Finding That Greenhouse Gas
Emissions from Aircraft Cause or
Contribute to Air Pollution That May
Reasonably Be Anticipated to Endanger
Public Health and Welfare
Response to Comments
&EPA
United States
Environmental Protection
Agency
-------
Finding That Greenhouse Gas
Emissions from Aircraft Cause or
Contribute to Air Pollution That May
Reasonably Be Anticipated to Endanger
Public Health and Welfare
Response to Comments
Climate Change Division
Office of Atmospheric Programs
U.S. Environmental Protection Agency
and
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
&EPA
United States
Environmental Protection
Agency
EPA-420-R-16-005
July 2016
-------
Finding that Greenhouse Emissions from Aircraft Cause or
Contribute to Air Pollution that May Reasonably be Anticipated to
Endanger Public Health and Welfare
Response to Comments Document
July 25, 2016
Table of Contents
1. Generally Support Endangerment and Cause or Contribute Findings 5
2. General Approach to the Science 6
3. The Endangerment Finding 8
3.1 Scope & Proposed Definition of "Air Pollution" 8
3.2 Additional Substances 13
3.2.1 Aerosols/black carbon 16
3.2.2 Water vapor, contrails 18
3.2.3 High-altitude NOx 21
3.3 Climate Science 23
3.4 Sectoral Impacts 31
4. The Cause or Contribute Finding 34
4.1 Scope & Proposed Definition of "Air Pollutant" 34
4.2 Analysis of Contribution 36
4.2.1 Including GHG Emissions from Combustion of International Aviation Bunker Fuels in the
U.S. Aircraft GHG Inventory 39
4.2.2. Finding Significant Contribution, or Establishing a Bright Line 40
4.2.3. Providing Context in Comparing Aircraft GHG Emissions to Other Sector GHG Emissions
41
4.2.4. Utilizing Multiple Databases for Global GHG Emissions 42
4.2.5. Past Aircraft Industry Fuel Efficiency Trends 43
4.3 Proposed Scope and Description of Covered Aircraft 45
4.3.1. Applicability Weight Thresholds Match Those of International CO2 Standard 45
4.3.2. Defining U.S. Covered Aircraft 46
4.3.3. Limiting the Contribution Finding to U.S. Covered Aircraft 47
4.4 Aircraft Emissions Projections 48
5. Miscellaneous Legal, Economic, and Other Comments 52
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FOREWORD
This document, together with the U.S. Environmental Protection Agency's (EPA's) Finding That
Greenhouse Gas Emissions From Aircraft Cause or Contribute To Air Pollution That May Reasonably Be
Anticipated To Endanger Public Health and Welfare, contains responses to public comments received on
the proposed finding, published at 80 Federal Register (FR) 37758 (July 1, 2015). The EPA accepted
comments on this proposal via online submission through regulations.gov as well as mail, e-mail, and fax,
and at one public hearing held in Washington, D.C. in August 2015. Copies of all comment letters
submitted and the transcript of the public hearing are available at the EPA Docket Center Public Reading
Room, or electronically through http://www.regulations.gov by searching Docket ID EPA-HQ-OAR-
2014-0828.
In light of the overlap between many comments, the EPA provided responses to all of the significant
issues raised by commenters, but did not provide responses to each individual commenter. Within each
comment summary, the EPA provides in parentheses lists of one or more Docket ID numbers for
comment letters that raised similar issues; however, these lists are not meant to be exhaustive and the
EPA does not individually identify each and every commenter who made a certain point in all instances,
particularly in cases where multiple commenters expressed essentially identical arguments.
The EPA's responses to comments are generally provided immediately following each summary of
comments. In some cases, the EPA has fully responded to specific comments or groups of similar
comments in the Federal Register Notice (FRN) for the final action, in which case the EPA references the
FRN rather than repeating those responses in this document. The responses presented in this document are
intended to augment the responses to comments that appear in the FRN for the final action and to address
comments not discussed in the FRN. Although portions of the Finding are paraphrased in this response to
comments (RTC) document, and some comments are discussed in the both documents, to the extent such
paraphrasing or overlap in comment responses introduces any confusion or apparent inconsistency it is
our intention that the FRN for the final action be viewed as definitive. This document, together with the
FRN, docket memoranda and related technical documents, should be considered collectively as the EPA's
response to all of the significant comments submitted on the EPA's proposed finding.
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List of Organizations Submitting Comments
Organization
Aerospace Industries
Association (AIA)
Aerospace Industries
Association of Brazil (AIAB)
Aerospace Industries
Association of Canada (AIAC)
Airbus S.A.S.
Aircraft Owners and Pilots
Association (AOPA)
AirFair
Airlines for America (A4A) and
the Air Line Pilots Association
(ALPA)
American Airlines, Inc
Association of European
Airlines (AEA)
Biogenic COi Coalition
Boeing Commercial Airplanes
California Air Resources Board
(ARE)
Cargo Airline Association
Center for Biological Diversity
Citizens United and Green
City of Santa Monica
Climate Viewer News
Corpirate Company - Climate
Environment Confirmation and
Responsibility (CECR)
Earth Guardians of Vermont
Earthjustice et al.
Embraer Aircraft Holding, Inc
Environmental Defense Fund
(EOF)
Friends of the Earth
General Aviation Manufacturers
Association
General Electric (GE)
Geoengineering Watch
Honeywell
Institute for Policy Integrity
Commenter Name(s)
Leslie Riegle
Walter B artels
Mark Beauregard
Charles Champion
David Oord
Steven Taber
Nancy Young
Howard Kass
N/A
N/A
Sheila Remes
Richard W. Corey
N/A
Doug Wolf
N/A
Kevin McKeown
James Lee
Harry Rhodes
Nicole Mardin
Sarah Burt
Gary J Spulak
Pamela Campos
Kate DeAngelis
Edward Smith
Gary Mercer and
Norman Liu
N/A
Chris Benich
Jason A. Schwartz
Docket File(s)
EPA-HQ-OAR-2014-0828-0570,
EPA-HQ-OAR-2014-0828-0797
EPA-HQ-OAR-2014-0828-0857
EPA-HQ-OAR-2014-0828-0858
EPA-HQ-OAR-2014-0828-0834,
EPA-HQ-OAR-2014-0828-0859
EPA-HQ-OAR-2014-0828-0751
EPA-HQ-OAR-2014-0828-0860
EPA-HQ-OAR-2014-0828-0747
EPA-HQ-OAR-2014-0828-0917
EPA-HQ-OAR-2014-0828-0736
EPA-HQ-OAR-2014-0828-0916
EPA-HQ-OAR-2014-0828-0568
EPA-HQ-OAR-2014-0828-0861
EPA-HQ-OAR-2014-0828-0732
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0915
EPA-HQ-OAR-2014-0828-0786
EPA-HQ-OAR-2014-0828-0359,
EPA-HQ-OAR-2014-0828-0484
EPA-HQ-OAR-2014-0828-0608
EPA-HQ-OAR-2014-0828-0920
EPA-HQ-OAR-2014-0828-0863
EPA-HQ-OAR-2014-0828-0745
EPA-HQ-OAR-2014-0828-0754,
EPA-HQ-OAR-2014-0828-0793
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0837
EPA-HQ-OAR-2014-0828-0932
EPA-HQ-OAR-2014-0828-0261
EPA-HQ-OAR-2014-0828-0757
EPA-HQ-OAR-2014-0828-0749
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Organization
International Air Transport
Association (IATA)
International Council on Clean
Transportation (ICCT)
Lissys Ltd.
Mass Comment Campaign
sponsored by anonymous 1
(email) - (24,558 duplicates)
National Air Carrier Association
(NACA)
National Association of Clean
Air Agencies (NACAA)
National Business Aviation
Association (NBAA)
National Tribal Air Association
(NTAA)
Natural Resources Defense
Council (NRDC)
Northeast States for Coordinated
Air Use Management
(NESCAUM)
Pratt & Whitney
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Private Citizen
Regional Airline Association
(RAA)
Rolls Royce
Sierra Club
StopSprayingUs-SF.com
The International Air Cargo
Association (TIACA)
Transport & Environment
UCLArts and Healing
Union of Concerned Scientists
Commenter Name(s)
Douglas E Lavin
Drew Kodjak
Dimitri Simos
N/A
A. Oakley Brooks
Nancy Seidman and
Barry Wallers tein
Douglas Can-
Bill Thompson
David Baake
Arthur Marin
Alan Epstein
Max Bliss
Steve Charnovitz
Rachael Gutierrez
Joseph Holmes
Michael Jimenez
James Lee
Mark and Leanna
McEnearney
Daniel Paschall
Michael Saraceno
Katherine Savarese
Liam Connolly
Paul Madden
Andres Restrepo
Patrick Roddie
N/A
Bill Hemming s
Ping Ho
David Babson
Docket File(s)
EPA-HQ-OAR-2014-0828-0548
EPA-HQ-OAR-2014-0828-0791
EPA-HQ-OAR-2014-0828-0203
EPA-HQ-O AR-20 14-0828-036 1 ,
EPA-HQ-OAR-2014-0828-0940
EPA-HQ-OAR-2014-0828-0546
EPA-HQ-OAR-2014-0828-0560
EPA-HQ-OAR-2014-0828-0567
EPA-HQ-OAR-2014-0828-0362
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0770
EPA-HQ-OAR-2014-0828-0569,
EPA-HQ-OAR-2014-0828-0794
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0147
EPA-HQ-OAR-2014-0828-0173
EPA-HQ-OAR-2014-0828-0412
EPA-HQ-OAR-2014-0828-0796
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0601
EPA-HQ-OAR-2014-0828-0174
EPA-HQ-OAR-2014-0828-0357
EPA-HQ-OAR-2014-0828-0178
EPA-HQ-OAR-2014-0828-0620,
EPA-HQ-OAR-2014-0828-0731
EPA-HQ-OAR-2014-0828-0795
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0927
(public hearing transcript)
EPA-HQ-OAR-2014-0828-0622
EPA-HQ-OAR-2014-0828-0862
EPA-HQ-OAR-2014-0828-0763
EPA-HQ-OAR-2014-0828-0526
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1. Generally Support Endangerment and Cause or Contribute
Findings
Organization: AirFair (EPA-HQ-OAR-2014-0828-0860)
California Air Resources Board (ARE) (EPA-HQ-OAR-2014-0828-0861, EPA-HQ-OAR-2014-0828-
0927 (public hearing transcript))
Cargo Airline Association (EPA-HQ-OAR-2014-0828-0732)
Center for Biological Diversity (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
City of Santa Monica (EPA-HQ-OAR-2014-0828-0786)
Earth Guardians of Vermont (EPA-HQ-OAR-2014-0828-0920)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Embraer Aircraft Holding, Inc (EPA-HQ-OAR-2014-0828-0745)
Friends of the Earth (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Institute for Policy Integrity (EPA-HQ-OAR-2014-0828-0749)
Lissys Ltd. (EPA-HQ-OAR-2014-0828-0203)
National Association of Clean Air Agencies (NACAA) (EPA-HQ-OAR-2014-0828-0560)
National Tribal Air Association (NTAA) (EPA-HQ-OAR-2014-0828-0362)
Natural Resources Defense Council (NRDC) (EPA-HQ-OAR-2014-0828-0927 (public hearing
transcript))
Northeast States for Coordinated Air Use Management (NESCAUM) (EPA-HQ-OAR-2014-0828-0770)
Private Citizen (EPA-HQ-OAR-2014-0828-0796)
Private Citizen (EPA-HQ-OAR-2014-0828-0173)
Private Citizen (EPA-HQ-OAR-2014-0828-0178)
Sierra Club (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Union of Concerned Scientists (EPA-HQ-OAR-2014-0828-0526)
Summary of Comments: Many commenters (0173, 0178, 0860, 0861, 0732, 0927, 0786, 0920, 0863,
0745, 0749, 0796, 0203, 0560, 0362, 0770, 0526) stated their general support or strong support for the
proposed endangerment and contribution findings. Some commenters discussed the scientific support for
the endangerment finding and the emissions data support for the contribution finding. A number of
commenters specifically noted that they support the findings because they view them as a first step in
establishing emissions standards under Clean Air Act (CAA) section 231; others described how the
findings will help address various concerns about the impacts of greenhouse gas (GHG) emissions on the
climate or the environment. One commenter, not persuaded that the U.S. Environmental Protection
Agency (EPA or the Agency) regulation of aircraft engine emissions is necessary, did not generally object
to this action because they recognized it was a part of the process for "future adoption of a CO2 standard
established by ICAO," which the commenter supports. A few commenters expressed appreciation for the
EPA using its authority under the CAA; one describes how the proposed endangerment and contribution
findings are consistent with other federal government policies to address climate change.
Response: The EPA agrees with the general statements of support from commenters for the
endangerment and contribution findings. The Agency acknowledges in section II. A of the Federal
Register Notice (FRN) for the final findings that these final endangerment and cause or contribute
findings for aircraft engine GHG emissions are a part of preparing for a subsequent domestic rulemaking
process under CAA section 231.
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2. General Approach to the Science
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
California Air Resources Board (ARE) (EPA-HQ-OAR-2014-0828-0861)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
National Association of Clean Air Agencies (NACAA) (EPA-HQ-OAR-2014-0828-0560)
Northeast States for Coordinated Air Use Management (NESCAUM) (EPA-HQ-OAR-2014-0828-0770)
Private Citizen (EPA-HQ-OAR-2014-0828-0796)
Summary of Comments: A number of commenters (0861, 0863, 0796, 0560, 0747) stated general
agreement with the EPA's approach to the scientific evidence underlying the endangerment finding.
Commenters (0861, 0796, 0560) support the EPA's statements in the proposal that the 2009
endangerment finding1 is firmly established and well-settled, and that there is no need to reopen or revisit
it in order to make an additional finding under section 231. One commenter (0861) states, "Since 2009,
new scientific assessments have further substantiated the detrimental effect of GHG emissions." One
comment letter (0863) calls GHGs "harmful pollution" and says, "As EPA acknowledges, the science is
abundantly clear, and there is no reason to delay any further a finding that reflects the overwhelming
scientific consensus." Another commenter (0770) also agrees with the Agency's use of scientific
assessments, arguing that they "reflect a robust peer review process, and incorporate thousands of climate
studies that have individually been peer reviewed as well.. .this large body of scientific evidence is
consistent with the Agency's peer review policy and OMB guidelines, as well as supported] by prior
legal precedent in Coalition for Responsible Regulation, Inc. v. EPA, 684 F.3d 102 (B.C. Cir. 2012)."
One comment letter (0747) states that the "EPA is required to rely upon the best scientific, statistical and
economic information available in making an endangerment finding, and EPA's analytical process and
the results forming the basis for such a finding must be transparent. The Proposed Endangerment Finding
appears to be consistent with that requirement."
Response: The EPA responds to comments stating general support for the Agency's approach to the
scientific evidence underlying the endangerment finding in section IV.A.I of the FRN. In addition to that
response in the FRN, the EPA provides further detail here. The EPA agrees that it uses the best available,
peer-reviewed science in Agency actions. We followed the EPA's Peer Review Policy,2 guidance in the
EPA's Peer Review Handbook,3 and Agency guidelines for information quality4 with respect to the
1 U.S. EPA, 2009: Endangerment and Cause or Contribute Findings for Greenhouse Gases Under section 202(a) of
the Clean Air Act; Final Rule, 74 FR 66496 (December 15, 2009) ("2009 Endangerment Finding").
2 U.S. EPA, 2006: Memorandum on Peer Review and Peer Involvement at the U.S. EPA, 4 pp. Available at
https://www.epa.gov/osa/memorandum-peer-review-and-peer-involvement-epa (last accessed July 11, 2016).
3 U.S. EPA, 2015: EPA Peer Review Handbook, Fourth Edition, 248 pp. Available at https://www.epa.gov/osa/peer-
review-handbook-4th-edition-2015-0 (last accessed July 11, 2016). Also, the EPA Science Advisory Board reviewed
this approach to the underlying technical and scientific information supporting this action, and concluded that the
approach had precedent and the action will be based on well-reviewed information. A copy of this letter and all
other relevant EPA peer review documentation is located in the docket for today's final action (EPA-HQ-OAR-
2014-0828).
4 Applicable guidance includes U.S. EPA 2012: Addendum to A Summary of General Assessment Factors for
Evaluating the Quality of Scientific and Technical Information, 9 pp. Available at
https://www.epa.gov/risk/guidance-evaluating-and-documenting-qualitv-existing-scientific-and-technical-
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scientific information supporting this action, which is primarily comprised of existing syntheses of the
literature by prominent scientific organizations that the EPA considers as highly influential scientific
assessments (HISAs). These assessments by the USGCRP, IPCC, and NRC draw synthesis conclusions
across thousands of individual peer-reviewed studies that appear in scientific journals, and the reports
themselves undergo additional peer review. We considered the processes and procedures employed by the
USGCRP, IPCC, and the NRC in terms of the EPA's five general assessment factors and information
quality factors of objectivity, integrity, utility, and transparency, including how these scientific
organizations have employed rigorous peer review processes. The EPA considers these assessments to
represent the best available science that maintains the highest level of adherence to Agency guidelines for
information quality. These assessments have been adequately peer reviewed in a manner commensurate
with the EPA's Peer Review Policy and guidance in the EPA's Peer Review Handbook. The EPA's Peer
Review Handbook explains that there are circumstances when peer review is generally not conducted for
HIS As; for example, if the Agency has determined that work has already been subject to adequate peer
review, as it has here, then it is generally not necessary to have further peer review of that information.
We also cite data from the annual Inventory of U.S. Greenhouse Gas Emissions and Sinks report (U.S.
Inventory), which we have determined to have been adequately reviewed in accordance the EPA's Peer
Review Handbook. We disaggregated data in one area of the U.S. Inventory for presentation purposes.
Though not considered influential scientific information, we chose to have the disaggregation
methodology peer reviewed in accordance with the EPA's Peer Review Handbook. The commenters'
views are consistent with section IV.A of the FRN, which describes how the Agency's approach to the
science provides assurance that the Administrator is basing her judgment on the best available, well-
vetted science that reflects the consensus of the climate science research community.
information (last accessed July 11, 2016) and U.S. EPA, 2002: Guidelines for Ensuring and Maximizing the
Quality, Objectivity, Utility, and Integrity of Information Disseminated by the EPA, 61 pp. Available at
https://www.epa.gov/qualitv/guidelines-ensuring-and-maximizing-qualitv-objectivitv-utilitv-and-integritv-
information (last accessed July 11, 2016).
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3. The Endangerment Finding
The EPA summarizes and responds to comments regarding the endangerment finding in the sections
below, 3.1,3.2, 3.3, and 3.4.
