distinction between the statutory scheme of the CAA permitting programs
at issue in that case and the mobile source programs under Title II of
the Act which were at issue in Massachusetts. Namely, the UARG opinion
notes that part of the Court's reasoning in Massachusetts was based on
its understanding that ``nothing in the Act suggested that regulating
greenhouse gases under [Title II] would conflict with the statutory
design. Title II would not compel EPA to regulate in any way that would
be `extreme,' `counterintuitive,' or contrary to `common sense.' . . .
At most, it would require EPA to take the modest step of adding
greenhouse-gas standards to the roster of new-motor-vehicle emission
regulations.'' 134 S.Ct. at 2441 (quoting Massachusetts, 549 U.S. at
531). Like Massachusetts, the statutory provisions for this action are
found in Title II, and closely parallel the structure and language of
the statutory program at issue in Massachusetts.\87\ Compare CAA
section 231(a)(2)(A) with 202(a)(1). Nor will reading the Title II
provision section 231(a)(2)(A) to extend to GHGs result in a regulatory
outcome that would be extreme, counterintuitive or contrary to common
sense. Instead, as the D.C. Circuit has previously ruled, the EPA's
discretion when establishing reasonable standards under section 231 is
exceptionally broad. See NACAA, 489 F.3d at 1230-32. In short, the UARG
opinion in no way precludes the EPA's interpretation that ``air
pollutant'' as used in CAA section 231(a)(2)(A) includes GHGs, but
rather supports that interpretation.
---------------------------------------------------------------------------

    \87\ Although this comment asserts that section 202(a) does not
include mention of ``air quality control region'' as other
provisions of section 231(a) do, that distinction is immaterial. As
described above, the use of that term in other paragraphs imposing
additional duties beyond those established by section 231(a)(2)(A)
does not affect what pollutants may be addressed under section
231(a)(2)(A).
---------------------------------------------------------------------------

    To the extent that the commenter is suggesting that the EPA should
exercise its discretion to interpret CAA section 231(a)(2)(A) to
exclude GHGs, the EPA declines to do so. The commenter has provided no
persuasive reason for such an exclusion. Moreover, to make the
threshold findings in this action, the EPA must, fundamentally, answer
only two questions: Whether the particular ``air pollution''--here, the
six well-mixed GHGs--``may reasonably be anticipated to endanger public
health or welfare,'' and whether emissions of those six well-mixed GHGs
from U.S. covered aircraft engines ``cause, or contribute to'' that
endangerment. See CRR, 648 F.3d at 117 (interpreting analogous
provisions in CAA section 202(a)). Because the EPA answers both of
these questions in the affirmative for emissions of the six well-mixed
GHGs from U.S. covered aircraft engines--based on extensive scientific
evidence and emissions information, as explained in detail in sections
IV and V below--it is appropriate and reasonable to make both
endangerment and cause or contribute findings under section
231(a)(2)(A) in this action.
    In sum, after considering all of the relevant information,
including that in public comments, the EPA interprets section
231(a)(2)(A) to include authority to address GHGs from U.S. covered
aircraft engines. This interpretation is consistent with both its own
and with judicial interpretations that the EPA's authority under the
analogous section 202(a) unambiguously extends to GHGs.

B. Air Pollutant, Public Health and Welfare

    The CAA defines both ``air pollutant'' and ``welfare.'' Air
pollutant is defined as: ``any air pollution agent or combination of
such agents, including any physical, chemical, biological,

radioactive (including source material, special nuclear material, and
byproduct material) substance or matter which is emitted into or
otherwise enters the ambient air. Such term includes any precursors to
the formation of any air pollutant, to the extent the Administrator has
identified such precursor or precursors for the particular purpose for
which the term `air pollutant' is used.'' CAA section 302(g). GHGs fit
well within this capacious definition. See Massachusetts v. EPA, 549
U.S. at 532. They are ``without a doubt'' physical chemical substances
emitted into the ambient air. Id. at 529. Section V below contains
further discussion of the term ``air pollutant'' for purposes of this
section 231(a)(2)(A) contribution finding, which uses the same
definition of air pollutant as the one the EPA adopted for purposes of
the 2009 Endangerment Finding.
    Regarding ``welfare,'' the CAA states that ``[a]ll language
referring to effects on welfare includes, but is not limited to,
effects on soils, water, crops, vegetation, manmade materials, animals,
wildlife, weather, visibility, and climate, damage to and deterioration
of property, and hazards to transportation, as well as effects on
economic values and on personal comfort and well-being, whether caused
by transformation, conversion, or combination with other air
pollutants.'' CAA section 302(h). This definition is quite broad.
Importantly, it is not an exclusive list due to the use of the term
``includes, but is not limited to . . .'' Effects other than those
listed here may also be considered effects on welfare.
    Moreover, the terms contained within the definition are themselves
expansive. For example, deterioration to property could include damage
caused by extreme weather events. Effects on vegetation could include
impacts from changes in temperature and precipitation as well as from
the spreading of invasive species or insects. Prior welfare effects
evaluated by the EPA in other contexts include impacts on vegetation,
as well as reduced visibility, changes in nutrient balance and acidity
of the environment, soiling of buildings and statues, and erosion of
building materials. See, e.g., Final Secondary National Ambient Air
Quality Standards for Oxides of Nitrogen and Sulfur, 77 FR 20218 (April
3, 2012); Control of Emissions from Nonroad Large Spark Ignition
Engines and Recreational Engines (Marine and Land-Based), 67 FR 68242
(November 8, 2002); Final Heavy-Duty Engine and Vehicle Standards and
Highway Diesel Sulfur Control Requirements, 66 FR 5002 (January 18,
2001).
    Although the CAA defines ``effects on welfare'' as discussed above,
there is no definition of ``public health'' in the Clean Air Act. The
Supreme Court has discussed the concept of ``public health'' in the
context of whether costs can be considered when setting NAAQS. Whitman
v. American Trucking Ass'n, 531 U.S. 457 (2001). In Whitman, the Court
imbued the term with its most natural meaning: ``the health of the
public.'' Id. at 466. When considering public health, the EPA has
looked at morbidity, such as impairment of lung function, aggravation
of respiratory and cardiovascular disease, and other acute and chronic
health effects, as well as mortality. See, e.g., Final National Ambient
Air Quality Standard for Ozone, 73 FR 16436 (March 27, 2008).

IV. The Administrator's Finding Under CAA Section 231 That Greenhouse
Gases Endanger Public Health and Welfare

    The Administrator finds, for purposes of CAA section 231(a)(2)(A),
that elevated concentrations of the six well-mixed GHGs constitute air
pollution that may reasonably be anticipated to endanger both the
public health and welfare of current and future generations. The
Administrator is making this finding specifically with regard to the
same definition of the ``air pollution'' under CAA section 231(a)(2) as
that used under CAA section 202(a)(1), namely the combined mix of
CO2, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride, which together are the root
cause and best understood drivers of human-induced climate change and
the resulting impacts on public health and welfare. The EPA received
public comments on this definition of air pollution from the proposed
findings, and summarizes responses to some of those key comments below;
fuller responses to public comments can be found in EPA's Response to
Comments document included in the docket. The Administrator addresses
other climate-forcing agents both in the 2009 Endangerment Finding \88\
and in this action; however, these substances are not included in the
air pollution definition used in this action for the reasons discussed
below in section IV.B.7.
---------------------------------------------------------------------------

    \88\ 74 FR at 66519-21.
---------------------------------------------------------------------------

    Section IV.A below discusses the EPA's approach to evaluating the
scientific evidence before it. Section IV.B discusses the scope and
nature of the relevant air pollution for the endangerment finding under
CAA section 231(a)(2)(A), including a discussion of other substances
with climate effects that were addressed but not included in the
definition of air pollution. Section IV.C summarizes the scientific
evidence that the air pollution is reasonably anticipated to endanger
both public health and welfare. Section IV.D summarizes the
Administrator's conclusion for purposes of section 231(a)(2)(A), in
light of the evidence, analysis, and conclusions that led to the 2009
Endangerment Finding as well as more recent evidence and consideration
of public comments, that emissions of the six well-mixed GHGs in the
atmosphere may reasonably be anticipated to endanger public health and
welfare.

A. The Science Upon Which the Agency Relied

    This finding under section 231(a)(2)(A) reflects the EPA's careful
consideration not only of the scientific and technical record for the
2009 Endangerment Finding, but also of science assessments released
since 2009, which, as illustrated below, strengthen and further support
the judgment that the six well-mixed GHGs in the atmosphere may
reasonably be anticipated to endanger public health and welfare. The
Administrator's view is that the body of scientific evidence amassed in
the record for the 2009 Endangerment Finding compellingly supports an
endangerment finding for the six well-mixed GHGs under CAA section
231(a)(2)(A). While the EPA is providing a summary of newer scientific
assessments below, the EPA is also relying on the same scientific and
technical evidence discussed in the notices for the 2009 Endangerment
Finding in these final findings for purposes of CAA section
231(a)(2)(A).\89\
---------------------------------------------------------------------------

    \89\ See sections III of the 2009 Proposed Endangerment Finding
and sections III and IV of the 2009 Endangerment Finding. 74 FR at
18894-18904 and 74 FR at 66510-36.
---------------------------------------------------------------------------

    The EPA is following the same approach toward technical and
scientific information in this finding under section 231(a)(2)(A) as it
used in the 2009 Endangerment Finding. More specifically, in the 2009
Endangerment Finding the EPA's approach to providing the technical and
scientific information to inform the Administrator's judgment regarding
the question of whether GHGs endanger public health and welfare was to
consider the recent, major assessments by the U.S. Global Change
Research Program (USGCRP), the IPCC, and the National Research Council
of the

National Academies of Sciences, Engineering, and Medicine (referred to
interchangeably as NRC or NAS) as the primary scientific and technical
basis informing the endangerment finding. These assessments draw
synthesis conclusions across thousands of individual peer-reviewed
studies that appear in scientific journals, and the reports themselves
undergo additional peer review. The EPA has considered the processes
and procedures employed by the USGCRP, IPCC, and the NRC in terms of
factors such as their objectivity, integrity, utility, and
transparency, including how they 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.\90\ These assessments have
been adequately peer reviewed in a manner commensurate with the EPA's
Peer Review Policy \91\ and guidance in the EPA's Peer Review
Handbook.\92\
---------------------------------------------------------------------------

    \90\ 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-quality-existing-scientific-and-technical-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/quality/guidelines-ensuring-and-maximizing-quality-objectivity-utility-and-integrity-information (last accessed July 11, 2016).
    \91\ 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
April 12, 2016).
    \92\ 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 April 12, 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).
---------------------------------------------------------------------------

    The EPA is giving careful consideration to all of the scientific
and technical information in the record. However, the Administrator
considers the major scientific assessments as the primary scientific
and technical basis of her endangerment decision. This 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. These assessments addressed the
scientific issues that the EPA was required to examine, were
comprehensive in their coverage of the GHG and climate change issues,
and underwent rigorous and exacting peer review by the expert
community, as well as rigorous levels of U.S. government review, in
which the EPA took part. The major findings of the USGCRP, IPCC, and
NRC assessments support the Administrator's determination that elevated
concentrations of GHGs in the atmosphere may reasonably be anticipated
to endanger the public health and welfare of current and future
generations. The EPA presented this scientific support at length in the
comprehensive record for the 2009 Endangerment Finding.
    The EPA reviewed ten administrative petitions for reconsideration
of the 2009 Endangerment Finding in 2010.\93\ In the Reconsideration
Denial, the Administrator denied those petitions on the basis of the
Petitioners' failure to provide substantial support for their argument
that the EPA should revise the 2009 Endangerment Finding and their
objections' lack of ``central relevance'' to the Finding.\94\ The EPA
prepared an accompanying three-volume Response to Petitions document to
provide additional information, often more technical in nature, in
response to the arguments, claims, and assertions by the Petitioners to
reconsider the Endangerment Finding.\95\
---------------------------------------------------------------------------

