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EO12866_GHG Emissions from Aircraft _2060-AS31 NPRM_ANPRM_FRN_

201527.docx 

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 87 and 1068

[EPA-HQ-OAR-2014-0828; FRL_9924-06-OAR]

RIN 2060-AS31

Proposed Finding that Greenhouse Gas Emissions from Aircraft Cause or Contribute to 

Air Pollution that May Reasonably Be Anticipated to Endanger Public Health and 

Welfare and Advance Notice of Proposed Rulemaking 

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule and advance notice of proposed rulemaking.

SUMMARY: In this action, the Administrator is proposing to determine that greenhouse gas 

concentrations in the atmosphere endanger the public health and welfare of current and future 

generations within the meaning of section 231(a) of the Clean Air Act. She proposes to make 

this finding specifically with respect to the same six well-mixed greenhouse gases (GHGs) --

carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur 

hexafluoride -- that together were defined as the air pollution in the 2009 Endangerment 

Finding under section 202(a) of the Clean Air Act and that together constitute the primary

cause of the climate change problem. The Administrator is also proposing to find that 
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greenhouse gas emissions from certain classes of engines used in aircraft are contributing to 

this mix of greenhouse gases in the atmosphere. Thus, she proposes to find that greenhouse 

gas emissions from those classes of engines are contributing to air pollution which endangers

public health and welfare under section 231(a) of the Clean Air Act. Concurrent with these 

proposed findings, the EPA is issuing an Advance Notice of Proposed Rulemaking to provide 

an overview of and seek input on a variety of issues related to setting an international CO2

standard for aircraft at the International Civil Aviation Organization (ICAO), ICAO’s 

progress in establishing global aircraft standards that achieve meaningful reductions in CO2

emissions, and (provided the EPA promulgates final endangerment and cause and contribute 

findings for aircraft engine GHG emissions) the potential use of section 231 of the Clean Air 

Act to adopt and implement corresponding aircraft engine GHG emission standards 

domestically, ensuring transparency and the opportunity for public comment.

DATES: Comments. Comments must be received on or before [Insert date 60 days after 

publication in the Federal Register]. 

Public Hearing. If anyone contacts the EPA requesting a public hearing by [insert 

date 10 days after publication in the Federal Register], we will hold a public hearing on 

[DATE and PLACE]. If a hearing is requested, the last day to pre-register in advance to 

speak at the hearing will be [insert date]. The hearing will start at 9:30 am local time and 

continue until everyone has had a chance to speak. Requests to speak will be taken the day of

the hearing at the hearing registration desk, although preferences on speaking times may not 

be able to be fulfilled. If you require the service of a translator or special accommodations 
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such as audio description, please let us know at the time of registration. If no one contacts the 

EPA requesting a public hearing to be held concerning this proposed rule by [insert date 10 

days after publication in the Federal Register], a public hearing will not take place. For 

further information on the public hearing or to register to speak at the hearing, please see

section I.B below or go to http://www.epa.gov/otaq/aviation.htm 

ADDRESSES: Comments. Submit your comments, identified by Docket ID No. EPA-HQOAR-2014-0828, by one of the following methods:

 Online: www.regulations.gov Follow the on-line instructions for submitting 

comments.

 Email: A-and-R-Docket@epamail.epa.gov Attention Docket ID No. EPA-HQ-OAR2014-0828.

 Fax: (202) 566-9744, Attention Docket ID No. EPA-HQ-OAR-2014-0828.

 Mail: U.S. Postal Service, send comments to Air and Radiation Docket and 

Information Center, Environmental Protection Agency, Mail Code: 28221T, 1200 

Pennsylvania Ave., NW, Washington, DC 20460. Attention Docket ID No. EPA-HQOAR-2014-0828. 

 Hand Delivery: U.S. Environmental Protection Agency, EPA West, EPA Docket 

Center, EPA West Building, Room 3334, 1301 Constitution Ave., NW., Washington, 

DC 20004. Attention Docket ID No. EPA-HQ-OAR-2014-0828. Such deliveries are 
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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-2014-0828. See 

section I.B on “Public Participation” for more information about submitting written 

comments. The 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 confidential business information (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

website is an “anonymous access” system, which means the EPA will not know your identity

or contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through http://www.regulations.gov, your email 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, 

the 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 the EPA cannot read your 

comment due to technical difficulties and cannot contact you for clarification, the EPA may

not be able to consider your comment. Electronic files should avoid the use of special 

characters, or any form of encryption, and be free of any defects or viruses. For additional 

information about the EPA’s public docket visit the EPA Docket Center homepage at:
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http://www.epa.gov/dockets. For additional instructions on submitting comments, go to 

section I.B of this document.

Docket. The EPA has established a docket for this rulemaking under Docket ID No. EPAHQ-OAR-2014-0828. All documents in the docket are listed in the 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, is not placed on the Internet and will be publicly available only in hard 

copy in the EPA’s docket. Publicly available docket materials are available either 

electronically in www.regulations.gov or in hard copy at the Air and Radiation Docket and 

Information Center, EPA/DC, EPA WJC West, Room 3334, 1301 Constitution Ave., NW, 

Washington, DC. The Public Reading Room 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: JoNell Iffland, Office of Transportation 

and Air Quality, Assessment and Standards Division (ASD), Environmental Protection 

Agency, 2000 Traverwood Drive, Ann Arbor, MI 48105; Telephone number: (734) 214-4454; 

Fax number: (734) 214-4816; E-mail address: iffland.jonell@epa.gov. Please use this contact 

information for general questions about this rulemaking, to request a hearing, to determine if a 

hearing will be held, and to register to speak at the hearing, if one is held. 

SUPPLEMENTARY INFORMATION: 
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TABLE OF CONTENTS

I. GENERAL INFORMATION

A. Does this Action Apply to Me?

B. Public Participation

1. What Should I Consider as I Prepare My Comments for the EPA?

2. Public Hearing

C. Did the EPA Conduct a Peer Review before Issuing this Notice?

D. Children’s Environmental Health

E. Environmental Justice

II. INTRODUCTION: OVERVIEW A CONTEXT FOR THIS PROPOSAL

A. Summary

B. Background Information Helpful to Understanding This Proposal

1. Greenhouse Gases and Their Effects

2. Statutory Basis for This Proposal

C. The EPA’s Responsibilities under the Clean Air Act

1. The EPA’s Regulation of Greenhouse Gases

2. Background on the Aircraft Petition, 2008 ANPR, and D.C. District Court 

Decision

D. U.S. Aircraft Regulations and the International Community

1. International Regulations and U.S. Obligations
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2. The International Community’s Reasons for Addressing Aircraft GHG 

Emissions

3. Relationship of the EPA’s Proposed Endangerment and Cause or 

Contribute Findings to International Aircraft Standards

E. The EPA’s Regulation of Aircraft Emissions

III. LEGAL FRAMEWORK FOR THIS ACTION

A. Section 231(a)(2)(A) —Endangerment and Cause or Contribute

1. The Statutory Language

2. How the Origin of the Current Statutory Language Informs the EPA’s 

Interpretation of Section 231(a)(2)(A)

3. Additional Considerations for the Cause or Contribute Analysis

B. Air Pollutant, Public Health and Welfare

IV. THE PROPOSED ENDANGERMENT FINDING UNDER CAA SECTION 231

A. Scientific Basis of the 2009 Endangerment Finding under CAA section 202(a)(1)

1. The Definition of Air Pollution in the 2009 Endangerment Finding

2. Public Health Impacts Detailed in the 2009 Endangerment Finding

3. Public Welfare Impacts Detailed in the 2009 Endangerment Finding

4. The Science Upon Which the Agency Relied

B. Recent Science Further Supports the Administrator’s Judgment that the Six WellMixed Greenhouse Gases Endanger Public Health and Welfare
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1. More Recent Evidence That Elevated Atmospheric Concentrations of the 

Six Greenhouse Gases Are the Root Cause of Observed Climate Change

2. More Recent Evidence that Greenhouse Gases Endanger Public Health

3. More Recent Evidence that Greenhouse Gases Endanger Public Welfare

4. Consideration of Other Climate Forcers

C. Summary of the Administrator’s Proposed Endangerment Finding under CAA 

Section 231

V. THE PROPOSED CAUSE OR CONTRIBUTE FINDING FOR GREENHOUSE 

GASES UNDER CAA SECTION 231

A. The Air Pollutant

1. Proposed Definition of Air Pollutant

2. How the Definition of Air Pollutant in the Endangerment Determination 

Affects Section 231 Standards.

B. Proposed Cause or Contribute Finding

1. The Administrator’s Approach in Making this Proposed Finding

2. Overview of Greenhouse Gas Emissions

3. Proposed Contribution Finding for the Single Air Pollutant Comprised of 

the of Six Well-Mixed Greenhouse Gases

4. Additional Considerations
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VI. ADVANCE NOTICE OF PROPOSED RULEMAKING: DISCUSSION OF 

ONGOING INTERNATIONAL PROCEEDINGS TO DEVELOP AIRCRAFT CO2

EMISSIONS STANDARD AND REQUEST FOR COMMENT

A. Purpose of the International Standard

B. Applicability of the International CO2 Emissions Standard

C. CAEP Discussion on In-Production Aircraft Applicability

1. Applicability to In-Production Aircraft and Date of Implementation

2. Reporting Requirement for New In-Production Aircraft

D. Metric System, Applicability, and Certification Requirement

1. CO2 Metric System

2. Applicability

3. Certification requirement

4. Regulating the Entire Aircraft Instead of the Engine

E. Stringency Options

F. Costs, Technology Responses for Stringency Options, and Cost-Effectiveness 

Analysis

1. Non-Recurring Costs (engineering development costs)

2. Technology Responses

3. Cost Effectiveness Analysis

G. Request for Comment on EPA’s Domestic Implementation of International CO2

Standards 
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VII. STATUTORY AUTHORITY AND EXECUTIVE ORDER REVIEWS

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 

13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act (PRA)

C. Regulatory Flexibility Act (RFA)

D. Funded Mandates Reform Act (UMRA)

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 (NTTAA)

J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations

K. Determination Under Section 307(d)

VIII. STATUTORY PROVISIONS AND LEGAL AUTHORITY
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I. General Information

A. Does this Action Apply to Me?

These proposed findings, if finalized, would trigger new duties that would apply to the 

EPA, but would not themselves apply new requirements to other entities outside the federal 

government. Specifically, if the EPA issues final findings that greenhouse gas emissions from 

certain classes of engines -- those used in certain aircraft -- cause or contribute to air pollution 

which endangers public health or welfare, then the EPA would have a duty under section 231 

of the Clean Air Act to promulgate aircraft engine emission standards applicable to emissions

of that air pollutant from those classes of engines. Only those standards would apply to and 

have an effect on other entities outside the federal government. Entities potentially interested 

in this proposed action are those that manufacture and sell aircraft engines and aircraft in the 

United States. Categories that may be regulated in a future regulatory action include: 

Category NAICSa Code SIC

b Code Examples of Potentially Affected Entities

Industry 3364412 3724 Manufacturers of new aircraft engines.

Industry 336411 3721 Manufacturers of new aircraft.

a North American Industry Classification System (NAICS).

b Standard Industrial Classification (SIC) code.

This table is not intended to be exhaustive, but rather provides a guide for readers 

regarding entities likely to be interested in today’s proposed action. This table lists the types 

of entities that the EPA is now aware could potentially have an interest in this proposed 

action. If the EPA issues final affirmative findings under section 231(a) regarding greenhouse 

gases, the EPA would then be required to undertake a separate notice and comment 
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rulemaking to issue emission standards applicable to greenhouse gas emissions from the 

classes of aircraft engines that the EPA finds cause or contribute in such a finding, and the 

FAA would be required to Prescribe regulations to insure compliance with these emissions 

standards pursuant to section 232 of the Clean Air Act. Other types of entities not listed in 

the table could also be interested and potentially affected by subsequent actions at some future 

time. If you have any questions regarding the scope of this proposed action, consult the 

person listed in the preceding FOR FURTHER INFORMATION CONTACT section.

B. Public Participation

The EPA requests comment on all aspects of the proposed aircraft endangerment and 

cause or contribute findings and the Advance Notice of Proposed Rulemaking (ANPR). This 

section describes how you can participate in this process.

If you submitted comments on the issues raised by this proposal in dockets for other, 

earlier Agency efforts (e.g., the 2009 Endangerment and Cause or Contribute Findings for 

Greenhouse Gases Under Section 202 of the Clean Air Act or 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-2014-0828) by the 

deadline if you want them to be considered. 
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1. What Should I Consider as I Prepare My Comments for the EPA?

We are opening a formal comment period by publishing this document. We will 

accept comments during the period indicated in the DATES section. If you have an interest 

in the proposed aircraft endangerment and cause or contribute findings and/or the ANPR

described in this document, we encourage you to comment on any aspect of this rulemaking.

Tips for Preparing Your Comments.

When submitting comments, remember to:

 Identify the rulemaking by docket number and other identifying information 

(subject heading, Federal Register date and page number).

 Follow directions - The agency may ask you to respond to specific questions or 

organize comments by referencing a Code of Federal Regulations (CFR) part 

or section number.

 Explain why you agree or disagree, suggest alternatives, and substitute 

language for your requested changes.

 Describe any assumptions and provide any technical information and/or data 

that you used.

 If you estimate potential costs or burdens, explain how you arrived at your 

estimate in sufficient detail to allow for it to be reproduced.
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 Provide specific examples to illustrate your concerns, and suggest alternatives.

 Explain your views as clearly as possible, avoiding the use of profanity or 

personal threats.

 Make sure to submit your comments by the comment period deadline 

identified.

Do not submit information to the EPA containing CBI through 

http://www.regulations.gov or e-mail. Clearly mark the part or all of the information that you 

claim to be CBI. For CBI information on a disk or CD-ROM that you mail to the 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, you must submit a copy of 

the comment that does not contain the information claimed as CBI 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. Public Hearing

If a hearing is held, it will provide interested parties the opportunity to present data, 

views or arguments concerning the proposed action. The EPA will make every effort to 

accommodate all speakers who arrive and register. Because this hearing, if held, will be at a 

U.S. government facility, individuals planning to attend the hearing should be prepared to 
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show valid picture identification to the security staff in order to gain access to the meeting 

room. In addition, you will need to obtain a property pass for any personal belongings you 

bring with you. Upon leaving the building, you will be required to return this property pass to 

the security desk. No large signs will be allowed in the building, cameras may only be used 

outside of the building and demonstrations will not be allowed on federal property for security 

reasons. 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 oral 

comments and supporting information presented at the public hearings. 

Speakers should notify Ms. JoNell Iffland (see FOR FURTHER INFORMATION 

contact) if they will need specific equipment, or if there are other special needs related to 

providing comments at the hearing. The EPA will provide equipment for commenters to 

show overhead slides or make computerized slide presentations if we receive special requests 

at least 3 business days in advance of the public hearing. Oral testimony will be limited to 5 

minutes for each commenter. The EPA requests that commenters provide the EPA with at 

least 2 copies of their oral testimony in hard copy form or a copy electronically. Verbatim 

transcripts of the hearings and written statements will be included in the docket for the 

rulemaking. The EPA will make every effort to follow the schedule as closely as possible on 

the day of the hearing; however, please plan for the hearing to run either ahead of schedule or 

behind schedule. Information regarding the hearing (including information as to whether or 

not one will be held) will be available at http://www.epa.gov/otaq/aviation.htm. Again, all 
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requests for a public hearing to be held must be received by [insert date 10 days after 

publication in the Federal Register].

C. Did the EPA Conduct a Peer Review before Issuing this Notice?

As outlined in section IV.A of this action, the EPA's approach to providing the 

technical and scientific information to inform the Administrator's judgment regarding the 

question of whether greenhouse gases endanger public health and welfare was to rely 

primarily upon the recent, major assessments by the U.S. Global Change Research Program 

(USGCRP), the Intergovernmental Panel on Climate Change (IPCC), and the National 

Research Council (NRC) of the National Academies. These assessments draw synthesis 

conclusions across thousands of individual peer-reviewed studies that appear in scientific 

journals, and the reports themselves undergo additional peer review. The EPA has considered 

the processes and procedures employed by the USGCRP, IPCC, and the NRC, and has 

determined that these assessments have been adequately peer reviewed in a manner 

commensurate with the EPA’s Peer Review Policy1and the guidelines in Office of

Management and Budget’s (OMB) Final Information Quality Bulletin for Peer Review

(“OMB Bulletin”) for highly influential scientific assessments. According to guidelines in the 

EPA's Peer Review Handbook, if the Agency has determined that information has already 

 

1 U.S. EPA, 2006: EPA Peer Review Policy. Available at http://www.epa.gov/peerreview/ (Last accessed May 

12, 2015).
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been subject to adequate peer review, then it is not necessary to have further peer review of 

that information.2

The EPA also cites data from its annual Inventory of U.S. Greenhouse Gas Emissions 

and Sinks report,

3 which the Agency has determined to have been adequately reviewed in 

accordance with the OMB Bulletin and the EPA’s Peer Review Handbook. For the 

presentation of emissions inventory information to support the cause or contribute finding, the

EPA disaggregated the existing data in one area of the GHG Inventory (for the General 

Aviation Jet Fuel Category) and had the disaggregation methodology and results peer 

reviewed in accordance with the EPA’s Peer Review Handbook. The EPA Science Advisory

Board reviewed this approach to the underlying technical and scientific information supporting 

this action, and concluded that the approach had precedent and the action will be based on wellreviewed information. All relevant peer review documentation is located in the docket for 

today's action (EPA-HQ-OAR-2014-0828).

D. Children’s Environmental Health

As described in detail in section IV of this preamble, the scientific evidence and 

conclusions in the USGCRP, IPCC, and the NRC assessment reports cited in the 2009 

 

2 U.S. EPA, 2012: EPA Peer Review Handbook, Third Edition. Available at 

http://www.epa.gov/peerreview/pdfs/peer_review_handbook_2012.pdf (Last accessed May 12, 2015). 

3

U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (Last accessed May 12, 

2015).
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Endangerment Finding4indicate that children are uniquely vulnerable to climate change

related health effects given behavioral, developmental, and physiological factors. The new 

assessment literature published since 2009 strengthens these conclusions by providing more 

detailed findings regarding children’s vulnerabilities and projected impacts they may

experience. 

These assessments describe how children’s unique physiological and developmental 

factors contribute to making them disproportionately vulnerable to climate change. Impacts 

to children are expected from heat waves, air pollution, infectious and waterborne illnesses, 

and mental health effects resulting from extreme weather events. In addition, the assessments

find that climate change will influence production of pollen that affects asthma and other 

allergic respiratory diseases, to which children are among those especially susceptible.

Additional health concerns may arise in low income households, especially those with 

children, if climate change reduces food availability and increases prices, leading to food 

insecurity within households.

E. Environmental Justice

As described in detail in section IV below, the scientific assessment literature cited in 

the 2009 Endangerment Finding concluded that certain populations are most vulnerable to 

 

4 U.S. EPA, 2009: Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section 

202(a) of the Clean Air Act; Final Rule, 74 FR 66496 (December 15, 2009) (“2009 Endangerment Finding”); 74 

FR 18886 (April 24, 2009) (“Proposed 2009 Endangerment Finding”).
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the health and welfare effects of climate change, including the elderly, the poor, and

indigenous peoples in the United States, particularly Alaska Natives. The more recent 

literature strengthens these conclusions by providing more detailed analysis regarding these 

populations’ vulnerabilities and projected impacts they may experience. In addition, the most 

recent literature provides new analysis about how some subpopulations defined jointly by 

ethnic/racial disparities and geographic location may be vulnerable to certain climate change 

health impacts.

These studies find that certain climate change related impacts—including heat waves, 

degraded air quality, and extreme weather events—may have disproportionate effects on 

some low-income subpopulations and some subpopulations defined by ethic/racial 

characteristics, raising environmental justice concerns. Factors that contribute to increased 

vulnerability to the health effects of climate change include limited resources to adapt and 

existing health disparities (e.g., higher prevalence of chronic health conditions such as 

diabetes). 

II. Introduction: Overview and Context for this Proposal

A. Summary

Pursuant to section 231(a)(2)(A) of the Clean Air Act (CAA or Act), the 

Administrator proposes to find that greenhouse gas (GHG) emissions from aircraft engines

used in certain types of aircraft (referred to as “covered aircraft” throughout this notice) 

contribute to air pollution that endangers public health and welfare. Covered aircraft would 
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be those aircraft to which ICAO has agreed the international CO2 standard would apply5: 

subsonic jet aircraft with a maximum takeoff mass (MTOM) greater than 5,700 kilograms,

and subsonic propeller-driven (e.g., turboprop) aircraft with a MTOM greater than 8,618 

kilograms. Examples of covered aircraft would include smaller jet aircraft such as the Cessna 

Citation CJ2+ and the Embraer E170, up to and including the largest commercial jet aircraftthe Airbus A380 and the Boeing 747. Other examples of covered aircraft would include 

larger turboprop aircraft, such as the ATR 72 and the Bombardier Q400. 

In this proposed action, the EPA relies primarily on the extensive scientific and 

technical evidence in the record supporting the Endangerment and Cause or Contribute 

Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act; Final Rule, 74 FR

66496 (collectively referred to as the 2009 Endangerment Finding in today’s action). This

includes the major, peer-reviewed scientific assessments that were used to address the 

question of whether GHGs in the atmosphere endanger public health and welfare, and on the 

analytical framework and conclusions upon which the EPA relied in making that finding. The 

Administrator’s view is that the body of scientific evidence amassed in the record for the 2009 

Endangerment Finding also compellingly supports an endangerment finding under CAA 

section 231(a). Furthermore, this proposed finding under section 231 reflects the EPA’s 

careful consideration not only of the scientific and technical record for the 2009 

 

5

ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standards, Circular 

(Cir) 337, AN/192, Available at: http://www.icao.int/publications/catalogue/cat_2015_en.pdf. The ICAO 

Circular 337 is found on page 85 of the catalog and is copyright protected; Order No. CIR337 (last accessed May 

12, 2015.
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Endangerment Finding, but also of science assessments released since 2009, which, as 

illustrated below, strengthen and further support the judgment that GHGs in the atmosphere 

may reasonably be anticipated to endanger public health and welfare. No information or 

analyses published since late 2009 suggest that it would be reasonable for the EPA to now 

reach a different or contrary conclusion for purposes of CAA section 231(a)(2)(A) than the 

Agency reached for purposes of section 202(a). However, as explained below, in proposing 

this finding for purposes of section 231, we are not reopening or revising our prior findings 

under CAA section 202. 

The Administrator is proposing to define the “air pollution” referred to in section 

231(a)(2)(A) of the CAA to be the mix of six well-mixed GHGs: CO2, methane, nitrous 

oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. This is the same 

definition that was used for the finding for purposes of section 202(a). It is the 

Administrator’s judgment that the total body of scientific evidence compellingly supports a 

positive endangerment finding that elevated concentrations of the six well-mixed GHGs

constitute air pollution that endangers both the public health and the public welfare of current 

and future generations within the meaning of section 231(a) of the Clean Air Act. 

Under section 231 of the CAA, the Administrator must also determine whether 

emissions of any air pollutant from a class or classes of aircraft engines cause or contribute to 

the air pollution that may reasonably be anticipated to endanger public health or welfare. 

Following the rationale outlined in the 2009 Endangerment Finding, the Administrator in this 

action is proposing to use the same definition of the air pollutant as was used for purposes of
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section 202(a) for purposes of making the cause or contribute determination under section 

231(a) – that is, the aggregate group of the same six well-mixed GHGs. Based on the data 

summarized in section V, the Administrator is proposing to find that GHG emissions from 

aircraft engines used in covered aircraft, contribute to the air pollution that endangers public 

health and welfare under section 231(a).

The Administrator’s proposed findings come in response to a citizen petition 

submitted by Friends of the Earth, Oceana, the Center for Biological Diversity, and 

Earthjustice (Petitioners) requesting that the EPA issue an endangerment finding and 

standards under section 231(a)(2)(A) of the Act for the GHG emissions from aircraft. The 

EPA is not proposing or taking action under any other provision of the CAA. Further, the

EPA anticipates that ICAO will adopt a final CO2 emissions standard in February 2016. This 

proposal, and any final endangerment and cause or contribute findings for aircraft engine 

GHG emissions, are also part of preparing for a possible subsequent domestic rulemaking 

process to adopt standards that are of at least equivalent stringency as the anticipated ICAO 

CO2 standards. Once an international standard is finalized by ICAO, member states are then 

required to adopt standards that are equivalent to those set by ICAO, or they may adopt 

standards that are more stringent than those agreed upon by ICAO once proper notification is 

given. Section II. D provides additional discussion of the international aircraft standardsetting process. 
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B. Background Information Helpful to Understanding This Proposal

1. Greenhouse Gases and Their Effects 

GHGs in the atmosphere effectively trap some of the Earth’s heat that would 

otherwise escape to space. GHGs are both naturally occurring and anthropogenic. The 

primary GHGs directly emitted by human activities include CO2, methane, nitrous oxide, 

hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. Of these six gases, two (CO2

and nitrous oxide) are emitted by aircraft engines. 

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 them from the 

atmosphere. Observations of the Earth’s globally averaged combined land and ocean surface 

temperature over the period 1880 to 2012 show a warming of 0.85 [0.65 to 1.06] degrees 

Celsius or 1.53 [1.17 to 1.91] degrees Fahrenheit.

6

The heating effect caused by the humaninduced buildup of these and other GHGs in the atmosphere, plus other human activities (e.g., 

land use change and aerosol emissions), is extremely likely (>95 percent likelihood) to be the 

cause of most of the observed global warming since the mid-20th century.

7 A detailed 

explanation of climate change and its impact on health, society, and the environment is 

 

6

“IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution 

of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, 

T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley 

(eds.)]. Cambridge University Press, 29 pp.

7

Ibid.
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included in the record for the 2009 Endangerment Finding. The relevant scientific information 

from that record has also been included in the docket for this proposed determination under 

CAA section 231 (EPA-HQ-OAR-02914-0828). Section IV of this preamble discusses this 

information, as well as information from the most recent scientific assessments, in the context 

of the Administrator’s proposed endangerment finding under CAA section 231.

The U.S. transportation sector is a significant contributor to total U.S. and global 

anthropogenic GHG emissions. In 2013, aircraft remained the single largest GHG-emitting 

transportation source not yet subject to any GHG regulations. Aircraft constitute a significant 

part of U.S. transportation emissions, accounting for 11 percent of all U.S transportation GHG 

emissions and representing more than 3 percent of total U.S. GHG emissions in 2013.

8

 

Globally, U.S. aircraft GHG emissions represent 29 percent of all global aircraft emissions 

and 0.5 percent of total global GHG emissions. Section V of this preamble provides detailed 

information on aircraft GHG emissions in the context of the Administrator’s proposed cause 

or contribute finding under CAA section 231. 

2. Statutory Basis for This Proposal

Section 231(a)(2)(A) of the CAA states that “The Administrator shall, from time to 

time, issue proposed emission standards applicable to the emission of any air pollutant from 

 

8 U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last accessed May 12, 

2015). 
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any class or classes of aircraft engines which in [her] judgment causes, or contributes to, air 

pollution which may reasonably be anticipated to endanger public health or welfare.”

Before the Administrator may issue standards addressing emissions of GHGs under 

section 231, 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 certain classes of aircraft engines 

cause or contribute to this air pollution.9 If the Administrator answers both questions in the 

affirmative, she must issue standards under section 231. See Massachusetts v. EPA, 549 U.S. 

497,533 (2007) (interpreting analogous provision in CAA section 202). Section III of this 

preamble summarizes the legal framework for this proposed action under CAA section 231. 

Typically, past endangerment and cause or contribute findings have been proposed 

concurrently with proposed standards under various sections of the CAA, including section 

231. 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 at 58 FR 28809, 28813–14 (May

17, 1993), Final Rule at 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, 

 

9 To 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 of GHGs in the atmosphere. The air 

pollutants, on the other hand, are the emissions of GHGs and can be thought of as the flow that changes the size 

of the total stock.
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Proposed Rule at 65 FR 35430 (June 2, 2000), and Final Rule 66 FR 5002 (January 18, 2001). 

However, there is no requirement that the Administrator propose the endangerment and cause 

or contribute findings concurrently with proposed standards. See 74 FR 66502-(December 26, 

2001), (explaining that nothing in section 202(a) requires the EPA to propose or issue 

endangerment and cause or contribute findings in the same rulemaking, and that Congress left 

the EPA discretion to choose an approach that satisfied the requirements of section 202(a)). 

The same analysis applies to section 231(a)(2)(A), which is analogous to section 202(a). The 

EPA is choosing to propose these findings at this time for a number of reasons, including its 

previous commitment to issue such proposed findings in response to a 2007 citizens’

petition.

10

 

The Administrator is applying the rulemaking provisions of CAA section 307(d) to 

this action, pursuant to CAA section 307(d)(1)(V), which provides that the provisions of 

307(d) apply to “such other actions as the Administrator may determine.” 11 Any standard 

setting rulemaking under section 231 will also be subject to the notice and comment 

rulemaking procedures under 307(d), as provided in CAA section 307(d)(1)(F) (applying the 

provisions of 307(d) to the promulgation or revision of any aircraft emission standard under 

 

10 Center for Biological Diversity, Center for Food Safety, Friends of the Earth, International Center for 

Technology Assessment, and Oceana, 2007: Petition for Rulemaking Under the Clean Air Act to Reduce the 

Emissions of Air Pollutants from Aircraft the Contribute to Global Climate Change, December 5. Available at 

http://www.epa.gov/otaq/aviation.htm (last accessed May 12, 2015).

11 As the Administrator is applying the provisions of section 307(d) to this rulemaking under section 

307(d)(1)(V), we need not determine whether those provisions would apply to this action under section 

307(d)(1)(F). 
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section 231). Thus, these proposed findings will be subject to the same rulemaking 

requirements that would apply if the proposed findings were part of a standard-setting 

rulemaking. 

C. The EPA’s Responsibilities under the Clean Air Act

The CAA provides broad authority to combat air pollution to protect public health and 

welfare. Cars, trucks, construction equipment, airplanes, and ships, as well as a broad range of 

electric generation, industrial, commercial and other facilities, are subject to various CAA 

programs. Implementation of the Act over the past four decades has resulted in significant 

reductions in air pollution while the nation’s economy has continued to grow.

1. The EPA’s Regulation of Greenhouse Gases

In Massachusetts v. EPA, 549 U.S. 497 (2007), the Supreme Court found that GHGs 

are air pollutants that can be regulated under the CAA. The Court held that the Administrator 

must determine whether emissions of GHGs from new motor vehicles cause or contribute to 

air pollution which may reasonably be anticipated to endanger public health and/or welfare, or 

whether the science is too uncertain to make a reasoned decision. In making these decisions, 

the Administrator was bound by the provisions of section 202(a) of the CAA. The Supreme 

Court decision resulted from a petition for rulemaking under section 202(a) filed by more than 

a dozen environmental, renewable energy, and other organizations. 
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Following the Supreme Court decision, the EPA proposed (74 FR 18886, April 24, 

2009) and then finalized (74 FR 66496, December 15, 2009) the 2009 Endangerment Finding, 

which can be summarized as follows: 

 Endangerment Finding: The Administrator found that the then-current and 

projected concentrations of the six key well-mixed GHGs — CO2, methane, 

nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride —

in the atmosphere threaten the public health and welfare of current and future 

generations.

 Cause or Contribute Finding: The Administrator found that the combined 

emissions of these well-mixed GHGs from new motor vehicles and new motor 

vehicle engines contribute to the GHG pollution which threatens public health 

and welfare. 

The findings did not themselves impose any requirements on industry or other 

entities. However, these findings compelled the EPA to promulgate GHG emission standards 

for new motor vehicles under section 202(a). Subsequently, in May of 2010 the EPA, in 

collaboration with the National Highway Traffic Safety Administration (NHTSA), finalized 

Phase 1 GHG emission standards for light-duty vehicles (2012-2016 model years).

12 This was 

followed in August 2011 by adoption of the first-ever GHG emission standards for heavy-

 

12 U.S. EPA, 2010: Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel 

Economy Standards; Final Rule, 75 FR 25324 (May 7, 2010).
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duty engines and vehicles (2014-2018 model years).

