[Federal Register Volume 80, Number 126 (Wednesday, July 1, 2015)]
[Proposed Rules]
[Pages 37757-37806]
[FR Doc No: 2015-15192]

Vol. 80

Wednesday,

No. 126

July 1, 2015

Part II

Environmental Protection Agency

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40 CFR Parts 87 and 1068

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; Proposed Rule

Federal Register / Vol. 80 , No. 126 / Wednesday, July 1, 2015 /
Proposed Rules

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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.

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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 greenhouse gas emissions
from certain classes of engines used in aircraft are contributing to
air pollution--the mix of greenhouse gases in the atmosphere--that
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 August 31,
2015.
    Public Hearing. The EPA will hold a public hearing on August 11,
2015 in Washington, DC, at the William Jefferson Clinton East Building,
Room 1153, 101 Constitution Avenue NW., Washington, DC 20004. If no one
contacts the EPA requesting to speak at the hearing for this proposal
by July 13, 2015 the public hearing will not take place and will be
cancelled with no further notice. Speakers should contact Ms. JoNell
Iffland (see FOR FURTHER INFORMATION CONTACT) to request to speak at
the hearing. The last day to pre-register in advance to speak at the
hearing will be August 6, 2015. The hearing will start at 10:00 a.m.
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 such as audio description, please let us know at
the time of registration. 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-HQ-OAR-2014-0828, by one of the following methods:
     Online: www.regulations.gov Follow the on-line
instructions for submitting comments.
     Email<EMAIL_ADDRESS>Attention Docket ID
No. EPA-HQ-OAR-2014-0828.
     Fax:<PHONE_NUMBER>, 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-HQ-OAR-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 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 email. The http://www.regulations.gov Web site 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: 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. EPA-HQ-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:<PHONE_NUMBER>; Fax number: (734)
214-4816; Email address<EMAIL_ADDRESS>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:

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 and 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
    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 Well-Mixed Greenhouse Gases Endanger Public Health and
Welfare
    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
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 the EPA's Domestic Implementation of
International CO2 Standards
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. UnFunded 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

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:

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                                                                                Examples of potentially affected
                Category                    NAICS \a\ Code      SIC \b\ Code                entities
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Industry................................            3364412               3724  Manufacturers of new aircraft
                                                                                engines.
Industry................................             336411               3721  Manufacturers of new aircraft.
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\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 this
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 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.
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.
     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 email. 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 show valid picture identification to the security staff
in order to gain access to the meeting room. Please note that the REAL
ID Act, passed by Congress in 2005, established new requirements for
entering federal facilities. These requirements took effect July 21,
2014. If your driver's license is issued by Alaska, American Samoa,
Arizona, Kentucky, Louisiana, Maine, Massachusetts, Minnesota, Montana,
New York, Oklahoma, or the state of Washington, you must present an
additional form of identification to enter the federal buildings where
the public hearings will be held. Acceptable alternative forms of
identification include: Federal employee badges, passports, enhanced
driver's licenses and military identification cards. 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 contact 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.
Oral testimony will be limited to no more than 10 minutes for each
commenter, although we may need to adjust the time for each speaker if
there is a large turnout. The EPA requests that commenters provide the
EPA with three copies of their oral testimony in hard copy form the day
of the hearing or an electronic copy in advance of the hearing date.
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, if we do not receive a request to
speak at the August 11, 2015 public hearing by July 13, 2015 the
hearing will be cancelled.

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 Policy \1\ and 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 been subject to
adequate peer review, then it is not necessary to have further peer
review of that information.\2\
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    \1\ U.S. EPA, 2006: EPA Peer Review Policy. Available at http://www.epa.gov/peerreview/ (Last accessed May 12, 2015).
    \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).
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    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 well-reviewed information. All relevant
peer review documentation is located in the docket for today's action
(EPA-HQ-OAR-2014-0828).
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    \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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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 Endangerment Finding \4\ indicate
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.
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    \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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    These assessments describe that children will be disproportionately
impacted by climate change given the unique physiological and
developmental factors that occur during this lifestage. 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.