    \93\ Administrative petitions are available from www3.epa.gov/climatechange/endangerment/petitions.html (last accessed June 21,
2016), and in the docket for the 2009 Endangerment Finding: EPA-HQ-
OAR-2009-0171.
    \94\ U.S. EPA, 2010: Denial of the Petitions to Reconsider the
Endangerment and Cause or Contribute Findings for Greenhouse Gases
Under section 202(a) of the Clean Air Act, 75 FR 49557 (August 13,
2010) (``Reconsideration Denial''). In that notice, the EPA
thoroughly considered the scientific and technical information
relevant to the petitions. In addition to the other information
discussed in the present notice, the EPA is also relying on the
scientific and technical evidence discussed in that prior notice for
purposes of its proposed determination under CAA section 231. See
section III of the Reconsideration Denial.
    \95\ The Response to Petitions document is available from
www3.epa.gov/climatechange/endangerment/petitions.html (last
accessed June 21, 2016), and in the docket for the 2009 Endangerment
Finding: EPA-HQ-OAR-2009-0171.
---------------------------------------------------------------------------

    The 2009 Endangerment Finding and the 2010 Reconsideration Denial
were challenged in a lawsuit before the D.C. Circuit.\96\ On June 26,
2012, the D.C. Circuit upheld the Endangerment Finding and the
Reconsideration Denial, ruling that the Finding (including the
Reconsideration Denial) was not arbitrary or capricious, was consistent
with the U.S. Supreme Court's decision in Massachusetts v. EPA (which
affirmed the EPA's authority to regulate GHGs) \97\ and the text and
structure of the CAA, and was adequately supported by the
administrative record.\98\ The D.C. Circuit also agreed with the EPA
that the Petitioners had ``not provided substantial support for their
argument that the Endangerment Finding should be revised.'' \99\ It
found that the EPA had based its decision on ``substantial scientific
evidence,'' observing that ``EPA's scientific evidence of record
included support for the proposition that greenhouse gases trap heat on
earth that would otherwise dissipate into space; that this `greenhouse
effect' warms the climate; that human activity is contributing to
increased atmospheric levels of greenhouse gases; and that the climate
system is warming,'' as well as providing extensive scientific evidence
for EPA's determination that anthropogenically induced climate change
threatens both public health and welfare.\100\ The D.C. Circuit further
noted that the EPA's reliance on assessments was consistent with the
methods decision-makers often use to make a science-based
judgment.\101\ Moreover, it supported the EPA's reliance on the major
scientific assessment reports conducted by USGCRP, IPCC, and NRC and
found:
---------------------------------------------------------------------------

    \96\ Coalition for Responsible Regulation, Inc. v. Environmental
Protection Agency, 684 F.3d 102 (D.C. Cir. 2012), reh'g en banc
denied, 2012 U.S. App. LEXIS 25997, 26313, 26315 (D.C. Cir. 2012)
(CRR).
    \97\ 549 U.S. 497 (2007).
    \98\ CRR, 684 F.3d at 117-27.
    \99\ Id. at 125.
    \100\ Id. at 120-121.
    \101\ Id. at 121.

    The EPA evaluated the processes used to develop the various
assessment reports, reviewed their contents, and considered the
depth of the scientific consensus the reports represented. Based on
these evaluations, the EPA determined the assessments represented
the best source material to use in deciding whether GHG emissions
may be reasonably anticipated to endanger public health or welfare .
. . It makes no difference that much of the scientific evidence in
large part consisted of ``syntheses'' of individual studies and
research. Even individual studies and research papers often
synthesize past work in an area and then build upon it. This is how
science works. The EPA is not required to re-prove the existence of
the atom every time it approaches a scientific question.\102\
---------------------------------------------------------------------------

    \102\ Id. at 120.

    In addition, the EPA's consideration of the major assessments to
inform the Administrator's judgment allowed for full and explicit
recognition of scientific uncertainty regarding the endangerment posed
by the atmospheric buildup of GHGs. The Administrator considered the
fact that ``some aspects of climate change science and the projected
impacts are more certain than others.'' \103\ The D.C. Circuit

subsequently noted that ``the existence of some uncertainty does not,
without more, warrant invalidation of an endangerment finding.'' \104\
---------------------------------------------------------------------------

    \103\ 74 FR at 66524.
    \104\ CRR, 684 F.3d at 121.
---------------------------------------------------------------------------

    As noted above, the Supreme Court granted some of the petitions for
certiorari that were filed, while denying others, but agreed to decide
only the question: ``Whether EPA permissibly determined that its
regulation of greenhouse gas emissions from new motor vehicles
triggered permitting requirements under the Clean Air Act for
stationary sources that emit greenhouse gases.'' \105\ Thus, the
Supreme Court did not disturb the D.C. Circuit's holding that affirmed
the 2009 Endangerment Finding.
---------------------------------------------------------------------------

    \105\ Utility Air Reg. Group v. EPA, 134 S. Ct. 2427, 2438
(2014) (internal marks and citations omitted). See also Virginia v.
EPA, 134 S. Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418
(2013), and CRR, 134 S. Ct. 468 (2013) (all denying cert.).
---------------------------------------------------------------------------

    Since the closure of the administrative record concerning the 2009
Endangerment Finding (including the denial of petitions for
reconsideration), a number of new major, peer-reviewed scientific
assessments have been released. The EPA carefully reviewed the updated
scientific conclusions in these assessments, largely to evaluate
whether they would lead the EPA in this CAA section 231(a)(2)(A)
finding to use a different interpretation of, or place more or less
weight on, the major findings reflected in the previous assessment
reports that underpinned the Administrator's judgment that the six
well-mixed GHGs endanger public health and welfare. The EPA reviewed
the following new major peer-reviewed scientific assessments:

 IPCC's 2013-2014 Fifth Assessment Report (AR5) \106\
---------------------------------------------------------------------------

    \106\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University
Press, 1535 pp, doi:10.1017/CBO9781107415324; IPCC, 2014: Climate
Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global
and Sectoral Aspects. Contribution of Working Group II to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change
[Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B.
Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and
L.L. White (eds.)]. Cambridge University Press, 1132 pp; IPCC, 2014:
Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B:
Regional Aspects. Contribution of Working Group II to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change
[Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach,
T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B.
Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and
L.L. White (eds.)]. Cambridge University Press, 688 pp; and IPCC,
2014: Climate Change 2014: Mitigation of Climate Change.
Contribution of Working Group III to the Fifth Assessment Report of
the Intergovernmental Panel on Climate Change [Edenhofer, O., R.
Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A.
Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J.
Savolainen, S. Schl[ouml]mer, C. von Stechow, T. Zwickel and J.C.
Minx (eds.)]. Cambridge University Press, 1435 pp.
---------------------------------------------------------------------------

 IPCC's 2012 ``Special Report on Managing the Risks of Extreme
Events and Disasters to Advance Climate Change Adaptation'' (SREX)
\107\
---------------------------------------------------------------------------

    \107\ IPCC, 2012: Managing the Risks of Extreme Events and
Disasters to Advance Climate Change Adaptation. A Special Report of
Working Groups I and II of the Intergovernmental Panel on Climate
Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken,
K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen,
M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, 582
pp.
---------------------------------------------------------------------------

 USGCRP's 2014 ``Climate Change Impacts in the United States:
the Third National Climate Assessment'' (NCA3) \108\
---------------------------------------------------------------------------

    \108\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, 841 pp.
---------------------------------------------------------------------------

 NRC's 2010 ``Ocean Acidification: A National Strategy to Meet
the Challenges of a Changing Ocean'' (Ocean Acidification) \109\
---------------------------------------------------------------------------

    \109\ NRC, 2010: Ocean Acidification: A National Strategy to
Meet the Challenges of a Changing Ocean. The National Academies
Press, 188 pp.
---------------------------------------------------------------------------

 NRC's 2011 ``Climate Change, the Indoor Environment, and
Health'' (Indoor Environment) \110\
---------------------------------------------------------------------------

    \110\ NRC Institute of Medicine, 2011: Climate Change, the
Indoor Environment, and Health. Washington, DC: The National
Academies Press, 272 pp.
---------------------------------------------------------------------------

 NRC's 2011 ``Report on Climate Stabilization Targets:
Emissions, Concentrations, and Impacts over Decades to Millennia''
(Climate Stabilization Targets) \111\
---------------------------------------------------------------------------

    \111\ NRC 2011: Climate Stabilization Targets: Emissions,
Concentrations, and Impacts over Decades to Millennia. The National
Academies Press, 298 pp.
---------------------------------------------------------------------------

 NRC's 2011 ``National Security Implications for U.S. Naval
Forces'' (National Security Implications) \112\
---------------------------------------------------------------------------

    \112\ NRC, 2011: National Security Implications of Climate
Change for U.S. Naval Forces. The National Academies Press, 226 pp.
---------------------------------------------------------------------------

 NRC's 2011 ``Understanding Earth's Deep Past: Lessons for Our
Climate Future'' (Understanding Earth's Deep Past) \113\
---------------------------------------------------------------------------

    \113\ NRC, 2011: Understanding Earth's Deep Past: Lessons for
Our Climate Future. The National Academies Press, 212 pp.
---------------------------------------------------------------------------

 NRC's 2012 ``Sea-Level Rise for the Coasts of California,
Oregon, and Washington: Past, Present, and Future'' (Sea Level Rise)
\114\
---------------------------------------------------------------------------

    \114\ NRC, 2012: Sea-Level Rise for the Coasts of California,
Oregon, and Washington: Past, Present, and Future. The National
Academies Press, 201 pp.
---------------------------------------------------------------------------

 NRC's 2013 ``Climate and Social Stress: Implications for
Security Analysis'' (Climate and Social Stress) \115\
---------------------------------------------------------------------------

    \115\ NRC, 2013: Climate and Social Stress: Implications for
Security Analysis. The National Academies Press, 280 pp.
---------------------------------------------------------------------------

 NRC's 2013 ``Abrupt Impacts of Climate Change'' (Abrupt
Impacts) \116\
---------------------------------------------------------------------------

    \116\ NRC, 2013: Abrupt Impacts of Climate Change: Anticipating
Surprises. The National Academies Press, 250 pp.
---------------------------------------------------------------------------

 NRC's 2014 ``The Arctic in the Anthropocene: Emerging Research
Questions'' (Arctic) \117\.
---------------------------------------------------------------------------

    \117\ NRC, 2014: The Arctic in the Anthropocene: Emerging
Research Questions. The National Academies Press, 220 pp.