13

 On August 29, 2012, the second phase 

of the GHG emission standards for light-duty vehicles (2017-2025 model years) was finalized

further reducing GHG emissions from light-duty vehicles.

14 In 2014, the President directed 

the EPA and the Department of Transportation to set standards by March 2016 that further 

increase fuel efficiency and reduce GHG emissions from medium- and heavy-duty vehicles.15

The GHG rules for cars and trucks have been supported by a broad range of 

stakeholders, including states, major automobile and truck manufacturers, and environmental 

and labor organizations. Together these new standards for cars and trucks are resulting in 

significant reductions in GHG emissions, and over the lifetime of these vehicles GHG 

emissions will have been reduced by 6 billion metric tons.16,17

On June 25, 2013, President Obama announced a Climate Action Plan that set forth a 

series of executive actions to further reduce GHGs, prepare the U.S. for the impacts of climate 

 

13 US EPA, 2011: Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and HeavyDuty Engines and Vehicles; Final Rule, 76 FR 57106 (September 15, 2011).

14 U.S. EPA, 2012: 2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions and Corporate 

Average Fuel Economy Standards; Final Rule, 77 FR 62623 (October 15, 2012).

15 Executive Office of the President, 2014: Remarks by the President on Fuel Efficiency Standards of Medium 

and Heavy-Duty Vehicles, Office of the Press Secretary, February 18. Available at: 

http://www.whitehouse.gov/the-press-office/2014/02/18/remarks-president-fuel-efficiency-standards-mediumand-heavy-duty-vehicl (last accessed May 12, 2015).

16 US EPA, 2012: Regulatory Impact Analysis: Final Rulemaking for 2017-2025 Light-Duty Vehicle Greenhouse 

Gas Emissions Standards and Corporation Average Fuel Economy Standards, August, Document No. EPA-420-

R-12-016, Table 7.4-2. Available at: http://www.epa.gov/oms/climate/documents/420r12016.pdf (last accessed 

May 12, 2015).

17 76 FR 57106 (September 15, 2011). 
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change, and lead international efforts to address global climate change.

18 As part of the 

Climate Action Plan, the President issued a Presidential Memorandum directing the EPA to 

work expeditiously to complete carbon pollution standards for the power sector.19 In 

response, in January 2014, the EPA proposed carbon pollution standards for new electric 

utility generating units.

20 This was followed in June 2014, by proposed standards to address 

carbon pollution from modified and reconstructed power plants21 and from existing power 

plants.22

In the Climate Action Plan, the President also indicated that the U.S. was working 

internationally to make progress in a variety of areas and specifically noted the progress being 

made by ICAO to develop global CO2 emission standards for aircraft.23 The proposed 

endangerment and cause or contribute findings for aircraft GHG emissions under section 

231(a)(2)(A) of the CAA are a preliminary but necessary first step to begin to address GHG 

emissions from the aviation sector, the highest-emitting category of transportation sources 

 

18 Executive Office of the President, 2013: The President’s Climate Action Plan, June 25. Available at: 

http://www.whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf (last accessed May 12, 

2015).

19 Executive Office of the President, 2013: Presidential Memorandum – Power Sector Carbon Pollution 

Standards, Office of the Press Secretary, June 25. Available at: http://www.whitehouse.gov/the-pressoffice/2013/06/25/presidential-memorandum-power-sector-carbon-pollution-standards (last accessed May 12, 

2015).

20 U.S. EPA, 2014: Standards of Performance for Greenhouse Gas Emissions From New

Stationary Sources: Electric Utility Generating Units; Proposed Rule, 79 FR 1430 (January 8, 2014).

21 U.S. EPA, 2014: Carbon Pollution Standards for Modified and Reconstructed Stationary 

Sources: Electric Utility Generating Units; Proposed Rules, 79 FR 34960 (June 18, 2014).

22 U.S. EPA, 2014: Carbon Pollution Emission Guidelines for Existing Stationary Sources:

Electric Utility Generating Units; Proposed Rule, 79 FR 34830 (June 18, 2014).

23

 Executive Office of the President, 2013: The President’s Climate Action Plan at 21, June. Available at: 

http://www.whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf (last accessed May 12, 

2015).
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that the EPA has not yet addressed. As presented in more detail in Section V of this 

preamble, total U.S. aircraft GHG emissions in 2013 represented 11 percent of GHG 

emissions from the U.S. transportation sector,

24 and in 2010, the latest year with complete 

global emissions data, U.S. aircraft GHG emissions represented 29 percent of global aircraft 

GHG emissions.

25, 26 U.S. aircraft GHG emissions are projected to increase by almost 50 

percent over the next two decades.27 See section V of this preamble for more information 

about the data sources that compose the aircraft GHG emissions inventory.

2. Background on the Aircraft Petition, 2008 ANPR, and D.C. District Court 

Decision

Section 231(a)(2)(A) of the CAA directs the Administrator of the EPA to, from time to 

time, propose aircraft engine emissions standards applicable to the emission of any air 

 

24 U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport (last accessed May 12, 

2015).

25 Ibid.

26 IPCC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the 

Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, 

Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. 

Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, pp. 

599-670.

27 As discussed in section V.B.2.c., fuel burn growth rates for air carriers and general aviation aircraft operating 

on jet fuel are projected to grow by 49 percent from 2010 to 2035, and this provides a scaling factor for growth 

in GHG emissions which would increase at a similar rate as the fuel burn by 2030, 2035, and 2040. 

FAA, 2015: FAA Aerospace Forecast Fiscal Years 2015-2035, 134 pp. Available at 

https://www.faa.gov/about/office_org/headquarters_offices/apl/aviation_forecasts/aerospace_forecasts/2015-

2035/media/2015_National_Forecast_Report.pdf (last accessed May 12, 2015). 
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pollutant from any classes of aircraft engines which in her judgment causes or contributes to 

air pollution which may reasonably be anticipated to endanger public health or welfare. 

On December 5, 2007, Friends of the Earth, Oceana, the Center for Biological 

Diversity, Earthjustice, and others (Petitioners) sent a letter to the EPA petitioning the Agency

to undertake rulemaking regarding GHG emissions from aircraft.28 Specifically, Petitioners 

requested that the EPA make a finding that GHG emissions from aircraft engines “may

reasonably be anticipated to endanger public health and welfare” and that the EPA promulgate 

standards for GHG emissions from aircraft.

Following the Supreme Court’s decision in Massachusetts v. EPA in 2007, the EPA 

issued an ANPR in 2008 presenting information relevant to potentially regulating GHGs 

under the Act, and soliciting public comment on how to respond to the Court’s ruling and the 

potential ramifications of the Agency’s decision to regulate GHGs under the CAA. This 

ANPR described and solicited comment on numerous petitions the Agency had received to 

regulate GHG emissions from both stationary and mobile sources, including aircraft. 73 FR 

44354, 44468-44473 (July 30, 2008). With regard to aircraft, the Agency sought comment on 

the impact of aircraft operations on GHG emissions and the potential for reductions in GHG 

emissions from these operations. 

 

28 Center for Biological Diversity, Center for Food Safety, Friends of the Earth, International Center for 

Technology Assessment, and Oceana, 2007: Petition for Rulemaking Under the Clean air Act to Reduce the 

Emissions of Air Pollutants from Aircraft the Contribute to Global Climate Change, December 5. Available at 

http://www.epa.gov/otaq/aviation.htm (last accessed May 12, 2015).
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In response to the ANPR, the EPA received comments from a wide range of aviation 

sector stakeholders including industry trade groups, individual manufacturers, states and local 

governments, and nongovernmental organizations (NGOs). Industry groups and individual 

manufacturers stressed that fuel costs (and market forces) created an economic incentive to 

reduce fuel consumption and thus GHG emissions. One industry association indicated its 

commitment to achieve an additional 30 percent fuel efficiency improvement by 2025. 

Another commenter identified engine technologies that were improving fuel efficiency by

more than 15 percent in the next generation of aircraft. With regard to CO2 engine emissions 

standards, these commenters felt that international CO2 standards for aircraft engines were not 

necessary and that, if pursued, such standards would burden the industry and necessitate the 

development of new test procedures if CO2 emissions standards were based on aircraft cruise 

conditions instead of landing and takeoff operations (LTO). Industry commenters also argued 

that other potential approaches to reducing aircraft related emissions, such as averaging of 

GHGs among existing aircraft fleets and cap-and-trade schemes as existed in the European 

Union, were beyond the scope of the EPA’s authority under section 231 of the CAA. Finally,

industry commenters noted that any program developed by the EPA should incentivize market 

forces and provide for flexibility. 

State/local governments and NGO commenters felt strongly that the EPA had clear 

authority to find endangerment under section 231 and that there were multiple options to 

reduce aircraft emissions, so that the Agency must set a GHG emissions standard for aircraft 

engines as states were preempted from doing so under CAA section 233. These commenters 
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also argued that GHG standards for aircraft engines could provide aircraft manufacturers the 

incentive to renew or redesign aircraft and to adopt advanced engines brought to market. In 

addition these commenters suggested that an engine GHG standard could be set as a function 

of thrust similar to ICAO’s standard for oxides of nitrogen (NOx)

29 and should also include 

provisions for an averaging, banking, and trading (ABT) program.

30

 Some commenters also 

stated their support for fleet-wide (in-use fleet) emission reductions through a cap-and-trade 

system. Finally, these stakeholders stated that, absent the EPA rulemaking, quick 

international actions were unlikely and that the EPA should engage internationally to push for 

action on reducing CO2 emissions from aircraft. 

On July 31, 2008, Earthjustice, on behalf of Petitioners, notified the EPA of its intent 

to file suit under CAA section 304(a) against the EPA for the Agency’s alleged unreasonable 

delay in responding to its aircraft petition and in making an endangerment finding under 

section 231. On June 11, 2010, Petitioners filed a complaint against the EPA in the U.S. 

District Court for the District of Columbia claiming that, among other things, the EPA had 

unreasonably delayed because it had failed to answer the 2007 Petition and to determine 

 

29 Sections II.D.1 and II.E provide an overview of the history ICAO’s regulation of aircraft engine NOx 

emissions from 1981 through 2012 and the EPA’s adoption of equivalent aircraft engine NOx standards under 

the CAA. 

30

 ABT programs have been utilized in a number of Clean Air Act programs to provide greater flexibilities that 

lower overall costs by allowing a manufacturer to comply with performance standards through averaging 

emissions among the vehicles it manufactures. Companies that achieve extra pollution reductions can bank these 

as ‘credits” and then ‘trade or sell’ emission credits to other companies, typically those that face higher costs to 

control pollution. Well-designed ABT programs can sometimes achieve greater emissions reductions at less cost 

and provide incentives for technology innovation.
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whether or not GHG emissions from aircraft cause or contribute to air pollution which may

reasonably be anticipated to endanger public health and/or welfare. 

The District Court found that while CAA section 231 generally confers broad 

discretion to the EPA in determining what standards to promulgate, section 231(a)(2)(A) 

imposed a nondiscretionary duty on the EPA to make a finding with respect to endangerment

from aircraft GHG emissions. Center for Biological Diversity, et al. v. EPA, 794 F. Supp. 2d 

151 (D.D.C. 2011). This ruling was issued in response to EPA’s motion to dismiss the case 

on jurisdictional grounds and did not address the merits of the Plaintiffs’ claims regarding the 

Agency’s alleged unreasonable delay. Therefore, it did not include an order for the EPA to 

make such a finding by a certain date. In a subsequent ruling on the merits, the Court found 

that the Plaintiffs had not shown that EPA had unreasonably delayed in making an 

endangerment determination regarding GHG emissions from aircraft. Center for Biological 

Diversity, et al. v. EPA, No. 1:10-985 (D.D.C. March, 20, 2012). Thus, the Court did not find 

the EPA to be liable based on the Plaintiffs’ claims and did not place the Agency under a 

remedial order to make an endangerment finding or to issue standards. The Plaintiffs did not 

appeal this ruling to the U.S. Court of Appeals for the D.C. Circuit.
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The EPA issued a Response to the Aircraft Petition31 on June 27, 2012 stating our 

intention to move forward with a proposed endangerment finding for aircraft GHG emissions

under section 231, while explaining that it would take the Agency significant time to complete 

this action. The EPA explained that the Agency would not begin this effort until after the 

U.S. Court of Appeals completed its then-pending review of the previous section 202 

Endangerment Finding, since the then-awaited ruling might provide important guidance for 

the EPA in conducting future GHG endangerment findings. The EPA further explained that 

after receiving the Court of Appeal’s ruling, it would take at least 22 months from that point 

for the Agency to conduct an additional finding regarding aircraft GHG emissions. 

Meanwhile, the Court upheld EPA’s section 202 findings in a decision of a threejudge panel on June 26, 2012, and denied petitions for rehearing of that decision on December 

20, 2012. Coalition for Responsible Regulation, Inc., v. EPA, 684 F.3d 102 (D.C. Cir. 2012), 

reh’g denied 2012 U.S. App. LEXIS 26315, 25997 (D.C. Cir 2012). 32 Given these rulings, we 

are proceeding with today’s proposed findings regarding aircraft engine GHG emissions as a 

further step toward responding to the Petition for Rulemaking. 

 

31

 U.S. EPA, 2012: Memorandum in Response to Petition Regarding Greenhouse Gas Emissions from Aircraft, 

June 14. Available at http://www.epa.gov/otaq/aviation.htm (last accessed May 12, 2015) and Docket EPA-HQOAR-2014-0828.

32 Petitions for certiorari were filed in the Supreme Court, and the Supreme Court granted six of those petitions 

but “agreed to decide only one question: ‘Whether EPA permissibly determined that its regulation of greenhouse 

gas emissions from new motor vehicles triggered permitting requirements under the Clean Air Act for stationary 

sources that emit greenhouse gases”. Utility Air Reg. Group v. EPA, 134 S. Ct. 2427, 2438 (2014); see also

Virginia v. EPA, 134 S. Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418 (2013), and CRR, 134 S. Ct. 

468 (2013) (all denying cert.). Thus, the Supreme Court did not disturb the D.C. Circuit’s holding that affirmed 

the 2009 Endangerment Finding.
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D. U.S. Aircraft Regulations and the International Community

The EPA and the Federal Aviation Administration (FAA) traditionally work within the 

standard-setting process of ICAO’s Committee on Aviation Environmental Protection 

(CAEP) to establish international emission standards and related requirements. Historically, 

under this approach, international emission standards have first been adopted by ICAO, and 

subsequently the EPA has initiated rulemakings under CAA section 231 to establish domestic 

standards equivalent to ICAO’s standards where appropriate. This approach has been 

affirmed as reasonable by the U.S. Court of Appeals for the D.C. Circuit. NACAA v. EPA, 

489 F.3d 1221, 1230-32 (D.C. Cir. 2007). After EPA promulgates aircraft engine emissions 

standards, CAA section 232 requires the FAA to issue subsequent regulations to ensure 

compliance with these standards when issuing certificates under its United States Code Title 

49 authority. In exercising the EPA’s standard-setting and FAA’s enforcement authorities, 

we expect to proceed using a similar approach for the future CAA section 231 aircraft engine 

GHG standard (which may take the form of a CO2 standard), provided the EPA issues final 

positive endangerment and cause or contribute findings under CAA section 231. This 

approach is contingent on ICAO’s adoption of an international aircraft CO2 standard that is 

consistent with CAA section 231 and is appropriate for domestic needs in the United States.

1. International Regulations and U.S. Obligations

As noted above, we have worked with the FAA since 1973, and later with ICAO, to 

develop domestic and international standards and other recommended practices pertaining to 
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aircraft engine emissions. ICAO is a United Nations (UN) specialized agency, established in 

1944 by the Convention on International Civil Aviation (Chicago Convention), “in order that 

international civil aviation may be developed in a safe and orderly manner and that 

international air transport services may be established on the basis of equality of opportunity 

and operated soundly and economically.”33 ICAO sets standards and regulations necessary

for aviation safety, security, efficiency, capacity and environmental protection, and serves as 

the forum for cooperation in all fields of international civil aviation. ICAO works with the 

Chicago Convention’s member States and global aviation organizations to develop 

international Standards and Recommended Practices (SARPs), which member States 

reference when developing their legally-enforceable national civil aviation regulations. The 

U.S. is currently one of 191 participating ICAO member States.34,35

In the interest of global harmonization and international air commerce, the Chicago 

Convention urges its member States to collaborate in securing the highest practicable degree 

of uniformity in regulations, standards, procedures and organization. The Chicago 

Convention also recognizes that member States may adopt standards that are more stringent 

than those agreed upon by ICAO. Any member State which finds it impracticable to comply

 

33ICAO, 2006: Convention on International Civil Aviation, Ninth Edition, Document 7300/9. Available at: 

http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015).

The ICAO Document 7300 is found on page 1 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. 7300 (. 

34 Members of ICAO’s Assembly are generally termed member States or contracting States. These terms are 

used interchangeably throughout this preamble.

35 There are currently 191 Contracting States according to ICAO’s website: www.icao.int (last accessed May 12, 

2015).
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in all respects with any international standard or procedure, or which deems it necessary to 

adopt regulations or practices differing in any particular respect from those established by an 

international standard, is required to give immediate notification to ICAO of the differences 

between its own practice and that established by the international standard.

36

 

ICAO’s work on the environment focuses primarily on those problems that benefit 

most from a common and coordinated approach on a worldwide basis, namely aircraft noise 

and engine emissions. Standards and Recommended Practices (SARPs) for the certification of 

aircraft noise and aircraft engine emissions are covered by Annex 16 of the Chicago 

Convention. To continue to address aviation environmental issues, in 2004, ICAO established 

three environmental goals: (1) limit or reduce the number of people affected by significant 

aircraft noise; (2) limit or reduce the impact of aviation emissions on local air quality; and (3) 

limit or reduce the impact of aviation greenhouse gas emissions on the global climate.

The Convention has a number of other features that govern international commerce. 

First, member States that wish to use aircraft in international transportation must adopt 

emissions standards and other recommended practices that are at least as stringent as ICAO’s

standards. Member States may ban the use of any aircraft within their airspace that does not 

 

36 ICAO, 2006: Doc 7300-Convention on International Civil Aviation, Ninth edition, Document 7300/9. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Document 7300 is found on page 1 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. 7300..
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meet ICAO standards.37 Second, member States are required to recognize the airworthiness 

certificates of any State whose standards are at least as stringent as ICAO’s standards, thereby

assuring that aircraft of any member State will be permitted to operate in any other member 

State.38 Third, to ensure that international commerce is not unreasonably constrained, a 

member State which elects to adopt more stringent domestic emission standards is obligated 

to notify ICAO of the differences between its standards and ICAO standards.39 

ICAO’s CAEP, which consists of Members and Observers from States, 

intergovernmental and non-governmental organizations representing aviation industry and 

environmental interests, undertakes ICAO’s technical work in the environmental field. The 

Committee is responsible for evaluating, researching, and recommending measures to the 

ICAO Council that address the environmental impacts of international civil aviation. CAEP’s 

terms of reference indicate that “CAEP’s assessments and proposals are pursued taking into 

account: technical feasibility; environmental benefit; economic reasonableness;

interdependencies of measures (for example, among others, measures taken to minimize noise 

 

37 ICAO, 2006: Convention on International Civil Aviation, Article 87, Ninth Edition, Document 7300/9. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015).The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.

38 ICAO, 2006: Convention on International Civil Aviation, Article 33, Ninth Edition, Document 7300/9. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.

39ICAO, 2006: Convention on International Civil Aviation, Article 38, Ninth Edition, Document 7300/9. 

Available at http://www.icao.int/publications/catalogue/cat_2015 en.pdf (last accessed May 12, 2015). The 

ICAO Document 7300 is found on page 1 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. 7300 . 
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and emissions); developments in other fields; and international and national programs.”

40

 

CAEP is composed of various task groups, work groups, and other committees whose

contributing members include atmospheric, economic, aviation, environmental, and other 

professionals interested in and knowledgeable about aviation and environmental protection. 

The ICAO Council reviews and adopts the recommendations made by CAEP. It then reports 

to the ICAO Assembly, the highest body of the Organization, where the main policies on 

aviation environmental protection are adopted and translated into Assembly Resolutions. 

At CAEP meetings, the U.S. is represented by the FAA and plays an active role.41 

The EPA has historically been a principal participant in various ICAO/CAEP working groups

and other international venues, assisting and advising FAA on aviation emissions, technology, 

and policy matters. In turn, the FAA assists and advises the EPA on aviation technology and 

certification matters. If ICAO adopts a CAEP proposal for a new environmental standard, it 

then becomes part of ICAO standards and recommended practices (Annex 16 to the Chicago 

Convention). 42,43

 

40 CAEP’s terms of reference are available at http://www.icao.int/environmentalprotection/Pages/Caep.aspx#ToR (last accessed May 12, 2015).

41Pursuant to the President’s memorandum of August 11, 1960 (and related Executive Order No. 10883 from 

1960), the Interagency Group on International Aviation (IGIA) was established to facilitate coordinated 

recommendations to the Secretary of State on issues pertaining to international aviation. The DOT/FAA is the 

chair of IGIA, and as such, the FAA represents the U.S. on environmental matters at CAEP. 

42 ICAO, 2008: Aircraft Engine Emissions, International Standards and Recommended Practices, Environmental 

Protection, Annex 16, Volume II, Third Edition, July. Available at

http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The ICAO Circular 

337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright protected; Order No. 

CIR337.
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The first international standards and recommended practices for aircraft engine 

emissions were recommended by CAEP’s predecessor, the Committee on Aircraft Engine 

Emissions (CAEE), and adopted by ICAO in 1981.

44

 These standards limited aircraft engine 

emissions of hydrocarbons, carbon monoxide, and NOx. The 1981 standards applied to newly

manufactured engines, which are those engines built after the effective date of the regulations 

– also referred to as in-production engines. In 1993, ICAO adopted a CAEP/2 proposal to 

tighten the original NOx standard by 20 percent and amend the test procedures.45 These 1993 

standards applied both to newly-certified turbofan engines, which are those engine models 

that received their initial type certificate after the effective date of the regulations - also 

referred to as newly-certified engines or new engine designs -- and to in-production engines, 

but with different effective dates for newly-certified engines and in-production engines. In 

1995, CAEP/3 recommended a further tightening of the NOx standards by 16 percent and 

additional test procedure amendments, but in 1997 the ICAO Council rejected this stringency

proposal and approved only the test procedure amendments. At the CAEP/4 meeting in 1998, 

the Committee adopted a similar 16 percent NOx reduction proposal, which ICAO approved 

 

43 CAEP develops new emission standards based on an assessment of the technical feasibility, cost, and 

environmental benefit of potential requirements.

44 ICAO, 2008: Aircraft Engine Emissions: Foreword, International Standards and Recommended Practices, 

Environmental Protection, Annex 16, Volume II, Third Edition, July. Available at 

http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The ICAO Annex 16 

Volume II is found on page 19 of the ICAO Products & Services 2015 catalog and is copyright protected; Order 

No. AN16-2.

45 CAEP conducts its work over a period of years. Each work cycle is numbered sequentially and that identifier 

is used to differentiate the results from one CAEP to another by convention. The first technical meeting on 

aircraft emission standards was CAEP’s successor, i.e., CAEE. The first meeting of CAEP, therefore, is referred 

to as CAEP/2.
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in 1998. The CAEP/4 standards applied only to new engine designs certified (or newlycertified engines) after December 31, 2003 (i.e., unlike the CAEP/2 standards, the CAEP/4 

requirements did not apply to in-production engines). In 2004, CAEP/6 recommended a 12

percent NOx reduction, which ICAO approved in 2005.

46,47

 The CAEP/6 standards applied to 

new engine designs certified after December 31, 2007. In 2010, CAEP/8 recommended a 

further tightening of the NOx standards by 15 percent for new engine designs certified after 

December 31, 2013.

48,49

 The Committee also recommended that the CAEP/6 standards be 

applied to in-production engines (eliminating the production of CAEP/4 compliant engines 

with the exception of spare engines). ICAO approved these recommendations in 2011, then 

equivalent standards (to CAEP/6 and CAEP/8 standards) were promulgated domestically in 

2012 by the EPA in consultation with FAA.50

 

46 CAEP/5 did not address new aircraft engine emission standards.

47 ICAO, 2008: Aircraft Engine Emissions, Annex 16, Volume II, Third Edition, July 2008, Amendment 5 

effective on July 11, 2005. Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last 

accessed May 12, 2015). The ICAO Annex 16 Volume II is found on page 19 of the ICAO Products & Services 

2015 catalog and is copyright protected; Order No. AN16-2.

48 CAEP/7 did not address new aircraft engine emission standards.

49 ICAO, 2010: Committee on Aviation Environmental Protection (CAEP), Report of the Eighth Meeting, 

Montreal, February 1-12, 2010, CAEP/8-WP/80 Available in Docket EPA-HQ-OAR-2010-0687.

50 ICAO, 2011: Aircraft Engine Emissions, Annex 16, Volume II, Third Edition, July 2008, Amendment 7 

effective on July 18, 2011. Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last 

accessed May 12, 2015).. The ICAO Annex 16 Volume II is found on page 19 of the ICAO Products & Services 

2015 catalog and is copyright protected; Order No. AN16-2/E/10 (last accessed February 5, 2015).

U.S. EPA, 2012: Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test 

Procedures; Final Rule, 77 FR 36342 (June 18, 2012). 
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2. The International Community’s Reasons for Addressing Aircraft GHG 

Emissions

In October 2010, the 37th Assembly (Resolution A37-19) of ICAO requested the 

development of an ICAO CO2 emissions standard.

51

 Also, Resolution A37-19 provided a 

framework towards the achievement of an environmentally sustainable future for international 

aviation. With this Resolution, the ICAO Assembly agreed to a global aspirational goal for 

international aviation of improving annual fuel efficiency by two percent and stabilizing CO2 

emissions at 2020 levels.52 The Resolution included the following statements regarding 

ICAO policies and practices related to climate change. 

“ -... ICAO and its member States recognize the importance of providing continuous 

leadership to international civil aviation in limiting or reducing its emissions that contribute to global 

climate change;

 

51A consolidated statement of continuing policies and practices related to environmental protection (known as 

Assembly Resolutions) is revised and updated by the Council every three years for adoption by the ICAO 

Assembly. ICAO, 2010: Resolutions Adopted by the Assembly, 37th Session, Montreal, September 29 - October 

8, 2010, Provisional Edition, November.

52 The global aspirational goal for international aviation of improving annual fuel efficiency by 2 percent is for 

the annual international civil aviation in-service fleet. Fuel efficiency is measured on the basis of the volume of 

fuel used per revenue tonne kilometer performed. (ICAO, CAEP, Aspirational Goals and Implementation 

Options, HLM-ENV/09-WP/5, High-Level Meeting on International Aviation and Climate Change, Presented by 

the Secretariat, Montreal, October 7 to 9, 2009. Available at 

http://www.icao.int/Meetings/AMC/MA/High%20Level%202009/hlmenv_wp005_en.pdf (last accessed May 12, 

2015).
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-Reemphasizing the vital role which international aviation plays in global economic and social 

development and the need to ensure that international aviation continues to develop in a sustainable 

manner; 

-... the ultimate objective of the United Nations Framework Convention on Climate Change 

(UNFCCC) is to achieve stabilization of greenhouse gas (GHG) concentrations in the atmosphere at a 

level that would prevent dangerous anthropogenic interference with the climate system; and

-Acknowledging that international aviation emissions, currently accounting for less than 2 per 

cent of total global CO2 emissions, are projected to grow as a result of the continued development of 

the sector.”

As the above statements indicate, reducing climate impacts of international aviation is 

a critical element of ICAO’s strategic objective of achieving environmental protection and 

sustainable development of air transport. ICAO is currently pursuing a comprehensive set of 

measures to reduce aviation’s climate impact, including alternative fuels, CO2 emissions 

technology-based standards, operational improvements, and market based measures. The 

development and adoption of a CO2 emissions standard is an important part of ICAO’s 

comprehensive set of measures.

3. Relationship of the EPA’s Proposed Endangerment and Cause or Contribute 

Findings to International Aircraft Standards

As described earlier, the EPA and the FAA work within the ICAO/CAEP standard 

setting process to establish international emission standards and related requirements. Under 
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this approach international emission standards have first been adopted by ICAO (with U.S. 

participation and agreement), and subsequently the EPA has initiated rulemakings under CAA 

section 231 to establish domestic aircraft engine emission standards that are of at least 

equivalent stringency as ICAO’s standards. This approach has been affirmed as reasonable 

by the U.S. Court of Appeals for the D.C. Circuit. NACAA v. EPA, 489 F.3d 1221, 1230-32 

(D.C. Cir. 2007). In exercising the EPA’s standard-setting authority, provided the EPA 

makes positive endangerment and cause or contribute findings under CAA section 231 and 

ICAO adopts an international aircraft CO2 standard that is consistent with CAA section 231 

and is appropriate for domestic needs in the United States, the EPA expects to proceed along a 

similar approach for the future CAA section 231 aircraft GHG standard (or aircraft CO2

standard). 

We anticipate that ICAO/CAEP will adopt a final aircraft CO2 emissions standard in 

February 2016. This proposal, and any final endangerment and cause or contribute finding 

for aircraft GHG emissions, are part of preparing for the possible subsequent domestic 

rulemaking process to adopt standards that are of at least equivalent stringency as the

anticipated ICAO CO2 standards. These findings, which are factual and science-based, 

encompass a determination of whether GHG emissions from aircraft cause or contribute to air 

pollution which may reasonably be anticipated to endanger public health or welfare. If 

positive findings are made, the EPA will be obligated under section 231 of the CAA to set 

emission standards applicable to GHG emissions from the classes of aircraft engines for 

which the EPA makes the cause or contribute finding. If positive findings are not made, the 
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EPA will not have triggered its obligation to set GHG emission standards under CAA section 

231. 

The EPA has worked diligently over the past four years within the ICAO/CAEP 

process on a range of technical issues regarding aircraft CO2 emission standards. The ANPR 

that accompanies this proposal, in Section VI, discusses the issues arising in the ongoing 

international proceedings and U.S. input to CAEP regarding the international CO2 standard to 

help ensure transparency about this process.. In addition, in the ANPR the EPA requests 

public comments on a variety of issues to assist the Agency in developing its position with 

regard to these issues and the aircraft engine GHG emission standards that it may potentially 

adopt under the CAA.

E. The EPA’s Regulation of Aircraft Emissions

As required by the CAA, the EPA has been engaged in reducing harmful air pollution 

from aircraft engines for over 40 years. In 1973, the EPA began to regulate gaseous exhaust 

emissions, smoke, and fuel venting from aircraft engines.53 We have occasionally revised 

these regulations. In a 1997 rulemaking, for example, we made our emission standards and 

test procedures more consistent with those of ICAO’s CAEP for turbofan engines used in 

commercial aviation with rated thrusts greater than 26.7 kilonewtons. These ICAO 

 

53 U.S. EPA, 1973: Emission Standards and Test Procedures for Aircraft; Final Rule, 38 FR 19088 (July 17,

1973).
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requirements are generally referred to as CAEP/2 standards.54 That action included new NOx

emission standards for newly manufactured commercial turbofan engines (as described 

earlier, those engines built after the effective date of the regulations that were already certified 

to pre-existing standards – also referred to as in-production engines)55 and for newly-certified 

commercial turbofan engines (as described earlier, those engine models that received their 

initial type certificate after the effective date of the regulations – also referred to as new 

engine designs).56 It also included a carbon monoxide emission standard for in-production 

commercial turbofan engines.57 In 2005, we promulgated more stringent NOx emission 

standards for newly-certified commercial turbofan engines.

58

 That final rule brought the U.S. 

standards closer to alignment with ICAO CAEP/4 requirements that became effective in 2004. 

In 2012, we issued more stringent two-tiered NOx emission standards for newly-certified and 

in-production commercial and non-commercial turbofan aircraft engines, and these NOx

 

54 The full CAEP membership meets every three years and each session is denoted by a numerical identifier. For 

example, the second meeting of CAEP is referred to as CAEP/2, and CAEP/2 occurred in 1994. 

55 This does not mean that in 1997 we promulgated requirements for the re-certification or retrofit of existing inuse engines.

