009 in Arlington, VA. The other 

hearing will be on May 21, 2009 in 

Seattle, WA. To obtain information 

about the public hearings or to register 

to speak at the hearings, please see the 

SUPPLEMENTARY INFORMATION section 

below or go to http://www.epa.gov/ 

climatechange/endangerment.html. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–HQ– 

OAR–2009–0171, by one of the 

following methods: 

• Federal eRulemaking Portal: http:// 

www.regulations.gov. Follow the online 

instructions for submitting comments. 

• E-mail: GHG–EndangermentDocket@epa.gov. 

• Fax: (202) 566–1741. 

• Mail: Environmental Protection 

Agency, EPA Docket Center (EPA/DC), 

Mailcode 6102T, Attention Docket ID 

No. EPA–HQ–OAR–2009–0171, 1200 

Pennsylvania Avenue, NW., 

Washington, DC 20460. 

• Hand Delivery: EPA Docket Center, 

Public Reading Room, EPA West 

Building, Room 3334, 1301 Constitution 

Avenue, NW., Washington, DC 20004. 

Such deliveries are only accepted 

during the Docket’s normal hours of 

operation, and special arrangements 

should be made for deliveries of boxed 

information. 

Instructions: Direct your comments to 

Docket ID No. EPA–HQ–OAR–2009– 

0171. EPA’s policy is that all comments 

received will be included in the public 

docket without change and may be 

made available online at http:// 

www.regulations.gov, including any 

personal information provided, unless 

the comment includes information 

claimed to be CBI or other information 

whose disclosure is restricted by statute. 

Do not submit information that you 

consider to be CBI or otherwise 

protected through http:// 

www.regulations.gov or e-mail. The 

http://www.regulations.gov Web site is 

an ‘‘anonymous access’’ system, which 

means EPA will not know your identity 

or contact information unless you 

provide it in the body of your comment. 

If you send an e-mail comment directly 

to EPA without going through http:// 

www.regulations.gov your e-mail 

address will be automatically captured 

and included as part of the comment 

that is placed in the public docket and 

made available on the Internet. If you 

submit an electronic comment, EPA 

recommends that you include your 

name and other contact information in 

the body of your comment and with any 

disk or CD–ROM you submit. If EPA 

cannot read your comment due to 

technical difficulties and cannot contact 

you for clarification, EPA may not be 

able to consider your comment. 

Electronic files should avoid the use of 

special characters, any form of 

encryption, and be free of any defects or 

viruses. 

Docket: All documents in the docket 

are listed in the http:// 

www.regulations.gov index. Although 

listed in the index, some information is 

not publicly available, e.g., CBI or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy. Publicly available docket 

materials are available either 

electronically in http:// 

www.regulations.gov or in hard copy at 

the Air Docket, EPA/DC, EPA West, 

Room B102, 1301 Constitution Ave., 

NW., Washington, DC. This Docket 

Facility is open from 8:30 a.m. to 4:30 

p.m., Monday through Friday, excluding 

legal holidays. The telephone number 

for the Public Reading Room is (202) 

566–1744, and the telephone number for 

the Air Docket is (202) 566–1742. 

FOR FURTHER INFORMATION CONTACT: 

Jeremy Martinich, Climate Change 

Division, Office of Atmospheric 

Programs (MC–6207J), Environmental 

Protection Agency, 1200 Pennsylvania 

Ave., NW., Washington, DC 20460; 

telephone number: (202) 343–9927; fax 

number: (202) 343–2202; e-mail 

address: ghgendangerment@epa.gov. 

Please use this contact information for 

general questions only. Official 

comments must be submitted using the 

instructions above. 

SUPPLEMENTARY INFORMATION: 

Additional Information on Public 

Hearings: The two public hearings will 

be held on May 18 in Arlington, VA, 

and on May 21, 2009, in Seattle, WA. 

Both hearings will begin at 9 a.m. and 

end at 8 p.m., respective local times. 

Addresses: The hearings will be held 

at the following locations: 


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1. Arlington, VA: One Potomac Yard, 

2777 S. Crystal Drive, Arlington, VA 

22202. 

2. Seattle, WA: Bell Harbor 

International Conference Center, 2211 

Alaskan Way, Pier 66, Seattle, WA 

98121. 

The public hearings will provide 

interested parties the opportunity to 

present data, views, or arguments 

concerning the proposed findings. The 

EPA may ask clarifying questions during 

the oral presentations, but will not 

respond to the presentations at that 

time. Written statements and supporting 

information submitted during the 

comment period will be considered 

with the same weight as any oral 

comments and supporting information 

presented at the public hearings. 

Written comments must be received by 

the last day of the comment period, as 

specified in the proposal. 

To obtain additional information 

about the public hearings or to register 

to speak at the hearings, please go to: 

http://www.epa.gov/climatechange/ 

endangerment.html. Alternatively, 

contact Jeremy Martinich at 202–343– 

9927. Verbatim transcripts of the 

hearings and written statements will be 

included in the rulemaking docket. 

What Should I Consider as I Prepare 

My Comments for EPA? 

1. Submitting CBI 

Do not submit this information to EPA 

through www.regulations.gov or e-mail. 

Clearly mark the part or all of the 

information that you claim to be 

confidential business information (CBI). 

For CBI information in a disk or CD 

ROM that you mail to EPA, mark the 

outside of the disk or CD ROM as CBI 

and then identify electronically within 

the disk or CD ROM the specific 

information that is claimed as CBI. In 

addition to one complete version of the 

comment that includes information 

claimed as CBI, a copy of the comment 

that does not contain the information 

claimed as CBI must be submitted for 

inclusion in the public docket. 

Information so marked will not be 

disclosed except in accordance with 

procedures set forth in 40 CFR part 2. 

2. Tips for Preparing Your Comments 

When submitting comments, 

remember to: 

• Explain your views as clearly as 

possible. 

• Describe any assumptions that you 

used. 

• Provide any technical information 

and/or data you used that support your 

views. 

• Provide specific examples to 

illustrate your concerns. 

• Offer alternatives. 

• Make sure to submit your 

comments by the comment period 

deadline identified. 

• To ensure proper receipt by EPA, 

identify the appropriate docket 

identification number in the subject line 

on the first page of your response. It 

would also be helpful if you provided 

the name, date, and Federal Register 

citation related to your comments. 

Table of Contents 

I. Introduction 

A. Summary 

B. Background Information Helpful to 

Understanding This Proposal 

1. Greenhouse Gases and Their Effects 

2. Statutory Basis for This Proposal 

3. The Supreme Court’s Decision in 

Massachusetts v. EPA 

a. The Petition of the International Center 

for Technology Assessment 

b. The Supreme Court’s Decision 

4. EPA’s Advance Notice of Proposed 

Rulemaking on Regulating Greenhouse 

Gases Under the Clean Air Act 

C. Solicitation of Comments 

II. Legal Framework for This Action 

A. Section 202(a)—Endangerment and 

Cause or Contribute 

1. The Statutory Language 

2. Origin of the Current Statutory Language 

a. Ethyl Corp. v. EPA 

b. The 1977 Clean Air Act Amendments 

3. Additional Considerations for the Cause 

or Contribute Analysis 

4. Comments on Elements of the 

Endangerment and Cause or Contribute 

Tests Made During the ANPR Public 

Comment Period 

B. Air Pollutant, Public Health and Welfare 

III. The Administrator’s Proposed 

Endangerment Finding 

A. Approach in Utilizing the Best 

Available Scientific Information 

B. The Air Pollution 

1. Common Features of the Six Key 

Greenhouse Gases 

2. Evidence That the Six Greenhouse Gases 

Are at Unprecedented Levels in the 

Atmosphere 

3. Evidence That Elevated Atmospheric 

Concentrations of the Six Greenhouse 

Gases Are the Root Cause of Observed 

Climate Change 

4. Other Climate Forcers 

a. Water Vapor 

b. The Ozone-Depleting Substances: CFCs, 

HCFCs and Halons 

c. Tropospheric Ozone 

d. Black Carbon 

e. Fluorinated Ethers and Recently 

Identified Greenhouse Gases 

C. The Administrator’s Proposed Finding 

That the Air Pollution Endangers Public 

Health and Welfare 

1. Evidence of Currently Observed Climatic 

and Related Effects 

2. Future Projected Climatic and Related 

Effects 

3. Impacts on Public Health 

4. Impacts on Public Welfare 

5. The Administrator’s Consideration of 

International Effects 

6. The Administrator’s Consideration of 

Key Uncertainties 

7. Summary 

IV. The Administrator’s Cause or Contribute 

Finding 

A. The Air Pollutant(s) 

1. Proposed Definition of Air Pollutant 

2. How the Definition of Air Pollutant in 

the Endangerment Determination Affects 

Section 202(a) Standards 

B. Proposed Cause or Contribute Finding 

1. Overview of Greenhouse Gas Emissions 

2. Overview of Section 202(a) Source 

Categories and Cause or Contribute 

Analysis 

3. Proposed Finding That Emissions of the 

Collective Group of Six Greenhouse 

Gases Contributes to Air Pollution 

Which May Reasonably Be Anticipated 

To Endanger Public Health and Welfare 

a. Total Greenhouse Gas Emissions From 

Section 202(a) Source Categories 

b. Proposed Contribution Finding for the 

Single Air Pollutant Comprised of the 

Collective Group of Six Greenhouse 

Gases 

4. Additional Consideration of Whether 

Each Greenhouse Gas as a Separate Air 

Pollutant Contributes to Air Pollution 

Which May Reasonably Be Anticipated 

To Endanger Public Health and Welfare 

a. Carbon Dioxide Emissions From Section 

202(a) Source Categories 

b. Methane Emissions From Section 202(a) 

Source Categories 

c. Nitrous Oxide Emissions From Section 

202(a) Source Categories 

d. HFC emissions From Section 202(a) 

Source Categories 

V. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution, or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

I. Introduction 

A. Summary 

Pursuant to section 202(a) of the 

Clean Air Act (CAA or Act), the 

Administrator proposes to find that the 

mix of six key greenhouse gases in the 

atmosphere may reasonably be 

anticipated to endanger public health 

and welfare. Specifically, the 

Administrator is proposing to define the 

‘‘air pollution’’ referred to in section 


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1As discussed later, EPA does not need to 

determine, and is not determining, whether impacts 

occurring outside the U.S. would be sufficient by 

themselves to justify the proposed endangerment 

finding. Instead the impacts occurring outside the 

U.S. are considered as providing additional support 

for the proposed finding, in a situation where, as 

here, the impacts occurring within the U.S. are 

sufficient on their own to warrant the proposed 

finding. Thus, the Administrator does not now take 

a position on the legal question whether 

international effects, on their own, would be 

sufficient to support an endangerment finding 

under the Clean Air Act. 

2According to Intergovernmental Panel on 

Climate Change (IPCC) terminology, ‘‘very likely’’ 

conveys a 90 to 99 percent probability of 

occurrence. ‘‘Virtually certain’’ conveys a greater 

than 99 percent probability, ‘‘likely’’ conveys a 66 

to 90 percent probability, and ‘‘about as likely as 

not’’ conveys a 33 to 66 percent probability. 

3To clarify the distinction between air pollution 

and air pollutant, the air pollution is the 

atmospheric concentrations and can be thought of 

as the total, cumulative stock problem of 

greenhouse gases in the atmosphere. The air 

pollutants, on the other hand, are the emissions of 

greenhouse gases and can be thought of as the flow 

that changes the size of the total stock. 

202(a) of the CAA to be the mix of six 

key directly emitted and long-lived 

greenhouse gases: Carbon dioxide (CO2), 

methane (CH4), nitrous oxide (N2O), 

hydrofluorocarbons (HFCs), 

perfluorocarbons (PFCs), and sulfur 

hexafluoride (SF6). It is the 

Administrator’s judgment that the total 

body of scientific evidence compellingly 

supports a positive endangerment 

finding for both public health and 

welfare. The Administrator reached this 

judgment by considering both observed 

and projected future effects, and by 

considering the full range of risks and 

impacts to public health and welfare 

occurring within the U.S., which by 

itself warrants this judgment. In 

addition, the scientific evidence 

concerning risks and impacts occurring 

outside the U.S., including risks and 

impacts that can affect people in the 

U.S., provides further support for this 

finding.1

Under section 202(a) of the CAA, the 

Administrator is to determine whether 

emissions of any air pollutant from new 

motor vehicles and their engines cause 

or contribute to air pollution which may 

reasonably be anticipated to endanger 

public health or welfare. The 

Administrator further proposes to find 

that combined emissions from new 

motor vehicles and new motor vehicle 

engines of four of these greenhouse 

gases—carbon dioxide, methane, nitrous 

oxide, and hydrofluorocarbons— 

contribute to this air pollution. The 

other greenhouse gases that are the 

subject of this proposal 

(perfluorocarbons and sulfur 

hexafluoride) are not emitted by motor 

vehicles. 

The Administrator’s proposed 

findings come in response to the 

Supreme Court’s decision in 

Massachusetts v. EPA, 549 U.S. 497 

(2007). That case involved a petition 

submitted by the International Center 

for Technology Assessment and 18 other 

environmental and renewable energy 

industry organizations requesting that 

EPA issue standards under section 

202(a) of the Act for the emissions of 

carbon dioxide, methane, nitrous oxide, 

and hydrofluorocarbons from new 

motor vehicles and engines. The 

proposed findings are in response to 

this petition and are for purposes of 

section 202(a). EPA is not proposing or 

taking action under any other provision 

of the Clean Air Act. 

B. Background Information Helpful to 

Understanding This Proposal 

1. Greenhouse Gases and Their Effects 

Greenhouse gases are gases that 

effectively trap some of the Earth’s heat 

that would otherwise escape to space. 

Greenhouse gases are both naturally 

occurring and anthropogenic. The 

primary greenhouse gases of concern 

directly emitted by human activities 

include carbon dioxide, methane, 

nitrous oxide, hydrofluorocarbons, 

perfluorocarbons, and sulfur 

hexafluoride. Of these six gases, four 

(carbon dioxide, methane, nitrous oxide, 

and hydrofluorocarbons) are emitted by 

motor vehicles. 

These six gases, once emitted, remain 

in the atmosphere for decades to 

centuries. Thus, they become well 

mixed globally in the atmosphere and 

their concentrations accumulate when 

emissions exceed the rate at which 

natural processes remove greenhouse 

gases from the atmosphere. The heating 

effect caused by the human-induced 

buildup of greenhouse gases in the 

atmosphere is very likely 2 the cause of 

most of the observed global warming 

over the last 50 years. A detailed 

explanation of climate change and its 

impact on health, society, and the 

environment is included in EPA’s 

technical support document (docket 

#OAR–2009–0171) and discussed in the 

context of the Administrator’s finding in 

Section III. 

The U.S. transportation sector is a 

significant contributor to total U.S. and 

global anthropogenic emissions of 

greenhouse gases. Transportation 

sources subject to regulation under 

section 202(a) of the Act are the second 

largest greenhouse gas-emitting sector in 

the U.S., after electricity generation, and 

accounted for 24 percent of total U.S. 

greenhouse gas emissions in 2006 (see 

table 1 in section IV below) (these 

emissions are compared on carbon 

dioxide equivalent basis; see footnote 18 

for an explanation). Detailed 

information on past, present, and 

projected greenhouse gas concentrations 

and emissions is provided in the 

Technical Support Document, and 

summarized in Sections III and IV, 

respectively. 

2. Statutory Basis for This Proposal 

Section 202(a)(1) of the CAA states 

that ‘‘The Administrator shall by 

regulation prescribe (and from time to 

time revise) * * * standards applicable 

to the emission of any air pollutant from 

any class or classes of new motor 

vehicles or new motor vehicle engines, 

which in [her] judgment cause, or 

contribute to, air pollution which may 

reasonably be anticipated to endanger 

public health or welfare.’’ 

Before the Administrator may issue 

standards addressing emissions of 

greenhouse gases from new motor 

vehicles or engines under section 

202(a), the Administrator must satisfy a 

two-step test. First, the Administrator 

must decide whether, in her judgment, 

the air pollution under consideration 

may reasonably be anticipated to 

endanger public health or welfare. 

Second, the Administrator must decide 

whether, in her judgment, emissions of 

an air pollutant from new motor 

vehicles or engines cause or contribute 

to this air pollution.3 If the 

Administrator answers both questions 

in the affirmative, she must issue 

standards under section 202(a). 

Massachusetts v. EPA, 549 U.S. at 533. 

Typically, the endangerment and 

cause or contribute findings have been 

proposed concurrently with proposed 

standards under various sections of the 

CAA, including section 202(a). 

Comment has been taken on these 

proposed findings as part of the notice 

and comment process for the emission 

standards. See, e.g., Rulemaking for 

non-road compression-ignition engines 

under section 213(a)(4) of the CAA, 

Proposed Rule 58 FR 28809, 28813–14 

(May 17, 1993), Final Rule 59 FR 31306, 

31318 (June 17, 1994); Rulemaking for 

highway heavy duty diesel engines and 

diesel sulfur fuel under sections 202(a) 

and 211(c) of the CAA, Proposed Rule 

65 FR 35430 (June 2, 2000), Final Rule 

66 FR 5002 (Jan. 18, 2001). However, 

there is no requirement that the 

Administrator propose the 

endangerment and cause or contribute 

findings with proposed standards. The 

Administrator is moving forward with 

this proposed endangerment finding 

and a cause or contribute determination 


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4Commenters on the Advanced Notice of 

Proposed Rulemaking on Regulating Greenhouse 

Gases under the Clean Air Act, 73 FR 44354 (2007), 

see Section I.B.4 below, argued that EPA is required 

to follow notice and comment requirements for the 

endangerment and cause or contribute findings. 

Without agreeing or disagreeing with the reasoning 

set forth in those comments, the Administrator is 

applying the rulemaking requirements of CAA 

section 307(d), including notice and comment, to 

today’s action. See, e.g., CAA sections 307(d)(1)(K) 

(applying 307(d) requirements to the promulgation 

or revisions of regulations under section 202), 

307(d)(1)(V) (the provisions of section 307(d) apply 

to ‘‘such other actions as the Administrator may 

determine.’’). 

while developing proposed standards 

under section 202(a). 

The Administrator is applying the 

rulemaking provisions of CAA section 

307(d) to this action.4 Thus, these 

proposed findings will be subject to the 

same rulemaking requirements that 

would apply if the proposed findings 

were part of the standard-setting 

rulemaking. Any standard setting 

rulemaking under section 202(a) will 

also be subject to these notice and 

comment rulemaking procedures. 

3. The Supreme Court’s Decision in 

Massachusetts v. EPA 

a. The Petition of the International 

Center for Technology Assessment 

On October 20, 1999, the International 

Center for Technology Assessment and 

18 other environmental and renewable 

energy industry organizations filed a 

‘‘Petition for Rulemaking and Collateral 

Relief Seeking the Regulation of 

Greenhouse Gas Emissions from New 

Motor Vehicles under Section 202(a) of 

the Clean Air Act.’’ The thrust of the 

petition was that four greenhouse 

gases—carbon dioxide, methane, nitrous 

oxide, and hydrofluorocarbons—are air 

pollutants as defined in CAA section 

302(g), that emissions of these 

greenhouse gases contribute to air 

pollution which is reasonably 

anticipated to endanger public health or 

welfare, that these greenhouse gases are 

emitted by new motor vehicles, and 

therefore that EPA has a mandatory duty 

to issue regulations under CAA section 

202(a) addressing these greenhouse 

gases. 

