[Federal Register Volume 81, Number 157 (Monday, August 15, 2016)]
[Rules and Regulations]
[Pages 54421-54475]
[FR Doc No: 2016-18399]

Vol. 81

Monday,

No. 157

August 15, 2016

Part V

 Environmental Protection Agency

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40 CFR Parts 87 and 1068

Finding That Greenhouse Gas Emissions From Aircraft Cause or Contribute
to Air Pollution That May Reasonably Be Anticipated To Endanger Public
Health and Welfare; Final Rule

Federal Register / Vol. 81 , No. 157 / Monday, August 15, 2016 /
Rules and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 87 and 1068

[EPA-HQ-OAR-2014-0828; FRL-9950-15-OAR]
RIN 2060-AS31

Finding That Greenhouse Gas Emissions From Aircraft Cause or
Contribute to Air Pollution That May Reasonably Be Anticipated To
Endanger Public Health and Welfare

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: In this action, the Administrator finds that elevated
concentrations of greenhouse gases in the atmosphere endanger the
public health and welfare of current and future generations within the
meaning of section 231(a)(2)(A) of the Clean Air Act (CAA, or Act). She
makes this finding specifically with respect to the same six well-mixed
greenhouse gases--carbon dioxide (CO2), 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 CAA and that together
constitute the primary cause of the climate change problem. The
Administrator also finds that emissions of those six well-mixed
greenhouse gases from certain classes of engines used in certain
aircraft are contributing to the air pollution--the aggregate group of
the same six greenhouse gases--that endangers public health and welfare
under CAA section 231(a)(2)(A).

DATES: These findings are effective on September 14, 2016.

ADDRESSES: 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 Web site. Although listed in the
index, some information is not publicly available, e.g., confidential
business information (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<PHONE_NUMBER>, and the telephone
number for the Air Docket is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: Lesley Jantarasami, Office of
Atmospheric Programs, Climate Change Division, Environmental Protection
Agency, 1200 Pennsylvania Ave. NW., Mail Code 6207-A, Washington, DC
20460; Telephone number:<PHONE_NUMBER>; Email address:
<EMAIL_ADDRESS>For additional information regarding these
final findings, please go to the Web site http://www3.epa.gov/otaq/climate/regs-aviation.htm.

SUPPLEMENTARY INFORMATION:

Judicial Review

    Under CAA section 307(b)(1), judicial review of this final action
is available only by filing a petition for review in the U.S. Court of
Appeals for the District of Columbia Circuit by October 14, 2016. This
final action is a nationally applicable action because it triggers the
EPA's statutory duty to promulgate aircraft engine emission standards
under CAA section 231, which are nationally applicable regulations and
for which judicial review will be available only in the U.S. Court of
Appeals for the District of Columbia Circuit. In the alternative, even
if this action were considered to be only locally or regionally
applicable, the Administrator determines that it has nationwide scope
and effect within the meaning of CAA section 307(b)(1) both because of
the obligation to establish standards under CAA section 231 that it
triggers and because it concerns risks from GHG pollution and
contributions to such pollution that occur across the nation. Under CAA
section 307(d)(7)(B), only an objection to this final action that was
raised with reasonable specificity during the period for public comment
can be raised during judicial review. This section also provides a
mechanism for us to convene a proceeding for reconsideration, ``[i]f
the person raising an objection can demonstrate to [EPA] that it was
impracticable to raise such objection within [the period for public
comment] or if the grounds for such objection arose after the period
for public comment (but within the time specified for judicial review)
and if such objection is of central relevance to the outcome of this
rule.'' Any person seeking to make such a demonstration to us should
submit a Petition for Reconsideration to the Office of the
Administrator, Environmental Protection Agency, Room 3000, William
Jefferson Clinton Building, 1200 Pennsylvania Ave. NW., Washington, DC
20460, with a copy to the person listed in the preceding FOR FURTHER
INFORMATION CONTACT section, and the Associate General Counsel for the
Air and Radiation Law Office, Office of General Counsel (Mail Code
2344-A) Environmental Protection Agency, 1200 Pennsylvania Ave. NW.,
Washington, DC 20460.

Table of Contents

I. General Information
    A. Does this action apply to me?
II. Introduction: Overview and Context for This Final Action
    A. Summary
    B. Background Information Helpful To Understanding This Final
Action
    C. The EPA's Responsibilities Under the Clean Air Act
    D. U.S. Aircraft Regulations and the International Community
III. Legal Framework for This Action
    A. Section 231(a)(2)(A)--Endangerment and Cause or Contribute
    B. Air Pollutant, Public Health and Welfare
IV. The Administrator's Finding Under CAA Section 231 That
Greenhouse Gases Endanger Public Health and Welfare
    A. The Science Upon Which the Agency Relied
    B. The Air Pollution Consists of Six Key Well-Mixed Greenhouse
Gases
    C. The Air Pollution Is Reasonably Anticipated To Endanger Both
Public Health and Welfare
    D. Summary of the Administrator's Endangerment Finding Under CAA
Section 231
V. The Administrator's Cause or Contribute Finding for Greenhouse
Gases Emitted by Certain Classes of Engines Used by Covered Aircraft
Under CAA Section 231
    A. The Air Pollutant
    B. The Administrator's Finding Under CAA Section 231(a)(2)(A)
That Greenhouse Gas Emissions From Certain Classes of Aircraft
Engines Used in Certain Aircraft Cause or Contribute to Air
Pollution That May Be Reasonably Anticipated To Endanger Public
Health and Welfare
    C. Response to Key Comments on the Administrator's Cause or
Contribute Finding
VI. 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. Congressional Review Act (CRA)
    L. Determination Under Section 307(d)
VII. Statutory Provisions and Legal Authority

I. General Information

A. Does this action apply to me?

    These final findings trigger new duties that apply to the EPA but
do not themselves apply new requirements to other entities outside the
federal government. Specifically, in issuing these final findings that
emissions of the six well-mixed GHGs from certain classes of engines
used in certain aircraft cause or contribute to air pollution which may
reasonably be anticipated to endanger public health or welfare, the EPA
becomes subject to a duty under CAA section 231 to propose and
promulgate aircraft engine emission standards applicable to emissions
of that air pollutant from those classes of engines. We are
anticipating indicating an expected timeline for proposed GHG emission
standards for the classes of aircraft engines included in the
contribution finding in EPA's Unified Agenda of Federal Regulatory and
Deregulatory Actions. Only those future standards will apply to and
have an effect on other entities outside the federal government.
Entities potentially interested in this final action include 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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    \1\ Manufacturers of new aircraft engines refers to
manufacturers of new type engines and in-production engines, and
manufacturers of new aircraft refers to manufacturers of new type
aircraft and in-production aircraft.