56 In the existing EPA regulations, 40 CFR part 87, newly-certified aircraft engines are described as engines of a 

type or model of which the date of manufacture of the first individual production model was after the 

implementation date. Newly manufactured aircraft engines are characterized as engines of a type or model for 

which the date of manufacturer of the individual engine was after the implementation date.

57 U.S. EPA, 1997: Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test 

Procedures; Final Rule, 62 FR 25355 (May 8, 1997).

58 U.S. EPA, 2005: Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test 

Procedures; Final Rule, 70 FR 2521 (November 17, 2005).
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standards align with ICAO’s CAEP/6 and CAEP/8 requirements that became effective in 

2013 and 2014, respectively.

59,60 

The EPA’s actions to regulate certain pollutants emitted from aircraft engines come 

directly from its authority in section 231 of the CAA, and we have aligned the U.S. emissions 

requirements with those promulgated by ICAO. In addressing CO2 emissions, however,

ICAO has moved to regulating a whole aircraft. This ICAO extension beyond pollutant 

emissions from engines to the whole aircraft was described in a 2013 ICAO circular.

61

 

Several factors are considered when addressing whole-aircraft CO2 emissions, as the CO2

emissions are influenced by aerodynamics, weight, and engine-specific fuel consumption. 

Since each of these factors may affect aircraft engine fuel consumption, they ultimately affect 

CO2 emissions. Rather than viewing CO2 as a measurable emission from engines, therefore, 

ICAO now addresses CO2 emissions as a characteristic applicable to the entirety of the aircraft 

based on fuel consumption. In this proposed action, we are giving advance notice that the 

EPA may propose to adopt domestic GHG emission standards (which may take the form of 

 

59 U.S. EPA, 2012: Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test 

Procedures; Final Rule, 77 FR 36342 (June 18, 2012).

60 While ICAO’s standards were not limited to “commercial” aircraft engines, our 1997 standards were explicitly 

limited to commercial engines, as our finding that NOx and carbon monoxide emissions from aircraft engines 

cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare 

was so limited. See 62 FR 25358. In the 2012 rulemaking, we expanded the scope of that finding and of our 

standards pursuant to Section 231(a)(2)(A) of the Clean Air Act to include such emissions from both commercial 

and non-commercial aircraft engines based on the physical and operational similarities between commercial and 

noncommercial civilian aircraft and to bring our standards into full alignment with ICAO’s.

61 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard, Circular 

(Cir) 337, AN/192. Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 

12, 2015). The ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is 

copyright protected; Order No. CIR337. 
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CO2 standards) for aircraft engines used in covered aircraft as an outgrowth of the 

international negotiations that commenced in 2010 under the auspices ICAO/CAEP. Such 

standards could then discharge the EPA’s duties under CAA sections 231(a)(2)(A) and 

231(a)(3), if triggered by final positive endangerment and cause or contribute findings, to 

“issue proposed emission standards applicable to the emission of” GHG from aircraft engines 

and to issue final “regulations with such modifications as [she] deems appropriate.” 

III. Legal Framework for This Action

The EPA has previously made an endangerment finding for GHGs under Title II of the 

CAA, in the 2009 Endangerment Finding for section 202(a) source categories. In the 2009 

Endangerment Finding, the EPA explained its legal framework for making an endangerment 

finding under section 202(a) of the CAA (79 FR 18886, 18890-94 (Apr. 24, 2009), and 79 FR 

66496, 66505-10 (Dec. 15, 2009)). The text in section 202(a) that was the basis for the 2009 

Endangerment Finding addresses “the emission of any air pollutant from any class or classes 

of new motor vehicles or new motor vehicle engines, which in [the Administrator’s] judgment 

cause, or contribute to, air pollution which may reasonably be anticipated to endanger public 

health or welfare.” Similarly, section 231(a)(2)(A) concerns “the emission of any air 

pollutant from any class or classes of aircraft engines which in [the Administrator’s] judgment 

causes, or contributes to, air pollution which may reasonably be anticipated to endanger 

public health or welfare.” Thus, the text of the CAA section concerning aircraft emissions in 
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section 231(a)(2)(A) mirrors the text of CAA section 202(a) that was the basis for the 2009 

Endangerment Finding. 

The EPA’s approach in the 2009 Endangerment Finding (described below in Sections 

III.A and III.B) was affirmed by the U.S. Court of Appeals for the D.C. Circuit in Coalition 

for Responsible Regulation, Inc. v. EPA, 684 F.3d 102 (D.C. Cir. 2012), reh’g denied 2012 

U.S. App. LEXIS 26313, 26315, 25997 (D.C. Cir 2012) (CRR). In particular, the D.C. Circuit 

ruled that the 2009 Endangerment Finding (including the agency’s denial of petitions for 

reconsideration of that Finding) was not arbitrary or capricious, was consistent with the U.S. 

Supreme Court’s decision in Massachusetts v. EPA and the text and structure of the CAA, and 

was adequately supported by the administrative record. CRR, 684 F.3d at 116-128. The D.C. 

Circuit found that the EPA had based its decision on “substantial scientific evidence” and 

noted that the EPA’s reliance on major scientific assessments was consistent with the methods 

that decision-makers often use to make a science-based judgment. Id. at 120-121. Petitions 

for certiorari were filed in the Supreme Court, and the Supreme Court granted six of those 

petitions but “agreed to decide only one question: ‘Whether EPA permissibly determined that 

its regulation of greenhouse gas emissions from new motor vehicles triggered permitting 

requirements under the Clean Air Act for stationary sources that emit greenhouse gases.’” 

Utility Air Reg. Group v. EPA, 134 S. Ct. 2427, 2438 (2014); see also Virginia v. EPA, 134 S. 

Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418 (2013), and CRR, 134 S. Ct. 468 

(2013) (all denying cert.). Thus, the Supreme Court did not disturb the D.C. Circuit’s holding 

that affirmed the 2009 Endangerment Finding. Accordingly, the Agency proposes that it is 
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reasonable to use that same approach under section 231(a)(2)(A)’s similar endangerment text, 

and as explained in the following discussion, is acting consistently with that judicially

sanctioned framework for purposes of this proposed section 231 finding. 

Two provisions of the CAA govern today’s proposal. Section 231(a)(2)(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 231(a)(2)(A). These statutory provisions are discussed below.

A. Section 231(a)(2)(A) —Endangerment and Cause or Contribute

As noted above, section 231(a)(2)(A) of the CAA (like section 202(a)) calls for the 

Administrator to exercise her judgment and make two separate determinations: first, whether 

the relevant kind of air pollution—here, GHGs—may reasonably be anticipated to endanger 

public health or welfare, and second, whether emissions of any air pollutant from classes of

the sources in question (aircraft engines under section 231 and new motor vehicles or engines 

under section 202) cause or contribute to this air pollution.62

The Administrator interprets the two-part test required under section 231(a)(2)(A) as

being the same as that explained in the 2009 Endangerment Finding. (See 74 FR 66505-06.) 

As in the section 202(a) context, this analysis entails a scientific judgment by the 

 

62 See CRR, 684 F.3d at 117 (explaining two-part analysis under section 202(a)).
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Administrator about the potential risks posed by GHG emissions to public health and welfare. 

See CRR, 684 F.3d at 117-118.63 

In making this scientific judgment, the Administrator is guided by five principles. 

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.64 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.65 At different points along this scale, the

Administrator is permitted to find endangerment. Accordingly, the Administrator need not set 

a precise or minimum threshold of risk or harm as part of making an endangerment finding, 

but rather may base her determination on “‘a lesser risk of greater harm . . . or a greater risk of 

 

63 When agencies such as the EPA make determinations based on review of scientific data within their technical 

expertise, those decisions are given an “extreme degree of deference” by the D.C. Circuit, and as that court noted 

in reviewing the 2009 endangerment finding, “although we perform a searching and careful inquiry into the facts 

underlying the agency’s decisions, we will presume the validity of the agency action as long as a rational basis 

for it is presented.” CRR, 684 F.3d at 120 (internal citations and marks omitted). 

64 See id. at 121-122.

65 See id. at 122-123 (noting that the § 202(a)(1) inquiry “necessarily entails a case-by-case, sliding scale 

approach” because endangerment is “‘composed of reciprocal elements of risk and harm, or probability and 

severity’” (quoting Ethyl Corp. v. EPA, 541 F.2d, 1, 18 (D.C. Cir. 1976)).
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lesser harm’ or any combination in between.” CRR, 684 F.3d at 123 (quoting Ethyl Corp. v. 

EPA, 541 F.2d, 1, 18 (D.C. Cir. 1976)). 

Third, 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.66 At the same time, the Administrator must exercise 

reasoned decision making, and avoid speculative inquiries. 

Fourth, 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. We additionally note that in making an 

endangerment finding, the Administrator is not limited to considering only those impacts that 

can be traced to the amount of air pollution directly attributable to the GHGs emitted by the 

subject source classes. Such an approach would collapse the two prongs of the test by

requiring that any climate change impacts upon which an endangerment determination is 

made result solely from the GHG emissions of aircraft. See 74 FR at 66542 (explaining the 

same point in the context of analogous language in section 202(a)). Similarly, the 

Administrator is not, in making the endangerment and cause or contribute findings, to 

 

66 See id. at 121-122.
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consider the effect of emissions reductions from the resulting standards.67 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.

Fifth, 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. As explained previously in the 2009 

Endangerment Finding and as reiterated below for this proposed section 231 finding,

vulnerable subpopulations face serious health risks as a result of climate change.

As the Supreme Court recognized in Massachusetts v. EPA, 549 U.S. at 534, the EPA 

may make an endangerment finding despite the existence of “some residual uncertainty” in 

the scientific record. See also CRR, 684 F. 2d at 122. Thus, this framework recognizes that

 

67 As the D.C. Circuit explained in reviewing the 2009 Endangerment Finding under analogous language in 

section 202(a): “At bottom, § 202(a)(1) requires EPA to answer only two questions: whether particular ‘air 

pollution’--here, greenhouse gases—‘may reasonably be anticipated to endanger public health or welfare,’ and 

whether motor-vehicle emissions ‘cause, or contribute to’ that endangerment.” CRR, 648 F.3d at 117.
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regulatory agencies such as the 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.); 

see also CRR, 684 F.3d at 121-122.

In the 2009 Endangerment Finding, the Administrator recognized that the scientific

context for an action addressing climate change was unique at that time because there was a 

very large and comprehensive base of scientific information that had been developed over 

many years through a global consensus process involving numerous scientists from many

countries and representing many disciplines. 74 FR at 66506. That informational base has 

since grown. The Administrator also previously recognized that there are varying degrees of 

uncertainty across many of these scientific issues, which remains true. It is in this context that 

she is exercising her judgment and applying the statutory framework in this proposed section 

231 finding. Further discussion of the language in section 231(a)(2)(A), and parallel language 

in 202(a), is provided below to explain more fully the basis for this interpretation, which the 

D.C. Circuit upheld in the 202(a) context.
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1. The Statutory Language

The interpretation described above flows from the statutory language itself. The 

phrase “may reasonably be anticipated” and the term “endanger” in section 231(a)(2)(A) (as 

in section 202(a)) 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. As the D.C. Circuit explained, the 

language “may reasonably be anticipated to endanger public health or welfare” in CAA § 

202(a) requires a “precautionary, forward-looking scientific judgment about the risks of a 

particular air pollutant, consistent with the CAA’s precautionary and preventive orientation.” 

CRR, 684 F.3d at 122 (internal citations omitted). The court determined that “[r]equiring that 

EPA find ‘certain’ endangerment of public health or welfare before regulating greenhouse 

gases would effectively prevent EPA from doing the job that Congress gave it in § 202(a)—

utilizing emission standards to prevent reasonably anticipated endangerment from maturing 

into concrete harm.” Id. The same language appears in section 231(a)(2)(A), and the same 

interpretation applies in that context. 

Moreover, by instructing the Administrator to consider whether emissions of an air

pollutant cause or contribute to air pollution in the second part of the two-part test, the Act 

makes clear that she need not find that emissions from any one sector or class of sources are 

the sole or even the major part of an air pollution problem. The use of the term “contribute”
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clearly indicates that such emissions need not be the sole or major cause of the pollution. 

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. 

Finally, when exercising her judgment in making both the endangerment and cause-orcontribute findings, 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. How the Origin of the Current Statutory Language Informs the EPA’s 

Interpretation of Section 231(a)(2)(A) 

In the proposed and final 2009 Endangerment Finding, the EPA explained that when 

Congress revised the section 202(a) language that governed that finding, along with other 

provisions, as part of the 1977 amendments to the CAA, it was responding to decisions issued 

by the D.C. Circuit in Ethyl Corp. v. EPA regarding the pre-1977 version of section 211(c) of 

the Act. 74 FR at 18891; see also 74 FR at 66506. Section 231 was one of those other CAA 

provisions included in the 1977 amendments; therefore, the Agency’s discussion for the 2009 

Endangerment Finding regarding the history of section 202 and how it supports the EPA’s 

approach is also relevant for section 231. The legislative history of those amendments, 

particularly the report by the House Committee on Interstate and Foreign Commerce, 

demonstrates that the EPA’s interpretation of the section 231(a)(2)(A) language as set forth 
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here in support of the Agency’s section 231 finding (which is the same as its interpretation of 

the parallel language in section 202(a) as explained in the 2009 Endangerment Finding), is 

fully consistent with Congress’ intention in crafting these provisions. 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).68

The legislative history clearly indicates that the House Committee believed the Ethyl 

Corp. decisions posed several “crucial policy questions” regarding the protection of public 

health and welfare. H.R. Rep. 95-294 at 48, 4 LH at 2515.

69 The following paragraphs 

summarize the en banc decision in Ethyl Corp. v. EPA and describe how the House 

Committee revised the endangerment language in the 1977 amendments to the CAA to serve 

several purposes consistent with that decision. In particular, the language: (1) emphasizes the 

preventive or precautionary nature of the CAA70; (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 

 

68 The committee explained that its action addressed not only section 211(c)(1)(A) but rather the entire proposal, 

and would thus apply its interpretation to all other sections of the Act relating to public health protection. 4 LH at 

2516. It also noted that it had used the same basic formulation in section 202 and section 231, as well as in other 

sections. Id. at 2517. 

69 The Supreme Court recognized that the current language in section 202(a)(1), which uses the same 

formulation as that in section 231(a)(2)(A), is “more protective” than the 1970 version that was similar to the 

section 211 language before the D.C. Circuit in Ethyl Corp. Massachusetts v. EPA, 549 U.S. at 506, fn 7. 

70 See 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”).
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uncertainties and limitations in information available to the Administrator. H.R. rep. 95-294 

at 49-50, 4 LH 2516-17.71

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.72 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.” 541 F.2d 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.

 

71 Congress 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 the CAA 

Amendments of 1977.

72 At 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). 
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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 the 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 the EPA that even without the language in 

section 202(a) (which is also in section 231(a)(2)(A)) regarding “cause or contribute to,” it 

was appropriate for the 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.

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. The Committee addressed those questions with the language that now appears in 

section 231(a)(2)(A) and several other CAA provisions—“emission of any air pollutant * * * 

which in [the Administrator’s] judgment causes, or contributes to, air pollution which may

reasonably be anticipated to endanger public health or welfare.”

As noted above in section III.A.1, the phrase “in [her] judgment” calls for the 

Administrator to make a comparative assessment of risks and projections of future 
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possibilities, consider uncertainties, and extrapolate from limited data. Thus, the 

Administrator must balance the likelihood of effects with the severity of the effects in 

reaching her judgment. The Committee emphasized that the Administrator’s exercise of 

“judgment”73 may include making projections, assessments and estimates that are reasonable, 

as opposed to a speculative or “‘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 above. 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 2518.74

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 

 

73 Throughout this Notice under CAA section 231, as throughout the previous Notices concerning the 2009

Endangerment Finding under section 202, 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 

judgment is solely a fact finding exercise; rather, the Administrator’s exercise of judgment is to consider and 

weigh multiple factors when applying the scientific information to the statutory criteria.

74 Thus, the statutory language does not require that the 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. See also CRR, 684 F.3d at 121-122.
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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 class 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, like the section 202(a) language that governed the 2009 Endangerment 

Finding, the statutory language in section 231(a)(2)(A) does not contain a modifier on its use 

of the term “contribute.” Unlike other CAA provisions, it does not require “significant”

contribution. Compare, 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 the EPA to identify

and then address what may well be many different sectors, classes, or groups of sources that 

are each part of the problem.
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As explained for the 2009 Endangerment Finding, the D.C. Circuit has discussed the 

concept of contribution in the CAA, and its case law supports the EPA’s interpretation that 

the level of contribution need not be significant. 74 FR at 66542. In Catawba County v. EPA, 

571 F.3d 20 (D.C. Cir. 2009), the court upheld EPA’s PM[2.5] attainment and nonattainment 

designation decisions, analyzing CAA section 107(d), which requires EPA to designate an 

area as nonattainment if it “contributes to ambient air quality in a nearby area” not attaining 

the national ambient air quality standards. Id. at 35. The court noted that it had previously 

held that the term “contributes” is ambiguous in the context of CAA language. See EDF v. 

EPA, 82 F.3d 451, 459 (DC Cir. 1996). “[A]mbiguities in statutes within an agency’s 

jurisdiction to administer are delegations of authority to the agency to fill the statutory gap in 

reasonable fashion.” 571 F.3d at 35 (citing Nat'l Cable & Telecomms. Ass'c v. Brand X 

Internet Servs, 545 U.S. 967, 980 (2005)). The court then proceeded to consider and reject 

petitioners' argument that the verb “contributes” in CAA section 107(d) necessarily connotes 

a significant causal relationship. Specifically, the D.C. Circuit again noted that the term is 

ambiguous, leaving it to EPA to interpret in a reasonable manner. In the context of this 

discussion, the court noted that “a contribution may simply exacerbate a problem rather than 

cause it * * *.” 571 F.3d at 39.

This is consistent with the D.C. Circuit’s discussion of 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 (D.C. Cir. 2004). In that case, industry argued that section 213(a)(3) requires a 

finding of a significant contribution before the EPA can regulate, while the EPA’s view was 
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that the CAA requires a finding only of contribution. Id. at 13. Section 213(a)(3), like section 

231(a)(2)(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 the 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 13.75 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 a cause or contribute finding. 

Id. at 14.76

Like the statutory language considered in Catawba County and Bluewater Network, as 

well as the section 202(a) language that governed the Agency’s previous findings for GHGs 

emitted by other types of mobile sources, section 231(a)(2)(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

 

75 Specifically, 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.

76 The 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.
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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.77 As noted above, in 

addressing provisions in section 202(a), the D.C. Circuit has explained that the Act at the 

endangerment finding step did not require the EPA to identify a precise numerical value or “a 

minimum threshold of risk or harm before determining whether an air pollutant endangers.” 

CRR, 684 F.3d at 122-123. Accordingly, EPA “may base an endangerment finding on ‘a 

lesser risk of greater harm . . . or a greater risk of lesser harm’ or any combination in 

between.” Id. (quoting Ethyl Corp., 541 F.2d at 18). Recognizing the substantial record of 

empirical data and scientific evidence that the EPA relied upon in the 2009 Endangerment 

Finding, the court determined that its “failure to distill this ocean of evidence into a specific 

number at which greenhouse gases cause ‘dangerous’ climate change is a function of the 

precautionary thrust of the CAA and the multivariate and sometimes uncertain nature of 

climate science, not a sign of arbitrary or capricious decision-making.” Id. at 123. As the 

language in section 231(a)(2)(A) is analogous to that in section 202(a), it is clearly reasonable 

to apply this interpretation to the endangerment determination under section 231(a)(2)(A). 

Moreover, the logic underlying this interpretation supports the general principle that under 

CAA section 231 the EPA is not required to identify a specific minimum threshold of 

 

77 Section V discusses the evidence in this case that supports the proposed finding of contribution. The 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.
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contribution from potentially subject source categories in determining whether their emissions 

“cause or contribute” to the endangering air pollution. The reasonableness of this principle is 

further supported by the fact that section 231 does not impose on the EPA a requirement to 

find that such contribution is “significant,” let alone the sole or major cause of the 

endangering air pollution. This context further supports the EPA’s interpretation that section 

231(a)(2)(A) requires some level of contribution that, while exceeding de minimis or trivial 

thresholds, does not need to rise to a pre-determined numerical level of significance. 

In addition, when exercising her judgment in making a cause or contribute 

determination, 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 or classes of sources at issue, the number of sources in the source 

sector or class, and the number and type of other source sectors or categories that may emit 

the air pollutant) when determining whether the emissions “justify regulation” under the 

CAA. See Catawba County, 571 F.3d at 39 (discussing EPA’s interpretation of the term 

“contribute” under CAA § 107(d) and finding it reasonable for the agency to adopt a totality

of the circumstances approach); see also 74 FR at 66542. Further discussion of this issue can 

be found in sections IV and V of this preamble.

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, 
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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 V below 

contains further discussion of the “air pollutant” for purposes of today’s section 231 proposed 

contribution finding, which uses the same definition of air pollutant as the EPA adopted for 

purposes of the 2009 Endangerment 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 could include impacts from changes in temperature and precipitation as 
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well as from the spreading of invasive species or insects. Prior welfare effects evaluated by 

the EPA in other contexts include impacts on vegetation, 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 Secondary National Ambient Air Quality

Standards for Oxides of Nitrogen and Sulfur, 77 FR 20218 (2012); 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).

Although the CAA defines “effects on welfare” as discussed above, there are no 

definitions of “public health” or “public welfare” in the Clean Air Act. The Supreme Court 

has discussed the concept of “public health” 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, the EPA has looked at 

morbidity, such as 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 (2008).

IV. The Proposed Endangerment Finding under CAA section 231

This section describes the Administrator’s proposed endangerment finding under CAA 

section 231(a)(2) and its basis. Beginning with the air pollution under consideration, the
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Administrator is proposing to use the same definition of the “air pollution” under CAA 

section 231(a)(2) as that used under CAA section 202(a)(1), namely the mix of six wellmixed GHGs mentioned above: CO2, methane, nitrous oxide, hydrofluorocarbons,

perfluorocarbons, and sulfur hexafluoride. As described in section IV.A below, it is the

Administrator’s view that the reasons detailed in the 2009 Endangerment Finding for 

defining the scope and nature of the air pollution to be these six well-mixed GHGs remain 

valid and well-supported by the current science and are therefore reasonable bases for 

adopting the same definition of “air pollution” in this section 231(a)(2)(A) finding.

Information from the new scientific assessments described in section IV.B below provides

further support that the six well-mixed GHGs are the primary cause and driver of climate 

change. The Administrator considered other climate-forcing agents both in the 2009 

Endangerment Finding and in today’s action; however, these substances are not included in 

the air pollution definition proposed today for the reasons discussed below in section IV.B.4.

The Administrator is proposing to find, for purposes of CAA section 231(a)(2)(A), that 

elevated concentrations of the six well-mixed GHGs constitute air pollution that endangers

both the public health and the public welfare of current and future generations. The 

Administrator’s view is that the body of scientific evidence amassed in the record for the 

2009 Endangerment Finding compellingly supports an endangerment finding under CAA 

section 231(a). Information from the new scientific assessments described in section IV.B

below provides further support and justification for this proposed finding.
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Section IV.A below summarizes the 2009 Endangerment Finding under CAA section 

202, explains the approach EPA took in compiling an extensive record to inform the 

Administrator’s judgment on that finding, and describes the recent judicial affirmation of the 

2009 Endangerment Finding. Section IV.B provides a summary of new scientific 

assessments that strengthen or provide further scientific evidence, in addition to that which 

the Administrator relied upon in making her prior judgment, for a finding that GHGs

endanger public health and welfare.78 Finally, section IV.C summarizes the Administrator’s 

conclusion for purposes of section 231, in light of the evidence, analysis, and conclusions 

that led to the 2009 Endangerment Finding as well as more recent evidence, that emissions of 

the six well-mixed GHGs in the atmosphere endanger public health and welfare.

A. Scientific Basis of the 2009 Endangerment Finding under CAA section 

202(a)(1)

In the 2009 Endangerment Finding, the Administrator found that elevated concentrations 

of the well-mixed GHGs in the atmosphere may reasonably be anticipated to endanger public 

health and welfare of current and future generations. See, e.g., 74 FR 66516. The 

Administrator reached this judgment by carefully considering a significant body of scientific 

evidence and public comments submitted to the Agency. The sections below summarize the 

 

78 While the EPA is providing a summary of newer scientific assessments below, the EPA is also relying on the 

same scientific and technical evidence discussed in the notices for the 2009 Endangerment Finding in this 

proposed finding for purposes of CAA section 231. See sections III of the 2009 Proposed Endangerment Finding

and sections III and IV of the 2009 Endangerment Finding.
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scope and nature of the relevant air pollution for the 2009 Endangerment Finding, as well as 

the public health and welfare considerations within the finding.

1. The Definition of Air Pollution in the 2009 Endangerment Finding

The Administrator defined the scope and nature of the relevant air pollution as the 

aggregate group of six key, well-mixed GHGs: CO2, methane, nitrous oxide,

hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.

79 The Administrator 

considered five primary reasons for focusing on this aggregate group as the air pollution in 

the 2009 Endangerment Finding: (1) they share common physical properties that influence

their climate effects; (2) on the basis of these common physical properties, they have been 

determined to be the primary cause of human-induced climate change, are the bestunderstood driver of climate change, and are expected to remain the key driver of future 

climate change; (3) they are the common focus of climate change science research and policy

analyses and discussions; (4) using the combined mix of these gases as the definition (versus 

an individual gas-by-gas approach) is consistent with the science, because risks and impacts

associated with GHG-induced climate change are not assessed on an individual gas-by-gas 

basis; and (5) using the combined mix of these gases is consistent with past EPA practice,

 

79 74 FR 66516.
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where separate substances from different sources, but with common properties, may be 

treated as a class (e.g., oxides of nitrogen, particulate matter, volatile organic compounds).80

The common physical properties these six GHGs share that are relevant to the climate 

change problem include the following: all are long-lived in the atmosphere; 81 all become 

globally well mixed in the atmosphere, resulting in similar GHG concentrations around the 

globe regardless of geographic location of emissions; all trap outgoing heat that would 

otherwise escape to space; and all are directly emitted as GHGs rather than becoming a GHG 

in the atmosphere after emission of a precursor gas. The Administrator acknowledged that 

other anthropogenic climate forcers also play a role in climate change but for various 

scientific and policy reasons, these substances were not included in the air pollution

definition.

82

As explained in more detail in the 2009 Endangerment Finding, the EPA made the 

judgment that the scientific evidence is compelling that elevated concentrations of heattrapping GHGs are the root cause of recently observed climate change and that the scientific 

 

80 74 FR 66517 to 66519.

81 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. According to the most recent IPCC Fifth 

Assessment Report (2014), methane has an atmospheric lifetime of about 12 years. One of the most commonly 

used hydrofluorocarbons (HFC–134a) has a lifetime of about 13 years. Nitrous oxide has a lifetime of around 

130 years; sulfur hexafluoride over 3,000 years; and some perfluorocarbons up to 10,000 to 50,000 years. CO2 is 

sometimes approximated as having a lifetime of roughly 100 years, but for a given amount of CO2 emitted a 

better description is that some fraction of the atmospheric increase in concentration is quickly absorbed by the 

oceans and terrestrial vegetation, some fraction of the atmospheric increase will only slowly decrease over a 

number of years, and a small portion of the increase will remain for many centuries or more.

82 74 FR 66519 to 66521.
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record showed that most of the observed increase in global average temperatures since the 

mid-20th century is very likely due to the observed increase in anthropogenic GHG

concentrations. The attribution of observed climate change to anthropogenic activities was

based on multiple lines of evidence.83 The first line of evidence arises from our basic 

physical understanding of the effects of changing concentrations of GHGs, natural factors, 

and other human impacts on the climate system. The second line of evidence arises from 

indirect, historical estimates of past climate changes that suggest that the changes in global 

surface temperature over the last several decades are unusual. The third line of evidence 

arises from the use of computer-based climate models to simulate the likely patterns of 

response of the climate system to different forcing mechanisms (both natural and 

anthropogenic).

2. Public Health Impacts Detailed in the 2009 Endangerment Finding

Climate change resulting from anthropogenic GHG emissions threatens multiple aspects 

of public health.84 In determining that the well-mixed GHG air pollution is reasonably 

anticipated to endanger public health for current and future generations, the Administrator 

noted her view that climate change can increase the risk of morbidity and mortality.85 In

making that public health finding, the Administrator considered direct temperature effects, 

 

83 74 FR 66518.

84 74 FR 66524 to 66530.

85 74 FR 66524.
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air quality effects, the potential for changes in vector-borne diseases, and the potential for 

changes in the severity and frequency of extreme weather events. In addition, the 

Administrator considered whether and how susceptible populations may be particularly at 

risk. As explained in more detail in the 2009 Endangerment Finding, with respect to direct 

temperature effects, by raising average temperatures, climate change increases the likelihood 

of heat waves, which are associated with increased deaths and illnesses. Climate change is 

also expected to lead to reductions in cold-related mortality. The 2009 Endangerment 

Finding, while noting uncertainty about how heat and cold related mortality would change in 

the future, also pointed to a USGCRP assessment report discussion that increases in heatrelated mortality due to global warming in the United States was unlikely to be compensated 

for by decreases in cold-related mortality (74 FR 66525). With regard to air quality effects,

climate change is expected to increase ozone pollution over broad areas of the country, 

including large metropolitan population centers, and thereby increase the risks of respiratory

infection, aggravation of asthma, and premature death. Other public health threats stem from

the potential for increased deaths, injuries, infectious and waterborne diseases, stress-related

disorders, and other adverse effects associated with increased hurricane intensity and 

increased frequency of intense storms and heavy precipitation associated with climate 

change. In addition, climate change is expected to be associated with an increase in the 

spread of food-, water-, and vector-borne diseases in susceptible populations. Climate 

change also has the potential to change aeroallergen production (for example, through 

lengthening the growing season for allergen-producing plants), and subsequent human 
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exposures could increase allergenic illnesses. Children, the elderly, and the poor are among 

the most vulnerable to climate-related health effects.

3. Public Welfare Impacts Detailed in the 2009 Endangerment Finding

Climate change resulting from anthropogenic GHG emissions also threatens multiple 

aspects of public welfare.86 In determining that the well-mixed GHG air pollution is 

reasonably anticipated to endanger public welfare for current and future generations, the

Administrator considered the multiple pathways by which GHG air pollution and resultant 

climate change affect public welfare by evaluating the numerous and far-ranging risks to 

food production and agriculture; forestry; water resources; sea level rise and coastal areas; 

energy, infrastructure, and settlements; and ecosystems and wildlife. The Administrator also 

considered impacts on the U.S. population from climate change effects occurring outside of 

the United States. As explained in more detail in the 2009 Endangerment Finding, the 

potential serious adverse impacts of extreme events, such as wildfires, flooding, drought, and 

extreme weather conditions provided strong support for the determination. Climate change 

is expected to place large areas of the country at serious risk of reduced water supplies, 

increased water pollution, and increased occurrence of extreme events such as floods and 

droughts. Coastal areas are expected to face increased risks from storm and flooding damage 

to property, as well as adverse impacts from rising sea level such as land loss due to 

 

86 74 FR 66530 to 66536.
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inundation, erosion, wetland submergence and habitat loss. Climate change is expected to 

result in an increase in peak electricity demand, and extreme weather from climate change 

threatens energy, transportation, and water resource infrastructure. Climate change may

exacerbate existing environmental pressures in certain settlements, particularly in Alaskan 

indigenous communities. Climate change is also very likely to fundamentally change U.S. 

ecosystems over the 21st century and to lead to predominantly negative consequences for 

biodiversity, ecosystem goods and services, and wildlife. Though there may be some 

benefits for agriculture and forestry in the next few decades, the body of evidence points 

towards increasing risks of net adverse impacts on U.S. food production, agriculture and 

forest productivity as average temperature continues to rise. Looking across all sectors 

discussed above, the risk and the severity of adverse impacts on public welfare are expected 

to increase over time. Lastly, these impacts are global and may exacerbate problems outside 

the United States that raise humanitarian, trade, and national security issues for the United 

States.