After an opportunity for public 

comment, EPA denied the petition in a 

notice issued on August 8, 2003. The 

Agency concluded that it lacked 

authority under the CAA to regulate 

greenhouse gases for purposes of global 

climate change, and that even if it did 

have the authority to set greenhouse gas 

emission standards for new motor 

vehicles, it would be unwise to do so at 

that time. The federal appeals court in 

Washington, DC, upheld EPA’s denial of 

the petition. 

b. The Supreme Court’s Decision 

In Massachusetts v. EPA, the Supreme 

Court reversed the lower court’s 

decision and held that EPA had 

improperly denied the petition. 549 U.S. 

497 (2007). The Court held that 

greenhouse gases are air pollutants 

under the CAA, and that the alternative 

grounds EPA gave for denying the 

petition were ‘‘divorced from the 

statutory text’’ and hence improper. 

Specifically, the Court held that 

carbon dioxide, methane, nitrous oxide, 

and hydrofluorocarbons fit the CAA’s 

‘‘sweeping definition of ‘air pollutant’ ’’ 

since they are ‘‘without a doubt 

‘physical [and] chemical * * * 

substances which [are] emitted into 

* * * the ambient air.’ The statute is 

unambiguous.’’ Id. at 529. The Court 

also rejected the argument that postenactment legislative developments 

even ‘‘remotely suggest[ed] that 

Congress meant to curtail [EPA’s] power 

to treat greenhouse gases as air 

pollutants.’’ Id. 

The Court further rejected the 

argument that EPA could not regulate 

motor vehicle emissions of the chief 

greenhouse gas, carbon dioxide, because 

doing so would essentially require 

control of vehicle fuel economy, and 

Congress delegated that authority to the 

Department of Transportation in the 

Energy Policy and Conservation Act. 

The Court held that the fact ‘‘that DOT 

sets mileage standards in no way 

licenses EPA to shirk its environmental 

responsibilities. EPA has been charged 

with protecting the public’s ‘health’ and 

‘welfare,’ 42 U.S.C. 7521(a)(1), a 

statutory obligation wholly independent 

of DOT’s mandate to promote energy 

efficiency.’’ Id. at 532 (citation omitted). 

The two obligations may overlap ‘‘but 

there is no reason to think the two 

agencies cannot both administer their 

obligations and yet avoid 

inconsistency.’’ Id. 

Turning to EPA’s alternative grounds 

for denial, the Court held that EPA’s 

decision on whether or not to grant the 

petition must relate to ‘‘whether an air 

pollutant ‘causes, or contributes to, air 

pollution which may reasonably be 

anticipated to endanger public health or 

welfare.’ ’’ Id. at 532–33. Thus, ‘‘[u]nder 

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 

explanation as to why it cannot or will 

not exercise its discretion to determine 

whether they do.’’ Id. at 533. The Court 

held that three of the four reasons EPA 

advanced as alternative grounds for 

denying the petition were unrelated to 

whether greenhouse gas emissions from 

new motor vehicles cause or contribute 

to air pollution that may reasonably be 

anticipated to endanger public health or 

welfare. Thus, EPA had failed to offer a 

reasoned explanation for its action. For 

example, the Court held that concerns 

related to foreign policy objectives had 

‘‘nothing to do with whether greenhouse 

gas emissions contribute to climate 

change’’ and hence could not justify the 

denial. Id. The Court further held that 

EPA’s generalized concerns about 

scientific uncertainty were likewise 

insufficient unless ‘‘the scientific 

uncertainty is so profound that it 

precludes EPA from making a reasoned 

judgment as to whether greenhouse 

gases contribute to global warming,’’ in 

which case EPA must so find. Id. at 534. 

The Supreme Court was careful to 

note that it was not dictating EPA’s 

action on remand, and was not deciding 

whether or not EPA must find that 

greenhouse gases endanger public 

health or welfare. Nor did the Court rule 

on ‘‘whether policy concerns can inform 

EPA’s actions in the event that it makes 

such a finding.’’ Id. at 534–35. The 

Court also observed that under CAA 

section 202(a), ‘‘EPA no doubt has 

significant latitude as to the manner, 

timing, content, and coordination of its 

regulations with those of other 

agencies.’’ Id. at 533. Nonetheless, any 

EPA decisions concerning the 

endangerment and cause or contribute 

criteria must be grounded in the 

requirements of CAA section 202(a). 

Since the Supreme Court’s decision in 

April 2007, some stakeholders have 

taken the position, including in 

comments on the Advance Notice of 

Proposed Rulemaking discussed below, 

that the Supreme Court did not 

foreclose EPA’s ability to deny the 

petition without addressing the 

endangerment question. For example, 

one industry group argued that EPA 

could deny the rulemaking petition 

based on statutory factors besides 

scientific uncertainty and those already 

rejected by the Court, but did not 

describe what those additional statutory 

factors may be or how they would 

support a denial of the ICTA petition. 

EPA does not agree with these 

interpretations of the Supreme Court’s 

decision. Moreover, commenters have 

not provided examples of additional 

statutory factors that they believe would 

justify denying the petition without 

addressing the endangerment and cause 

or contribute criteria. Today the 

Administrator is addressing these 

criteria, and is proposing to find that the 

mix of six key greenhouse gases in the 

atmosphere may reasonably be 

anticipated to endanger public health 


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5Cf. Massachusetts v. EPA, 549 U.S. at 525 n.23, 

citing Mountain States Legal Foundation v. 

Glickman, 92 F.3d 1228, 1234 (D.C. Cir. 1996) (‘‘The 

more drastic the injury that government action 

makes more likely, the lesser the increment in 

probability to establish standing’’); Village of Elk 

Grove Village v. Evans, 997 F.2d 328, 329 (7th Cir. 

1993) (‘‘[E]ven a small probability of injury is 

sufficient to create a case or controversy—to take a 

suit out of the category of the hypothetical— 

provided of course that the relief sought would, if 

granted, reduce the probability.’’). 

and welfare due overwhelmingly to the 

effects of climate change. Furthermore, 

the Administrator is proposing to find 

that emissions of greenhouse gases by 

motor vehicles collectively contribute to 

the air pollution that endangers public 

health and welfare. 

4. EPA’s Advance Notice of Proposed 

Rulemaking on Regulating Greenhouse 

Gases Under the Clean Air Act 

On July 30, 2008, EPA published an 

Advance Notice of Proposed 

Rulemaking on ‘‘Regulating Greenhouse 

Gas Emissions under the Clean Air Act’’ 

(73 FR 44354) (ANPR). The ANPR 

presented information relevant to, and 

solicited public comment on, a wide 

variety of issues regarding the potential 

regulation of greenhouse gases under 

the CAA, including EPA’s response to 

the Supreme Court’s decision in 

Massachusetts v. EPA. Section V of the 

ANPR contained an earlier version of 

much of the material in this proposal, 

including the legal framework, a 

summary of the science of climate 

change, and an illustration of how the 

Administrator could analyze the cause 

or contribute element using information 

regarding the greenhouse gas emissions 

of the portion of the U.S. transportation 

sector covered by section 202(a). A July 

2008 version of the Technical Support 

Document (TSD) for this proposal was 

also in the docket for the ANPR (EPA– 

HQ–OAR–2008–0318). 

The ANPR also contained a summary 

of much of the work EPA had done in 

2007 regarding draft greenhouse gas 

emission standards for light duty 

vehicles and trucks under section 202(a) 

of the Act. As noted earlier, EPA is 

currently developing proposed 

emissions standards related to today’s 

proposal. EPA expects that these 

proposed standards will be ready to 

propose for public comment several 

months from now. 

Finally, the ANPR also discussed 

pending petitions under various 

sections of the Act requesting that EPA 

regulate greenhouse gas emissions from 

other mobile sources, as well as 

stationary source rulemakings (recently 

completed, ongoing or remanded) in 

which commenters suggested EPA 

regulate greenhouse gas emissions. EPA 

is continuing to evaluate its response to 

those other pending petitions and 

rulemakings and will address them in 

later actions. 

C. Solicitation of Comments 

The Administrator requests comments 

on all aspects of this action. She 

requests comment on the data on which 

the proposed findings are based, the 

methodology used in obtaining and 

analyzing the data, and the major legal 

interpretations and policy 

considerations underlying the proposed 

findings. 

II. Legal Framework for This Action 

Two provisions of the CAA govern 

today’s proposal. Section 202(a) sets 

forth a two-part predicate for regulatory 

action under that provision: 

endangerment and cause or contribute. 

Section 302 of the Act contains 

definitions of the terms air pollutant 

and welfare used in section 202(a). 

These statutory provisions are discussed 

below. 

A. Section 202(a)—Endangerment and 

Cause or Contribute 

As noted above, section 202(a) of the 

CAA calls for the Administrator to 

exercise her judgment and make two 

separate determinations: first, whether 

air pollution may reasonably be 

anticipated to endanger public health or 

welfare, and second whether emissions 

of any air pollutant from new motor 

vehicles or engines cause or contribute 

to this air pollution. 

Based on the text of this provision 

and its legislative history, the 

Administrator interprets the two-part 

test as follows. First, the Administrator 

is required to protect public health and 

welfare. She is not asked to wait until 

harm has occurred but instead must be 

ready to take regulatory action to 

prevent harm before it occurs. The 

Administrator is thus to consider both 

current and future risks. Second, the 

Administrator is to exercise judgment 

by weighing risks, assessing potential 

harms, and making reasonable 

projections of future trends and 

possibilities. It follows that when 

exercising her judgment the 

Administrator balances the likelihood 

and severity of effects. This balance 

involves a sliding scale; on one end the 

severity of the effects may be significant, 

but the likelihood low, while on the 

other end the severity may be less 

significant, but the likelihood high. 

Under either scenario, the 

Administrator is permitted to find 

endangerment. If the harm would be 

catastrophic, the Administrator is 

permitted to find endangerment even if 

the likelihood is small. In the context of 

climate change, for example, the 

Administrator should take account of 

the most catastrophic scenarios and 

their probabilities. As explained below, 

however, it is not necessary to rely on 

low-probability outcomes in order to 

find endangerment here.5

Because scientific knowledge is 

constantly evolving, the Administrator 

may be called upon to make decisions 

while recognizing the uncertainties and 

limitations of the data or information 

available, as risks to public health or 

welfare may involve the frontiers of 

scientific or medical knowledge. At the 

same time, the Administrator must 

exercise reasoned decision making, and 

avoid speculative or crystal ball 

inquiries. Third, the Administrator is to 

consider the cumulative impact of 

sources of a pollutant in assessing the 

risks from air pollution, and is not to 

look only at the risks attributable to a 

single source or class of sources. Fourth, 

the Administrator is to consider the 

risks to all parts of our population, 

including those who are at greater risk 

for reasons such as increased 

susceptibility to adverse health effects. 

If vulnerable subpopulations are 

especially at risk, the Administrator is 

entitled to take that point into account 

in deciding the question of 

endangerment. Here too, both likelihood 

and severity of adverse effects are 

relevant, and here too, catastrophic 

scenarios and their probabilities should 

be considered. As explained below, 

vulnerable subpopulations face serious 

health risks as a result of climate 

change. 

This framework recognizes that 

regulatory agencies such as EPA must be 

able to deal with the reality that 

‘‘[m]an’s ability to alter his environment 

has developed far more rapidly than his 

ability to foresee with certainty the 

effects of his alterations.’’ See Ethyl 

Corp v. EPA, 541 F.2d 1, 6 (D.C. Cir.), 

cert. denied 426 U.S. 941 (1976). Both 

‘‘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 Massachusetts v. EPA, 

549 U.S. at 506, n.7 (citing Ethyl Corp.). 

To be sure, the concept of ‘‘expected 

value’’ has its limitations in this 

context, but it is useful insofar as it 

suggests that when severe risks to the 

public health and welfare are involved, 

the Administrator need not wait as 

evidence continues to accumulate. 

The Administrator recognizes that the 

context for this action is unique. There 

is a very large and comprehensive base 

of scientific information that has been 


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6At the time of the 1973 rules requiring the 

reduction of lead in leaded gasoline, section 

211(c)(1)(A) of the CAA stated that the 

Administrator may promulgate regulations that: 

‘‘control or prohibit the manufacture, introduction 

into commerce, offering for sale, or sale of any fuel 

or fuel additive for use in a motor vehicle or motor 

vehicle engine (A) if any emissions product of such 

fuel or fuel additive will endanger the public health 

or welfare * * * .’’ CAA 211(c)(1)(A) (1970) 

(emphasis added). The italicized language in the 

above quote is the relevant language revised by the 

1977 amendments. 

7The Supreme Court recognized that the current 

language in section 202(a)(1) is ‘‘more-protective’’ 

than the 1970 version that was similar to the section 

211 language before the DC Circuit in Ethyl Corp. 

Massachusetts v. EPA, 549 U.S. at 506, fn 7. 

8See H.R. Rep. 95–294 at 49, 4 LH at 2516 (‘‘To 

emphasize the preventive or precautionary nature 

of the Act, i.e. to assure that regulatory action can 

effectively prevent harm before it occurs’’). 

9Congress also standardized this language across 

the various sections of the CAA which address 

emissions from both stationary and mobile sources. 

H.R. Rep. 95–294 at 50, 4 LH at 2517; Section 401 

of CAA Amendments of 1977. 

developed over many years through a 

global consensus process involving 

numerous scientists from many 

countries and representing many 

disciplines. She also recognizes that 

there are varying degrees of uncertainty 

across many of these scientific issues. It 

is in this context that she is exercising 

her judgment and applying the statutory 

framework. Further discussion of the 

language in section 202(a) and its 

legislative history is provided below, to 

explain more fully the basis for this 

interpretation. 

1. The Statutory Language 

The interpretation described above 

flows from the statutory language itself. 

The phrase ‘‘may reasonably be 

anticipated’’ and the term ‘‘endanger’’ 

authorize, if not require, the 

Administrator to act to prevent harm 

and to act in conditions of uncertainty. 

They do not limit her to merely reacting 

to harm or to acting only when certainty 

has been achieved; indeed, the 

references to anticipation and to 

endangerment imply that to fail to look 

to the future or to less than certain risks 

would be to abjure the Administrator’s 

statutory responsibilities. Moreover, by 

instructing the Administrator to 

consider whether emissions of an air 

pollutant cause or contribute to air 

pollution, the statute is clear that she 

need not find that emissions from any 

one sector or group of sources are the 

sole or even the major part of an air 

pollution problem. The use of the term 

contribute clearly indicates that a lower 

threshold than a finding that such 

emissions are the sole or major cause is 

a sufficient basis to make the required 

finding. Finally, the phrase ‘‘in [her] 

judgment’’ authorizes the Administrator 

to weigh risks and to consider 

projections of future possibilities, while 

also recognizing uncertainties and 

extrapolating from existing data. When 

exercising her judgment the 

Administrator balances the likelihood 

and severity of effects. Notably, the 

phrase ‘‘in [her] judgment’’ modifies 

both ‘‘may reasonably be anticipated’’ 

and ‘‘cause or contribute.’’ 

2. Origin of the Current Statutory 

Language 

When Congress revised section 202(a) 

and other provisions of the CAA as part 

of the 1977 amendments to the CAA, it 

was responding to an opinion issued by 

the D.C. Circuit regarding the pre-1977 

version of section 211(c) of the Act. The 

legislative history of those amendments, 

particularly the report by the House 

Committee on Interstate and Foreign 

Commerce, demonstrate that EPA’s 

interpretation is fully consistent with 

Congress’ intention in crafting this a 

provision See H.R. Rep. 95–294 (1977), 

as reprinted in 4 A Legislative History 

of the Clean Air Act Amendments of 

1977 (1978) at 2465 (hereinafter ‘‘LH’’). 

a. Ethyl Corp. v. EPA 

In revising the statutory language, 

Congress relied heavily on the en banc 

decision in Ethyl Corp. v. EPA, which 

reversed a 3-judge panel opinion 

regarding an EPA rule restricting the 

content of lead in leaded gasoline.6

After reviewing the relevant facts and 

law, the full court evaluated the 

statutory language at issue to see what 

level of ‘‘certainty [was] required by the 

Clean Air Act before EPA may act.’’ Id. 

at 7. 

The petitioners argued that the 

statutory language ‘‘will endanger’’ 

required proof of actual harm, and that 

the actual harm had to come from 

emissions from the fuels in and of 

themselves. Id. at 12, 29. The en banc 

court rejected this approach, finding 

that the term ‘‘endanger’’ allowed the 

Administrator to act when harm is 

threatened, and did not require proof of 

actual harm. Id. at 13. ‘‘A statute 

allowing for regulation in the face of 

danger is, necessarily, a precautionary 

statute.’’ Id. Optimally, the court held, 

regulatory action would not only 

precede, but prevent, a perceived threat. 

Id. 

The court also rejected petitioner’s 

argument that any threatened harm 

must be ‘‘probable’’ before regulation 

was authorized. Specifically, the court 

recognized that danger ‘‘is set not by a 

fixed probability of harm, but rather is 

composed of reciprocal elements of risk 

and harm, or probability and severity.’’ 

Id. at 18. Next, the court held that EPA’s 

evaluation of risk is necessarily an 

exercise of judgment, and that the 

statute did not require a factual finding. 

Id. at 24. Thus, ultimately, the 

Administrator must ‘‘act, in part on 

‘factual issues,’ but largely ‘on choices 

of policy, on an assessment of risks, 

[and] on predictions dealing with 

matters on the frontiers of scientific 

knowledge * * * .’’ Id. at 29 (citations 

omitted). Finally, the en banc court 

agreed with EPA that even without the 

language in section 202(a) regarding 

‘‘cause or contribute to,’’ it was 

appropriate for EPA to consider the 

cumulative impact of lead from 

numerous sources, not just the fuels 

being regulated under section 211(c). Id. 

at 29–31. 

b. The 1977 Clean Air Act Amendments 

The dissent in the original Ethyl Corp. 

decision and the en banc opinion were 

of ‘‘critical importance’’ to the House 

Committee which proposed the 

revisions to the endangerment language 

in the 1977 amendments to the CAA. 

H.R. Rep. 95–294 at 48, 4 LH at 2515. 

In particular, the Committee believed 

the Ethyl Corp. decision posed several 

‘‘crucial policy questions’’ regarding the 

protection of public health and 

welfare.’’ Id. 7 The Committee addressed 

those questions with the language that 

now appears in section 202(a) and 

several other CAA provisions— 

‘‘emission of any air pollutant * * * , 

which in [the Administrator’s] judgment 

cause, or contribute to, air pollution 

which may reasonably be anticipated to 

endanger public health or welfare.’’ 