3.1 Scope & Proposed Definition of "Air Pollution"
Organization: Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570, EPA-HQ-
OAR-2014-0828-0797)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Biogenic CO2 Coalition (EPA-HQ-OAR-2014-0828-0916)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
California Air Resources Board (ARE) (EPA-HQ-OAR-2014-0828-0861)
Cargo Airline Association (EPA-HQ-OAR-2014-0828-0732)
Climate Viewer News (EPA-HQ-OAR-2014-0828-0359, EPA-HQ-OAR-2014-0828-0484)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Embraer Aircraft Holding, Inc (EPA-HQ-OAR-2014-0828-0745)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
General Electric (GE) (EPA-HQ-OAR-2014-0828-0932)
The International Air Cargo Association (TIACA) (EPA-HQ-OAR-2014-0828-0622)
International Air Transport Association (IATA) (EPA-HQ-OAR-2014-0828-0548)
National Association of Clean Air Agencies (NACAA) (EPA-HQ-OAR-2014-0828-0560)
National Tribal Air Association (NTAA) (EPA-HQ-OAR-2014-0828-0362)
Northeast States for Coordinated Air Use Management (NESCAUM) (EPA-HQ-OAR-2014-0828-0770)
Private Citizen (EPA-HQ-OAR-2014-0828-0796)
Private Citizen (EPA-HQ-OAR-2014-0828-0173)
Regional Airline Association (RAA) (EPA-HQ-OAR-2014-0828-0620, EPA-HQ-OAR-2014-0828-0731)
Summary of Comments: A commenter (0916) expressed concern about the EPA's proposed
endangerment finding because it does not differentiate between carbon dioxide (CO2) emissions that
result from the combustion of fossil fuels and those that result from the "combustion of biomass or
biofuels derived from herbaceous crops or crop residues, as well as biogenic CO2 emissions associated
with the production, gathering and processing of crops or crop residues used in bio-based products
including fuels."5 They argue that such crop-related biogenic CO2 emissions should be categorically
excluded from the endangerment finding because the COa released back to the atmosphere when emitted
from crop-derived biogenic sources contains the same carbon that was previously removed or sequestered
from CO2 in the atmosphere; does not contribute to elevated atmospheric concentrations of the six well-
mixed GHGs; and thus is "harmless from a global warming standpoint." The commenter requests that the
EPA "expressly confirm that such exclusion prevents any standard of performance for aircraft resulting
from any endangerment or cause or contribute finding under section 231 from making such CO2 'a
pollutant subject to regulation' for purposes of the PSD and Title V permitting programs under the Clean
Air Act." The commenter requests that if the EPA does not exclude such CO2 emissions categorically, the
5 Biogenic CO2 Coalition, 2015: Comments on EPA's 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, 80 FR 37757 (July 1, 2015). Docket ID number EPA-HQ-OAR-2014-0828-
0916. Available at www.regulations.gov (last accessed April 11, 2016).
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Agency should complete the development of and apply its Biogenic Accounting Framework within the
context of this endangerment finding and any future standards under section 231. The commenter cites
and incorporates by reference their previously submitted comment letter on the EPA's Proposed
Standards of Performance for Greenhouse Gas Emissions from New Stationary Sources: Electric
Generating Units (79 FR 1430, January 8, 2014), stating that their letter is the basis for their comments on
the CAA section 231 proposal.
Response: The EPA responded to public comments on 79 FR 1430 in the context of that rulemaking; all
documents in the record for that action can be found in Docket ID No. EPA-HQ-OAR-2013-0495 at
www.regulations.gov. Here, the EPA responds to comments pertaining specifically to the CAA section
231 proposal. The EPA responds to comments regarding crop-related biogenic CO2 emissions or the
source of the air pollution, for defining the relevant air pollution considered in the endangerment finding,
in section IV.B.6 of the FRN. In addition to that response in the FRN, the EPA provides further detail
here.
The EPA developed the 2014 revised report, Framework for Assessing Biogenic C02 Emissions from
Stationary Sources (hereafter, Framework}, to present a methodological framework for assessing the
extent to which the production, processing, and use of biogenic material at stationary sources for energy
production results in a net atmospheric contribution of biogenic CO2 emissions. The EPA is currently
engaged in a second round of targeted peer review with the Science Advisory Board on the Framework.
However, for the reasons stated in section IV.B.6 of the FRN, any findings reached in the context of the
Framework's technical process would not change the primary scientific basis of the definition of the air
pollution for purposes of this endangerment finding (i.e., that COa and the other well-mixed GHGs share
common physical characteristics and that no matter the original source of the CO2, the behavior of the
CO2 molecules in the atmosphere in terms of radiative forcing, chemical reactivity, and atmospheric
lifetime is effectively the same). Thus, the EPA does not agree that the Framework is relevant in the
context of this endangerment finding.
Because the EPA is not creating such an exclusion in this action, it need not address what the scope or
effect of such an exclusion might be for the Prevention of Significant Deterioration (PSD) and title V
permitting programs. Regulation of such substances under these permitting programs is a separate issue
that the EPA is handling separately. Moreover, as the EPA explained in a 2010 rulemaking, it does not
view an endangerment finding or cause and contribute finding under CAA section 202(a) alone as making
the requirements of the major source permitting programs applicable to a pollutant. See 75 Fed. Reg.
17012-13 (April 2, 2010) (discussing PSD) and id. at 17023 (applying a similar approach for title V as for
PSD). The same interpretation applies for an endangerment finding under CAA section 231, in light of the
analogous language and structure of sections 202 and 231. Moreover, as for the 2009 findings, while the
endangerment and contribution findings for aircraft GHG emissions under section 231(a)(2)(A) are a
necessary preliminary step to establishing emissions standards under CAA section 231, this action is not
establishing such emissions standards or otherwise establishing requirements for the actual control of
aircraft GHG emissions.
Summary of Comments: One comment letter (0747) agreed with the Agency's evaluation of the
six GHGs based on their common attributes and the fact that according to the commenter, "no new
developments in the scientific or technical literature since 2009 support expanding the scope of the
endangerment analysis beyond these six GHGs." However, the commenter questioned the EPA's decision
to aggregate the six gases rather than considering them individually for purposes of making the findings.
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Response: The EPA responds to comments suggesting a separate endangerment analysis for each of the
six well-mixed gases individually in section IV.B.6 of the FRN. In addition to that response in the FRN,
the EPA agrees with the commenter that the scientific assessment literature supports the Administrator's
decision to define the air pollution as the combined mix of the six well-mixed GHGs. The EPA is not at
this time taking final action to determine whether other climate forcers should be found to represent air
pollution within the meaning of CAA section 231(a)(2)(A).
Summary of Comments: Many commenters (0173, 0362, 0560, 0570, 0770, 0796, 0861, 0863, 0927)
stated their general support for the EPA's inclusion of the six well-mixed GHGs in the definition of the
"air pollution" for purposes of the endangerment finding under section 231(a)(2)(A) of the CAA. Many of
those commenters also supported the EPA's focusing of the scope of this endangerment finding on those
six well-mixed GHGs. One commenter states, "no new developments in the scientific or technical
literature since 2009 support expanding the scope of the endangerment analysis beyond these six GHGs."
Response: The EPA agrees that the best available science represented by the findings of the recent, major
assessments by the U.S. Global Change Research Program (USGCRP), the Intergovernmental Panel on
Climate Change (IPCC), and the National Research Council of the National Academies of Sciences,
Engineering, and Medicine (referred to interchangeably as NRC or NAS) support the Administrator's
decision to define the air pollution as the combined mix of the six well-mixed GHGs. The Administrator's
definition of air pollution for purposes of section 231(a)(2)(A) is made in light of (1) the evidence,
analysis, and conclusions that led to the 2009 endangerment finding; (2) more recent evidence from
scientific assessments published since 2009; and (3) consideration of public comments. The
Administrator reasonably and appropriately considered all information before her, and she maintains her
view that defining the scope and nature of the air pollution to be these six well-mixed GHGs remains
valid and well-supported by the current science.
Summary of Comments: Many commenters (0548, 0570, 0568, 0732, 0745, 0837, 0932, 0620, 0622)
highlighted the fact that aircraft engines emit only two of the six GHGs that together are defined as the
"air pollution" and "air pollutant" for purposes of the endangerment and contribution findings under
section 231 of the CAA. Commenters pointed out that the majority of emissions are CO2, and they
described nitrous oxide (NaO) emissions as "nominal (< 1 percent)," "trace," or "relatively small." Some
commenters ultimately conclude that the scope of the EPA's endangerment finding is acceptable, while
others say it would be more logical for the EPA to limit itself to CO2 or to CO2 and N2O (either by
limiting the definition of "air pollution" or by limiting the scope of future regulations).
Response: The EPA responds to comments regarding limiting the definition of air pollution in this action
to CO2 (or to CO2 and N2O), and limiting the scope of future regulations to CO2 in section IV.B.6 of the
FRN. In addition to that response in the FRN, the EPA provides further detail here. The EPA disagrees
with commenters suggesting that the fact that aircraft N2O emissions are about 1 percent of aircraft GHG
emissions means that N2O should be excluded from the scope of the relevant air pollution in this
endangerment finding. The endangerment and contribution findings constitute a two-step test. The first
step is for the Administrator to 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.
As part of the endangerment test, the Administrator must define the scope and nature of the relevant air
pollution for which she then must make a scientific judgment regarding the potential risks posed to public
health and welfare. As explained in section III. A of the FRN, the Administrator is to consider the
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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. 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. The EPA has defined the air
pollution as the aggregate group of the six well-mixed GHGs for five primary reasons detailed in
section IV.B of the FRN. In brief, they are: (1) they share common physical properties that influence their
climate effects; (2) on the basis of these common physical properties, they have been determined to be the
root cause of human-induced climate change, are the best-understood driver of climate change, and are
expected to remain the primary driver of future climate change; (3) they are the common focus of climate
change science research and policy analyses and discussions; (4) using the combined mix of these gases
as the definition (versus an individual gas-by-gas approach) is consistent with the science, because risks
and impacts associated with GHG-induced climate change are not assessed on an individual gas-by-gas
basis; and (5) using the combined mix of these gases is consistent with past EPA practice, where separate
substances from different sources, but with common properties, may be treated as a class (e.g., oxides of
nitrogen, paniculate matter, volatile organic compounds). The focus on the six well-mixed GHGs as the
air pollution is based on their shared physical characteristics and common attributes relevant to climate
change science and policy, which are not affected by consideration of the magnitude of the emissions
contributing to the air pollution. Accordingly, the EPA reasonably evaluated the six well-mixed GHGs
together in this endangerment finding, even though the subject source category for these findings only
emits two of those gases and emissions of one of those gases are approximately 1 percent of GHG
emissions from this category. The fact that these six substances within the definition of GHGs share
common, relevant attributes is true regardless of the subject source categories. Moreover, the
reasonableness of grouping these six chemicals as the relevant air pollution does not turn on the particular
source category or categories at issue for a particular action. Comments regarding the scope of the
definition of air pollutant are addressed in section 4.1 of this response to comments (RTC) document. The
EPA responds to comments regarding limiting the definition of air pollutant in this action to CO2 (or to
CO2 and N2O) and limiting the scope of future regulations to CO2 in section V.A.2 of the FRN.
Summary of Comments: A commenter (0359) disagreed with the scope of the endangerment finding and
requests a focus on the following: (1) metal particulates (e.g., lead, barium, aluminum); (2) trade-secret,
toxic chemicals used as additives in aviation fuel; and (3) cloud formation from jet exhaust (i.e., aviation-
induced cloudiness). The commenter argued that these substances or effects associated with aircraft
emissions pose a greater risk to human health than the six well-mixed GHGs, and states, "If the EPA is
truly concerned about aviation-induced climate change, they will regulate the production of contrails and
cirrus clouds which change our climate to a much greater extent than the sum of the six greenhouse gases
named in this Proposal." The commenter cited multiple examples of health risks such as Alzheimer's
disease, autism, and cancer that the commenter claims are associated with the aviation fuel itself, the
burning of aviation fuel, and the effects on clouds from burning aviation fuel. The commenter argued that
the EPA should consider other sources of scientific information conducted outside of a laboratory setting,
and provides links to information that the commenter claims verify the health impacts from the substances
listed above. The commenter concludes that such information supports focusing the endangerment finding
under CAA section 231 on the three categories identified above rather than the six well-mixed GHGs.
Response: The EPA responds to comments requesting inclusion of other climate forcing substances
associated with aircraft emissions beyond the six well-mixed GHGs in section IV.B.7 of the FRN. In
addition to those responses, the EPA provides further detail here.
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The EPA disagrees with the comments suggesting that the scientific and technical information considered
by the Administrator was limited to studies in a laboratory, and that the three categories of substances or
effects associated with aircraft emissions mentioned in the comment are more important drivers of climate
change than the six well-mixed GHGs. The EPA is giving careful consideration to all of the scientific and
technical information in the record, including information provided by the public. However, for the
reasons stated in section IV. A of the FRN, the Administrator considers recent, major assessments by the
USGCRP, IPCC, and the NRC as the primary scientific and technical basis informing the endangerment
finding. These major climate science assessments synthesize thousands of individual peer-reviewed
studies published in scientific journals that use a variety of methods (e.g., field observations, computer
modeling). The authors evaluate these individual studies in order to draw more general and overarching
conclusions about the state of science. The reports themselves then undergo additional peer review to
ensure that they accurately convey the consensus conclusions on what the body of scientific literature tells
us.
Contrary to the comment's suggestion, as reflected in these major climate science assessments, there is
extensive scientific evidence in the climate change assessment literature that the six well-mixed GHGs
together constitute the largest anthropogenic driver of climate change. The EPA has no reason to believe
that the assessment reports do not represent the best source material to determine the state of science and
the consensus view of the world's scientific experts on the issues central to making an endangerment
decision with respect to the six well-mixed GHGs. The commenter's suggestion that the scientific and
technical information considered by the Administrator was limited to studies in a laboratory is incorrect,
since the assessments incorporate many types of studies. The studies cited by the commenter do not
provide evidence challenging or refuting the role of the six well-mixed GHGs in driving climate change,
and do not support the claim that the Administrator should have drawn a different conclusion regarding
the scope of the relevant air pollution. The EPA responds to comments regarding the climate science
aspects of how emissions of metal particulates may play a role in cloud formation and the climate effects
of contrails/aviation-induced cloudiness in section 3.3 of this RTC document. The EPA responds to
comments regarding possible direct health effects of substances or effects associated with aircraft in
section 5.3 of this RTC document. The EPA is not at this time taking final action to determine whether
substances other than the six well-mixed GHGs that may have climate effects should be found to
represent air pollution within the meaning of CAA section 231(a)(2)(A).
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3.2 Additional Substances
Organization: Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570, EPA-HQ-
OAR-2014-0828-0797)
AirFair(EPA-HQ-OAR-2014-0828-0860)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
General Electric (GE) (EPA-HQ-OAR-2014-0828-0932)
International Air Transport Association (IATA) (EPA-HQ-OAR-2014-0828-0548)
Summary of Comments: A number of commenters (0548, 0570, 0747, 0837, 0932) agree with the
EPA's proposal that aerosol particles including black carbon, high-altitude nitrogen oxides (NOx), and
water vapor should not at this time be included in the definition of "air pollution" and "air pollutant"
under the endangerment and contribution findings because of significant scientific uncertainties regarding
their net effect on the Earth's climate. One commenter (0747) argues that their exclusion is appropriate
because "these agents do not fit into the regulatory and policy paradigm represented by the six well-mixed
GHGs identified in the Proposed Endangerment Finding: they are short-lived; they are not as well-studied
or understood as the six well-mixed GHGs; and the best science available indicates that their effect on
climate is either low/negligible or significant uncertainty surrounds their forcing effects." Another
commenter (0837) states that in some instances the radiative forcing effects of these substances can be
either positive or negative, and thus the "the science is not yet settled" on the contribution of these
additional substances to climate change.
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. The EPA has reviewed the commenters' points and finds that they are
generally consistent with the description in section IV.B.7 of the FRN regarding these substances' short-
lived nature in the atmosphere, and the scientific uncertainties about their net climate effects (as
illustrated by estimates of radiative forcing that span both positive and negative values). In section IV.B.4
in the proposal and in section IV.B.7 in the FRN, the EPA cited evidence from the assessment literature
that these other substances have radiative forcing effects that are dependent on a number of factors such
as the location and time of emissions, but the EPA did not describe their climate effects as "low" or
"negligible" in either section.
Summary of Comments: A joint comment letter (0863) argues that the EPA failed to properly examine
water vapor, NOx, and black carbon in its proposed endangerment and contribution findings, and thus
failed to do what the Agency committed to in its response to a 2007 petition from many of the joint
commenters.6 The commenter concludes that the discussion of the scientific uncertainties contained in the
proposal does not constitute careful examination of the issues. The commenter states that the EPA should
have quantified and included the effect of these additional substances in the Agency's discussion of
drivers of climate change. The commenter requests that the EPA finalize the proposed endangerment
finding for the six well-mixed GHGs, and also to "embark upon a second endangerment finding that both
6U.S. EPA (2012). Memorandum in Response to Petition Regarding Greenhouse Gas Emissions from Aircraft.
Available at https://www3.epa.gov/otaq/documents/aviation/oar-12-001 -0157-aircraft-petition-response-
memorandum.pdf (last accessed June 9, 2016).
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analyzes the effect of the remaining gases and addresses a broader range of aircraft, including smaller
turboprop and jet aircraft, piston-engine aircraft, and helicopters."
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. In this discussion, the EPA provides a careful examination of the issues that is
based on the best available, well-vetted science that reflected the consensus of the climate science
research community. As described there, the EPA considered the following assessments to obtain the best
estimates of these substances' radiative forcing: the IPCC Fifth Assessment Report (AR5),7 the IPCC
2007 Fourth Assessment Report (AR4),8 the IPCC Special Report: Aviation and the Global Atmosphere
(IPCC 1999),9 the NRC's Advancing the Science of Climate Change (NRC 2010),10 and the NRC's
Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project (NRC 1999).n The
USGCRP assessments have not dealt specifically with such emissions at high altitude. The EPA presented
quantitative estimates of radiative forcing from the assessment literature for aviation-induced cloudiness,
aviation-induced particles (including black carbon), and high-altitude NOx in the proposal and in
section IV.B.7 of the FRN. The EPA also cited evidence from the assessments that the climatic impact of
these other substances is dependent on a number of factors (including location and time of emission), and
that the scientific uncertainty for quantitative estimates of radiative forcing is greater for these other short-
lived substances (as illustrated by estimates of radiative forcing that span both positive and negative
values) than for the six well-mixed GHGs. The commenter provides no evidence that the EPA missed or
mischaracterized conclusions of the assessments regarding aviation climate impacts. Accordingly, the
EPA disagrees with the comment that it should at this time have included the effects of these additional
substances in the analysis of the drivers of climate change in this action. The EPA will take the
commenter's request to undertake a second endangerment finding that addresses the "remaining gases"
and a broader range of aircraft, under advisement and consideration among its other duties and priorities,
but is not prepared at this time to either reject or grant that request.
Summary of Comments: A commenter (0860) stated that aviation has radiative forcing effects that
contribute to climate change not only through the effects of CO2 emissions, but also through NOx
emissions, emissions of aerosols and their precursors (soot and sulfate), and increased cloudiness in the
form of persistent linear contrails and induced-cirrus cloudiness. The commenter noted that the EPA's
estimates of aircraft's contribution to climate change in terms of radiative forcing would be larger if these
other substances were taken into account, but did not ask the EPA to change its analysis or how it defines
"air pollution" and "air pollutant" under the endangerment and contribution findings.