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                                                                               Examples of Potentially Affected
                  Category                   NAICS \a\ Code   SIC \b\ Code               Entities \1\
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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
final action. This table lists the types of entities that the EPA is
now aware could potentially have an interest in this final action. By
issuing these final findings under CAA section 231(a)(2)(A) regarding
emissions of greenhouse gases from aircraft engines, the EPA is now
required to undertake a separate notice and comment rulemaking to
propose and issue emission standards applicable to greenhouse gas
emissions from the classes of aircraft engines subject to the findings,
and the Federal Aviation Administration (FAA) is to prescribe
regulations to ensure compliance with EPA's future emissions standards
pursuant to CAA section 232. 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 final action, consult the person listed in the preceding
FOR FURTHER INFORMATION CONTACT section.

II. Introduction: Overview and Context for This Final Action

A. Summary

    Pursuant to CAA section 231(a)(2)(A), the Administrator finds that
emissions of the six well-mixed \2\ greenhouse gases (GHGs) from
certain classes of aircraft engines used in certain types of aircraft
(referred to interchangeably as ``covered aircraft'' or ``US covered
aircraft'' throughout this document) contribute to air pollution that
may reasonably be anticipated to endanger the public health and welfare
of current and future generations. This final action follows the
Administrator's proposed findings,\3\ and responds to public comments
submitted to the EPA following that proposal. It is based on careful
consideration of the scientific evidence, as well as a thorough review
of the public comments. In light of the large number of comments
received and overlap between many comments, EPA has not responded to
each comment individually. Instead, EPA has summarized and provided
responses to each significant argument, assertion and question
contained within the totality of these comments. Covered aircraft are
those aircraft to which the International Civil Aviation Organization
(ICAO) has agreed the recently recommended international CO2
standard will apply \4\: 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 include smaller jet aircraft such as the
Cessna Citation CJ3+ 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 include larger turboprop aircraft,
such as the ATR 72 and the Bombardier Q400.
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    \2\ The term ``well-mixed GHGs''--used both in the definition of
``air pollution'' in the endangerment finding and in the definition
of ``air pollutant'' in the cause or contribute finding--is based on
the fact that these gases are sufficiently long lived in the
atmosphere such that, once emitted, concentrations of each gas
become well mixed throughout the entire global atmosphere. These
shared attributes are one of five primary reasons that the EPA
considers the six gases as an aggregate group rather than as
individual gases. See section IV.B for more information on the
definition of ``air pollution'' and section V.A for more information
on the definition of the ``air pollutant.''
    \3\ U.S. EPA, 2015: 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, 80 Federal
Register (FR) 37758 (July 1, 2015).
    \4\ ICAO, 2013: CAEP/9 Agreed Certification Requirement for the
Aeroplane CO2 Emissions Standards, Circular (Cir) 337, 40 pp, AN/
192, Available at: http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed May 9, 2016). The ICAO Circular 337
is found on page 87 of the catalog and is copyright protected; Order
No. CIR337.
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    In this final action, the EPA is informed by and places
considerable weight on the extensive scientific and technical evidence
in the record supporting the 2009 Endangerment and Cause or Contribute
Findings under CAA section 202(a) (hereafter, collectively referred to
as the 2009 Endangerment Finding).\5\ This includes the major, peer-
reviewed scientific assessments that were used to address the question
of whether elevated concentrations of GHGs in the

atmosphere endanger public health and welfare under CAA section 202(a),
as well as 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)(2)(A). Furthermore, this finding under section
231(a)(2)(A) 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 the public health
and welfare of current and future generations. No information or
assessments 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).
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    \5\ 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).
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    The Administrator defines the ``air pollution'' referred to in
section 231(a)(2)(A) of the CAA to be the combined mix of
CO2, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride (henceforth the six ``well-
mixed GHGs''). 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 welfare of current and future generations within
the meaning of CAA section 231(a)(2)(A). The Administrator is not at
this time making a finding regarding whether other substances emitted
from aircraft engines cause or contribute to air pollution which may
reasonably be anticipated to endanger public health or welfare.
    Under CAA section 231(a)(2)(A), 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 using 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)(2)(A)--that is, the aggregate group of the same six well-mixed
GHGs. With respect to this pollutant, based on the data summarized in
section V.B, the Administrator finds that emissions of the six well-
mixed GHGs from aircraft engines used in covered aircraft contribute to
the air pollution that endangers public health and welfare under
section 231(a)(2)(A). The Administrator is not at this time making a
cause or contribute finding regarding GHG emissions, or emissions of
other substances, from engines used in non-covered aircraft.
    The Administrator's final 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 CAA section
231(a)(2)(A) for the GHG emissions from aircraft. Further, the EPA
anticipates that the 39th ICAO Assembly will approve a final
CO2 emissions standard in October 2016, and that
subsequently, ICAO will formally adopt the final CO2
emissions standard in March 2017. These final endangerment and cause or
contribute findings for aircraft engine GHG emissions are also part of
preparing for a subsequent domestic rulemaking process under CAA
section 231. If an international standard is approved and finalized by
ICAO, member states that wish to use aircraft in international
transportation will then be required under the Chicago Convention \6\
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. This document does not
take action or respond to comments on the 2015 U.S. EPA Aircraft
Greenhouse Gas Emissions Advance Notice of Proposed Rulemaking
(henceforth the ``2015 ANPR''),\7\ which discussed such standards.
Technical issues and comments for the 2015 ANPR would be addressed in a
future notice of proposed rulemaking related to such standards.
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    \6\ ICAO, 2006: Convention on International Civil Aviation,
Ninth Edition, Document 7300/9, 114 pp. Available at: http://www.icao.int/publications/Documents/7300_9ed.pdf (last accessed May
9, 2016).
    \7\ 80 FR 37758 (July 1, 2015).
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B. Background Information Helpful to Understanding This Final Action

1. Greenhouse Gases and Their Effects
    GHGs in the atmosphere have the effect of trapping 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 degrees
Celsius or 1.53 degrees Fahrenheit.\8\ The Intergovernmental Panel on
Climate Change's (IPCC) 2013-2014 Fifth Assessment Report concluded
that 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.\9\ Further information about 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 determination under CAA section 231(a)(2)(A) (EPA-HQ-OAR-2014-
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 endangerment finding under CAA section
231.
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    \8\ 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.
    \9\ Ibid.
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    The U.S. transportation sector constitutes a meaningful part of
total U.S. and global anthropogenic GHG emissions. In 2014, aircraft
remained the single largest GHG-emitting transportation source not yet
subject to any GHG standards. Aircraft clearly contribute to U.S.
transportation emissions, accounting for 12 percent of all U.S.
transportation GHG emissions and representing more than 3 percent of
total U.S. GHG emissions in 2014.\10\