    From its review, the EPA finds that these new assessments are
largely consistent with, and in many cases strengthen and add to, the
already compelling and comprehensive scientific evidence detailing the
role of the six well-mixed GHGs in driving climate change, explained in
the 2009 Endangerment Finding.
1. Response to Key Comments on the EPA's Approach to the Science
    Here we summarize key public comments regarding the approach to the
science--see the Response to Comments document for the Agency's full
responses to comments. Several commenters agreed and no commenters
disagreed with the EPA's approach to the science for making an
endangerment decision specifically with respect to the six well-mixed
GHGs (see section IV.B.7 for a summary of key public comments and our
responses to commenters who argued that the science supports expanding
the scope of the endangerment finding to include other climate forcers
beyond the six well-mixed GHGs). They specifically mentioned their
support for the EPA's approach to considering the scientific and
technical information in the record of the 2009 Endangerment Finding--
primarily the recent, major assessments by the USGCRP, the IPCC, and
the NRC--as well as the most recent scientific assessments for
additional support and justification. For the reasons stated in section
IV.A above, the EPA agrees with the commenters that this approach
ensures that the Administrator considers the best available scientific
and technical information.

B. The Air Pollution Consists of Six Key Well-Mixed Greenhouse Gases

    The Administrator must define the scope and nature of the relevant
air pollution for the endangerment finding under CAA section
231(a)(2)(A). In this

final action, the Administrator finds that the air pollution is the
combined mix of six well-mixed GHGs, which together are the root cause
and best understood drivers of human-induced climate change and the
resulting impacts on public health and welfare. These six GHGs--
CO2, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride--are considered an aggregate
group for purposes of this finding. 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, for which key comments and responses are summarized in
sections IV.B.6 and 7 below. The Administrator considered five primary
reasons in the 2009 Endangerment Finding for focusing on this aggregate
group as the air pollution: (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, particulate matter, volatile organic
compounds).\118\ After consideration of all information before her,
including public comments, as explained below, the Administrator
maintains her view that these five reasons for defining the scope and
nature of the air pollution to be these six well-mixed GHGs remain
valid and well supported by the current science and are therefore
reasonable bases for adopting the same definition of ``air pollution''
in this section 231(a)(2)(A) finding as that under section 202(a)(1).
The following subsections summarize the five reasons detailed in the
2009 Endangerment Finding and as appropriate, summarize additional
supporting information from the recent scientific assessments published
since 2009.
---------------------------------------------------------------------------

    \118\ 74 FR at 66517-19.
---------------------------------------------------------------------------

1. Common Physical Properties of the Six Greenhouse Gases
    The six GHGs share common physical properties that are relevant to
the climate change problem. They all are sufficiently long lived in the
atmosphere such that, once emitted, concentrations of each gas become
globally well mixed in the atmosphere.\119\ A well-mixed gas has
relatively uniform concentrations in the atmosphere anywhere around the
globe, with little local or regional variation except immediately next
to sources or sinks. A given amount of a well-mixed gas emitted
anywhere will have similar impacts on global concentrations regardless
of the geographic location of emission. All six GHGs trap outgoing heat
that would otherwise escape to space, and all are directly emitted from
a source as a GHG rather than becoming a GHG in the atmosphere after
emission of a precursor gas. This fundamental scientific understanding
of the intrinsic physical, chemical, and atmospheric properties of the
six GHGs has not changed and remains supported by the more recent
climate change assessments.
---------------------------------------------------------------------------

    \119\ The properties ``long lived'' and ``well mixed'' used in
this document mean that the gas has a lifetime in the atmosphere
sufficient to become globally well mixed throughout the entire
atmosphere, which requires a minimum atmospheric lifetime of about
one year. Atmospheric lifetime is a measure of how long a type of
molecule is likely to remain in the atmosphere before it breaks
down, reacts with other gases, or is absorbed by Earth's surface.
The IPCC often refers interchangeably to the six well-mixed GHGs as
long-lived GHGs; however, the IPCC and others in the international
climate change community, such as the United Nations Environment
Programme, also refer to methane and some HFCs as ``near-term
climate forcers,'' ``short-lived climate forcers,'' or ``short-lived
climate pollutants.'' These terms refer to those compounds whose
impacts on Earth's climate occurs primarily with the first decade
after their emission. According to the IPCC AR5 (2014), methane has
an atmospheric lifetime of about 12 years. One of the most commonly
used hydrofluorocarbons (HFC-134a) has a lifetime of about 13 years.
Thus, methane and some HFCs are both short- and long-lived GHGs--
i.e., they have lifetimes long enough to become globally well mixed
in the atmosphere, but short enough to primarily affect Earth's
climate within a decade after their emission. For comparison,
nitrous oxide has a lifetime of around 130 years; sulfur
hexafluoride over 3,000 years; and some perfluorocarbons up to
10,000 to 50,000 years. CO2 is sometimes approximated as
having a lifetime of roughly 100 years, but for a given amount of
CO2 emitted, a better description is that some fraction
of the atmospheric increase in concentration is quickly absorbed by
the oceans and terrestrial vegetation, some fraction of the
atmospheric increase will only slowly decrease over a number of
years, and a small portion of the increase will remain for many
centuries or more.
---------------------------------------------------------------------------

2. The Six Well-Mixed Greenhouse Gases Are the Primary and Best
Understood Driver of Current and Projected Climate Change
    The Administrator judges that the scientific evidence is compelling
that together the six well-mixed GHGs constitute the largest
anthropogenic driver of climate change. In addition, the six well-mixed
GHGs are the best-understood driver of climate change because they have
well-understood physical properties as described above that govern
their climate effect (e.g., their radiative forcing, a measure of their
total net effect on the global energy balance). As explained in more
detail in the 2009 Endangerment Finding,\120\ the Administrator made
the judgment that the scientific evidence is compelling that elevated
concentrations of heat-trapping GHGs are the root cause of recently
observed climate change and that the scientific record showed that most
of the observed increase in global average temperatures since the mid-
20th century is very likely due to the observed increase in
anthropogenic GHG concentrations. The attribution of observed climate
change to anthropogenic activities was based on multiple lines of
evidence. The first line of evidence arises from our basic physical
understanding of the effects of changing concentrations of GHGs,
natural factors, and other human impacts on the climate system. The
second line of evidence arises from indirect, historical estimates of
past climate changes that suggest that the changes in global surface
temperature over the last several decades are unusual. The third line
of evidence arises from the use of computer-based climate models to
simulate the likely patterns of response of the climate system to
different forcing mechanisms (both natural and anthropogenic). Observed
increases in global average air temperatures are driving observed
climate impacts like widespread melting of snow and ice and rising
global average sea level. The Administrator also considered these
observed changes as additional evidence of the unequivocal warming of
the climate system driven primarily by elevated atmospheric GHG
concentrations because the consistency of these observed changes in
physical and biological systems and the observed significant warming
cannot be explained entirely due to natural variability or other
confounding non-climate factors.
---------------------------------------------------------------------------

    \120\ 74 FR at 66517-18.

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

    In addition, as described in more detail in the 2009 Endangerment
Finding,\121\ the Administrator made the judgment that the scientific
evidence is compelling that six GHGs are expected to remain the primary
driver of future climate change and that, without substantial and near-
term efforts to significantly reduce emissions, it can be expected that
atmospheric concentrations of the six GHGs will continue to climb and
thus lead to ever greater rates of climate change. Given the long
atmospheric lifetime of the six well-mixed GHGs, which range from
roughly a decade to centuries, future atmospheric GHG concentrations
for the remainder of this century and beyond will be influenced not
only by future emissions but indeed by present-day and near-term
emissions. Consideration of future plausible scenarios, and how our
current GHG emissions essentially commit present and future generations
to cope with an altered atmosphere and climate, reinforces the
Administrator's judgment that it is appropriate to define the
combination of the six key greenhouse gases as the air pollution. Most
future scenarios that assume no explicit GHG mitigation actions (beyond
those already enacted) project increasing global GHG emissions over the
century, which in turn result in climbing GHG concentrations.
Concentrations of the six well-mixed GHGs increase even for those
scenarios where annual emissions toward the end of the century are
assumed to be lower than current annual emissions.
---------------------------------------------------------------------------

    \121\ 74 FR at 66518-19.
---------------------------------------------------------------------------

    The EPA has also carefully reviewed the recent assessments of the
IPCC, USGCRP, and NRC. The EPA finds that these recent assessments
support and strengthen the evidence cited in the 2009 Endangerment
Finding that current atmospheric GHG concentrations are now at elevated
and essentially unprecedented levels primarily as a result of both
historic and current anthropogenic emissions. The 2014 USGCRP NCA3
states, ``Atmospheric levels measured at Mauna Loa in Hawai`i and at
other sites around the world reached 400 parts per million in 2013,
higher than the Earth has experienced in over a million years.'' \122\
Such concentrations are the primary driver of observed changes in
Earth's climate system, namely increased global average temperatures
that drive climate impacts like widespread melting of snow and ice and
rising global average sea level (discussed in more detail in section
IV.C). The recent assessments of the IPCC, USGCRP, and NRC also
describe how these six well-mixed GHGs play a dominant role in future
warming of the climate system. The USGCRP NCA3 makes the following
finding with very high confidence: ``The magnitude of climate change
beyond the next few decades depends primarily on the amount of heat-
trapping gases emitted globally, and how sensitive the Earth's climate
is to those emissions.'' \123\ Key findings from the recent assessments
regarding global and U.S. trends are described briefly below.
---------------------------------------------------------------------------

    \122\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 739.
    \123\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p.20. See also p. 736: ``Past emissions of heat-trapping
gases have already committed the world to a certain amount of future
climate change. How much more the climate will change depends on
future emissions and the sensitivity of the climate system to those
emissions.''
---------------------------------------------------------------------------

a. Key Observed Trends Driven Primarily by the Six Well-Mixed GHGs
    According to the IPCC AR5, observations of the Earth's globally
averaged combined land and ocean surface temperature over the period
1880 to 2012 show a warming of 0.85 [0.65 to 1.06] degrees Celsius or
1.53 [1.17 to 1.91] degrees Fahrenheit.\124\ The IPCC AR5 concludes
that the increase in atmospheric GHG concentrations since 1750, plus
other human activities (e.g., land use change and aerosol emissions),
has had a radiative forcing effect estimated to be 2.3 Watts per square
meter (W/m\2\) in 2011.\125\ Radiative forcing is a measure of a
substance's total net effect on the global energy balance for which a
positive number represents a warming effect and a negative number
represents a cooling effect. The IPCC's estimate is an increase from
the previous 2007 IPCC Fourth Assessment Report (AR4) total net
estimate of 1.6 W/m\2\ that was referred to in the record for the 2009
Endangerment Finding. The reasons for this increase include continued
increases in GHG concentrations, as well as reductions in the estimated
negative forcing due to aerosol particles. The IPCC AR5 rates the level
of confidence \126\ in their radiative forcing estimates as ``high''
for methane and ``very high'' for CO2 and nitrous oxide.
---------------------------------------------------------------------------

    \124\ ``IPCC, 2013: Summary for Policymakers. In: Climate Change
2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor,
S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, 29 pp.
    \125\ Ibid.
    \126\ The IPCC expresses levels of confidence using five
qualifiers: Very low, low, medium, high, and very high. These levels
are based on a qualitative evaluation of the robustness of the
evidence (considering the type, amount, quality, and consistency of
evidence such as data, mechanistic understanding, theory, models,
and expert judgment) and the degree of agreement among the findings.
---------------------------------------------------------------------------