7 IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change [Stacker, T.F., D. Qin, G.-K. Plattner, M.
Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University Press,
1535 pp.
8 IPCC, 2007: Climate Change 2007: The Physical Scientific Basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen,
M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)] Cambridge University Press, 996 pp.
9 IPCC, 1999: Aviation and the Global Atmosphere, Special Report to the Intergovernmental Panel on Climate
Change [Penner, I.E., D.H. Lister, DJ. Griggs, DJ.Dokken, M.McFarland (eds.)] Cambridge University Press, 373
pp.
10 NRC, 2010: Advancing the Science of Climate Change. The National Academies Press, 528 pp.
11 NRC, 1999: Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project. The National
Academies Press, 54 pp.
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Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. The EPA finds that the current climate science assessment literature does not
support a definitive conclusion that the net climate effects in terms of radiative forcing of aircraft
emissions would be larger if the effects of NOx, water vapor, and aerosols and their precursors were taken
into account. The EPA presented quantitative estimates of radiative forcing from the assessment literature
for aviation-induced cloudiness, aviation-induced particles, and high-altitude NOx in the proposal and in
section IV.B.7 of the FRN, noting their large uncertainty ranges. The EPA also described in detail the
scientific uncertainties regarding these estimates, including that their net climatic impact is dependent on
a number of factors such as location and time of emission, and that estimates of their radiative forcing
spans both positive and negative values. The radiative forcing effects of these substances are also
discussed in greater detail in section 3.3, "Climate Science," of this RTC document.
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3.2.1 Aerosols/Black Carbon
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
International Council on Clean Transportation (ICCT) (EPA-HQ-OAR-2014-0828-0791)
Summary of Comments: A commenter (0747) agrees with the EPA regarding scientific uncertainty of
the net climate effects of black carbon and supports not including black carbon in the definition of air
pollution for the purposes of this endangerment finding under CAA section 231(a). The commenter's
reasons include (1) that there is limited information available on the radiative forcing, global warming
potential (GWP) and global temperature potential (GTP) of black carbon; (2) that there are uncertainties
with regard to understanding and quantifying the effects of black carbon, which include aerosol-radiation
interaction, aerosol-cloud interactions, and albedo; and (3) that despite growing estimates of global mean
radiative forcing from black carbon recently, there is "substantial disagreement among experts about the
effects and climate impacts of black carbon," and these uncertainties extend to the "human health benefits
and climate change mitigation co-benefits associated with reducing black carbon." The commenter also
argues that there are no appropriate means, metrics, or methods available for quantifying black carbon
emissions from aircraft, which they argue provides further evidence that black carbon "simply does not fit
EPA's paradigm for the six GHGs identified in the Proposed Endangerment Finding, all of which can be
quantified and compared on a CC^e basis."
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 of the FRN. The commenter's arguments regarding scientific uncertainty for quantitative
radiative forcing estimates for aircraft black carbon emissions are generally consistent with the EPA's
discussion in section IV.B.7. The EPA cited evidence from the assessment literature that the magnitude of
aerosol effects can vary immensely with location and season of emissions, and that estimates of their total
climate forcing effect have a large uncertainty range. Varying GWP estimates are additional information
alongside other measures of radiative forcing demonstrating this variation in aerosol effects. While the
GWP is a measure of the heat absorbed over a given time period due to emissions of a gas, the GTP is a
measure of the temperature change at the end of that time period (again, relative to CO2).While the
calculation of the GTP is more complicated than that for the GWP, as it requires modeling how much the
climate system responds to increased concentrations of GHGs (the climate sensitivity) and how quickly
the system responds (based in part on how the ocean absorbs heat), GTP-based estimates also provide
similar information. The EPA responds to other comments regarding the global radiative forcing effects
of aircraft black carbon emissions in section 3.3, "Climate Science," of this RTC document. The other
uncertainties cited by the commenter (e.g., human health co-benefits of potential black-carbon mitigation
policies) are outside the scope of the EPA's inquiry in this action, which is focused on climate change
risks and impacts.
Regarding the commenter's statements about not being able to measure black carbon using CCV
equivalent (CO2e) units not fitting into "EPA's paradigm for the six GHGs," we reiterate that the EPA has
defined the air pollution as the aggregate group of the six well-mixed GHGs for five primary reasons
detailed in section IV.B of the FRN. In brief, they are: (1) they share common physical properties that
influence their climate effects; (2) on the basis of these common physical properties, they have been
determined to be the root cause of human-induced climate change, are the best-understood driver of
climate change, and are expected to remain the primary driver of future climate change; (3) they are the
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common focus of climate change science research and policy analyses and discussions; (4) using the
combined mix of these gases as the definition (versus an individual gas-by-gas approach) is consistent
with the science, because risks and impacts associated with GHG-induced climate change are not assessed
on an individual gas-by-gas basis; and (5) using the combined mix of these gases is consistent with past
EPA practice, where separate substances from different sources, but with common properties, may be
treated as a class (e.g., NOx, paniculate matter (PM), volatile organic compounds). The ability to quantify
and compare substances using CCVe units is not one of the foundational reasons for the Agency's focus
on the six well-mixed GHGs. As explained in section IV.B.7 of the FRN, the EPA is not at this time
including these substances in the definition of air pollution in this action, for reasons including that
aerosols such as black carbon do not share these five properties with the six well-mixed GHGs.
Summary of Comments: One commenter (0863) argued that the EPA should have included black carbon
in this endangerment finding, stating that the "EPA has noted that black carbon—the most strongly light-
absorbing component of PM—'is an important climate forcing agent.' As black carbon 'darkens bright
surfaces such as snow and ice,' it could have severe effects on ice melt and warming in the Arctic." The
commenter requested the EPA to complete a separate endangerment finding analyzing other climate
forcers emitted from aircraft, including black carbon. Another commenter (0791) agreed with the EPA's
discussion of scientific uncertainties on the climate impact of NOx and water vapor, but states, "The
science regarding the contribution of black carbon to climate change, on the other hand, is clear. Black
carbon was identified as the second most important contributor to anthropogenic emissions in 2013. To
reflect the latest science, EPA should consider promulgating a separate aviation endangerment finding
focusing on black carbon in the near future."
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the EPA will take the commenter's request to undertake a second
endangerment finding focusing on black carbon under advisement and consideration among its other
duties and priorities. However, it is not prepared at this time to either reject or grant that request. The
Agency's basis for not including climate forcers other than the six well-mixed GHGs in the current action
is described in section IV.B.7 in the FRN. The EPA cited evidence from the assessment literature about
the direct radiative forcing effects of high-altitude emissions of the two primary aviation-induced
particles, sulfates and black carbon, and acknowledged that black carbon can have warming effects
through darkening bright surfaces such as snow and ice, which reduces reflectivity. The EPA also cited
evidence from the assessment literature that the magnitude of aerosol effects can vary immensely with
location and season of emissions and that that estimates of its total climate forcing effect have a large
uncertainty range. Thus, the EPA disagrees with comments suggested that the Agency's discussion did
not accurately reflect the state of the science, and notes that the commenters provide no evidence that the
EPA missed or mischaracterized conclusions of the assessments regarding aviation climate impacts,
including impacts from black carbon. It is also worth noting that in contrast to the commenter's statement
that black carbon is the second, most-important contributor to anthropogenic emissions, the IPCC AR5
estimated the total warming effect of black carbon at 0.64 W/m2, which is below both the estimates for
CO2 (1.68 W/m2) and methane (0.97 W/m2, including indirect effects).12
12 See Table 8.SM.8 on p. 8SM-13 in IPCC, 2013: Anthropogenic and Natural Radiative Forcing Supplementary Material. 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 [Stacker, 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, 44 pp.
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3.2.2 Water Vapor, Contrails
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
Transport & Environment (EPA-HQ-OAR-2014-0828-0862)
Summary of Comments: A commenter (0747) agreed that the EPA acted appropriately when it did not
include water vapor in its proposed endangerment and cause or contribute findings and stated that the
rationale given in the 2009 endangerment finding still apply. The commenter argued that the EPA should
focus on anthropogenic climate-forcing agents and that "anthropogenic water vapor emissions from
aircraft are an insignificant climate forcing agent." The commenter cites as additional support for these
arguments the fact that the EPA's U.S. National Emissions Inventory, the State of California, and the
United Nations Framework Convention on Climate Change (UNFCCC) GHG reporting guidelines all do
not require reporting of anthropogenic water vapor emissions. The commenter states that the most recent
IPCC assessment (AR5) excludes water vapor from a list of anthropogenic emissions that cause radiative
forcing and estimates "negligible" radiative forcing from contrails and contrail-induced cirrus clouds that
is "even less than previously thought." Citing the IPCC AR5, the commenter notes that radiative forcing
from contrails is estimated at 0.01 (0.005 to 0.03) W/m2, that combined contrails and contrail-induced
cirrus radiative forcing is 0.05 (0.02 to 0.15) W/m2, and that "[m]ore recent estimates tend to indicate
somewhat smaller radiative forcing than assessed in the AR4."
The commenter states that the IPCC categorizes its radiative forcing estimates for contrails as "very
uncertain," with medium confidence levels and low agreement, and describes the evidence on radiative
forcing from combined contrails and contrail-induced cirrus as "even less certain, with low agreement and
a low confidence level." The commenter concludes that EPA's decision not to include contrails is
"entirely consistent with its obligation to use the best available science as required by Information Quality
Act," and that "both the low radiative forcing and the high degree of scientific uncertainty associated with
any positive climate forcing consequence supports EPA's decision to exclude contrails, contrail-induced
cirrus clouds and induced cirrus clouds from its Proposed Endangerment and Cause or Contribute
Findings."
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. The commenter's arguments regarding scientific uncertainty for quantitative
radiative forcing estimates for aircraft water vapor emissions are generally consistent with the EPA's
discussion in section IV.B.7. The EPA cited evidence from the assessment literature that aviation-induced
cloudiness associated with water vapor emissions has uncertain radiative forcing effects that are
dependent on a number of factors such as the location and time of emission, but the EPA did not describe
their climate effects as "insignificant" or "negligible." In addition, the EPA's basis for not including water
vapor in the current action does not include the consideration of emissions inventory reporting guidelines.
The EPA responds to comments regarding the radiative forcing effects of anthropogenic water vapor
emissions in section 3.3, "Climate Science," of this RTC document.
Summary of Comments: A commenter (0747) argues that there is no basis for properly including water
vapor emissions from aircraft in the endangerment finding because even if man-made water vapor were
reduced, this would not materially decrease the concentration of water vapor in the atmosphere. The
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commenter argues that it is this background concentration of water vapor that represents the only
potentially significant aspect of radiative forcing, not anthropogenic water vapor emissions.
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. The EPA has defined the air pollution as the aggregate group of the six well-
mixed GHGs for five primary reasons detailed in section IV.B of the FRN. In brief: (1) they share
common physical properties that influence their climate effects; (2) on the basis of these common
physical properties, they have been determined to be the root cause of human-induced climate change, are
the best-understood driver of climate change, and are expected to remain the primary driver of future
climate change; (3) they are the common focus of climate change science research and policy analyses
and discussions; (4) using the combined mix of these gases as the definition (versus an individual gas-by-
gas approach) is consistent with the science, because risks and impacts associated with GHG-induced
climate change are not assessed on an individual gas-by-gas basis; and (5) using the combined mix of
these gases is consistent with past EPA practice, where separate substances from different sources, but
with common properties, may be treated as a class (e.g., NOx, PM, volatile organic compounds). The
focus on the six well-mixed GHGs as the air pollution is based on their shared physical characteristics and
common attributes relevant to climate change science and policy, which does not include the
consideration of the potential climate effects of reducing aircraft emissions of specific substances. The
EPA responds to comments regarding the radiative forcing effects of anthropogenic water vapor
emissions in section 3.3, "Climate Science," of this RTC document.
Summary of Comments: A commenter (0568) states that water vapor has not been determined to be a
pollutant under the CAA, arguing that—in addition to the scientific uncertainties described in the
proposal—this is justification to exclude water vapor from the proposed endangerment and contribution
findings.
Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN. The EPA's rationale does not include consideration of policy effects; rather, it
is based on considerations of the state of the science. For water vapor, these considerations include the
fact that the state of the science, as represented in the assessment literature at present, continues to
highlight significant scientific uncertainties regarding the total net forcing effect of water vapor.
Furthermore, as explained in the previous response and in more detail in section IV.B of the FRN, the
EPA has defined the air pollution as the aggregate group of the six well-mixed GHGs for five primary
reasons; these reasons do not include whether a substance has or has not previously been determined to be
an air pollutant under the CAA.
Summary of Comments: A commenter (0862) argues that the climate effects of contrails "cannot be
ignored in rulemaking that aims to reduce the impact of aviation on climate change, even though there
remains uncertainty about quantifying the effect."
Response: The EPA is not ignoring potential climate effects from contrails in this action. Although the
EPA is not at this time taking final action to determine whether climate forcers other than the six well-
mixed GHGs should be found to represent air pollution within the meaning of CAA section 231(a)(2)(A),
the Agency's basis for not including them in the current action is described in section IV.B.7 in the FRN.
As stated in that section, the state of the science as represented in the assessment literature at present
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continues to highlight significant scientific uncertainties regarding the total net forcing effect of water
vapor, NOX, and aerosol particles when emitted at high altitudes. The dependence of the effects on where
and when the substance is emitted, and the complex temporal and spatial patterns that result, mean that
the current level of understanding regarding these short-lived substances is much lower than for the six
well-mixed GHGs. Given these scientific uncertainties at present, the Agency is not including these
constituents in the definition of air pollution for purposes of the endangerment finding under
section 231(a)(2)(A) of the CAA; rather, the EPA is focusing this action on the six well-mixed GHGs that
together constitute the largest anthropogenic driver of climate change.
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3.2.3 High-Altitude NOx
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570, EPA-HQ-OAR-2014-0828-
0797)
International Air Transport Association (IATA) (EPA-HQ-OAR-2014-0828-0548)
International Council on Clean Transportation (ICCT) (EPA-HQ-OAR-2014-0828-0791, EPA-HQ-OAR-
2014-0828-0927 (public hearing transcript))
Summary of Comments: Some commenters (0548, 0570) state their agreement that the EPA did not
include NOx in the definition of air pollution for this endangerment finding under CAA section 231,
noting that NOx is already regulated by the International Civil Aviation Organization (ICAO), the Federal
Aviation Administration (FAA), and by the EPA under CAA section 231 based on local air quality
impacts.
Response: The EPA agrees that in 2012, the Agency adopted its revised NOx emissions standards for
aircraft gas turbine engines with rated thrusts greater than 26.7 kilonewtons, and the prior NOx standards
the EPA had adopted in a series of prior rulemakings, relied on the Agency's prior finding that NOx
emissions from aircraft contributed to local air quality impacts. To the extent that the commenters are
noting this fact as justification to exclude NOx from the proposed endangerment and contribution
findings regarding GHG emissions, the EPA disagrees that the prior findings must have that result.
Although the EPA is not at this time taking final action to determine whether climate forcers other than
the six well-mixed GHGs should be found to represent air pollution within the meaning of CAA
section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 in the FRN, including section IV.BJ.d in the FRN, which describes the scientific
uncertainties about the net climate effects of NOx.
Summary of Comments: A few commenters (0548, 0747) agree with not including NOx in the
definition of air pollution in this endangerment finding given the substantial scientific uncertainties
regarding the net climate effects of high-altitude NOx emissions. One comment letter (0747) states that
the balance between the climate warming and cooling effects of NOx is "not uniform or predictable"
because it depends on geographic, altitude, and temporal factors in addition to local concentrations of
ozone and methane. The commenter explains that "NOx is a precursor to ozone formation, which has
been recognized to have a warming effect," but "reduces methane lifetimes and contributes to the
formation of nitrate aerosols, which has a recognized cooling effect." The commenter also states, that
"NOx also likely affects the climate through nitrogen deposition, contributing to increased carbon sinks."
The commenter cited the IPCC AR5 and IPCC 1999 Assessment, arguing that their use of two different
metrics (radiative forcing and GWP) to quantify the overall climate impact of NOx illustrates "the
speculative nature of any effort to evaluate potential climate impacts." Regarding quantifying these
various effects to determine an overall net effect of NOx emissions on the climate, the commenter
questioned the radiative forcing and GWP estimates that the EPA included in the proposal for this action
for aircraft emissions of NOx, but also stated that it does not affect their conclusions that generally agree
with the EPA's discussion of scientific uncertainty. In addition, the commenter argues that NOx
emissions affect the climate only indirectly via their effects on methane and ozone, and that "Consistent
with its overall approach to the endangerment analysis, EPA should therefore continue to focus on
emissions that have been determined to directly contribute to climate change."
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Response: Although the EPA is not at this time taking final action to determine whether climate forcers
other than the six well-mixed GHGs should be found to represent air pollution within the meaning of
CAA section 231(a)(2)(A), the Agency's basis for not including them in the current action is described in
section IV.B.7 of the FRN. The commenter's arguments regarding scientific uncertainty for quantitative
radiative forcing estimates for aircraft NOx emissions are generally consistent with the EPA's discussion
in section IV.B.7. The EPA cited evidence from the assessment literature about the geographic,
altitudinal, and temporal factors leading to differential NOx impacts on methane, aerosol, and ozone
formation and destruction, and therefore on the Earth's climate. These factors contribute to the
uncertainty regarding net NOx impacts on climate. The EPA agrees with the commenter that nitrogen
deposition may have additional ecosystem effects through fertilization, but also notes that increased ozone
concentrations resulting from NOx emissions may have damaging effects on ecosystems. These effects
(and their related climate impacts) are less well-quantified than the radiative forcing effects discussed in
the FRN, contributing to the overall scientific uncertainties regarding the net climate effects of aircraft
NOx emissions. This comment is consistent with the EPA not at this time including NOx in the definition
of air pollution in this finding regarding GHG emissions.
Regarding the commenter's questions about the radiative forcing and GWP estimates that the EPA
included in the proposal for aircraft emissions, it should be noted that the EPA cites from the same
assessment reports that the commenter cites—the IPCC AR5 and the IPCC 1999. The EPA cited the 1999
assessment since no more recent quantitative estimates are available for the globally averaged radiative
forcing for high-altitude aircraft emissions of NOX. The EPA cited the IPCC AR5 report as additional
context and more recent evidence of the scientific uncertainties regarding the net climate effects of
aircraft NOx emissions (since the GWP estimates range from negative to positive, indicating uncertainty
whether the net effect is one of warming or cooling).