E. Environmental Justice

    As described in detail in section IV below, the scientific evidence
and conclusions in the USGCRP, IPCC, and the NRC assessment reports
cited in the 2009 Endangerment Finding indicate that certain
populations are most vulnerable to 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
assessment reports strengthen these conclusions by providing more
detail regarding these populations' vulnerabilities and projected
impacts they may experience.
    In addition, the most recent assessment reports provide new
analysis about how low-income populations and some populations defined
jointly by ethnic/racial characteristics and geographic location are
vulnerable to certain climate change health impacts, raising
environmental justice concerns. Factors that contribute to increased
vulnerability to the health effects of climate change include limited
resources to adapt to and recover from climate impacts, as well as
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 be those aircraft to which ICAO has agreed the international
CO2 standard would apply: \5\ 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 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.
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    \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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    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,
(December 15, 2009) (collectively

referred to as the 2009 Endangerment Finding in this 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 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 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 of at least
equivalent stringency to those set by ICAO. Section II. D provides
additional discussion of the international aircraft standard-setting
process.

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 human-induced 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 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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    The U.S. transportation sector constitutes a meaningful part of
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 clearly contribute to 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.
---------------------------------------------------------------------------

    \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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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 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,
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\
---------------------------------------------------------------------------

    \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.
    \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).
---------------------------------------------------------------------------

    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 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.
---------------------------------------------------------------------------

    \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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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.
    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 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-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\
---------------------------------------------------------------------------

    \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).
    \13\ US EPA, 2011: Greenhouse Gas Emissions Standards and Fuel
Efficiency Standards for Medium- and Heavy-Duty Engines and
Vehicles; Final Rule, 76 Federal Register 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-medium-and-heavy-duty-vehicl
(last accessed May 12, 2015).

---------------------------------------------------------------------------

    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
---------------------------------------------------------------------------

    \16\ U.S. EPA. ``EPA and NHTSA Set Standards to Reduce
Greenhouse Gases and Improve Fuel Economy for Model Years 2017-2025
Cars and Light Trucks.'' Office of Transportation and Air Quality
Document No. EPA-420-F-12-051, August 2012. Available at http://www.epa.gov/otaq/climate/documents/420f12051.pdf (last accessed May
26, 2015). See also 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).
---------------------------------------------------------------------------

    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 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 plants \21\ and from existing power plants.\22\
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    \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 26, 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-press-office/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).
---------------------------------------------------------------------------

    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 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.
---------------------------------------------------------------------------

    \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).
    \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. Schl[ouml]mer, 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).
---------------------------------------------------------------------------

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 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.
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    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.
    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
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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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 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.
    The EPA issued a Response to the Aircraft Petition \31\ 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.
---------------------------------------------------------------------------

    \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-HQ-OAR-2014-0828.
---------------------------------------------------------------------------

    Meanwhile, the Court upheld EPA's section 202 findings in a
decision of a three-judge 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 this proposed findings
regarding aircraft engine GHG emissions as a further step toward
responding to the Petition for Rulemaking.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

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 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
---------------------------------------------------------------------------

    \33\ ICAO, 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 Web site: www.icao.int (last accessed May 12, 2015).
---------------------------------------------------------------------------

    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 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\
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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 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\
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    \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.
    \39\ ICAO, 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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    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
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.
---------------------------------------------------------------------------

    \40\ CAEP's terms of reference are available at http://www.icao.int/environmental-protection/Pages/Caep.aspx#ToR (last
accessed May 12, 2015).
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    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
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    \41\ Pursuant 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.
    \43\ CAEP develops new emission standards based on an assessment
of the technical feasibility, cost, and environmental benefit of
potential requirements.
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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 in 1998. The CAEP/4 standards applied only to new engine
designs certified (or newly-certified 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\
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    \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.
    \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.
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    \51\ A 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).