Globally, U.S. aircraft GHG emissions represent 29 percent of all
global aircraft GHG 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 cause or
contribute finding under CAA section 231(a)(2)(A).
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    \10\ U.S. EPA, 2016: Inventory of U.S. Greenhouse Gas Emissions
and Sinks: 1990-2014, 1,052 pp., U.S. EPA Office of Air and
Radiation, EPA 430-R-16-002, April 2016. Available at: www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html (last accessed
June 14, 2016).
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2. Statutory Basis for This Final Action
    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 propose and issue final standards
addressing emissions of an air pollutant 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.\11\ If the
Administrator answers both questions in the affirmative, she must
propose and issue final 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 document summarizes the legal
framework for this final action under CAA section 231. Typically, past
endangerment and cause or contribute findings have been proposed and
promulgated concurrently with proposed and promulgated standards under
various sections of the CAA, including section 231. In those actions,
public comment was taken on the proposed findings as part of the notice
and comment process for the proposed 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 at 66 FR 5002 (January 18, 2001). However, there
is no requirement that the Administrator propose or finalize the
endangerment and cause or contribute findings concurrently with the
related standards. See 74 FR 66502 (December 15, 2009). As explained in
the 2009 Endangerment Finding, nothing in section 202(a) requires the
EPA to propose or issue endangerment and cause or contribute findings
in the same rulemaking, and Congress left the EPA discretion to choose
an approach that satisfied the requirements of section 202(a). See id.
The same analysis applies to section 231, which is analogous to section
202(a). The EPA is choosing to finalize these findings at this time for
a number of reasons, including its previous commitment to issue such
findings in response to a 2007 citizens' petition.\12\
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    \11\ To clarify the distinction between air pollution and air
pollutant, in the context of GHGs, the air pollution is the
atmospheric concentrations and can be thought of as the total,
cumulative stock of GHGs in the atmosphere. The air pollutant, on
the other hand, is the emissions of GHGs and can be thought of as
the flow that changes the size of the total stock.
    \12\ 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 31, 2007.
Available at http://www3.epa.gov/otaq/aviation.htm (last accessed
April 8, 2016). EPA, 2012: Response to the Petition for Rulemaking
Under the Clean Air Act to Reduce the Emission of Air Pollutants
from Aircraft that Contribute to Global Climate Change, June 14,
2012. Available at http://www3.epa.gov/otaq/aviation.htm (last
accessed April 8, 2016).
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    The Administrator has applied 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.'' \13\ CAA section 307(d)
provides specific procedural requirements for the EPA to follow in
taking certain rulemaking actions under the CAA, that apply in lieu of
the otherwise applicable provisions of the Administrative Procedure
Act, 5 U.S.C. 553-557, and 706. See, CAA section 307(d)(1). 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 findings were subject to the same rulemaking
procedures and requirements, as applicable, as would have applied if
they had been part of a standard-setting rulemaking.
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    \13\ 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 and the environment. Cars, trucks,
construction equipment, airplanes, and ships, as well as a broad range
of electricity generation, industrial, commercial and other facilities,
are subject to various CAA programs. Many of these programs are
targeted at ensuring protection of public health and welfare with a
margin of safety, others are directed at encouraging improved
industrial emissions performance and use of lesser polluting
technologies and processes, and some address the prevention of adverse
environmental effects. Implementation of the Act over the past four
decades has resulted in significant reductions in air pollution that
have benefited human health and the environment. The EPA's duties
regarding aircraft air pollution emissions under CAA section 231
reflect a combination of the CAA's goals to protect public health and
welfare and encourage improved emissions performance. This is shown by
section 231(a)(2)(A)'s directive that EPA first identify whether
emissions of aircraft engine air pollutants cause or contribute to air
pollution that may reasonably be anticipated to endanger public health
or welfare (which is broadly defined in section 302(h) of the CAA).\14\
This is also shown by section 231(b)'s subsequent requirement that
EPA's standards, which may require improved emissions performance over
the status quo, provide sufficient time for the development and
application of requisite technology to meet emission standards, after
consideration of costs.
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    \14\ Previously the EPA has made the prerequisite endangerment
and cause or contribute findings under CAA section 231(A) that
formed the basis to begin addressing the issue of various aircraft
pollutants including NOX aircraft pollution. U.S. EPA,
``Control of Air Pollution from Aircraft and Aircraft Engines,
Emission Standards and Test Procedures for Aircraft.'' Final Rule,
38 FR 19088 (July 17, 1973). See also section IV.B.7.d of this
preamble for a discussion of previous NOX section 231(A)
findings.
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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 combined mix in the
atmosphere of the six well-mixed GHGs--CO2, methane, nitrous
oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride--
endanger the public health and welfare of current and future
generations.
     Cause or Contribute Finding: The Administrator found that
the combined emissions of the six well-mixed GHGs from new motor
vehicles and new motor vehicle engines contribute to the GHG pollution
which threatens public health and welfare.
    The Administrator made both of these findings with respect to the
six well-mixed GHGs, recognizing that CAA section 202(a) sources emit
only four of the six substances. 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).\15\ 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).\16\ On August 29, 2012,
the EPA finalized the second phase of the GHG emission standards for
light-duty vehicles (2017-2025 model years), further reducing GHG
emissions from light-duty vehicles.\17\ In 2014, the President directed
the EPA and the Department of Transportation to set standards in 2016
that further increase fuel efficiency and reduce GHG emissions from
medium- and heavy-duty vehicles.\18\
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    \15\ 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).
    \16\ U.S. EPA, 2011: Greenhouse Gas Emissions Standards and Fuel
Efficiency Standards for Medium- and Heavy-Duty Engines and
Vehicles; Final Rule, 76 FR 57106 (September 15, 2011).
    \17\ 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).
    \18\ 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:
https://www.whitehouse.gov/the-press-office/2014/02/18/remarks-president-fuel-efficiency-standards-medium-and-heavy-duty-vehicl
(last accessed April 27, 2016).
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    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 more than 6.25 billion metric
tons.19 20
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    \19\ U.S. EPA, 2012: EPA and NHTSA Set Standards to Reduce
Greenhouse Gases and Improve Fuel Economy for Model Years 2017-2025
Cars and Light Trucks. Document No. EPA-420-F-12-051, 10 pp.
Available at http://www.epa.gov/otaq/climate/documents/420f12051.pdf
(last accessed April 27, 2016). See also Table 7.4-2 in U.S. EPA,
2012: Regulatory Impact Analysis: Final Rulemaking for 2017-2025
Light-Duty Vehicle Greenhouse Gas Emissions Standards and
Corporation Average Fuel Economy Standards, EPA-420-R-12-016, 555
pp. Available at: https://www3.epa.gov/otaq/climate/documents/420r12016.pdf (last accessed April 27, 2016).
    \20\ U.S. EPA, 2011: Greenhouse Gas Emissions Standards and Fuel
Efficiency Standards for Medium-and Heavy-Duty Engines and Vehicles;
Final Rule, 76 FR 57106 (September 15, 2011).
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    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.\21\ 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.\22\ In August 2015, after notice and
comment rulemaking, the EPA finalized two carbon pollution rulemakings:
One for new, modified, and reconstructed electric utility generating
units \23\ and another for existing power plants.\24\
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    \21\ Executive Office of the President, 2013: The President's
Climate Action Plan, June 25, 21 pp. Available at: http://www.whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf (last accessed April 8, 2016).
    \22\ 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 April 8, 2016).
    \23\ U.S. EPA, 2015: Standards of Performance for Greenhouse Gas
Emissions From New, Modified, and Reconstructed Stationary Sources:
Electric Utility Generating Units; Final Rule, 80 FR 64510 (October
23, 2015).
    \24\ U.S. EPA, 2014: Carbon Pollution Emission Guidelines for
Existing Stationary Sources: Electric Utility Generating Units;
Final Rule, 80 FR 64661 (October 23, 2015). On February 9, 2016 the
Supreme Court stayed this rule pending judicial review. The Court's
stay order does not articulate a basis for the stay and does not
address the merits of the rule.
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    In the Climate Action Plan, the President also indicated that the
United States 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.\25\ The
final 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 document, total U.S. aircraft GHG emissions in
2014 represented 12 percent of GHG emissions from the U.S.
transportation sector,\26\ and in 2010, the latest year with complete
global emissions data, U.S. aircraft GHG emissions represented 29
percent of global aircraft GHG emissions.27 28 U.S. aircraft
GHG emissions are projected to increase by 43 percent over the next two
decades.\29\