    The new assessments also have greater confidence since the 2009
Endangerment Finding in attributing recent warming to human causes. The
IPCC AR5 stated that it is extremely likely (>95 percent likelihood)
that human influences have been the dominant cause of warming since the
mid-20th century, which is an even stronger statement than the AR4
conclusion that it is very likely (>90 percent likelihood) that most of
the increase in temperature since the mid-20th century was due to the
observed increase in anthropogenic GHG concentrations. The AR4
conclusion was referred to in the record for the 2009 Endangerment
Finding. In addition, the IPCC AR5 found that concentrations of
CO2 and several other of the major GHGs are higher than they
have been in at least 800,000 years. This is an increase from what was
reported in IPCC AR4, which found higher concentrations than in at
least 650,000 years.
    The USGCRP NCA3 states that there is very high confidence \127\
that the global climate change of the past 50 years is primarily due to
human activities. Human activities are affecting climate through
increasing atmospheric levels of heat-trapping GHGs, through changing
levels of various particles that can have either a heating or cooling
influence on the atmosphere, and through activities such as land use
changes that alter the reflectivity of the Earth's surface and cause
climatic warming and cooling effects. The USGCRP concludes that
``considering all known natural and human drivers of climate since
1750, a strong net warming from long-lived greenhouse gases produced by
human activities dominates the recent climate record.'' \128\
---------------------------------------------------------------------------

    \127\ The NCA expresses levels of confidence using four
qualifiers: low, medium, high, and very high. These levels are based
on the strength and consistency of the observed evidence; the skill,
range, and consistency of model projections; and insights from peer-
reviewed sources.
    \128\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 741.
---------------------------------------------------------------------------

    These recent and strong conclusions attributing recent observed
global warming to human influence have been made despite what some have
termed a

warming slowdown or ``hiatus'' over the past 15 years or so. The IPCC
AR5 notes that global mean surface temperature exhibits substantial
natural decadal and interannual variability. Short-term variability
does not alter conclusions about the long-term climate trend that the
IPCC AR5 finds after its review of independently verified observational
records: ``Each of the past three decades has been successively warmer
at the Earth's surface than all the previous decades in the
instrumental record, and the first decade of the 21st century has been
the warmest.'' 129 130
---------------------------------------------------------------------------

    \129\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University
Press, p. 161.
    \130\ Furthermore, we note that according to both NOAA and NASA,
2015 was the warmest year in the modern instrumental record for
globally averaged surface temperature, breaking the record
previously held by 2014. This now means that the last fifteen years
have been fifteen of the sixteen warmest years on record. Available
at http://www.ncdc.noaa.gov/sotc/global/201513 (last accessed April
11, 2016).
---------------------------------------------------------------------------

    Temperature trends at the global level have also been observed
regionally and in the United States. In the Northern Hemisphere, the
IPCC AR5 finds that the last 30 years were likely the warmest 30-year
period of the last 1400 years. The USGCRP NCA3 states with very high
confidence that ``U.S. average temperature has increased by 1.3 [deg]F
to 1.9 [deg]F since record keeping began in 1895; most of this increase
has occurred since about 1970. The most recent decade was the nation's
warmest on record.'' \131\ The USGCRP also notes that the rate of U.S.
temperature increase over the past 4 to 5 decades has been greater than
the rate observed in earlier decades.
---------------------------------------------------------------------------

    \131\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 28.
---------------------------------------------------------------------------

b. Key Projections Based Primarily on Future Scenarios of the Six Well-
Mixed GHGs
    Future temperature changes will depend on what path the world
follows with respect to GHG emissions and associated levels of GHG
concentrations in the atmosphere. The NRC Climate Stabilization Targets
assessment concludes that CO2 emissions are currently
altering the atmosphere's composition and will continue to alter
Earth's climate for thousands of years. The NRC Understanding Earth's
Deep Past assessment finds that ``the magnitude and rate of the present
greenhouse gas increase place the climate system in what could be one
of the most severe increases in radiative forcing of the global climate
system in Earth history.'' \132\ A key future projection of this
assessment is that by the end of the century, if no emissions
reductions are made, CO2 concentrations are projected to
increase to levels that Earth has not experienced for more than 30
million years. In its high emission scenario, the IPCC AR5 projects
that global temperatures by the end of the century will likely be 2.6
to 4.8 degrees Celsius (4.7 to 8.6 degrees Fahrenheit) warmer than
today. Temperatures on land and in northern latitudes will likely warm
even faster than the global average.
---------------------------------------------------------------------------

    \132\ NRC, 2011: Understanding Earth's Deep Past: Lessons for
Our Climate Future. The National Academies Press, p. 138.
---------------------------------------------------------------------------

    For the United States, the USGCRP NCA3 concludes, ``Warming is
ultimately projected for all parts of the nation during this century.
In the next few decades, this warming will be roughly 2 [deg]F to 4
[deg]F in most areas. By the end of the century, U.S. warming is
projected to correspond closely to the level of global emissions:
roughly 3 [deg]F to 5 [deg]F under lower emissions scenarios (B1 or RCP
4.5) involving substantial reductions in emissions, and 5 [deg]F to 10
[deg]F for higher emissions scenarios (A2 or RCP 8.5) that assume
continued increases in emissions; the largest temperature increases are
projected for the upper Midwest and Alaska.'' \133\
---------------------------------------------------------------------------

    \133\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 29.
---------------------------------------------------------------------------

3. The Six Well-Mixed GHGs Are Currently the Common Focus of the
Climate Change Science and Policy Communities
    The six well-mixed GHGs are currently the common focus of climate
science and policy analyses and discussions. Grouping them is
consistent with the focus of international and domestic climate science
research enterprises like the IPCC and USGCRP. The IPCC and USGCRP
assessment reports assess the climate change effects on health,
society, and the environment as a result of human-induced climate
change driven primarily by the group of six gases.
    Grouping them is also consistent with the focus of climate policy.
The United Nations Framework Convention on Climate Change (UNFCCC),
signed and ratified by the United States in 1992, requires its
signatories to ``develop, periodically update, publish and make
available . . . national inventories of anthropogenic emissions by
sources and removals by sinks of all greenhouse gases not controlled by
the Montreal Protocol, using comparable methodologies . . .'' \134\ To
date, the primary focus of UNFCCC actions and discussions has been on
the six well-mixed GHGs, including the recent Paris Agreement in which
Parties agreed to undertake nationally determined contributions to
achieving the goal of ``global peaking of GHG emissions as soon as
possible'' in order to reach a long-term global temperature
target.\135\ Domestically, the EPA has been developing standards for
GHG emissions from mobile and stationary sources under the Clean Air
Act since finalizing the 2009 Endangerment Finding.
---------------------------------------------------------------------------

    \134\ United Nations Framework Convention on Climate Change,
1992: Article 4(1)(a) of the United Nations Framework Convention on
Climate Change, p. 10. Available at https://unfccc.int/files/essential_background/background_publications_htmlpdf/application/pdf/conveng.pdf (last accessed April 11, 2016).
    \135\ United Nations Framework Convention on Climate Change,
2015: Adoption of the Paris Agreement Conference of the Parties
Twenty-first session Paris, FCCC/CP/2015/L.9/Rev.1, 12 December
2015, Available at: https://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf (last accessed April 8, 2016).
---------------------------------------------------------------------------

4. Defining Air Pollution as the Aggregate Group of Six GHGs Is
Consistent With Evaluation of Risks and Impacts Due to Human-Induced
Climate Change
    Based on her review of the science described in detail above in
section IV.B.2, the Administrator judges that the six well-mixed GHGs
constitute the largest anthropogenic driver of climate change and play
a dominant role in observed and projected changes in Earth's climate
system. Thus, the Administrator finds, as she did in the 2009
Endangerment Finding, that because the six well-mixed GHGs are
collectively the primary driver of current and projected human-induced
climate change, the current and future risks (here described in section
IV.C below) due to human-induced climate change--whether these risks
are associated with increases in temperature, changes in precipitation,
a rise in sea levels, changes in the frequency and intensity of weather
events, or more directly with the elevated GHG concentrations
themselves--can be associated with this definition of air pollution.
Due to the cumulative purpose of the statutory language, even if the
Administrator were to look at the atmospheric

concentration of each GHG individually, she would still consider the
impact of the concentration of a single GHG in combination with that
caused by the other GHGs.
5. Defining Air Pollution as the Aggregate Group of Six GHGs Is
Consistent With Past EPA Practice
    Treating the air pollution as the aggregate of the well-mixed GHGs
is consistent with other provisions of the CAA and previous EPA
practice under the CAA, where separate emissions from different sources
but with common properties may be treated as a class (e.g., particulate
matter (PM)). This approach addresses the total, cumulative effect that
the elevated concentrations of the six well-mixed GHGs have on climate
and, thus, on different elements of health, society, and the
environment. The EPA treats, for example, PM as a common class of air
pollution; PM is a complex mixture of extremely small particles and
liquid droplets. Particle pollution is made up of a number of
components, including acids (such as nitrates and sulfates), organic
chemicals, metals, and soil or dust particles.
6. Response to Key Comments on Defining the Air Pollution as the
Aggregate Group of the Six Well-Mixed Greenhouse Gases
    Many commenters agreed with the EPA that the ``air pollution'' for
purposes of the endangerment finding under section 231(a)(2)(A) of the
CAA should be defined as the six well-mixed GHGs. Several commenters
discussed the fact that aircraft engines emit only two of the six well-
mixed GHGs. Commenters pointed out that the majority of aircraft
emissions are CO2, while nitrous oxide emissions are
described as ``nominal (<1%)'' or ``trace.'' Some commenters ultimately
concluded that the EPA's approach to defining the air pollution as an
aggregate group of six gases is acceptable, but that the scope of
future regulations should be limited to CO2. One commenter
agreed with the Agency's evaluation of the six GHGs based on their
common attributes, but questioned the EPA's decision to aggregate the
six gases rather than considering them individually for purposes of
making the findings. Other commenters disagreed with the EPA and
requested limiting the definition of air pollution in this action to
CO2 or to CO2 and nitrous oxide.
    The EPA disagrees with comments regarding changing the definition
of the air pollution to limit it to only those GHGs that are emitted
from aircraft or to CO2 only. The EPA has explained both in
the 2009 Endangerment Finding and in the proposed findings under CAA
section 231(a)(2)(A) that the definition of the air pollution is based
on shared characteristics and common attributes relevant to climate
change science and policy \136\--which is not affected by the identity
of the source(s) of the emissions contributing to the air pollution.
The EPA recognized in the proposed findings that aircraft emit two of
the six well-mixed GHGs but stated that nonetheless it is entirely
reasonable and appropriate, and in keeping with the 2009 Endangerment
Finding and past EPA practice, for the Administrator to group into a
single class those substances that possess shared relevant properties,
even though they are not all emitted from the classes of sources before
her.\137\ After considering all the comments, this continues to be the
EPA's view. Moreover, this approach to defining air pollution (and air
pollutant, as described below) as a grouping of many substances is not
unique to GHGs but rather is common practice under the CAA, for example
for particulate matter and volatile organic compounds.
---------------------------------------------------------------------------

    \137\ 80 FR at 37774, 37785 and 37787.
---------------------------------------------------------------------------

    The five primary reasons for grouping the six well-mixed GHGs are
explained in detail above in sections IV.B.1 through IV.B.5. Because
the well-mixed GHGs are collectively the primary driver of current and
projected human-induced climate change, all current and future risks
due to human-induced climate change can be associated with this
definition of air pollution. Thus, this grouping is consistent with
evaluation of the scientific issues that the EPA is required to examine
in this endangerment finding, namely the risks and impacts due to
human-induced climate change. As discussed above, the key scientific
evidence and observations that are the basis of this finding focus on
the combined six well-mixed GHGs, and did not assess risks and impacts
associated with greenhouse gas-induced climate change using an
individual gas approach. Accordingly, we are not undertaking a separate
endangerment analysis for each of the six well-mixed gases
individually.
    The question of limits to the scope of future regulations is
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
this question 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
emission standards in response to EPA's anticipated future notice of
proposed rulemaking on standards. Although this final action
establishes 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 form that such standards may take.
    Another commenter expressed concern about EPA's proposed
endangerment finding because it does not differentiate between
CO2 emissions that result from combustion of fossil fuels
and those that result from ``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.'' \138\ The commenter argues that such crop-related
biogenic CO2 emissions should be excluded from the
endangerment 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.
---------------------------------------------------------------------------