The legislative history clearly 

indicates that the Committee intended 

the language to serve several purposes 

consistent with the en banc decision in 

Ethyl Corp. In particular, the language 

(1) emphasizes the preventive or 

precautionary nature of the CAA 8; (2) 

authorizes the Administrator to 

reasonably project into the future and 

weigh risks; (3) assures the 

consideration of the cumulative impact 

of all sources; (4) instructs that the 

health of susceptible individuals, as 

well as healthy adults, should be part of 

the analysis; and (5) indicates an 

awareness of the uncertainties and 

limitations in information available to 

the Administrator. H.R. Rep. 95–294 at 

49–50, 4 LH at 2516–17.9

As noted above, the phrase ‘‘in [her] 

judgment’’ calls for the Administrator to 

make a comparative assessment of risks 

and projections of future possibilities, 

consider uncertainties, and extrapolate 

from limited data. Thus, the 

Administrator must balance the 

likelihood of effects with the severity of 


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10 Throughout this Notice the judgments on 

endangerment and cause or contribute are described 

as a finding or findings. This is for ease of reference 

only, and is not intended to imply that the 

Administrator’s exercise of judgment in applying 

the scientific information to the statutory criteria is 

solely a factual finding; while grounded squarely in 

the science of climate change, these judgments also 

embody policy considerations. 

11Thus, contrary to the position set forth by at 

least one commenter on the Greenhouse Gas ANPR, 

the statutory language does not require that EPA 

prove the effects of climate change ‘‘beyond a 

reasonable doubt.’’ Indeed, such an approach is 

inconsistent with the concepts of reasonable 

anticipation and endangerment embedded in the 

statute. 

12Specifically, the decision noted that 

‘‘ ‘contribute’ means simply ‘to have a share in any 

act or effect,’ WEBSTER’S THIRD NEW 

INTERNATIONAL DICTIONARY 496 (1993), or ‘to 

have a part or share in producing,’ 3 OXFORD 

ENGLISH DICTIONARY 849 (2d ed. 1989).’’ Id. at 

13. 

13The court explained, ‘‘[t]he repeated use of the 

term ‘significant’ to modify the contribution 

required for all nonroad vehicles, coupled with the 

omission of this modifier from the ‘cause, or 

contribute to’ finding required for individual 

categories of new nonroad vehicles, indicates that 

Congress did not intend to require a finding of 

‘significant contribution’ for individual vehicle 

categories.’’ Id. at 13. 

14Section IV discusses the evidence in this case 

that supports the proposed finding of contribution. 

EPA need not determine at this time the 

circumstances in which emissions would be trivial 

or de minimis and would not warrant a finding of 

contribution. 

the effects in reaching her judgment. 

The Committee emphasized that 

‘‘judgment’’ is different from a factual 

‘‘finding.’’ 10 The Administrator may 

make projections, assessments and 

estimates that are reasonable, as 

compared to a ‘‘ ‘crystal ball’ inquiry.’’ 

Moreover, procedural safeguards apply 

to the exercise of judgment, and final 

decisions are subject to judicial review. 

Also, the phrase ‘‘in [her] judgment’’ 

modifies both the phrases ‘‘cause and 

contribute’’ and ‘‘may reasonably be 

anticipated,’’ as discussed below. H.R. 

Rep. 95–294 at 50–51, 4 LH at 2517–18. 

As the Committee further explained, 

the phrase ‘‘may reasonably be 

anticipated’’ points the Administrator in 

the direction of assessing current and 

future risks rather than waiting for proof 

of actual harm. This phrase is also 

intended to instruct the Administrator 

to consider the limitations and 

difficulties inherent in information on 

public health and welfare. H.R. Rep. 95– 

294 at 51, 4 LH at 2518.11

Finally, the phrase ‘‘cause or 

contribute’’ ensures that all sources of 

the contaminant which contribute to air 

pollution are considered in the 

endangerment analysis (e.g., not a single 

source or category of sources). It is also 

intended to require the Administrator to 

consider all sources of exposure to a 

pollutant (for example, food, water, and 

air) when determining risk. Id. 

3. Additional Considerations for the 

Cause or Contribute Analysis 

By instructing the Administrator to 

consider whether emissions of an air 

pollutant cause or contribute to air 

pollution, the statute is clear that she 

need not find that emissions from any 

one sector or group of sources are the 

sole or even the major part of an air 

pollution problem. The use of the term 

contribute clearly indicates a lower 

threshold than the sole or major cause. 

Moreover, the statutory language in 

section 202(a) does not contain a 

modifier on its use of the term 

contribute. Unlike other CAA 

provisions, it does not require 

‘‘significant’’ contribution. See, e.g., 

CAA sections 111(b); 213(a)(2), (4). 

Congress made it clear that the 

Administrator is to exercise her 

judgment in determining contribution, 

and authorized regulatory controls to 

address air pollution even if the air 

pollution problem results from a wide 

variety of sources. While the 

endangerment test looks at the entire air 

pollution problem and the risks it poses, 

the cause or contribute test is designed 

to authorize EPA to identify and then 

address what may well be many 

different sectors or groups of sources 

that are each part of the problem. 

The DC Circuit Court of Appeals has 

discussed the concept of contribution in 

the context of CAA section 213 and 

rules for nonroad vehicles. In Bluewater 

Network v. EPA, 370 F.3d 1 (DC Cir. 

2004), industry argued that section 

213(a)(3) requires a finding of a 

significant contribution before EPA can 

regulate, while EPA’s view was that the 

CAA requires a finding only of 

contribution. Id. at 13. Section 213(a)(3), 

like section 202(a), is triggered by a 

finding that certain sources ‘‘cause, or 

contribute to,’’ air pollution, while an 

adjacent provision, section 213(a)(2), is 

triggered by a finding of a ‘‘significant’’ 

contribution. The court looked at the 

‘‘ordinary meaning of ‘contribute’ ’’ 

when upholding EPA’s reading. After 

referencing dictionary definitions of 

contribute, the court also noted that 

‘‘[s]tanding alone, the term has no 

inherent connotation as to the 

magnitude or importance of the relevant 

‘share’ in the effect; certainly it does not 

incorporate any ‘significance’ 

requirement.’’ 370 F.3d at 13.12 The 

court found that the bare ‘‘contribute’’ 

language invests the Administrator with 

discretion to exercise judgment 

regarding what constitutes a sufficient 

contribution for the purpose of making 

an endangerment finding. Id. at 14.13

Like section 213(a)(3), section 202(a) 

refers to contribution and does not 

specify that the contribution must be 

significant before an affirmative finding 

can be made. To be sure, any finding of 

a ‘‘contribution’’ requires some 

threshold to be met; a truly trivial or de 

minimis ‘‘contribution’’ might not count 

as such. The Administrator therefore 

has ample discretion in exercising her 

reasonable judgment and determining 

whether, under the circumstances 

presented, the cause or contribute 

criterion has been met.14 In the past, the 

Administrator has evaluated the 

emissions of the source or sources in 

different ways, based on the particular 

circumstances involved. For instance, in 

some mobile source rulemakings, the 

Administrator has used the percent of 

emissions from the regulated mobile 

source category compared to the total 

mobile source inventory for that air 

pollutant as the best way to evaluate 

contribution. See, e.g., 66 FR 5001 

(2001) (heavy duty engine and diesel 

sulfur rule). In other instances the 

Administrator has looked at the percent 

of emissions compared to the total 

nonattainment area inventory of the air 

pollution at issue. See, e.g., 67 FR 

68,242 (2002) (snowmobile rule). EPA 

has found that air pollutant emissions 

that amount to 1.2 percent of the total 

inventory ‘‘contribute.’’ Bluewater 

Network, 370 F.3d at 15 (‘‘For 

Fairbanks, this contribution was 

equivalent to 1.2 percent of the total 

daily CO inventory for 2001.’’). 

While these prior actions are 

instructive, they do not establish bright 

line emission levels above which a 

positive contribution determination 

must be made, or below which a 

contribution determination could not be 

made. The Administrator may 

determine that emissions at a certain 

level or percentage contribute to air 

pollution in one set of circumstances, 

while also judging that the same level or 

percentage of another air pollutant in a 

different circumstances and involving 

different air pollution does not 

contribute. When exercising her 

judgment, the Administrator not only 

considers the cumulative impact, but 

also looks at the totality of the 

circumstances (e.g., the air pollutant, 

the air pollution, the nature of the 

endangerment, the type of source 

category, the number of sources in the 

source category, and the number and 

type of other source categories that may 

emit the air pollutant) when 

determining whether the emissions 

‘‘justify regulation’’ under the CAA. 

Further discussion of this issue can be 

found in Section IV. 


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15Numerous comments on the ANPR discussed 

the endangerment and cause or contribute findings, 

and set forth how various stakeholders believe EPA 

is compelled to make those findings. EPA has 

reviewed the comments on the ANPR, and EPA 

appreciates the work that went into them. While we 

are not responding to every comment received in 

today’s proposal, the Agency is taking this 

opportunity to respond to a few key comments 

related to the test that some stakeholders believe 

guides the Administrator when undertaking an 

endangerment analysis and cause or contribute 

evaluation. As noted above, commenters should 

submit to the docket for today’s action any 

comments they want EPA to consider as it makes 

a decision on this proposed determination. 

16EPA also rejects the comment that EPA has 

defined ‘‘contribute’’ as resulting in a ‘‘humanly 

perceptible’’ difference. See Regional Haze 

Regulations and Guidelines for Best Available 

Retrofit Technology [BART] Determinations, 70 FR 

39104 (2005). In that rule, EPA noted that a 1.0 

deciview change in visibility is humanly 

perceptible in virtually all situations. Based on this, 

EPA concluded that for a state making a 

contribution finding for an individual source under 

section 169A(b)(2)(A), it would be unreasonable to 

determine that a source emitting pollution that 

resulted in a 0.5 deciview change in visibility did 

not ‘‘contribute’’ to visibility impairment. Id. at 

39120. In fact, EPA noted that ‘‘[i]f ‘causing’ 

visibility impairment means causing a humanly 

perceptible change in visibility, * * * then 

‘contributing’ to visibility impairment must mean 

having some lesser impact * * * that need not rise 

to the level of human perception.’’ Id. at 39120, fn 

32. The Agency did not establish a test that required 

human perception before contribution could be 

found. 

4. Comments on Elements of the 

Endangerment and Cause or Contribute 

Tests Made During the ANPR Public 

Comment Period 

Certain comments submitted on the 

ANPR15 argued that when evaluating 

endangerment and cause or contribute, 

the Administrator is limited to 

considering only those impacts that can 

be traced to the amount of air pollution 

directly attributable to the greenhouse 

gases emitted by new motor vehicles 

and engines. Such an approach 

collapses the two prongs of the test by 

requiring that any climate change 

impacts upon which an endangerment 

determination is made result solely from 

the greenhouse gas emissions of motor 

vehicles. It essentially eliminates the 

‘‘contribute’’ part of the ‘‘cause or 

contribute’’ portion of the test. This 

approach was clearly rejected by the en 

banc court in Ethyl Corp. 541 F.2d at 29 

(rejecting the argument that the 

emissions of the fuel additive to be 

regulated must ‘‘in and of itself, i.e. 

considered in isolation, endanger[s] 

public health.’’). Moreover, it conflicts 

with an enumerated purpose of the 1977 

CAA Amendments: ‘‘To assure 

consideration of the cumulative impact 

of all sources of a pollutant in setting 

ambient and emission standards, not 

just the extent of the risk from the 

emissions from a single source or class 

of sources of the pollutant; * * *’’ H.R. 

Rep. 95–294 at 49–50, 4 LH at 2516–17. 

Nor does EPA agree with comments 

that argue the Administrator cannot 

make a positive endangerment or 

contribution determination unless the 

emissions reductions required by the 

resulting standards would ‘‘effectively 

mitigate’’ or ‘‘fruitfully attack’’ the 

impacts underlying the endangerment 

determination. Again, such an approach 

fails to appreciate the holistic approach 

that Congress adopted in 1977. 

Moreover, as the Supreme Court 

recognized, ‘‘[a]gencies, like 

legislatures, do not generally resolve 

massive problems in one fell regulatory 

swoop.’’ Massachusetts v. EPA, 549 U.S. 

at 524 (citations omitted).16 The 

threshold endangerment and cause or 

contribute criteria are separate and 

distinct from the standard setting 

criteria that apply if the threshold 

findings are met, and they serve a 

different purpose. Indeed, the more 

serious the endangerment to public 

health and welfare, the more important 

it may be that action be taken to address 

the actual or potential harm even if no 

one action alone can solve the problem, 

and a series of actions is called for. 

Importantly, these various narrow 

approaches to the endangerment and 

cause or contribute criteria would 

effectively preclude the Administrator 

from ever making a positive finding for 

a global phenomenon like climate 

change because the regulatory actions 

would always be limited to just part of 

the picture. Indeed, they would 

preclude the Administrator from making 

a positive finding for any complex 

pollution problem that cannot be solved 

by one regulatory action alone. This is 

contrary to Congress’ direction that the 

Administrator consider the whole 

picture when exercising her judgment 

about the critical issues of cause or 

contribute and endangerment to public 

health and welfare. 

B. Air Pollutant, Public Health and 

Welfare 

The CAA defines both ‘‘air pollutant’’ 

and ‘‘welfare.’’ Air pollutant is defined 

as: ‘‘Any air pollution agent or 

combination of such agents, including 

any physical, chemical, biological, 

radioactive (including source material, 

special nuclear material, and byproduct 

material) substance or matter which is 

emitted into or otherwise enters the 

ambient air. Such term includes any 

precursors to the formation of any air 

pollutant, to the extent the 

Administrator has identified such 

precursor or precursors for the 

particular purpose for which the term 

‘air pollutant’ is used.’’ CAA section 

302(g). Greenhouse gases fit well within 

this capacious definition. See 

Massachusetts v. EPA, 549 U.S. at 532. 

They are ‘‘without a doubt’’ physical 

chemical substances emitted into the 

ambient air. Id. at 529. Section IV below 

contains further discussion on today’s 

proposed definition of ‘‘air pollutant’’ 

for purposes of the contribution finding. 

Regarding ‘‘welfare’’, the CAA states 

that ‘‘[a]ll language referring to effects 

on welfare includes, but is not limited 

to, effects on soils, water, crops, 

vegetation, man-made materials, 

animals, wildlife, weather, visibility, 

and climate, damage to and 

deterioration of property, and hazards to 

transportation, as well as effects on 

economic values and on personal 

comfort and well-being, whether caused 

by transformation, conversion, or 

combination with other air pollutants.’’ 

CAA section 302(h). This definition is 

quite broad. Importantly, it is not an 

exclusive list due to the use of the term 

‘‘includes, but is not limited to, * * *.’’ 

Effects other than those listed here may 

also be considered effects on welfare. 

Moreover, the terms contained within 

the definition are themselves expansive. 

For example, deterioration to property 

could include damage caused by 

extreme weather events. Effects on 

vegetation can include impacts from 

changes in temperature and 

precipitation as well as from the 

spreading of invasive species or insects. 

Prior welfare effects evaluated by EPA 

include impacts on vegetation generally, 

and changes in crop and forestry 

specifically, as well as reduced 

visibility, changes in nutrient balance 

and acidity of the environment, soiling 

of buildings and statues, and erosion of 

building materials. See, e.g., Final 

National Ambient Air Quality Standard 

for Ozone, 73 FR 16436 (2007); Control 

of Emissions from Nonroad Large Spark 

Ignition Engines and Recreational 

Engines (Marine and Land-Based), 67 

FR 68242 (2002); Final Heavy-Duty 

Engine and Vehicle Standards and 

Highway Diesel Sulfur Control 

Requirements, 66 FR 5002 (2001). 

There is no definition of public health 

in the Clean Air Act. The Supreme 

Court has discussed the concept in the 

context of whether costs can be 

considered when setting National 

Ambient Air Quality Standards. 

Whitman v. American Trucking Ass’n, 

531 U.S. 457 (2001). In Whitman, the 

Court imbued the term with its most 

natural meaning: ‘‘the health of the 

public.’’ Id. at 466. 

When considering public health, EPA 

has looked at morbidity, such as 


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17U.S. EPA (2002), EPA/260R–02–008 http:// 

www.epa.gov/quality/informationguidelines/ 

documents/EPA_InfoQualityGuidelines.pdf. 

impairment of lung function, 

aggravation of respiratory and 

cardiovascular disease, and other acute 

and chronic health effects, as well as 

mortality. See, e.g., Final National 

Ambient Air Quality Standard for 

Ozone, 73 FR 16436 (2007). 

III. The Administrator’s Proposed 

Endangerment Finding 

This section describes the basis for 

the proposed endangerment finding, by 

laying out the scientific evidence and 

the Administrator’s rationale for 

reaching this judgment. The first section 

describes the approach EPA has taken in 

gathering and synthesizing the best 

available scientific information to 

inform the Administrator’s judgment, 

the next section describes the proposed 

definition of the air pollution, and the 

third section discusses the scientific 

evidence and the Administrator’s 

reasons for judging that the air pollution 

is reasonably anticipated to endanger 

both public health and public welfare. 

A. Approach in Utilizing the Best 

Available Scientific Information 

EPA has developed a technical 

support document (TSD) which 

synthesizes major findings from the best 

available scientific assessments that 

have gone through rigorous and 

transparent peer review. The TSD 

therefore relies most heavily on the 

major assessment reports of both the 

Intergovernmental Panel on Climate 

Change (IPCC) and the U.S. Climate 

Change Science Program (CCSP). EPA 

took this approach rather than 

conducting a new assessment of the 

scientific literature. The IPCC and CCSP 

assessments base their findings on the 

large body of many individual, peerreviewed studies in the literature, and 

then the IPCC and CCSP assessments 

themselves go through a transparent 

peer-review process. The TSD was in 

turn reviewed by a dozen federal 

government scientists, who have 

contributed significantly to the body of 

climate change literature, and indeed to 

our common understanding of this 

problem. The information in the TSD 

has therefore been developed and 

prepared in a manner that is consistent 

with EPA’s Guidelines for Ensuring and 

Maximizing the Quality, Objectivity, 

Utility and Integrity of Information 

Disseminated by the Environmental 

Protection Agency.17 Furthermore, 

relying most heavily on the assessment 

reports that reflect the scientific 

literature more broadly guards against 

an overreliance on and narrow 

consideration of individual studies. 

An earlier version of this TSD was 

publicly released on July 30, 2008, to 

accompany the ANPR. The July 2008 

version of the TSD has been updated to 

reflect the findings of 11 additional 

CCSP reports that have since been 

published, and to incorporate more 

recent climate data from U.S. federal 

agencies. This addresses a number of 

concerns raised by commenters about 

the July 2008 version of the TSD, 

arguing that it relied too heavily on the 

IPCC Fourth Assessment Report 

(published 2007), which some argued 

was either not current enough or not 

specific enough to U.S. conditions. We 

note that the IPCC North American 

chapter (of the Working Group II 

volume) on impacts, adaptation and 

vulnerability covers the U.S. and 

Canada (not Mexico) and that the 

general findings in that chapter (drawn 

from many individual studies for the 

U.S.) are indeed applicable to U.S. 

conditions. Even with more recent 

information available, the IPCC Fourth 

Assessment Report remains a standard 

reference, essentially serving as the 

benchmark against which new findings 

over the next few years will be 

compared. Therefore it also serves as a 

robust and valuable reference for 

purposes of this proposal. The TSD has 

also been edited or updated in a number 

of places to reflect specific comments 

received on the July 2008 version, and 

to reflect comments from an additional 

round of review by the federal scientists 

following the incorporation of the more 

recent scientific findings. 