See section V of this preamble for more information about the data
sources that comprise the aircraft GHG emissions inventory.
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    \25\ Executive Office of the President, 2013: The President's
Climate Action Plan, June 25, 21 pp. Available at http://www.whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf (last accessed April 8, 2016).
    \26\ U.S. EPA, 2016: Inventory of U.S. Greenhouse Gas Emissions
and Sinks: 1990-2014, 1,052 pp., U.S. EPA Office of Air and
Radiation, EPA 430-R-16-002, April 2016. Available at: www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html (last accessed
June 14, 2016).
    \27\ Ibid.
    \28\ 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.
    \29\ As discussed in section V.B.4.c, fuel burn growth rates for
air carriers and general aviation aircraft operating on jet fuel are
projected to grow by 43 percent from 2010 to 2036, and this provides
a scaling factor for growth in GHG emissions which would increase at
a similar rate as the fuel burn by 2030, 2036, and 2040. FAA, 2016:
FAA Aerospace Forecast Fiscal Years 2016-2036, 94 pp. Available at
https://www.faa.gov/data_research/aviation/aerospace_forecasts/media/FY2016-36_FAA_Aerospace_Forecast.pdf (last accessed April 8,
2016).
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2. Background on the Aircraft Petition, the 2008 Advance Notice of
Proposed Rulemaking, and the 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.\30\ 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.
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    \30\ 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, 26 pp.
Available at http://www.epa.gov/otaq/aviation.htm (last accessed
April 8, 2016) and Docket EPA-HQ-OAR-2014-0828.
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    Following the Supreme Court's decision in Massachusetts v. EPA in
2007, the EPA issued an advance notice of proposed rulemaking (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, 44468-73 (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.
    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 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 the 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 the 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
District of Columbia Circuit (also called the ``D.C. Circuit'' in this
document).
    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.
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    \31\ U.S. EPA, 2012: Memorandum in Response to Petition
Regarding Greenhouse Gas Emissions from Aircraft, June 14, 11 pp.
Available at http://www.epa.gov/otaq/aviation.htm (last accessed
April 8, 2016) and Docket EPA-HQ-OAR-2014-0828.
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    Meanwhile, the Court of Appeals upheld the 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 these findings
regarding aircraft engine GHG emissions as a further step toward
responding to the 2007 Petition for Rulemaking.
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    \32\ Petitions for certiorari were filed in the Supreme Court,
and the Supreme Court granted six of those petitions but ``agreed to
decide only one question: `Whether EPA permissibly determined that
its regulation of greenhouse gas emissions from new motor vehicles
triggered permitting requirements under the Clean Air Act for
stationary sources that emit greenhouse gases.' '' Utility Air Reg.
Group v. EPA, 134 S. Ct. 2427, 2438 (2014); see also Virginia v.
EPA, 134 S. Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418
(2013), and CRR, 134 S. Ct. 468 (2013) (all denying cert.). Thus,
the Supreme Court did not disturb the D.C. Circuit's holding that
affirmed the 2009 Endangerment Finding.
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D. U.S. Aircraft Regulations and the International Community

    The EPA and the FAA traditionally work within the standard-setting
process of ICAO's Committee on Aviation Environmental Protection (CAEP
or the Committee) to establish international emission standards and
related requirements, which individual nations later adopt into
domestic law in fulfillment of their obligations under the Convention
on International Civil Aviation (Chicago Convention). 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 that are at least
as stringent as ICAO's standards. This approach has been affirmed as a
reasonable way to implement the Agency's duties under CAA section 231
by the U.S. Court of Appeals for the D.C. Circuit. Nat'l Ass'n of Clean
Air Agencies (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 regulations to ensure compliance with
these standards when issuing certificates under its authority under
Title 49 of the United

States Code. These final endangerment and cause or contribute findings
for aircraft GHG emissions are in preparation for this domestic
emissions standards rulemaking process.
1. International Regulations and U.S. Obligations
    The EPA has 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 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
international standards and regulations 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 United States is currently one of 191 participating
ICAO member states.34 35
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    \33\ ICAO, 2006: Convention on International Civil Aviation,
Ninth Edition, Document 7300/9, 114 pp. Available at: http://www.icao.int/publications/Documents/7300_9ed.pdf (last accessed
April 20, 2016).
    \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 April 8, 2016).
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    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 that 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\
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    \36\ ICAO, 2006: Doc 7300-Convention on International Civil
Aviation, Ninth Edition, Document 7300/9, 114 pp. Available at
http://www.icao.int/publications/Documents/7300_9ed.pdf (last
accessed April 8, 2016).
---------------------------------------------------------------------------