4. The Science Upon Which the Agency Relied

As outlined in section III.A of the 2009 Endangerment Finding,

87 the EPA's approach 

to providing the technical and scientific information to inform the Administrator's judgment 

regarding the question of whether GHGs endanger public health and welfare was to rely 

 

87 74 FR 66510 to 66512.
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primarily upon the recent, major assessments by the USGCRP, the IPCC, and the NRC. 

These assessments addressed the scientific issues that the EPA was required to examine, 

were comprehensive in their coverage of the GHG and climate change issues, and underwent 

rigorous and exacting peer review by the expert community, as well as rigorous levels of 

U.S. government review, in which the EPA took part. Primary reliance on the major 

scientific assessments provided assurance that the Administrator was basing her judgment on 

the best available, well-vetted science that reflected the consensus of the climate science 

research community. The major findings of the USGCRP, IPCC, and NRC assessments 

supported the Administrator’s determination that elevated concentrations of GHGs in the 

atmosphere may reasonably be anticipated to endanger the public health and welfare of 

current and future generations. The EPA presented this scientific support at length in the 

comprehensive record for the 2009 Endangerment Finding. Relevant sections of documents

from the 2009 Endangerment Finding record have been placed in the docket for this 

proposed finding under CAA section 231.

The EPA then reviewed ten administrative petitions for reconsideration of the 

Endangerment Finding in 2010.88 In the Reconsideration Denial, the Administrator denied 

those petitions on the basis of the Petitioners’ failure to provide substantial support for their

argument that the EPA should revise the Endangerment Finding and their objections’ lack of 

 

88 Administrative petitions are available from http://www.epa.gov/climatechange/endangerment/petitions.html

(last accessed May 12, 2015), and in the docket for the 2009 Endangerment Finding: EPA-HQ-OAR-2009-017.
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“central relevance” to the Finding. The EPA prepared an accompanying three-volume 

Response to Petitions document to provide additional information, often more technical in 

nature, in response to the arguments, claims, and assertions by the Petitioners to reconsider 

the Endangerment Finding.89

The 2009 Endangerment Finding and the 2010 Reconsideration Denial were 

challenged in a lawsuit before the U.S. Court of Appeals for the D.C. Circuit.90 On June 26, 

2012, the Court upheld the Endangerment Finding and the Reconsideration Denial, ruling 

that the Finding (including the Reconsideration Denial) was not arbitrary or capricious, was 

consistent with the U.S. Supreme Court's decision in Massachusetts v. EPA (which affirmed

the EPA’s authority to regulate greenhouse gases)91 and the text and structure of the 

CAA, and was adequately supported by the administrative record.92 The Court also agreed 

with the EPA that the Petitioners had “not provided substantial support for their argument 

that the Endangerment Finding should be revised.”93 The Court found that the EPA had 

based its decision on “substantial scientific evidence,” observing that “EPA’s scientific 

evidence of record included support for the proposition that greenhouse gases trap heat on

 

89 U.S. EPA, 2010: Denial of the Petitions to Reconsider the Endangerment and Cause or Contribute Findings 

for Greenhouse Gases Under section 202(a) of the Clean Air Act, 75 FR 49557 (August 13, 2010) 

(“Reconsideration Denial”). In that notice, the EPA throughly considered the scientific and technical 

information relevant to the petitions. In addition to the other information discussed in the present notice, the 

EPA is also relying on the scientific and technical evidence discussed in that prior notice for purposes of its 

proposed determination under CAA section 231. See section III of the Reconsideration Denial. 

90 Coalition for Responsible Regulation, Inc. v. Environmental Protection Agency, 684 F.3d 102 (D.C. Cir. 

2012),), reh'g en banc denied, 2012 U.S. App. LEXIS 25997, 26313, 26315 (D.C. Cir. 2012) (CRR).

91 549 U.S. 497 (2007).

92 CRR, 684 F.3d at 117-27.

93 Id. at 125
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earth that would otherwise dissipate into space; that this ‘greenhouse effect’ warms the 

climate; that human activity is contributing to increased atmospheric levels of greenhouse 

gases; and that the climate system is warming,” as well as providing extensive scientific 

evidence for EPA’s determination that anthropogenically induced climate change threatens 

both public health and welfare.94 The court further noted that the EPA's reliance on 

assessments was consistent with the methods decision-makers often use to make a sciencebased judgment.95 Moreover, the Court supported the EPA's reliance on the major scientific 

assessment reports conducted by USGCRP, IPCC, and NRC and found:

The EPA evaluated the processes used to develop the various 

assessment reports, reviewed their contents, and considered the depth of the 

scientific consensus the reports represented. Based on these evaluations, the 

EPA determined the assessments represented the best source material to use in 

deciding whether GHG emissions may be reasonably anticipated to endanger 

public health or welfare. * * * It makes no difference that much of the 

scientific evidence in large part consisted of “syntheses” of individual studies 

and research. Even individual studies and research papers often synthesize past 

work in an area and then build upon it. This is how science works. The EPA is 

 

94 Id. at 120-121.

95 Id. at 121
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not required to re-prove the existence of the atom every time it approaches a 

scientific question.96

In addition, the EPA’s reliance on the major assessments to inform the 

Administrator's judgment allowed for full and explicit recognition of scientific uncertainty

regarding the endangerment posed by the atmospheric buildup of GHGs. The Administrator 

considered the fact that “some aspects of climate change science and the projected impacts 

are more certain than others.”97 The D.C. Circuit subsequently noted that “the existence of 

some uncertainty does not, without more, warrant invalidation of an endangerment 

finding.”98

As noted above the Supreme Court granted some of the petitions for certiorari that 

were filed, while denying others, but agreed to decide only the question: “Whether EPA 

permissibly determined that its regulation of greenhouse gas emissions from new motor 

vehicles triggered permitting requirements under the Clean Air Act for stationary sources 

that emit greenhouse gases.”99 Thus, the Supreme Court did not disturb the D.C. Circuit’s 

holding that affirmed the 2009 Endangerment Finding. 

 

96 Id. at 120.

97 74 FR at 66524

98 CRR, 684 F.3d at 121.

99 Utility Air Reg. Group v. EPA, 134 S. Ct. 2427, 2438 (2014) (internal marks and citations omitted). See also

Virginia v. EPA, 134 S. Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418 (2013), and CRR, 134 S. Ct. 

468 (2013) (all denying cert.). 
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B. Recent Science Further Supports the Administrator’s Judgment that the Six 

Well-Mixed Greenhouse Gases Endanger Public Health and Welfare

Since the closure of the administrative record concerning the 2009 Endangerment 

Finding (including the denial of petitions for reconsideration), a number of new major, peerreviewed scientific assessments have been released. The EPA carefully reviewed the 

updated scientific conclusions in these assessments, largely to evaluate whether they would 

lead the EPA in this CAA section 231(a)(2)(A) finding to propose a different interpretation 

of, or place more or less weight on, the major findings reflected in the previous assessment 

reports that underpinned the Administrator’s judgment that the six well-mixed GHGs

endanger public health and welfare. From its review, the EPA finds that these new 

assessments are largely consistent with, and in many cases strengthen and add to, the already

compelling and comprehensive scientific evidence detailing the role of the six well-mixed 

GHGs in driving climate change, detailed in the 2009 Endangerment Finding. Therefore, the

new scientific assessments do not provide any reasonable basis on which to propose under 

CAA section 231(a)(2)(A) a different conclusion than the one the EPA reached in 2009 

under CAA section 202(a). Rather, they provide further support for today’s proposed 

finding under section 231. In particular, the new assessments discussed in this preamble 

provide additional detail regarding public health impacts, particularly on groups and people 

at certain lifestages especially vulnerable to climate change including children, the elderly, 

low-income communities and individuals, indigenous groups, and communities of color. 
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The subsections below present brief summaries of the relevant key findings from the

new major peer-reviewed scientific assessments, which include the following: 

 IPCC's 2013-2014 Fifth Assessment Report (AR5)100

 IPCC’s 2012 “Special Report on Managing the Risks of Extreme Events and 

Disasters to Advance Climate Change Adaptation” (SREX)101

 USGCRP's 2014 “Climate Change Impacts in the United States: the Third 

National Climate Assessment” (NCA3)

102

 NRC’s 2010 “Ocean Acidification: A National Strategy to Meet the Challenges 

of a Changing Ocean” (Ocean Acidification)103

 

100 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth 

Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University 

Press, 1535 pp, doi:10.1017/CBO9781107415324; IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and 

Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment 

Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, 

M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. 

Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, 1132 pp; IPCC, 

2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of 

Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, 

V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, 

R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. 

Cambridge University Press, 688 pp; and IPCC, 2014: Climate Change 2014: Mitigation of Climate Change.

Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, 

S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx 

(eds.)]. Cambridge University Press, 1435 pp.

101 IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A 

Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [Field, C.B., V. 

Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, 

M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, 582 pp.

102 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. 
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 NRC’s 2011 “Climate Change, the Indoor Environment, and Health” (Indoor 

Environment)104

 NRC’s 2011 “Report on Climate Stabilization Targets: Emissions, 

Concentrations, and Impacts over Decades to Millennia” (Climate Stabilization 

Targets)105

 NRC’s 2011 “National Security Implications for U.S. Naval Forces” (National 

Security Implications)106

 NRC’s 2011 “Understanding Earth's Deep Past: Lessons for Our Climate Future” 

(Understanding Earth's Deep Past)107

 NRC’s 2012 “Sea Level Rise for the Coasts of California, Oregon, and 

Washington: Past, Present, and Future” (Sea Level Rise)108

 NRC’s 2013 “Climate and Social Stress: Implications for Security Analysis” 

(Climate and Social Stress)109

 NRC’s 2013 “Abrupt Impacts of Climate Change” (Abrupt Impacts)110

 

103 NRC, 2010: Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean. The 

National Academies Press, 188 pp.

104 NRC Institute of Medicine, 2011: Climate Change, the Indoor Environment, and Health. Washington, DC: 

The National Academies Press, 272 pp.

105 NRC 2011: Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to 

Millennia. The National Academies Press, 298 pp.

106 NRC, 2011: National Security Implications of Climate Change for U.S. Naval Forces. The National 

Academies Press, 226 pp.

107 NRC, 2011: Understanding Earth's Deep Past: Lessons for Our Climate Future. The National Academies

Press, 212 pp.

108 NRC, 2012: Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future. 

The National Academies Press, 201 pp.

109 NRC, 2013: Climate and Social Stress: Implications for Security Analysis. The National Academies Press, 

280 pp.
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 NRC’s 2014 “The Arctic in the Anthropocene: Emerging Research Questions” 

(Arctic)111.

1. More Recent Evidence That Elevated Atmospheric Concentrations of the Six 

Greenhouse Gases Are the Root Cause of Observed Climate Change

The EPA has carefully reviewed the recent assessments regarding elevated 

concentrations of the six well-mixed GHGs in the atmosphere. The EPA finds that the new 

assessments of the IPCC, USGCRP, and NRC support and strengthen the science underlying 

the 2009 Endangerment Finding that the six well-mixed GHGs are the root cause of recently

observed climate change. Key findings are described briefly here.

According to the IPCC AR5, observations of the Earth’s globally averaged combined 

land and ocean surface temperature over the period 1880 to 2012 show a warming of 0.85 

[0.65 to 1.06] degrees Celsius or 1.53 [1.17 to 1.91] degrees Fahrenheit.112 The IPCC AR5 

concludes that the global average net effect of the increase in atmospheric GHG

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,

 

110 NRC, 2013: Abrupt Impacts of Climate Change: Anticipating Surprises. The National Academies Press, 250 

pp.

111 NRC, 2014: The Arctic in the Anthropocene: Emerging Research Questions. The National Academies Press, 

220 pp.

112 “IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis.

Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex 

and P.M. Midgley (eds.)]. Cambridge University Press, 29 pp.
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referred to as “forcing,” is estimated to be 2.3 Watts per square meter (W/m2), which has 

increased from the previous 2007 IPCC Fourth Assessment Report (AR4) total net estimate 

of 1.6 Watts per square meter (W/m2) that was referred to in the record for the 2009 

Endangerment Finding. The reasons for this increase include continued increases in GHG 

concentrations, as well as reductions in the estimated negative forcing due to aerosols. The 

IPCC AR5 rates the level of confidence113 in their radiative forcing estimates as “high” for 

methane and “very high” for CO2 and nitrous oxide. 

The new assessments also have greater confidence in attributing recent warming to 

human causes. The IPCC AR5 stated that it is extremely likely (>95 percent likelihood) that 

human influences have been the dominant cause of warming since the mid-20th century, 

which is a stronger statement than the AR4 conclusion that it is very likely (>90 percent 

likelihood) that most of the increase in temperature since the mid-20th century was due to the 

increase in GHG concentrations. The AR4 conclusion was referred to in the record for the 

2009 Endangerment Finding. In addition, the IPCC AR5 found that concentrations of CO2

and several other of the major GHGs are higher than they have been in at least 800,000 years. 

This is an increase from what was reported in IPCC AR4, which found higher concentrations 

than in at least 650,000 years.

 

113 The IPCC expresses levels of confidence using five qualifiers: very low, low, medium, high, and very high. 

These levels are based on a qualitative evaluation of the robustness of the evidence (considering the type, 

amount, quality, and consistency of evidence such as data, mechanistic understanding, theory, models, and 

expert judgment) and the degree of agreement among the findings.
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The USGCRP NCA3 states that there is very high confidence114 that the global 

climate change of the past 50 years is primarily due to human activities. Human activities 

are affecting climate through increasing atmospheric levels of heat-trapping gases, through 

changing levels of various particles that can have either a heating or cooling influence on the 

atmosphere, and through activities such as land use changes that alter the reflectivity of the 

Earth’s surface and cause climatic warming and cooling effects. The USGCRP concludes 

that “considering all known natural and human drivers of climate since 1750, a strong net 

warming from long-lived greenhouse gases produced by human activities dominates the 

recent climate record.”115

These recent and strong conclusions attributing recent observed global warming to 

human influence have been made despite what some have termed a warming slowdown or 

“hiatus” over the past 15 years or so. The IPCC AR5 notes that global mean surface 

temperature exhibits substantial natural decadal and interannual variability, such that trends 

based on short records are very sensitive to the beginning and end dates and do not in general 

reflect long-term climate trends. As an example, the IPCC AR5 notes that the rate of 

warming over the 15 year period from 1998-2012 was less than that over the period 1951-

2012. This short term variability does not alter the long-term climate trend that the IPCC 

 

114 The NCA expresses levels of confidence using four qualifiers: low, medium, high, and very high. These 

levels are based on the strength and consistency of the observed evidence; the skill, range, and consistency of 

model projections; and insights from peer-reviewed sources.

115 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 741 
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AR5 finds after its review of independently verified observational records: “Each of the past 

three decades has been successively warmer at the Earth’s surface than all the previous

decades in the instrumental record, and the first decade of the 21st century has been the 

warmest.”116, 117

The NRC Climate Stabilization Targets assessment concludes that CO2 emissions are 

currently altering the atmosphere's composition and will continue to alter Earth’s climate for 

thousands of years. The NRC Understanding Earth’s Deep Past assessment finds that “the 

magnitude and rate of the present greenhouse gas increase place the climate system in what 

could be one of the most severe increases in radiative forcing of the global climate system in 

Earth history.”118 This assessment finds that if no emissions reductions are made CO2

concentrations by the end of the century are projected to increase to levels that Earth has not 

experienced for more than 30 million years. 

2. More Recent Evidence that Greenhouse Gases Endanger Public Health

The EPA has carefully reviewed the key conclusions in the recent assessments

regarding human-induced climate change risks and impacts on public health. The EPA finds 

 

116 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth 

Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University 

Press, p. 161.

117 Furthermore, we would note that according to both NOAA and NASA, 2014 was the warmest year in the 

modern instrumental record for globally averaged surface temperature, and that the ten warmest years, with the 

exception of 1998, have now occurred since 2000. Available at 

http://www.giss.nasa.gov/research/news/20150116/ (last accessed May 12, 2015). 

118 NRC, 2011: Understanding Earth's Deep Past: Lessons for Our Climate Future. The National Academies

Press, p. 138.
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that the new assessments are consistent with or strengthen the underlying science considered 

in the 2009 Endangerment Finding regarding public health effects from changes in 

temperature, air quality, extreme weather, and climate-sensitive diseases and aeroallergens.

These key findings are described briefly here.

Regarding temperature effects, the conclusions of the assessment literature cited in the 

2009 Endangerment Finding were uncertain with respect to the exact balance of how heatversus cold-related mortality will change in the future, but noted that the available evidence 

suggested that the increased risk from heat would exceed the decreased risk from cold in a

warming climate. The most recent assessments now have greater confidence that increases in 

heat-related mortality will be larger than the decreases in cold-related mortality. The 

USGCRP NCA3 concludes that, “While deaths and injuries related to extreme cold events are 

projected to decline due to climate change, these reductions are not expected to compensate 

for the increase in heat-related deaths.”119 The IPCC AR5 also notes a potential benefit of 

climate change could include “modest reductions in cold-related mortality and morbidity in 

some areas due to fewer cold extremes (low confidence),”120 but that, “[o]verall, we conclude 

 

119 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 224.

120 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral 

Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on 

Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, 

K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and 

L.L. White (eds.)]. Cambridge University Press, p. 713.
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that the increase in heat-related mortality by mid-century will outweigh gains due to fewer 

cold periods.”121

Regarding air quality effects, the assessment literature cited in the 2009 Endangerment 

Finding concluded that climate change is expected to increase regional ozone pollution, with 

associated risks in respiratory illnesses and premature death, but that the directional effect of 

climate change on ambient particulate matter levels was less certain. The USGCRP NCA3 

similarly concludes that, “Climate change is projected to harm human health by increasing

ground-level ozone and/or particulate matter air pollution in some locations. ... There is less

certainty in the responses of airborne particles to climate change than there is about the 

response of ozone.”

122 The IPCC AR5 finds that ozone and particulate matter have been 

associated with adverse health effects in many locations in North America, and that ozone

concentrations could increase under future climate change scenarios if emissions of precursors 

were held constant. For particulate matter, both the USGCRP NCA3 and IPCC AR5 discuss 

increasing wildfire risk under climate change, and explain that wildfire smoke exposure can 

lead to various respiratory and cardiovascular impacts. The NRC Indoor Environment 

assessment identifies potential adverse health risks associated with climate-change induced 

alterations in the indoor environment, including possible exposure to air pollutants like ozone 

via changes in outdoor air quality. Other risks include potential for alterations in indoor 

 

121 Ibid. at p. 721.

122 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 222.
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allergens due to climate change-related increases in outdoor pollen levels, potential chemical 

exposures due to greater use of pesticides to address changes in geographic ranges of pest 

species, and dampness/mold associated symptoms and illness due to potential flooding and 

water damage in buildings from potential climate change-related increases in storm intensity

and extreme precipitation events.

Regarding extreme weather events (e.g., storms, heavy precipitation, and, in some 

regions of the United States, floods and droughts), the conclusions of the assessment literature 

cited in the 2009 Endangerment Finding found potential for increased deaths, injuries, 

infectious and waterborne diseases, and stress-related disorders. Similarly, the USGCRP 

NCA3 discusses elevated waterborne disease outbreaks and the potential for mold 

contamination and degraded indoor air quality following heavy precipitation. Other impacts 

include mortality associated with flooding and impacts on mental health, such as anxiety and 

post-traumatic stress disorder. The IPCC AR5 also discusses death and injury in coastal 

zones and regions vulnerable to inland flooding. The USGCRP NCA3 and the IPCC AR5 

both find that climate change may increase exposure to and health risks associated with 

drought conditions, which includes impacts from wildfires, dust storms, extreme heat events, 

flash flooding, degraded water quality, reduced water quantity, and water-related diseases. 

The IPCC SREX assessment projects further increases in some extreme weather and climate 

events during this century, and specifically notes that changes in extreme weather events have 

implications for disaster risk in the health sector.
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The effects of climate change on climate-sensitive diseases were also cited in the 2009 

Endangerment Finding, including a likely increase in the spread of several food and waterborne pathogens among susceptible populations, and the potential for range expansion of 

some zoonotic disease carriers such as the Lyme disease-carrying tick. The new assessment 

literature similarly focuses on increased exposure risk for some diseases under climate 

change, finding that increasing temperatures may expand or shift the ranges of some disease 

vectors like mosquitoes, ticks, and rodents. The IPCC AR5 notes that climate change may

influence the “growth, survival, persistence, transmission, or virulence of pathogens”

123 that 

cause food and water-borne disease. The USGCRP NCA3 notes that uncertainty remains 

regarding future projections of increased human burden of vector-borne disease, given 

complex interacting factors such as “local, small-scale differences in weather, human 

modification of the landscape, the diversity of animal hosts, and human behavior that affects 

vector-human contact, among other factors.”124

Regarding aeroallergens, the assessment literature cited in the 2009 Endangerment 

Finding found potential for climate change to affect the prevalence and severity of allergy

symptoms, but that definitive data or conclusions were lacking on how climate change might 

 

123 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral 

Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on 

Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, 

K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and 

L.L. White (eds.)]. Cambridge University Press, p. 726.

124 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 225.
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impact aeroallergens in the United States. The most recent assessments now have greater

confidence in the likelihood that climate change would influence production of pollen, which 

in turn could affect asthma and other allergic respiratory symptoms such as allergic rhinitis, as 

well as effects on conjunctivitis and dermatitis. Both the USGCRP NCA3 and the IPCC AR5 

found that increasing temperature has lengthened the allergenic pollen season for ragweed, 

and that increased CO2 by itself can elevate production of plant-based allergens. The IPCC 

AR5 concludes that in North America, “warming will lead to further changes in the seasonal 

timing of pollen release (high confidence).”

125

The assessment literature cited in the 2009 Endangerment Finding concluded that 

certain populations, including children, the elderly, and the poor, are most vulnerable to 

climate-related health effects. The 2009 Endangerment Finding also described climate 

change impacts facing indigenous peoples in the United States, particularly Alaska Natives. 

The new assessment literature strengthens these conclusions by providing more detailed 

findings regarding these populations’ vulnerabilities and projected impacts they may

experience. 

The most recent assessment literature of the USGCRP and IPCC provides new 

information on how low-income populations and some communities of color may be 

 

125 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. 

Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. 

Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. 

White (eds.)]. Cambridge University Press, Cambridge, p. 1465-1466.
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uniquely vulnerable to climate change health impacts. The USGCRP NCA3 finds that, 

“Climate change will, absent other changes, amplify some of the existing health threats the 

nation now faces. Certain people and communities are especially vulnerable, including 

children, the elderly, the sick, the poor, and some communities of color.”

126 According to 

the USGCRP NCA3, certain climate change related impacts—including heat waves,

degraded air quality, and extreme weather events—have disproportionate effects on lowincome populations and some communities of color, raising environmental justice concerns. 

Limited resources make low-income populations more vulnerable to ongoing climate-related 

threats, less able to adapt to anticipated changes, and less able to recover from climate 

change impacts. Low-income populations also face higher prevalence of chronic health 

conditions than higher income groups, which increases their vulnerability to the health 

effects of climate change. Factors that contribute to increased vulnerability of some 

communities of color to the health effects of climate change include existing health 

disparities (e.g., higher prevalence of chronic health conditions) and other inequities related 

to increased exposure to health threats and cumulative stresses. The IPCC AR5 identifies a 

number of contributing factors to the increased vulnerability of black Americans to heatrelated deaths, including higher prevalence of chronic health conditions such as diabetes, 

lower incomes that may restrict access to air conditioning during heat waves, and 

 

126 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 221.
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community-level factors such as local crime rates and disruptions to social networks that 

affect local resilience and ability to recover from impacts.

The USGCRP NCA3 also finds that climate change, in addition to chronic stresses 

such as extreme poverty, is affecting indigenous peoples’ health in the United States through 

impacts such as reduced access to traditional foods, decreased water quality, and increasing 

exposure to health and safety hazards. The IPCC AR5 finds that climate change-induced 

warming in the Arctic and resultant changes in environment (e.g., permafrost thaw, effects 

on traditional food sources) have significant observed and projected impacts on the health 

and well-being of Arctic residents, especially indigenous peoples. Small, remote, 

predominantly-indigenous communities are especially vulnerable given their “strong 

dependence on the environment for food, culture, and way of life; their political and

economic marginalization; existing social, health, and poverty disparities; as well as their 

frequent close proximity to exposed locations along ocean, lake, or river shorelines.”127 In

addition, increasing temperatures and loss of Arctic sea ice increases the risk of drowning for 

those engaged in traditional hunting and fishing.

The USGCRP NCA3 concludes that children will suffer disproportionately from 

climate change given the unique physiological and developmental factors that occur during 

 

127 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. 

Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. 

Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. 

White (eds.)]. Cambridge University Press, Cambridge, p. 1581.
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this lifestage. Impacts on children are expected from heat waves, air pollution, infectious

and waterborne illnesses, and mental health effects resulting from extreme weather events. 

The IPCC AR5 indicates that children are among those especially susceptible to most 

allergic diseases, as well as health effects associated with heat waves, storms, and floods. 

The IPCC finds that additional health concerns may arise in low income households, 

especially those with children, if climate change reduces food availability and increases 

prices, leading to food insecurity within households.

Both the USGCRP and IPCC conclude that climate change will increase health risks 

facing the elderly. Older people are at much higher risk of mortality during extreme heat 

events. Pre-existing health conditions also make older adults susceptible to cardiac and

respiratory impacts of air pollution and to more severe consequences from infectious and 

waterborne diseases. Limited mobility among older adults can also increase health risks

associated with extreme weather and floods.

3. More Recent Evidence that Greenhouse Gases Endanger Public Welfare

The EPA has carefully reviewed the recent scientific conclusions in the assessments

regarding human-induced climate change impacts on public welfare.128 The EPA finds that 

 

128 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 language is quite broad. Importantly, it is not an exclusive list due to the 
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they are largely consistent with or strengthen the underlying science supporting the 2009 

Endangerment Finding regarding public welfare effects on food production and agriculture; 

forestry; water resources; sea level rise and coastal areas; energy, infrastructure, and

settlements; ecosystems and wildlife; and impacts on the U.S. population from climate change 

effects occurring outside of the United States. These key findings are described briefly here. 

Regarding agriculture, the assessment literature cited in the 2009 Endangerment 

Finding found potential for increased CO2 levels to benefit yields of certain crops in the shortterm, but with considerable uncertainty. The body of evidence pointed towards increasing 

risk of net adverse impacts on U.S. food production and agriculture over time, with the 

potential for significant disruptions and crop failure in the future. The most recent 

assessments now have greater confidence that climate change will negatively affect U.S. 

agriculture over this century. Specifically, the USGCRP NCA3 concludes, “While some U.S. 

regions and some types of agricultural production will be relatively resilient to climate change 

over the next 25 years or so, others will increasingly suffer from stresses due to extreme heat, 

drought, disease, and heavy downpours. From mid-century on, climate change is projected to 

have more negative impacts on crops and livestock across the country.”

129 The IPCC AR5 

concludes, “Overall yields of major crops in North America are projected to decline modestly

by mid-century and more steeply by 2100 among studies that do not consider adaptation (very

 

use of the term ‘‘includes, but is not limited to, * * *.’’ Effects other than those listed here may also be 

considered effects on welfare.

129 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 16.
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high confidence).”

130 The IPCC AR5 notes that in the absence of extreme events, climate 

change may benefit certain regions and crops, but that in North America significant harvest 

losses have been observed due to recent extreme weather events. In addition, the IPCC SREX 

assessment specifically notes that projected changes in extreme weather events will increase

disaster risk in the agriculture sector.

Regarding forestry, the assessment literature cited in the 2009 Endangerment Finding 

found that near term benefits to forest growth and productivity in certain parts of the country

from elevated CO2 concentrations and temperature increases to date are offset by longer term 

risks from wildfires and the spread of destructive pests and disease that present serious 

adverse risks for forest productivity. The most recent assessments provide further support for 

this conclusion. Both the USGCRP NCA3 and the IPCC AR5 conclude that climate change is 

increasing risks to forest health from fire, tree disease and insect infestations, and drought. 

The IPCC AR5 also notes risks to forested ecosystems associated with changes in

temperature, precipitation amount, and CO2 concentrations, which can affect species and 

ecological communities, leading to ecosystem disruption, reorganization, movement or loss.

The NRC Arctic assessment states that climate change is likely to have a large negative 

impact on forested ecosystems in the high northern latitudes due to the effects of permafrost

 

130 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. 

Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. 

Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. 

White (eds.)]. Cambridge University Press, Cambridge, p. 1462.
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thaw and greater wildfire frequency, extent, and severity. The NRC Climate Stabilization 

Targets assessment found that for an increase in global average temperature of 1 to 2°C above 

pre-industrial levels, the area burnt by wildfires in western North America will likely more 

than double.

Regarding water resources, the assessment literature cited in the 2009 Endangerment 

Finding concluded that increasing temperatures and increased variability in precipitation 

associated with climate change will impact water quality and quantity through changes in 

snowpack, increased risk of floods, drought, and other concerns such as water pollution.

Similarly, the new assessments further support projections of water resource impacts 

associated with increased floods and short-term drought in most U.S. regions. The USGCRP 

NCA3 also finds that, “[c]limate change is expected to affect water demand, groundwater 

withdrawals, and aquifer recharge, reducing groundwater availability in some areas.”

131 The 

IPCC AR5 finds that in part of the western United States, “water supplies are projected to be

further stressed by climate change, resulting in less water availability and increased drought 

conditions.”132 The IPCC AR5 also projects that climate change will degrade surface water 

 

131 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 70.

132 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. 

Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate 

Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. 

Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. 

White (eds.)]. Cambridge University Press, Cambridge, p. 1456-1457.
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quality, including the Great Lakes, and will negatively affect drinking water 

treatment/distribution and sewage collection systems.

The assessment literature cited in the 2009 Endangerment Finding found that the 

most serious potential adverse effects to coastal areas are the increased risk of storm surge 

and flooding in coastal areas from sea level rise and more intense storms. Coastal areas also 

face other adverse impacts from sea level rise such as land loss due to inundation, erosion, 

wetland submergence, and habitat loss. The most recent assessments provide further 

evidence in line with the science supporting the 2009 Endangerment Finding. The USGCRP 

NCA3 finds that, “Sea level rise, combined with coastal storms, has increased the risk of 

erosion, storm surge damage, and flooding for coastal communities, especially along the 

Gulf Coast, the Atlantic seaboard, and in Alaska.”

133

The IPCC AR5, the USGCRP NCA3, and three of the new NRC assessments provide 

estimates of projected global sea level rise. These estimates, while not always directly 

comparable as they assume different emissions scenarios and baselines, are at least 40 percent 

larger than, and in some cases more than twice as large as, the projected rise estimated in the

IPCC AR4 assessment, which was referred to in the 2009 Endangerment Finding.134 The 

NRC Sea Level Rise assessment projects a global sea level rise of 0.5 to 1.4 meters by 2100, 

 

133 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 9.

134 The 2007 IPCC AR4 assessment cited in 2009 Endangerment Finding estimated a projected sea level rise of 

between 0.18 and 0.59 meters by the end of the century, relative to 1990. It should be noted that in 2007, the

IPCC stated that including poorly understood ice sheet processes could lead to an increase in the projections.
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which is sufficient to lead to a relative rise in sea level even around the northern coasts of

Washington State, where the land is still rebounding from the disappearance of the great ice 

sheets. The NRC National Security Implications assessment suggests that “the Department of 

the Navy should expect roughly 0.4 to 2 meters global average sea-level rise by

2100.”

135 The NRC Climate Stabilization Targets assessment states that an increase of 3°C 

will lead to a sea level rise of 0.5 to 1 meter by 2100. While these NRC and IPCC 

assessments continue to recognize and characterize the uncertainty inherent in accounting for 

ice sheet processes, these revised estimates are consistent with the assessments underlying the 

2009 Endangerment Finding.

Regarding climate impacts on energy, infrastructure, and settlements, the 2009 

Endangerment Finding cited the assessment literature’s findings that temperature increases 

will change heating and cooling demand; that declining water quantity may adversely impact 

the availability of cooling water and hydropower in the energy sector; and that changes in 

extreme weather events will threaten energy, transportation, water, and other key societal 

infrastructure, particularly on the coast. The most recent assessments provide further 

evidence in line with the science supporting the 2009 Endangerment Finding. For example, 

the USGCRP NCA3 finds that, “Coastal infrastructure, including roads, rail lines, energy

infrastructure, airports, port facilities, and military bases, are increasingly at risk from sea

 

135 NRC, 2011: National Security Implications of Climate Change for U.S. Naval Forces. The National 

Academies Press, p. 28.
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level rise and damaging storm surges.”