    \138\ 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).
---------------------------------------------------------------------------

    The EPA reiterates that the Administrator defines the relevant air
pollution considered in the endangerment finding as the aggregate group
of the six well-mixed GHGs based on shared physical characteristics and
common attributes relevant to climate change science and policy, which
is not affected by consideration of the sources of the emissions
contributing to the air pollution. In the record for the 2009
Endangerment Finding, the Agency stated that ``all CO2
emissions, regardless of source, influence radiative forcing equally
once it reaches the atmosphere and therefore there is no distinction
between biogenic and non-biogenic CO2 regarding the
CO2 and the other well-mixed GHGs within the definition of
air pollution that is reasonably anticipated to endanger public health
and welfare.'' \139\ The EPA continues to hold that position in these

findings, which is supported by the evidence before it. First, the fact
that these CO2 emissions originate from combustion of
carbon-based fuels created through different processes is not relevant
to defining the air pollution that is reasonably anticipated to
endanger public health and welfare. The origin and constitution of a
fuel prior to its combustion and subsequent emission into the
atmosphere has no bearing on the fact that CO2 and the other
well-mixed GHGs are all sufficiently long lived to become well mixed in
the atmosphere, directly emitted, of well-known radiative forcing, and
generally grouped and considered together in climate change scientific
and policy forums as the primary driver of climate change. Moreover, as
explained in section IV.C of this document, the endangerment arises
from the elevated concentrations of the six well-mixed GHGs in the
atmosphere. A molecule of biogenic CO2 has the same
radiative forcing effect as a molecule of fossil-fuel derived
CO2. In other words, 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. Any differential
treatment of biogenic CO2 in the context of the endangerment
finding would be inconsistent with the primary scientific basis for the
grouping of the six well-mixed GHGs as a single class for purposes of
identifying the air pollution (and air pollutant, as explained below).
A more detailed response to the issues raised in this comment can be
found in the Response to Comments document in the docket.
---------------------------------------------------------------------------

    \139\ EPA, 2009. Response to Comments document, Volume 9: The
Endangerment Finding, EPA-HQ-OAR-2009-0171-11676. Available at
www.regulations.gov (last accessed April 11, 2016).
---------------------------------------------------------------------------

7. Other Climate Forcers Not Being Included in the Definition of Air
Pollution for This Finding
    Both in the 2009 Endangerment Finding and in this action, the
Administrator recognizes that there are other substances in addition to
the six well-mixed GHGs that are emitted from human activities and that
affect Earth's climate (referred to as climate forcers). However, as
described in more detail in the 2009 Endangerment Finding and in the
proposed findings under CAA section 231(a)(2)(A),\140\ these substances
do not fit within one or more of the five primary reasons for focusing
on this aggregate group as the air pollution. As described in the
following subsections, we received comments on the omission of water
vapor, NOX, and aerosol particles emitted from aircraft from
the proposed definition of air pollution for this finding, but not on
the omission of other climate forcers. After considering public
comments and additional information in the new assessments regarding
the climate-relevant substances outside the group of the six well-mixed
GHGs, it is the Administrator's view that the reasons stated in the
2009 Endangerment Finding \141\ for not including these substances in
the scope of the GHG air pollution still apply at this time.
---------------------------------------------------------------------------

    \140\ 74 FR at 66519-21 and 80 FR at 37781-84.
    \141\ 74 FR at 66519-21.
---------------------------------------------------------------------------

    As the EPA acknowledged in the proposed findings,\142\ some short-
lived substances--namely water vapor, NOX emitted at high
altitude, and aerosol particles including black carbon--have physical
properties that result in their having different, and often larger,
climate effects when emitted at high altitudes. For example, the
assessment literature indicates that aerosol particles, including black
carbon, emitted at high altitudes have more interactions with clouds
and therefore have different effects on the global energy balance than
do particles emitted at the surface. However, the very properties that
lead to differential climate effects depending on the altitude of
emission--properties that are different from those of the six well-
mixed GHGs--lead to more uncertainty in the scientific understanding of
these short-lived substances' total effect on Earth's climate. The
short-lived nature of these substances means that, unlike GHGs that are
sufficiently long lived to become well mixed in the atmosphere, the
climatic impact of the substance is dependent on a number of factors
such as the location and time of its emission. The magnitude, and often
the direction (positive/warming or negative/cooling), of the globally
averaged climate impact will differ depending on the location of the
emission due to the local atmospheric conditions (e.g., due to
differing concentrations of other compounds with which the emissions
can react, background humidity levels, or the presence or absence of
clouds). In addition, for emissions at any given location, the spatial
and temporal pattern of the climate forcing will be heterogeneous,
again often differing in direction (for example, in the case of
NOX emissions, the near-term effect in the hemisphere in
which the emissions occur is usually warming due to increased ozone
concentrations, but the longer term effects, and effects in the other
hemisphere, are often cooling due to increased destruction of methane).
More detail on the uncertainties relating to the climate effects of
these short-lived substances is provided in the subsections below in
response to public comments and in the Response to Comments document.
---------------------------------------------------------------------------

    \142\ 80 FR at 37781-84.
---------------------------------------------------------------------------

    Overall, 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,
NOX, and aerosol particles when emitted at high altitudes.
The dependence of the effects on where the substance is emitted, and
the complex temporal and spatial patterns that result, mean that the
current level of understanding regarding these short-lived substances
is much lower than for the six well-mixed GHGs. Given the
aforementioned 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.
    Many public comments either supported or opposed inclusion of other
substances in addition to the six well-mixed GHGs in the definition of
air pollution, and some specifically suggested water vapor,
NOX, and aerosol particles as additional substances to
include in that definition. The Agency's full responses to those
comments can be found in the Response to Comments document; key
comments and responses are summarized below.
a. Response to Key Comments on Including Other Climate Forcers in the
Definition of Air Pollution
    Some commenters argued that the proposed findings under CAA section
231(a)(2)(A) did not demonstrate careful examination of the scientific
issues with regard to those short-lived substances that have different
climate effects when emitted at high altitudes, and that a more
thorough analysis should lead the EPA to conclude that water vapor,
NOX, and black carbon also drive climate change in addition
to the six well-mixed GHGs. These comments stated that the EPA should
have quantified and included the effect of high-altitude water vapor,
NOX, and black carbon in the Agency's discussion of drivers
of climate change. Another commenter argued that the EPA should include
metal particulates (specifically lead, barium, and aluminum) in the
definition of air pollution for this finding because of their role in
aviation-induced cloudiness, which the commenter argues has a larger
effect on climate change than the six well-mixed GHGs.
    Although the EPA is not at this time taking final action to
determine whether these other climate forcers should be found to
represent air pollution within

the meaning of CAA section 231(a)(2)(A), the EPA disagrees with these
comments suggesting that the Agency did not carefully examine the
scientific issues and information supporting its current endangerment
finding in regard to these substances. Consistent with the approach
described in the proposed findings and for the reasons discussed above,
the Administrator considers the major peer-reviewed scientific
assessments of the IPCC and NRC as the primary scientific and technical
basis informing the endangerment finding and providing the current
state of scientific understanding of the differential climate effects
that water vapor, NOX, and aerosols such as black carbon
have when emitted at high altitudes. The EPA has considered the
following assessment reports to obtain the best estimates of these
substances' net impact on the climate system, which is generally
discussed in terms of radiative forcing: The IPCC AR5, the IPCC 2007
Fourth Assessment Report (AR4),\143\ the IPCC Special Report: Aviation
and the Global Atmosphere (IPCC 1999),\144\ the NRC's Advancing the
Science of Climate Change (NRC 2010),\145\ and the NRC's Atmospheric
Effects of Aviation: A Review of NASA's Subsonic Assessment Project
(NRC 1999).\146\ The USGCRP assessments have not dealt specifically
with emissions at high altitude.
---------------------------------------------------------------------------

    \143\ 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.
    \144\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)]
Cambridge University Press, 373 pp.
    \145\ NRC, 2010: Advancing the Science of Climate Change. The
National Academies Press, 528 pp.
    \146\ NRC, 1999: Atmospheric Effects of Aviation: A Review of
NASA's Subsonic Assessment Project. The National Academies Press, 54
pp.
---------------------------------------------------------------------------

    As described previously in section IV.A of this document, the
Administrator's consideration of the major scientific assessments
provides assurance that the Administrator is basing her judgment on the
best available, well-vetted science that reflected the consensus of the
climate science research community. These scientific assessments
addressed the scientific issues that the EPA was required to examine,
were comprehensive in their coverage of the GHG and climate change
issues, and underwent rigorous and exacting peer review by the expert
community, as well as rigorous levels of U.S. government review, in
which the EPA took part. The commenters provide no compelling arguments
against this approach, which underwent judicial review and was upheld
as described in section IV.A of this document. The assessments
synthesize literally thousands of individual studies to convey the
consensus conclusions on what the body of scientific literature tells
us, and the commenters did not provide evidence that we had missed or
mischaracterized conclusions of the assessments regarding aviation
impacts.
    The state of the science as represented in the assessment
literature supports the EPA's reasons for defining the air pollution as
the aggregate group of the six well-mixed GHGs, which include their
common physical properties relevant to climate change (i.e., directly
emitted and sufficiently long lived to become well mixed in the
atmosphere), the fact that these gases are considered the primary
drivers of climate change, and the fact that these gases remain the
best understood drivers of anthropogenic climate change. Water vapor,
NOX, aerosol particles, or aviation-induced cloudiness
associated with metal particulates do not share these common
attributes, and are each associated with substantial scientific
uncertainty. Accordingly, although the EPA is not making a final
determination on whether these additional substances should be found to
be air pollution within the meaning of CAA section 231(a)(2)(A), the
EPA is not at this time changing or expanding the definition of the air
pollution to include these additional substances. The following
subsections provide additional discussion of the state of the science
as represented in the assessment literature regarding the climatic
effects of these substances when emitted at high altitudes.
b. Responses to Key Comments on Changes in Clouds From High Altitude
Emissions of Water Vapor and Particles
    Some commenters supported the EPA's summary of the scientific
assessment literature and agreed that there are substantial scientific
uncertainties regarding net climate effects of aviation-induced
cloudiness from high altitude emissions of water vapor and particles.
Other commenters disagreed and argued that there is clear scientific
evidence that aviation-induced cloudiness associated with high altitude
emissions of water vapor drives climate change and should be included
in the definition of air pollution. One commenter disagrees and argues
that, due to their effect on aviation-induced cloudiness and climate
change, metal particulates should be included in the definition of air
pollution.
    The EPA disagrees with the comments regarding changing or expanding
the definition of the air pollution employed in this endangerment
finding to include these additional substances. For the reasons stated
above, the Administrator considers the scientific assessment literature
as the primary scientific and technical basis informing the
endangerment finding and providing the state of climate science on
aviation-induced cloudiness. Section IV.B.4 of the proposed findings
under CAA section 231(a)(2)(A) \147\ explained that aviation-induced
cloudiness (sometimes called AIC) refers to all changes in cloudiness
associated with aviation operations, which are primarily due to the
effects of high altitude emissions of water vapor and particles
(primarily sulfates and black carbon). Changes in cloudiness affect the
climate by both reflecting solar radiation (cooling) and trapping
outgoing longwave radiation (warming). Unlike the warming effects
associated with GHGs that are sufficiently long lived to become well
mixed in the atmosphere, the climate effects associated with changes in
cloud cover are more regional and temporal in nature. The assessment
literature describes three main components of aviation-induced
cloudiness--persistent contrails, contrail-induced cirrus, and induced
cirrus. Aircraft engine emissions of water vapor at high altitudes
during flight can lead to the formation of condensation trails, or
contrails, under certain conditions such as ice-supersaturated air
masses with specific humidity levels and temperature.
---------------------------------------------------------------------------