    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. 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 GHG emissions on the global climate.
    The Chicago 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, the Chicago
Convention indicates that member states are required to recognize the
airworthiness certificates of any state whose standards are at least as
stringent as ICAO's standards.\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, 114 pp. Available at
http://www.icao.int/publications/Documents/7300_9ed.pdf (last
accessed April 8, 2016).
    \38\ ICAO, 2006: Convention on International Civil Aviation,
Article 33, Ninth Edition, Document 7300/9, 114 pp. Available at
http://www.icao.int/publications/Documents/7300_9ed.pdf (last
accessed April 8, 2016).
    \39\ ICAO, 2006: Convention on International Civil Aviation,
Article 38, Ninth Edition, Document 7300/9, 114 pp. Available at
http://www.icao.int/publications/Documents/7300_9ed.pdf (last
accessed April 8, 2016).
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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\ 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. 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).41 42
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    \40\ ICAO: CAEP Terms of Reference. Available at http://www.icao.int/environmental-protection/Pages/Caep.aspx#ToR (last
accessed April 27, 2016).
    \41\ ICAO, 2008: Aircraft Engine Emissions, International
Standards and Recommended Practices, Environmental Protection, Annex
16, Volume II, Third Edition, July, 110 pp. Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed
April 8, 2016). The ICAO Annex 16 Volume II is found on page 19 of
the ICAO Products & Services 2016 catalog and is copyright
protected; Order No. AN16-2.
    \42\ CAEP develops new emission standards based on an assessment
of the technical feasibility, cost, and environmental benefit of
potential requirements.
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    At CAEP meetings, the United States is represented by the FAA and
plays an active role.\43\ 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 environmental policy matters. In turn, the FAA assists and advises
the EPA on aviation environmental issues, technology and certification
matters.
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    \43\ 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.
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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 (HC), carbon monoxide (CO), and oxides of
nitrogen (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), and ICAO approved these
recommendations in 2011.\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, 110 pp.
Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed April 8, 2016). The ICAO Annex 16
Volume II is found on page 19 of the ICAO Products & Services 2016
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,
110 pp. Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed April 8, 2016). The ICAO Annex 16
Volume II is found on page 19 of the ICAO Products & Services 2016
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, 2014: Aircraft Engine Emissions, Annex 16, Volume II,
Third Edition, July 2008, Amendment 8, 108 pp. CAEP/8 corresponds to
Amendment 7 effective on July 18, 2011. Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed
April 8, 2016). The ICAO Annex 16 Volume II is found on page 19 of
the ICAO Products & Services 2016 catalog and is copyright
protected; Order No. AN16-2/E/11.
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2. The International Civil Aviation Organization'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\ The Resolution 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 up to the year 2050, and stabilizing
CO2 emissions at 2020 levels.\52\ 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 lower-carbon 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.
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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 2010.
    \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, 2009: Aspirational Goals and
Implementation Options, Working Paper HLM-ENV/09-WP/5, 5 pp.
Available at http://www.icao.int/Meetings/AMC/MA/High%20Level%202009/hlmenv_wp005_en.pdf (last accessed April 8,
2015).
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3. EPA's Regulation of Aircraft Emissions and the Relationship of the
Final Endangerment and Cause or Contribute Findings to International
Aircraft Standards
    As required by the CAA, the EPA has been engaged in reducing
harmful air pollution from aircraft engines for over 40 years,
regulating gaseous exhaust emissions, smoke, and fuel venting from
aircraft engines.\53\ We have periodically 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.\54\ These ICAO requirements are generally referred to
as CAEP/2 standards.\55\ The 1997 rulemaking 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) \56\ 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).\57\ It also included a CO emission standard for in-production
commercial turbofan engines.\58\ In 2005, we promulgated more stringent
NOX emission standards for newly certified commercial
turbofan
---------------------------------------------------------------------------

    \53\ U.S. EPA, 1973: Emission Standards and Test Procedures for
Aircraft; Final Rule, 38 FR 19088 (July 17, 1973).
    \54\ U.S. EPA, 1997: Control of Air Pollution from Aircraft and
Aircraft Engines; Emission Standards and Test rocedures; Final Rule,
62 FR 25355 (May 8, 1997).
    \55\ 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.
    \56\ This does not mean that in 1997 we promulgated requirements
for the re-certification or retrofit of existing in-use engines.
    \57\ 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.
    \58\ 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).

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engines.\59\ 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.60 61 The EPA's
actions to regulate certain pollutants emitted from aircraft engines
come directly from the authority in section 231 of the CAA, and we have
aligned the U.S. emissions requirements with those promulgated by ICAO.
All of these previous emission standards have generally been considered
anti-backsliding standards (most aircraft engines meet the standards),
which are technology-following.
---------------------------------------------------------------------------

    \59\ U.S. EPA, 2005: Control of Air Pollution from Aircraft and
Aircraft Engines; Emission Standards and Test Procedures; Final
Rule, 70 FR 69664 (November 17, 2005).
    \60\ 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).
    \61\ 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 CAA section 231(a)(2)(A) 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.
---------------------------------------------------------------------------

    In addressing CO2 emissions, ICAO has moved to
regulating a whole aircraft. ICAO explained its decision to regulate
pollutant emissions from the whole aircraft in a 2013 ICAO
circular.\62\ Several factors are considered when addressing whole-
aircraft CO2 emissions, as CO2 emissions are
influenced by aerodynamics, weight, and engine technology. Since the
aircraft-specific characteristics of aerodynamics and weight affect
fuel consumption, they ultimately affect CO2 engine exhaust
emissions. Rather than viewing CO2 as a measurable emission
from the engine alone, ICAO addresses CO2 emissions as an
aircraft-specific characteristic based on fuel consumption.
---------------------------------------------------------------------------

    \62\ ICAO, 2013: CAEP/9 Agreed Certification Requirement for the
Aeroplane CO2 Emissions Standard, Circular (Cir) 337, 40 pp, AN/192.
Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed April 8, 2016). The ICAO Circular 337
is found on page 87 of the ICAO Products & Services 2016 catalog and
is copyright protected; Order No. CIR337.
---------------------------------------------------------------------------

    The EPA has worked diligently over the past six years within the
ICAO/CAEP process on a range of technical issues regarding aircraft
CO2 emission standards. The 2015 ANPR discussed the issues
arising from those international proceedings and requested public
comment on a variety of issues to assist the Agency in developing its
position with regard to these issues, to help ensure transparency and
obtain views on aircraft engine GHG emission standards that it might
potentially adopt under the CAA.
    As described in the 2015 ANPR, in 2013 CAEP agreed on a metric \63\
to compare CO2 emissions from aircraft. The CO2
metric value is a comparative metric meant to differentiate between
generations of aircraft and to equitably capture improvements in
aerospace technology that contribute to a reduction in the airplane
CO2 emissions. The CO2 metric is not intended for
use as a direct measure of CO2 emissions rates or
operational fuel burn, rather it is a comparative measure of technology
on different aircraft.
---------------------------------------------------------------------------

    \63\ The CO2 metric is the average of three cruise
test points normalized by a dimensionless parameter representing
aircraft fuselage size. The units of the metric value are kilograms
of fuel burned per kilometer flown. However, because the metric is a
normalized value it cannot be used to estimate operational fuel burn
or emission rates of aircraft. The metric value is described in
detail in both ICAO Circular 337 and in section D of the 2015 ANPR.
ICAO, 2013: CAEP/9 Agreed Certification Requirement for the
Aeroplane CO2 Emissions Standard, Circular (Cir) 337, 40 pp., AN/
192, Available at http://www.icao.int/publications/catalogue/cat_2016_en.pdf (last accessed April 27, 2016). The ICAO Circular
337 is found on page 87 of the catalog and is copyright protected;
Order No. CIR337.
---------------------------------------------------------------------------