136 The NRC Arctic assessment identifies threats to 

human infrastructure in the Arctic from increased flooding, erosion, and shoreline ice pileup, or ivu, associated with summer sea ice loss and the increasing frequency and severity of 

storms. 

Regarding ecosystems and wildlife, the assessment literature cited in the 2009 

Endangerment Finding found that climate change will predominantly adversely impact both 

terrestrial and marine biodiversity and the ability of these ecosystems to provide goods and 

services. The NRC Arctic assessment states that major marine and terrestrial biomes will 

likely shift pole ward, with significant implications for changing species composition, food 

web structures, and ecosystem function. The NRC Climate Stabilization Targets assessment 

found that coral bleaching will increase due both to warming and ocean acidification. The 

NRC Understanding Earth's Deep Past assessment notes four of the five major coral reef 

crises of the past 500 million years were caused by acidification and warming that followed

GHG increases of similar magnitude to the emissions increases expected over the next 

hundred years. Similarly, the NRC Ocean Acidification assessment finds that “[t]he 

chemistry of the ocean is changing at an unprecedented rate and magnitude due to 

anthropogenic CO2 emissions; the rate of change exceeds any known to have occurred for at 

 

136 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 9.
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least the past hundreds of thousands of years.”137 The assessment notes that the full range of 

consequences is still unknown, but the risks “threaten coral reefs, fisheries, protected species, 

and other natural resources of value to society.”

138 The IPCC AR5 also projects biodiversity

losses in marine ecosystems, especially in the Arctic and tropics.

In general, climate change impacts related to public welfare are expected to be 

unevenly distributed across different regions of the United States and have a greater impact 

on certain populations, such as indigenous peoples and the poor. The USGCRP NCA3 finds 

climate change impacts such as the rapid pace of temperature rise, coastal erosion and 

inundation related to sea level rise and storms, ice and snow melt, and permafrost thaw are 

affecting indigenous people in the United States. Particularly in Alaska, critical 

infrastructure and traditional livelihoods are threatened by climate change and, “[i]n parts of 

Alaska, Louisiana, the Pacific Islands, and other coastal locations, climate change impacts 

(through erosion and inundation) are so severe that some communities are already relocating 

from historical homelands to which their traditions and cultural identities are tied.”

139 The 

IPCC AR5 notes, “Climate-related hazards exacerbate other stressors, often with negative 

outcomes for livelihoods, especially for people living in poverty (high confidence). Climaterelated hazards affect poor people’s lives directly through impacts on livelihoods, reductions 

 

137 NRC, 2010: Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean. The 

National Academies Press, p. 5.

138 Ibid.

139 Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the 

United States: The Third National Climate Assessment. U.S. Global Change Research Program, p. 17.
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in crop yields, or destruction of homes and indirectly through, for example, increased food 

prices and food insecurity.”

140

In the 2009 Endangerment Finding, the Administrator considered impacts on the U.S.

population from climate change effects occurring outside of the United States, such as 

national security concerns that may arise as a result of climate change impacts in other regions 

of the world. The most recent assessments provide further evidence in line with the science 

supporting the 2009 Endangerment Finding. The NRC Climate and Social Stress assessment 

found that it would be “prudent for security analysts to expect climate surprises in the coming 

decade . . . and for them to become progressively more serious and more frequent 

thereafter.”

141 The NRC National Security Implications assessment recommends preparing 

for increased needs for humanitarian aid; responding to the effects of climate change in 

geopolitical hotspots, including possible mass migrations; and addressing changing security 

needs in the Arctic as sea ice retreats. 

In addition, the NRC Abrupt Impacts report examines the potential for tipping points, 

thresholds beyond which major and rapid changes occur in the Earth’s climate system, as 

well as in natural and human systems that are impacted by the changing climate. The Abrupt 

 

140 IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral 

Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on 

Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, 

K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and 

L.L. White (eds.)]. Cambridge University Press, p. 796.

141 NRC, 2013: Climate and Social Stress: Implications for Security Analysis. The National Academies Press, p. 

18.
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Impacts report did find less cause for concern than some previous assessments regarding 

some abrupt events within the next century such as disruption of the oceanic Atlantic 

Meridional Overturning Circulation (AMOC) and sudden releases of high-latitude methane 

from hydrates and permafrost. But, the same report found that the potential for abrupt 

changes in ecosystems, weather and climate extremes, and groundwater supplies critical for 

agriculture now seem more likely, severe, and imminent. The assessment found that some 

abrupt changes were already underway (e.g., Arctic sea ice retreat and increases in extinction 

risk due to the speed of climate change), and cautioned that even abrupt changes such as the 

AMOC disruption that are not expected in this century can have severe impacts if/when they

happen.

4. Consideration of Other Climate Forcers 

Both in the 2009 Endangerment Finding and in this action, the Administrator 

recognizes that there are other substances in addition to the six well-mixed GHGs that are 

emitted from human activities and affect Earth’s climate (referred to as climate forcers). 

These can be grouped into two categories: (1) other substances with similar physical 

properties to the six well-mixed GHGs—these include the ozone-depleting substances of 

chlorofluorocarbons, hydrochlorofluorocarbons, and halons, as well as nitrogen trifluoride 

and similar recently identified substances; and (2) short-lived substances—tropospheric ozone 

and its precursor gases, water vapor, and aerosol particles and precursors. For some shortlived substances — namely, water vapor; NOx; and aerosol particles including black carbon— 

their physical properties result in these substances having different, and often larger, climate 
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effects when emitted at high altitudes. However, the very properties that lead to differential 

climate effects depending on the altitude of emission—properties that are different from those 

of the six well-mixed, long-lived GHGs—lead to more uncertainty in the scientific 

understanding of these short-lived substances’ total effect on Earth’s climate. More detail is 

provided below.

As described in section III.B of the 2009 Endangerment Finding and in section IV.A.1 

of this preamble, the primary reasons for defining the air pollution as the aggregate group of 

the six well-mixed GHGs include their common physical properties relevant to climate 

change (i.e., long-lived, well-mixed, directly emitted), the fact that these gases are considered 

the primary drivers of climate change, and the fact that these gases remain the best understood 

drivers of anthropogenic climate change. The common physical properties of the six wellmixed GHGs not only support grouping them together as a class, but also contribute to their 

higher degree of scientific understanding related to climate change, relative to short-lived 

substances that are not well-mixed, or substances that are formed indirectly rather than being 

directly emitted. After considering additional information in the new assessments regarding

the climate-relevant substances outside the basket of the six well-mixed GHGs, it is the 

Administrator’s view that the reasons originally stated for not including these substances in 

the scope of the GHG air pollution still apply at this time. For example, nitrogen triflouride 

and some other recently discovered substances are not as well studied or understood as the six 

well-mixed GHGs. Similarly, for tropospheric ozone—a short-lived gas in the atmosphere 

that is not directly emitted (it forms from emissions of various precursor gases)—the 
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understanding and quantification of the link between precursor emissions and climate change 

is not as strong as for the six well-mixed GHGs. 

Regarding the short-lived substances with different climate effects when emitted at 

high altitudes, the Aircraft Petition (see section II of this preamble) mentions the effects of

water vapor and NOx on clouds and atmospheric chemistry. The major peer-reviewed 

scientific assessments of the IPCC and NRC provide the current state of scientific 

understanding of these effects; the USGCRP assessments have not dealt specifically with 

emissions at high altitude. The EPA considered the following assessment reports to obtain the 

best estimates of these substances’ net impact on the climate system, which is generally 

discussed in terms of radiative forcing: the IPCC AR5, the IPCC 2007 Fourth Assessment 

Report (AR4),142 the IPCC Special Report: Aviation and the Global Atmosphere (IPCC 

1999),143 the NRC’s Advancing the Science of Climate Change (NRC 2010),144 and the 

NRC’s Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project 

(NRC 1999).145 In addition to high altitude water vapor and NOx, the literature indicates that 

aerosol particles, including black carbon, emitted at high altitudes have more interactions with 

 

142 IPCC, 2007: Climate Change 2007: The Physical Scientific Basis. Contribution of Working Group I to the 

Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. 

Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)] Cambridge University Press, 

996 pp.

143 IPCC, 1999: Aviation and the Global Atmosphere, Special Report to the Intergovernmental Panel on Climate 

Change [Penner, J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)] Cambridge University Press,

373 pp.

144 NRC, 2010: Advancing the Science of Climate Change. The National Academies Press, 528 pp.

145 NRC, 1999: Atmospheric Effects of Aviation: A Review of NASA’s Subsonic Assessment Project. The National 

Academies Press, 54 pp.
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clouds and therefore have different effects on the global energy balance than do particles 

emitted at the surface.

The state of the science as represented in the assessment literature highlights

significant scientific uncertainties regarding the total net forcing effect of water vapor, NOx, 

and aerosol particles when emitted at high altitudes. Given these uncertainties, the Agency is 

not including them in the proposed definition of air pollution for purposes of the 

endangerment finding under section 231 of the CAA. The short-lived nature of these 

substances means that, unlike the long-lived GHGs, the climatic impact of the substance is 

dependent on a number of factors such as the location and time of its emission. The 

magnitude, and often the direction (positive/warming or negative/cooling), of the globally

averaged climate impact will differ depending on the location of the emission due to the local 

atmospheric conditions (e.g., due to differing concentrations of other compounds with which 

the emissions can react, background humidity levels, or the presence or absence of clouds).

In addition, for emissions at any given location, the spatial and temporal pattern of the climate 

forcing will be heterogeneous, again often differing in direction (for example, in the case of 

NOx emissions, the near term effect in the hemisphere in which the emissions occur is usually 

warming due to increased ozone concentrations, but the longer term effects, and effects in the 

other hemisphere, are often cooling due to increased destruction of methane). As the climatic 

effects of these substances when emitted at high altitudes were not addressed at length in the 

2009 Endangerment Finding, the following subsections briefly summarize the findings of the 
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major scientific assessments regarding these substances’ climatic effects at altitude and the 

various sources of uncertainty surrounding these estimates. 

a. Changes in Clouds from High Altitude Emissions of Water Vapor and Particles

Aviation-induced cloudiness (sometimes called AIC) refers to all changes in 

cloudiness associated with aviation operations, which are primarily due to the effects of high 

altitude emissions of water vapor and particles (primarily sulfates and black carbon). 

Changes in cloudiness affect the climate by both reflecting solar radiation (cooling) and 

trapping outgoing longwave radiation (warming). Unlike the warming effects associated with 

the six long-lived, well-mixed GHGs, the warming effects associated with changes in cloud 

cover are more regional and temporal in nature. The three key components of aviationinduced cloudiness are persistent contrails, contrail-induced cirrus, and induced cirrus.

Aircraft engine emissions of water vapor at high altitudes during flight can lead to the 

formation of condensation trails, or contrails, under certain conditions such as icesupersaturated air masses with specific humidity levels and temperature. The NRC estimates 

that persistent contrails increased cloudiness above the United States by two percent between 

1950 and 1988, with similar results reported over Europe.146 As stated above, clouds can 

have both warming and cooling effects, and persistent contrails were once considered to have 

significant net warming effects. However, more recent estimates suggest a smaller overall 

 

146 NRC, 1999: Atmospheric Effects of Aviation: A Review of NASA’s Subsonic Assessment Project. The National 

Academies Press, 54 pp.
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climate forcing effect of persistent contrails. The IPCC AR5 best estimate for the global 

mean radiative forcing from contrails is 0.01 W/m2(medium confidence and with an 

uncertainty range of 0.005 to 0.03 W/m2).147 To put this number into context, some examples 

of other IPCC AR5 best estimates for global mean radiative forcing include: 1.68 W/m2for 

CO2 (very high confidence and with an uncertainty range of 1.33 to 2.03 W/m2), 0.97 W/m2

for methane (high confidence and with an uncertainty range of 0.74 to 1.20 W/m2), and 0.17

W/m2for nitrous oxide (very high confidence and with an uncertainty range of .013 to 0.21 

W/m2).148 In addition, the NRC (2010) assessment suggested that contrails may affect 

regional diurnal temperature differences, but this has been called into question by the recent 

findings presented in the IPCC AR5, which suggests that aviation contrails do not have an 

effect on mean or diurnal range of surface temperatures (medium confidence). 

Persistent contrails also sometimes lose their linear form and develop into cirrus 

clouds, an effect referred to as contrail-induced cirrus. Studies to date have been unable to 

isolate this climate forcing effect, but the IPCC AR5 provides a combined contrail and 

 

147 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth 

Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University 

Press, 1535 pp. 

148 IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution 

of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, 

T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley 

(eds.)]. Cambridge University Press, 29 pp
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contrail-induced cirrus best estimate of 0.05 W/m2(low confidence and with an uncertainty 

range of 0.02 and 0.15 W/m2).

149

 

Particles emitted or formed in the atmosphere as a result of aircraft emissions may also 

act as ice nuclei and modify naturally forming cirrus clouds, an effect referred to as “induced 

cirrus.” The two primary aviation-induced particles are sulfates and black carbon, and their 

effects on cirrus cloud modification is an area of active research. There are significant 

challenges in estimating the climatic impacts of induced cirrus; for example, the 2007 IPCC 

AR4 characterizes our knowledge of the natural freezing modes in cirrus conditions as 

“poor,” and notes that cirrus cloud processes are not well represented in global models.150

Neither IPCC AR4 nor AR5 provided global or regional estimates related to this forcing. 

Given differences in scientific understanding of the three components of aviationinduced cloudiness, the more recent assessments have not provided estimates of the net 

climate forcing effect of changes in clouds from high altitude emissions of water vapor and 

 

149 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth 

Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University 

Press, 1535 pp. 

150 IPCC, 2007: Climate Change 2007: The Physical Scientific Basis. Contribution of Working Group I to the 

Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. 

Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)] Cambridge University Press, 

996 pp.
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particles. Going back to the 1999 IPCC assessment, the science is characterized as “very

uncertain” with a range for the best estimate between 0 to 0.040 W/m2.

151

b. Direct Radiative Forcing Effects of High Altitude Particle Emissions 

The 2009 Endangerment Finding noted that much of the uncertainty range surrounding 

the estimate of total net forcing due to all human activities was due to uncertainties about the 

cooling and warming effects of aerosols152 (though from all sources, not just aircraft). The 

Finding noted that the magnitude of aerosol effects can vary immensely with location and 

season of emissions, and also discussed black carbon as a specific type of aerosol particle, 

noting that estimates of its total climate forcing effect have a large uncertainty range.153 Here, 

we discuss the direct radiative forcing effects of high altitude emissions of the two primary

aviation-induced particles, sulfates and black carbon.

Aircraft emit precursor gases that convert to sulfate particles in the atmosphere, such 

as sulfur dioxide. Sulfate particles have direct effects on the climate by scattering solar 

radiation, which results in cooling. The more recent assessments have not quantified this 

effect from aviation. Going back to the 1999 IPCC assessment, the direct effect of sulfate 

 

151 IPCC, 1999: Aviation and the Global Atmosphere, Special Report to the Intergovernmental Panel on 

Climate Change [Penner, J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)] Cambridge 

University Press, 373 pp.

152 74 FR 66517

153 74 FR 66520
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aerosols from aviation for the year 1992 is estimated at -0.003 W/m2 with an uncertainty 

range between -0.001 and -0.009 W/m2.

154

 

Black carbon emissions from aviation, which are produced by the incomplete 

combustion of jet fuel, primarily absorb solar radiation and heat the surrounding air, resulting 

in a warming effect. The more recent assessments have not quantified this effect from 

aviation. The 1999 IPCC assessment estimates the global mean radiative forcing of black 

carbon emissions to be 0.003 W/m2 with uncertainty spanning 0.001 to 0.009 W/m2.

155

 The 

IPCC 1999 assessment suggests that because the contribution of black carbon in the 

stratosphere (which actually contribute to cooling of the surface rather than warming) was not 

included in its calculations, its estimates of radiative forcing were likely to be too high. 

c. Changes in Atmospheric Chemistry from High Altitude Nitrogen Oxides 

Emissions

Emissions of NOx do not themselves have warming or cooling effects, but affect the 

climate through catalyzing changes in the chemical equilibrium of the atmosphere. High 

altitude emissions of NOx increase the concentration of ozone, which has a warming effect in 

the short term. Elevated NOx concentrations also lead to an increased rate of destruction of 

methane, which has a cooling effect in the long-term. The reduced methane concentrations 

 

154 IPCC, 1999: Aviation and the Global Atmosphere, Special Report to the Intergovernmental Panel on 

Climate Change [Penner, J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)] Cambridge 

University Press, 373 pp.

155 Ibid.
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eventually contribute to decreases in ozone, which also decreases the long-term net warming 

effect. Thus, the net radiative impact of NOx emissions depends on the balance between the 

reductions in methane versus the production of ozone, which in turn depends on the time scale 

under consideration. Quantifying these impacts is an area of active study with large 

uncertainties. The quantification of the net global effect of NOx is difficult because the 

atmospheric chemistry effects are heavily dependent on highly localized atmospheric 

properties and mixing ratios. Because the background atmospheric concentration of NOx is

important for quantifying the impact of aviation NOx emissions on ozone and methane 

concentrations, the location of aircraft emissions would be an important additional factor. In 

addition, NOx has different residence times in the atmosphere depending on the altitude at 

which it is emitted. The residence time of NOx in the upper troposphere, or roughly the cruise 

altitude for jet aircraft, is on the order of several days. Going back to the IPCC 1999

assessment, the globally averaged radiative forcing estimates for aircraft emissions of NOx in 

1992 were 0.023 W/m2 for O3-induced changes (uncertainty range of 0.011 to 0.046 W/m2), 

and -0.014 W/m2for methane-induced changes (uncertainty range of -0.005 to -0.042 

W/m2).

156

The IPCC AR5 presents the impact of aviation NOx emissions using a different metric, 

global warming potential (GWP), which is a measure of the warming impact of a pulse of 

 

156 IPCC, 1999: Aviation and the Global Atmosphere, Special Report to the Intergovernmental Panel on 

Climate Change [Penner, J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)] Cambridge 

University Press, 373 pp.
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emissions of a given substance over 100 years relative to the same mass of CO2. The AR5 

presents a range from -21 to +75 for GWP of aviation NOx.

157 The uncertainty in sign 

indicates uncertainty whether the net effect is one of warming or cooling. This report further 

suggests that at cruise altitude there is strong regional sensitivity of ozone and methane to 

NOx, particularly notable at low latitudes. 

The Administrator notes that NOx emissions are already regulated under the EPA’s 

rules implementing CAA section 231, at 40 CFR part 87. The prerequisite endangerment and 

cause or contribute findings that formed the basis for these standards, however, did not rely 

upon any conclusions regarding the climate forcing impacts of NOx, but rather the role of NOx

emissions as a precursor to ozone formation in areas that did not meet the National Ambient 

Air Quality Standard (NAAQS) for ozone.

158 The continuing significant uncertainties 

regarding NOx as a climate forcer do not undermine the Agency’s prior conclusion under 

CAA section 231 that emissions of NOx from aircraft engines cause or contribute to air 

pollution which may reasonably be anticipated to endanger public health or welfare due to 

their contribution to ozone concentrations that exceed the NAAQS. 

 

157 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth 

Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University 

Press, 1535 pp. 

158 U.S. EPA, “Control of Air Pollution from Aircraft and Aircraft Engines, Emission Standards and Test 

Procedures for Aircraft.” Final Rule, 38 FR 19088, July 17, 1973.
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d. Summary

Overall, the state of the science as represented in the assessment literature highlights

significant scientific uncertainties regarding the total net forcing effect of water vapor, NOx, 

and aerosol particles, when emitted at high altitudes. The dependence of the effects on where 

the substance is emitted, and the complex temporal and spatial patterns that result, mean that 

the current level of understanding regarding these short-lived substances is much lower than 

for the six long-lived, well-mixed GHGs. Given the aforementioned scientific uncertainties at 

present, the Agency is not including these constituents in the proposed definition of air 

pollution for purposes of the endangerment finding under section 231 of the CAA.

C. Summary of the Administrator’s Proposed Endangerment Finding under CAA 

Section 231

In sum, the Administrator proposes to find, for purposes of CAA section 231(a)(2)(A), 

that elevated atmospheric concentrations of the six well-mixed GHGs constitute air pollution

that endangers both the public health and the public welfare of current and future 

generations. In this proposed action under CAA section 231(a)(2)(A), the EPA relies 

primarily on the extensive scientific and technical evidence in the record supporting the 2009 

Endangerment Finding, including the major, peer-reviewed scientific assessments used to 

address the question of whether GHGs in the atmosphere endanger public health and welfare, 

and on the analytical framework and conclusions upon which the EPA relied in making that 

finding. This proposed finding under section 231 accounts for the EPA’s careful 
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consideration not only of the scientific and technical record for the 2009 Endangerment 

Finding, but also of new, major scientific assessments issued since closing the administrative 

record for the 2009 Endangerment Finding. No recent information or analyses published 

since late 2009 suggest that it would be reasonable for the EPA to now reach a different or 

contrary conclusion for purposes of CAA section 231(a)(2)(A) than the Agency reached for 

purposes of section 202(a). In proposing this finding for purposes of section 231, we are not 

reopening or revisiting our 2009 Endangerment Finding. To the contrary, in light of the 

recent judicial decisions upholding those findings, the EPA believes the 2009 Endangerment 

Finding is firmly established and well settled.159 Moreover, there is no need for the EPA to 

reopen or revisit that finding for purposes of making an additional finding under section 231

of the CAA. Therefore, public comments addressing this finding for purposes of section 

231(a)(2)(A) should be limited to the section 231 context; the EPA will not consider or

respond to comments on this proposal that seek a reevaluation of our 2009 Endangerment 

Finding for purposes of section 202(a).

V. The Proposed Cause or Contribute Finding for Greenhouse Gases under

CAA Section 231

As noted above, the Administrator has proposed to define the air pollution for 

purposes of the endangerment finding under CAA section 231 to be the aggregate of six well-

 

159 CRR, 684 F.3d at 117 (D.C. Cir. 2012), reh'g en banc denied, 2012 U.S. App. LEXIS 25997, 26313, 26315 

(D.C. Cir. 2012); see also Utility Air Reg. Group v. EPA, 134 S. Ct. at 2438 (2014). 
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mixed GHGs in the atmosphere. The second step of the two-part endangerment test for this 

proposed finding is for the Administrator to determine whether the emission of any air 

pollutant from certain classes of aircraft engines causes or contributes to this air pollution. 

This is referred to as the cause or contribute finding, and is the second proposed finding by 

the Administrator in this action. 

Section V.A of this proposal describes the Administrator’s reasoning for using the 

same definition and scope of the GHG air pollutant that was used in the 2009 Endangerment 

Finding. Section V.B puts forth the Administrator’s proposed finding that emissions of wellmixed GHGs from classes of aircraft engines used in covered aircraft contribute to the air 

pollution which endangers public health and welfare.

A. The Air Pollutant

1. Proposed Definition of Air Pollutant

Under section 231, the Administrator is to determine whether emissions of any air 

pollutant from any class or classes of aircraft engines cause or contribute to air pollution 

which may reasonably be anticipated to endanger public health or welfare. As with the 2009 

Endangerment Finding that the EPA conducted for purposes of CAA section 202(a), when 

making a cause or contribute finding under section 231(a)(2), the Administrator must first 

define the air pollutant being evaluated. The Administrator has reasonably and logically

considered the relationship between the GHG air pollution and air pollutant: while the air 

pollution is the concentration (e.g., stock) of the well-mixed GHGs in the atmosphere, the air 
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pollutant is the same combined grouping of the well-mixed GHGs, the emissions of which are 

analyzed for contribution (e.g., the flow into the stock). See 74 FR at 66537 (similar 

discussion with respect to the finding for section 202). Thus, for purposes of section 231, the 

Administrator is proposing to use the same definition of the air pollutant that was used in the 

2009 Endangerment Finding, namely, the aggregate group of the same six GHGs: CO2, 

methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. See 

74 FR at 66536-66537 (discussing the definition of the GHG air pollutant with respect to the 

finding for section 202). That is, as for the 2009 Endangerment Finding, the Administrator is 

proposing to define a single air pollutant made up of these six GHGs. 

To reiterate what the Agency has previously stated on this subject, this collective 

approach for the contribution test is consistent with the treatment of GHGs by those studying 

climate change science and policy, where it is common practice to evaluate GHGs on a 

collective, CO2-equivalent basis.160 This collective approach to defining the air pollutant is 

not unique; grouping of many substances with common attributes as a single pollutant is 

common practice under the CAA, for example with particulate matter and volatile organic 

 

160 As detailed in the 2009 Endangerment Finding proposal (74 FR 18904 (April 24, 2009) and continuing today, 

the UNFCCC, the U.S. and other Parties report their annual emissions of the six GHGs in CO2-equivalent units. 

This facilitates comparisons of the multiple GHGs from different sources and from different countries, and 

provides a measure of the collective warming potential of multiple GHGs. Emissions of different GHGs are 

compared using GWPs, which as described in section IV.B of this preamble are measures of the warming impact

of a pulse of emissions of a given substance over 100 years relative to the same mass of CO2. Therefore, GWPweighted emissions are measured in teragrams of CO2 equivalent (Tg CO2eq). The EPA’s Greenhouse Gas 

Reporting Program (http://www.epa.gov/ghgreporting/index.html, (last accessed May 12, 2015)) also reports 

GHG emissions on a CO2-equivalent basis, recognizing the common and collective treatment of the six GHGs.
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compounds (VOC). As noted in section IV, these substances share common attributes that 

support their grouping as the air pollution for purposes of the endangerment finding. These 

same common attributes also support the Administrator grouping the six GHGs for purposes 

of defining the air pollutant for the proposed cause or contribute finding under CAA section 

231. 

The Administrator recognizes that in this case, the aircraft engines covered by this 

notice emit two of the six gases, but not the other four gases. Nonetheless, it is entirely

appropriate, and in keeping with the 2009 Endangerment Finding and past EPA practice, for 

the Administrator to define the air pollutant in a manner that recognizes the shared relevant 

properties of all these six gases, even though they are not all emitted from the classes of 

sources before her.161 For example, a source may emit only 20 of the possible 200-plus 

chemicals that meet the definition of VOC in the EPA’s regulations, but that source is 

evaluated based on its emissions of VOC and not on its emissions of the 20 chemicals by

name. The fact that these six substances within the definition of GHGs share common, 

relevant attributes is true regardless of the type of sources being evaluated for contribution. 

By proposing to use the definition of the air pollutant as comprised of the six GHGs with 

common attributes, the Administrator is taking account of these shared attributes and how 

they are relevant to the air pollution that endangers public health and welfare.

 

161

 In the 2009 Endangerment Finding, the Administrator found that four of the six gases that were included in 

the definition of the air pollutant were emitted by section 202 sources. 74 FR 66496, 66537 (December 15, 

2009).
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2. How the Definition of Air Pollutant in the Endangerment Determination 

Affects Section 231 Standards.

Under section 231(a), the Administrator is required to set “emission standards 

applicable to the emission of any air pollutant” from classes of aircraft engines that the 

Administrator determines causes or contributes to air pollution that endangers public health or 

welfare. If the Administrator makes a final determination under section 231 that the 

emissions of the GHG air pollutant from certain classes of aircraft engines contribute to the 

air pollution that may reasonably be anticipated to endanger public health and welfare, then 

she is called on to set standards applicable to the emissions of this air pollutant. The term 

“standards applicable to the emissions 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 231 to set standards that either control the emissions of the group of six 

well-mixed gases as a whole and/or control emissions of individual 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 GHGs, weighted by CO2 equivalent. Depending on 

the circumstances, however, it may be appropriate to set standards for certain individual 

gases, or some combination of group and individual standards. These and other similar 

approaches could appropriately be considered in setting a standard or standards applicable to 

the emissions of the group of GHGs 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 aircraft 
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emissions, such as rate and variability, the kind and availability of control technology, and 

other matters relevant to setting standards under section 231.162

B. Proposed Cause or Contribute Finding 

1. The Administrator’s Approach in Making this Proposed Finding

As it did for the 2009 Endangerment Finding, and consistent with prior practice and 

current science, the EPA uses annual emissions as a reasonable proxy for contributions to the 

air pollution, i.e., elevated atmospheric concentrations of GHGs. Cumulative anthropogenic

emissions are primarily responsible for the observed change in concentrations in the 

atmosphere (i.e., the fraction of a country’s or an economic sector’s cumulative emissions 

compared to the world’s GHG emissions over a long time period will be roughly equal to the 

fraction of the change in concentrations attributable to that country or economic sector); 

likewise, annual emissions are a reasonable proxy for annual incremental changes in 

atmospheric concentrations. 

 

162

 In setting GHG emissions standards for model years 2012-2016 light-duty vehicles, the EPA set fleet-wide 

average CO2 equivalentstandards for cars and trucks based on a technology assessment analysis which indicated 

that there was a wide range of technologies available for manufacturers to use when upgrading vehicles to reduce 

CO2 emissions and improve fuel economy. The final standards were based on CO2 emissions-footprint curves, 

where each vehicle has a different CO2 emissions compliance target depending on its footprint value (related to 

the size of the vehicle). The EPA also set standards to cap tailpipe nitrous oxide, methane emissions, and 

provided compliance credits to manufacturers who improved air conditioning systems, such as through reduced 

refrigerant leakage (hydrofluorocarbons) and indirect CO2 emissions related to the increased load on the engine. 

75 FR 25324 (May 7, 2010).
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There are a number of possible ways of assessing whether air pollutants cause or 

contribute to the air pollution which may reasonably be anticipated to endanger public health 

and welfare, and no single approach is required or has been used exclusively in previous 

determinations under the CAA. Because the air pollution against which the contribution is 

being evaluated is the six well-mixed GHGs, the logical starting point for any contribution 

analysis is a comparison of the emissions of the air pollutant from the section 231 category to 

the total U.S. and total global emissions of the six GHGs. The Administrator recognizes that 

there are other valid comparisons that can be considered in evaluating whether emissions of 

the air pollutant cause or contribute to the combined concentration of the six GHGs. To 

inform the Administrator’s assessment, section V.B.2 presents the following types of simple 

and straightforward comparisons of U.S. aircraft GHG emissions:

 As a share of current total U.S. GHG emissions; 

 As a share of current U.S. transportation GHG emissions; 

 As a share of current total global GHG emissions; and

 As a share of the current global transportation GHG emissions. 

All annual GHG emissions data are reported on a CO2-equivalent (CO2eq) basis, 

which as described above is a commonly accepted metric for comparing different GHGs. 

This approach is consistent with how EPA determined contribution for GHGs under section 

202 of the CAA in 2009. 
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2. Overview of Greenhouse Gas Emissions 

Atmospheric concentrations of CO2 and other GHGs are now at essentially 

unprecedented levels compared to the distant and recent past.163 This is the unambiguous 

result of human emissions of these gases. Global emissions of well-mixed GHGs have been 

increasing, and are projected to continue increasing for the foreseeable future. According to 

IPCC AR5, total global (from all major emitting sources including forestry and other land 

use) emissions of GHGs in 2010 were about 49,000 teragrams164 of CO2 equivalent (Tg

CO2eq).

165 This represents an increase in global GHG emissions of about 29 percent since 

1990 and 23 percent since 2000. In 2010, total U.S. GHG emissions were responsible for 

about 14 percent of global GHG emissions (and about 12 percent when factoring in the effect 

of carbon sinks from U.S. land use and forestry).

Because 2010 is the most recent year for which IPCC emissions data are available, we 

provide 2011 estimates from another widely used and recognized global dataset, the World 

 

163 IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution 

of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, 

T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley 

(eds.)]. Cambridge University Press, p. 11.

164 One teragram (Tg) = 1 million metric tons = 1 megatonne (Mt). 1 metric ton = 1,000 kg = 1.102 short tons = 

2,205 lbs. 