    \147\ 80 FR at 37782-83.
---------------------------------------------------------------------------

    The NRC estimated that persistent contrails increased cloudiness
above the United States by two percent between 1950 and 1988, with
similar results reported over Europe.\148\ As stated above, clouds can
have both warming and cooling effects, and persistent contrails were
once considered to have significant net warming effects. However, more
recent estimates suggest a smaller overall climate forcing effect of
persistent contrails. The IPCC AR5 best estimate for the global mean
radiative forcing from contrails is 0.01 W/m\2\ (medium confidence and
with an uncertainty range of 0.005 to 0.03 W/m\2\). \149\ To put both
the magnitude and

large uncertainty range of this number for the first of the three
components of aviation-induced cloudiness into context, some examples
of other IPCC AR5 best estimates for global mean radiative forcing
include: 1.68 W/m\2\ for CO2 (very high confidence and with
an uncertainty range of 1.33 to 2.03 W/m\2\), 0.97 W/m\2\ for methane
(high confidence and with an uncertainty range of 0.74 to 1.20 W/m\2\),
and 0.17 W/m\2\ for nitrous oxide (very high confidence and with an
uncertainty range of .013 to 0.21 W/m\2\).\150\ In addition, the NRC
(2010) assessment suggested that contrails may affect regional diurnal
temperature differences, but this has been called into question by the
recent findings presented in the IPCC AR5, which suggests that aviation
contrails do not have an effect on mean or diurnal range of surface
temperatures (medium confidence).
---------------------------------------------------------------------------

    \148\ NRC, 1999: Atmospheric Effects of Aviation: A Review of
NASA's Subsonic Assessment Project. The National Academies Press, 54
pp.
    \149\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University
Press, 1535 pp.
    \150\ IPCC, 2013: Summary for Policymakers. In: Climate Change
2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor,
S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, 29 pp.
---------------------------------------------------------------------------

    Persistent contrails also sometimes lose their linear form and
develop into cirrus clouds, an effect referred to as contrail-induced
cirrus. Studies to date have been unable to isolate this second of
three main climate forcing components of aviation-induced cloudiness,
but the IPCC AR5 provides a combined contrail and contrail-induced
cirrus best estimate of 0.05 W/m\2\ (low confidence and with an
uncertainty range of 0.02 and 0.15 W/m\2\).\151\
---------------------------------------------------------------------------

    \151\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].Cambridge University
Press, 1535 pp.
---------------------------------------------------------------------------

    Particles emitted or formed in the atmosphere as a result of
aircraft emissions (primarily sulfates and black carbon) may also act
as ice nuclei and modify naturally forming cirrus clouds, an effect
referred to as ``induced cirrus.'' This third of three main climate
forcing components of aviation-induced cloudiness is an area of active
research, and there are significant challenges in estimating the
climatic impacts of cirrus cloud modification. Neither IPCC AR4 nor AR5
provided global or regional estimates related to this forcing, with the
AR5 stating that ``it is deemed too uncertain to be further assessed
here.''\152\ The 2007 IPCC AR4 characterizes our knowledge of the
natural freezing modes in cirrus conditions as ``poor'' and notes that
cirrus cloud processes are not well represented in global models.\153\
---------------------------------------------------------------------------

    \152\ Ibid.
    \153\ 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.
---------------------------------------------------------------------------

    Given differences in scientific understanding of the three main
components of aviation-induced cloudiness, the more recent assessments
have not provided quantitative estimates of the overall net climate
forcing effect of changes in clouds from high altitude emissions of
water vapor and particles. Going back to the 1999 IPCC assessment's
quantitative estimates, the science is characterized as ``very
uncertain'' with a range for the best estimate between 0 to 0.040 W/
m\2\.\154\ Thus, based on its consideration of the scientific evidence
and all the comments on this issue, the EPA agrees with those
commenters that indicate there are substantial scientific uncertainties
regarding net effects of the three components of aviation-induced
cloudiness on the climate system. These uncertainties result in the
Agency's not being prepared at this time to determine whether these
additional substances are air pollution within the meaning of CAA
section 231(a)(2)(A) and not including them within the definition of
``air pollution'' being employed in this endangerment finding.
---------------------------------------------------------------------------

    \154\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)]
Cambridge University Press, 373 pp.
---------------------------------------------------------------------------

c. Responses to Key Comments on Direct Radiative Forcing Effects of
High Altitude Particle Emissions
    Some commenters supported the EPA's summary of the scientific
uncertainties regarding the net direct radiative forcing effects of
aviation-induced particles including black carbon. Other commenters
disagreed and argued that there is clear scientific evidence that black
carbon in particular drives climate change and should be included in
the definition of air pollution.
    The EPA disagrees with comments regarding changing or expanding the
definition of the air pollution employed in this endangerment finding
to include aviation-induced particles like black carbon. For the
reasons stated above, the Administrator considers the scientific
assessment literature as the primary scientific and technical basis
informing the endangerment finding and providing the state of climate
science regarding the direct radiative forcing effects of high altitude
emissions of the two primary aviation-induced particles, sulfates and
black carbon. Section IV.B.4 of the proposed findings under CAA section
231(a)(2)(A) \155\ explained that aircraft emit precursor gases that
convert to sulfate particles in the atmosphere, such as sulfur dioxide.
Sulfate particles have direct effects on the climate by scattering
solar radiation, which is a negative radiative forcing that ultimately
results in cooling. The more recent assessments have not identified a
quantitative best estimate for this negative radiative forcing effect
specifically from aviation, as it is an active area of scientific study
with large uncertainties. Going back to the 1999 IPCC assessment's
quantitative estimates, the direct radiative forcing effect of sulfate
aerosols from aviation for the year 1992 is estimated at -0.003 W/m\2\
with an uncertainty range between -0.001 and -0.009 W/m\2\.\156\
---------------------------------------------------------------------------

    \155\ 80 FR at 37783.
    \156\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J.Dokken, M.McFarland (eds.)]
Cambridge University Press, 373 pp.
---------------------------------------------------------------------------

    Similarly, the proposed findings under CAA section 231(a)(2)(A)
explained that black carbon emissions from aviation, which are produced
by the incomplete combustion of jet fuel, primarily absorb solar
radiation and heat the surrounding air, resulting in a warming effect
(positive radiative forcing). The more recent assessments have not
identified a quantitative best estimate for this effect specifically
from aviation, as it is an area of active scientific study with large
uncertainties. Going back to the 1999 IPCC assessment's quantitative
estimates, the global mean radiative forcing of black carbon emissions
from aircraft is estimated to be 0.003 W/m\2\ with uncertainty spanning
0.001 to 0.009 W/m\2\.\157\ The IPCC 1999 assessment suggests that
because the contribution of black carbon in the stratosphere (which
actually contributes to cooling of the Earth's surface rather than
warming) was not included in its calculations, its estimates of
radiative forcing were likely to be too high.
---------------------------------------------------------------------------

    \157\ Ibid.
---------------------------------------------------------------------------

    In addition, the 2009 Endangerment Finding did not include aerosols
in the

definition of air pollution, noting that much of the uncertainty range
surrounding the best estimate of total net forcing due to all human
activities was due to uncertainties about the cooling and warming
effects of aerosols \158\ (though from all sources, not just aircraft).
The 2009 Endangerment Finding also stated that the magnitude of aerosol
effects can vary immensely with location and season of emissions,
noting that estimates of its total climate forcing effect have a large
uncertainty range.\159\ Regarding black carbon specifically, the 2009
Endangerment Finding noted that it does not share common physical and
chemical attributes with the six well-mixed GHGs because it is an
aerosol particle (not a gas) that has different physical, chemical, and
atmospheric properties. Black carbon affects the climate differently
than GHGs that are sufficiently long lived to become well mixed in the
atmosphere. In contrast to its indirect warming and cooling effects via
clouds, black carbon causes a direct warming effect primarily by
absorbing incoming and reflected sunlight (whereas GHGs that are
sufficiently long lived to become well mixed in the atmosphere cause
warming by trapping outgoing, infrared heat), and by darkening bright
surfaces such as snow and ice, which reduces reflectivity. Black carbon
is short-lived, remaining in the atmosphere for only about a week, and
does not become well-mixed in the atmosphere. There are also concerns
in the international climate science and policy communities about how
to treat the short-lived black carbon emissions alongside GHGs--for
example, what are the appropriate metrics to compare the warming and/or
climate effects of the different substances, given that, unlike GHGs
that are sufficiently long lived to become well mixed in the
atmosphere, the magnitude of aerosol effects can vary immensely with
location and season of emissions.
---------------------------------------------------------------------------

    \158\ 74 FR at 66517.
    \159\ 74 FR at 66520.
---------------------------------------------------------------------------

    Thus, although the EPA is not at this time prepared to make a final
determination on whether black carbon should be found to be air
pollution within the meaning of CAA section 231(a)(2)(A), based on its
consideration of the scientific evidence and all the comments on this
issue, and consistent with its conclusion in the 2009 Endangerment
Finding, the EPA disagrees with commenters that ask for black carbon to
be included in the definition of the air pollution as part of this
endangerment finding. Because aerosols such as black carbon and
sulfates are fundamentally different from and do not share the relevant
properties that support grouping the six well-mixed GHGs together as a
class, and scientific uncertainties remain regarding the net radiative
forcing effects of these substances (whether in general or when emitted
at high altitudes), the EPA is not at this time including them in the
definition of air pollution employed in this finding. However, because
of these uncertainties the Agency is not at this time taking final
action to determine whether these additional substances should be found
to represent air pollution within the meaning of CAA section
231(a)(2)(A).
d. Responses to Key Comments on Changes in Atmospheric Chemistry From
High Altitude Nitrogen Oxides Emissions
    Most commenters supported the EPA's summary of the scientific
uncertainties regarding the changes in atmospheric chemistry from high
altitude NOX emissions. At least one commenter disagreed and
argued that there is clear scientific evidence that the effects of
NOX emissions on ozone production have a significant climate
forcing effect. They concluded that NOX should therefore be
included in an endangerment finding.
    The EPA disagrees with comments to the extent that they suggest
including NOX in this endangerment finding by changing or
expanding the definition of the air pollution. NOX emissions
have different, and potentially larger, climate effects when emitted at
high altitudes and about 90 percent of aircraft NOX is
emitted in flight (not during landing and takeoff),\160\ meaning its
relevance for climate change is primarily in relation to emissions at
high altitude. The atmospheric lifetime of NOX emitted near
the surface is on the order of a few hours, while in the upper
troposphere, or roughly the cruise altitude for jet aircraft, it is on
the order of several days.
---------------------------------------------------------------------------