    Using this metric, CAEP considered and analyzed 10 different
stringency levels for both in-production and new type standards,
comparing aircraft with a similar level of technology on the same
stringency level. These levels were generically referred to numerically
from ``1'' as the least stringent to ``10'' as the most stringent,
which correspond to the upper and lower lines of constant technology,
respectively, from the 2015 ANPR. The 2015 ANPR described the range of
stringency levels under consideration at CAEP as falling into three
categories as follows: (1) CO2 stringency levels that could
impact \64\ only the oldest, least efficient aircraft in-production
around the world, (2) middle range CO2 stringency levels
that could impact many aircraft currently in-production and comprising
much of the current operational fleet, and (3) CO2
stringency levels that could impact aircraft that have either just
entered production or are in final design phase but will be in-
production by the time the international CO2 standards
becomes effective.\65\
---------------------------------------------------------------------------

    \64\ As described in the 2015 ANPR, the aircraft shown in
[Figure II.1 and II.2] are in-production and current in-development.
These aircraft could be impacted by an in-production standard in
that, if they were above the standard, they would need to either
implement a technology response or go out of production. For a new
type only standard there will be no regulatory requirement for these
aircraft to respond.
    \65\ 80 FR at 37797.
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    At its meeting in February of 2016, CAEP agreed on an initial set
of international standards to regulate CO2 emissions from
aircraft.\66\ It was agreed that these international standards should
apply to both new type and in-production aircraft. The applicability
date for the in-production standard was agreed to be later than for the
new type standard. CAEP explained that this will allow manufacturers
and certification authorities additional preparation time to
accommodate the standards. The new type and in-production stringency
levels for smaller and larger aircraft were agreed to be set at
different levels to reflect the range of technology being used and the
availability of new fuel burn reduction technologies that vary across
aircraft of differing size and weight. Table II.1 provides a brief
overview of the applicability dates and stringency levels of the
standards agreed to at ICAO/CAEP. As described earlier, CAEP considered
and analyzed 10 different stringency levels for both in-production and
new type standards (from 1 as the least stringent to 10 as the most
stringent).
---------------------------------------------------------------------------

    \66\ Further, the EPA anticipates that the 39th ICAO Assembly
will approve these CO2 emissions standards in October
2016, and that subsequently, ICAO will formally adopt these
CO2 emissions standards in March 2017.

           Table II.1--Stringency Levels and Applicability Dates for ICAO/CAEP CO2 Emission Standards
----------------------------------------------------------------------------------------------------------------
                                                                                In-production
                                                                                aircraft
                                           Aircraft MTOM thresholds     New type aircraft \67\        maximum
                                                     (kg)                maximum permitted CO2     permitted CO2
                                                                             metric value          metric value

----------------------------------------------------------------------------------------------------------------
Stringency Level........................  >5,700 to <60,000.........  \A\ 5.....................           \B\ 3
                                          Horizontal Transition \68\  \C\.......................             \D\
                                          60,000 to ~70,000.........
                                          > ~70,000.................  \E\ 8.5...................           \F\ 7
Applicability Date......................  Application for a new type  2020......................            2023
                                           certificate or a change    (2023 for planes with less
                                           to an existing type         than 19 seats).
                                           certificate.
                                          Production Cut Off........  n/a.......................            2028
----------------------------------------------------------------------------------------------------------------
\A\ Equation of ICAO Stringency Option #5: MV = 10-2.73780+(0.681310*\l\\o\\g\ 10(\M\\T\\O\\M\))+(-
  0.0277861*(\l\\o\\g\ 10(\M\\T\\O\\M\))2)
\B\ Equation of ICAO Stringency Option #3: MV = 10-2.57535+(0.609766*\l\\o\\g\ 10(\M\\T\\O\\M\))+(-
  0.0191302*(\l\\o\\g\ 10(\M\\T\\O\\M\))2)
\C\ Equation of New Type transition--60,000 to 70,395 kg: MV = 0.764
\D\ Equation of In-production transition--60,000 to 70,107 kg: MV = 0.797
\E\ Equation of ICAO Stringency Option #8.5: MV = 10-2.57535+(0.609766*\l\\o\\g\ 10(\M\\T\\O\\M\))+(-
  0.0191302*(\l\\o\\g\ 10(\M\\T\\O\\M\))2)
\F\ Equation of ICAO Stringency Option #7: MV = 10-1.39353+(-0.020517*\l\\o\\g\
  10(\M\\T\\O\\M\))+(0.0593831*(\l\\o\\g\ 10(\M\\T\\O\\M\))2)

    Figures II.1 and II.2 show a graphical depiction of both the new
type and in-production standards compared against the lines of constant
technology described in the 2015 ANPR and CO2 metric value
levels of current (as of February 2016) in-production and in-
development \69\ aircraft. The aircraft data shown were generated by
the EPA using a commercially available aircraft modeling tool called
PIANO.\70\ It should be noted that a number of the aircraft currently
shown as in-production are expected to go out of production and be
replaced by known in-development aircraft prior to both the new type
and the in-production CO2 standards going into effect
internationally.
---------------------------------------------------------------------------

    \67\ ``In Development'' aircraft shown in Figures II.1 and II.2
are the aircraft that were in development by manufacturers at the
time the 2015 ANPR was published.
    \68\ Stringency lines above and below 60,000 kilograms (MTOM)
are connected by a horizontal transition starting at 60,000
kilograms (MTOM) and continuing right (increasing mass) until it
intersects with the next level.
    \69\ Aircraft that are currently in-development but will be in
production by the applicability dates. These could be new types or
significant partial redesigned aircraft.
    \70\ PIANO (Project Interactive Analysis and Optimization),
Aircraft Design and Analysis Software by Dr. Dimitri Simos, Lissys
Limited, UK, 1990-present; Available at www.piano.aero (last
accessed April 8, 2016). This is a commercially available aircraft
design and performance software suite used across the industry and
academia. This model contains non-manufacturer provided estimates of
performance of various aircraft.
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BILLING CODE 6560-50-P

[GRAPHIC] [TIFF OMITTED] TR15AU16.000

[GRAPHIC] [TIFF OMITTED] TR15AU16.001

BILLING CODE 6560-50-C
    In this final action, the EPA is promulgating findings under
section 231(a)(2) that emissions of the six well-mixed GHGs from
certain classes of engines used in covered aircraft cause or contribute
to endangering air pollution. The EPA is not yet issuing proposed or
final emission standards, nor is the EPA taking final action that
prejudges what future standards will be. Instead, the EPA's final
endangerment and cause or contribute findings for aircraft GHG
emissions are in preparation for a subsequent, expected domestic

rulemaking process to adopt future GHG emissions standards. If the ICAO
Assembly, in October 2016, approves the final CO2 standards
and subsequently ICAO formally adopts the final CO2
standards in March 2017, the EPA's standards will need to be at least
as stringent as the ICAO CO2 aircraft standards for the
United States to meet its treaty obligations under the Chicago
Convention. As a result of these positive findings, the EPA is
obligated under section 231 of the CAA to set emission standards
applicable to GHG emissions from the classes of aircraft engines
included in the contribution finding, no matter the outcome of ICAO's
future actions in October 2016 and March 2017.