165 IPCC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the 

Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, 

Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. 

Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, 1435 

pp.
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Resources Institute’s (WRI) Climate Analysis Indicators Tool (CAIT),166 for comparison. 

According to WRI/CAIT, the total global GHG emissions in 2011 were 43,816 Tg of CO2eq, 

representing an increase in global GHG emissions of about 42 percent since 1990 and 30 

percent since 2000 (excluding land use, land use change and forestry). These estimates are 

generally consistent with those of IPCC. In 2011, WRI/CAIT data indicate that total U.S. 

GHG emissions were responsible for about 16 percent of global emissions, which is also 

generally in line with the percentages using IPCC’s 2010 estimate described above. 

According to WRI/CAIT, current U.S. GHG emissions rank only behind China’s, which was 

responsible for 24 percent of total global GHG emissions. 

The Inventory of U.S. Greenhouse Gas Emissions and Sinks Report167 (hereinafter 

“U.S. Inventory”), in which 2013 is the most recent year for which data are available, 

indicates that total U.S. GHG emissions increased by 5.7 percent from 1990 to 2013 (or by

about 4.7 percent when including the effects of carbon sinks), and emissions increased from 

2012 to 2013 by 1.8 percent. This 2012 to 2013 increase was attributable to multiple factors 

including an increase in carbon intensity of fuels consumed for electricity generation, a small 

increase in vehicle miles traveled and vehicle fuel use, and a colder winter leading to an

increase in heating requirements. The U.S. Inventory also shows that while overall U.S. GHG 

 

166 World Resources Institute (WRI) Climate Analysis Indicators Tool (CAIT) Data Explorer (Version 2.0). 

Available at http://cait.wri.org (last accessed May 12, 2015). 

167 U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last accessed May 12, 

2015).
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emissions grew between 1990 and 2013, transportation GHG emissions grew at a significantly 

higher rate, 15 percent, more rapidly than any other U.S. sector. Within the transportation 

sector, aircraft remain the single largest source of GHG emissions not yet subject to any GHG 

regulations.

Section V.B.2.a which follows describes U.S. aircraft GHG emissions within the 

domestic context, while section V.B.2.b describes these same GHG emissions in the global 

context. Section V.B.2.c addresses future projections of aircraft GHG emissions.

a. U.S. Aircraft GHG Emissions Relative to U.S. GHG Transportation and Total U.S.

GHG Inventory

Relying on data from the U.S. Inventory, we compare U.S. aircraft GHG emissions to 

the transportation sector and to total U.S. GHG emissions as an indication of the role this

source plays in the total domestic contribution to the air pollution that is causing climate 

change. In 2013, total U.S. GHG emissions from all sources were 6,774 Tg CO2eq. As stated 

above, total U.S. GHG emissions have increased by almost 6 percent between 1990 and 2013,

while U.S. transportation GHG emissions from all categories have grown 15 percent since 

1990. The U.S. transportation sector was the second largest GHG emitting sector (behind 

electricity generation), contributing 1,911 Tg CO2eq or about 30 percent of total U.S. GHG 

emissions in 2013. This sectoral total and the total U.S. GHG emissions include emissions 
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from combustion of U.S. international bunker fuels, which are fuels used for transport 

activities, from aviation (both commercial and military) and marine sources.

168 Consistent 

with IPCC guidelines for common and consistent accounting and reporting of GHGs under 

the UNFCCC, the “U.S. international aviation bunker fuels” category includes emissions from 

combustion of fuel purchased in and used by aircraft departing from the United States, 

regardless of whether they are a U.S. flagged carrier. Total U.S. aircraft emissions constitute 

a significant part of the U.S. transportation sector’s emissions, accounting for 216 Tg CO2eq 

or 11 percent of such emissions (see Table V.1.). In 2013, emissions from aircraft (216 Tg

CO2eq) were the third largest transportation source of GHGs within the United States, behind 

light-duty vehicles and medium- and heavy-duty trucks (totaling 1,494 Tg CO2eq). 

For purposes of making this cause or contribute finding, the EPA is focused on, and 

proposes to include, a set of aircraft engine classes used in types of aircraft as described 

below, which corresponds to the scope of the international CO2 emissions standard 

contemplated by ICAO. 

As mentioned earlier in section II.D, traditionally the EPA (and FAA) participates at 

ICAO in the development of international standards, and then where appropriate, the EPA 

 

168 According to IPCC guidelines for common and consistent accounting and reporting of GHGs under the 

UNFCCC, the total U.S. GHG emissions from the U.S. Inventory that is reported to the UNFCCC excludes

international bunker fuel emissions (aviation and marine international bunker fuel emissions) from the reported 

total national GHG emissions. However, the total U.S. GHG emissions in this proposed cause or contribute 

finding section of today’s action do include international bunker fuel emissions because we want to capture the 

full contribution of U.S. emissions, including those from U.S. aircraft.
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establishes domestic aircraft engine emission standards under CAA section 231 of at least 

equivalent stringency to ICAO’s standards. An international CO2 emissions standard is 

anticipated in February 2016, and provided that the EPA makes a positive endangerment 

finding and ICAO adopts an international CO2 emissions standard that is both consistent with 

CAA section 231 and appropriate for domestic needs, we would expect to proceed with 

promulgating a CO2 emissions standard (or GHG standard) of at least equivalent stringency

domestically. As described later in section VI.D, the thresholds of applicability for the 

international CO2 emissions standard are based on gross weight as follows: for subsonic jet 

aircraft, a maximum takeoff mass (MTOM) greater than 5,700 kilograms; and for subsonic 

propeller driven (e.g., turboprop) aircraft, a MTOM greater than 8,618 kilograms.169 

Applying these gross weight thresholds, our proposed cause or contribute finding applies to 

GHG emissions from classes of engines used in covered aircraft. Examples of covered 

aircraft would include smaller jet aircraft such as the Cessna Citation CJ2+ and the Embraer 

E170, up to the largest commercial jet aircraft -- the Airbus A380 and the Boeing 747. Other 

examples of covered aircraft would include larger turboprop aircraft, such as the ATR 72 and 

the Bombardier Q400. Our intention is for the scope of the contribution finding to correspond 

to the aircraft engine GHG emissions that are from aircraft that match the applicability

thresholds for the international aircraft CO2 standard. As such we have also identified aircraft 

 

169 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard, Circular 

(Cir) 337, AN/192, Available at http://www.icao.int/publications/ICAO Products & Services 2015 

catalogue/cat_2015en.pdf (last accessed May 12, 2015). The ICAO Circular 337 is found on page 85 of the 

ICAO Products & Services 2015 catalog and is copyright protected; Order No. CIR337 . 
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that are not covered aircraft for purposes of our proposed contribution finding. That includes 

aircraft that fall below the international applicability thresholds: smaller turboprop aircraft, 

such as the Beechcraft King Air 350i, and smaller jet aircraft, such as the Cessna Citation M2. 

In addition, ICAO (with U.S. participation) has agreed to exclude “piston-engine aircraft,” 

“helicopters,” and “military aircraft”

170 from the types of aircraft that would be covered by the 

anticipated ICAO standards.171 These aircraft would not be covered aircraft and consequently, 

we are also not including GHG emissions from classes of engines used in these types of 

aircraft in our proposed cause or contribute finding. 

Thus, for the purposes of the cause or contribute finding, the EPA proposes to include 

GHG emissions from aircraft engines used in covered aircraft in the scope of this proposed 

cause or contribute finding. This is an equivalent scope of applicability as that contemplated 

by ICAO. The majority of the GHG emissions from all classes of aircraft engines would be 

covered by this scope of applicability. Below we describe the contribution of these U.S. 

covered aircraft GHG emissions to U.S. GHG emissions, and later in section V.B.2.b we 

discuss the contribution of these U.S. covered aircraft emissions to global GHG emissions.

In 2013, GHG emissions from U.S. covered aircraft (which includes U.S. international 

aviation bunker fuels in certain cases) comprised 90 percent (195 Tg CO2eq) of total U.S. 

 

170 ICAO regulations only apply to civil aviation (aircraft and aircraft engines), and consequently, ICAO 

regulations do not apply to military aircraft.

171 The applicability of the anticipated international CO2 standard would be limited to subsonic aircraft, and 

would not extend to supersonic aircraft. Since space vehicles (or spacecraft) will be operated at supersonic 

speeds, space vehicles would not be covered by the anticipated international CO2 standard.
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aircraft GHG emissions172 and 10 percent of total U.S. transportation sector GHG emissions 

(See Table V.1.). Overall, U.S. covered aircraft comprised the third largest source of GHG 

emissions in the U.S. transportation sector behind only the light-duty vehicle and mediumand heavy-duty truck sectors, which is the same ranking as total U.S. aircraft.

173 The U.S. 

covered aircraft also represent 3 percent of total U.S. GHG emissions, which is approximately

equal to the contribution from total U.S. aircraft of 3.2 percent (Table V.1).

174

It is important to note that in regard to the six well-mixed GHGs (CO2, methane, 

nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride), only two of 

these gases -- CO2 and nitrous oxide -- are reported as non-zero emissions for total aircraft and 

 

172 Eastern Research Group, Incorporated (ERG), U.S. Jet Fuel Use and CO2 Emissions Inventory for Aircraft 

Below ICAO CO2 Standard Thresholds, Final Report, EPA Contract Number EP-D-11-006, May 7, 2015.

173 Compared independently, total U.S. aircraft GHG emissions and U.S. covered aircraft GHG emissions are 

both ranked the third largest source in the U.S. transportations sector, behind only light-duty vehicle and 

medium- and heavy-duty truck sectors.

174 Total U.S. aircraft GHG emissions and U.S. covered aircraft GHG emissions were from 12 to 32 percent 

greater in 2000 and 2005 than in 1990. These increases in aircraft GHG emissions are primarily because aircraft 

operations (or number of flights) grew by similar amounts during this time period. Also, total U.S. aircraft GHG 

emissions and U.S. covered aircraft GHG emissions were from 10 to 17 percent greater in 2000 and 2005 than in 

2013. These decreases in aircraft GHG emissions are partly because aircraft operations decreased by similar 

amounts during this time period. In addition, the decreases in aircraft emissions are due in part to improved 

operational efficiency that results in more direct flight routing, improvements in aircraft and engine technologies 

to reduce fuel burn and emissions, and the accelerated retirement of older, less fuel efficient aircraft. 

Also, the U.S. transportation GHG emissions were changing at similar rates as total U.S. aircraft GHG emissions 

and U.S. covered aircraft GHG emissions for these same time periods, and thus, the aircraft GHG emissions 

share of U.S. Transportation remains approximately constant (over these time periods).

(U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, Last accessed May 12, 

2015; U.S. FAA. 2015, APO Terminal Area Forecast Summary Report - Forecast Issued January 2015, 

http://aspm.faa.gov/apowtaf/ .).
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covered aircraft.175 CO2 represents 99 percent of all GHGs from both total aircraft (214 Tg

CO2eq) and U.S. covered aircraft (193 Tg CO2eq), and nitrous oxide represents about one

percent from total aircraft (2 Tg CO2eq) and covered aircraft (1.8 Tg CO2eq). Modern aircraft 

do not emit methane,176 and hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride

are not products of aircraft engine combustion.

Table V.1

COMPARISONS OF U.S. AIRCRAFT GHG EMISSIONS TO TOTAL U.S. 

TRANSPORTATION AND TOTAL U.S. GHG EMISSIONS 

1990 2000 2005 2010 2011 2012 2013

Total U.S. Aircraft GHG emissions

(Tg CO2eq)

Share of U.S. Transportation

Share of total U.S. Inventory

228

14%

3.6%

262

13%

3.6%

254

12%

3.4%

216

11%

3.1%

215

11%

3.1%

212

11%

3.2%

216

11%

3.2%

 

175 U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last accessed May 12, 

2015).

176 Emissions of methane from jet fuels are no longer considered to be emitted (based on the latest studies) across 

the time series from aircraft gas turbine engines burning jet fuel A at higher power settings (EPA, Recommended 

Best Practice for Quantifying Speciated Organic Gas Emissions from Aircraft Equipped with Turbofan, Turbojet 

and Turboprop Engines, EPA-420-R-09-901, May 27, 2009 (see 

http://www.epa.gov/otaq/regs/nonroad/aviation/420r09901.pdf (last accessed May 12, 2015)). Based on this 

data, methane emissions factors for jet aircraft were reported as zero to reflect the latest emissions testing data. 

Also, the 2006 IPCC Guidelines indicate the following: “Methane (CH4) may be emitted by gas turbines during 

idle and by older technology engines, but recent data suggest that little or no CH4 is emitted by modern engines.” 

(IPCC, 2006: IPCC Guidelines for National Greenhouse Gas Inventories, The National Greenhouse Gas 

Inventories Programme, The Intergovernmental Panel on Climate Change, H.S. Eggleston, L. Buendia, K. Miwa, 

T Ngara, and K. Tanabe (eds.). Hayama, Kanagawa, Japan.) The EPA uses an emissions factor of zero to 

maintain consistency with the IPCC reporting guidelines, while continuing to stay abreast of the evolving 

research in this area. For example, one recent study has indicated that modern aircraft jet engines operating at 

higher power modes consume rather than emit methane (Santoni et al., 2011: Aircraft Emissions of Methane and 

Nitrous Oxide during the Alternative Aviation Fuel Experiment, Environ. Sci. Technol., 45, pp. 7075-7082).
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U.S. Covered Aircraft GHG emissions

(Tg CO2eq)

Share of U.S. aircraft GHG emissions 

Share of U.S. Transportation

Share of total U.S. Inventory

169

74%

10%

2.6%

223

85%

11%

3%

217

85%

10%

2.9%

190

88%

9.7%

2.7%

193

90%

10%

2.8%

190

90%

9.9%

2.9%

195

90%

10%

2.9%

Transportation Sector emissions

(Tg CO2eq)

Share of total U.S. Inventory

1,659

26%

2,044

28%

2,137

29%

1,966

28%

1,932

28%

1,907

29%

1,911

28%

Total U.S. GHG emissions 6,406 7,315 7,464 7,017 6,889 6,652 6,744

 

b. U.S. Aircraft GHG Emissions Relative to Global Aircraft GHG Inventory and the 

Total Global GHG Inventory

For background information and context, we first provide information on the 

contribution of GHG emissions from global aircraft and the global transportation sector to 

total global GHG emissions, and describe how this compares to the emissions from aircraft

that would be covered by the anticipated ICAO CO2 standard. We then compare U.S. aircraft 

GHG emissions to the global aircraft sector, to the global transport sector, and to total global

GHG emissions as an indication of the role this source plays in the total global contribution to 

the air pollution that is causing climate change. As in the preceding section, we present 

comparisons from both total U.S. aircraft and U.S. covered aircraft. 

According to IPCC AR5, global aircraft GHG emissions in 2010 were 11 percent of 

global transport GHG emissions and 2 percent of total global GHG emissions. Data from 
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ICAO’s 2013 Environmental Report indicate that the vast majority of global emissions from 

the aircraft sector are emitted by the types of aircraft that would be covered by the anticipated 

ICAO CO2 standard (“ICAO covered aircraft”).177 When compared to global data from IPCC 

AR5, worldwide GHG emissions from ICAO covered aircraft represented about 93 percent 

(688 Tg CO2eq) of global aircraft GHG emissions,

178 10 percent of global transport GHG 

emissions, and 1.5 percent of total global GHG emissions in 2010.

Comparing data from the U.S. Inventory to IPCC AR5, we find that total U.S. aircraft

GHG emissions represented about 29 percent of global aircraft GHG emissions, about 3.1 

percent of global transport GHG emissions, and about 0.5 percent of total global GHG

emissions in 2010 (see Table V.2). For U.S. covered aircraft in 2010 GHG emissions 

represented about 26 percent of global aircraft GHG emissions, 2.7 percent of global transport 

GHG emissions, and 0.5 percent of total global GHG emissions (see Table V.2). Because 

2010 is the most recent year for which IPCC emissions data are available, we also made 

comparisons using 2011 estimates from WRI/CAIT and the International Energy Agency

(IEA)179 and found that they yield very similar results.180

 

177 ICAO CAEP, 2013: ICAO Environmental Report 2013, Aviation and Climate Change, 224 pp. Available at 

http://cfapp.icao.int/Environmental-Report-2013/ (last accessed May 12, 2015).

178 Worldwide GHG emissions from ICAO covered aircraft include emissions from both international and 

domestic aircraft operations around the world. 

179 International Energy Agency, Data Services. Available at http://data.iea.org (last accessed May 12, 2015).

180 Data from WRI/CAIT and IEA show that, in 2011, total U.S. aircraft emissions represented about 28 percent 

of global aircraft GHG emissions, about 3.7 percent of global transport GHG emissions, and about 0.5 percent of 

total global GHG emissions. U.S. covered aircraft represented about 25 percent of global aircraft GHG 
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Table V.2

COMPARISONS OF U.S.AIRCRAFT GHG EMISSIONS TO TOTAL GLOBAL 

GREENHOUSE GAS EMISSIONS IN 2010

2010

(Tg CO2 eq)

Total U.S. 

Aircraft 

Share (%)

U.S. Covered 

Aircraft

Share (%)

Global Aircraft 

Share (%)

Global Aircraft GHG 

emissions

743 29 26 --

Global Transport 

GHG emissions

7,000 3.1 2.7 11

Total Global GHG 

emissions

49,000 0.5 0.5 2

For additional background information and context, we used 2011 WRI/CAIT and 

IEA data to make comparisons between the aircraft sector and the emissions inventories of 

entire countries and regions. When compared to entire countries, total global aircraft GHG 

emissions in 2011 ranked 9

th overall, behind only China, United States, India, Russian 

Federation, Japan, Brazil, Germany, and Indonesia, and ahead of about 175 other countries. 

Total U.S. aircraft GHG emissions have historically been and continue to be by far the largest 

contributor to global aircraft GHG emissions. Total U.S. aircraft GHG emissions are about 7 

times higher than aircraft GHG emissions from China, which globally is the second ranked 

country for aircraft GHG emissions, and about 5 times higher than aircraft GHG emissions 

from all of Asia. U.S. covered aircraft GHG emissions are about 6 times more than aircraft 

 

emissions, 3.3 percent of global transport GHG emissions, and 0.5 percent of total global GHG emissionsin 

2011.
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GHG emissions from China, and about 4 times more than aircraft GHG emissions from all of 

Asia. If U.S. covered aircraft emissions of GHGs were ranked against total GHG emissions 

for entire countries, these covered aircraft emissions would rank ahead of Belgium, Czech 

Republic, Ireland, Sweden and about 150 other countries in the world. 

c. Aircraft GHG Emissions Are Projected to Increase in the Future

While overall GHG emissions from U.S. covered aircraft increased by about 13

percent from 1990 to 2010, the portion attributable to U.S. international aviation bunker 

fuels181 increased by about 90 percent.182 During this same time period, global aircraft GHG

emissions grew by about 40 percent, and the portion attributable to global international 

aviation bunker fuels increased by 80 percent.183,184 Notwithstanding the substantial growth 

in GHG emissions from U.S. international aviation bunker fuels, U.S. covered aircraft

 

181 The U.S. international aviation bunker fuels category includes emissions from combustion of fuel purchased 

in and used by aircraft departing from the United States, regardless of whether they are a U.S. flagged carrier. 

GHG emissions from U.S. international aviation bunker fuels are a subset of GHG emissions from U.S. covered 

aircraft. From 1990 to 2010, GHG emissions from U.S. covered aircraft increased from 169 to 190 Tg CO2eq, 

and GHG emissions from the portion attributable to U.S. international aviation bunker fuels grew from 30 to 58 

Tg CO2eq during this same time period. From 1990 to 2011, GHG emissions from U.S. covered aircraft 

increased from 169 to 192 Tg CO2eq (about 14 percent), and GHG emissions from the portion attributable to 

U.S. international aviation bunker fuels grew from 30 to 62 Tg CO2eq (about 110 percent).

182 U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, 564 pp. Available at 

http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last accessed May 12, 

2015). 

183 IPCC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the 

Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, 

Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. 

Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, pp.

599-670.

184 According to IEA, from 1990 to 2011, global aircraft GHG emissions grew by about 50 percent, and global 

international aviation bunker fuels increased by 80 percent. International Energy Agency Data Services, 

Available at http://data.iea.org (last accessed May 12, 2015, 2015). 
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emissions have not increased as much as global aircraft emissions primarily because the U.S. 

aviation market was relatively mature compared to the markets in Europe and other emergent 

markets, and because during this time period the U.S. commercial air carriers suffered several 

major shocks that reduced demand for air travel.185,186 After consolidation and restructuring 

in recent years, the U.S. commercial air carriers have regained profitability and are forecasted 

by the FAA to grow more over the next 20 to 30 years.

187

 With regard to global aircraft GHG 

emissions, the aviation markets in Asia/Pacific, Europe (where airline deregulation has 

stimulated significant new demands in this period), and the Middle East (and other emerging 

markets) have been growing rapidly, and the global market is expected to continue to grow 

significantly over the next 20 to 30 years.188 

Recent studies estimate that both ICAO covered aircraft and U.S. covered aircraft will 

experience substantial growth over the next 20 to 30 years in their absolute fuel burn, and that 

this will translate into increased GHG emissions. ICAO estimates that the global fuel burn 

from ICAO covered aircraft will increase by about 120 percent from 2010 to 2030 and by

 

185 FAA, 2014: FAA Aerospace Forecast Fiscal Years 2014-2034, 129 pp. Available at 

https://www.faa.gov/about/office_org/headquarters_offices/apl/aviation_forecasts/aerospace_forecasts/2014-

2034/media/2014_FAA_Aerospace_Forecast.pdf (last accessed May 12, 2015).

186 These shocks include the September 11 terror attacks, significant increases in fuel prices, debt restructuring in 

Europe and U.S., and a global recession. 

187 According to the FAA Aerospace Forecast 2014-2034, in 2013 U.S. air carriers were profitable for the fourth 

consecutive year. 

188 According to the FAA Aerospace Forecast 2014-2034, the International Air Transport Association (IATA) 

reports that world air carriers (including U.S. airlines) are expected to register an operating profit for 2013. 

Based on financial data compiled by ICAO and IATA, between 2004 and 2013 world airlines produced 

cumulative operating profits (with nine years out of ten posting gains) and net profits (with six years out of ten 

posting gains).
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about 210 percent from 2010 to 2040 (for a scenario with moderate technology and 

operational improvements).189 The FAA projects that the fuel consumption from U.S. air 

carriers and general aviation aircraft operating on jet fuel will grow by 49 percent from 2010

to 2035, corresponding to an average annual increase rate in fuel consumption of 1.6

percent.190 These aircraft groups (U.S. air carriers and general aviation aircraft operating on 

jet fuel) are of similar scope to the U.S. covered aircraft whose engine GHG emissions are the 

subject of this proposed finding. Using fuel burn growth rates provided above as a scaling 

factor for growth in GHG emissions (globally and nationally), it is estimated that GHG

emissions from ICAO covered aircraft and U.S. covered aircraft would increase at a similar 

rate as the fuel burn by 2030, 2035, and 2040. 

3. Proposed Contribution Finding for the Single Air Pollutant Comprised of the 

of Six Well-Mixed Greenhouse Gases 

Taking into consideration the data summarized in section V.B.2 above, the 

Administrator proposes to find that GHG emissions from classes of engines used in U.S. 

covered aircraft, which are subsonic jet aircraft with a maximum takeoff mass (MTOM) 

greater than 5,700 kilograms and subsonic propeller driven (e.g., turboprop) aircraft with a 

MTOM greater than 8,618 kilograms, contribute to the air pollution that endangers public 

 

189 ICAO CAEP, 2013: ICAO Environmental Report 2013, Aviation and Climate Change, 224 pp. Available at 

http://cfapp.icao.int/Environmental-Report-2013/ (last accessed May 12, 2015).

190 FAA, 2015: FAA Aerospace Forecast Fiscal Years 2015-2035, 134 pp. Available at 

https://www.faa.gov/about/office_org/headquarters_offices/apl/aviation_forecasts/aerospace_forecasts/2015-

2035/media/2015_National_Forecast_Report.pdf (last accessed May 12, 2015).
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health and welfare. The Administrator is not at this time proposing a contribution finding for

GHG emissions from engines not used in covered aircraft (i.e., those used in smaller

turboprops, smaller jet aircraft, piston-engine aircraft, helicopters and military aircraft). We 

solicit comment on the scope of the proposed contribution finding, whether a broader 

contribution finding (e.g., including all engines used in aircraft certified by the FAA) would 

be appropriate, and the extent to which EPA has discretion to establish standards pursuant to a 

contribution finding that do not impose requirements on every engine or class of engines

within the scope of that finding.

It is the Administrator’s judgment that the collective GHG emissions from the classes 

of engines used in U.S. covered aircraft are significant, whether the comparison is domestic 

(10 percent of all U.S. transportation GHG emissions, representing 3 percent of total U.S. 

emissions) or global (26 percent of total global aircraft GHG emissions representing 3 percent 

of total global transportation emissions and 0.5 percent of all global GHG emissions). The 

proposed scope of GHG emissions from engines used in U.S. covered aircraft under this cause 

or contribute finding would result in the vast majority (90 percent) of U.S. aircraft GHG

emissions being included in this determination. 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. GHG emissions from engines used in U.S. covered 

aircraft will become globally well-mixed in the atmosphere, and thus will have an effect not 

only on the U.S. regional climate but also on the global climate as a whole, for years and 

indeed many decades to come. It is the Administrator’s view that the cause or contribute test 
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used here under CAA section 231 can follow the same reasoning that was used in the 2009 

GHG cause or contribute finding under CAA section 202; that is, the Administrator believes a 

positive cause or contribute finding for GHG emissions from engines used in U.S. covered 

aircraft is justified whether only the domestic context is considered, only the global context is 

considered, or both the domestic and global GHG emissions comparisons are viewed in 

combination. 

As was the case in 2009, no single GHG source category dominates on the global 

scale, and many (if not all) individual GHG source categories could appear small in 

comparison to the total, when, in fact, they could be very important contributors in terms of 

both absolute emissions or in comparison to other source categories, globally or within the 

United States. If the United States 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.”191 Moreover, as the Supreme Court 

explained in Massachusetts v. EPA, agencies commonly take an incremental approach to 

resolving large issues, stating that, “[a]gencies, like legislatures, do not generally resolve 

massive problems in one fell regulatory swoop. ... They instead whittle away at them over 

time, refining their preferred approach as circumstances change and as they develop a more 

nuanced understanding of how best to proceed.” 549 U.S. 497, 524 (2007) (citations 

 

191 74 FR 66543 (December 15, 2009).
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omitted). The Administrator continues to believe that these unique, global aspects of the 

climate change problem—including that from a percentage perspective there are no 

dominating sources emitting GHGs and few sources that would even be considered to be 

close to dominating—tend to support consideration of contribution to the air pollution at 

lower percentage levels than EPA typically encounters when analyzing contribution towards a 

more localized air pollution problem. Thus, the Administrator, similar to the approach taken

in the 2009 GHG cause or contribute finding under CAA section 202, is placing significant 

weight on the fact that engines used in U.S. covered aircraft contribute 3 percent of total U.S. 

GHG emissions for the proposed contribution finding and comprise the single largest 

transportation source in the United States that has not yet been regulated for GHG emissions. 

4. Additional Considerations

The Administrator is also concerned that reasonable estimates of GHG emissions from 

engines used in U.S. covered aircraft are projected to grow over the next 20 to 30 years. 

Given the projected growth in aircraft emissions compared to other sectors, it is reasonable for 

the Administrator to consider future emissions projections as adding weight to her primary

reliance on annual emissions. Recent projections reveal that by 2035 GHG emissions from all 

aircraft and U.S. covered aircraft engines are likely to increase by almost 50 percent.192 By

 

192 As discussed in Section V.B.2.c fuel burn growth rates for air carriers and general aviation aircraft operating 

on jet fuel are projected to grow by 49 percent from 2010 to 2035 and this provides a scaling factor for growth in 

GHG emissions which would increase at a similar rate as the fuel burn by 2030, 2035, and 2040.
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contrast, it is estimated that by 2035 the light duty vehicle sector will see a 30 percent 

reduction in GHG emissions from the 2010 baseline, while the heavy duty vehicle sector will 

experience a 33 percent increase in GHG emissions from the 2010 baseline (this projected 

increase does not reflect the impact of GHG reductions anticipated from the Phase 2 heavy 

duty GHG standards that have not yet been promulgated). In addition, by 2035 the rail sector 

is projected to experience a 6 percent reduction in GHG emissions from 2010 baseline.

193

Because the projected growth in aircraft engine GHG emissions from U.S. covered aircraft 

appears to be greater in percentage terms than other transportation sources, this future 

consideration adds weight to the Administrator’s proposed positive contribution finding. 

VI. Advance Notice of Proposed Rulemaking: Discussion of Ongoing 

International Proceedings to Develop Aircraft CO2 Emissions Standard and 

Request for Comment 

For more than four years, the EPA and FAA have been engaged with the ICAO’s 

Committee on Aviation Environmental Protection (ICAO/CAEP) to establish an international 

CO2 emissionsstandard which the EPA could then consider proposing for adoption under its 

 

193 U.S. Energy Information Administration (EIA), 2015: Annual Energy Outlook (AEO) 2015 with projections 

to 2040, DOE/EIA-0383, 154 pp. EIA’s reference case (used as the baseline in this comparison) assumes fuel 

economy levels for light duty vehicles required to meet federal light duty GHG standards for years 2012-2025, 

and for heavy duty trucks GHG standards for years 2014-2018, plus improvements in vehicles and engines for 

all subsectors due to availability of fuel-saving technologies and fuel price effects. EIA counts biofuels as zero 

tailpipe GHG emissions. Because the comparison in this section focuses on tailpipe emissions, we include them

here, at volumes as forecast in the AEO 2015 reference case. Available at http://www.eia.gov/forecasts/aeo/ (last 

accessed May 12, 2015).
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section 231 authority of the CAA. This section of today’s notice serves as an ANPR to

discuss the key issues of the ongoing international proceedings prior to February 2016, when 

ICAO/CAEP is expected to finalize an international aircraft CO2 standard. An ANPR is 

intended to solicit comments and/or information from the public prior to an agency

determining whether to propose a rulemaking. As such, an ANPR does not propose or impose 

any regulatory requirements. The EPA may choose to develop an ANPR for actions (such as 

the promulgation of standards pursuant to CAA section 231 to implement an international 

aircraft CO2 standard domestically) which are still in the early stages of development and for 

which public input may be particularly helpful. This also helps ensure transparency, while 

assisting the EPA in obtaining input from a wide range of stakeholders as we continue work 

within CAEP to establish an international CO2 aircraft standard. The EPA is seeking 

comments for all interested parties, including small businesses, on a variety of issues related 

to setting an international CO2 standard for aircraft, including whether such standards should 

apply to in-production aircraft instead of new aircraft types only, the appropriate effective 

dates for the potential international CO2 standard, as well as the appropriate stringency levels.

 CAEP met an important milestone at its 9th meeting (CAEP/9) in 2013 in reaching an

agreement on the appropriate metric to be used in assessing fuel efficiency (or CO2

emissions)194 of an engine/aircraft combination. They also reached agreement on a mature 

 

194 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard, Circular 

(Cir) 337, AN/192, Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 
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certification requirement195 to evaluate CO2 emissions for new aircraft types and also agreed 

on certain aspects of the scope of applicability of the CO2 emissions standard; however, work 

on applicability options for in-production aircraft continues.

At the CAEP Steering Group meeting in November 2013, there was agreement on a 

set of stringency options to be used for the cost-effectiveness analysis, and at the Steering 

Group meeting in September 2014 there was a decision on the associated inputs for costs and 

technology responses to be utilized in the cost-effectiveness analysis of these stringency

options. This analysis, and work on the applicability of the standard to in-production aircraft 

and the certification requirement are scheduled to be completed prior to the 10th CAEP 

meeting (CAEP/10) in February 2016. As described in section II.A, the EPA and the FAA 

traditionally work within the ICAO/CAEP standard-setting process to establish international 

emission standards and related requirements. Under this approach, international emission 

standards have first been adopted by ICAO, and subsequently the EPA has initiated 

rulemakings under CAA section 231 to establish domestic standards that are of at least equal 

stringency as ICAO’s standards. This approach has been affirmed as reasonable by the U.S. 