Regarding the commenter's statements about whether NOx should be excluded because it primarily
impacts climate through indirect effects on methane and ozone rather than direct effects of elevated levels
of NOx on climate, we reiterate that the EPA has defined the air pollution as the aggregate group of the
six well-mixed GHGs for five primary reasons detailed in section IV.B of the FRN and briefly
summarized elsewhere in this RTC document. We note that the fact that the six well-mixed GHGs are
directly emitted from a source as a GHG (rather than becoming a GHG in the atmosphere after emissions
of a precursor gas) is one aspect of one of the five primary reasons for the aggregate grouping, but does
not form the sole basis for including or excluding substances from the definition of air pollution in this
finding.
Summary of Comments: One commenter (0927) argues that the understanding of the climate impact of
aviation NOx emissions would be improved by additional emissions data. The commenter suggests that
the EPA should collect domestic cruise NOx data.
Response: Drawing from the scientific assessment literature, the EPA discusses the factors that contribute
to scientific uncertainties regarding the net climate effects of aircraft NOx emissions in section IV.B.7 of
the FRN. A lack of cruise emissions data was not identified as one of those factors, and the commenter
does not explain exactly how the scientific understanding of climate effects of NOx would be improved
by such data.
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3.3 Climate Science
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
Earth Guardians of Vermont (EPA-HQ-OAR-2014-0828-0920)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Friends of the Earth (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Geoengineering Watch (EPA-HQ-OAR-2014-0828-0261)
Private Citizen (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Private Citizen (EPA-HQ-OAR-2014-0828-0357)
StopSprayingUs-SF.com (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Summary of Comments: Commenters (0357, 0863) discussed the radiative forcing effects of aircraft
emissions, agreeing with the EPA that CO2 and other GHGs contribute to warming of the global climate
system through the greenhouse effect and trapping heat on the Earth. One commenter (0863) specifically
noted that N2O is a powerful, long-lived GHG with a warming effect 300 times that of CO2. One
commenter (0357) noted that other aircraft emissions include soot, sulfur, metals, and nitrogen oxides,
and that of these emissions, NOx and black carbon have also contributed to global warming. Another
commenter characterized estimates of the radiative forcing effects of aircraft black carbon emissions as
having "significant scientific uncertainties and disparate interpretations," stating that "Recent research
indicates that the radiative forcing from black carbon on snow and ice is weaker than previously thought
and has been decreasing over the past several decades. It also reveals that black carbon concentrations in
the Arctic atmosphere have been declining."
Response: The EPA agrees that the six well-mixed GHGs, including N2O, contribute to warming of the
global climate system, and finds that these comments are generally consistent with the discussion in
section IV.B.2 of the FRN. Regarding other aircraft emissions, the EPA presented quantitative estimates
of radiative forcing from the assessment literature for aviation-induced cloudiness, aviation-induced
particles, and high-altitude NOx in the proposal and in section IV.B.7 of the FRN. The EPA also cited
evidence from the assessments that the climatic impact of these other substances is dependent on a
number of factors (including location and time of emissions) and that the scientific uncertainty for
quantitative estimates of radiative forcing is greater for these other short-lived substances than for the six
well-mixed GHGs, reflecting greater uncertainty regarding climate impacts of emissions of these other
substances. The commenter is correct that the IPCC AR5 assesses black carbon impacts on snow and ice
to have a global radiative forcing effect of 0.04 W/m2 (with an uncertainty range of 0.02 to 0.09 W/m2),
which is smaller than the IPCC AR4's estimate of +0.1 W/m2 (plus/minus 0.1), and that black carbon
concentrations in the Arctic (at least in the Western Hemisphere) have likely declined since the 1980s.
However, as the EPA explains in section IV.B.7.C of the FRN, snow and ice albedo forcing is only one
aspect of how black carbon affects the climate. Other mechanisms include indirect warming and cooling
effects via clouds and a direct warming effect primarily via absorbing incoming and reflected sunlight.
Overall, commenters provide no evidence that the EPA missed or mischaracterized conclusions of the
assessments regarding aviation climate impacts.
Summary of Comments: One comment letter (0863) noted that high-altitude aircraft emissions may
have a disproportionately higher warming effect and impact on the Earth's climate when emitted into the
"climatically sensitive upper troposphere and lower stratosphere."
Response: The EPA recognizes that emissions of some substances such as ozone precursors and aerosols
at high altitudes can have different impacts than emissions of the same substances at the surface due to a
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longer residence time in the atmosphere and different background conditions. While in some cases, the
difference will lead to more warming, in others it could lead to more cooling, and the climate impacts of
such emissions can be sensitive to the time and place of emissions. The EPA presented quantitative
estimates of radiative forcing from the assessment literature for aviation-induced cloudiness, aviation-
induced particles, and high-altitude NOx in the proposal and in section IV.B.7 of the FRN. The EPA also
cited evidence from the assessments that the climatic impact of these other substances is dependent on a
number of factors (including location and time of emissions) and that the scientific uncertainty for
quantitative estimates of radiative forcing is greater for these other short-lived substances than for the six
well-mixed GHGs. The commenter provides no evidence that the EPA missed or mischaracterized
conclusions of the assessments regarding aviation climate impacts.
Summary of Comments: One comment letter (0863) stated that NOx "contribute^] to the formation of
ground-level ozone or smog," which has a significant climate-forcing effect in the upper troposphere and
lower stratosphere."
Response: The EPA recognizes that NOx emissions can contribute to the formation of tropospheric
ozone, which does have a climate warming effect, though NOx emissions also contribute to the
destruction of methane in the atmosphere, creating a counterbalancing cooling effect. The EPA presented
quantitative estimates of radiative forcing from the assessment literature for changes in atmospheric
chemistry from high-altitude NOx emissions, among other things, in the proposal and in section IV.B.7 of
the FRN. The EPA also cited evidence from the assessments that the climatic impact of these other
substances is dependent on a number of factors (including location and time of emissions) and that the
scientific uncertainty for quantitative estimates of radiative forcing is greater for these other short-lived
substances than for the six well-mixed GHGs. The commenter provides no evidence that the EPA missed
or mischaracterized conclusions of the assessments regarding aviation climate impacts.
Summary of Comments: Commenters (0357, 0863, 0927, 0747) discussed various views on how
aviation emissions affect the formation of contrails or clouds and the resultant radiative forcing effects or
other climate or weather effects.
• A commenter stated that aviation-induced contrail cirrus clouds reflect incoming short wave solar
radiation, but block outgoing long wave infrared, thereby affecting earth's climate and
temperature. The commenter stated that the warming effect of these contrail cirrus clouds could
"increase surface and lower atmospheric temperatures by 0.36 to 0.54 degrees Fahrenheit per
decade."13 The commenter also cited Burkhardt and Karcher (2011)14 to describe how contrail
cirrus clouds prevent other clouds from forming by using the surrounding atmosphere water
vapor.
• A commenter stated, "According to various investigations, such as the 1998 subsonic contrail and
clouds effects special study, they have noted that, apart from water and CO2, there are metal
particles, including zinc, aluminum, and titanium, also soot, sulfates, etc., found in the [aircraft]
exhaust blooms, contributing as nuclei for ice crystals to form contrails. Contrails, haziness, and
cloud blankets, which eventually—which certainly do affect changes to the weather, altering
rainfall, altering temperatures, reducing droughts, reducing frost, et cetera, and ultimately
affecting climate change."
13 Minnis, P., Ayers, J.K., Palikonda, R. and Phan, D., 2004: Contrails, Cirrus Trends, and Climate. Journal of
Climate, 17, 1671-1685. http://dx.doi.org/10.1175/1520-0442(2004)017<1671:CCTAC>2.0.CO;2
14 Burkhardt, U. and Karcher, B., 2011: Global radiative forcing from contrail cirrus. Nature Climate Change 1,
54-58. doi:10.1038/nclimatel068.
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A commenter stated that the IPCC AR4 only accounted for linear contrails in its estimates of
contrail radiative forcing and did not account for aviation-induced contrail cirrus clouds, which
can evolve into cirrus clouds indistinguishable from those formed naturally. The commenter cites
Boucher (2011)15 to argue that spreading contrails "may be causing more climate warming today
than all the carbon dioxide emitted by aircraft since the start of aviation."
A commenter cited the work of Hay wood et al. (2009)16 to describe that "A single aircraft
operating in conditions favorable for persistent contrail formation appears to exert a contrail-
induced radiative forcing some 5000 times greater than recent estimates of the average persistent
contrail radiative forcing from the entire civil aviation fleet." The commenter concluded, "Cirrus
clouds trap heat and likely have a greater climate change impact than CO2."
A commenter stated, "With regard to contrails, the EPA suggests that AR5 shows no net effect of
contrails on radiative forcing (p. 37783). However, the AR5 say this: 'Persistent contrails from
aviation contribute a RF of +0.01 (+0.005 to +0.03) W/m2 for year 2011, and the combined
contrail and contrail-cirrus RF from aviation is assessed to be +0.05 (+0.02 to +0.15) W/m2
(IPCC, 2013, p. 574). Though the direct effect of contrails alone is rather small, contrail induced
cirrus has a much larger effect and will not develop without contrails." The commenter suggests
that effect could "easily double aviation's historic CCVonly contribution to RF."
A commenter argued that the "IPCC downplays the effects of contrails on our climate" and that
there "are gaping holes in climate science." The commenter notes that aerosol cloud interactions
are one of the main uncertainties in climate research, including how contrails transition into cirrus
clouds. The commenter states that scientific understanding is rapidly evolving with recent
research results finding evidence of lead (from tetraethyl lead used in light aircraft fuels) in cirrus
clouds, as well as "a whole host of different metals." The commenter cites Czcizo et al. (2013)17
to note that even small amounts of metal particulates may have major effects on cirrus clouds,
stating "because mineral dust and metallic particles are such a small amount of the particulate
matter, just a percent or two, it means that you only have to change about a percent or two of the
particles to get a big effect on these clouds." The commenter argues that this recent research
challenges the validity of IPCC's contrail assumptions and that in order to address the real impact
of aviation on climate change, the EPA should seriously consider high-altitude metals and cirrus
cloud condensation nuclei (which are likely coming from leaded AVGAS and jet exhaust).
A commenter stated that water vapor "is the most important greenhouse gas responsible for the
natural, background greenhouse effect" and that it "could potentially have a larger impact at
higher altitudes" when it forms contrails. The commenter quotes a statement from Lee et al.
(2009)18 that compared to only aircraft CO2 emissions, "aviation has a larger impact on radiative
forcing" due to contrails and aviation-induced cirrus cloud formation.
A commenter stated, "It also should be kept in mind when discussing climate change, especially
with respect to aviation, that water vapor is estimate[d] [to] contribute anywhere from 36 percent
15 Boucher, 2011: Atmospheric science: Seeing through contrails. Nature Climate Change, 1, 24-25.
doi:10.1038/nclimatel078.
16 Haywood, J.M., Allan, R.P., Bornemann, J., Forster, P.M., Francis, P.N., Milton, S., Radel, G., Rap, A., Shine,
K.P. and Thorpe, R., 2009: A case study of the radiative forcing of persistent contrails evolving into contrail-
induced cirrus. Journal of Geophysical. Research, 114, D24201.doi:10.1029/2009JD012650.
17 Cziczo, D.J., Froyd, K.D., Hoose, C., Jensen, E.J., Diao, M., Zondlo, M.A., Smith, J.B., Twohy, C.H. and
Murphy, D.M., 2013: Clarifying the dominant sources and mechanisms of cirrus cloud formation. Science, 340,
1320-1324. doi: 10.1126/science. 1234145.
18 Lee, D.S., Fahey, D.W., Forster, P.M., Newton, P.J., Wit, R.C., Lim, L.L., Owen, B. and Sausen, R., 2009:
Aviation and global climate change in the 21st century. Atmospheric Environment, 43, 3520-3537.
doi:10.1016/j.atmosenv.2009.04.024
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to 72 percent of the greenhouse effect. This is important because the radiative forcing effect of
cirrus cloud formation from the aircraft is a significant contributor to the greenhouse effect."
• A commenter argued that "any climate forcing effect of water vapor does not arise from
anthropogenic emissions of water vapor" and that "To the extent water vapor acts as a climate
forcing agent, it does so independent of direct aviation emissions. Rather, when other GHGs
cause temperature increases, the atmosphere can hold more water vapor. These resulting
concentrations of water vapor represent the only potentially significant aspect radiative forcing,
not anthropogenic water vapor emissions." The commenter then stated that the most recent IPCC
assessment (AR5) excludes water vapor from a list of anthropogenic emissions that cause
radiative forcing and estimates "negligible" radiative forcing from contrails and contrail-induced
cirrus clouds that is "even less than previously thought." They cited the IPCC's estimates for
radiative forcing from contrails: 0.01 (0.005 to 0.03) W/m2. They stated that combined contrails
and contrail-induced cirrus radiative forcing estimates are 0.05 (0.02 to 0.15) W/m2 and that
"[m]ore recent estimates tend to indicate somewhat smaller radiative forcing than assessed in the
AR4."
Two commenters also describe what they see as an increasing trend in contrails over the past few
decades:
• A commenter stated that "the extra contrails and cloud generation is excessive in correlation with
the increasing, growing aviation use" and that "In 2000, NASA scientists demonstrated that the
typical width of a contrail was 22Km or 72,178ft (Duda et al. 2004)19. The commenter went on to
describe a study of contrails following the September 11th attack when air traffic was shutdown,
which found that the spread of six contrail clouds could cover 7,722sq miles/20,OOOsqKm
(Minnis et al. 2002).20
• A commenter stated, "The dark blue skies of our childhood have been replaced with a milky
white haze, crisscrossed with fast expanding persistent contrails, stretching from horizon to
horizon, and spreading out to cover the sky. These trails can stretch for thousands of miles and
can be seen by anyone visiting NASA.gov. These trails persist, regardless of altitude,
temperature, humidity, or other atmospheric conditions. Persistent contrails used to be rare but
have now become an everyday phenomenon all over the world. If physics hasn't changed, what
has?"
Response: A number of commenters describe their views regarding the impact of contrails on climate
change, the climate effect of water vapor in general, and the observed trends in contrail formation. In the
proposal and in section IV.B.7 of the FRN, the EPA presented quantitative estimates of radiative forcing
from the assessment literature for aviation-induced cloudiness that results primarily from high-altitude
emissions of water vapor and particles. As described in section IV.B.7 of the FRN, aviation-induced
cloudiness includes three components: persistent contrails, contrail-induced cirrus, and induced cirrus.
The EPA also cited evidence from the assessments that the climatic impact of short-lived substances such
as water vapor is dependent on a number of factors (including location and time of emissions) and that the
scientific uncertainty for quantitative estimates of radiative forcing is greater for these other short-lived
19 Duda, D.P., Minnis, P., Nguyen, L. and Palikonda, R., 2004. A case study of the development of contrail clusters
over the Great Lakes. Journal of Atmospheric Sciences, 61, 1132-1146. http://dx.doi.org/10.1175/1520-
0469(2004)061<1132:ACSOTD>2.0.CO;2
20 Minnis et al., 2002: Spreading of isolated contrails during the 2001 air traffic shutdown. Abstract for the 10th
Conference on Aviation, Range, and Aerospace Meteorology, 13-16 May, Portland, OR. Available at: http://www-
pm.larc.nasa.gov/sass/pub/conference/Minnis.abs. ARAMS.02.pdf (last accessed July 11, 2016).
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substances than for the six well-mixed GHGs. The arguments and information provided by the
commenters does not demonstrate that the EPA missed or mischaracterized conclusions of the
assessments regarding aviation climate impacts. Moreover, the EPA has explained its reasons for using
the major scientific assessments as the primary scientific and technical basis to inform this endangerment
finding in section IV.A of the FRN. Contrary to the comments' implications, the EPA continues to find
that the appropriate approach with respect to issues related to aviation-induced cloudiness in this
endangerment finding, and after reviewing these comments and the cited information, the EPA does not
find evidence in them that the Administrator should change the definition of "air pollution" used in this
finding. Three overarching issues raised by the commenters are addressed in more detail below.
(1) Contrails and climate:
As explained in greater detail in section IV.B.T.b of the FRN, contrails differ from the six well-mixed
GHGs that the Administrator has defined as the air pollution in this action. For example, contrails have
different physical properties (contrails are not long-lived or well-mixed, nor are they directly emitted),
contrails are not as well-understood nor are they the primary driver of climate change, and contrails are
not a primary focus of climate change science and policy communities. Commenters have not provided
evidence that the EPA has mischaracterized the ways in which contrails are different from the six well-
mixed GHGs that the Administrator has defined as the air pollution in this action.
One commenter claimed that the EPA suggested that the IPCC AR5 shows no net effect of contrails on
radiative forcing, but the number that the commenter cites as the IPCC AR5 best estimate of combined
contrail and contrail-cirrus radiative forcing of 0.05 W/m2, which is the same number that the EPA cited
in its proposal (see 80 FR at 37783) and in section IV.BJ.b of the FRN.
Another commenter characterized the estimate of 0.01 W/m2 from contrails and 0.05 W/m2 from contrails
and contrail-induced cirrus as "negligible" radiative forcing and states that the IPCC AR5 excluded
anthropogenic emissions of water vapor from the list of compounds that cause radiative forcing. While
the EPA cited evidence from the assessment literature that aviation-induced cloudiness associated with
water vapor emissions has uncertain radiative forcing effects that are dependent on a number of factors
such as the location and time of emissions, the EPA did not describe their climate effects as "negligible."
Regarding the commenter's statement that the IPCC AR5 found a smaller radiative forcing than assessed
in the IPCC AR4, the EPA notes that while the AR5 does have a lower estimate of cirrus forcing for an
equal quantity of aviation activity, because aviation activity has increased, the best estimate of the contrail
effect from AR5 (0.01 W/m2) is equal to that made in the AR4 report. Moreover, the IPCC AR5 no longer
presents the possibility that contrail forcing would be negative. In addition, the IPCC AR5 presents an
estimate for forcing from induced cirrus clouds, which IPCC AR4 did not. While the IPCC AR5 does
state that water vapor from activities such as irrigation, power plant cooling, and fossil fuel combustion
has a negligible effect on concentrations of water vapor in the atmosphere, the IPCC AR5 also explicitly
discusses the impacts of water vapor emissions from aircraft in the context of emissions in the
stratosphere and as a contributor to contrails. This is because emissions of water vapor at altitude can
have different effects from emissions at ground level.
Commenters cited one news article and three peer-reviewed studies for the EPA to consider as evidence
that contrails and contrail-induced cirrus have a large impact on climate: Haywood et al. (2009), Lee et al.
(2009), Boucher (2011), and Burkhardt and Karcher (2011). Boucher (2011) is a news piece that
describes Burkhardt and Karcher (2011). For Haywood et al. (2009), Lee et al. (2009), and Burkhardt and
Karcher (2011), commenters do not demonstrate that these studies contradict the assessment literature.