--. . . 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;
--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 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 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 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 standards align with ICAO's CAEP/6 and CAEP/8
requirements that became effective in 2013 and 2014,
respectively.59 60
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    \53\ U.S. EPA, 1973: Emission Standards and Test Procedures for
Aircraft; Final Rule, 38 FR 19088 (July 17, 1973).
    \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 in-use 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).
    \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 (May 8, 1997). 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.
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    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 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.''
---------------------------------------------------------------------------

    \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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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 (74 FR 18886, 18890-94 (April 24, 2009), and
74 FR 66496, 66505-10 (December 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 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 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 this 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\
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    \62\ See CRR, 684 F.3d at 117 (explaining two-part analysis
under section 202(a)).
---------------------------------------------------------------------------

    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, December 15, 2009.) As in
the section 202(a) context, this analysis entails a scientific judgment
by the Administrator about the potential risks posed by GHG emissions
to public health and welfare. See CRR, 684 F.3d at 117-118.\63\
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    \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).
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    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.
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    \64\ See id. at 121-122.
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    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 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)).
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    \65\ See id. at 122-123 (noting that the Sec.  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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    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.
---------------------------------------------------------------------------

    \66\ See id. at 121-122.
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    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 66542, December 15, 2009 (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 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.
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    \67\ As the D.C. Circuit explained in reviewing the 2009
Endangerment Finding under analogous language in section 202(a):
``At bottom, Sec.  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.
---------------------------------------------------------------------------

    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 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 66506, December
15, 2009. 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.
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 Sec.  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 Sec.  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'' 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-or-contribute 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 18891, (April 24, 2009); see also 74
FR 66506, (December 15, 2009). 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 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\
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    \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.
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    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 CAA \70\; (2) authorizes the Administrator
to reasonably project into the future and weigh risks; (3) assures the
consideration of the cumulative impact of all sources; (4) instructs
that the health of susceptible individuals, as well as healthy adults,
should be part of the analysis; and (5) indicates an awareness of the
uncertainties and limitations in information available to the
Administrator. H.R. rep. 95-294 at 49-50, 4 LH 2516-17.\71\
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    \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'').
    \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.
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    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.
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    \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 petitioners argued that the statutory language ``will
endanger'' required proof of actual harm, and that the actual harm had
to come from emissions from the fuels in and of themselves. Id. at 12,
29. The en banc court rejected this approach, finding that the term
``endanger'' allowed the Administrator to act when harm is threatened,
and did not require proof of actual harm. Id. at 13. ``A statute
allowing for regulation in the face of danger is, necessarily, a
precautionary statute.'' Id. Optimally, the court held, regulatory
action would not only precede, but prevent, a perceived threat. Id.
    The court also rejected petitioner's argument that any threatened
harm must be ``probable'' before regulation was authorized.
Specifically, the court recognized that danger ``is set not by a fixed
probability of harm, but rather is composed of reciprocal elements of
risk and harm, or probability and severity.'' Id. at 18. Next, the
court held that 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 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.
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    \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.
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    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\
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    \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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    Finally, the phrase ``cause or contribute'' ensures that all
sources of the contaminant which contribute to air pollution are
considered in the endangerment analysis (e.g., not a single source or
category of sources). It is also intended to require the Administrator
to consider all sources of exposure to a pollutant (for example, food,
water, and air) when determining risk. Id.
3. Additional Considerations for the Cause or Contribute Analysis
    By instructing the Administrator to consider whether emissions of
an air pollutant cause or contribute to air pollution, the statute is
clear that she need not find that emissions from any one sector or
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.
    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 66542, December 15, 2009. 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 (D.C. 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 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\
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    \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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    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 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 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.
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    \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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    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
Sec.  107(d) and finding it reasonable for the agency to adopt a
totality of the circumstances approach); see also 74 FR at 66542,
(December 15, 2009). 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, 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 this
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