Court of Appeals for the D.C. Circuit. Provided the EPA makes a positive endangerment 

 

12, 2015) . The ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is 

copyright protected; Order No. CIR337. Section 3.2 of this Circular states the following: “An important Phase 1 

milestone in the development of the CO2 Standard was reached on 11 July 2012, when the CAEP Steering Group 

agreed unanimously on a CO2 metric system to measure the aeroplane fuel burn performance and therefore the 

CO2 emissions produced.”

195 ICAO defines a certification requirement as a combination of metric, procedures, instrumentation and 

measurement methodology, and compliance requirements. 
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finding196 under CAA section 231 and ICAO adopts an international aircraft CO2 standard 

that is consistent with CAA section 231 and U.S. domestic needs, we would expect to proceed 

with a similar approach promulgating a CO2 emissions standard (or GHG standard) of at least 

equivalent stringency domestically. 

A. Purpose of the International Standard

At the CAEP Steering Group meeting in 2011, the U.S provided a paper 

recommending that CAEP agree that the purpose of the international CO2 emissions standard

be “to achieve CO2 emissions reductions from the aviation sector beyond expected ‘business 

as usual’ – i.e., a standard that achieves CO2 emissions reductions from the aviation sector 

beyond what would be achieved in the absence of a standard. This would be analyzed using

ICAO criteria of technical feasibility, environmental benefit, cost effectiveness, and impacts 

of interdependencies.”197 The Steering Group accepted the U.S. proposal for the purpose of 

the international CO2 standard, and it is expected to be included in the standard setting

process. The metric system, stringency options, costs, technology responses (inputs to be 

utilized in the cost-effectiveness analysis), and applicability ultimately chosen will all have an 

effect on whether the international CO2 emissions standard adheres to this stated purpose of 

the standard. The U.S. continues to support the adoption of an international CO2 emissions 

 

196 As shorthand in today’s notice, in many places we will use the term “endangerment finding” for both 

endangerment and cause or contribute findings.

197 CAEP (U.S. Working Paper), “U.S. Position on the Development of ICAO’S Aircraft CO2 Standard,” CAEPSG/20112-WP/25, Presented by the United States, U.S. Working Paper for CAEP Steering Group meeting, 

Beijing, China, 12 to 16 September 2011.
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standard that meets this stated purpose, and the EPA requests comment on this continued 

support. The EPA requests comment on how to achieve the purpose of the standard.

B. Applicability of the International CO2 Emissions Standard

We are requesting comment on whether the aircraft CO2 standard should apply to inproduction aircraft, including aircraft with any engineered fuel efficiency improvements (e.g.,

different engines, redesigned wings, or engine performance improvement packages, etc.) or 

whether the aircraft CO2 standard should apply only to completely new aircraft type designs. 

In-production aircraft and new aircraft types are defined as follows: 

- In-production aircraft: those aircraft types which have already received a Type 

Certificate, and for which manufacturers either have existing undelivered sales orders 

or would be willing and able to accept new sales orders.

198,199,200

 

198 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015).The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.

199 As described earlier in section D, in existing U.S. aviation emissions regulations, in-production means newlymanufactured or built after the effective date of the regulations -- and already certified to pre-existing standards 

(if emission standards were established previously).). This is similar to the current CAEP definition for inproduction aircraft types for purposes of the CO2 standard.

200 According to ICAO Cir 337, a Type Certificate is “[a] document issued by a Contracting State to define the 

design of an aircraft type and to certify that this design meets the appropriate airworthiness requirements of that 

State”.
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- New aircraft types: aircraft types that have applied for a Type Certificate201 after the 

effective date of a standard and that have never been manufactured prior to the 

effective date of a standard.

In addition, for context, out of production aircraft are those aircraft types which have 

already received a Type Certificate, but for which manufacturers either have no existing 

undelivered sales orders or would not be willing and able to accept new sales orders. These 

aircraft are aircraft types that are no longer in active production.202

As described earlier in section II.E, CAEP’s Steering Group meeting in 2010 agreed 

that the scope of applicability for the international aircraft CO2 standard will be subsonic jets 

with an applicability weight threshold of maximum takeoff mass (MTOM) greater than 

5,700kg (12,566lb) and turboprop aircraft with a MTOM greater than 8,618kg (19,000lb). 

CAEP also agreed that the international CO2 standard will apply to new aircraft types, but not 

 

201 A Type Certificate is a design approval process whereby the FAA ensures the manufacturer’s designs meet 

the minimum requirements for aircraft safety and environmental regulations. This is typically issued only once 

for each aircraft, and modified as needed as an aircraft is modified over the course of its production life. This 

Type Certificate (for new aircraft types) would be the initial or new Type Certificate for this aircraft.

202 Out of production aircraft that are still in operational use would become subject to the international standard 

only if the standard applied to “in-use” aircraft, which it will not since CAEP has agreed that the international 

aircraft CO2 standard should not apply to out of production aircraft types. Note, the EPA’s CAA section 231 

aircraft engine standards have applied to in-use aircraft only in very limited situations, such as the prohibition 

against fuel venting at 40 CFR 87.11 and smoke number standards at 40 CFR 87.31. Note, however, that unlike 

the EPA’s authority to promulgate emission standards for motor vehicles under CAA section 202(a) or for 

nonroad engines and vehicles under section 213(a), section 231 of the CAA does not restrict the EPA’s authority 

to set standards for only new aircraft.
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apply to out of production aircraft types, and that applying the standard to in-production

aircraft types should not be ruled out.203 

It is important to further describe the difference between new aircraft types and inproduction aircraft. There are three categories of aircraft under consideration when describing 

a CO2 standard: new aircraft types submitted for certification (known as clean sheet designs), 

those with lesser levels of design change, such as a new series in an established type and 

model (considered to be significant partial redesigns), or an aircraft with incremental 

improvements.204 New aircraft types or new type designs are significant investments for 

manufacturers and are used for new and significantly different designs (also characterized as 

complete redesigns). Significant partial redesigns may be characterized as a new or later 

series of an established model that may incorporate newly designed wings and give 

purchasers more choices of engines. Incremental improvements are less extensive changes to 

an aircraft such as performance improvement packages that may be added to an aircraft or 

engine at some point during the production cycle.

 

203 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard.

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.

204 New aircraft types fall under the initial or new Type Certificate, and significant partial redesigns and 

incremental improvements fall under an amended Type Certificate. Significant partial redesigns would be a new

or later series of an established model, and incremental improvements would be a part of the same series as the 

established model.
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New aircraft types or new type designs are infrequent. The most recent new type 

designs introduced in service, such as the Airbus A380 in 2007, the Boeing 787 in 2011, and 

the original Boeing 777 in 1995,

205,206,207 indicate that it is unlikely a new type design will 

seek certification in the next 10 to 15 years.208 (New aircraft types (and similarly for 

significant partial redesigns) typically yield large fuel burn reductions – 10 percent to 20

percent over the prior generation they replace, and as one might expect, these significant fuel 

burn reductions do not happen frequently. Also, aircraft development programs are 

expensive. It is not unusual for new type designs to take 8-10 years to develop, from 

preliminary design to entry into service. 209,210) Significant partial redesigns do not occur 

often, but are slightly more frequent than new type designs. For example, after the current 

 

205 Boeing, 2011: Boeing Unveils First 787 to Enter Service for Japan Airlines, December 14. Available at 

http://boeing.mediaroom.com/2011-12-14-Boeing-Unveils-First-787-to-Enter-Service-for-Japan-Airlines (last 

accessed May 12, 2015).

206 The Independent, 2012: BA reveals Airbus A380 superjumbo flight plans, by Peter Woodman, December 11. 

Available at http://www.independent.co.uk/travel/news-and-advice/ba-reveals-airbus-a380-superjumbo-flightplans-8405961.html (last accessed May 12, 2015).

207 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.

208 Ibid.

209 Ibid.

210 Analysts estimate a new single aisle would have cost $10-12 billion to develop. The A380 and 787 are 

estimated to each have cost around $20 billion to develop; the A350 is estimated to have cost $15 billion, 

excluding engine development. Due to the large development cost of a totally new aircraft design, 

manufacturers are opting to re-wing or just re-engine their aircraft (significant partial redesigns). Boeing is said 

to have budgeted $5 billion for the re-wing of the 777 and Airbus and Boeing have budgeted $1-2 billion each 

for the re-engine of the A320 and the 737, respectively (excluding engine development costs). Embraer has 

publically stated they will need to spend $1-2 billion to re-wing the EMB-175 and variants. (ICF International, 

CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract Number EP-C-12-011, 

March 17, 2015.)
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significant partial redesign wave211 has passed (which includes the Boeing 747-8, Boeing 737 

Max, Airbus 320 Neo, and Boeing 777-X), we do not currently have knowledge of many 

additional significant partial redesigns anticipated over the next decade (as the previous wave 

of significant partial redesigns included the Boeing 777-200LR in 2004, 777-300ER in 2006, 

737NG in 1998, Airbus 319 in 1996, and Airbus 330-200 in 1998).212,213 Incremental 

improvements will likely be frequent and occur in the near term. One approach CAEP is 

considering would be to limit the applicability of any international CO2 standard to only new 

type designs (or new aircraft types). Under this approach the international CO2 standard 

would not apply to significant partial redesigned aircraft and incremental improvements. 

Under another approach CAEP is considering, CAEP would also apply the international CO2

standard to in-production aircraft (in addition to new aircraft types). Significant partial 

redesigned aircraft and incremental improvements would be characterized as changes made to 

in-production aircraft; thus, these categories of aircraft (or these changes) would need to meet 

the international CO2 standard under this approach (or they would need to meet the standard if 

 

211 In general, design waves are prompted by the combination of market demand for new aircraft performance 

needs (e.g., more seats for longer range) and the age of existing aircraft, and design waves are typically enabled 

by advances in propulsion technology.

212 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.

213 Insofar as we are going through a wave of major redesign and service entry now, prospects for further stepfunction improvements will be low in the coming 10-15 years. (ICF International, CO2 Analysis of CO2-

Reducing Technologies for Aircraft, Final Report, EPA Contract Number EP-C-12-011, March 17, 2015.)
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it also applied to in-production aircraft).

214

 The EPA requests comments on the applicability

approaches that CAEP is considering.

Another approach for applicability of the international CO2 standard that CAEP could 

adopt (or CAEP is considering) would be an approach based on criteria addressing significant 

changes to aircraft designs, which could be considered an applicability requirement different 

than that for new aircraft types only and in-production aircraft. This alternative approach 

could redefine a new aircraft type for CO2 purposes to include in-production aircraft that have 

a significant change in CO2 emissions, thus including in-production aircraft in the 

applicability of the CO2 standard. The alternative approach could even cover significant 

partial redesigned aircraft, depending upon the definition. CAEP’s current mature 

certification requirement for the international CO2 standard215 provides further detail on 

technology changes to aircraft that would affect the aircraft’s CO2 metric value. A changed 

version of an aircraft could be defined as follows: an aircraft which incorporates changes in 

type design that may adversely affect216 its CO2 emissions. This possible definition could also 

note the following: (1) where the proposed change in design, configuration, power or mass is 

 

214 As described earlier, CAEP has not ruled out applying the international CO2 standard to in-production aircraft 

types, which are aircraft types that have already received a Type Certificate and are produced after the effective 

date of the standard. In-production aircraft types would include significant partial redesigned aircraft and 

incremental improvements. CAEP is currently considering and analyzing in-production applicability.

215 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337. See Chapter 1.

216 Due to substantial market forces to alleviate any adverse effects on aircraft fuel burn or CO2 emissions, 

adverse changes are rare.
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so extensive that a substantially new investigation of compliance with the applicable 

airworthiness regulations is required, the aircraft should be considered to be a new type design 

rather than a changed version, and (2) “adversely” refers to an increase in CO2 emissions of 

more than an amount (or percentage) that has yet to be determined (this amount or criterion is 

still being considered by CAEP). The EPA requests comment on this change-based criteria

approach, including how to identify those changes that would result in treating in-production 

aircraft as new types subject to the standard.

If CAEP were to limit the scope of applicability to new aircraft types only (and 

without the significant change criteria approach described above), the international CO2

standard would not apply to later series aircraft with redesigned wings, aircraft that are 

available with different engines, or aircraft that undergo incremental improvements. 

Following are several examples that illustrate this situation. The re-engined Boeing 737 Max 

is an example of a significant partial redesigned aircraft that is expected to enter into service 

in 2017.217 This aircraft would fall under the original Boeing 737 Type Certificate that was 

issued in 1967 (and entered into service in 1968) -- or more specifically it would fall under an 

amended Type Certificate, and it would not be considered a new aircraft type as defined by

CAEP. The current in-production 737s (Next Generation 737s or commonly abbreviated as 

737 NGs) feature newer engines, have redesigned wings, and entered service in 1998 under 

 

217Boeing, 737 Max Program “LEAPS” into Engine Testing, Article by Eric Olson, July 11, 2014. Available at 

http://www.boeing.com/boeing/Features/2014/07/bca_737max_leap_07_11_14.page, (last accessed May 12, 

2015).
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the original 737 Type Certificate that was issued in 1967, and these also were not considered a 

new type aircraft when they were introduced in 1998.218 Another example of an aircraft that 

does not qualify as a new type is the Boeing 747-8 aircraft, that entered into service in 2011, 

and which included a new wing, new engines, and a lengthened fuselage but fell under an 

amended Type Certificate for the original Boeing 747 that was certified in 1969 (and entered 

into service in 1969). An example of incremental improvements to in-production aircraft, is 

the Boeing Next Generation 737 performance improvement package which was implemented 

between 2011 and 2013 and the Boeing 767-300 winglets that entered into service in 2008, 

both of which improve aircraft fuel efficiency. There are many other examples that exist for 

different manufacturers and aircraft around the world as well, but for conciseness, we are 

limiting our discussion to these above examples. These examples illustrate the typical 

certification for significant partial redesigns and incremental improvements by various aircraft 

certificating or certifying authorities (or national airworthiness authorities) around the world. 

Using CAEP’s current definition of new aircraft types (clean sheet designs, which are 

completely new aircraft) we cannot today identify the first aircraft to which the new standard 

would apply. As the examples above illustrate, new aircraft types are infrequent,

219 and there 

 

218 The original 737 entered service in 1968. The 737 Classic entered service in 1984, and it had new high 

bypass engines, an updated wing, and other aerodynamic improvements. The 737 NGs entered service in 1998, 

and they featured a new wing and updated engines. Several mid-life upgrades were produced for the 737 NGs, 

offering improved range, payload, and efficiency. (ICF International, CO2 Analysis of CO2-Reducing 

Technologies for Aircraft, Final Report, EPA Contract Number EP-C-12-011, March 17, 2015.)

219 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.
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are no currently announced new type designs that are expected to be introduced after the 

implementation dates being analyzed by CAEP – 2020 and 2023. Furthermore, based on 

provisions to which CAEP has already agreed,

220 new aircraft types subject to the CO2

standard would be aircraft that submit an application for a Type Certificate after the 

implementation dates of 2020 and 2023 (dates for the stringency analysis) which would likely

result in entry into service dates of about 2025 or 2028.221 If the international CO2 standard is 

applied only to new aircraft types, then CO2 emissions would not be expected to begin to

deviate from business-as-usual (in comparison to CO2 emissions reductions that would be 

achieved in the absence of a standard) before 2025. Therefore, an international standard 

developed for only new aircraft types may not actually apply to any new aircraft for at least a

decade. Even if a few new type aircraft are introduced in this timeframe, it will take even 

longer for these aircraft to comprise any significant portion of the fleet. Therefore, applying

an international standard which applies only to new aircraft types will likely result in no 

additional CO2 reductions beyond what would have occurred absent a CO2 standard, either for 

 

220 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard, 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337. Section 1.5 states that that the date to be used in determining the applicability of 

the CO2 standard is the date the application for a Type Certificate was submitted to the certificating authority 

having jurisdiction over the manufacturer responsible for the aircraft design. Section 1.6 specifies that an

application shall be effective for the period specified in the designation of the airworthiness regulations 

appropriate to the aircraft type. An application for a Type Certificate is valid for 5 years.

221 These dates assume 5 years from application for the aircraft Type Certificate to entry into service, which is 

how long an application is valid for a Type Certificate.
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the near- and mid-term, about 5 to 10 years from 2016, or even in the longer-term of 20 years 

plus.

222,223

 

The EPA requests comments on the timeframes for introducing new aircraft types and 

their subsequent penetration into the fleet. Are there any aircraft manufacturer 

announcements that we missed in regard to new aircraft types that will be introduced or apply 

for a Type Certificate after 2020 and 2023 (or new aircraft types that will be introduced or 

apply for a Type Certificate five years after these dates)?224 If so, what are these new aircraft 

types? How many new types are projected to enter the fleet in this timeframe and what 

portion of the fleet will they represent? 

The alternative approach described earlier (addressing changes in design of inproduction aircraft) may offer an opportunity to cover more aircraft in an earlier timeframe 

(including significant partial redesigns), but it is unclear what effect this approach would have 

on additional CO2 emissions reductions compared to a standard for only new aircraft types. 

 

222 Approximate time-scales are considered to be 5 years for near-term, 10 years for mid-term, and 20 years or 

more for long-term.

223 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.

224 In November 2014, Boeing indicated that it would replace the 737 with a new aircraft type in 2030. Earlier 

this decade, Boeing was assessing an all new clean sheet 737 replacement, but eventually they decided to reengine the 737 (the 737 Max) instead. (Flight Club, Paul Thompson, Here's The Skinny On What's Next For 

Boeing, November 16, 2014. Available at http://flightclub.jalopnik.com/heres-the-skinny-on-whats-next-forboeing-1656206527 (last accessed May 12, 2015), Also, Wichita Business Journal, Daniel McCoy, Boeing 

planning 737 MAX replacement by 2030 — What it could mean for Spirit AeroSystems, November 5, 2014. 

Available at http://www.bizjournals.com/wichita/blog/2014/11/boeing-planning-737-max-replacement-by-2030-

what.html) (last accessed May 12, 2015). We would consider this as a Boeing projection or sketching out of 

plans for a new aircraft type, but it is not a commitment from Boeing. 
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The EPA requests comments on the timeframe for CO2 emissions reductions and the likely

share of annual aircraft production (or share of in-production aircraft built annually) that 

would be affected under the alternative approach.

Applying an emission standard to in-production aircraft (provided the EPA makes a 

positive endangerment finding under CAA section 231(a)), means that all aircraft that are 

built (in-production) after the effective date of the regulation would need to certify and 

comply with the standard to remain in production. This includes (as described earlier) inproduction aircraft with incremental improvements (though we reiterate this would not 

include in-use aircraft). As an example of in-production aircraft, the Gulfstream G650, which 

is currently in production and expected to remain so after 2020, would need to certify and 

comply with the new CO2 standard. In the next section we discuss in more detail how 

applicability to in-production aircraft could work.

C. CAEP Discussion on In-Production Aircraft Applicability

At the request of the CAEP Steering Group meeting in November 2013, CAEP began 

work on defining potential options to implement applicability requirements for in-production 

aircraft. Subsequently, based on the options provided to the 2014 Steering Group meeting, 

CAEP decided that it should continue to investigate potential in-production aircraft 

applicability options, and that these should be presented at the July 2015 Steering Group 

meeting, so that a decision can be taken at the 10th meeting of CAEP (CAEP/10) in February

2016 regarding whether the international CO2 standard will apply to in-production aircraft. 
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There are a wide range of options under consideration, including both mandatory and 

voluntary options for reporting and certification processes for in-production aircraft 

applicability, but the 2014 Steering Group meeting requested that CAEP focus on defining the 

mandatory options (in contrast to options such as voluntary reporting and certification).

1. Applicability to In-Production Aircraft and Date of Implementation

At the 2014 Steering Group meeting, CAEP also agreed that 2023 represented the 

earliest possible date for an in-production aircraft standard to allow time to promulgate 

domestic regulations and process manufacturer certification applications. CAEP did not rule 

out later dates though and could consider implementation dates for an in-production aircraft 

CO2 standard later than 2023 (CAEP could consider applicability dates for in-production 

aircraft that are five years following the new aircraft type applicability date, i.e. dates ranging 

from 2023 to 2028).

The EPA seeks comments on both a 2023 implementation date and on possible later 

implementation dates for an in-production domestic CO2 (or GHG) aircraft engine emissions 

standard that would be adopted under CAA section 231,

225 the impact the date of 

implementation might have on per-aircraft GHG or CO2 emissions rates and the ability of a

 

225 Traditionally, international emission standards have first been adopted by ICAO, and subsequently the EPA 

had initiated rulemakings under CAA section 231 to establish domestic standards equivalent to ICAO’s 

standards where appropriate. Provided ICAO adopts an international aircraft CO2 standard that is consistent with 

CAA section 231 and it is appropriate for domestic needs in the United States, we expect to proceed along a 

similar approach for the future CAA section 231 aircraft engine CO2 standard (or aircraft engine GHG standard), 

provided the EPA issues final positive endangerment and cause or contribute findings under CAA section 231.
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domestic GHG or CO2 standard to achieve aircraft emission reductions beyond what would 

occur in the absence of such a standard. Would the dates of applicability (e.g., 2023 to 2028) 

be consistent with the purpose of the standard as accepted by CAEP? 

As described in section VI.F.2, the technologies considered for the CAEP analyses

are those technologies that will be widely used on in-production aircraft by 2016 or shortly

thereafter.226 The EPA requests comments regarding whether applying an international CO2

standard to in-production aircraft is consistent with the purpose of the standard as accepted by

the CAEP Steering Group meeting in 2011: “to achieve CO2 emission reductions from the 

aviation sector beyond expected ‘business as usual’... analyzed using ICAO criteria of 

technical feasibility, environmental benefit, cost effectiveness, and impacts of 

interdependencies.”227 The International Coalition for Sustainable Aviation (ICSA),

228 which 

is a CAEP Observer organization, submitted papers to CAEP that analyzed this issue. Also, a

 

226 CAEP determined in 2012 that all technology responses would have to be based on technology that would be 

in common use by the time the standard was to be decided upon in 2016 or shortly thereafter. This generation of 

technology was defined within CAEP as a Technology Readiness Level (TRL) 8 – an actual system completed 

and qualified through test and demonstration – by 2016 or shortly thereafter.

227CAEP (U.S. Working Paper), “U.S. Position on the Development of ICAO’S Aircraft CO2 Standard,” CAEPSG/20112-WP/25, Presented by the United States, U.S. Working Paper for CAEP Steering Group meeting, 

Beijing, China, 12 to 16 September 2011. Available at http://www.epa.gov/otaq/aviation.htm. (last accessed 

May 12,2015).

International Coalition for Sustainable Aviation (ICSA), “ICAO’S CO2 Standard as Part of a Basket of 

Measures to Meet Emission Reduction Goals”, ICAO Assembly - 38th Session, Executive Committee, Agenda 

Item 17 - Environmental Protection, A38-WP/297, EX/99, September 19, 2013.

228 The International Coalition for Sustainable Aviation (ICSA) is a structured network of environmental nongovernmental organizations (NGOs) who share a common concern with civil aviation’s contribution to air 

quality issues, climate change and noise, and who are committed to developing and providing technical 

expertise, common policy positions and strategies with a view to reducing emissions and noise from aviation. 

See http://www.icsa-aviation.org/ (last accessed May 12, 2015)..
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member of ICSA229 has developed similar analyses which indicate that applying the 

international standard only to new aircraft types would likely result in no additional CO2

reductions beyond what would have occurred absent a CO2 standard, either for the near- and 

mid-term, about 5 to 10 years from 2016, or even in the longer-term of 20 years plus. This 

occurs, the ICCT states, because the development cycles for new aircraft are very lengthy and 

it is not unusual for new aircraft to take 8 to 10 years to develop from preliminary design to 

entry into service and once in service it takes significant time for new aircraft types to 

penetrate the fleet.230,231 Another study funded by the EPA corroborates this analysis.232 The 

EPA requests comments on whether applying the international CO2 aircraft standard only to 

new aircraft types would be consistent with the accepted purpose of the international standard

(the purpose of the standard that has been accepted by the CAEP Steering Group). Lastly, the 

EPA requests comment on the appropriateness of following either of these approaches

(applicability to only new aircraft types or both new aircraft types and in-production aircraft) 

in any future CAA section 231 aircraft engine CO2 or GHG emissions standard (if either 

 

229 The International Council on Clean Transportation (ICCT) is a member of ICSA, and ICCT is an independent 

nonprofit organization founded to provide research and technical and scientific analysis to environmental 

regulators. See http://www.theicct.org/ (last accessed May 12, 2015).

230 ICCT, Efficiency Trends for New Commercial Jet Aircraft 1960 to 2008, November 2009. Available at 

http://www.theicct.org/sites/default/files/publications/ICCT_Aircraft_Efficiency_final.pdf (last accessed May 

12, 2015).

231 ICCT, “Could ICAO’s CO2 Standard Not Actually Cover Any Aircraft? Yes, If Nobody’s Watching”. Blog, 

December 9, 2014. Available at http://www.theicct.org/blogs/staff/could-icaos-co2-standard-not-cover-anyaircraft (last accessed May 12, 2015).

232 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.
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approach is adopted by ICAO, and provided the EPA makes positive endangerment and 

contribution finding findings under CAA section 231).

Also, there have been concerns raised in CAEP about applying the international CO2

standard to in-production aircraft. These concerns include (a) the added resource burden on 

certificating authorities to process manufacturers’ certification applications, which will be 

more numerous compared to new aircraft types; and (b) the potential added costs to 

manufacturers to certify in-production aircraft. The EPA requests comment on these two 

concerns, including providing supporting documentation on the extent of these concerns and

any other issues the commenters may identify with applying the international CO2 standard to 

in-production aircraft.

2. Reporting Requirement for New In-Production Aircraft

CAEP is working to define mandatory in-production aircraft options, and one possible 

option is a reporting requirement233,234,235 for in-production aircraft CO2 emissions rates

 

233 Currently, CAEP is developing a publicly available database for aircraft CO2 emissions (CAEP is now 

considering format, parameters, etc. for the database), but submissions to this database by aircraft manufacturers 

would be voluntary. There will not be a CAEP mandatory reporting requirement associated with this potential 

CO2 database. In addition, if the international aircraft CO2 standard applies to only new aircraft types, it could 

be many years before any data exists in this database. 

234 For many years, ICAO has maintained an Aircraft Engine Emissions Databank for landing and takeoff 

certificated emissions values of NOx, hydrocarbon, carbon monoxide, and smoke number (ICAO and the EPA 

also have aircraft engine emission standards for these pollutants). It contains certified emissions data voluntarily 

reported from each aircraft engine manufacturer. This databank is available at https://easa.europa.eu/documentlibrary/icao-aircraft-engine-emissions-databank (last accessed May 12, 2015).

235 In 2012, the EPA promulgated annual reporting requirements for aircraft engine emissions of NOx, 

hydrocarbon, carbon monoxide, and smoke number and related parameters. One of the reasons that the EPA 
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(measured according to the aircraft test procedure that was agreed upon at CAEP/9) as an 

alternative to establishing an aircraft CO2 standard for in-production aircraft. Although a

reporting requirement provides policy relevant information, it does not necessarily translate 

into specific emissions reductions. The EPA recognizes that only a mandatory standard for 

in-production aircraft would ensure that the aircraft CO2 standard reduces per-aircraft CO2

emissions rates. However, a reporting requirement could be an important component of an inproduction aircraft CO2 standard, especially if it is implemented shortly after an in-production 

aircraft standard is adopted. It would ensure that CO2 emissions rates data are gathered 

quickly prior to an effective date for the final standard (tracking CO2 emissions rates is 

beneficial for the reasons discussed later in this section and for potentially assisting with the 

assessment of a future CO2 standard). The EPA requests comment on an aircraft 

manufacturer reporting requirement that is implemented soon after the adoption of an inproduction international aircraft CO2 standard, as a component of the in-production aircraft 

CO2 standard.

In 2009 the EPA promulgated a final GHG reporting rule that applies to many sectors 

in the United States, including manufacturers of heavy-duty and offroad vehicles and engines, 

 

issued these reporting requirements was due to the varying amount of voluntary data reported by aircraft engine 

manufacturers. (U.S. EPA, “Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards 

and Test Procedures.” Final Rule, 77 FR 36342, (June 18, 2012)).
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and manufacturers of aircraft engines.

236,237

 The EPA’s experience with reporting programs 

indicates that the EPA and the public would be able to track CO2 emissions rates trends (i.e., 

trends of aircraft cruise fuel burn rates) from aircraft over time. Requiring the reporting of 

aircraft CO2 emissions rates trends from aircraft over time is appropriate and feasible. 

Requiring aircraft manufacturers to report aircraft CO2 emissions rates shortly after an inproduction international aircraft standard is adopted would enable and expedite the tracking 

and understanding of these emission trends. In addition, reporting programs typically raise 

awareness of emissions and can improve the understanding of the factors that influence 

emission rates as well as the actions that can be taken to reduce emissions. When similar 

methods for monitoring, measurement, and reporting are applied across an industry, it can 

lead to more consistent, accurate, and timely data to inform decision-making for individual 

manufacturers and the EPA (including a comparison of the CO2 emissions rates from aircraft 

 

236EPA’s 2009 rule on Mandatory Reporting of Greenhouse Gases included engine manufacturers for the 

following mobile source sectors: highway heavy-duty (engine and vehicle), non-road, aircraft, locomotive, 

marine, snowmobiles, and motorcycles. Manufacturers of aircraft jet engines of rated output (or thrust) greater 

than 26.7 kilonewtons are required under this program to report annually to the EPA CO2 and NOx emissions 

from aircraft engines during the landing and takeoff cycle. Manufacturers of these engines were already 

measuring and recording CO2 emissions as part of existing criteria air pollutant emission requirements for the 

landing and takeoff cycle, but prior to this 2009 rule, these data were not reported to the EPA. Manufacturers 

voluntarily reported the data to ICAO, but there was no assurance that the EPA would receive this information, 

and thus, the 2009 rule required reporting of the aircraft engine CO2 and NOx emissions during the landing and 

takeoff cycle to the EPA. 

237 An aircraft manufacturer reporting requirement for in-production aircraft CO2 emission rates would require 

the reporting of aircraft CO2 emissions during the cruise phase of operation to the EPA. The majority of aircraft 

CO2 emissions occurs in the cruise phase of operation, and thus, reporting CO2 emission rates from this phase 

will improve our ability to track full-flight aircraft CO2 emission rates over time (in addition to reporting the 

aircraft engine CO2 emissions during the landing and takeoff cycle). Also, the aircraft test procedure that was 

agreed upon at CAEP/9 now enables us to measure aircraft CO2 emissions during cruise. 
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of various manufacturers). Thus, a reporting requirement could potentially contribute to 

efforts to identify and implement future aircraft CO2 emission reduction opportunities. 

Independent of action that CAEP may or may not take in February 2016, the EPA 

could under its CAA section 114(a) authority pursue a reporting requirement for aircraft 

cruise GHG or CO2 emissions rates– to ensure we have GHG or CO2 emissions rates data on 

all in-production aircraft (and any new aircraft types that enter service).238 The EPA could 

use the same metric agreed to at CAEP/9 (and in ICAO circular 337). This will be described 

in detail in VI.D.1 below. In general, the EPA asks for comment on a mandatory reporting 

requirement for in-production aircraft GHG or CO2 emissions rates – either as part of the 

CAEP international standard or as an independent domestic requirement to be adopted by the 

EPA. If the EPA were to pursue this requirement independently from CAEP, what lead time 

would be appropriate for manufacturers to report the GHG or CO2 emissions rates from all of 

their in-production aircraft239 (after we promulgate such a requirement)? Additionally, if we 

were to pursue such an independent reporting requirement, should we require the annual 

reporting of the GHG or CO2 emissions rates from in-production aircraft (and any new type 

 

238 This GHG or CO2 emissions rate data will help to track trends, raise awareness, better understand the 

technology in the fleet, etc.