The radiative forcing estimate in Burkhardt and Karcher (2011) is 0.038 W/m2 for the contribution of
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spreading contrails. Lee et al. (2009) estimated a contribution of induced cirrus of 0.033 W/m2. Haywood
et al. (2009) was a case study of a single exceptional event, but the authors noted that 5,000 such events
would have to occur a year to generate 0.01 W/m2 of forcing. These estimates are generally consistent
with the estimate of 0.01 W/m2 for contrails and the estimate of 0.05 W/m2 for contrails plus cirrus that
were developed by the IPCC AR5 and cited by the EPA in the FRN. Moreover, the IPCC AR5 cites all
three of the referenced peer-reviewed studies. Accordingly, the IPCC AR5 took these studies into
account, and that assessment, which the EPA discusses at length in the FRN, reflects the scientific
community's evaluation of these studies in the context of the larger body of scientific research on these
topics.
Several commenters have claimed that the forcing from contrails exceeds the forcing from aviation CO2,
with one quoting Boucher (2011). However, Boucher (2011) also stated that "It is important to note,
however, that the emitted carbon dioxide would continue to exert a warming influence for much longer
than contrails." This means that if aircraft were to be grounded today, the contrail effect would disappear
immediately, but the CO2 forcing from historical emissions from aircraft would remain. Alternatively, if
current patterns of aviation were maintained indefinitely, the contrail effect would remain constant while
the CO2 effect would continue to grow.
A commenter also discussed a recently published paper (Cziczo et al., 2013) about metallic particles and
aviation-induced cirrus clouds. This paper was not cited by the IPCC AR5 as it was published after the
cut-off date for papers to be included in Working Group I of the IPCC AR5. The study experimentally
determined the composition of nuclei in cirrus ice crystals, and found that most of the seed particles were
either mineral dust or metallic particles. The research suggests both that metallic particle emissions might
be particularly effective at seeding clouds, but also that organic carbon and sulfate emissions might
actually coat natural mineral dust in the atmosphere and make the dust less effective as an ice nucleus.
While the EPA acknowledges that this research is of interest, it is an emerging area of science, and the
results have not yet been demonstrated to be robust by the larger assessment community. Moreover, while
the study provided new information about which types of particles emitted by aircraft might be the most
effective contributors to cloudiness, this research does not provide any new information regarding net
global radiative forcing resulting from contrails or cirrus clouds.
The EPA considered the studies on the climate effects of contrails submitted by commenters and believes
that they are consistent with the conclusions of the scientific assessments cited in the FRN regarding the
radiative forcing impacts of contrails. The studies cited by commenters do not provide evidence that the
EPA mischaracterized the ways in which contrails differ from the six well-mixed GHGs.
(2) Contrail trends:
Some commenters also cited studies examining contrail trends, claiming that the climate effect of
contrails has been growing. The commenters do not demonstrate that these studies contradict any of the
information relied on by the Administrator, nor do they provide evidence that the Administrator should
have changed the definition of air pollution in this endangerment finding. The EPA cited an NRC
estimate that contrails had increased cloudiness above the United States by two percent between 1950 and
1988. This is consistent with the work by the researchers that the commenters cited, such as Minnis et al.
(2004), which found that observed cirrus coverage in the United States increased from 20.1 percent to
21.9 percent (a difference of 1.8 percent) between 1971 and 1995.
(3) Water vapor other than contrails:
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One commenter submitted an argument in favor of the importance of water vapor, asserting that water
vapor contributes 36 percent to 72 percent of the greenhouse effect. Another commenter argued that
aviation emissions of water vapor would not contribute to radiative forcing because water vapor
concentrations are determined by temperatures (i.e., water vapor is a feedback mechanism, not a forcing
effect on the global energy balance). The EPA cited evidence from the assessment literature that water
vapor emitted at high altitude can contribute to climate change. Importantly, in relation to the submissions
by the commenters, this analysis is specific to water vapor emitted at high altitude. In the 2009
endangerment finding, the EPA noted that direct human emissions of water vapor at low altitudes has
little effect because the water content of the air is on average a function of temperature and partial
pressure (see 74 FR at 66520). However, in the same document, the EPA noted that water produced at
higher altitudes could potentially have a larger impact through contribution to contrails (see 74 FR at
66520). In the proposal and in section IV.BJ.b of the FRN for these findings under section 231, the EPA
presented estimates of radiative forcing resulting from all aviation emissions contributions to contrails,
including emissions of water vapor. Commenters have not demonstrated that the EPA has
mischaracterized the estimates of the assessments regarding the impact of water vapor, nor that these
assessments were in error.
Summary of Comments: Commenters (0261, 0920, 0927) state that aircraft contrails are a form of
geoengineering through modification of the Earth's weather and climate. Commenters argue that climate
change is caused by this form of geoengineering rather than by increasing atmospheric concentrations of
anthropogenic GHG emissions. One commenter states that, "Climate has always had natural variability
and weather extremes" and suggests that weather extremes can be artificially stimulated by such
geoengineering. Other commenters state that the climatic effects of such geoengineering include reducing
sunlight and heat, reducing global warming, or refreezing the Arctic.
Response: The EPA has reviewed the estimates of the contribution of aircraft contrails to changes in
weather and climate provided in the major scientific assessments, as described in section IV.BJ.b of the
FRN. The EPA has also compared the magnitude of these estimates (central estimate of 0.05 W/m2) to the
magnitude of the forcing from several GHGs, such as COa (central estimate of 1.68 W/m2). While
increased cloudiness resulting from contrails could reduce sunlight at the surface by small amounts,
contrary to the commenters' suggestions, the EPA finds no credible evidence to suggest that contrails
produced by aircraft would cause more climate change than CO2, which is one of the six well-mixed
GHGs included in the definition of air pollution in this notice, or that contrails would reduce global
warming or refreeze the Arctic. The commenters provide no evidence that the EPA has mischaracterized
the science in the assessments, or that the assessments are mistaken.
Summary of Comments: One commenter (0927) states that CO2 is a "harmless trace gas essential to all
life on Earth." The commenter states that CO2 represents "just 3 percent of the planet's greenhouse gases,
while 95 percent of it is water."
Response: The commenter provides no evidence to support the claim that CO2 is harmless and does not
address or provide criticism of all the evidence provided by the EPA in the proposed endangerment
finding regarding the climate effects of elevated atmospheric concentrations of CO2 and the other well-
mixed GHGs. The EPA discusses the evidence from the assessment literature that together, the six well-
mixed GHGs constitute the largest and best-understood anthropogenic driver of climate change in
section IV.B.2 of the FRN. The commenter provides no support for the assertion that CO2 represents only
3 percent of the planet's GHGs. Published estimates of the total contribution of CO2 to global radiative
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forcing are about 20 percent (Schmidt et al., 2010).21 Moreover, CO2 and the other long-lived GHGs have
collectively been the dominant drivers of changes to global radiative forcing over the past 100 years.
21 Schmidt, G.A., Ruedy, R.A., Miller, R.L. andLacis, A.A., 2010. Attribution of the present-day total greenhouse
effect. Journal of Geophysical Research: Atmospheres, 115, D20. doi: 10.1029/2010JD014287
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3.4 Sectoral Impacts
Organization: Center for Biological Diversity (EPA-HQ-OAR-2014-0828-0927 (public hearing
transcript))
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Environmental Defense Fund (EDF) (EPA-HQ-OAR-2014-0828-0754, EPA-HQ-OAR-2014-0828-0793)
National Tribal Air Association (NTAA) (EPA-HQ-OAR-2014-0828-0362)
Northeast States for Coordinated Air Use Management (NESCAUM) (EPA-HQ-OAR-2014-0828-0770)
Private Citizen (EPA-HQ-OAR-2014-0828-0173)
Private Citizen (EPA-HQ-OAR-2014-0828-0412)
Private Citizen (EPA-HQ-OAR-2014-0828-0796)
Sierra Club (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Summary of Comments: Several commenters (0412, 0927, 0770, 0863) describe multiple impacts of
human-induced climate change that are happening now or are projected. One states, "Climate change is
already upon us with unprecedented temperature increases, rising sea levels, extraordinary rates of species
extinction, and more extreme weather events." A commenter cites findings from the IPCC on sea level
rise, temperature rise, and reduction in snow and ice as support for stating, "there is broad continuing
consensus that manmade emissions are contributing to adverse changes in climate and that these impacts
will get worse over time without corrective action." Another commenter cites recent peer-reviewed
journal articles (including the USGCRP's Third National Climate Assessment) as support for stating that
"climate change will displace coastal populations as sea levels rise; increase the frequency of flooding,
hurricanes, drought and other natural disasters; strain a limited water supply; and threaten food security by
fostering spread of disease and pests." A commenter argues that "action is needed to avoid the worst
impacts of climate change, which include rising sea levels, more extreme temperatures and weather
events, mass displacement of people, more severe droughts and floods, enhanced formation of harmful air
pollution, and widespread extinctions of plant and animal species." Another commenter states that
"human emissions have already set in motion changes in Antarctic land ice which will inundate every
coastal community on Earth in catastrophic ways."
Response: The EPA agrees that the types of higher-level impacts discussed by the commenter,
particularly rising temperatures, sea level rise, and changes in precipitation patterns and extreme events,
are generally consistent with the discussion of health and welfare impacts of climate change found in
sections IV.C.l and IV.C.2 of the FRN.
Summary of Comments: A commenter (0796) discusses air quality impacts of climate change and
agrees with the EPA that "climate change will likely increase regional ozone pollution, and the associated
risks in respiratory illnesses and premature death, discounting the fact that the directional effect of climate
change on ambient particulate matter levels remains less certain." In addition, the commenter highlights
that the IPCC AR5 found that "ozone and particulate matter have been associated with adverse health
effects in many locations in North America, and that ozone concentrations could increase under future
climate change scenarios if emissions of precursors were held constant," and that the NRC Indoor
Environment assessment "identifies potential adverse health risks associated with climate-change induced
alterations in the indoor environment, including possible exposure to air pollutants like ozone via changes
in outdoor air quality."
Response: The EPA finds that the comments are generally consistent with the discussion of health
impacts of climate-induced changes in air quality that can be found in section IV.C. 1 of the FRN.
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Summary of Comments: A commenter (0754) argued that climate change brings significant risks for the
aviation industry and that while the EPA "should consider the full spectrum of the impacts of climate
change, impacts to aviation itself have particular relevance under section 231 of the Clean Air Act" and
"warrant consideration in EPA's final Endangerment Finding." The commenter discusses various climate
impacts that could affect airport or airline operations, citing two NRC assessments, the IPCC, the
USGCRP National Climate Assessment, and a Delta Airlines annual report:
• The commenter stated, "Higher temperatures will reduce air density, reducing lift and
contributing to flight cancellations or more restricted payloads, especially at high-altitude
airports. Intense heat can cause runways to buckle. Increased precipitation and sea level rise can
submerge runways, disrupting air travel or forcing temporary airport closures. More intense
tropical storms can damage or temporarily close airports. Increased wildfires in drought-
susceptible regions will reduce visibility and can close airports. In far northern locations, such as
Alaska, where air transport use is disproportionately high, warming temperatures will have a
deleterious impact on airstrips built on permafrost, and may undermine runway foundations. All
of these risks create significant safety concerns for the aviation sector and its regulators."
• The commenter discussed changes in extreme weather associated with climate change and how
this can "cripple multiple airports," resulting in massive human impacts and economic costs to
both the airline industry and the economy as a whole, which the commenter states was observed
during Hurricane Katrina and Hurricane Sandy.
• The commenter noted that "13 of the nation's 47 busiest airports - one in four of these airports -
have at least one runway that is low enough to be inundated by a moderate-to-high storm surge."
• The commenter stated that "increases in frequency, severity or duration of thunderstorms,
hurricanes, typhoons or other severe weather events, including from changes in the global
climate, could result in increases in fuel consumption to avoid such weather, turbulence-related
injuries, delays and cancellations, any of which would increase the potential for greater loss of
revenue and higher costs."
• The commenter stated that "Increased storminess at airports, particularly those located in coastal
regions, may increase the number of weather related delays and cancellations and increase
maintenance and repair costs. Clear-air turbulence will increase in the Atlantic corridor leading to
longer and bumpier trips. The impact of climate change on airport pavement is very similar to
paved roads. The effect of temperature and increase[d] precipitation intensity on airports imposes
a risk to the entire facility if pavements are not adapted to these increases. Increasing carbon
dioxide concentrations in the atmosphere are expected to increase the frequency and intensity of
turbulence, which is already responsible for costing airlines tens of millions of dollars and
injuring (occasionally fatally) hundreds of passengers each year— Increased temperatures,
flooding and extreme weather events arising from climate change threaten the health of airport
and airline workers, damage airport runways and other critical air traffic control equipment,
overwhelm stormwater systems, impair airplane performance, increase the risk of vehicle crashes,
and, as a result, disrupt traffic, restrict public transportation, and threaten human lives and local
economies."
Response: The EPA finds that the points raised and information provided by the commenter on climate
impacts to airports and the airline industry are helpful examples of some of the potential downstream
impacts of climate change. However, the Agency has chosen not to focus the discussion of climate
impacts in the endangerment finding on any particular industry or sector of the economy. Instead, the
EPA provides a higher-level summary of expected impacts to categories like "energy, infrastructure, and
settlements" that would encompass airports and other transportation infrastructure in the course of
evaluating the broader question of whether the air pollution may reasonably be anticipated to endanger
public health or welfare. The EPA finds that the types of higher-level impacts discussed by the
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commenter, particularly rising temperatures and changes in the frequency and severity of extreme
weather, are generally consistent with the discussion of health and welfare impacts of climate change
found in sections IV.C.l and IV.C.2 of the FRN.
Summary of Comments: A commenter (0173) highlighted the impacts of climate change on vulnerable
populations, specifically identifying the elderly, poor, very young, and indigenous groups. The
commenter expressed concern for these communities, citing language from the EPA's proposed findings
that "These groups tend to have 'limited resources to adapt and recover from climate impacts, as well as
existing health disparities" (see 80 FR at 37761). The commenter stated a desire to see these vulnerable
populations "take control of their own environmental future by being engaged in an equal manner to other
communities."
Response: The EPA finds that the comments are generally consistent with the discussion of health and
welfare impacts of climate change on vulnerable populations that can be found in sections IV.C. 1 .a and
IV.C.2.aoftheFRN.
Summary of Comments: Another commenter (0362) noted that "climate change affects the health of the
nation, but that Native American and Alaska Native communities are much more severely and directly
impacted by climate change as compared to the general population." The commenter specifically
identified that "Indian Tribes are seeing the effects of climate change through increased storm surges,
erosion, and flooding; prolonged droughts never seen in modern times; and increased fires and insect pest
outbreaks in their forests." The commenter stated that "Tribal communities are integrated into the
ecosystems of North America; and many Tribal economies are heavily dependent on the use of fish,
wildlife, and native plants. Even where Tribal economies are integrated into the national economy, Tribal
cultural identities continue to be deeply rooted in the natural environment. As climate change disrupts
biological communities, the survival of some Tribes as distinct cultures may be at risk. The loss of
traditional cultural practices, due to climate-driven die-off or range shift of culturally significant plant and
animal species, may prove to be too much for some Tribal cultures to withstand on top of other external
pressures that they face." The commenter expressed appreciation for the EPA's discussion of vulnerable
populations in the proposal, especially in light of "the federal government's trust responsibility to Native
American tribes, and as recognized by EPA in the 2014 EPA Policy on EJ'for Working with Federally
Recognized Tribes and Indigenous Peoples."
Response: The EPA finds that the comments are generally consistent with the discussion of health and
welfare impacts of climate change on vulnerable populations—in particular for indigenous communities,
including those in Alaska—that can be found in sections IV.C.l.a and IV.C.2.a of the FRN.
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4. The Cause or Contribute Finding
The EPA summarizes and responds to comments regarding the cause or contribute finding (also referred
to as the contribution finding) in the sections below, 4.1, 4.2, 4.3, and 4.4.
4.1 Scope & Proposed Definition of "Air Pollutant"
Organization: Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570, EPA-HQ-
OAR-2014-0828-0797)
Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
AirFair(EPA-HQ-OAR-2014-0828-0860)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Biogenic CO2 Coalition (EPA-HQ-OAR-2014-0828-0916)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
California Air Resources Board (ARE) (EPA-HQ-OAR-2014-0828-0861)
Cargo Airline Association (EPA-HQ-OAR-2014-0828-0732)
Embraer Aircraft Holding, Inc. (EPA-HQ-OAR-2014-0828-0745)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
General Electric (GE) (EPA-HQ-OAR-2014-0828-0932)
The International Air Cargo Association (TIACA) (EPA-HQ-OAR-2014-0828-0622)
International Air Transport Association (IATA) (EPA-HQ-OAR-2014-0828-0548)
Private Citizen (EPA-HQ-OAR-2014-0828-0796)
Regional Airline Association (RAA) (EPA-HQ-OAR-2014-0828-0620, EPA-HQ-OAR-2014-0828-0731)
Summary of Comments: Similar to the discussion in section 3.1 for the definition of "air pollution" for
purposes of the endangerment finding under CAA section 231(a)(2)(A), some commenters (0570, 0568,
0747, 0732, 0745, 0837, 0932, 0548, 0620, and 0622) highlighted the fact that aircraft engines emit only
two of the six well-mixed GHGs that together are defined as the "air pollutant" for purposes of the cause
or contribute finding under section 231(a)(2)(A) of the CAA. Commenters (0570, 0568, 0747, 0732,
0745, 0837, 0932, 0548, 0620, 0622, 0751, and 0861) point out that the majority of emissions are CO2,
while N2O emissions are described as "nominal (< 1 percent)" or "trace." Some commenters (0570, 0747,
0745, 0837, 0932, 0620, and 0622) ultimately concluded that the EPA's approach to defining the air
pollutant as an aggregate group of six gases is acceptable, but that the scope of future regulations should
be limited to CO2. One commenter letter (0747) supported the Agency's evaluation of the six GHGs
based on their common attributes, but questioned the EPA's decision to aggregate the six gases for
purposes of making the findings for a sector that emits only two of the GHGs, and where N2O emissions
are < 1 percent of aircraft GHG emissions. Other commenters (0828 and 0837) disagreed with the EPA
and requested limiting the definition of air pollutant in this action to CO2 or to CO2 and N2O. Some
commenters (0860 and 0796) agreed with the EPA's definition of air pollutant.
Response: The EPA responds to comments regarding limiting the definition of air pollutant in this action
to CO2 (or to CO2 and N2O), and limiting the scope of future regulations to CO2 in section V.A.2 of the
FRN.
Summary of Comments: The Biogenic CO2 Coalition (0916) expressed concern about the EPA's
proposed contribution finding because it does not differentiate between CO2 emissions that result from the
combustion of fossil fuels and those that result from the "combustion of biomass or biofuels derived from
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herbaceous crops or crop residues, as well as biogenic CO2 emissions associated with the production,
gathering and processing of crops or crop residues used in bio-based products including fuels."22 They
argue that such crop-related biogenic CO2 emissions should be excluded from the contribution finding
because the CO2 released back to the atmosphere when emitted from crop-derived biogenic sources
contains the same carbon that was previously removed or sequestered from CO2 in the atmosphere, and
thus does not contribute to elevated atmospheric concentrations of the six well-mixed GHGs.