Regarding the scope of the relevant 

scientific findings, EPA took the 

approach that the timeframe under 

consideration should be consistent with 

the timeframe over which greenhouse 

gases may influence the climate (i.e., 

observed effects and projected effects 

over the next several decades and 

indeed at least for the remainder of this 

century). Moreover, the analysis was not 

restricted to only those climate and 

public health or welfare effects which 

may be attributable solely to greenhouse 

gas emissions from section 202(a) 

sources under the Act. In addition, 

although the primary focus for 

evaluation of risks and impacts to 

public health or welfare was on the 

U.S., careful consideration was also 

given to the global context. 

Finally, climate policy or societal 

responses to any known or perceived 

risks and impacts to public health or 

welfare, which may or may not be 

implemented in the future—whether 

through planned adaptation or 

greenhouse gas mitigation measures— 

were not explicitly assessed in the 

endangerment analysis. Some observed 

and projected effects or risks due to 

climate change reported in the TSD and 

summarized below do have embedded 

within them assumptions about 

autonomous behavioral or management 

changes to cope with climate change. 

We have noted these situations in the 

TSD. However, it is the Administrator’s 

position that the purpose of the 

endangerment analysis is to assess the 

risks posed to public health and welfare, 

rather than to estimate how various 

adaptation and greenhouse gas 

mitigation policies may ameliorate or 

exacerbate any endangerment that 

exists. Indeed, the presumed need for 

adaptation and greenhouse gas 

mitigation to occur to avoid, lessen or 

delay the risks and impacts associated 

with human-induced climate change 

presupposes that there is endangerment 

to public health or welfare. The 

Administrator therefore disagrees with 

commenters on the ANPR who argue 

that when considering whether the 

atmospheric concentration of 

greenhouse gases may reasonably be 

anticipated to endanger public health or 

welfare, she must consider the impact 

from the regulation of greenhouse gases 

under the CAA following an 

endangerment finding. The 

Administrator also believes it is 

inappropriate, in considering whether 

greenhouse gases endanger public 

health or welfare, to consider potential 

private behavior aimed at alleviating 

some of the effects of climate change. 

Just as the Administrator would not 

consider, for example, the availability of 

asthma medication in determining 

whether criteria air pollutants endanger 

public health, so the Administrator will 

not consider private behavior in the 

endangerment determination at hand. 

On the contrary, ameliorative steps of 

that kind would attest to the fact of 

endangerment. 

To be sure, private adaptation might 

be considered as a relevant factor in 

deciding on the proper regulatory 

approach, although the Administrator 

need not decide that here. Determining 

whether there are adverse public health 

and welfare impacts due to the 

existence of air pollution is a separate 

matter from considering the appropriate 

approaches for responding to any such 

impacts and the possible repercussions 

of those approaches. The proposed 

approach suggested by commenters 

essentially would insert extra-statutory 

considerations into the endangerment 

analysis. 


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18 We use ‘‘long-lived’’ here to mean that the gas 

has a lifetime in the atmosphere sufficient to 

become globally well mixed throughout the entire 

atmosphere, which requires a minimum 

atmospheric lifetime of about one year. IPCC also 

refers to these six greenhouse gases as long-lived. 

Methane has an atmospheric lifetime of roughly a 

decade. One of the most commonly used 

hydrofluorocarbons (HFC–134a) has a lifetime of 14 

years. Nitrous oxide has a lifetime of 114 years; 

sulfur hexafluoride over 3,000 years; and some 

PFCs up to 10,000 to 50,000 years. Carbon dioxide 

is generally thought to have a lifetime of roughly 

100 years, but for a given amount of carbon dioxide 

emitted some fraction is quickly absorbed by the 

oceans and terrestrial vegetation and the remainder 

will only slowly decay in the atmosphere after 

several years, and indeed some portion will remain 

in the atmosphere for many centuries. 

19 Global warming potentials (GWPs) for each 

greenhouse gas have been estimated by IPCC so that 

emissions of these gases can be compared to one 

another on a CO2-equivalent basis. The GWP 

represents the cumulative heating effect of a gas 

over a specified timeframe in the atmosphere (100 

years), relative the heating effect caused by carbon 

dioxide, the reference gas. Carbon dioxide is 

assigned a GWP of 1, whereas methane has a GWP 

of 21. The GWP of sulfur hexafluoride is 23,900. 

20 Due to the cumulative purpose of the statutory 

language, even if the Administrator were to look at 

the atmospheric concentration of each greenhouse 

gas individually, she would still consider the 

impact of the concentration of a single greenhouse 

gas in combination with that caused by the other 

greenhouse gases. 

B. The Air Pollution 

In applying the endangerment test to 

greenhouse gases under section 202(a), 

the Administrator must define the scope 

and nature of the relevant air pollution 

that must be evaluated. For this action, 

the Administrator is proposing that the 

air pollution be defined as the combined 

mix of six key directly-emitted and 

long-lived greenhouse gases which 

together constitute the root cause of 

human-induced climate change: carbon 

dioxide (CO2), methane, nitrous oxide, 

hydrofluorocarbons, perfluorocarbons, 

and sulfur hexafluoride. The 

Administrator acknowledges that there 

are other anthropogenic climate forcers 

which play a role in climate change 

(discussed below), but that for today’s 

action these other climate forcers are not 

the priority and may need to be 

evaluated further. What follows is a 

summary of key scientific findings from 

the TSD and the Administrator’s 

rationale for the proposed definition of 

air pollution. 

1. Common Features of the Six Key 

Greenhouse Gases 

There are a number of scientific and 

policy reasons why the Administrator is 

proposing that the air pollution for this 

endangerment finding be defined as the 

combination of the six greenhouse 

gases. These six greenhouse gases are 

well studied by and have been the 

primary focus of climate change 

research, and are therefore the 

Administrator’s first priority in 

addressing endangerment for 

greenhouse gases. These six greenhouse 

gases share common physical properties 

relevant to the climate change problem: 

all are long-lived 18 in the atmosphere; 

all become globally well mixed in the 

atmosphere regardless of where the 

emissions occur; all trap outgoing heat 

that would otherwise escape to space; 

and all are directly emitted as 

greenhouse gases rather than forming as 

a greenhouse gas in the atmosphere after 

emission of a precursor gas. Because of 

these properties, the climate effects of 

these greenhouse gases are generally 

better understood than the climate 

effects associated with most other 

climate-forcing agents (described in 

more detail in subsection 4 below). 

As discussed above, carbon dioxide is 

the most important greenhouse gas 

directly emitted by human activities in 

terms of its total additional heating 

effect being exerted on the climate. 

However, the other greenhouse gases are 

stronger heat-trapping gases compared 

to carbon dioxide on a per mass basis, 19

and are responsible for a sizable fraction 

of the total anthropogenic climatic 

heating effect caused to date. 

Collectively, increased atmospheric 

concentrations of methane, nitrous 

oxide, hydrofluorocarbons, 

perfluorocarbons, and sulfur 

hexafluoride have exerted an additional 

heating effect on the global climate 

since pre-industrial times that is about 

40 percent as large as the additional 

carbon dioxide heating effect, according 

to the IPCC. Of these non-CO2

greenhouse gases, methane is the most 

important in terms of its total additional 

heating effect. Under all future 

scenarios, carbon dioxide is projected to 

remain the dominant driver of climate 

change for the remainder of this 

century. 

Because these six greenhouse gases 

share common properties and are the 

key driver of human-induced climate 

change, they have been the common 

focus of climate change science and 

policy to date. The United Nations 

Framework Convention on Climate 

Change (UNFCCC) addresses these six 

long-lived, well-mixed greenhouse gases 

not controlled by the Montreal Protocol 

on Substances that Deplete the Ozone 

Layer. The IPCC scientific assessments 

focus primarily on these six greenhouse 

gases and their effects on climate. 

Treating the air pollution as the mix 

of the six greenhouse gases is consistent 

with other provisions of the Act and 

previous EPA practice under the Act, 

where separate air pollutants from 

different sources but with common 

properties may be treated as a class (e.g., 

Class I and Class II substances under 

Title VI). This approach addresses the 

cumulative effect that the elevated 

concentrations of the six greenhouse 

gases have on climate, and thus on 

different elements of health, society and 

the environment.20

The scientific literature that assesses 

the potential risks and end-point 

impacts of human-induced climate 

change does not typically assess these 

impacts on a gas-by-gas basis. It is true 

that estimates are available for how 

individual greenhouse gases and other 

climate-forcing agents are contributing 

to the anthropogenic heating (or 

cooling) effect being exerted on the 

global climate. However, as one moves 

farther down the causal chain towards 

end-point risks and impacts to human 

health, society and the environment, 

such impacts, whether observed or 

projected, are typically not attributed to 

the temperature increase or other 

climatic change due to the elevated 

atmospheric concentration of just one of 

the greenhouse gases. 

2. Evidence That the Six Greenhouse 

Gases Are at Unprecedented Levels in 

the Atmosphere 

Given the long atmospheric lifetime 

and global mixing of greenhouse gases, 

global average atmospheric 

concentrations are an important metric 

by which to measure changes in 

atmospheric composition. Current 

atmospheric greenhouse gas 

concentrations are now at elevated 

levels as a result of both historic and 

current anthropogenic emissions. The 

global atmospheric carbon dioxide 

concentration has increased about 38 

percent from pre-industrial levels to 

2009, and almost all of the increase is 

due to anthropogenic emissions. The 

current (year 2009) carbon dioxide 

concentration is 386 parts per million 

(ppm) and has recently been increasing 

by about 2.0 ppm per year. The global 

atmospheric concentration of methane 

has increased by 149 percent since preindustrial levels (through 2007), and the 

nitrous oxide concentration has 

increased 23 percent (through 2007). 

The observed concentration increase in 

these gases can also be attributed 

primarily to anthropogenic emissions. 

The industrial fluorinated gases, 

hydrofluorocarbons, perfluorocarbons, 

and sulfur hexafluoride, are almost 

entirely anthropogenic in origin, and 

have relatively low atmospheric 

concentrations but are increasing 

rapidly; concentrations of many of these 

gases have increased by large factors 


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21According to IPCC terminology, ‘‘very likely’’ 

conveys a 90 to 99 percent probability of 

occurrence. ‘‘Virtually certain’’ conveys a greater 

than 99 percent probability, and ‘‘likely’’ conveys 

a 66 to 90 percent probability. 

22 CCSP (2008) Reanalysis of Historical Climate 

Data for Key Atmospheric Features: Implications for 

Attribution of Causes of Observed Change. A Report 

by the U.S. Climate Change Science Program and 

the Subcommittee on Global Change Research 

[Randall Dole, Martin Hoerling, and Siegfried 

Schubert (eds.)]. National Oceanic and Atmospheric 

Administration, National Climatic Data Center, 

Asheville, NC, 156 pp. 

23This CCSP report used likelihood terminology 

that is consistent with that used by IPCC where 

‘‘likely’’ also conveys a 66 to 90 percent probability 

of occurrence. 

(between 4.3 and 1.3) between 1998 and 

2005. 

Historic data that go back many 

thousands of years show that current 

atmospheric concentrations of the two 

most important directly emitted, longlived greenhouse gases (carbon dioxide 

and methane) are well above the natural 

range of atmospheric concentrations 

compared to the last 650,000 years. 

Atmospheric greenhouse gas 

concentrations have been increasing 

because human emissions have been 

outpacing the ability of the natural 

environment to remove greenhouse 

gases from the atmosphere over 

timescales of decades to centuries. 

The Administrator recognizes these 

scientific findings that the current 

global atmospheric concentrations of the 

six greenhouse gases are now at 

unprecedented and record-high levels 

compared to both the recent and distant 

past. It is also unambiguous that the 

current elevated greenhouse gas 

concentrations are the primary result of 

human activities. 

Total concentrations of these 

greenhouse gases are projected to 

continue climbing, and thus to continue 

pushing unprecedented levels upwards 

for the foreseeable future under different 

plausible assumptions of U.S. and 

global greenhouse gas-emitting 

activities. Given the long atmospheric 

lifetime of the six greenhouse gases, 

significant changes in total greenhouse 

gas global atmospheric concentrations 

do not come about quickly (i.e., within 

a few years). Future atmospheric 

greenhouse gas concentrations—not 

only for the remainder of the current 

century but indeed for decades and in 

some cases centuries well beyond 

2100—will be influenced by our present 

and near-term greenhouse gas 

emissions. Consideration of future 

plausible scenarios, and how our 

current greenhouse gas emissions 

essentially commit present and future 

generations to cope with an altered 

atmosphere and climate, reinforces the 

Administrator’s judgment that it is 

appropriate to define the combination of 

the six key greenhouse gases as the air 

pollution. 

3. Evidence That Elevated Atmospheric 

Concentrations of the Six Greenhouse 

Gases Are the Root Cause of Observed 

Climate Change 

The scientific evidence is compelling 

that elevated concentrations of heattrapping greenhouse gases are the root 

cause of recently observed climate 

change. This is different from historic 

drivers of climate change, such as 

cyclical changes in the Earth’s orbit, 

which have occurred over thousands of 

years. 

The global average net effect of the 

increase in atmospheric greenhouse gas 

concentrations, plus other human 

activities (e.g., land use change and 

aerosol emissions), on the global energy 

balance since 1750 has been one of 

warming. This total net heating effect, 

referred to as forcing, is estimated to be 

1.6 Watts per square meter (W/m2), with 

much of the range surrounding this 

estimate due to uncertainties about the 

cooling and warming effects of aerosols. 

The combined radiative forcing due to 

the cumulative increase in atmospheric 

concentrations of carbon dioxide, 

methane, and nitrous oxide over the 

period 1750 to 2005 is 2.30 W/m2. The 

positive radiative forcing due to carbon 

dioxide is the largest (1.66 W/m2). 

Methane is the second largest source of 

positive radiative forcing (0.48 W/m2). 

Nitrous oxide has a positive radiative 

forcing of 0.16 W/m2. The rate of 

increase in forcing due to these three 

greenhouse gases during the industrial 

era is, according to IPCC, very likely 21

to have been unprecedented in more 

than 10,000 years. 

Warming of the climate system is now 

unequivocal, as is evident from 

observations of increases in global 

average air and ocean temperatures, 

widespread melting of snow and ice, 

and rising global average sea level. 

Global mean surface temperatures have 

risen by 0.74 °C (1.3 °F) over the last 100 

years. Eight of the ten warmest years on 

record have occurred since 2001. Global 

mean surface temperature was higher 

during the last few decades of the 20th 

century than during any comparable 

period during the preceding four 

centuries. 

Most of the observed increase in 

global average temperatures since the 

mid-20th century is very likely due to 

the observed increase in anthropogenic 

greenhouse gas concentrations. Global 

observed temperatures over the last 

century can be reproduced only when 

model simulations include both natural 

and anthropogenic forcings, that is, 

simulations that remove anthropogenic 

forcings are unable to reproduce 

observed temperature changes. Thus, 

most of the warming cannot be 

explained by natural variability, such as 

variations in solar activity. 

In addition to attributing recent global 

warming to anthropogenic greenhouse 

gas influence at the global scale, both 

the IPCC and CCSP reports attributed 

recent North American warming to 

elevated greenhouse gas concentrations. 

A 2008 CCSP report 22 found that for 

North America, ‘‘more than half of this 

warming [for the period 1951–2006] is 

likely 23 the result of human-caused 

greenhouse gas forcing of climate 

change.’’ 

Therefore, by defining air pollution as 

the six greenhouse gases, the 

Administrator is identifying the 

fundamental and underlying driver of 

human-induced climate change, which 

in turn, as described below, poses risks 

to human health, society, and the 

environment. The Administrator 

believes that the proposed definition of 

air pollution captures the root of the 

problem, and addresses the part of the 

problem that is best understood, 

scientifically speaking, and that is 

already the focus of scientists and 

policy analysts involved in studying 

climate change. Because the six 

greenhouse gases are collectively the 

primary driver of the climate change 

problem, all current and future risks due 

to human-induced climate change— 

whether these risks are associated with 

increases in temperature, changes in 

precipitation, a rise in sea levels, 

changes in the frequency and intensity 

of weather events, or more directly with 

the elevated greenhouse gas 

concentrations themselves—can be 

associated with this definition of ‘‘air 

pollution.’’ This does not imply that 

other anthropogenic climate forcers, 

discussed below, would pose no risks. 

EPA has considered whether other 

climate-forcing agents in addition to the 

six greenhouse gases should be included 

in this proposed definition of air 

pollution, and for the reasons discussed 

below is not proposing to include them 

in the definition of air pollution for 

purposes of this proposed 

endangerment finding. 

4. Other Climate Forcers 

There are other greenhouse gases and 

aerosols that have warming (and 

cooling) effects but are not being 

included in the proposed definition of 

air pollution. These include water 

vapor, chlorofluorocarbons (CFCs), 

hydrochlorofluorocarbons (HCFCs), 


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halons, tropospheric ozone (O3), black 

carbon, and other short-lived precursor 

gases. For each of these substances, 

there are different scientific and policy 

reasons why these substances are not 

being included in the proposed 

definition of air pollution for purposes 

of section 202(a). 

a. Water Vapor 

Water vapor is the most abundant 

naturally occurring greenhouse gas and 

therefore makes up a significant share of 

the natural, background greenhouse 

effect. However, direct water vapor 

emissions from human activities have 

only a negligible effect on atmospheric 

concentrations of water vapor, whereas 

direct emissions of the six greenhouse 

gases have significantly altered the 

global atmospheric concentrations of 

those gases, as detailed above. 

Significant changes to global 

atmospheric concentrations of water 

vapor can occur indirectly through 

human-induced global warming, which 

then increases the amount of water 

vapor in the atmosphere because a 

warmer atmosphere can hold more 

moisture. Therefore, changes in water 

vapor concentrations are not an initial 

driver of climate change, but rather an 

effect of climate change which then acts 

as a positive feedback that further 

enhances warming. For this reason, the 

IPCC does not list direct emissions of 

water vapor as an anthropogenic forcing 

agent of climate change, but does 

include this water vapor feedback 

mechanism in response to humaninduced warming in all modeling 

scenarios of future climate change. 

Based on this recognition that 

anthropogenic emissions of water vapor 

are a negligible driver of anthropogenic 

climate change, EPA’s annual Inventory 

of U.S. Greenhouse Gas Emissions and 

Sinks does not include water vapor, and 

greenhouse gas inventory reporting 

guidelines under the UNFCCC do not 

require data on water vapor emissions. 

Water vapor may be an issue of 

concern when it is emitted by aircraft at 

high altitudes, where, under certain 

conditions, it can lead to the formation 

of condensation trails, referred to as 

contrails. Similar to high-altitude, thin 

clouds, contrails have a warming effect. 