239 In this case, manufacturers would need to report the GHG or CO2 emission rates for in-production aircraft 

(aircraft types which have already received a Type Certificate, and for which manufacturers either have existing 

undelivered sales order or would be willing and able to accept new sales orders) that are built after a certain date, 

which has yet to be determined but would likely be a date that occurs shortly after we promulgate the 

requirement.
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aircraft)

240 to enable us to track any updates? We are not at this time proposing to promulgate 

such a requirement in advance of ICAO’s decision. Due to the possibility of ICAO’s 

adoption of a reporting requirement, we believe it is reasonable to await the outcome of that 

decision in order to determine whether to strictly follow ICAO’s possible reporting 

requirement or make changes to it in the form of an additional U.S. domestic requirement, as 

appropriate. 

D. Metric System, Applicability, and Certification Requirement 

The CO2 metric system and mature certification procedure were agreed upon by

CAEP in 2013.241 This section describes the metric system that was developed, the scope of 

aircraft to be covered by the international CO2 standard, the certification test procedures that 

would be used to demonstrate compliance with the international CO2 standard, and CAEP’s 

decision to focus on the entire aircraft for the international CO2 standard. 

1. CO2 Metric System

The metric system was developed to cover a wide range of aircraft types, designs, 

technology, and uses. To do this, the metric system was designed to differentiate between 

generations of aircraft and to equitably capture improvements in aerospace technology

 

240 By applying a reporting requirement to in-production aircraft after a certain implementation date, this 

reporting requirement also includes new aircraft types that are produced after this implementation date.

241 As described earlier, the certification requirement is the combination of metric, procedures, instrumentation 

and measurement methodology, and compliance requirements. We are using the terms metric system and 

certification test procedures to describe these elements of the certification requirement.
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(structural, propulsion, and aerodynamic) that contribute to a reduction in the airplane CO2

emissions. In addition, the metric system accommodates a wide range of technologies and 

designs which manufacturers may choose to implement to reduce CO2 emissions from their 

aircraft. 

The metric system agreed to at CAEP uses multiple Specific Air Range (SAR) test 

points to represent cruise fuel burn. SAR is a traditional measure of aircraft cruise 

performance which measures the distance an aircraft can travel for a unit of fuel. This is 

similar to the instantaneous “miles per gallon” readings in many cars today. However, here 

the inverse of SAR is used (1/SAR); therefore a lower metric value represents a better fuel 

efficiency. The SAR data are gathered at three gross weight points. The three equally

weighted points are used to represent a range of day to day aircraft operations.242 The 

functional form of the metric system is provided below.

   =



1





.

Equation 1: CO2 Metric 

 

242 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015).. The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.
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(1/SAR)avg is calculated at 3 gross weight fractions of Maximum Takeoff Mass 

(MTOM):

High gross mass: 92% MTOM

Mid gross mass: Average of high gross mass and low gross mass

Low gross mass: (0.45 * MTOM) + (0.63 * (MTOM^0.924))

The Reference Geometric Factor (RGF) is a measure of the fuselage size on a given 

aircraft. In analyzing various metric system options it was found that in some instances, 

namely stretch aircraft, changes in aircraft size, and thus capability, were not reflected in 

changes to the aircraft’s gross weight (MTOM). To account for these occurrences, and the 

variety of methods that manufacturers may use to make such a change, an adjustment factor 

was added (the RGF with a 0.24 exponent used in the metric system). 

2. Applicability

CAEP has decided the scope of applicability for a future international CO2 standard 

should be subsonic jet and propeller-driven aircraft meeting the following criteria: 

All subsonic jet aircraft over 12,566 lbs (5,700kg) MTOM.

All subsonic propeller driven (e.g. turboprop) aircraft over 19,000 lbs (8618 kg) 

MTOM, except amphibious airplanes and those designed and used for fire-fighting 

operations. 
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No military aircraft will be subject to this international standard. 

3. Certification requirement

CAEP has developed a mature certification requirement243 that would allow for the 

determination of an aircraft CO2 metric value for any aircraft meeting the applicability criteria 

set forth above. This certification requirement incudes the metric system and test procedure. 

The test procedure was based upon industry’s current best practices for establishing the cruise 

performance of their aircraft, and input from certification authorities. These procedures 

include specifications for aircraft conformity, weighing, fuel specifications, test condition 

stability criteria, required confidence intervals, measurement instrumentation required, and 

corrections to reference conditions. 

These CO2 test procedures are based upon manufacturer’s existing practices when 

certifying new aircraft. This means that there is a very heavy reliance on dedicated flight 

testing of the aircraft. This potentially poses challenges for the certification of in-production 

aircraft. Manufacturers have stated that there could be logistical challenges associated with 

the certification of aircraft for CO2 that have previously been type certificated (e.g. procuring 

and instrumenting an aircraft for flight testing). To address this, the EPA is currently working 

 

243 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR337.
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within CAEP to encourage the development of a modified or separate equivalent certification

test procedure that would reduce this burden on manufacturers and allow for quicker/simpler 

certification of in-production types. 

4. Regulating the Entire Aircraft Instead of the Engine

The CO2 metric system intends to equitably reward improvements in aircraft 

technologies that reduce emissions, including advances in structures (aircraft weight), 

propulsion (engine specific fuel consumption), and aerodynamics. These three factors are key

to the overall aircraft CO2 emissions. In addition, CAEP has indicated ( and EPA agrees) that 

it is best to consider the aircraft as a whole instead of only the aircraft engine technology in 

addressing factors that influence CO2 emissions, because of the effects and interaction these 

key factors have on the aircraft CO2 emissions from engines.

244

 The three factors -- and 

technology categories that improve these factors -- are described as follows:

245

Structures: reducing basic aircraft weight to increase the commercial payload or 

extend range for the same amount of thrust and fuel burn; 

 

244 ICAO, 2013: CAEP/9 Agreed Certification Requirement for the Aeroplane CO2 Emissions Standard. 

Available at http://www.icao.int/publications/catalogue/cat_2015_en.pdf (last accessed May 12, 2015). The 

ICAO Circular 337 is found on page 85 of the ICAO Products & Services 2015 catalog and is copyright 

protected; Order No. CIR33..

245 ICAO, Environmental Report 2010 – Aviation and Climate Change, 2010, which is located at 

http://www.icao.int/environmental-protection/Pages/EnvReport10.aspx (last accessed May 12, 2015).
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Propulsion (thermodynamic and propulsion efficiency): advancing the overall specific 

performance of the engine, to reduce the fuel burn per unit of delivered thrust; and

Aerodynamics: advancing the aircraft aerodynamics, to reduce drag and its associate

impacts on thrust.

Specific examples of technologies that affect these three factors help to further 

illustrate that it is best to consider the aircraft as a whole in addressing CO2 emissions. For 

structural improvements, aircraft manufacturers have shown significant weight reduction 

results over time due to the progressive introduction of new technologies such as: advanced 

alloys and composite materials, improved and new manufacturing processes and techniques 

(including integration and global evaluation simulation), and new systems (e.g. fly-bywire).246,247

 

For propulsion improvements, technologies include enhanced compressors (e.g., 

intercooled compressors) and reduced hub-tip ratio fans.

248

 As another example, 

manufacturers seek higher operating pressure ratios (OPR) to improve combustion and engine 

cycle refinements. 

 

246 Ibid.

247 Fly-by-wire refers to a system which transmits signals from the cockpit to the aircraft's control surfaces 

electronically rather than mechanically. AirlineRatings.com, Available at http://www.airlineratings.com/did-youknow.php?id=18 (last accessed on May 12, 2015, 2015).

248 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.
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For aerodynamics, friction and lift-dependent drag are the biggest contributors to 

aerodynamic drag. Advances in aerodynamics enable significant lift-dependent drag 

reduction by maximizing effective wing span extension. For example, wing-tip devices can 

give an increase in the effective aerodynamic span of wings, particularly where wing lengths 

are limited by airport gate sizes. Manufacturers are also looking at ways of decreasing the 

drag caused by skin friction. An example of a technology to improve aircraft local skin 

friction is to utilize riblets (which are micro-grooves on the surface) to maintain laminar flow 

via Natural Laminar Flow and Hybrid Laminar Flow Control (HLFC) to reduce turbulent skin 

friction. 249 The first production example of a HLFC system went into service on the new 

Boeing 787-9 in 2014.

E. Stringency Options

At the Steering Group meeting in November 2013, CAEP agreed to analyze a range of 

CO2 stringency options that cover the full range of aircraft in-production and in-development 

around the world (within the applicable weight thresholds and categories), and this includes 

the wide range of technology that is currently in the aircraft fleet.250 Generally, the stringency

options that are being evaluated fall into three categories as follows: (1) CO2 stringency

 

249 ICAO, Environmental Report 2010 – Aviation and Climate Change, 2010, which is located at 

http://www.icao.int/environmental-protection/Pages/EnvReport10.aspx.(last accessed May 12, 2015).

250 The ICAO standard has the following applicability weight thresholds: maximum takeoff mass greater than

5,700 kilograms for subsonic jet aircraft and maximum takeoff mass greater than 8,618 kilograms for 

turboprops. 
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levels that could impact251 only the oldest, least efficient aircraft in-production around the 

world, (2) middle range CO2 stringency levels that could impact many aircraft currently inproduction and comprising much of the current operational fleet, and (3) CO2 stringency

levels that could impact aircraft that have either just entered production or are in final design 

phase but will be in-production by the time the international CO2 standard becomes effective. 

We are requesting comment on the level(s) at which the CO2 stringency options should be set, 

what factors should be considered in establishing the stringency of the CO2 standard, and on 

their potential relationship to any future CAA section 231 standard.

The figures below are intended to show the range of stringency levels under 

consideration at CAEP and CO2 metric value levels of today’s in-production and indevelopment252 aircraft. The data shown were generated by the EPA using a commercially 

available aircraft modeling tool called PIANO.253 This model contains non-manufacturer 

provided estimates of the performance of various aircraft. In contrast, CAEP is using 

manufacturer-provided estimates of the aircraft metric value performance. 

 

251 The aircraft shown in these charts are in-production and current in-development. These aircraft could be 

impacted by an in-production standard in that, if they were above the standard, they would need to either 

implement a technology response or go out of production. For a new type only standard there will be no 

regulatory requirement for these aircraft to respond. 

252 Aircraft that are currently in-development but will be in production by the applicability dates. These could be 

new types or significant partial redesigned aircraft. 

253 PIANO (Project Interactive Analysis and Optimization), Aircraft Design and Analysis Software by Dr. 

Dimitri Simos, Lissys Limited, UK, 1990-present; Available at www.piano.aero (last accessed May 12, 2015). 

This is a commercially available aircraft design and performance software suite used across the industry and 

academia.
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The stringency options under consideration at CAEP are functions of the aircraft CO2

Metric Value and have a correlating parameter of MTOM. They are upwards sloping and 

have a “kink” at 60,000 kilograms MTOM. The “kink” was included in the stringency

options as a technical approach to reflect the different behaviors observed between the larger 

and smaller aircraft. 

The official stringency options under consideration at CAEP have not been cleared for 

release outside of the participating members since deliberations on the standard are still 

ongoing (proceedings are expected to be completed at CAEP/10 in February 2016). To show 

the relative efficiency of the aircraft, Figure 1 and Figure 2 below show the aircraft metric 

values254versus MTOM. In place of the official stringency options under consideration, lines 

of constant technology are used to notionally show how the stringency options were set across 

the fleet. These lines reflect the three ranges of options discussed above. Lower metric 

values, for a given MTOM, represent an increased fuel efficiency. Figure 1 shows the makeup 

of the current production fleet and the in-development aircraft. This is what CAEP is using as 

the starting point for modeling the affect of the CO2 standard. Figure 2 shows what the EPA 

expects the market to look like in 2023, considering the publicly announced plans by industry

to replace existing aircraft with new products.

 

254 Metric values were generated using PIANO.
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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 100000 200000 300000 400000 500000 600000

Aircraft Metric Value (lo

wer value reflects lo

wer C

O2 emissions)

MTOM (kg)

Lines of Constant Technology Level

In Production

In Development
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Figure 1 – Lines of constant technology level over the 2014 in-production and in 

development fleet

Figure 2 – Lines of constant technology level over the predicted 2023 in-production fleet

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 100000 200000 300000 400000 500000 600000

Aircraft Metric Value (lo

wer value reflects lo

wer C

O2 emissions

MTOM (kg)

Lines of Constant Technology

Level

In Production
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A standard set near the upper-most line of constant technology in Figures 1 and 2 

would affect a very modest number of aircraft, namely the oldest, least efficient types. Many

of the aircraft that would be affected by such a stringency level are being produced in very

limited numbers and may not be eligible to operate in U.S. air space (e.g., Russian and 

Ukraine aircraft).

Aircraft around the middle two lines of constant technology in Figures 1 and 2 reflect 

the performance of many aircraft that are currently in production and compose much of the 

current operational fleet. The current generation of single aisle aircraft from Boeing and 

Airbus are in this middle range.

Aircraft near the lowest line of constant technology in Figures 1 and 2 reflect the 

most advanced aircraft currently for sale on the market. These are aircraft that have either just 

entered production or are still in-development but will be in-production by the effective date 

of a potential in-production the standard. The replacement single aisle aircraft and new twin 

aisle aircraft from Boeing and Airbus are modeled to be clustered around the lowest line.

While Figures 1 and 2 show the ranges of stringency under consideration and how 

aircraft fall within those ranges, because of the scale, it is hard to see the range of technology

present in the fleet. Therefore Figure 3 and 4 expand the view and show percent differences

between the four constant technology lines represented in Figures 1 and 2. This allows for a 

clearer view of best and worst performing aircraft; Figure 3 provides the perspective from the 
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current in-production and in-development fleet, and Figure 4 projects out to the 2023 fleet. In 

addition, these figures allow one to compare the technology level and efficiency of aircraft 

with differing MTOMs. 
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Figure 3 – Percent from least efficient line of constant technology for the 2014 in 

production and in development fleet

-35%

-30%

-25%

-20%

-15%

-10%

-5%

0%

5%

0 100000 200000 300000 400000 500000 600000

Margin to

Highest Line of C

onstant Technology

MTOM (kg)

Lines of Constant

Technology Level

In Production

In Development
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Figure 4 - Percent from least efficient line of constant technology for the predicted 2023 

in–production fleet

-35%

-30%

-25%

-20%

-15%

-10%

-5%

0%

5%

0 100000 200000 300000 400000 500000 600000

Margin to

Highest Line of C

onstant Technology

MTOM (kg)

Lines of Constant

Technology Level

In Production
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 The EPA requests comment on a range of stringency options within the constant 

technology lines identified in Figure 1 and Figure 2, on their potential impact, and on their 

potential relationship to any future CAA section 231 standard.

CAEP is considering the possibility of adopting two separate CO2 stringency levels, 

one for new type aircraft and one for in-production aircraft. This would allow stringencies to 

be set for both new types and in-production aircraft at a level closer to what could be achieved 

by each aircraft type. Issues surrounding the potential for in-production standards are 

discussed in section VI.C.1. 

There is ongoing discussion on what appropriate levels of stringency may be for new 

type and in-production aircraft. Any final decisions will have to wait until the full analysis 

has been conducted at CAEP. As explained in sections VI.B and VI.C.1, new types are 

infrequently developed and typically represent a step change in technology. It may be possible 

to set a level of stringency that is reasonable for in-production aircraft to meet, but at the same 

time provide an incentive for new type aircraft to improve. However, this is challenging to 

develop because of the significant efficiency improvements typically seen between inproduction and new type aircraft. The EPA requests comment on the potential for developing 

a standard with two stringency levels at CAEP. 

The development of a new aircraft type standard must take into consideration the 

standard’s potential effect on any future type designs. Even the most stringent option under 

consideration at CAEP is still based on technology available today. Any new type aircraft that 
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may be developed and certified 10 years or more from now would be expected to use more 

advanced fuel efficient technology that is not yet developed or tested. 

The implications for an in-production standard are more significant in the near term 

for manufacturers. Aircraft currently in-production, and not meeting the level of an inproduction standard, would need to be modified to meet the standard to remain in production; 

this would take time and resources from the manufacturers. The full implications of this have 

not yet been resolved in CAEP. However, we expect that the effect on aircraft CO2 emissions 

would be minimal for less stringent options. The aircraft with the highest CO2 metric values 

generally rely on older technology and were designed in the 1980’s to early 1990’s. Many of 

these aircraft are also expected to be replaced with updated versions in the near future, before 

a CO2 standard would be implemented and go into effect. The EPA requests comment on the 

levels at which in-production and new type standards might be set and on what factors should 

be considered in establishing the stringency. 

F. Costs, Technology Responses for Stringency Options, and Cost-Effectiveness 

Analysis

The EPA has been involved in CAEP’s effort to analyze the CO2 stringency options 

and the potential costs and environmental impacts that would result from both new type only

CO2 standards and in-production international CO2 standards. CAEP is still determining the 

best way to conduct portions of this analysis. The inputs that have been developed by the 

CAEP include non-recurring costs data and technology responses for the various stringency
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options under consideration. This section describes the development of these inputs. The 

EPA requests comments on how the modeling should be conducted to differentiate inproduction and new type scenarios. 

1. Non-Recurring Costs (engineering development costs) 

CAEP developed a single cost estimate that could be used for all aircraft as a function 

of MTOM and percent metric value improvement required. Based on past practice, industry

provided estimates for developing clean sheet designs and significant partial redesigns, only

including high level information that has been made available to the public. This was 

considered to be a top down estimate because it included all aircraft development costs 

(airworthiness certification, noise, etc.) not just those for CO2 improvements. 

Since the initial dataset provided by industry only included major changes (or major 

improvements), the EPA saw the need to supplement this dataset with an estimate of CO2-

only changes (or CO2-only improvements), which was considered to be a bottom up estimate. 

These changes would be much smaller, on the order of a few percent, and could be applied to 

in-production aircraft at a cost much lower than projected by industry. The EPA contracted 

with ICF International to develop an estimate of the cost to modify in production aircraft to 

comply with a CO2 standard. ICF International conducted a detailed literature search, 

conducted a number of interviews with industry leaders, and did its own modeling to estimate 
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the cost of making modifications to in production aircraft.255 The results from this peerreviewed study (small changes) were then combined with inputs from the industry and the 

other CAEP participants (large changes) to develop the CO2 technology response and cost 

estimation. For the cost estimation, the CAEP combined the two different methodologies to 

develop the final cost surface. 256

A top-down approach is being used to model large changes to aircraft design, such as 

what would be seen in significant partial redesigns or new types. For significant partial 

redesigns that result in new series of an established model, these types of changes may

include: redesigned wings, new engine options, longer fuselages, improved aerodynamics, or 

reduced weight. When making significant changes to an aircraft many other changes and 

updates get wrapped into the process that do not have an effect on the CO2 emissions of the 

aircraft, and significant partial redesigns may not have been spurred by changes to fuel 

efficiency (CO2 reductions). This confluence of changes led CAEP to agree that it was 

reasonable to use the full development cost for a new type (clean sheet) or significant partial 

redesign for major changes. Total costs for past projects were used to estimate non-recurring 

cost for the CAEP analysis. This type of aircraft improvement/development program has 

historically ranged approximately from 1 to 15 billion Dollars (U.S.) depending on the size of 

the aircraft and scope of the improvements desired. 

 

255 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.

256 The two datasets were merged together and a single cost surface was then generated to calculate the cost to 

modify any aircraft based on the MTOM, and percent metric value change needed.
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A bottom-up approach was used, by CAEP, to model smaller incremental metric value 

changes to aircraft design. The CAEP agreed that the above top-down approach would not be 

the best approach for minor changes or incremental improvements because the significant 

design efforts include many changes that would not be required for smaller CO2 reductions. 

The EPA used the information gathered by ICF International to provide input on the cost for 

individual technologies which were used to build up non-recurring costs for these incremental 

improvements (a bottom-up approach). The technologies available to make incremental 

improvements to aircraft is wide ranging and aircraft specific. Some examples of technologies 

that could be integrated into an aircraft for incremental improvements include improved fan 

blade design or reduction in turbine clearances in the engine, reducing the gap between 

control surfaces, carbon brake pads, or advanced wing tip devices. As an example, ICF 

International estimated that depending on the additive nature of specific technologies and the 

magnitude improvement required, the cost to incrementally improve the Boeing 767 could 

range from approximately 230 million to 1.3 billion US dollars (3.5% to 11% metric value 

improvement).

257

2. Technology Responses

When CAEP started to develop the technology responses for the stringency options, a 

determination needed to be made on what level of technology could be considered as a 

 

257 ICF International, CO2 Analysis of CO2-Reducing Technologies for Aircraft, Final Report, EPA Contract 

Number EP-C-12-011, March 17, 2015.
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response to the standard. At the outset, CAEP decided the international CO2 standard would 

be a technology following standard, rather than a technology forcing one. This means that the 

international standard would reflect a level of emissions performance that is already achieved 

by some portion of current in-production aircraft. Additionally, CAEP determined in 2012

that all technology responses would have to be based on technology that would be in common 

use by the time the standard was to be decided upon in 2016 or shortly thereafter. This 

generation of technology was defined within CAEP as a Technology Readiness Level (TRL) 

8

258

– an actual system completed and qualified through test and demonstration – by 2016 or 

shortly thereafter. This means that the technology responses considered for the future 

international CO2 standard, going into effect internationally in 2020 or 2023 for new types 

and potentially in 2023 or later for in-production, are based on what will be in operation by

2016 or shortly thereafter. EPA is requesting comment on the approach of establishing a new 

type standard in 2020 or 2023 and in-production standard for 2023 and later based on proven 

2016 technologies. In addition the EPA requests comment on how the purpose of the 

standard, to achieve reductions beyond what would have been achieved in the absence of a 

standard, can be attained considering the technology response assumptions agreed to at 

CAEP. 

 

258 TRL8 is a measure of Technology Readiness Level. “Actual system completed and "mission qualified" 

through test and demonstration in an operational environment.” TRL is a scale of 1 to 9, 1 being conceptual 

design, and 9 being successful operational use. As defined by NASA. 
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3. Cost Effectiveness Analysis

CAEP is currently conducting the cost effectiveness analysis for new-type and inproduction aircraft. With rare exceptions CAEP has historically developed new type only 

standards. To model cost impacts of a new type standard, CAEP has historically used an 

assumption that the in-production aircraft will respond to the new type standard, even though 

the standard would not apply to them and has assumed that the aviation sector is competitive 

enough that market forces will drive manufacturers to voluntarily upgrade their fleet to meet 

any new type aircraft standard. This scenario is modeled no differently from a mandatory inproduction standard. The EPA requests comment on modeling cost and environmental 

impacts of new-type standards based on the assumed attainment of such emissions levels by

in-production aircraft.

Because CAEP has modeled all in-production aircraft as responding by the 

implementation date of the new-type standard, CAEP has by definition, performed an inproduction analysis. More stringent options for new-type aircraft may be restricted due to the 

assumed in-production impacts.

CAEP has recognized that its past methods for modeling a new-type only standard (by 

assuming in-production aircraft comply) may not be sufficient for the CO2 standard analysis. 

Thus, CAEP developed new methods to model what cost and environmental impacts would 

result from only new types being regulated under a new-type emission standard. CAEP is still 

determining the best way to conduct an analysis of impacts only on new types using the 
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agreed upon technology responses and cost estimates. The EPA requests comments on how 

to model cost impacts for only new types for the future international CO2 standard, if it were 

to apply only to new types. The EPA also requests comment on how the modeling should be 

conducted to differentiate in-production and new type scenarios. 

G. Request for Comment on the EPA’s Domestic Implementation of International 

CO2 Standards

As described earlier in section II.E, traditionally international emission standards for 

aircraft engines have first been adopted by ICAO, and subsequently the EPA has initiated 

rulemakings to establish domestic standards that are of at least equal stringency as ICAO’s 

engine standards. However, the Chicago Convention,259 which established ICAO, recognizes 

that ICAO member states may adopt their own unique standards that are more stringent than 

ICAO standards. A participating member state (or nation) that adopts more stringent 

standards is obligated to notify ICAO of the differences between its standards and ICAO’s

standards.

260

 

 

259 ICAO, 2006: Doc 7300-Convention on International Civil Aviation, Ninth edition, Document 7300/9. 

Available at: http://www.icao.int/publications/ICAO Products & Services 2015 catalogue/cat_2015en.pdf (last 

accessed May 12, 2015). The ICAO Document 7300 is found on page 1 of the ICAO Products & Services 2015 

catalog and is copyright protected; Order No. 7300.

260 According to the Chicago Convention, a participating member State that adopts regulations or practices 

differing in any particular respect from those established by an international standard is obligated to notify ICAO 

of the differences between its standards and ICAO’s standards. However, member States that wish to use 

aircraft in international transportation must adopt emissions standards and other recommended practices that are 

at least as stringent as ICAO’s standards. Member States may ban the use of any aircraft within their airspace 

that does not meet ICAO standards.
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Section 231(b) of the CAA requires that any emission standards “take effect after such 

period as the Administrator finds necessary (after consultation with the Secretary of 

Transportation) to permit the development and application of the requisite technology, giving 

appropriate consideration to the cost of compliance during such period.” 42 U.S.C. 7571(b). 

Section 231(a)(2)(B) provides that the Administrator shall consult with the Administrator of 

the FAA on standards, and “shall not change the aircraft engine emission standards if such 

change would significantly increase noise and adversely affect safety.” 42 U.S.C. 

7571(a)(2)(B). 

As discussed in the 2005 rule (CAEP/4 aircraft engine NOx standard),

261 the EPA 

needs to have a technical basis for expecting the standards will be achievable in a specific 

period of time. While the statutory language of section 231 is not identical to other provisions 

in title II of the CAA that direct the EPA to establish technology-based standards for various 

types of mobile sources, the EPA interprets its authority under section 231 to be similar to 

those provisions that grant us significant discretion to identify a reasonable balance of 

specified emissions reduction, and cost without adversely affecting safety or increasing noise.

See, e.g., Husqvarna AB v. EPA, 254 F.3d 195 (DC Cir. 2001) (upholding the EPA’s 

promulgation of technology-based standards for small non-road engines under section 

213(a)(3) of the CAA). In this regard, we note CAEP’s intent for the purpose of the 

 

261 U.S. EPA, “Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test 

Procedures;” Final Rule, 70 FR 2521, November 17, 2005.
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international CO2 standard (as accepted by the CAEP Steering Group in 2011), which is to 

achieve aircraft CO2 emissions reductions beyond that which would have occurred in the 

absence of a standard. 

In ruling on a petition for judicial review of the 2005 rule,

262 the U.S. Court of 

Appeals for the D.C. Circuit held that the EPA’s approach in that action of tracking the ICAO 

standards was reasonable and permissible under the CAA. NACAA v. EPA, 489 F.3d 1221, 

1230-32 (D.C. Cir. 2007). The Court also held that section 231 of the CAA confers a broad 

degree of discretion on the EPA to adopt aircraft emission standards that the Agency

determines are reasonable. Id. 

Although the EPA has traditionally established domestic standards that track the 

ICAO standards, for purposes of having a robust ANPPR process, we ask for comment on the 

possibility of the EPA adopting a more stringent aircraft engine emissions standard than 

ICAO, provided ICAO/CAEP promulgates a standard in 2016 and the EPA makes a positive 

endangerment finding. In the same vein, the EPA requests that commenters consi der the 

following factors (among others): the potential to reflect the CO2 emissions performance of 

products from U.S. manufacturers, competitive advantages and disadvantages for U.S.

manufacturers, certification reciprocity with certificating authorities of other nations, and the 

EPA’s role in the ongoing ICAO negotiations. In addition, the EPA asks for comment on 

 

262 Ibid.
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what action the EPA should take if the ICAO/CAEP process fails to result in the adoption of 

an aircraft CO2 emissions standard.

VII. Statutory Authority and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 

13563: Improving Regulation and Regulatory Review

This action is a significant regulatory action because it raises novel policy issues. 

Accordingly, it was submitted to the Office of Management and Budget (OMB) for review. 

This action proposes a finding that GHG emissions from aircraft cause or contribute to air 

pollution that may be reasonably anticipated to endanger public health and welfare along with 

an ANPR which provides an overview of the international efforts to reduce GHG emissions, 

progress to date in establishing global aircraft standards that achieve meaningful CO2

reductions and, if the EPA finds that aircraft GHG emissions do cause or contribute to 

endangerment, the potential use of CAA section 231 to implement these standards 

domestically ensuring transparency and the opportunity for public comment. Any changes 

made in response to OMB recommendations have been documented in the docket. 

B. Paperwork Reduction Act (PRA)

This action does not impose an information collection burden under the PRA. The 

proposed endangerment and cause or contribute findings under CAA section 231 do not

contain any information collection activities. 
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C. Regulatory Flexibility Act (RFA) 

I certify that this action will not have a significant economic impact on a substantial 

number of small entities under the RFA. This action will not impose any requirements on 

small entities. The proposed endangerment and cause or contribute findings under CAA 

section 231 do not in-and-of-themselves impose any new requirements but rather set forth the 

Administrator’s proposed determination that GHG emissions from certain classes of aircraft 

engines—those used in U.S. covered aircraft—cause or contribute to air pollution that may be 

reasonably anticipated to endanger public health and welfare. Accordingly, this action affords 

no opportunity for the 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 (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 

1531–1538, and does not significantly or uniquely affect small governments. The action 

imposes no enforceable duty on any state, local or tribal governments or the private sector.

E. Executive Order 13132: Federalism

This action 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.
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F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

This action does not have tribal implications as specified in Executive Order 13175. 

The proposed endangerment and cause or contribute findings under CAA section 231 do not 

in-and-of-themselves impose any new requirements but rather set forth the Administrator’s 

proposed determination that GHG emissions from certain classes of aircraft engines—those 

used in U.S. covered aircraft—cause or contribute to air pollution that may be reasonably 

anticipated to endanger public health and welfare. Thus, Executive Order 13175 does not 

apply to this action. 

G. Executive Order 13045: Protection of Children from Environmental Health Risks 

and Safety Risks 

This action is not subject to Executive Order 13045 because it is not economically

significant as defined in Executive Order 12866. The Administrator considered climate 

change risks to children as part of this proposed endangerment finding under CAA section 

231. This action’s discussion of climate change impacts on public health and welfare is found 

in section IV of this preamble. Specific discussion with regard to children are contained in 

sections IV and I.D of the preamble titled “Children’s Environmental Health.” A copy of all 

documents pertaining to the impacts on children’s health from climate change have been 

placed in the public docket for this action. 
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H. Executive Order 13211: Actions Concerning Regulations That Significantly 

Affect Energy Supply, Distribution or Use

This action is not a “significant energy action” because it is not likely to have a 

significant adverse effect on the supply, distribution or use of energy. Further, we have 

concluded that this action is not likely to have any adverse energy effects because the 

proposed endangerment and cause or contribute findings under section 231 do not in-and-of 

themselves impose any new requirements but rather set forth the Administrator’s proposed 

determination that GHG emissions from certain classes of aircraft engines—those used in 

U.S. covered aircraft— cause or contribute to air pollution that may be reasonably anticipated 

to endanger public health and welfare.

I. National Technology Transfer and Advancement Act (NTTAA)

This action does not involve technical standards. 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations

The EPA believes this action will not have potential disproportionately high and 

adverse human health or environmental effects on minority, low-income, or indigenous

populations because this action does not affect the level of protection provided to human 

health or the environment. The Administrator considered climate change risks to minority, 

low-income, and indigenous populations as part of this proposed endangerment finding under 

CAA section 231. This action’s discussion of climate change impacts on public health and 
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welfare is found in section IV of the preamble. Specific discussion with regard to minority, 

low-income, and indigenous populations are found in sections IV and I.E of this preamble 

titled “Environmental Justice.” A copy of all documents pertaining to the impacts on these 

communities from climate change have been placed in the public docket for this action. 

K. Determination Under Section 307(d)

Section 307(d)(1)(V) of the CAA provides that the provisions of section 307(d) apply 

to “such other actions as the administrator may determine.” Pursuant to section 307(d)(1)(V),

the Administrator determines that this action is subject to the provisions of section 307(d).

VIII. Statutory Provisions and Legal Authority

Statutory authority for this action comes from 42 U.S.C. sections 7571, 7601 and 

7607. 

Dated: _______________________________

_____________________________________

Gina McCarthy,

Administrator