Response: The EPA responds to comments regarding crop-related biogenic CO2 emissions or the source
of the air pollutant (and air pollution), for defining the relevant air pollutant (and air pollution) considered
in the findings, in sections IV.B.6 and V.A.2 of the FRN, as well as in section 3.1 of this RTC document.
22 Biogenic COi Coalition, 2015: Comments on EPA's 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, 80 FR 37757 (July 1, 2015). Docket ID number EPA-HQ-OAR-2014-0828-0916. Available at
www.regulations.gov (last accessed April 11, 2016).
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4.2 Analysis of Contribution
Organization: Airbus S.A.S. (EPA-HQ-OAR-2014-0828-0834, EPA-HQ-OAR-2014-0828-0859)
Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
AirFair(EPA-HQ-OAR-2014-0828-0860)
Environmental Defense Fund (EDF) (EPA-HQ-OAR-2014-0828-0754, EPA-HQ-OAR-2014-0828-0793)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
International Council on Clean Transportation (ICCT) (EPA-HQ-OAR-2014-0828-0791)
National Association of Clean Air Agencies (NACAA) (EPA-HQ-OAR-2014-0828-0560)
National Tribal Air Association (NTAA) (EPA-HQ-OAR-2014-0828-0362)
Natural Resources Defense Council (NRDC) (EPA-HQ-OAR-2014-0828-0927 (public hearing
transcript))
Private Citizen (EPA-HQ-OAR-2014-0828-0174)
Summary of Comments: AOPA (0751) commented that the EPA should describe the contribution of
general aviation aircraft GHG emissions to the U.S. transportation GHG emissions, particularly
discussing the contribution from general aviation aircraft that operate on aviation gasoline (piston-engine
general aviation aircraft). AOPA claimed that general aviation aircraft are less than 1.5 percent of total
U.S. GHG emissions, and piston-engine general aviation aircraft are 0.09 percent of total U.S. GHG
emissions.
Response: The EPA describes in section V.B of the FRN that for the purposes of this cause or contribute
finding under CAA section 231(a)(2)(A), the EPA is including emissions of the six well-mixed GHGs
from classes of engines used in U.S. covered aircraft, which are subsonic jet aircraft with a maximum
takeoff mass (MTOM) greater than 5,700 kilograms and subsonic propeller driven (e.g., turboprop)
aircraft with a MTOM greater than 8,618 kilograms. The cause or contribute finding is a prerequisite
under CAA section 231 for the EPA to adopt standards of at least equivalent stringency to those set by
ICAO. Accordingly, in this finding, the EPA is focusing on matching the scope of its contribution finding
to the applicability thresholds of the international standard. The covered aircraft match the applicability
(or MTOM) thresholds of the international aircraft CO2 standard. Also, we have identified aircraft that are
not covered aircraft for the purposes of this contribution finding. This includes aircraft that fall below the
international applicability thresholds: smaller turboprop aircraft, such as the Beechcraft King Air 350i;
and smaller jet aircraft, such as the Cessna Citation M2. In addition, similar to ICAO, we are not in this
action addressing "piston-engine aircraft," "helicopters," and "military aircraft."23
The MTOM thresholds for covered aircraft include some (but not all) general aviation aircraft using jet
fuel but, as described earlier, general aviation aircraft using aviation gasoline (piston-engine general
aviation aircraft) would not be covered. According to the Inventory of U.S. Greenhouse Gas Emissions
23 U.S. EPA, 2016: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014, 558 pp. Available at
http://www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventorv-2016-Main-Text.pdf (last
accessed April 22, 2016). The EPA has determined that the U.S. Inventory has been adequately reviewed in
accordance with the EPA's Peer Review Handbook. For the presentation of emissions inventory information in this
contribution finding, the EPA disaggregated the existing data in one area of the U.S. Inventory (for the General
Aviation Jet Fuel Category) and had the disaggregation methodology peer reviewed in accordance with the EPA's
Peer Review Handbook. The EPA Science Advisory Board reviewed this approach to the underlying technical and
scientific information supporting this action, and concluded that the approach had precedent and the action will be
based on well-reviewed information. All relevant peer-review documentation is located in the docket for today's
final action (EPA-HQ-OAR-2014-0828).
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and Sinks Report,24 in 2014 GHG emissions from all U.S. general aviation aircraft using jet fuel and
aviation gasoline represent 1 percent of total U.S. transportation GHG emissions and 9 percent of total
U.S. aircraft GHG emissions. General aviation aircraft using jet fuel represent 8 percent of total
U.S. aircraft GHG emissions. General aviation aircraft that use jet fuel and are above the MTOM
thresholds (as described above) represent 6 percent of total U.S. aircraft GHG emissions and 7 percent of
U.S. covered aircraft GHG emissions.25 General aviation aircraft that use aviation gasoline (portion of
non-covered aircraft) represent 1 percent of total U.S. aircraft GHG emissions and 0.1 percent of
U.S. transportation GHG emissions.
The FRN describes the contribution of aircraft above the MTOM thresholds without specifically
discussing the contribution from the general aviation category that is above the MTOM thresholds, so that
the scope of the findings is not inadvertently mischaracterized. The term "general aviation aircraft" is not
specifically defined in EPA and FAA regulations and, thus, describing the general aviation contribution in
the findings could lead to varying interpretations of the scope of the findings. By mainly describing
covered aircraft as those aircraft above the MTOM thresholds, the EPA believes it is appropriately
identifying the aircraft covered by the findings, while avoiding any potential confusion on the scope of
the findings. This action makes no judgment regarding whether the GHG emissions of aircraft such as
"piston engine aircraft," "helicopters," and "military aircraft" contribute to endangering GHG air
pollution, or whether other aircraft that are below the MTOM thresholds so contribute.
Summary of Comments: Some commenters (0754, 0791, 0560, 0362, 0927, 0837, and 0174) support the
cause or contribution analysis in the proposed findings. They agree with the percentages of contribution
provided -- including total U.S. aircraft GHG emissions and U.S. covered aircraft GHG
emissions relative to total U.S. GHG emissions, U.S. transportation GHG emissions, and global GHG
emissions (global aircraft, global transport, and total global GHG emissions). Also, they support the
percentages provided of global aircraft GHG emissions in relation to global transport and total global
GHG emissions. In contrast, some commenters (0834 and 0860) disagreed with the EPA's description of
the contribution of aircraft GHG emissions for these same comparisons. These commenters
either indicated that the EPA overstated or understated the contribution from aircraft GHG emissions.
Response: Section V.B of the FRN has the EPA's description and analysis of aircraft's contribution to
U.S. and global GHG emissions. For the U.S. GHG emissions inventories in the FRN, the EPA relied
upon the Inventory of U.S. Greenhouse Gas Emissions and Sinks Report (U.S. Inventory).26 The EPA has
determined that the U.S. Inventory has been adequately reviewed in accordance with the EPA's Peer
Review Handbook. For the presentation of emissions inventory information in this contribution finding,
the EPA disaggregated the existing data in one area of the U.S. Inventory (for the General Aviation Jet
Fuel Category)27 and had the disaggregation methodology peer reviewed in accordance with the EPA's
Peer Review Handbook. The EPA Science Advisory Board reviewed this approach to the underlying
technical and scientific information supporting this action, and concluded that the approach had precedent
24ICAO regulations only apply to civil aviation (aircraft and aircraft engines); consequently, ICAO regulations do
not apply to military aircraft.
25 ERG, 2015: U.S. Jet Fuel Use and CO2 Emissions Inventory for Aircraft Below ICAO CO2 Standard Thresholds,
Final Report, EPA Contract Number EP-D-11-006, 38 pp.
26 U.S. EPA, 2016: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014, 558 pp. Available at
http://www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2016-Main-Text.pdf (last
accessed April 22, 2016).
27 ERG, 2015: U.S. Jet Fuel Use and CO2 Emissions Inventory for Aircraft Below ICAO CO2 Standard Thresholds,
Final Report, EPA Contract Number EP-D-11-006, 38 pp.
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and the action will be based on well-reviewed information. A copy of this letter and all other relevant
EPA peer review documentation is located in the docket for the FRN (EPA-HQ-OAR-2014-0828).
For the global GHG emissions inventories, we were informed by IPCC's 2013-2014 Fifth Assessment
Report (AR5)28 and place considerable weight upon the IPCC AR5 data on global GHG emissions. We
also considered but placed less emphasis on the World Resources Institute's (WRI) Climate Analysis
Indicators Tool (CAIT), and the International Energy Agency (IEA)29 emissions data, which in
comparison have a different aggregation of underlying data, but are available for more recent years. The
approach of considering the major scientific assessments, including IPCC's assessments, provides
assurance that the Administrator's judgment is informed by the best available, well-vetted science that
reflects the consensus of the climate science research community. The major findings of the assessments,
including IPCC's assessment, support the Administrator's findings in this action. While the EPA uses the
IPCC data as the primary data source for informing this contribution finding, it has reasonably used
additional data sources from widely used and recognized global datasets to provide context and
information from more recent years. These additional data supplement and confirm the IPCC data, as
they are generally in line with IPCC. Ultimately, whether the Agency utilizes the IPCC data alone or the
WRI/CAIT dataset (and IEA data) alone, or both datasets together, it would have no material effect on the
emissions comparisons discussed in the FRN, and the Administrator would make the same contribution
finding.
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. Schlomer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, 1435 pp.
29 World Resources Institute (WRI) Climate Analysis Indicators Tool (CAIT) Data Explorer (Version 2.0).
Available at http://cait.wri.org (last accessed January 19, 2016). International Energy Agency, Data Services.
Available at http://data.iea.org (last accessed January 21, 2016).
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4.2.1 Including GHG Emissions from Combustion of International Aviation
Bunker Fuels in the U.S. Aircraft GHG Inventory
Organization: Airbus S.A.S. (EPA-HQ-OAR-2014-0828-0834, EPA-HQ-OAR-2014-0828-0859)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
Summary of Comments: Some commenters (0834, 0747, and 0837) claimed that the EPA's choice of
data for the cause or contribute analysis was selective and biased. They contended that emissions resulting
from the combustion of international aviation bunker fuels should not be a part of the U.S. covered
aircraft GHG inventory or of the total U.S. aircraft GHG inventory, since the EPA's own U.S. Inventory
for UNFCCC reporting purposes does not include emissions from combustion of these fuels in the
national GHG totals and reports them separately to the UNFCCC, pursuant to UNFCCC inventory
reporting guidelines.30 Consequently, the commenters asserted that the total emissions from domestic
commercial aircraft account for less than 2 percent (1.7 percent) of total U.S. aircraft GHG emissions.
Because of this, some commenters believe that EPA inappropriately specified that the U.S. covered
aircraft GHG emissions represent 3 percent of the total U.S. GHG emissions.
Response: The EPA responds to comments regarding the inclusion of GHG emissions from the
combustion of international aviation bunker fuels in U.S. covered aircraft GHG emissions (and total U.S.
aircraft GHG emissions) in section V.C.2.a of the FRN.
30 EPA GHG Emissions Inventory at A-31 (reporting and methods) is available at:
http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventorv-2015-Annex-2-Emissions-Fossil-
Fuel-Combustion.pdf (last accessed April 8, 2016).
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4.2.2. Finding Significant Contribution, or Establishing a Bright Line
Organization: Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
General Electric (GE) (EPA-HQ-OAR-2014-0828-0932)
Summary of Comments: Some commenters (0751, 0747, 0568, and 0932) stated that aircraft GHG
emissions are extremely small relative to both domestic and global GHG emissions in the aggregate.
Also, one comment letter (0747) questioned whether there is a reasoned basis for EPA to find that GHG
emissions from U.S. aircraft cause or contribute to air pollution that endangers public health and welfare
when assessed not only relative to contributions from other sectors, but also relative to climate impacts.
For example, this comment letter indicated the EPA estimates that total U.S. aircraft GHG emissions
accounted for about 0.5 percent of total global GHG emissions in 2010. Thus, the commenters (0751,
0747, 0568, and 0932) stated that the total U.S. aircraft GHG emissions contributions from the U.S.
aviation sector are extremely small relative to total global GHG emissions, or negligible as a percentage
of total global GHG emissions.
Response: The EPA responds to comments regarding whether finding a contribution for purposes of
CAA section 231 requires that the EPA find a "significant" contribution or establish a bright line
threshold of contribution in section V.C.2.b of the FRN.
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4.2.3. Providing Context in Comparing Aircraft GHG Emissions to Other
Sector GHG Emissions
Organization: Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Center for Biological Diversity (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Environmental Defense Fund (EDF) (EPA-HQ-OAR-2014-0828-0754, EPA-HQ-OAR-2014-0828-0793)
Friends of the Earth (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Northeast States for Coordinated Air Use Management (NESCAUM) (EPA-HQ-OAR-2014-0828-0770)
Private Citizen (EPA-HQ-OAR-2014-0828-0357)
Sierra Club (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Summary of Comments: Some commenters (0751 and 0747) asserted that the EPA did not show
important context in comparing covered aircraft GHG emissions to other mobile source categories' GHG
emissions. They claimed the EPA does not describe what they consider the very low level of aircraft
emissions in general relative to emissions from other sources. The commenters assert that, for example,
the EPA does not point out that the growth in emissions from U.S. medium-duty and heavy-duty trucks
since 1990 is 53 percent greater than the GHG emissions from the U.S. commercial aircraft sector today,
and 18 percent higher than the total U.S. aircraft (or entire U.S. aviation sector) GHG emissions today.
Some commenters (0927, 0863, 0754, 0770, and 0357) supported the context that the EPA provided for
such comparisons, including the statement that aircraft GHG emissions are the third-largest source of
GHG emissions in the U.S. transportation sector.
Response: The EPA responds to comments regarding providing context in comparing aircraft GHG
emissions to other sector GHG emissions in section V.C.2.C of the FRN.
Summary of Comments: One comment letter (0747) stated that it is inappropriate and misleading to
compare U.S. aircraft GHG emissions with those of other, individual countries. They indicated that to
fairly compare the U.S. airlines' GHG emissions contribution, EPA should analyze, as ICAO does,
contributions from other world regions with comparable land masses and levels of economic activity. (In
terms of landmass, the U.S. ranks third globally, behind only Russia and Canada.) However, some
commenters (0927, 0863, 0754, and 0770) supported the EPA's comparison of U.S. aircraft GHG
emissions to those of individual countries.
Response: The EPA responds to comments regarding comparing U.S. aircraft GHG emissions to other,
individual countries GHG emissions in section V.C.2.C of the FRN.
Summary of Comments: One comment letter (0747) asserted that the methodology EPA and its
consultant, Eastern Research Group (ERG), utilized to determine emissions from "non-covered aircraft"
is unclear.
Response: In a memorandum to the docket (EPA-HQ-OAR-2014-0828) entitled, Methods Utilized in
Calculations for GHG Emissions Inventories for Final Aircraft GHG Contribution Finding (July 20,
2016), the EPA has provided additional description of the methods utilized to calculate U.S. covered
aircraft GHG emissions, and through that description has also provided additional information on what it
considered non-covered aircraft GHG emissions for purposes of these findings. These additional details
will make the EPA's methods clearer.
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4.2.4. Utilizing Multiple Databases for Global GHG Emissions
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
Summary of Comments: Some commenters (0747 and 0837) claimed that the mix of data from different
years utilizing emissions data from IPCC, WRI/CAIT, and IEA was confusing and potentially misleading.
They allege that the EPA assertion that the different datasets are "generally consistent" is not supported
by a reasoned analysis tied to the record, and they claim that this calls EPA's overall analysis into
question.
Response: The EPA responds to comments regarding the use of multiple databases for the cause or
contribute finding in section V.C.2.d of the FRN.
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4.2.5. Past Aircraft Industry Fuel Efficiency Trends
Organization: Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570)
Airbus S.A.S (EPA-HQ-OAR-2014-0828-0834)
AirFair(EPA-HQ-OAR-2014-0828-0860)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
American Airlines, Inc. (EPA-HQ-OAR-2014-0828-0917)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
Cargo Airline Association (EPA-HQ-OAR-2014-0828-0732)
Embraer Aircraft Holding, Inc. (EPA-HQ-OAR-2014-0828-0745)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
General Electric (EPA-HQ-OAR-2014-0828-0932)
Honeywell (EPA-HQ-OAR-2014-0828-0757)
National Air Carrier Association (NACA) (EPA-HQ-OAR-2014-0828-0546)
National Business Aviation Association (NBAA) (EPA-HQ-OAR-2014-0828-0567)
Regional Airline Association (RAA) (EPA-HQ-OAR-2014-0828-0620)
The International Air Cargo Association (TIACA) (EPA-HQ-OAR-2014-0828-0622)
Summary of Comments: Some commenters (0747, 0837, 0570, 0917, 0568, 0732, 0745, 0932, 0757,
0546, 0567, and 0620) claimed that the U.S. aviation industry has a strong environmental track record and
has significantly improved fuel efficiency and decreased emissions compared to 40 to 60 years ago, and
that the EPA barely acknowledges industry's strong fuel efficiency record. Some commenters (0568,
0570, 0745, and 0622) asserted that current commercial aircraft are 70 percent more fuel efficient than
aircraft from 50 to 60 years ago with lowered emissions. Also, some commenters (0747, 0620, 0546,
0732, and 0917) argued that the U.S. airlines have achieved these significant improvements in fuel
efficiency and decreased emissions while adding capacity. In 2014, according to some commenters (0747,
0732, 0546, and 0620), the U.S. airlines carried 20 percent more passengers and cargo compared to 2000,
while emitting 8 percent less CO2 and improving their fuel efficiency by 31 percent during this same time
period. Furthermore, one comment letter (0747) claimed that in comparison to 1972, the North American
airlines now carry 6 times more payload using 60 percent less fuel per flight. One commenter (0834)
asserted that absolute fuel consumption and emissions from U.S. domestic aircraft operations have
decreased in the last 8 to 10 years, while emissions from international aviation bunker fuels has increased.
Another commenter (0837) asserted that U.S. commercial aircraft emissions increased slightly between
2012 and 2013, but have decreased 18 percent since 2007. One comment letter (0747) claimed that when
considering the relative contribution of U.S. aircraft, it is important to recognize that the United States has
commercial airline fleets that are among the most efficient fleets worldwide.
In contrast, another commenter (0860) stated that industry claims of fuel-efficiency progress need to be
evaluated closely and may be overstated, due to the choice of baseline aircraft by industry. Instead, the
2005 study by the Netherlands National Aerospace Laboratory (NLR) entitled, Fuel Efficiency of
Commercial Aircraft: An Overview of Historical and Future Trends, indicated that this fuel-efficiency
improvement is about 55 percent. Thus, relying on improvements in fuel efficiency may not be an
effective method to compensate for the impact that aviation has on climate change. Also, the commenter
indicated that environmental groups state that aviation is more climate-intensive than surface
transportation. They specified that the aviation industry's results include assumptions regarding load
factors, occupancy rates, and fuel-efficiency rates in comparison to those of surface transportation that
skew the results.