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 finds 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 final section 231 finding.
    Two provisions of the CAA govern this final action. 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, the six well-mixed 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.\71\
---------------------------------------------------------------------------

    \71\ 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. 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.\72\
---------------------------------------------------------------------------

    \72\ 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
courts. As the D.C. Circuit 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).
---------------------------------------------------------------------------

    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.\73\ The Administrator is thus to consider both
current and future risks.
---------------------------------------------------------------------------

    \73\ See id. at 121-122.
---------------------------------------------------------------------------

    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.\74\ 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)).
---------------------------------------------------------------------------

    \74\ 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)).
---------------------------------------------------------------------------

    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.\75\ At the same time, the Administrator must exercise
reasoned decision making, and avoid speculative inquiries.
---------------------------------------------------------------------------

    \75\ See id. at 121-122.
---------------------------------------------------------------------------

    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 subject source classes. Such an approach would
collapse the two prongs of the test by requiring that any climate
change impacts upon which an endangerment determination is made result
solely from the GHG emissions of aircraft. See 74 FR at 66542
(explaining the same point in the context of analogous language in
section 202(a)). Similarly, the Administrator is not, in making the
endangerment and cause or contribute findings, to consider the effect
of emissions reductions from the resulting standards.\76\ 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.
---------------------------------------------------------------------------

    \76\ 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 and welfare 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 section 231 finding, vulnerable
subpopulations face serious health and welfare 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.'' 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.' ''
Massachusetts v. EPA, 549 U.S. at 506, n.7 (citing Ethyl Corp.); see
also CRR, 684 F.3d at 121-122.
    In the 2009 Endangerment Finding, the Administrator recognized that
the scientific context for an action addressing climate change was
unique at that time because there was a very large and comprehensive
base of scientific information that had been developed over many years
through a global consensus process involving numerous scientists from
many countries and representing many disciplines. 74 FR at 66506. That
informational base has since grown. The Administrator also previously
recognized that there are varying degrees of uncertainty across many of
these scientific issues, which remains true. It is in this context that
she is exercising her judgment and applying the statutory framework in
this final 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 section 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
GHGs would effectively prevent EPA from doing the job that Congress
gave it in [section] 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. In addition, the absence of
the term ``significantly'' or any other word that modifies
``contribute'' shows that the EPA need not find that contributing
emissions cross a minimum percentage- or mass-based threshold to be
cognizable. 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 at

18891; see also 74 FR at 66506. 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 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). The committee
explained that its action addressed not only section 211(c)(1)(A) but
rather the entirety of the proposed legislative amendments, and stated
that the committee's bill would thus apply the interpretation of
section 211(c)(1)(A) in the en banc decision in Ethyl Corp. 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.
As both CAA sections 231 and 202 were included in the 1977 amendments,
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. EPA's interpretation of section 231 is the
same as its interpretation of the parallel language in section 202(a),
which is explained in the 2009 Endangerment Finding. See 74 FR at
18891; see also 74 FR at 66506.
    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.\77\ 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;\78\ (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.\79\
---------------------------------------------------------------------------

    \77\ 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.
    \78\ 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'').
    \79\ 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.
---------------------------------------------------------------------------

    In revising the statutory language, Congress relied heavily on the
en banc decision in Ethyl Corp. v. EPA, which reversed a three-judge
panel opinion regarding an EPA rule restricting the content of lead in
leaded gasoline.\80\ 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.
---------------------------------------------------------------------------

    \80\ 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 section 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 found, regulatory
action would not only precede, but prevent, a perceived threat. Id.
    The court also rejected petitioners' 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'' \81\ 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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    \81\ Throughout this document 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.\82\
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    \82\ 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.'' This
contrasts with other CAA provisions that expressly require
``significant'' contribution. Compare, e.g., CAA sections
110(a)(2)(D)(i)(I); 111(b); 213(a)(2), (4). In the absence of specific
language regarding the degree of contribution, the Administrator is to
exercise her judgment in determining contribution. Congress clearly
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 in
this context need not be significant. 74 FR at 66542. In Catawba County
v. EPA, 571 F.3d 20 (D.C. Cir. 2009), the court upheld EPA's
PM2.5 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'' that does not meet 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 from classes of new nonroad
engines or vehicles to ozone or carbon monoxide concentrations before
the EPA can regulate those engines or vehicles, while the EPA's view
was that the CAA requires a finding only of contribution. Id. at 13.
Section 213(a)(3)'s regulatory authority for specific classes of
nonroad engines or vehicles, like that of section 231(a)(2)(A) for
classes of aircraft engines, is triggered by a finding that certain
sources ``cause, or contribute to,'' air pollution, whereas an adjacent
provision, section 213(a)(2), is triggered by a finding of a
``significant'' contribution from all new and existing nonroad engines
and vehicles. The court looked at the ``ordinary meaning of
`contribute' '' when upholding the EPA's reading of section 213(a)(3).
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.\83\ The court found that the bare
``contribute'' language in section 213(a)(3) 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.\84\
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    \83\ 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.
    \84\ 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 measureable amount of pollutant
emissions to be resulting from the analyzed source category; a truly
trivial or de minimis ``contribution'' might not count as such
(although such a small level is not presented by the facts of today's
findings). 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.\85\ 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) does not require some level of contribution
that rises to a pre-determined numerical level or percentage- or mass-
based portion of the overall endangering air pollution.
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    \85\ Section V discusses the evidence in this case that supports
the 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.
---------------------------------------------------------------------------