    \160\ FAA, System for Assessing Aviation's Global Emissions,
Version 1.5, Global Aviation Emissions Inventories for 2000 through
2004, FAA-EE-2005-02, September 2005, revised March 2008, available
at http://www.faa.gov/about/office_org/headquarters_offices/apl/researc/models/sage/ (Last accessed April 14, 2016).
---------------------------------------------------------------------------

    Section IV.B.4 of the proposed findings under CAA section
231(a)(2)(A) \161\ explained that emissions of NOX do not
themselves have warming or cooling effects, but affect the climate
through catalyzing changes in the chemical equilibrium of the
atmosphere. High altitude emissions of NOX increase the
concentration of ozone, which has a warming effect in the short term.
Elevated NOX concentrations also lead to an increased rate
of destruction of methane, which has a cooling effect in the long-term.
The reduced methane concentrations eventually contribute to decreases
in ozone, which also decreases the long-term net warming effect. Thus,
the net radiative impact of NOX emissions depends on the
balance between the reductions in methane versus the production of
ozone, which in turn depends on the time scale under consideration.
---------------------------------------------------------------------------

    \161\ 80 FR at 37783-84.
---------------------------------------------------------------------------

    For the reasons stated above, the Administrator considers the
scientific assessment literature as the primary scientific and
technical basis informing the endangerment finding and providing the
state of climate science regarding how emissions of NOX
affect the climate system. Quantifying these impacts is an area of
active scientific study with large uncertainties. The quantification of
the net global effect of NOX is difficult because the
atmospheric chemistry effects are heavily dependent on highly localized
atmospheric properties and mixing ratios. Because the background
atmospheric concentration of NOX is important for
quantifying the impact of NOX emissions on ozone and methane
concentrations, the location of aircraft emissions is an important
additional factor. Going back to the IPCC 1999 assessment since no more
recent quantitative estimates are available, the globally averaged
radiative forcing estimates for high-altitude aircraft emissions of
NOX in 1992 were 0.023 W/m\2\ for ozone-induced changes
(uncertainty range of 0.011 to 0.046 W/m\2\), and -0.014 W/m\2\ for
methane-induced changes (uncertainty range of -0.005 to -0.042 W/
m\2\).\162\
---------------------------------------------------------------------------

    \162\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J. Dokken, M. McFarland (eds.)]
Cambridge University Press, 373 pp.
---------------------------------------------------------------------------

    The IPCC AR5 presents the impact of aviation high-altitude
NOX emissions using a different metric, global warming
potential (GWP), which is a measure of the warming impact of a pulse of
emissions of a given substance over 100 years relative to the same mass
of CO2. The AR5 presents a range from -21 to +75 for GWP of
aviation NOX.\163\ The uncertainty in sign indicates
uncertainty

whether the net effect is one of warming or cooling. This report
further suggests that at cruise altitude there is strong regional
sensitivity of ozone and methane to NOX, particularly
notable at low latitudes.
---------------------------------------------------------------------------

    \163\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, 1535 pp.
---------------------------------------------------------------------------

    Thus, although the EPA is not prepared to determine whether
NOX emissions at high altitude should be found to be air
pollution within the meaning of CAA section 231(a)(2)(A), based on its
consideration of the scientific evidence and all the comments on this
issue, and consistent with its conclusion in the 2009 Endangerment
Finding, the EPA disagrees with commenters that assert that
NOX should be included at this time in the definition of the
air pollution for this finding. NOX does not share the
relevant properties that support grouping the six well-mixed GHGs
together as a class. NOX is not classified as a GHG because
it influences the climate system indirectly through production of ozone
rather than directly through trapping outgoing heat. In addition,
NOX does not have a sufficiently long atmospheric lifetime
to become well-mixed in the atmosphere and significant scientific
uncertainties remain regarding its net radiative forcing effects.
    The Administrator notes that NOX emissions are already
regulated under the EPA's rules implementing CAA section 231, at 40 CFR
part 87, due to their impacts during landing and take-off operations
(LTO). The prerequisite endangerment and cause or contribute findings
that formed the basis for these standards, however, did not rely upon
any conclusions regarding the climate forcing impacts of
NOX, but rather the role of LTO NOX emissions as
a precursor to ozone formation in areas that did not meet the NAAQS for
ozone.\164\ The continuing significant uncertainties regarding high
altitude NOX emissions, which are emitted during cruise
operations rather than during LTO, as a climate forcer do not undermine
the Agency's prior conclusion under CAA section 231(a)(2)(A) that
emissions of NOX from aircraft engines cause or contribute
to air pollution which may reasonably be anticipated to endanger public
health or welfare due to their contribution to ozone concentrations
that exceed the NAAQS. This final finding does not revise or reopen the
Agency's prior findings under CAA section 231(a)(2)(A) that emissions
of NOX from aircraft engines cause or contribute to air
pollution which may reasonably be anticipated to endanger public health
or welfare due to their contribution to ozone concentrations that
exceed the NAAQS.
---------------------------------------------------------------------------

    \164\ U.S. EPA, ``Control of Air Pollution from Aircraft and
Aircraft Engines, Emission Standards and Test Procedures for
Aircraft.'' Final Rule, 38 FR 19088 (July 17, 1973).
---------------------------------------------------------------------------

C. The Air Pollution is Reasonably Anticipated To Endanger Both Public
Health and Welfare

    The Administrator finds that elevated atmospheric concentrations of
the six well-mixed GHGs may reasonably be anticipated to endanger the
public health and welfare of current and future generations within the
meaning of CAA section 231(a)(2)(A). This section describes the major
pieces of scientific evidence supporting the Administrator's
endangerment finding, discusses both the public health and welfare
aspects of the endangerment finding, and addresses a number of key
issues the Administrator considered when evaluating the state of the
science.
    The EPA is informed by and places considerable weight on the
extensive scientific and technical evidence in the record supporting
the 2009 Endangerment Finding, including the major, peer-reviewed
scientific assessments used to address the question of whether GHGs in
the atmosphere endanger public health and welfare, and on the
analytical framework and conclusions upon which the EPA relied in
making that finding. This final finding under CAA section 231(a)(2)(A)
accounts for the EPA's careful consideration of the scientific and
technical record for the 2009 Endangerment Finding, of the new, major
scientific assessments issued since closing the administrative record
for the 2009 Endangerment Finding, and of public comments. No recent
information or assessments published since late 2009 or provided by
commenters suggest that it would be reasonable for the EPA to now reach
a different or contrary conclusion for purposes of CAA section
231(a)(2)(A) than the one the EPA reached in 2009 under CAA section
202(a). Rather, they provide further support for this final finding
under section 231(a)(2)(A). In particular, the new assessments
discussed in this document provide additional detail regarding public
health impacts, particularly on groups and people especially vulnerable
to climate change, including children, the elderly, low-income
communities and individuals, indigenous groups, and communities of
color.
    Following the same decision framework and analysis that we followed
for the 2009 Endangerment Finding, as detailed in section IV.B of that
finding,\165\ here we summarize the general approach used by the
Administrator in reaching the judgment that a positive endangerment
finding should be made for purposes of CAA section 231(a)(2)(A), as
well as the specific rationale for finding that the GHG air pollution
may reasonably be anticipated to endanger both public health and
welfare. First, the Administrator finds the scientific evidence linking
anthropogenic emissions and resulting elevated atmospheric
concentrations of the six well-mixed GHGs to observed global and
regional temperature increases and other climate changes to be
sufficiently robust and compelling. The Administrator is basing her
finding on the total weight of scientific evidence and what the science
has to say regarding the nature and potential magnitude of the risks
and impacts across all climate-sensitive elements of public health and
welfare, now and projected out into the foreseeable future. The
Administrator has considered the state of the science on how
anthropogenic emissions and the resulting elevated atmospheric
concentrations of the six well-mixed GHGs may affect each of the major
risk categories, include human health, air quality, food production and
agriculture, forestry, water resources, sea level rise and coastal
areas, the energy sector, infrastructure and settlements, and
ecosystems and wildlife. The Administrator understands that the nature
and potential severity of impacts can vary across these different
elements of public health and welfare, and that they can vary by
region, as well as over time.
---------------------------------------------------------------------------

    \165\ 74 FR at 66523-36.
---------------------------------------------------------------------------

    The Administrator is therefore aware that, because human-induced
climate change has the potential to be far-reaching and multi-
dimensional, not all risks and potential impacts can be characterized
with a uniform level of quantification or understanding, nor can they
be characterized with uniform metrics. Thus, the Administrator is not
necessarily placing the greatest weight on those risks and impacts
which have been the subject of the most study or quantification.
Rather, given this variety in not only the nature and potential
magnitude of risks and impacts, but also in our ability to
characterize, quantify and project into the future such impacts, the
Administrator must use her judgment to weigh the threat in each of the
risk categories, weigh the potential benefits where relevant, and
ultimately to judge whether these risks and

benefits, when viewed in total, endanger public health and/or welfare.
    First, the Administrator has not established a specific threshold
metric for the different categories of risk and impacts, which are
referred to as impact sectors. The potential for both adverse and
beneficial effects is considered, as well as the relative magnitude of
such effects, to the extent that the relative magnitudes can be
quantified or characterized. Furthermore, given the multiple ways in
which the buildup of anthropogenic GHG emissions in the atmosphere can
cause effects (e.g., via elevated CO2 concentrations,
temperature increases, precipitation increases, sea level rise, and
changes in extreme events), these multiple pathways are considered. The
Administrator has balanced and weighed the varying risks and effects
for each impact sector. She has judged whether there is a pattern
across the sector that supports or does not support an endangerment
finding, and if so whether the support is of more or less weight. In
cases where there is a potential for both benefits and risks of harm,
the Administrator has balanced these factors by determining whether
there appears to be any directional trend in the overall evidence that
would support placing more weight on one than the other, taking into
consideration all that is known about the likelihood of the various
risks and effects and their seriousness. In all of these cases, the
judgment is largely qualitative in nature and is not reducible to
precise metrics or quantification.
    Regarding the timeframe for the endangerment test, it is the
Administrator's view that both current and future conditions must be
considered. The Administrator is thus taking the view that the
endangerment period of analysis extend from the current time to the
next several decades and in some cases to the end of this century. This
consideration is also consistent with the timeframes used in the
underlying scientific assessments. The future timeframe under
consideration is consistent with the atmospheric lifetime and climate
effects of the six well-mixed GHGs and also with our ability to make
reasonable and plausible projections of future conditions. The
Administrator acknowledges that some aspects of climate change science
and the projected impacts are more certain than others. Our state of
knowledge is strongest for recently observed, large-scale changes.
Uncertainty tends to increase in characterizing changes at smaller
(regional) scales relative to large (global) scales. Uncertainty also
increases as the temporal scales move away from the present, either
backward or more importantly forward in time. Nonetheless, the current
state of knowledge of observed and past climate changes and their
causes enables projections of plausible future changes under different
scenarios of anthropogenic forcing for a range of spatial and temporal
scales. The subsections below summarize the scientific information on
climate change impacts to public health and welfare that inform the
Administrator's judgment, as well as the key public comments and Agency
responses. The Agency's full responses to public comments can be found
in the Response to Comments document.
1. The Air Pollution is Reasonably Anticipated To Endanger Public
Health
    The Administrator finds under CAA section 231(a)(2)(A) that the
well-mixed GHG air pollution is reasonably anticipated to endanger
public health, for both current and future generations. The
Administrator finds that the public health of current generations is
endangered and that the threat to public health for both current and
future generations will mount over time as GHGs continue to accumulate
in the atmosphere and result in ever greater rates of climate change.
The Administrator continues to find robust scientific evidence in the
assessment literature that climate change can increase the risk of
morbidity and mortality and believes that these public health impacts
can and should be considered when determining endangerment to public
health under CAA section 231(a)(2)(A). As described in section IV.B.1
of the 2009 Endangerment Finding, the Administrator is not limited to
only considering whether there are any direct health effects such as
respiratory or toxic effects associated with exposure to GHGs.
    Here we summarize information from the scientific assessment
literature cited in the 2009 Endangerment Finding showing that climate
change resulting from anthropogenic GHG emissions threatens multiple
aspects of public health.\166\ In determining that the well-mixed GHG
air pollution is reasonably anticipated to endanger public health for
current and future generations under CAA section 202(a), the
Administrator noted her view that climate change can increase the risk
of morbidity and mortality. In making that public health determination,
the Administrator considered direct temperature effects, air quality
effects, the potential for changes in vector-borne diseases, and the
potential for changes in the severity and frequency of extreme weather
events. In addition, the Administrator considered whether and how
susceptible populations may be particularly at risk. As explained in
more detail in the 2009 Endangerment Finding, with respect to direct
temperature effects, by raising average temperatures, climate change
increases the likelihood of heat waves, which are associated with
increased deaths and illnesses. Climate change is also expected to lead
to reductions in cold-related mortality. The 2009 Endangerment Finding,
while noting uncertainty about how heat and cold related mortality
would change in the future, also pointed to a USGCRP assessment report
discussion that increases in heat-related mortality due to global
warming in the United States were unlikely to be compensated for by
decreases in cold-related mortality. With regard to air quality
effects, climate change is expected to increase ozone pollution over
broad areas of the country, including large metropolitan population
centers, and thereby increase the risks of respiratory infection,
aggravation of asthma, and premature death. Other public health threats
stem from the potential for increased deaths, injuries, infectious and
waterborne diseases, stress-related disorders, and other adverse
effects associated with increased hurricane intensity and increased
frequency of intense storms and heavy precipitation associated with
climate change. In addition, climate change is expected to be
associated with an increased risk of food-, water-, and vector-borne
diseases in susceptible populations. Climate change also has the
potential to change aeroallergen production (for example, through
lengthening the growing season for allergen-producing plants), and
subsequent human exposures could increase allergenic illnesses.
Children, the elderly, and the poor are among the most vulnerable to
climate-related health risks and impacts. The Administrator placed
weight on the fact that these certain groups are most vulnerable to
these climate-related health effects.
---------------------------------------------------------------------------