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Response: Section V.B of the FRN describes past improvements in U.S. aircraft GHG emissions. In
particular, U.S. covered aircraft GHG emissions grew by 15 percent between 1990 and 2014, but total
U.S. aircraft GHG emissions decreased by 3 percent over this same time period. Also, section V.B. of the
FRN indicates that total U.S. aircraft GHG emissions and U.S. covered aircraft GHG emissions were
from 10 to 15 percent greater in 2000 and 2005 than in 2014. The EPA provided the
accompanying explanation that these decreases in aircraft GHG emissions are partly because aircraft
operations decreased by similar amounts during this time period. Also, the EPA indicated that these
decreases in aircraft GHG emissions are due in part to improved operational efficiency that results in
more direct flight routing, improvements in aircraft and engine technologies to reduce fuel burn and
emissions, and the accelerated retirement of older, less fuel-efficient aircraft. (In fact, the EPA explained
that U.S. covered aircraft emissions decreased from 2000 to 2010 (13 percent), but then have increased
from 2010 to 2014 (3 percent).)31 The EPA has considered these comments and the cited information
about improvements in U.S. aircraft GHG emissions and fuel efficiency in making this contribution
finding.
In addition, according to the 2009 study by the International Council on Clean Transportation (ICCT),32
the average fuel efficiency of new passenger aircraft has about doubled on both a seat-km (passengers
only) and ton-km (passengers plus freight) basis since 1960. New aircraft fuel efficiency
has improved substantially in two of the last five decades, but it has stagnated in recent years. Fuel
efficiency, on average, has remained flat on a seat-km basis, and it has shown limited improvement
annually on a ton-km basis since 2000. Thus, the ICCT study indicates that the rate of fuel-burn
improvement has decreased in recent years.
31 U.S. EPA, 2016: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014, 558 pp. Available at
http://www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2016-Main-Text.pdf (last
accessed April 22, 2016).
32 International Council on Clean Transportation, 2009: Efficiency Trends for New Commercial Jet Aircraft 1960 to
2008, 20 pp., Available at
http://www.theicct.org/sites/default/files/publications/ICCT Aircraft Efficiency final.pdf (last accessed May 24,
2016).
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4.3 Proposed Scope and Description of Covered Aircraft
The EPA summarizes and responds to comments regarding the scope of the contribution finding and the
definition of covered aircraft for the finding in subsections 4.3.1,4.3.2, and 4.3.3.
4.3.1. Applicability Weight Thresholds Match Those of International CO2
Standard
Organization: Aerospace Industries Association (AIA) (EPA-HQ-OAR-2014-0828-0570, EPA-HQ-
OAR-2014-0828-0797)
Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
Summary of Comments: One comment letter (0863) stated that the EPA should undertake another cause
or contribute finding for a broader range of aircraft not covered in our proposed finding, including smaller
turboprop aircraft (such as the Beechcraft King Air 350i), smaller jet aircraft (such as the Cessna Citation
M2), piston-engine aircraft, and helicopters. These commenters stated, however, that this comment did
not affect the validity of the conclusions in the proposed finding. Other commenters (0570, 0751, and
0837) indicated that they support the scope of our proposed cause or contribute finding matching the
applicability (MTOM) thresholds of the international CO2 standard.
Response: The EPA responds to comments regarding the scope of the cause or contribute
finding matching the applicability thresholds of the international CO2 standard in section V.C.I.a of the
FRN.
Summary of Comments: One comment letter (0837) indicated that the EPA's example of Cessna
Citation CJ2+ as an example of smaller jet aircraft that is a U.S. covered aircraft was incorrect, because
the CJ2+ has an MTOM below the proposed applicability threshold for subsonic jet aircraft, which is
5,700 kilograms.
Response: The EPA agrees that this smaller jet aircraft example of the CJ2+ was in error in the proposed
findings, and in the FRN this has been corrected by using the example of a Cessna Citation CJ3+, which
has an MTOM of 6,291 kilograms.
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4.3.2. Defining U.S. Covered Aircraft
Organization: Airbus S.A.S. (EPA-HQ-OAR-2014-0828-0834, EPA-HQ-OAR-2014-0828-0859)
Summary of Comments: A commenter (0834) stated that they understand that the scope of the finding
corresponds to the aircraft engine GHG emissions that are from aircraft that match the applicability
thresholds (or MTOM thresholds) for the international aircraft CO2 standard; however, they requested
clarification on the difference between "U.S. covered aircraft" and "non-U.S. covered aircraft." This
commenter requested clarification on whether U.S. covered aircraft means aircraft made in the United
States, registered in the United States, operated by an entity holding an air carrier certificate issued by the
United States, operated by an air carrier in the National Air Space, or operated by anyone in the U.S.
(National) Air Space. The commenter expressed that the EPA must explain the basis for its definition, and
its claimed authority to regulate U.S. covered aircraft.
Response: The EPA responds to these comments regarding defining U.S. covered aircraft in section
V.C.l.boftheFRN.
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4.3.3. Limiting the Contribution Finding to U.S. Covered Aircraft
Organization: Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-
2014-0828-0747)
Summary of Comments: One comment letter (0747) questioned whether the EPA should limit the scope
of the contribution finding to those aircraft matching the MTOM applicability thresholds adopted by
ICAO, and asserted that the EPA should wait until the standard setting phase to exercise discretion as to
what classes of aircraft engines should be covered by standards. These commenters argued that the EPA
has authority to set aircraft engine GHG emissions standards, following a cause or contribute finding, that
do not impose requirements on every engine or class of aircraft engine within the scope of that finding.
They also argued that in this instance there does not seem to be a sufficiently reasoned basis for EPA to
exclude the non-covered aircraft for purposes of making the cause or contribute finding.
Response: The EPA responds to comments on limiting the cause or contribute finding to U.S. covered
aircraft in section V.C.I.c of the FRN. In addition, the EPA stresses that it is not in this action making a
finding that emissions from the non-covered aircraft do not contribute to the GHG air pollution, or
making any other final decision regarding those aircraft. Consequently, whether the EPA would have
discretion to not establish emissions standards for such aircraft were they included within the set of
aircraft subject to the present finding is a question that is outside the scope of this action.
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4.4 Aircraft Emissions Projections
Organization: Airbus S.A.S. (EPA-HQ-OAR-2014-0828-0834, EPA-HQ-OAR-2014-0828-0859)
Aircraft Owners and Pilots Association (AOPA) (EPA-HQ-OAR-2014-0828-0751)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Center for Biological Diversity (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Environmental Defense Fund (EDF) (EPA-HQ-OAR-2014-0828-0754, EPA-HQ-OAR-2014-0828-0793)
International Council on Clean Transportation (ICCT) (EPA-HQ-OAR-2014-0828-0791)
Natural Resources Defense Council (NRDC) (EPA-HQ-OAR-2014-0828-0927 (public hearing
transcript))
Sierra Club (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Summary of Comments: One comment letter (0834) raised an issue with the EPA's use of the 2014
FAA Aerospace Forecast33 (hereinafter, 2014 FAA forecast), saying that the EPA does not correlate
increased fuel burn with increased traffic. It indicated that the 2014 FAA forecast states that air carrier
"revenue passenger miles ('RPMs') are forecast to grow at an average of 2.8 percent per year through
2034. That is, American carriers will be carrying more people and more cargo farther when compared to
the amount of fuel the airlines burn getting them there." It cited its company traffic forecast to show that
the majority of growth will occur outside of the United States, and projected that worldwide traffic will
increase at a rate of 4.6 percent annually while the United States traffic will increase at 2.5 percent, thus
decreasing the U.S. share of global emissions. It stated that the majority of growth in emissions will
therefore be outside the reach of U.S. federal regulation. Therefore, the comment letter concluded that the
EPA's finding exaggerates the contribution of the U.S. industry and makes it appear to be more
responsible for global GHG emissions.
In addition, another comment letter (0747) stated that the EPA has not demonstrated that the assumptions
underlying the 2015 FAA forecasts34 are well-founded and how projections of GHG emissions were
made. It noted that past FAA traffic growth and jet fuel consumptions forecasts have been shown to over-
predict the traffic. Further, it stated that demand for air travel is contingent on a number of factors, thus
making accurate estimates of future air traffic, and therefore fuel burn and GHG emissions, not feasible.
Consequently, the comment letter claimed that the EPA's use of a scaling factor and extrapolation to
predict future emissions rates is insufficient.
Also, the comment letter (0747) questioned the EPA's authority under section 231 of the CAA to consider
future emissions while making a cause or contribute finding. It argued that the statutory language appears
to apply to current aircraft emissions instead of future emissions.
Response: The EPA acknowledges, as described in section V.B.4.C of the FRN, that there are differing
growth rates between U.S. markets and international markets due to the maturity of the U.S. market.
33 FAA, 2014: FAA Aerospace Forecast Fiscal Years 2014-2034, pages 5, 13, and 15. Available at
http://www.faa.gov/data research/aviation/aerospace forecasts/media/2014 faa aerospace forecastpdf
(last accessed April 8, 2016). This 2014 FAA forecast and the 2015 FAA forecast were both referenced in the
proposed findings.
34 FAA, 2015: FAA Aerospace Forecast Fiscal Years 2015-2035, 132 pp. Available at
https://www.faa.gov/data research/aviation/aerospace forecasts/media/2015 National Forecast Report Final.pdf
(last accessed July 5, 2016).
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However, in regard to the comments that the U.S. share of global GHG emissions is expected to decrease,
as a percentage, we believe this obscures the fact that on an absolute basis the U.S. aircraft GHG
emissions (total U.S. aircraft GHG emissions and U.S. covered aircraft GHG emissions) are still expected
to grow significantly (increase by 43 percent over the next two decades) in the foreseeable future.35
In regard to the comments that question the underlying assumptions of the 2015 FAA forecast, the EPA
believes that, as of July 1, 2015 (the date the proposed findings were published), the 2015 FAA forecast
was the best publicly available data on projections for U.S. aircraft fuel consumption (utilized as a
surrogate for projections in aircraft GHG emissions). Currently, the 2016 FAA forecast is the latest and
best publicly available data on forecasts for U.S. aircraft fuel consumption. Similar to the 2015 FAA
forecast (page 76), the 2016 FAA forecast report (page 39) states that, "[t]he forecasts in this document
are forecasts of aviation demand, driven by models built on forecasts of economic activity. There are
many assumptions in both the economic forecasts and in the FAA models that could impact the degree to
which these forecasts are realized." In addition, the 2016 FAA forecast (as well as the 2015 FAA
forecast) report discusses the assumptions in the FAA models and how they may affect the projections,
and as with any model or study, this is appropriate for describing the potential uncertainty with the
forecast. Moreover, the FAA forecast has been published annually for many years, and each year FAA
takes into account stakeholder input to improve the forecast. Therefore, the EPA believes this reaffirms
that the 2015 FAA forecast was the best available data on U.S. aircraft fuel consumption projections in
the public domain, as of July 1, 2015, and the 2016 FAA forecast is currently the best available data on
U.S. aircraft fuel consumption projections.
For the comments on the EPA's authority to consider future emissions projections for the contribution
finding, as described in section V.B.3 of the FRN, given the projected growth in aircraft emissions
compared to other sectors, it is reasonable for the Administrator to consider future emissions projections
as further support for her assessment of historical and current annual emissions (recent emissions from the
current fleet), and informing her contribution determination under CAA section 231(a)(2)(A). Also,
because the projected growth in aircraft engine GHG emissions from U.S. covered aircraft is greater in
percentage terms than percentages from other transportation sources, this consideration of projected
future emissions adds further support to the Administrator's cause or contribute finding. Ultimately, the
historical annual and current GHG emissions inventories from U.S. covered aircraft are sufficient to
support the Administrator's cause or contribute finding in the FRN. That is, the Administrator's
contribution finding is also supported by the historical and current annual GHG emissions from U.S.
covered aircraft to GHG inventories, alone, even without considering projected future emissions. But it is
appropriate and reasonable for the EPA to also address and rely upon projected future emissions as an
additional basis for the contribution finding, partly because the future standards the EPA will promulgate
may apply to future aircraft types and future manufactured aircraft that are not yet in use and that have not
yet begun emitting GHGs. The EPA's treatment of historical and current annual aircraft GHG emissions
is a reasonable set of information for the EPA to consider in making its contribution finding under CAA
section 231, just as using the emissions inventory from the current fleet of motor vehicles as a surrogate
for a projection of the inventory from new motor vehicles was reasonable in the 2009 Endangerment
Finding under CAA section 202 (see 74 FR at 66543). But as the EPA is able in this action to also project
future aircraft GHG emissions (including those expected from future manufactured aircraft), there is no
35 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 July 11, 2016). This 2016 and 2014 FAA forecasts are referred to in the FRN, and the 2014 and 2015
FAA forecasts were referred to in the proposed findings.
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reason for the EPA to not include discussion of this projection in its contribution finding and consider
whether it further supports the contribution finding.
Summary of Comments: One comment letter (0751) indicated that general aviation aircraft emissions
are projected to only contribute between 1 and 3 percent to the worldwide aviation sector's fuel
consumption and CO2 emissions in 2050. It also stated that general aviation aircraft's contribution to
global transportation emissions is likely to be orders of magnitude smaller.
Response: The EPA explains elsewhere in this document that the contribution finding's scope is not
defined by the use of terms such as "general aviation," but rather by MTOM thresholds that result in some
general aviation aircraft being considered covered aircraft. The EPA acknowledges the commenter's
statements about global percent contributions projected for general aviation aircraft GHG emissions in
2050. According to the 2016 FAA forecast,36 the U.S. fuel consumption for total general aviation aircraft
(using jet fuel and aviation gasoline) is projected to increase by 50 percent from 2010 to 2036, and the
U.S. fuel consumption for general aviation aircraft using only jet fuel is estimated to increase by 58
percent in this same time period. As mentioned previously in this section, total U.S. aircraft GHG
emissions and U.S. covered aircraft GHG emissions are projected to grow by 43 percent over this same
timeline. Given this estimated growth in U.S. GHG emissions and the projected increases in U.S. general
aviation aircraft fuel consumption, we anticipate that U.S. general aviation aircraft will have a similar
contribution to U.S. aircraft GHG emissions (total U.S. aircraft GHG emissions and U.S. covered aircraft
GHG emissions) in the next few decades as they do today. We do not currently have access to global
GHG emissions projections for general aviation aircraft, but we anticipate that general aviation aircraft's
percent global contribution in the long-term would follow similar trends as in the United States.
In regard to the comment letter's statements about the significance of general aviation aircraft's
contribution to total GHG emissions, the EPA responds to these comments in section 4.2.2 (also see
section 4.2 for further details).
Summary of Comments: One comment letter (0747) noted that the "US airline industry is part of a
global aviation coalition that aims to achieve 1.5 percent annual fleet average fuel efficiency
improvements through 2020" and to achieve carbon neutral growth after 2020. As part of this effort, it
stated that it is committed to working with other stakeholders to deploy "commercially viable,
environmentally friendly alternative jet fuel" to reduce GHG emissions.
Response: The EPA commends the airline industry on its goal of carbon neutral growth after 2020. The
EPA is encouraged that the airline industry recognizes the public health and welfare concerns from
aircraft engine GHG emissions. However, the goals mentioned by the commenter do not change the
contribution finding. The EPA notes that this goal as articulated relies significantly on the use of
alternative jet fuels to make up the balance of the GHG emissions deficit created by increased air traffic.37
36 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_F AA_Aerospace_Forecast.pdf
(last accessed April 8, 2016). This FAA forecast is referred to in the FRN.
37ICAO CAEP, 2013: ICAO Environmental Report 2013, Aviation and Climate Change, 212 pp., Chapter 1 -
Aviation and Environment Outlook. Available at http://cfapp.icao.int/Environmental-Report-2013/ (last accessed
April 8, 2016).
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The alternative fuel market (or suppliers) is not expected to have the production capacity needed for this
deficit in the foreseeable future.38
The EPA would also encourage the airline industry to review its GHG emissions reduction targets as well.
Setting an ambitious, long-term goal sufficiently in the future (20-30 years), with near- and mid-term
goals would be a proactive approach to move forward with the industry's commitments to reduce growth
in GHG emissions from aviation.
Summary of Comments: Some commenters (0927, 0754, and 0791) stressed the importance of the
projected growth in aircraft GHG emissions and the importance of trying to curb it. These commenters
indicated how light-duty and heavy-duty vehicles are expected to see significant reductions in GHG
emissions by 2035, while there is expected to be substantial growth in air traffic, operations, and aircraft
GHG emissions by 2030 and continuing to 2050.
Response: The EPA agrees with these commenters about the importance of the projected substantial
growth in aircraft GHG emissions. These comments reaffirm the EPA's consideration of future emissions
projections as information to further support its assessment of historical and current annual actual
emissions (recent emissions from the current fleet) for the contribution finding.
38 US DOT, 2012: Alternative Jet Fuel Scenario Analysis Report, 7-13 pp., Executive Summary. Available at
http://ntl.bts.gov/lib/46000/46500/46597/DOT-VNTSC-FAA-12-01 .pdf (last accessed July 8, 2016)
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5. Miscellaneous Legal, Economic, and Other Comments
Organization: Airbus S.A.S. (EPA-HQ-OAR-2014-0828-0834, EPA-HQ-OAR-2014-0828-0859)
AirFair, Inc. (EPA-HQ-OAR-2014-0828-0860)
Airlines for America (A4A) and the Air Line Pilots Association (ALPA) (EPA-HQ-OAR-2014-0828-
0747)
Biogenic CO2 Coalition (EPA-HQ-OAR-2014-0828-0916)
Boeing Commercial Airplanes (EPA-HQ-OAR-2014-0828-0568)
Cargo Airline Association (EPA-HQ-OAR-2014-0828-0732)
Citizens United and Green (EPA-HQ-OAR-2014-0828-0915)
Climate Viewer News (EPA-HQ-OAR-2014-0828-0359, EPA-HQ-OAR-2014-0828-0484)
Corpirate Company - Climate Environment Confirmation and Responsibility (CECR) (EPA-HQ-OAR-
2014-0828-0608)
Earth Guardians of Vermont (EPA-HQ-OAR-2014-0828-0920)
Earthjustice et al. (EPA-HQ-OAR-2014-0828-0863)
Friends of the Earth (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
General Aviation Manufacturers Association (EPA-HQ-OAR-2014-0828-0837)
Geoengineering Watch (EPA-HQ-OAR-2014-0828-0261)
Institute for Policy Integrity (EPA-HQ-OAR-2014-0828-0749)
Lissys Ltd. (EPA-HQ-OAR-2014-0828-0203)
National Tribal Air Association (NTAA) (EPA-HQ-OAR-2014-0828-0362)
Private Citizen (EPA-HQ-OAR-2014-0828-0357)
Private Citizen (EPA-HQ-OAR-2014-0828-0173)
Private Citizen (EPA-HQ-OAR-2014-0828-0147)
Private Citizen (EPA-HQ-OAR-2014-0828-0601)
Private Citizen (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Sierra Club (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
StopSprayingUs-SF.com (EPA-HQ-OAR-2014-0828-0927 (public hearing transcript))
Transport & Environment (EPA-HQ-OAR-2014-0828-0862)
UCLArts and Healing (EPA-HQ-OAR-2014-0828-0763)
Union of Concerned Scientists (EPA-HQ-OAR-2014-0828-0526)
Summary of Comments: Some commenters (0860, 0927, and 0362) strongly supported the proposed
GHG endangerment and contribution findings, stating that the CAA clearly authorizes the EPA to make
these findings under section 231(a)(2)(A) and pointing to the previous 2009 Endangerment Findings
under section 202(a)(l) as precedent for doing so, given the identical language found in each
section. Most commenters (0747, 0568, and 0732) stated that the CAA authorized the EPA to make the
proposed aircraft endangerment finding. One commenter (0732), while agreeing that the EPA
possessed authority under CAA section 231 (a) to make such findings, also stated that the EPA's
jurisdiction that follows from such authority is limited to then setting standards for the emissions
emanating from aircraft engines and does not authorize the EPA to, "otherwise regulate air
transportation."