Extensive cirrus clouds can also develop 

from aviation contrails, and increases in 

cirrus cloud cover would also have a 

warming effect. The IPCC Fourth 

Assessment Report estimated a very 

small positive heating effect for linear 

contrails, with a low degree of scientific 

understanding. Unlike the warming 

effects associated with the six longlived, well-mixed greenhouse gases, the 

warming effects associated with 

contrails or contrail-induced cirrus 

cloud cover are more regional and 

temporal in nature. EPA has received a 

petition under the Act to consider the 

regulation of aircraft emissions (water 

vapor and NOx) that lead to formation 

of contrails (in addition to aircraft 

greenhouse gas emissions), and EPA 

plans to evaluate this issue further. At 

this time, the Administrator is not 

proposing to include aircraft-related 

contrails or emissions that are not 

greenhouse gases within the definition 

of air pollution for purposes of section 

202(a). 

b. The Ozone-Depleting Substances: 

CFCs, HCFCs and Halons 

Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and 

halons are ozone-depleting substances 

that have been responsible for the 

depletion of stratospheric ozone, which 

prevents harmful forms of ultraviolet 

radiation from reaching the Earth’s 

surface. The Montreal Protocol on 

Substances that Deplete the Ozone 

Layer is an international agreement that 

controls these substances. In the U.S., 

these substances are being controlled 

and phased out under Title VI of the 

Act. Despite their ozone-depleting 

properties, which the six greenhouse 

gases in the definition of air pollution 

do not share, these substances share 

other common physical properties with 

the six greenhouse gases: They are also 

long-lived in the atmosphere; well 

mixed throughout the global 

atmosphere; are directly emitted by 

anthropogenic sources; and have been 

responsible for a share of the humaninduced heating effect to date. However, 

these substances have not been a 

priority for the scientists and policy 

analysts involved in studying climate 

change, and they are not a priority for 

the Administrator for this action. The 

UNFCCC does not address these 

substances and instead defers their 

treatment to the Montreal Protocol. The 

Administrator is not proposing to 

include these substances in the 

definition of air pollution with this 

action, but will continue to consider 

these issues. 

c. Tropospheric Ozone 

Increased concentrations of 

tropospheric O3 are estimated to be 

causing a significant anthropogenic 

warming effect. However, unlike the 

long-lived six greenhouse gases, 

tropospheric O3 has a short atmospheric 

lifetime (hours to weeks) and therefore 

its concentrations are more variable over 

space and time. For these reasons, its 

global heating effect and contribution to 

climate change tend to entail greater 

uncertainty compared to the wellmixed, long-lived greenhouse gases. 

Tropospheric O3 is also not a directly 

emitted greenhouse gas, but rather 

undergoes secondary formation in the 

atmosphere from the emission of 

precursor gases such as nitrogen oxides 

(NOX) and volatile organic compounds 

(VOCs). For these reasons, the 

Administrator is not including 

tropospheric O3 in the proposed 

definition of air pollution with this 

action. 

d. Black Carbon 

Black carbon is not a greenhouse gas 

but an aerosol particle that results from 

incomplete combustion of the carbon 

contained in fossil fuels, and remains in 

the atmosphere for only about a week. 

Black carbon is a component of 

particulate matter (PM), which is 

regulated as a criteria air pollutant 

under the Act. Scientific studies have 

found an association between exposure 

to PM and significant health problems. 

Black carbon causes a warming effect 

by absorbing incoming sunlight 

(whereas greenhouse gases cause 

warming by trapping outgoing, infrared 

heat), and by darkening bright surfaces 

such as snow and ice, which reduces 

reflectivity. This latter effect in 

particular has been raising concerns 

about the role black carbon may be 

playing in observed warming and ice 

melt in the Arctic. 

Black carbon is co-emitted with other 

pollutants, especially organic carbon, 

which all tend to have a direct cooling 

effect on climate because they reflect 

and scatter incoming sunlight. However, 

black carbon, per unit mass, is a more 

effective warming agent than organic 

carbon is a cooling agent. The IPCC 

Fourth Assessment Report estimated 

that co-emissions of organic carbon may 

be offsetting about 40 percent of black 

carbon’s warming effect on a global 

average. The ratio of black carbon to 

organic carbon varies by fuel type and 

by combustion efficiency, such that 

different emission sources will have 

different net climate effects; likewise, 

different emission reduction measures 

will have different net climate effects. 

Furthermore, because black carbon is 

short lived in the atmosphere, the net 

climate effect of a black carbon emission 

source will also depend on location; for 

example, emissions that deposit on 

snow and ice, or get lofted above cloud 

surfaces, could have a stronger warming 

effect. Like other aerosols, black carbon 

can also affect the reflectivity and 

lifetime of clouds. How black carbon 

and other aerosols, such as sulfates, 

alter cloud properties is a key source of 

uncertainty in quantifying the total 


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18898 Federal Register / Vol. 74, No. 78 / Friday, April 24, 2009 / Proposed Rules 

24 Ramanathan V. and G. Carmichael (2008) 

Global and regional climate changes due to black 

carbon. Nature Geoscience, 1: 221–227. 

human influence on the global climate. 

This total cloud indirect effect caused 

by all aerosols (e.g., sulfates, black 

carbon and organic carbon) is estimated 

to be causing a net cooling effect, with 

a large range of uncertainty. Given these 

reasons, there is considerably more 

uncertainty associated with black 

carbon’s warming effect compared to the 

estimated warming effect of the six longlived greenhouse gases. 

Given the number of science issues 

for black carbon that are different than 

for the six greenhouse gases, the 

Administrator is not proposing to 

include black carbon in the definition of 

air pollution for purposes of section 

202(a) with this action. However, EPA is 

already undertaking work to further 

evaluate the role of black carbon in 

climate change, in addition to its role as 

an element of the already-regulated 

PM2.5. Indeed, a recent study 24

referenced in the TSD estimated that 

black carbon is having a much stronger 

direct warming effect (160 percent 

higher on a global average) compared to 

IPCC’s estimate. EPA has also received 

petitions to specifically address black 

carbon emissions under the Act from 

marine and aviation sources, and EPA 

plans to respond to these petitions in a 

separate action. 

e. Fluorinated Ethers and Recently 

Identified Greenhouse Gases 

Fluorinated ethers are used in 

electronics, anesthetics, and as heat 

transfer fluids. Like the six greenhouse 

gases included in the proposed 

definition of air pollution, these 

fluorinated compounds have heattrapping properties and can also be 

long-lived in the atmosphere. In many 

cases these fluorinated gases are used in 

expanding industries (e.g., electronics) 

or as substitutes for hydrofluorocarbons. 

Also, new compounds that have 

greenhouse gas attributes continue to be 

discovered, such as nitrogen trifluoride 

(NF3). The IPCC has now assigned 

global warming potentials (GWPs) to 

both fluorinated ethers and NF3. 

However, the total global radiative 

forcing contribution of these 

compounds is not yet available to 

compare with the anthropogenic heating 

effect caused by the six greenhouse 

gases. The Administrator is not 

proposing to include these gases in the 

definition of air pollution with this 

action. 

C. The Administrator’s Proposed 

Finding That the Air Pollution 

Endangers Public Health and Welfare 

The scientific evidence clearly 

indicates that atmospheric levels of the 

six greenhouse gases are at 

unprecedented elevated levels due to 

human activities, and that most of the 

observed global and continental 

warming can be attributed to this 

anthropogenic rise in greenhouse gases. 

The information presented here builds 

on these facts that support the proposed 

definition of air pollution. 

Based on the total weight of evidence, 

which is briefly summarized here and 

set forth in more detail in the TSD, it is 

the Administrator’s judgment that 

current and projected levels of the mix 

of the six greenhouse gases endanger the 

public health and welfare of current and 

future generations. 

The Administrator’s proposed 

endangerment finding is based on the 

entire range of observed risks and 

potential harms to public health and 

welfare. The Administrator is not basing 

her proposal on any one impact, but 

instead is weighing the evidence 

collectively and determining that as a 

whole it clearly indicates that the air 

pollution at issue endangers public 

health and welfare now and in the 

future. 

Furthermore, the Administrator is 

taking into account a number of key 

considerations that provide guidance on 

how to weigh and interpret the 

collective body of scientific evidence for 

today’s proposal, namely: The observed 

record of climate change and our ability 

to attribute these changes to the 

observed anthropogenic buildup of 

greenhouse gases in the atmosphere; 

plausible future changes in climate over 

the next several decades and beyond 

given both the accumulation of 

greenhouse gases in the atmosphere to 

date plus expected increases in 

concentrations under different scenarios 

of future greenhouse gas emission 

pathways; the level of certainty with 

which we can reasonably project both 

near- and long-term climate change; our 

ability to identify known risks to public 

health and welfare, both today and in 

the future in light of a continually 

changing climate; the vulnerability of 

particularly susceptible populations and 

regions; the likelihood that such risks to 

both public health and welfare are 

happening now and will happen in the 

future; the magnitude of such risks and 

impacts to public health and welfare; 

and finally a consideration of how key 

gaps in our knowledge of current, but 

especially future, effects factor into an 

endangerment decision. 

The following discussion sets forth 

the Administrator’s rationale for making 

this proposed endangerment finding, 

including a description of the 

supporting scientific findings showing 

evidence of the effects that elevated 

greenhouse gas concentrations are 

having currently and are projected to 

have in the future, and the implications 

of these effects for public health and 

welfare. 

1. Evidence of Currently Observed 

Climatic and Related Effects 

There is compelling evidence that a 

number of climate and physical changes 

are occurring now that can be attributed 

to the anthropogenic rise in atmospheric 

greenhouse gases, and other changes 

that are consistent with the direction of 

change expected from warming and 

human-induced climate change. These 

observed changes described below can 

adversely affect and pose risks to both 

public health and welfare. 

The global indicators of change go 

beyond the well-established surface air 

temperature rise discussed above. 

Observational evidence from all 

continents and most oceans shows that 

many natural systems are being affected 

by regional climate changes, particularly 

temperature increases. Observations 

show that changes are occurring in the 

amount, intensity, frequency, and type 

of precipitation. There is strong 

evidence that global sea level gradually 

rose in the 20th century and is currently 

rising at an increased rate. Widespread 

changes in extreme temperatures have 

been observed in the last 50 years. 

Globally, cold days, cold nights, and 

frost have become less frequent, while 

hot days, hot nights, and heat waves 

have become more frequent. 

Satellite data since 1978 show that 

annual average Arctic sea ice extent has 

shrunk by 2.7 ± 0.6 percent per decade, 

with larger decreases in summer of 7.4 

± 2.4 percent per decade. The latest data 

from NASA indicate Arctic sea ice set 

a record low in September 2007, 38 

percent below the 1979–2007 average. 

In September 2008, Arctic sea ice 

reached its second lowest extent on 

record. 

Like global mean temperatures, U.S. 

air temperatures have warmed during 

the 20th and into the 21st century. 

According to official data from NOAA’s 

National Climatic Data Center: 

• U.S. average annual temperatures 

are now approximately 1.25 °F (0.69 °C) 

warmer than at the start of the 20th 

century, with an increased rate of 

warming over the past 30 years. The rate 

of warming for the entire period of 

record (1895–2008) is 0.13 °F/decade 

while the rate of warming increased to 


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25Backlund, P., A. Janetos, D.S. Schimel, J. 

Hatfield, M.G. Ryan, S.R. Archer, and D. 

Lettenmaier (2008) Executive Summary. In: The 

effects of climate change on agriculture, land 

resources, water resources, and biodiversity in the 

United States. A Report by the U.S. Climate Change 

Science Program and the Subcommittee on Global 

Change Research. Washington, DC., USA, 362 pp. 

26 Karl, T.R., G.A. Meehl, T.C. Peterson, K.E. 

Kunkel, W.J. Gutowski, Jr., D.R. Easterling (2008) 

Executive Summary in Weather and Climate 

Extremes in a Changing Climate. Regions of Focus: 

North America, Hawaii, Caribbean, and U.S. Pacific 

Islands. T.R. Karl, G.A. Meehl, C.D. Miller, S.J. 

Hassol, A.M. Waple, and W.L. Murray (eds.). A 

Report by the U.S. Climate Change Science Program 

and the Subcommittee on Global Change Research, 

Washington, DC. 

0.58 °F/decade (0.32 °C/decade) for the 

period from 1979–2008. 

• 2005–2007 were exceptionally 

warm years (among the top 10 warmest 

on record), while 2008 was slightly 

warmer than average (the 39th warmest 

year on record), 0.2 °F (0.1 °C) above the 

20th century (1901–2000) mean. 

• The last ten 5-year periods (2004– 

2008, 2003–2007, 2002–2006, 2001– 

2005, 2000–2004, 1999–2003, 1998– 

2002, 1997–2001, 1996–2000, and 1995– 

1999), were the warmest 5-year periods 

in the 114 years of national records, 

demonstrating the anomalous warmth of 

the last 15 years. 

Over the contiguous U.S., total annual 

precipitation increased at an average 

rate of 6.5 percent over the period 1901– 

2006. It is likely that there have been 

increases in the number of heavy 

precipitation events within many land 

regions, even in those where there has 

been a reduction in total precipitation 

amount, consistent with a warming 

climate. 

Sea level has been rising along most 

of the U.S. Atlantic and Gulf coasts. In 

the mid-Atlantic region from New York 

to North Carolina, tide-gauge 

observations indicate that relative sealevel rise (the combination of global sealevel rise and land subsidence) rates 

were higher than the global mean and 

generally ranged between 2.4 and 4.4 

millimeters per year, or about 0.3 meters 

(1 foot) over the twentieth century. 

Climate changes are very likely 

already affecting U.S. water resources, 

agriculture, land resources, and 

biodiversity as a result of climate 

variability and change. A 2008 CCSP 

report 25 that examined these observed 

changes concluded, ‘‘[t]he number and 

frequency of forest fires and insect 

outbreaks are increasing in the interior 

West, the Southwest, and Alaska. 

Precipitation, stream flow, and stream 

temperatures are increasing in most of 

the continental U.S. The western U.S. is 

experiencing reduced snowpack and 

earlier peaks in spring runoff. The 

growth of many crops and weeds is 

being stimulated. Migration of plant and 

animal species is changing the 

composition and structure of arid, polar, 

aquatic, coastal, and other ecosystems.’’ 

Regarding observed changes in 

extreme events, another 2008 CCSP 

report 26 stated the following: ‘‘Many 

extremes and their associated impacts 

are now changing. For example, in 

recent decades most of North America 

has been experiencing more unusually 

hot days and nights, fewer unusually 

cold days and nights, and fewer frost 

days. Heavy downpours have become 

more frequent and intense. Droughts are 

becoming more severe in some regions, 

though there are no clear trends for 

North America as a whole. The power 

and frequency of Atlantic hurricanes 

have increased substantially in recent 

decades, though North American 

mainland land-falling hurricanes do not 

appear to have increased over the past 

century. Outside the tropics, storm 

tracks are shifting northward and the 

strongest storms are becoming even 

stronger.’’ 

2. Future Projected Climatic and Related 

Effects 

Because atmospheric greenhouse gas 

concentrations are expected to climb for 

the foreseeable future, temperatures will 

continue to rise and the overall rate and 

magnitude of human-induced climate 

change will likely increase, such that 

risks to public health and welfare will 

likewise grow over time so that future 

generations will be especially 

vulnerable; their vulnerability will 

include potentially catastrophic harms. 

Projected effects here focus on the next 

several decades and the timeframe out 

to 2100. 

The majority of future reference-case 

scenarios (assuming no explicit 

greenhouse gas mitigation actions 

beyond those already enacted) project 

an increase of global greenhouse gas 

emissions over the century, with 

climbing greenhouse gas concentrations. 

Long-lived gas concentrations increase 

even for those scenarios where annual 

emissions toward the end of the century 

are assumed to be lower than current 

annual emissions. Indeed, for a given 

amount of CO2 released today, about 

half will be taken up by the oceans and 

terrestrial vegetation over the next 30 

years, a further 30 percent will be 

removed over a few centuries, and the 

remaining 20 percent will only slowly 

decay over time such that it will take 

many thousands of years to remove from 

the atmosphere. Carbon dioxide is 

expected to remain the dominant 

anthropogenic driver of climate change 

over the course of the 21st century. The 

heating effect associated with the nonCO2 greenhouse gases is still significant 

and growing over time. 

Future warming over the course of the 

21st century, even under scenarios of 

low emissions growth, is very likely to 

be greater than observed warming over 

the past century (Figure 1). Through 

about 2030, the global warming rate is 

affected little by the choice of different 

future emission scenarios, according to 

IPCC. By mid-century, the choice of 

scenario becomes more important for 

the magnitude of the projected warming; 

About a third of that warming is 

projected to be due to climate change 

that is already committed. By the end of 

the century, projected average global 

warming (compared to average 

temperature around 1990) varies 

significantly depending on emissions 

scenario and climate sensitivity 

assumptions, ranging from 1.8 to 4.0 °C 

(3.2 to 7.2 °F), with an uncertainty range 

of 1.1 to 6.4 °C (2.0 to 11.5 °F), according 

to the IPCC. 


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Global mean precipitation is expected 

to increase with global warming. 

However, there are substantial spatial 

and seasonal variations. Increases in the 

amount of precipitation are very likely 

in high latitudes, while decreases are 

likely in the mid-latitudes and semi-arid 

low latitudes including much of the 

already water-stressed southwestern 

U.S., continuing observed patterns in 

recent trends. Drought is expected to 

increase in the western U.S., where 

water availability to meet demands for 

agricultural and municipal water needs 

is already limited. Another projected 

impact in the western U.S. is decreased 

water availability due to a range of interconnected factors. These include: 

decreased snowpack, earlier snowmelt 

resulting in peak winter and decreased 

summer flows, which will disrupt and 

limit water storage capacity and will 

create additional challenges for water 

allocation among competing uses 

(agricultural, municipal, industrial, 

ecological). Rising sea levels could lead 

to salt water intrusion of coastal ground 

aquifers, which would further reduce 

freshwater availability for municipal 

and agricultural use among coastal 

communities that depend on these 

aquifers. 

By the end of the century, sea level is 

projected by IPCC to rise between 0.18 

and 0.59 meters relative to around 1990 

in the absence of increased dynamic ice 

sheet loss. Recent rapid changes at the 

edges of the Greenland and West 

Antarctic ice sheets show acceleration 

of flow and thinning. While 

understanding of these ice sheet 

processes is incomplete, their inclusion 

in models would likely lead to 

increased sea-level projections for the 

end of the 21st century. Sea ice is 

projected to shrink in the Arctic under 

all IPCC emission scenarios. 

All of the U.S. is very likely to warm 

during this century, and most areas of 

the U.S. are expected to warm by more 

than the global average. The largest 

warming through 2100 is projected to 

occur in winter over northern parts of 

Alaska. In western, central and eastern 

regions of North America, the projected 

warming has less seasonal variation and 

is not as large, especially near the coast, 

consistent with less warming over the 

oceans. 

The U.S is projected to see an overall 

average increase in the intensity of 

precipitation events, which is likely to 

increase the risk of flood events, though 

projections for specific regions are very 

uncertain. 

As the climate warms, glaciers will 

lose mass owing to dominance of 

summer melting over winter 

precipitation increases, contributing to 

sea level rise. 

For North American coasts, sea level 

rise may be similar to the global mean, 

with slightly higher rates in western 

Alaska. The projected rate of sea level 


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27According to the IPCC lexicon, ‘‘very high 

confidence’’ conveys at least a 9 out of 10 chance 

of being correct. ‘‘High confidence’’ conveys an 8 

out of 10 chance of being correct, and ‘‘medium 

confidence’’ a 5 out of 10 chance. 

rise off the low-lying U.S. South 

Atlantic and Gulf coasts is also higher 

than the global average. 