    \166\ 74 FR at 66524-30.
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    The EPA concludes that the 2009 Endangerment Finding's discussion
under CAA section 202(a) is equally persuasive for purposes of CAA
section 231(a)(2)(A). In addition, the EPA has carefully reviewed the
key conclusions in the recent assessments regarding public health risks
and the current and projected health impacts from human-

induced climate change. The EPA finds that the new assessments are
consistent with or strengthen the underlying science considered in the
2009 Endangerment Finding regarding public health effects from changes
in temperature, air quality, extreme weather, and climate-sensitive
diseases and aeroallergens, further supporting an endangerment finding
under CAA section 231(a)(2)(A). These key findings are described
briefly here.
    The USGCRP NCA3 finds that, ``Climate change threatens human health
and well-being in many ways, including impacts from increased extreme
weather events, wildfire, decreased air quality, threats to mental
health, and illnesses transmitted by food, water, and diseases carriers
such as mosquitoes and ticks. Some of these health impacts are already
underway in the United States.'' \167\ Regarding temperature effects,
the USGCRP NCA3 states, ``The effects of temperature extremes on human
health have been well documented for increased heat waves, which cause
more deaths, hospital admissions and population vulnerability.'' \168\
The conclusions of the assessment literature cited in the 2009
Endangerment Finding were uncertain with respect to the balance of
future heat- versus cold-related mortality associated with climate
change, but they noted that the available evidence suggested that the
increased risk from heat would exceed the decreased risk from cold in a
warming climate. The most recent assessments now have greater
confidence that increases in heat-related mortality likely will be
larger than the decreases in cold-related mortality, further supporting
this endangerment finding under CAA section 231(a)(2)(A). The USGCRP
NCA3 concludes, ``While deaths and injuries related to extreme cold
events are projected to decline due to climate change, these reductions
are not expected to compensate for the increase in heat-related
deaths.'' \169\ The IPCC AR5 also notes a potential benefit of climate
change could include ``modest reductions in cold-related mortality and
morbidity in some areas due to fewer cold extremes (low confidence),''
\170\ but that, ``[o]verall, we conclude that the increase in heat-
related mortality by mid-century will outweigh gains due to fewer cold
periods.'' \171\
---------------------------------------------------------------------------

    \167\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 221.
    \168\ Ibid at p. 252.
    \169\ Ibid at p. 224.
    \170\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros,
D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee,
K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N.
Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, p. 713.
    \171\ Ibid. at p. 721.
---------------------------------------------------------------------------

    Regarding air quality effects, the assessment literature cited in
the 2009 Endangerment Finding concluded that climate change is expected
to increase regional ozone pollution, with associated risks in
respiratory illnesses and premature death, but that the directional
effect of climate change on ambient particulate matter levels was less
certain. One of the more recent assessments, the USGCRP NCA3, similarly
concludes, ``Climate change is projected to harm human health by
increasing ground-level ozone and/or particulate matter air pollution
in some locations. . . . There is less certainty in the responses of
airborne particles to climate change than there is about the response
of ozone.'' \172\ The IPCC AR5 finds that ozone and particulate matter
have been associated with adverse health effects in many locations in
North America, and that ozone concentrations could increase under
future climate change scenarios if emissions of precursors were held
constant. For particulate matter, both the USGCRP NCA3 and IPCC AR5
discuss increasing wildfire risk under climate change and explain that
wildfire smoke exposure can lead to various respiratory and
cardiovascular impacts. The USGCRP NCA3 states, ``The effects of
wildfire on human health have been well documented with increases in
wildfire frequency, leading to decreased air quality and negative
health impacts.'' \173\ 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 due to changes in outdoor air
quality. Other risks include potential for alterations in indoor
allergens due to climate change-related increases in outdoor pollen
levels, potential chemical exposures due to greater use of pesticides
to address changes in geographic ranges of pest species, and dampness/
mold associated symptoms and illness due to potential flooding and
water damage in buildings from projected climate change-related
increases in storm intensity and extreme precipitation events in some
regions of the United States. Each of these assessments further
supports finding endangerment under CAA section 231(a)(2)(A).
---------------------------------------------------------------------------

    \172\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 222.
    \173\ Ibid at p. 252.
---------------------------------------------------------------------------

    Regarding extreme weather events (e.g., storms, heavy
precipitation, and, in some regions of the United States, floods and
droughts), the conclusions of the assessment literature cited in the
2009 Endangerment Finding found potential for increased deaths,
injuries, infectious and waterborne diseases, and stress-related
disorders. The more recent assessments further support this conclusion
for purposes of CAA section 231(a)(2)(A). The USGCRP NCA3 finds that
``Heavy downpours are increasing nationally, especially over the last
three to five decades. Largest increases are in the Midwest and
Northeast. Increases in the frequency and intensity of extreme
precipitation events are projected for all U.S. regions.'' \174\ The
USGCRP NCA3 identifies that: ``Elevated waterborne disease outbreaks
have been reported in the weeks following heavy rainfall, although
other variables may affect these associations. Water intrusion into
buildings can result in mold contamination that manifests later,
leading to indoor air quality problems.'' \175\ Other risks include
mortality associated with flooding and impacts on mental health, such
as anxiety and post-traumatic stress disorder. The IPCC AR5 also
discusses increased risk of death and injury in coastal zones and
regions vulnerable to inland flooding. The USGCRP NCA3 and the IPCC AR5
both find that climate change may increase exposure to health risks
associated with drought conditions, which includes impacts from
wildfires, dust storms, extreme heat events, and flash flooding.
Droughts can lead to reduced water quantity and degraded water quality,
thereby increasing the risk of water-related diseases. The IPCC SREX
assessment projects further increases in some extreme weather and
climate events during this century, and it specifically notes that
changes in extreme weather events have implications for disaster risk
in the health sector.
---------------------------------------------------------------------------

    \174\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 36.
    \175\ Ibid at p. 224-225.
---------------------------------------------------------------------------

    The potential for changes in climate-sensitive diseases was also
cited in the 2009 Endangerment Finding. This included an increase in
the spread of several food and water-borne pathogens,

which can affect susceptible populations. Also noted was the potential
for range expansion of some zoonotic disease carriers such as the Lyme
disease-carrying tick. The new assessment literature similarly focuses
on increased exposure risk for some diseases under climate change,
finding that increasing temperatures may expand or shift the ranges of
some disease vectors like mosquitoes, ticks, and rodents. The IPCC AR5
notes that climate change may influence the ``growth, survival,
persistence, transmission, or virulence of pathogens'' \176\ that cause
food and water-borne disease. The USGCRP NCA3 notes that uncertainty
remains regarding future projections of increased human burden of
vector-borne disease, given complex interacting factors such as
``local, small-scale differences in weather, human modification of the
landscape, the diversity of animal hosts, and human behavior that
affects vector-human contact, among other factors.'' \177\ This new
assessment literature further supports finding endangerment under CAA
section 231(a)(2)(A).
---------------------------------------------------------------------------

    \176\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros,
D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee,
K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N.
Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, p. 726.
    \177\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 225.
---------------------------------------------------------------------------

    Regarding aeroallergens, the assessment literature cited in the
2009 Endangerment Finding found potential for climate change to affect
the prevalence and severity of allergy symptoms, but definitive data or
conclusions were lacking on how climate change might impact
aeroallergens in the United States. Further supporting an endangerment
finding under CAA section 231(a)(2)(A), the most recent assessments now
express greater confidence that climate change influences the
production of pollen, which in turn could affect the incidence of
asthma and other allergic respiratory illnesses such as allergic
rhinitis, as well as effects on conjunctivitis and dermatitis. Both the
USGCRP NCA3 and the IPCC AR5 found that increasing temperature has
lengthened the allergenic pollen season for ragweed, and that increased
CO2 by itself can elevate production of plant-based
allergens. The IPCC AR5 concludes that in North America, there is high
confidence that ``warming will lead to further changes in the seasonal
timing of pollen release.'' \178\
---------------------------------------------------------------------------

    \178\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, Cambridge, pp. 1465-1466.
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a. Health Impacts of Climate Change on Vulnerable Populations
    In the 2009 Endangerment Finding, the EPA cited the assessment
literature's conclusions regarding the fact that certain populations,
including children, the elderly, and the poor, are most vulnerable to
climate change-related health effects. The 2009 Endangerment Finding
also described climate change impacts facing indigenous peoples in the
United States, particularly Alaska Natives. The new assessment
literature strengthens these conclusions and further supports an
endangerment finding under CAA section 231(a)(2)(A) by providing more
detailed findings regarding these populations' vulnerabilities and the
projected impacts they may experience. In addition, the most recent
assessment reports provide new analysis about how some populations
defined jointly by ethnic/racial characteristics and geographic
location may be vulnerable to certain climate change health impacts.
The following paragraphs summarize information from the most recent
assessment reports on these vulnerable populations.
    The USGCRP NCA3 finds, ``Climate change will, absent other changes,
amplify some of the existing health threats the nation now faces.
Certain people and communities are especially vulnerable, including