Joint commenters (0747) questioned whether EPA had adequately set forth the basis for its authority
under CAA section 231 to address pollutants other than pollutants for which the EPA has established
National Ambient Air Quality Standards (NAAQS), and asked that the EPA address this issue in light of
the recent Supreme Court opinion in Utility Air Regulatory Group v. EPA, which they quote as stating
that Massachusetts v. EPA "does not strip EPA of authority to exclude greenhouse gases from the class of
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regulable air pollutant under other parts of the Act where their inclusion would be inconsistent with the
statutory scheme." 134 S. Ct. 2427, 2441 (2014). These commenters argued that the use of the term "air
quality control regions" in CAA sections 231(a)(l)(A) and 231(a)(3) suggests that Congress intended to
authorize the EPA to issue standards only for pollutants for which a NAAQS has been established.
Finally, a commenter (0834) stated that if the EPA issued standards more stringent than ICAO and the
European Aviation Safety Agency (EASA), the United States would be substituting its judgment for that
of ICAO and would have seized EASA's role as the certificating agency of aircraft developed by the
commenter under European authority.
Response: The EPA agrees with commenters that the CAA clearly authorizes the EPA to make GHG
endangerment and contributions findings with regard to aircraft GHG emissions under 231(a)(2)(A). 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. 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." The legal framework for these findings is discussed in section III of the FRN.
With regard to the commenter who stated that the EPA's jurisdiction to regulate aircraft emissions is
limited to setting standards for emissions emanating from aircraft engines and does not authorize the EPA
to "otherwise regulate air transportation," we note that 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 emissions 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. 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.
(See U.S. EPA, 1973: Emissions Standards and Test Procedures for Aircraft; Final Rule, 38 FR 19088
(July 17, 1973).) The Administrator's decisions regarding the GHG air pollution and the GHG air
pollutant for these findings under section 231(a)(2)(A) are explained in detail in sections IV and V of the
FRN, respectively. To the extent this commenter is suggesting that these findings should only include
substances emitted by aircraft engines, section V.A.2 of the FRN explains that the definition of the
relevant air pollutant under section 231(a)(2)(A) may group into a single class multiple substances that
possess shared relevant properties, even though they are not all emitted from section 231(a)(2) sources.
Finally, we note that these final GHG endangerment and contribution 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 is triggering a duty under CAA
section 231 to propose and promulgate aircraft engine emissions standards applicable to emissions of that
air pollutant from those classes of engines.
After consideration of comments claiming that EPA's authority to address aircraft engine emissions is
restricted to NAAQS pollutants, the EPA disagrees with the argument that Congress intended to only
authorize the EPA to address NAAQS pollutants under section 231(a)(2)(A). The EPA fully responds to
this comment in section III.A.4 of the FRN. Finally, with regard to the comment warning EPA against
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issuing standards more stringent that those selected by ICAO, the EPA notes that this rulemaking does not
set any emissions standards for aircraft engines. 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, but the EPA is not yet issuing
proposed or final aircraft engine emissions standards. Nor is the EPA taking final action that prejudges
what future standards will be, nor that determines that the EPA's future standards will be the same as
ICAO's expected standards. 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 aircraft engine GHG standards. If the ICAO Assembly, in October 2016, approves the final
CO2 standards and subsequently ICAO formally adopts the final COa standards in March 2017, the EPA's
standards will need to be at least as stringent as the ICAO aircraft COa 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 (a) of the CAA to set emissions 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.
Summary of Comments: A commenter (0834) stated that their company seeks to ensure air transport
continues to be an eco-efficient means of transport while noting that millions of jobs and the U.S. GDP
are supported by the air travel industry. A commenter (0927) noted that because jet fuel is the number one
cost for commercial airlines their members have a powerful incentive to continue to reduce CO2 output in
addition to the need to act responsibly toward the environment. Another commenter (0863) urged EPA to
promptly set greenhouse gas emissions standards for aircraft arguing that immediate action is more cost-
effective than deferred action which will necessitate additional mitigation costs and also increase the
difficulty of the transition to low longer-term emissions levels. A commenter (0837) noted their
commitment to health and development of a sustainable general aviation industry worldwide and
explained that general aviation plays and important role in economic growth by serving the transportation
needs of communities, companies, and individuals worldwide. They reported on a study they had
commissioned and which was released in February 2015. This report found that general aviation in the
U.S. employs or supports about 1.1 million jobs and $219 billion dollars in US economic output every
year. They also noted that general aviation manufacturers shipped 1,631 airplanes, worth $11.69 billion
dollars with the majority being of the type and size that would be covered aircraft under the EPA's
proposed contribution finding.
Response: The EPA appreciates the economic data submitted by these commenters. Under CAA section
231(b), we will be required in promulgating future aircraft GHG standards to give appropriate
consideration to the cost of compliance with the standards. The endangerment and contribution findings,
however, are based on scientific considerations, and do not themselves impose burdens or costs on any
non-federal entity. Therefore, the comments on anticipated costs and benefits of future standards are not
within the scope of this action. As we discuss in section II.C. of the FRN, CAA section 231(a)(2)(A)
directs the EPA to 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. Then CAA section
231(b) requires that the 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
emissions standards, after appropriate consideration of the cost of compliance within that period.
Summary of Comments: One commenter (0747) expressed surprise that U.S. airlines were not included
in the Federal Register list of entities that the EPA believes would be affected by the proposed
Endangerment Finding and ANPR, stating that both US airlines and pilots have an interest in the potential
environmental impacts of the findings and any standards that result from them—including the costs of
meeting standards, the impact of such on aircraft pricing, and insuring that the U.S. approach is consistent
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with the international standards. Several commenters (0862, 0927) discussed the scientific consensus
surrounding the reality of climate change as well as the type of actions that will be necessary to reduce the
risks and well as the worst impacts of climate change. These commenters urged the Obama
Administration to take decisive action to address climate change while also acknowledging
that the Administration has recognized the need for urgent action, regulations, and executive action.
A number of commenters discussed the importance of transparency (0173, 0203, 0362). One of these
commenters, noting the technical and complex nature of studies associated with the 2009 Endangerment
Finding and the technical nature of the current proposal, urged the EPA to ensure the general public can
understand the goals of the proposal and how it would be implemented. Another commenter spoke of the
need for transparency when quantifying aviation CO2 emissions to ensure that they are technically
meaningful and urged the EPA to assemble a more complete inventory than what was attempted by
ICAO.
Response: With regard to the commenter who indicated they should be included in the list of entities that
would be affected by the proposed endangerment finding, the EPA notes that the table found in section
LA of the FRN is not intended to be exhaustive, but rather provides a guide for readers. We acknowledge
in section LA of the FRN that other type of entities, not listed in the table, could also be interested and
potentially affected by subsequent actions at some future time. In this regard, this final endangerment
finding triggers new duties that apply only to the federal government and do not apply any new
requirements to other entities outside the federal government. Section LA provides additional discussion
on this topic.
The EPA agrees with commenters who stated that addressing climate change is an urgent matter. Section
II.C.l of the FRN describes the many actions undertaken by EPA to address climate change since
2007, as well as the efforts by the Obama Administration through the Climate Action Plan and
Presidential Memorandum to develop additional executive actions to further reduce GHGs. The EPA also
agrees with commenters who discussed the importance and need for transparency when discussing
climate change science and when proposing and finalizing GHG-related rulemakings. The 2015
ANPR discussed the issues arising from the ICAO/CAEP international process, including a range of
technical issues regarding aircraft CO2 emissions standards, to assist the Agency in developing its
position with regard to these issues, and also to help ensure transparency, and obtain views on aircraft
engine GHG emissions standards that it might potentially adopt under the CAA. With regard to
comprehensive emissions inventories for aircraft emissions, the EPA believes it has quantified the GHG
emissions of aircraft in a meaningful, clear, and comprehensive manner. In addition to relying on the
latest IPCC data for associated aircraft emissions, to ensure completeness and the most up-to date
information, the EPA also provided 2012 estimates from other widely used and recognized global
datasets, the World Resources Institute's Climate Analysis Indicators Tool (WRICAIT). To further
inform the Administrator's assessment, section V.B.4 of the FRN presents various comparisons of
covered U.S. aircraft emissions: as a share of current total U.S. GHG emissions; as a share of current U.S.
transportation GHG emissions; as a share of current total global GHG emissions; and as a share of the
current global transportation GHG emissions.
Summary of Comments: One commenter (0362) wrote that the proposal has implications for Native
American tribes with regard to environmental justice communities and because climate-driven disruptions
of biological communities are having an effect on the treaty rights of Indian Tribes, especially those
treaties that preserve hunting, fishing, and gathering rights for tribes on their lands and in the usual and
accustomed areas. The commenter noted that some tribes are finding that animals and/or plants on which
they depend for their cultural practices and identity have either migrated onto other lands or disappeared
altogether. Pointing to Executive Order (EO) 13175, the commenter stated that the EPA was required to
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develop a process to ensure "meaningful and timely input" by tribes into regulatory policies that have
tribal implications, and the commenter urged the EPA to provide significant opportunities for consultation
with tribes regarding finalizing these findings and subsequently when developing regulations to control
aircraft emissions and other sources of GHGs. This commenter strongly supported the proposed
endangerment finding.
Response: With regard to comments regarding environmental justice and Native American tribes, the
EPA addresses comments regarding climate change impacts that tribes are facing, such as those to
culturally important plant and animal species, in section 3.4 "Sectoral Impacts" of this RTC document.
These findings were developed in accordance with the EPA Policy on Consultation and Coordination
with Indian Tribes.39 The Agency engaged in coordination activities with the National Tribal Air
Association. The EPA stated in the proposal and in the final findings in the FRN that this action will not
have potential disproportionately high and adverse human health or environmental effects on minority,
low-income, or indigenous populations and that EO 13175 does not apply to this action. The EPA made
these determinations because this action does not affect the level of protection provided to human health
or the environment, and thus it does not have tribal implications as specified in EO 13175. The final
endangerment and cause or contribute findings under CAA section 231(a)(2)(A) do not themselves
impose any new requirements but rather set forth the Administrator's determination that GHG emissions
from certain classes of aircraft engines—those used in U.S. covered aircraft—cause or contribute to air
pollution that may be reasonably anticipated to endanger public health and welfare. However, the
Administrator did consider climate change risks to minority, low-income, and indigenous populations as
part of these endangerment and contribution findings under CAA section 231(a)(2)(A). In addition, the
EPA wishes to assure the commenter that it is committed to continued dialogue with regard to issues,
including climate change related matters, that impact tribes in the U.S. This action's discussion of climate
change impacts on public health and welfare is found in section IV.C of the FRN. Specific discussion
with regard to minority, low-income, and indigenous populations is found in sections IV.C. 1.a and
IV.C.2.aoftheFRN.
Summary of Comments: A commenter (0749) described a petition that they submitted to the Agency in
2009, which made several requests including for the EPA to make endangerment and contribution
findings under CAA section 231. This commenter asks the Agency to clarify whether it considers these
final findings to respond to that part of their petition request. Another commenter (0927) requested that
the EPA adopt the proposed endangerment finding "within no more than three months of the comment
period."
Response: The EPA's final endangerment and contribution findings come in response to a separate
earlier citizen petition submitted by Friends of the Earth, Oceana, the Center for Biological Diversity, and
Earthjustice requesting that the EPA issue a GHG endangerment finding and standards under section
231(a)(2)(A) of the Act for GHG emissions from aircraft engines. The portion of the 2009 petition
submitted by the commenter referenced above is the same request made previously by the first petitioners.
Regarding the comment about timing for adopting the proposed endangerment findings, the Agency
39 EPA (2011). EPA Policy on Consultation and Coordination with Indian Tribes. Available at:
https://www.epa.gov/sites/production/files/2013-08/documents/cons-and-coord-with-indian-tribes-policy.pdf (last
accessed June 9, 2016).
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finalized this action according to the regulatory development processes outlined in the EPA's Action
Development Process Guidelines.40
Summary of Comments: Many commenters (0147, 0916, 0747, 0834, 0863, 0860, 0568, 0732, 0526,
0601) discuss the EPA's future regulatory standards for GHG emissions from aircraft engines, including,
for example, their views on the EPA's duty or authority under the Clean Air Act to undertake regulation,
what types of pollutants and what types of aircraft the future standards should cover, and various
considerations that the Agency should take into account when establishing aircraft emissions standards,
such as aircraft safety, engine and aircraft efficiency, and the carbon intensity of aircraft fuels.
Response: Comments related to future regulations under CAA section 231 are outside of the scope of this
action because the EPA has neither proposed nor is finalizing in this action any such regulatory standards.
This final action does not itself impose any requirements on source categories under CAA section 231.
Thus, the EPA anticipates that these questions could be raised and considered, as needed, in the standard-
setting phase of the regulatory process, and the EPA will consider comments submitted on the issue of the
appropriate form of emissions standards and other related issues in response to EPA's anticipated future
notice of proposed rulemaking on standards. Although this final action trigger a duty for the EPA to
promulgate standards for the GHG emissions from engines used by covered aircraft, the findings do not
pre-judge the standards.
Summary of Comments: Some commenters (0927, 0915,0359, 0608, 0920, 0261) argued that the EPA's
proposal does not address the negative human health effects of exposure to chemicals, heavy metals,
metal particulates/aerosols, biological agents or other toxic substances that the commenters contend are
associated with aircraft fuel, fuel additives, emissions, aviation-induced cloudiness/contrails, and/or
intentional spraying or geoengineering activities. These commenters describe a number of health
conditions—including Alzheimer's disease, cancer, autism, respiratory and heart disease, migraines,
nosebleeds, and ear ringing—that they argue result from exposure to these substances. In addition to
health impacts, one commenter (0920) notes loss of wildlife and trees in the State of Vermont from
exposure to these substances, while another commenter (0359) notes impacts from aviation-induced
cloudiness on the solar energy industry, tourism industry, and astronomy activities. This commenter
requests that the EPA participate in both domestic and international discussions about aircraft
geoengineering activities.
Response: The EPA's findings address the questions of whether concentrations of the six well-mixed
GHGs in the atmosphere may reasonably be anticipated to endanger public health and welfare and
whether emissions of the same six well-mixed GHGs from certain classes of engines used in covered
aircraft cause or contribute to the endangering air pollution. Thus, in this final action the EPA summarizes
the key findings from the scientific assessment literature regarding the role of the six well-mixed GHGs in
driving climate change and the observed and projected impacts of that climate change, including to public
health. Specifically, in section IV.C.l of the FRN, the EPA summarizes the key findings of the more
recent assessments regarding a range of health risks associated with climate change-induced changes in
air quality, increases in temperatures, changes in extreme weather events, increases in food and water
borne pathogens, and changes in aeroallergens; and that certain populations are more vulnerable to
climate change health risks and impacts. The question of whether other substances that may be emitted by
aircraft engines cause or contribute to air pollution which may reasonably be anticipated to endanger
40 EPA (2011). Guidance for EPA Staff on Developing Quality Actions. EPA Office of Policy.
https://vosemite.epa.gov/sab%5CSABPRODUCT.nsf/5088B3878A90053E8525788E005EC8D8/$File/adpQ3-00-
ll.pdf (last accessed June 6, 2016).
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public health or welfare is outside the scope of this final action. The request for involvement in domestic
or international discussions about geoengineering is also not within the scope of the EPA's final
endangerment and contribution findings for the six well-mixed GHGs.
Summary of Comments: Some commenters (0357, 0763) voiced concern with the negative health
effects due to the air quality impacts of aircraft emissions occurring near airports or when aircraft are
idling or taxiing on the ground, or departing or arriving. These commenters discussed the impact
of aircraft emissions of ultra-fine particulates, carbon monoxide, sulfuric acid, volatile organic
compounds and nitrogen dioxide and urged the EPA to take action to address these threats to human
health and local air quality. Another commenter (0359) noted the negative health impacts of lead and
argues that the EPA has a misplaced focus on climate change while "doing nothing to address ongoing
lawsuits over leaded aviation gasoline."
Response: For informational purposes, the EPA provides the following discussion regarding its current
program addressing aircraft emissions' impacts on local air quality. However, this final action does not
address local air quality impacts of aircraft non-GHG emissions, and takes no action regarding the
existing program. Consequently, comments on that program are outside the scope of this final action.
Regarding comments on the local air quality impacts of aircraft, jet aircraft contribute much less air
pollution than that from motor vehicles; however, their overall emissions are increasing every year as air
travel becomes more popular. In addition, jet aircraft can contribute significantly to ground-level ambient
air pollution in the immediate vicinity of an airport, especially emissions of NOx and hydrocarbons (HCs)
which contribute to the formation of ozone. Additional detailed information on aircraft emissions can be
found in an EPA report, "Evaluation of Air Pollutant Emissions from Subsonic Commercial Jet Aircraft"
(April 1999). This report is also available at EPA's aviation web site at: www.epa.gov/otaq/aviation.htm.
It provides an estimation of the contribution of aircraft to air quality emissions in ten urban areas.
The aircraft emissions standards for gas turbine (jet) engines which power civil aircraft have been in place
for about twenty years. Before an engine type can be sold or installed on an aircraft, the manufacturer
must test its emissions on the ground by simulating in-use conditions. The EPA sets the emissions
standards for the engines, and the FAA monitors the certification process and enforces the standards.
Emissions standards apply to essentially all commercial aircraft, comprising scheduled and freight
airlines. Controls on engine smoke and prohibitions on fuel venting were instituted in 1974 and have been
revised several times since then. Beginning in 1984, limits were placed on the amount of unburned HCs
which turbine engines can emit per landing and takeoff cycle. In May of 1997, the EPA adopted the then
existing emissions standards for gas turbine engines of the ICAO for NOx and carbon monoxide (CO). In
addition, in November 2005 and May 2012, EPA adopted more stringent NOx standards, which were
previously adopted by ICAO.
With regard to lead emissions, EPA has been petitioned to issue an endangerment finding for aircraft lead
emissions and currently intends to issue a 2017 notice of proposed rulemaking. If the endangerment
finding is positive, then we would subsequently evaluate and propose engine emissions standards. For
more information, go to: https://www3.epa.gov/otaq/aviation.htm#lead.
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