Based on a range of models, it is likely 

that tropical cyclones (tropical storms 

and hurricanes) will become more 

intense, with stronger peak winds and 

more heavy precipitation associated 

with ongoing increases of tropical sea 

surface temperatures. Storm surge levels 

are likely to increase due to projected 

sea level rise. Frequency changes in 

hurricanes are currently too uncertain 

for confident projections. 

3. Impacts on Public Health 

Many of the observed and projected 

changes in climate and climate-sensitive 

systems discussed above pose serious 

risks to public health. The following 

discussion outlines specific public 

health concerns raised by observations 

and plausible future outcomes, 

recognizing the statutory requirement 

that the Administrator consider how 

sensitive or susceptible populations 

may be particularly at risk. As our 

discussion of increasing temperatures 

suggests, the adverse effects of 

greenhouse gas emissions are expected 

to mount over time. The findings of the 

IPCC, and of many others, indicate that 

risks to public health will be more 

severe in 20 years than in ten years, 

more severe in 30 years than in 20 years, 

more severe in 40 years than in 30 years, 

and so forth. There is disagreement 

about whether and when increases in 

adverse effects will be linear or 

nonlinear; on some projections, 

nonlinear increases in such effects can 

reasonably be expected at some future 

point. We believe that existing evidence 

supports a finding that there are current 

adverse effects. This evidence also 

supports a finding that these effects will 

become more serious over the next 

several decades, in some cases out to 

2100. 

To be clear, ambient concentrations of 

carbon dioxide and the other 

greenhouse gases, whether at current 

levels or at projected ambient levels 

under scenarios of high emissions 

growth over time, do not cause direct 

adverse health effects such as 

respiratory or toxic effects. All public 

health risks and impacts described here 

as a result of elevated atmospheric 

concentrations of greenhouse gases 

occur via climate change. The pathway 

or mechanism occurs through changes 

in climate, but the end result is an 

adverse effect on the health of the 

population. Thus these effects from 

climate change are appropriately 

denoted public health effects. It is 

important to acknowledge that effects 

on ‘‘welfare’’ do not always entail 

effects on ‘‘public health,’’ and the 

Administrator does not mean to 

interpret ‘‘public health’’ to include 

‘‘welfare’’ effects as such. Today’s 

interpretation does not collapse the two 

categories—many ‘‘welfare’’ effects do 

not and cannot involve public health. 

The Administrator simply means to 

recognize, with the scientific 

community, that concentrations of 

greenhouse gases endanger public 

health through a wide range of 

pathways. 

As described above, there is evidence 

that unusually hot days and nights and 

heat waves have become more frequent 

in the U.S. Severe heat waves are 

projected to intensify in magnitude and 

duration over the portions of the U.S. 

where these events already occur, with 

likely increases in mortality and 

morbidity. The populations most 

sensitive to hot temperatures are older 

adults, the chronically sick, the very 

young, city-dwellers, those taking 

medications that disrupt 

thermoregulation, the mentally ill, those 

lacking access to air conditioning, those 

working or playing outdoors, and the 

socially isolated. 

The Administrator also acknowledges 

that warming temperatures may bring 

about some health benefits. Both 

extremely cold days and extremely hot 

days are dangerous to human health. 

But at least in the short run, modest 

temperature increases may produce 

health benefits in the U.S. (and 

elsewhere). Although the IPCC projects 

reduced human mortality from cold 

exposure through 2100, it is currently 

difficult to ascertain the balance 

between increased heat-related 

mortality and decreased cold-related 

mortality. With respect to health, 

different regions will be affected in 

different ways. The Administrator does 

not believe that it is now possible to 

quantify the various effects. Because the 

risks from unusually hot days and 

nights, and from heat waves, are very 

serious, it is reasonable to find on 

balance that these risks support a 

finding that public health is endangered 

even if it is also possible that modest 

temperature increases will have some 

beneficial health effects. 

Increases in regional ozone pollution 

in the U.S. relative to ozone levels 

without climate change are expected 

due to higher temperatures and a 

modification of meteorological factors. 

Increases in regional ozone pollution 

increase the risks of respiratory 

infection, aggravation of asthma, and 

premature death. EPA does have in 

place National Ambient Air Quality 

Standards (NAAQS) for ozone, which 

are premised on the harmfulness of 

ozone to public health and welfare. 

These standards and their 

accompanying regulatory regime have 

helped to reduce the dangers from 

ozone in the U.S. Substantial challenges 

remain with respect to achieving the air 

quality protection promised by the 

NAAQS for ozone. These challenges 

will be exacerbated by climate change. 

There will likely be an increase in the 

spread of several food and water-borne 

pathogens (e.g., Salmonella, Vibrio) 

among susceptible populations 

depending on the pathogens’ survival, 

persistence, habitat range and 

transmission under changing climate 

and environmental conditions. The 

primary climate-related factors that 

affect these pathogens include 

temperature, precipitation, extreme 

weather events, and shifts in their 

ecological regimes. 

Climate change, including the direct 

changes in carbon dioxide 

concentrations themselves, could 

impact the production, distribution, 

dispersion and allergenicity of 

aeroallergens and the growth and 

distribution of weeds, grasses and trees 

that produce them. These changes in 

aeroallergens and subsequent human 

exposures could affect the prevalence 

and severity of allergy symptoms. 

However, the scientific literature does 

not provide definitive data or 

conclusions on how climate change 

might impact aeroallergens and 

subsequently the prevalence of 

allergenic illnesses in the U.S. 

The IPCC reports with very high 

confidence 27 that climate change 

impacts on human health in U.S. cities 

will be compounded by population 

growth and an aging population. The 

CCSP reports that climate change has 

the potential to accentuate the 

disparities already evident in the 

American health care systems as many 

of the expected health effects are likely 

to fall disproportionately on the poor, 

the elderly, the disabled, and the 

uninsured. 

Within settlements experiencing 

climate change stressors, certain parts of 

the population may be especially 

vulnerable based on their 

circumstances. These include the poor, 

the elderly, the very young, those 

already in poor health, the disabled, 

those living alone, those with limited 

rights and power (such as recent 

immigrants with limited English skills), 

and/or indigenous populations 

dependent on one or a few resources. 


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These potential impacts of climate 

change have taken on added meaning in 

light of the risk that hurricanes are 

likely to become more severe with 

climate change, and in light of our 

heightened awareness about how 

vulnerable the U.S. Gulf Coast can be. 

Some have argued that a positive 

endangerment finding for public health 

cannot be made because the health 

effects associated with elevated 

atmospheric concentrations of 

greenhouse gases occur via climate 

change, and not directly through 

inhalation or other exposure to the 

greenhouse gases themselves. These 

commenters argue that because 

‘‘climate’’ is included in the definition 

of welfare, the Act requires that all 

effects which may flow from a welfare 

effect must themselves be considered a 

welfare effect. The Administrator 

disagrees with this narrow view of the 

endangerment criteria. Mortality and 

morbidity that result from the effects of 

climate change are clearly public health 

problems. It would be anomalous to 

argue that a person who is injured or 

dies from heat exhaustion or increased 

exposure to a pathogen has not suffered 

a health impact. In addition, 

tropospheric ozone is already regulated 

under the Act as a criteria air pollutant 

in part due to its adverse impacts on 

public health. It is estimated that 

climate change can exacerbate 

tropospheric ozone levels in some parts 

of the U.S. The Administrator rejects a 

position that would treat the adverse 

effects on the health of individuals 

caused by tropospheric ozone as 

something other than a public health 

threat because they are exacerbated by 

climate change. 

4. Impacts on Public Welfare 

The Act defines ‘‘effects on welfare’’ 

as including, but not limited to, ‘‘effects 

on soils, water, crops, vegetation, 

manmade materials, animals, wildlife, 

weather, visibility, and climate, damage 

to and deterioration of property, and 

hazards to transportation, as well as 

effects on economic values and on 

personal comfort and well-being * * *’’ 

CAA Section 302(h). It is clear that 

current and projected levels of 

greenhouse gases and resultant climate 

change are already adversely affecting, 

and will continue to adversely affect, 

public welfare within the meaning of 

the Act. As noted, the adverse effects of 

greenhouse gases are expected to 

increase over time with growing 

temperatures. This point holds for 

welfare as it does for health. In the 

future, the adverse effects will increase 

and perhaps accelerate; projected risks 

focus on the next several decades and 

out to 2100. 

As heavy rainfall events are expected 

to become more intense, there is an 

increased risk of flooding, greater runoff 

and erosion, and thus the potential for 

adverse water quality effects. 

Climate change will likely further 

constrain already over-allocated water 

resources in some sections of the U.S., 

increasing competition among 

agricultural, municipal, industrial, and 

ecological uses. Although current water 

management practices in the U.S. are 

generally advanced, particularly in the 

West, climate change increasingly 

creates conditions well outside of 

historical observations. Rising 

temperatures will diminish snowpack 

and increase evaporation, affecting 

seasonal availability of water. In the 

Great Lakes and major river systems, 

lower levels are likely to exacerbate 

challenges relating to water quality, 

navigation, recreation, hydropower 

generation, water transfers, and binational relationships. Higher water 

temperatures, increased precipitation 

intensity, and longer periods of low 

flows can exacerbate many forms of 

water pollution. Decreased water supply 

and lower water levels are likely to 

exacerbate challenges relating to 

navigation in the U.S. 

CCSP concluded that, with increased 

CO2 and temperature, the life cycle of 

grain and oilseed crops will likely 

progress more rapidly. But, as 

temperature rises, these crops will 

increasingly begin to experience failure, 

especially if climate variability 

increases and precipitation lessens or 

becomes more variable. Furthermore, 

the marketable yield of many 

horticultural crops—e.g., tomatoes, 

onions, fruits—is very likely to be more 

sensitive to climate change than grain 

and oilseed crops. The IPCC reported 

that moderate climate change in the 

early decades of the century is projected 

to increase aggregate yields of rain-fed 

agriculture in North America as a whole 

by 5–20 percent, but with important 

variability among regions. However, like 

CCSP, IPCC further stated that major 

challenges are projected for crops that 

are near the warm end of their suitable 

range or depend on highly utilized 

water resources. 

Higher temperatures will very likely 

reduce livestock production during the 

summer season, but these losses will 

very likely be partially offset by warmer 

temperatures during the winter season. 

Climate change has very likely 

increased the size and number of forest 

fires, insect outbreaks, and tree 

mortality in the interior west, the 

Southwest, and Alaska, and will 

continue to do so. An increased 

frequency of disturbance is at least as 

important to ecosystem function as 

incremental changes in temperature, 

precipitation, atmospheric CO2, nitrogen 

deposition, and ozone pollution. IPCC 

reported that overall forest growth for 

North America as a whole will likely 

increase modestly (10–20 percent) as a 

result of extended growing seasons and 

elevated CO2 over the next century, but 

with important spatial and temporal 

variation. 

In addition to human health effects, 

tropospheric ozone increases as a result 

of temperature increases and other 

climatic changes can have significant 

adverse effects on crop yields, pasture 

and forest growth and species 

composition. 

Coastal communities and habitats will 

be increasingly stressed by climate 

change impacts interacting with 

development and pollution. Sea level is 

rising along much of the U.S. coast, and 

the rate of change will increase in the 

future, exacerbating the impacts of 

progressive inundation, storm-surge 

flooding, and shoreline erosion. Coastal 

aquifers and estuaries are vulnerable to 

salt water intrusion due to rising sea 

levels, which could compromise water 

sources used for municipal drinking 

water, agricultural crops, and other 

human uses. Storm impacts are likely to 

be more severe, especially along the 

Gulf and Atlantic coasts. Salt marshes, 

other coastal habitats, and dependent 

species are threatened by sea-level rise, 

fixed structures blocking landward 

migration, and changes in vegetation. 

Population growth and rising value of 

infrastructure in coastal areas increases 

vulnerability to climate variability and 

future climate change. 

Water infrastructure, including 

drinking water and wastewater 

treatment plants, and sewer and 

stormwater management systems, may 

be at greater risk of flooding, sea level 

rise and storm surge, low flows, and 

other factors that could impair 

functioning. For example, some of these 

impacts are already being experienced 

in Alaska, where rapidly melting 

permafrost has damaged and disrupted 

drinking water distribution systems and 

wastewater infrastructure. 

Ocean acidification is projected to 

continue, resulting in the reduced 

biological production of marine 

calcifiers, including corals. 

Climate change is likely to affect U.S. 

energy use (e.g., heating and cooling 

requirements), and energy production 

(e.g., effects on hydropower), physical 

infrastructures and institutional 

infrastructures. Climate change will 

likely interact with and possibly 


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28A recent economic study that has received 

considerable attention in the climate change 

research community (Weitzman, The Review of 

Economics and Statistics, 2009) has determined that 

if the probability distribution of the magnitude of 

possible impacts has a ‘‘fat tail’’, then the expected 

utility of reducing the probability of that tail 

becomes astronomical. The study determined that 

anthropogenic climate change is a plausible 

candidate for such a ‘‘fat tailed’’ damage function. 

exacerbate ongoing environmental 

change and environmental pressures in 

settlements, particularly in Alaska 

where indigenous communities are 

facing major environmental changes 

from sea ice loss and coastal erosion 

that threaten traditional ways of life. 

Over the 21st century, changes in 

climate will cause some species to shift 

north and to higher elevations and 

fundamentally rearrange U.S. 

ecosystems. Differential capacities to 

adapt to range shifts and constraints 

from development, habitat 

fragmentation, invasive species, and 

broken ecological connections will alter 

ecosystem structure, composition, 

function, and services. 

The Administrator acknowledges that 

as for human health, so too for welfare: 

moderate temperature increases may 

have some benefits, particularly for 

agriculture and forestry over the short 

term, as summarized above in this 

section and discussed in more detail in 

the Technical Support Document in Part 

IV, sections 9(a) and 10(a). This 

possibility is not inconsistent with a 

judgment that greenhouse gases in the 

atmosphere endanger welfare. Beneficial 

effects can coexist with harmful effects, 

and it is not necessary to reach a firm 

conclusion, for particular domains and 

sectors, about the net result in order to 

reach an overall conclusion in favor of 

endangerment. 

5. The Administrator’s Consideration of 

International Effects 

The Administrator judges that the 

impacts to public health and welfare 

occurring within the U.S. alone warrant 

her proposed endangerment finding. In 

addition, the Administrator believes 

that consideration of climate change 

effects in other world regions adds 

support for today’s proposal, but that 

consideration of international impacts is 

not necessary in order to reach a 

judgment that there is endangerment to 

public health and welfare. Thus, the 

Administrator does not now take a 

position on the legal question whether 

international effects, on their own, 

would be sufficient to support an 

endangerment finding. Some of the 

world’s regions are expected to face 

greater impacts due to climate change 

because they are more vulnerable. Even 

apart from the effects of climate change 

on other world regions—effects which 

are considerable—the Administrator 

also believes many of these impacts 

could raise economic, trade, 

humanitarian and even national security 

issues for the U.S. 

The IPCC identifies the most 

vulnerable world regions as the Arctic, 

because of high rates of projected 

warming on natural systems; Africa, 

especially the sub-Saharan region, 

because of current low adaptive 

capacity (e.g., lack of infrastructure and 

resources) as well as climate change; 

small islands, due to high exposure of 

population and infrastructure to risk of 

sea-level rise and increased storm surge; 

and Asian mega deltas, due to large 

populations and high exposure to sea 

level rise, storm surge and river 

flooding. 

On a global basis, according to the 

IPCC, projected climate change-related 

impacts are likely to affect the health of 

millions of people, particularly those 

with low adaptive capacity, as a result 

of a number of factors including 

increased cardio respiratory diseases 

due to higher concentrations of groundlevel ozone brought on by higher 

temperatures, and by more frequent and 

intense heat waves. Food production is 

expected to be much more vulnerable to 

climate change in poorer regions of the 

world compared to food production in 

the U.S. The IPCC also identified that 

the coasts around the world are 

experiencing the adverse consequences 

of hazards related to climate and sea 

level. Coastal settlements are highly 

vulnerable to extreme events, such as 

storms which impose substantial costs 

on coastal societies. Ecosystems and 

species around the world are very likely 

to show a wide range of vulnerabilities 

to climate change, depending on the 

extent to which climate change alters 

conditions that could cross critical 

thresholds. The most vulnerable 

ecosystems include coral reefs, sea-ice 

ecosystems, high-latitude boreal forests, 

and mountain ecosystems where there is 

no possibility of migrating to adapt to 

climate change. 

Climate change impacts in certain 

regions of the world may exacerbate 

problems that raise humanitarian, trade 

and national security issues for the U.S. 

Climate change has been described as a 

potential threat multiplier regarding 

national security issues. This is because, 

as noted above, climate change can 

aggravate existing problems in certain 

regions of the world such as poverty, 

social tensions, general environmental 

degradation, and conflict over 

increasingly scarce water resources. 

6. The Administrator’s Consideration of 

Key Uncertainties 

There are many inherent uncertainties 

associated with characterizing both the 

observed and projected risks and 

impacts to public health and welfare 

due to current and projected greenhouse 

gas concentrations. Both probability and 

severity are not easy to specify. It is 

difficult to attribute any single past 

event (hurricane, flood, drought, or heat 

wave) to elevated greenhouse gas 

concentrations even if it is understood 

that anthropogenic climate change has 

already made such events more likely or 

more extreme. The precise rate and 

magnitude of future climate change, for 

both the globe and for the U.S., remain 

uncertain, even in the hypothetical case 

where current greenhouse gas 

concentrations would remain constant 

over the next several decades. Projecting 

the exact magnitude of a particular 

impact due to climate change is difficult 

due to what are often long time frames 

to consider, the uncertain nature of how 

the system or sector will be affected by 

climate change, and uncertainties about 

how other factors (e.g., income levels, 

technologies, demographics) will 

change over time which can in turn 

affect the vulnerability of the system or 

sector to climate change. 

Many uncertainties could push in the 

direction of either greater or lesser risks 

as they become better understood. EPA 

has acknowledged the possibility of 

beneficial effects on both health and 

welfare. Other possibilities include 

catastrophic events. Examples of such 

key uncertainties involve how the 

frequency of hurricanes and other 

extreme weather events may change in 

a changing climate, the potential to 

trigger thresholds for abrupt climate 

change (e.g., disintegration of the 

Greenland Ice Sheet or collapse of the 

West Antarctic Ice Sheet), and how 

responsive the climate ultimately will 

be to the heating effect being caused by 

anthropogenic greenhouse gases. Even if 

the probability of extremely high-impact 

events may be small, the existence of 

such high impact events, and the 

potential for other currently unknown 

catastrophic impacts that could 

plausibly result from record-high 

atmospheric greenhouse gas levels, 

substantially bolsters the case for an 

endangerment finding with respect to 

greenhouse gases.28 These uncertainties 

will be with us for the foreseeable 

future. However, Congress expected the 

Administrator to consider uncertainties 

and extrapolate from limited data. It 

also recognized that there are inherent 

limitations and difficulties in 

information on public health and 

welfare, but nonetheless expected the 


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Administrator to exercise her judgment 

based on the information available. 