    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
and weight of evidence (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
section 107(d) and finding it reasonable for the agency to apply a
totality of the circumstances approach); see also 74 FR at 66542.
Further discussion of this issue can be found in sections IV and V of
this preamble.
4. Summary of Responses to Key Legal Comments on the Interpretation of
the CAA Section 231(a) Endangerment and Cause or Contribute Test
    Here we summarize key public comments regarding the legal
interpretation of CAA section 231(a)(2)(A) that supports this finding
and the Agency's response. The Response to Comments document contains
the Agency's full response to comments on this topic.
    Some commenters strongly supported the proposed findings. These
comments stated, for example, that the proposed findings were clearly
authorized under CAA section 231(a)(2)(A) and further noted that the
U.S. Supreme Court had upheld EPA's authority under section 202(a) of
the CAA to make an endangerment finding with regard to GHG emissions
from motor vehicles and that the findings required under section
202(a)(1) are the same as the findings required under section
231(a)(2)(A). Another commenter, however, questioned the EPA's
authority to make endangerment and cause or contribute findings for
GHGs, stating that the EPA had not sufficiently explained its authority
to address pollutants other than NAAQS under CAA section 231. This
commenter made the following points in support of this view. First, the
comment pointed to the use of the term ``air quality control regions''
in CAA sections 231(a)(1)(A) and 231(a)(3) as suggesting that Congress
intended to authorize EPA to issue standards only for pollutants for
which a NAAQS has been established. Second, the comment stated that the
EPA should address this issue in light of a recent Supreme Court case,
Utility Air Regulatory Grp. v. EPA, 134 S.Ct. 2427 (2014).
    After consideration of these comments, we disagree with the
argument that Congress intended to only authorize the EPA to address
NAAQS pollutants under section 231(a)(2)(A). That provision of the Act
requires the EPA to issue 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.'' CAA
section 231(a)(2)(A) (emphasis added). Looking to that plain language,
there is nothing that limits the scope of the air pollutants that can
be found to contribute to possible endangerment, and therefore which
the EPA may be required to regulate, under that section to NAAQS
pollutants. To the contrary, the language is clear that the EPA would
be required to regulate aircraft engine emissions of ``any air
pollutant'' as long the pre-requisite endangerment and cause or
contribute findings are made. ``Air pollutant'' is not defined in
section 231; instead, the definition under CAA section 302(g) applies,
which states in relevant part that `` `air pollutant' means any air
pollutant agent or combination of such agents, including any physical,
chemical . . . substance or matter which is emitted into or otherwise
enters ambient air.'' CAA section 302(g) (emphasis added). Interpreting
this provision in Massachusetts v. EPA, the U.S. Supreme Court observed
that ``[o]n its face, the definition embraces all airborne compounds of
whatever stripe, and underscores that intent through the repeated use
of the word `any.' '' 549 U.S. 497, 529 (2007). It further stated that
``[b]ecause greenhouse gases fit well within'' this ``capacious
definition of `air pollutant' '' the EPA has the statutory authority to
regulate the emission of such gases from new motor vehicles under CAA
section 202(a)(2). Id. at 532. As noted above, sections 231(a)(2)(A)
and 202(a)(1) have parallel structures, use substantially the same
language, and use the same definition of air pollutant. As that
definition is ``unambiguous'' in its inclusion of GHGs, Massachusetts,
549 U.S. at 529, the Act clearly authorizes the EPA to make these
findings for GHGs under CAA section 231(a)(2)(A). Moreover, one U.S.
District Court has also ruled that the EPA has a duty to determine
whether aircraft engine emissions of GHGs cause or contribute to
endangerment, and that ruling was not appealed to the D.C. Circuit.
Center for Biological Diversity, et al. v. EPA, 794 F. Supp. 2d 151
(D.D.C. 2011). Consequently, the statutory language imposing the EPA's
duties under section 231(a)(2)(A), and relevant case law in the GHG
context, do not support the commenter's limited reading of the EPA's
authority under that language.
    The commenter points to the use of the term ``air quality control
regions'' in nearby paragraphs of CAA sections 231(a)(1)(A) and (a)(3)
to support its suggestion that Congress intended to limit the EPA's
analysis and regulatory authority to NAAQS pollutants in section
231(a)(2)(A). That argument is flawed for several reasons. The
commenter points to section 231(a)(1), which relates to a study the EPA
was to conduct of emissions of air pollutants from aircraft, and to
section 231(a)(3), which requires the EPA to hold public hearings with
respect to proposed standards under section 231(a)(2) in ``air quality
control regions . . . most seriously affected by aircraft emissions''
to the extent practicable. These obligations are imposed in addition to
those imposed by section 231(a)(2)(A), and their separate establishment
does not by that fact narrow the EPA's scope of authority regarding its
obligations imposed under section 231(a)(2)(A). They are additive, not
subtractive, duties. Moreover, one of those added

duties, to investigate the extent to which aircraft emissions affect
air quality in air quality control regions under section 231(a)(1)(A),
was a one-time duty that corresponded to NAAQS that have long-since
been revised, whereas the EPA's duty to propose and promulgate aircraft
emission standards is a continuing one to be conducted ``from time to
time'' under section 231(a)(2)(A). The commenter provides no reasoning
to explain why these provisions imposing additional duties should be
read to limit the scope of section 231(a)(2) beyond their proximity.
Sections 231(a)(1) and (a)(3) do not speak to what pollutants may be
addressed under section 231(a)(2). Further, there is no incompatibility
between the use of the term ``air quality control regions'' in those
provisions to identify geographic areas where certain activities are to
occur and making the endangerment and cause or contribute findings for
GHGs that are finalized in this action. In fact, the EPA long ago
discharged its one-time duty under CAA section 231(a)(1)(A) \86\ and,
after proposing new aircraft engine emission standards, could also meet
its obligations to hold public hearings in the air quality control
regions most seriously affected by aircraft emissions, to the extent
practicable, all while meeting its obligations under section
231(a)(2)(A). Accordingly, the EPA does not interpret sections
231(a)(1) and (a)(3) to limit the scope of the duties and authority
established by section 231(a)(2) to NAAQS pollutants. Further, the EPA
has previously implemented section 231(a)(2) to reach air pollutants
for which no NAAQS exists and has applied that provision to establish
standards for non-NAAQS pollutants, such as smoke. See, e.g., 40 CFR
87.21(a)-(c), (e), 87.23(a)-(c), and 87.31(a)-(c) emission standards
for smoke. The EPA's regulation of non-NAAQS smoke emissions from
aircraft engines has never been judicially challenged. Finally, even if
the Act were ambiguous, which it is not, the EPA's interpretation of
section 231(a)(2) to include authority to address GHGs, is reasonable
for the reasons described above.
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    \86\ USEPA, 1973: Aircraft Emissions: Impact On Air Quality And
Feasibility Of Control. U.S. Environmental Protection Agency, 102
pp. Available at http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=2000T6Z0.txt (last accessed April 26, 2016).
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    The U.S. Supreme Court's opinion in UARG cited by the commenter
does not change this analysis. The commenter misinterprets the UARG
decision to mean that for purposes of determining applicability of the
CAA's Prevention of Significant Deterioration (PSD) preconstruction
permitting program, ``air pollutant'' meant only pollutants for which
NAAQS had been established. The UARG decision, however, does not limit
PSD applicability to only NAAQS pollutants. In fact, the Court
recognized that such theories had been advanced during the course of
that litigation but expressly declined to consider them in its
decision. See 134 S.Ct. 2427, 2442 n.6 (2014). Rather, in UARG, the
Court's holding pertained only to GHGs. More specifically, the Court
held that the EPA may not treat GHGs as an air pollutant for the
specific purpose of determining whether a source is a major source (or
a modification thereof) and thus required to obtain a PSD permit or an
operating permit under title V of the CAA. Id. at 2449.
    Further, the regulatory context that was addressed in UARG is
distinguishable from that of this action. In UARG, the Court explained
that Massachusetts does not prevent an Agency from using statutory
context to infer that in some provisions ``air pollutant'' refers only
to those airborne substances that ``may sensibly be encompassed within
the particular regulatory program.'' 134 S.Ct. at 2441. However, the
commenter offers no reason why GHG emissions from U.S. covered aircraft
could not ``sensibly be encompassed'' under CAA section 231; nor is the
EPA aware of any such reasons. In fact, UARG itself recognizes a