At the same time, there is a broad base 

of scientific evidence that has been 

reviewed extensively by the scientific 

community, which supports the 

findings discussed about how 

anthropogenic increases in greenhouse 

gases are affecting the climate and the 

key risks to public health and welfare 

that human-induced climate change 

pose. The Administrator believes that 

the scientific findings in totality provide 

compelling evidence of human-induced 

climate change, and that serious risks 

and potential impacts to public health 

and welfare have been clearly 

identified, even if they cannot always be 

quantified with confidence. The 

Administrator’s proposed endangerment 

finding is based on weighing the 

scientific evidence, considering the 

uncertainties, and balancing any 

benefits to human health, society and 

the environment that may also occur. 

Given the evolution of climate change 

science over the past 15 years or more, 

the Administrator believes the evidence 

of discernible human influence on the 

global climate, and the risks that such 

climate change poses, has become more 

compelling, and therefore believes the 

evidence that there is endangerment to 

the public health and welfare of current 

and future generations has likewise 

become more compelling in step with 

our increasing understanding of the 

climate change problem. 

7. Summary 

The Administrator concludes that, in 

the circumstances presented here, the 

case for finding that greenhouse gases in 

the atmosphere endanger public health 

and welfare is compelling and, indeed, 

overwhelming. The scientific evidence 

described here is the product of decades 

of research by thousands of scientists 

from the U.S. and around the world. 

The evidence points ineluctably to the 

conclusion that climate change is upon 

us as a result of greenhouse gas 

emissions, that climatic changes are 

already occurring that harm our health 

and welfare, and that the effects will 

only worsen over time in the absence of 

regulatory action. The effects of climate 

change on public health include 

sickness and death. It is hard to imagine 

any understanding of public health that 

would exclude these consequences. The 

effects on welfare embrace every 

category of effect described in the Clean 

Air Act’s definition of ‘‘welfare’’ and, 

more broadly, virtually every facet of 

the living world around us. And, 

according to the scientific evidence 

relied upon in making this finding, the 

probability of the consequences is 

shown to range from likely to virtually 

certain to occur. This is not a close case 

in which the magnitude of the harm is 

small and the probability great, or the 

magnitude large and the probability 

small. In both magnitude and 

probability, climate change is an 

enormous problem. The greenhouse 

gases that are responsible for it endanger 

public health and welfare within the 

meaning of the Clean Air Act. 

IV. The Administrator’s Cause or 

Contribute Finding 

As noted above, the Administrator has 

proposed to define the air pollution for 

purposes of the endangerment finding to 

be the mix of six key greenhouse gases 

in the atmosphere. The Administrator 

must also define the air pollutant or 

pollutants for purposes of making the 

cause or contribute determination. In 

this section, the air pollutant(s) that 

may cause or contribute to the proposed 

definition of air pollution are discussed. 

As noted earlier, to help appreciate 

the distinction between these terms, the 

air pollution can be thought of as the 

total, cumulative stock in the 

atmosphere. The air pollutants, on the 

other hand, are the emissions and can 

be thought of as the flow that changes 

the size of the total stock. EPA did not 

conduct climate modeling analyses to 

determine what fraction of global 

greenhouse gas concentrations are due 

to the emissions from section 202(a) 

source categories. Rather, consistent 

with prior practice and with current 

science, EPA used emissions as a 

perfectly reasonable proxy for 

contributions to atmospheric 

concentrations. Indeed, cumulative 

emissions are responsible for the 

cumulative change in the stock of 

concentrations in the atmosphere (i.e., 

the fraction of a country’s or an 

economic sector’s cumulative emissions 

compared to the world’s greenhouse gas 

emissions over a long time period will 

be directly proportional to that fraction 

of the change in concentrations 

attributable to that country or economic 

sector); likewise, annual emissions are a 

perfectly reasonable proxy for annual 

incremental changes in atmospheric 

concentrations. 

A. The Air Pollutant(s) 

This section discusses the proposed 

definition of the air pollutant for the 

cause or contribute finding as the 

collective class of six greenhouse gases 

rather than the individual greenhouse 

gases. 

1. Proposed Definition of Air Pollutant 

When making a cause or contribute 

finding under section 202(a), the 

Administrator must first look at the 

emissions from the source category and 

decide how to define the air pollutant 

being evaluated. In this case, the source 

category emits four gases, which share 

common physical properties relevant to 

climate change: all are long-lived in the 

atmosphere; all become globally well 

mixed in the atmosphere; all trap 

outgoing heat that would otherwise 

escape to space; and all are directly 

emitted as greenhouse gases rather than 

forming as a greenhouse gas in the 

atmosphere after emission of a precursor gas. There are other gases which 

share these common properties which 

are not emitted by the section 202(a) 

source categories. Nonetheless, it is 

entirely appropriate for the 

Administrator to define the air pollutant 

in a manner that recognizes the shared 

relevant properties of all of these six 

gases, even though they are not all 

emitted from the source category before 

her. 

The Administrator is proposing to 

define a single air pollutant that is the 

collective class of the six greenhouse 

gases. It is the Administrator’s judgment 

that this collective approach for the 

contribution test is most consistent with 

the treatment of greenhouse gases by 

those studying climate change science 

and policy, where it has become 

common practice to evaluate 

greenhouse gases on a collective CO2- 

equivalent basis. For example, under the 

UNFCCC, the U.S. and other Parties 

report their annual emissions of the six 

greenhouse gases in CO2-equivalent 

units. This facilitates comparisons of the 

multiple greenhouse gases from 

different sources and from different 

countries, and provides a measure of the 

collective warming potential of multiple 

greenhouse gases. There are also several 

federal and state climate programs, such 

as EPA’s Climate Leaders program and 

California’s Climate Action Registry that 

encourage firms to report (and reduce) 

emissions of all six greenhouse gases. 

Furthermore, the Administrator recently 

signed (March 10, 2009) the Proposed 

Greenhouse Gas Mandatory Reporting 

Rule, which proposes the reporting of 

greenhouse gas emissions on a CO2- 

equivalent basis above certain CO2- 

equivalent thresholds, thereby also 

recognizing the common and collective 

treatment of the six greenhouse gases. 

This proposed definition of air 

pollutant is not unique, as EPA has 

previously treated a class of substances 

with similar impacts on the 

environment as a single pollutant (e.g., 

particulate matter, volatile organic 

compounds). These six greenhouse 

gases are being considered collectively 

in the endangerment determination 


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29At this time, a final positive endangerment 

finding would not make the air pollutant found to 

cause or contribute to air pollution that endangers 

a regulated pollutant under the CAA’s Prevention 

of Significant Deterioration (PSD) program. See 

memorandum entitled ‘‘EPA’s Interpretation of 

Regulations that Determine Pollutants Covered By 

Federal Prevention of Significant Deterioration 

(PSD) Permit Program’’ (Dec. 18, 2008). EPA is 

reconsidering this memorandum and will be 

seeking public comment on the issues raised in it. 

That proceeding, not this rulemaking, would be the 

appropriate venue for submitting comments on the 

issue of whether a final, positive endangerment 

finding under section 202(a) of the Act should 

trigger the PSD program, and the implications of the 

definition of air pollutant in that endangerment 

finding on the PSD program. 

30One teragram (Tg) = 1 million metric tons. 1 

metric ton = 1,000 kg = 1.102 short tons = 2,205 

lbs. 

31 Long-lived greenhouse gases are compared and 

summed together on a CO2 equivalent basis by 

multiplying each gas by its Global Warming 

Potential (GWPs), as estimated by IPCC. In 

accordance with UNFCCC reporting procedures, the 

U.S. quantifies greenhouse gas emissions using the 

100-year time frame values for GWPs established in 

the IPCC Second Assessment Report. 

because they share the same relevant 

properties regarding their effect on the 

global climate and the associated 

changes throughout the climate system 

that can result. Thus, the Administrator 

believes it is appropriate to consider the 

six greenhouse gases as constituents of 

a single air pollutant. 

The Administrator recognizes that 

only four of the six greenhouse gases 

covered in the definition of air pollution 

are emitted by section 202(a) source 

categories. It is not unusual for a 

particular source category to emit only 

a subset of a class of substances that 

constitute a single air pollutant. For 

example, a source may emit only 20 of 

the possible 200 plus chemicals that 

meet the definition of volatile organic 

compound (VOC) in the regulations, but 

that source is evaluated based on its 

emissions of ‘‘VOCs,’’ and not its 

emissions of the 20 chemicals by name. 

Nonetheless, the Administrator 

recognizes that each greenhouse gas 

could be considered a separate air 

pollutant. Thus, although proposing to 

define air pollutant as the class of six 

greenhouse gases, and basing the 

proposed contribution finding on that 

air pollutant, the Administrator also 

considered each greenhouse gas 

individually, as discussed below. 

2. How the Definition of Air Pollutant 

in the Endangerment Determination 

Affects Section 202(a) Standards 

The Administrator believes that she 

has significant discretion when 

establishing greenhouse gas emission 

standards under section 202(a) with 

respect to whether the greenhouse gases 

are treated as a single collective 

pollutant or each greenhouse gas is 

defined as a separate air pollutant. 

Under section 202(a), the Administrator 

is required to set ‘‘standards applicable 

to the emission of any air pollutant’’ 

that the Administrator determines 

causes or contributes to air pollution 

that endangers. If the Administrator 

defines the air pollutant as the 

collection of six greenhouse gases, and 

makes the appropriate cause or 

contribute and endangerment findings 

for section 202(a) sources, then she is 

called on to set standards applicable to 

the emission of this air pollutant. The 

term ‘‘standards applicable to the 

emission of any air pollutant’’ is not 

defined, and the Administrator has the 

discretion to interpret it in a reasonable 

manner to effectuate the purposes of 

section 202(a). 

If the Administrator defines the air 

pollutant as the group of greenhouse 

gases, she believes she would have the 

discretion to set standards that either 

control the emissions of the group as a 

whole, and/or standards that control 

emissions of individual greenhouse 

gases, as constituents of the class. For 

example, it might be appropriate to set 

a standard that measures and controls 

the aggregate emissions of the group of 

greenhouse gases, weighted by CO2

equivalent. Depending on the 

circumstances, however, it may be 

appropriate to set standards for 

individual gases, or some combination 

of group and individual standards. 

These and other similar approaches 

could appropriately be considered 

setting a standard or standards 

applicable to the emission of the group 

of greenhouse gases that are defined as 

the air pollutant. The Administrator 

would consider a variety of factors in 

determining what approach to take in 

setting the standard or standards; for 

example she would consider the 

characteristics of the vehicle or engine 

emissions, such as rate and variability, 

the kind and availability of control 

technology, and other matters relevant 

to setting standards under section 

202(a). Likewise, taking into 

consideration the circumstances 

involved, the Administrator could 

determine that it was appropriate to set 

separate standards, a group standard, or 

some combination of those, in a case 

where each greenhouse gas was 

considered a separate air pollutant.29

B. Proposed Cause or Contribute 

Finding 

1. Overview of Greenhouse Gas 

Emissions 

In 2006, U.S. greenhouse gas 

emissions were 7,054 teragrams 30 of 

CO2 equivalent 31 (TgCO2eq). The 

dominant gas emitted is CO2, mostly 

from fossil fuel combustion. Methane is 

the second largest component of U.S. 

emissions, followed by N2O, and the 

fluorinated gases (HFCs, PFCs, and SF6). 

Electricity generation is the largest 

emitting sector (2,378 TgCO2eq or 34 

percent of total U.S. greenhouse gas 

emissions), followed by transportation 

(1,970 TgCO2eq or 28 percent) and 

industry (1,372 TgCO2eq or 19 percent). 

Land use, land use change and forestry 

offset almost 13 percent of total U.S. 

emissions through net sequestration. 

Total U.S. greenhouse gas emissions 

have increased by almost 15 percent 

between 1990 and 2006. The electricity 

generation and transportation sectors 

have contributed most to this increase. 

Total global greenhouse gas emissions 

in 2005 (the most recent year for which 

data for all countries and all greenhouse 

gases are available) were 38,726 

TgCO2eq. This represents an increase in 

global greenhouse gas emissions of 

about 26 percent since 1990 (excluding 

land use, land use change and forestry). 

In 2005, total U.S. greenhouse gas 

emissions were responsible for 18 

percent of global emissions, ranking 

only behind China, which was 

responsible for 19 percent of global 

greenhouse gas emissions. 

2. Overview of Section 202(a) Source 

Categories and Cause or Contribute 

Analysis 

The relevant mobile sources under 

section 202 (a)(1) of the Clean Air Act 

are ‘‘any class or classes of new motor 

vehicles or new motor vehicle engines, 

* * * .’’ CAA § 202(a)(1) (emphasis 

added). The motor vehicles and motor 

vehicle engines (hereinafter ‘‘Section 

202(a) source categories’’) addressed are: 

• Passenger cars 

• Light-duty trucks 

• Motorcycles 

• Buses 

• Medium/heavy-duty trucks 

As noted earlier, in the past the 

requisite contribution findings have 

been proposed concurrently with 

proposing emission standards for the 

relevant mobile source category. Thus, 

the prior contribution findings often 

focused on a subset of the section 202(a) 

(or other section) source categories. 

Today’s proposed cause or contribute 

finding, however, is for all of the section 

202(a) source categories and the 

Administrator is considering emissions 

from all of these source categories in the 

proposed determination. 

Sources covered by section 202(a) of 

the Act emit four of the six greenhouse 

gases that in combination comprise the 

air pollutant being considered in the 

cause or contribute analysis: Carbon 


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32 Emissions of hydrofluorocarbons result from 

the use of HFCs in cooling systems designed for 

passenger comfort, as well as auxiliary systems for 

refrigeration. 

33WRI (2007) Climate Analysis Indicators Tool 

(CAIT). Available at http://cait.wri.org. Accessed 

February 20, 2009. 

34Emissions of different greenhouse gases are 

compared using global warming potentials (GWPs). 

The GWP of a greenhouse gas is defined as the ratio 

of the time-integrated radiative forcing from the 

instantaneous release of 1 kilogram (kg) of a trace 

substance relative to that of 1 kg of a reference gas 

(IPCC 2001). The reference gas used is CO2, and 

therefore GWP-weighted emissions are measured in 

teragrams of CO2 equivalent (TgCO2eq.). 

35 Detailed combined greenhouse gas emissions 

data for Section 202(a) source categories are 

presented in Appendix B of the Technical Support 

Document. 

dioxide, methane, nitrous oxide, and 

hydrofluorocarbons.32 To support the 

Administrator’s assessment, EPA has 

analyzed historical data of these 

greenhouse gases for motor vehicles and 

motor vehicle engines in the U.S. from 

1990 to 2006. The source of the U.S. 

greenhouse gas emissions data is the 

Inventory of U.S. Greenhouse Gas 

Emissions and Sinks: 1990–2006, 

published in 2008 (hereinafter ‘‘U.S. 

Inventory’’). The source of global 

greenhouse gas emissions data, against 

which a number of comparisons are 

made, is the Climate Analysis Indicators 

Tool of the World Resources Institute 

(2007).33

There are a number of possible ways 

of assessing ‘‘cause or contribute’’ and 

no single approach is required or has 

been used exclusively in previous 

determinations under the Act. Because 

the air pollution against which the 

contribution is being evaluated is the 

mix of six greenhouse gas 

concentrations, the logical starting point 

for any contribution analysis is a 

comparison of the emissions of the air 

pollutant from the section 202(a) 

category to the total, global emissions of 

the six greenhouse gases. The 

Administrator recognizes that there are 

other valid comparisons that can and 

should be considered in evaluating 

whether emissions of the air pollutant 

cause or contribute to the combined 

concentration of the six greenhouse 

gases. To inform the Administrator’s 

assessment, the following types of 

comparisons for both the collective and 

individual emissions of greenhouse 

gases from section 202(a) source 

categories are provided: 

• As a share of total current global 

aggregate emissions of the six 

greenhouse gases included in the 

proposed definition of air pollution; 

• As a share of total current U.S. 

aggregate emissions of the six 

greenhouse gases; and 

• As a share of the total current global 

transportation emissions of the six 

greenhouse gases. 

In addition, when reviewing each 

greenhouse gas as an individual 

pollutant, the Administrator also 

considered the following comparisons: 

• As a share of current global 

emissions of that individual greenhouse 

gas; 

• As a share of total section 202(a) 

source category emissions of the six 

greenhouse gases; and 

• As a share of current U.S. emissions 

of that individual greenhouse gas, 

including comparisons to the magnitude 

of emissions of that greenhouse gas from 

other non-transport related source 

categories. 

Note that for global comparisons, all 

emissions are from the year 2005, the 

most recent year for which data for all 

greenhouse gas emissions and all 

countries are available. For comparisons 

within the U.S., all emissions are for the 

year 2006, the most recent year for 

which U.S. data are currently available. 

All values for emission numbers 

represent total annual emissions. All 

annual emissions data are being 

considered on a CO2 equivalent basis, 

which is a commonly accepted metric 

for comparing different greenhouse 

gases, both in the U.S. annual 

greenhouse gas Inventory and with 

international greenhouse gas inventories 

from other Parties to the UNFCCC.34

Future projected emissions are not used 

in this cause or contribute analysis, 

because they are uncertain and current 

emissions data are a valid proxy for 

near-term emissions. This approach is 

consistent with how contribution has 

been assessed in previous actions under 

the Clean Air Act. 

Some comments on the ANPR argued 

that when evaluating the contribution 

from new motor vehicles and engines, 

the Administrator needs to project what 

emissions would be after 

implementation of the fuel efficiency 

standards in the Energy Independence 

and Security Act of 2007 (EISA). Other 

comments noted that the Administrator 

should recognize that in the future the 

denominator of global aggregate 

emissions of greenhouse gases will 

increase as the numerator of new motor 

vehicle and engine emissions decreases. 

As noted above, the Administrator 

believes that the traditional practice of 

considering the recent motor vehicle 

emissions inventory as a surrogate for 

estimates for new motor vehicles and 

engines is appropriate. In general, the 

focus of the contribution test should be 

on current and near-term emissions. The 

current and near term emissions from 

the section 202(a) sources can be 

expected to impact atmospheric 

concentrations for many decades to 

come, given the long atmospheric life of 

the greenhouse gases. The 

Administrator is aware of the 

requirements of EISA, and she has 

concluded that the expected reductions 

in emissions from section 202(a) source 

categories would not affect her 

determination regarding cause or 

contribution. In addition to looking at 

absolute emissions comparisons, the 

Administrator also considered other 

relevant factors, as described below. 

3. Proposed Finding That Emissions of 

the Collective Group of Six Greenhouse 

Gases Contributes to Air Pollution 

Which May Reasonably Be Anticipated 

To Endanger Public Health and Welfare 

a. Total Greenhouse Gas Emissions 

From Section 202(a) Source Categories 

As discussed above, the 

Administrator is proposing to define air 

pollutant for purposes of the 

contribution finding as the collective 

group of six greenhouse gases. Section 

202(a) source categories emit four of the 

greenhouse gases (CO2, CH4, N2O, and 

HFCs), therefore the emissions of the 

single air pollutant are the collective 

emissions of these four greenhouse 

gases. This section summarizes 

information on total section 202(a) 

source category emissions of greenhouse 

gases within that definition.35

In 2006, section 202(a) source 

categories collectively were the second 

largest greenhouse gas-emitting sector 

within the U.S. (behind the electricity 

generating sector), emitting 1,665 

TgCO2eq and representing 24 percent of 

total U.S. greenhouse gas emissions 

(Table 1). Between 1990 and 2006, total 

greenhouse gas emissions from 

passenger cars decreased 0.9 percent, 

while emissions from light-duty trucks 

increased 57 percent, largely due to the 

increased use of sport-utility vehicles 

and other light-duty trucks. 

Globally in 2005, section 202(a) 

source category greenhouse gas 

emissions represented 28 percent of 

global transport greenhouse gas 

emissions and 4.3 percent of total global 

greenhouse gas emissions (Table 2). The 

global transport sector was 14 percent of 

all global greenhouse gas emissions in 

2005. If U.S. section 202(a) source 

category greenhouse gas emissions were 

ranked against total greenhouse gas 

emissions for entire countries, U.S. 

section 202(a) emissions would rank 

behind only China, the U.S. as a whole, 

Russia and India, and would rank ahead 


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of Japan, Brazil, Germany and every 

other country in the world. 

TABLE 1—SECTORAL COMPARISON TO TOTAL U.S. GREENHOUSE GAS (GHG) EMISSIONS (TGCO2E) 

U.S. Emissions 1990 1995 2000 2001 2002 2003 2004 2005 2006 

Section 202(a) GHG emissions ... 1231.9 1364.4 1568.1 1576.8 1617.9 1629.7 1667.4 1670.0 1665.4 

Share of U.S. (%) ................. 20.0% 21.0% 22.3% 22.8% 23.2% 23.3% 23.6% 23.4% 23.6% 

Electricity Sector emissions ......... 1859.1 1989.7 2328.9 2290.9 2300.4 2329.4 2363.4 2430.0 2377.8 

Share of U.S. (%) ................. 30.2% 30.6% 33.1% 33.1% 33.0% 33.3% 33.4% 34.1% 33.7% 

Industrial Sector emissions .......... 1460.3 1478.0 1432.9 1384.3 1384.9 1375.5 1388.9 1354.3 1371.5 

Share of U.S. (%) ................. 23.8% 22.8% 20.4% 20.0% 19.8% 19.7% 19.6% 19.0% 19.4% 

Total US GHG emissions ........................... 6148.3 6494.0 7032.6 6921.3 6981.2 6998.2 7078.0 7129.9 7054.2 

TABLE 2—COMPARISON TO GLOBAL

GREENHOUSE GAS (GHG) EMISSIONS (TGCO2E) 

2005 

Sec 

202(a) 

share 

All US GHG emissions ..... 7,130 23.4% 

Global transport GHG 

emissions ...................... 5,909 28.3% 

All global GHG emissions 38,726 4.3% 

b. Proposed Contribution Finding for 

the Single Air Pollutant Comprised of 

the Collective Group of Six Greenhouse 

Gases 

Based on the data summarized above, 

the Administrator proposes to find that 

the emissions of the defined air 

pollutant from new motor vehicles and 

engines contribute to the air pollution 

previously discussed. As noted above, 

the Administrator recognizes that only 

four of the six greenhouse gases covered 

in the definition of air pollution are 

emitted by section 202(a) source 

categories, and has made her 

determination based on the combined 

contribution of these four greenhouse 

gases. It is not unusual for a particular 

source category to emit only a subset of 

a class of substances that constitute a 

single air pollutant (for example, 

volatile organic compounds). 

It is the Administrator’s judgment that 

the collective greenhouse gas emissions 

from section 202(a) source categories are 

significant, whether the comparison is 

global (over 4 percent of total 

greenhouse gas emissions) or domestic 

(24 percent of total greenhouse gas 

emissions). The Administrator believes 

that consideration of the global context 

is important for the cause or contribute 

test but that the analysis should not 

solely consider the global context. 

Greenhouse gas emissions from section 

202(a) source categories, or from any 

other U.S. source, will become globally 

mixed in the atmosphere, and thus will 

have an effect not only on the U.S. 

regional climate but on the global 

climate as a whole, and indeed for years 

and decades to come. The Administrator 

believes that these unique, global 

aspects of the climate change problem 

tend to support a finding that lower 

levels of emissions should be 

considered to contribute to the air 

pollution than might otherwise be 

considered appropriate when 

considering contribution to a local or 

regional air pollution problem. 

Importantly, because no single 

greenhouse gas source category 

dominates on the global scale, many (if 

not all) individual greenhouse gas 

source categories could appear too small 

to matter, when, in fact, they could be 

very significant contributors in terms of 

both absolute emissions or in 

comparison to other similar source 

categories within the U.S. If the U.S. 

and the rest of the world are to combat 

the risks associated with global climate 

change, contributors must do their part 

even if their contributions to the global 

problem, measured in terms of 

percentage, are smaller than typically 

encountered when tackling solely 

regional or local environmental issues. 

Total U.S. greenhouse gas emissions 

make up about 18 percent of the world’s 

greenhouse gas emissions, and 

individual sources within the U.S. will 

be subsets of that 18 percent. The 

Administrator is placing significant 

weight on the fact that section 202(a) 

source categories contribute to 24 

percent of total U.S. greenhouse gas 

emissions for the proposed contribution 

finding. 

4. Additional Consideration of Whether 

Each Greenhouse Gas as a Separate Air 

Pollutant Contributes to Air Pollution 

Which May Reasonably Be Anticipated 

To Endanger Public Health and Welfare 

As noted above, the Administrator 

also considered whether emissions of 

individual greenhouse gas from section 

202(a) source categories, separately, 

would contribute to the air pollution 

defined above. This section discussed 

the contribution of each of the four 

individual greenhouse gases emitted by 

Section 202(a) source categories. 

a. Carbon Dioxide Emissions From 

Section 202(a) Source Categories 

Carbon dioxide is emitted from motor 

vehicles and motor vehicle engines 

during the fossil fuel combustion 

process. During combustion, the carbon 

stored in the fuels is oxidized and 

emitted as CO2 and smaller amounts of 

other carbon compounds. 

In 1990, Section 202(a) source 

categories emitted 23 percent of total 

U.S. CO2 emissions, behind only the 

electricity generation sector (36 

percent). In 2006, Section 202(a) source 

categories remained the second largest 

sector, growing to 26 percent of total 

U.S. CO2 emissions. 

Carbon dioxide is the dominant 

greenhouse gas emitted from Section 

202(a) source categories (94 percent of 

total U.S. Section 202(a) source category 

greenhouse gas emissions in 2006). 

Carbon dioxide emissions from these 

source categories grew by 32 percent 

between 1990 and 2006, largely due to 

increased carbon dioxide emissions 

from light-duty trucks (61 percent since 

1990) and medium/heavy-duty trucks 

(76 percent). 

In 2005, carbon dioxide from section 

202(a) source categories in the U.S. were 

responsible for 4 percent of global 

aggregate greenhouse gas emissions (a 

similar percentage compared to the U.S. 

share of global greenhouse gas 

emissions when considering all 

greenhouse gas emissions from U.S. 

section 202(a) sources). Section 202(a) 

source category carbon dioxide 

emissions are a significantly larger share 

of global transportation greenhouse gas 

emissions (27 percent) than the 

corresponding share of all U.S. CO2

emissions to the global total (18 

percent), reflecting the comparatively 

larger size of the transport sector in the 

U.S. compared to the global average. 

If the Administrator were to evaluate 

carbon dioxide as a separate air 

pollutant, she would consider the 


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emissions from section 202(a) source 

categories to contribute to the air 

pollution, placing primary weight on 

the fact that carbon dioxide is so 

dominant among all section 202(a) 

greenhouse gas emissions (94 percent) 

and contributes to a significant share of 

all U.S. carbon dioxide emissions (26 

percent) and global greenhouse gas 

emissions (4 percent). 

b. Methane Emissions From Section 

202(a) Source Categories 

Methane emissions from motor 

vehicles are a function of the methane 

content of the motor fuel, the amount of 

hydrocarbons passing uncombusted 

through the engine, and any postcombustion control of hydrocarbon 

emissions (such as catalytic converters). 

In 2006, methane emissions from 

section 202(a) source categories were 

0.11 percent of total greenhouse gas 

emissions from U.S. motor vehicles and 

motor vehicle engines. Methane 

emissions from these source categories 

decreased by 58 percent between 1990 

and 2006, largely due to decreased 

methane emissions from passenger cars 

(62 percent) and light-duty trucks (51 

percent). In 2006, methane emissions 

from these source categories equaled 

0.32 percent of total U.S. methane 

emissions and 0.03 percent of total U.S. 

greenhouse gas emissions. 

Methane emissions from Section 

202(a) source categories were less than 

0.01 percent of total global greenhouse 

gas emissions in 2005. When compared 

to the smaller subsets of global 

transportation emissions, and global 

methane emissions, section 202(a) 

source category methane emissions were 

about 0.03 percent in both cases in 

2005. 

If the Administrator were to evaluate 

methane as a separate air pollutant, she 

would consider the emissions from 

section 202(a) source categories to 

contribute to the air pollution. The 

Administrator would place primary 

weight on the same reason that the 

Administrator promotes the reduction of 

methane and other non-CO2 greenhouse 

gas emissions from sources with 

relatively low but potent emissions, as 

manifested in its domestic methane 

partnership programs and the 

international Methane to Markets 

Partnership, which was launched in 

2004. Specifically, these emissions are 

at a level that contributes to the climate 

change problem and there are valuable 

reductions available from these levels. 

As noted above, consideration of the 

global nature of greenhouse gas 

emissions and climate change means 

that a percentage contribution of 

specific gases and sectors would be 

expected to be much smaller than for 

previous rulemakings when the nature 

of the air pollution was national, 

regional or local. 

c. Nitrous Oxide Emissions From 

Section 202(a) Source Categories 

Nitrous oxide is a product of the 

reaction that occurs between nitrogen 

and oxygen during fuel combustion. 

Nitrous oxide (and nitrogen oxide 

(NOX)) emissions from motor vehicles 

and motor vehicle engines are closely 

related to fuel characteristics, air-fuel 

mixes, combustion temperatures, and 

the use of pollution control equipment. 

For example, some types of catalytic 

converters installed to reduce motor 

vehicle NOX, CO, and hydrocarbon 

emissions can promote the formation of 

nitrous oxide. 

In 2006, nitrous oxide emissions from 

section 202(a) source categories 

accounted for 1.8 percent of total 

greenhouse gas emissions from U.S. 

motor vehicles and motor vehicle 

engines. Nitrous oxide emissions from 

these source categories decreased by 27 

percent between 1990 and 2006, largely 

due to decreased emissions from 

passenger cars (39 percent) and lightduty trucks (10 percent). In 2006, 

nitrous oxide emissions from these 

source categories equaled 8.0 percent of 

total U.S. nitrous oxide emissions. In 

fact, Section 202(a) source categories are 

the second largest U.S source of N2O, 

behind only agricultural soil 

management (which represented 72 

percent of total nitrous oxide emissions 

in 2006). 

In 2005, nitrous oxide emissions from 

U.S. section 202(a) source categories 

were 0.08 percent of total global 

greenhouse gas emissions. Also in 2005, 

U.S. section 202(a) sources accounted 

for 1.0 percent of global N2O emissions 

and 0.6 percent of global transportation 

greenhouse gas emissions. 

If the Administrator were to evaluate 

nitrous oxide as a separate air pollutant, 

she would consider the emissions from 

section 202(a) source categories to 

contribute to the air pollution, placing 

primary weight on the fact that nitrous 

oxide emissions from section these 

source categories are significant in terms 

of their contribution to U.S. (and global) 

emissions of that particular gas. 

Although Section 202 emissions of 

nitrous oxide appear small on a global 

basis, they were 8.0 percent of total U.S. 

N2O emissions in 2006, second only to 

agricultural soil management (which 

represented 72 percent of total nitrous 

oxide emissions in 2006). In addition, as 

mentioned in the previous discussion of 

methane, given the vast number of 

sources and sectors that emit 

greenhouse gases around the world, 

even sources which represent a small 

percentage of U.S. or global emissions 

can be considered to contribute to the 

larger problem. 

d. HFC Emissions From Section 202(a) 

Source Categories 

Hydrofluorocarbons (a term which 

encompasses a group of eleven related 

compounds) are progressively replacing 

CFCs and HCFCs in section 202(a) 

cooling and refrigeration systems as 

they are being phased out under the 

Montreal Protocol and Title VI of the 

Clean Air Act. For example, HFC–134a 

has become a replacement for CFC–12 

in mobile air conditioning systems. A 

number of HFC blends, containing 

multiple compounds, have also been 

introduced. The emissions pathway can 

be complex, with hydrofluorocarbons 

being emitted to the atmosphere during 

charging of cooling and refrigeration 

systems, during operation, and during 

decommissioning and disposal. 

Section 202(a) source categories of 

hydrofluorocarbons accounted for 4.2 

percent of total greenhouse gas 

emissions from U.S. motor vehicles and 

motor vehicle engines in 2006. 

Hydrofluorocarbons were not used in 

motor vehicles in 1990, but by 2006 

emissions had increased to 70 TgCO2e 

(this represents an increase of 270 

percent between 1995 and 2006). In 

2006, hydrofluorocarbon emissions from 

these source categories equaled 56 

percent of total U.S. hydrofluorocarbon 

emissions, making it the single largest 

source category of U.S. 

hydrofluorocarbon emissions. 

In 2005, hydrofluorocarbons from 

section 202(a) source categories were 

0.18 percent of total global greenhouse 

gas emissions. When compared to the 

smaller subset of global transportation 

emissions, section 202(a) source 

category hydrofluorocarbon emissions 

were 1.3 percent in 2005. However, U.S. 

section 202(a) HFC sources equaled 18 

percent of global hydrofluorocarbon 

emissions, making it the largest source 

of global hydrofluorocarbon emissions. 

If the Administrator were to evaluate 

hydrofluorocarbons as a separate air 

pollutant, she would consider the 

emissions from section 202(a) source 

categories to contribute to the air 

pollution, placing primary weight on 

the fact that hydrofluorocarbon 

emissions from these source categories 

are the largest U.S. and global source of 

that particular gas, and emissions have 

grown 270 percent since 1995. If the 

decision were made that these 

emissions do not contribute because 

hydrofluorocarbon emissions under 

section 202(a) make up just 0.18 percent 


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of global greenhouse gas emissions it 

would be inconsistent with the U.S. 

practice of encouraging 

hydrofluorocarbon emission reductions. 

Indeed, if the Administrator determined 

that hydrofluorocarbon emissions from 

section 202(a) source categories did not 

contribute, it would be unlikely that she 

would find contribution for 

hydrofluorocarbons from any other 

source of these (and other fluorinated) 

greenhouse gases. For these reasons, the 

Administrator believes the global 

context remains important to consider, 

but that more weight should placed on 

a contribution analysis done within the 

domestic context. 

V. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under Executive Order (EO) 12866 

(58 FR 51735, October 4, 1993), this 

action is a ‘‘significant regulatory 

action’’ because it raises novel policy 

issues. Accordingly, EPA submitted this 

action to the Office of Management and 

Budget (OMB) for review under EO 

12866 and any changes made in 

response to OMB recommendations 

have been documented in the docket for 

this action. 

B. Paperwork Reduction Act 

This action does not impose an 

information collection burden under the 

provisions of the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. Burden is 

defined at 5 CFR 1320.3(b). The final 

endangerment finding would not 

impose an information collection 

request on any person. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this action on small entities, small 

entity is defined as: (1) A small business 

as defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

Because this proposed action will not 

impose any requirements, the 

Administrator certifies that this 

proposed action will not have a 

significant economic impact on a 

substantial number of small entities. 

This proposed action will not impose 

any requirements on small entities. The 

endangerment and contribution findings 

do not in-and-of-themselves impose any 

new requirements but rather set forth 

the Administrator’s determination on 

whether greenhouse gases in the 

atmosphere may reasonably be 

anticipated to endanger public health or 

welfare, and whether emissions of 

greenhouse gases from new motor 

vehicles and engines contribute to this 

air pollution. Accordingly, the proposed 

action affords no opportunity for EPA to 

fashion for small entities less 

burdensome compliance or reporting 

requirements or timetables or 

exemptions from all or part of the 

proposal. 

D. Unfunded Mandates Reform Act 

This action contains no Federal 

mandates under the provisions of Title 

II of the Unfunded Mandates Reform 

Act of 1995 (UMRA), 2 U.S.C. 1531– 

1538 for State, local, or tribal 

governments or the private sector. The 

action imposes no enforceable duty on 

any State, local or tribal governments or 

the private sector. Therefore, this action 

is not subject to the requirements of 

sections 202 or 205 of the UMRA. 

This action is also not subject to the 

requirements of section 203 of UMRA 

because it contains no regulatory 

requirements that might significantly or 

uniquely affect small governments. 

E. Executive Order 13132: Federalism 

Executive Order 13132, entitled 

‘‘Federalism’’ (64 FR 43255, August 10, 

1999), requires EPA to develop an 

accountable process to ensure 

‘‘meaningful and timely input by State 

and local officials in the development of 

regulatory policies that have federalism 

implications.’’ ‘‘Policies that have 

federalism implications’’ is defined in 

the Executive Order to include 

regulations that have ‘‘substantial direct 

effects on the States, on the relationship 

between the national government and 

the States, or on the distribution of 

power and responsibilities among the 

various levels of government.’’ 

This proposed endangerment 

determination does not have federalism 

implications. It will not have substantial 

direct effects on the States, on the 

relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. Thus, Executive 

Order 13132 does not apply to this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000). Thus, Executive Order 13175 

does not apply to this action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

EPA interprets EO 13045 (62 FR 

19885, April 23, 1997) as applying only 

to those regulatory actions that concern 

health or safety risks, such that the 

analysis required under section 5–501 of 

the EO has the potential to influence the 

regulation. This action is not subject to 

EO 13045 because it does not establish 

an environmental standard intended to 

mitigate health or safety risks. Although 

the Administrator considered health 

and safety risks as part of this proposed 

endangerment finding, the proposed 

finding itself does not impose a 

standard intended to mitigate those 

risks. 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution, or Use 

This action is not a ‘‘significant 

energy action’’ as defined in Executive 

Order 13211 (66 FR 28355 (May 22, 

2001)), because it is not likely to have 

a significant adverse effect on the 

supply, distribution, or use of energy. 

This action does not impose 

requirements on these activities. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (‘‘NTTAA’’), Public Law 

104–113, 12(d) (15 U.S.C. 272 note) 

directs EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. NTTAA directs EPA to provide 

Congress, through OMB, explanations 

when the Agency decides not to use 


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available and applicable voluntary 

consensus standards. 

This proposed rulemaking does not 

involve technical standards. Therefore, 

EPA is not considering the use of any 

voluntary consensus standards. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order (EO) 12898 (59 FR 

7629 (Feb. 16, 1994)) establishes federal 

executive policy on environmental 

justice. Its main provision directs 

federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this 

proposed endangerment determination 

will not have disproportionately high 

and adverse human health or 

environmental effects on minority or 

low-income populations. Nonetheless, 

when developing the proposed 

endangerment determination, the 

Administrator considered the impacts of 

climate change on minority or lowincome populations. 

Dated: April 17, 2009. 

Lisa P. Jackson, 

Administrator. 

[FR Doc. E9–9339 Filed 4–23–09; 8:45 am] 

BILLING CODE 6560–50–P 


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