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 are no definitions of ``public health'' or ``public welfare'' in
the Clean Air Act. The Supreme Court has discussed the concept of
``public health'' in the context of whether costs can be considered
when setting National Ambient Air Quality Standards. Whitman v.
American Trucking Ass'n, 531 U.S. 457 (2001). In Whitman, the Court
imbued the term with its most natural meaning: ``The health of the
public.'' Id. at 466. When considering public health, the EPA has
looked at morbidity, such as impairment of lung function, aggravation
of respiratory and cardiovascular disease, and other acute and chronic
health effects, as well as mortality. See, e.g., Final National Ambient
Air Quality Standard for Ozone, 73 FR 16436, March 27, 2008.

IV. The Proposed Endangerment Finding Under CAA Section 231

    This section describes the Administrator's proposed endangerment
finding under CAA section 231(a)(2) and its basis. Beginning with the
air pollution under consideration, the Administrator is proposing to
use the same definition of the ``air pollution'' under CAA section
231(a)(2) as that used under CAA section 202(a)(1), namely the mix of
six well-mixed GHGs mentioned above: CO2, methane, nitrous
oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.
As described in section IV.A below, it is the Administrator's view that
the reasons detailed in the 2009 Endangerment Finding for defining the
scope and nature of the air pollution to be these six well-mixed GHGs
remain valid and well-supported by the current science and are
therefore reasonable bases for adopting the same definition of ``air
pollution'' in this section 231(a)(2)(A) finding. Information from the
new scientific assessments described in section IV.B below provides
further support that the six well-mixed GHGs are the primary cause and
driver of climate change. The Administrator considered other climate-
forcing agents both in the 2009 Endangerment Finding and in this
action; however, these substances are not included in the air pollution
definition proposed in this action for the reasons discussed below in
section IV.B.4.
    The Administrator is proposing to find, for purposes of CAA section
231(a)(2)(A), that elevated concentrations of the six well-mixed GHGs
constitute air pollution that endangers both the public health and the
public welfare of current and future generations. The Administrator's
view is that the body of scientific evidence amassed in the record for
the 2009 Endangerment Finding compellingly supports an endangerment
finding under CAA section 231(a). Information from the new scientific
assessments described in section IV.B below provides further support
and justification for this proposed finding.
    Section IV.A below summarizes the 2009 Endangerment Finding under
CAA section 202, explains the approach EPA took in compiling an
extensive record to inform the Administrator's judgment on that
finding, and describes the recent judicial affirmation of the 2009
Endangerment Finding. Section IV.B provides a summary of new scientific
assessments that strengthen or provide further scientific evidence, in
addition to that which the Administrator relied upon in making her
prior judgment, for a finding that GHGs endanger public health and
welfare.\78\ Finally, section IV.C summarizes the Administrator's
conclusion for purposes of section 231, in light of the evidence,
analysis, and conclusions that led to the 2009 Endangerment Finding as
well as more recent evidence, that emissions of the six well-mixed GHGs
in the atmosphere endanger public health and welfare.
---------------------------------------------------------------------------

    \78\ While the EPA is providing a summary of newer scientific
assessments below, the EPA is also relying on the same scientific
and technical evidence discussed in the notices for the 2009
Endangerment Finding in this proposed finding for purposes of CAA
section 231. See sections III of the 2009 Proposed Endangerment
Finding and sections III and IV of the 2009 Endangerment Finding.
---------------------------------------------------------------------------

A. Scientific Basis of the 2009 Endangerment Finding Under CAA Section
202(a)(1)

    In the 2009 Endangerment Finding, the Administrator found that
elevated concentrations of the well-mixed GHGs in the atmosphere may
reasonably be

anticipated to endanger public health and welfare of current and future
generations. See, e.g., 74 FR 66516, December 15, 2009. The
Administrator reached this judgment by carefully considering a
significant body of scientific evidence and public comments submitted
to the Agency. The sections below summarize the scope and nature of the
relevant air pollution for the 2009 Endangerment Finding, as well as
the public health and welfare considerations within the finding.
1. The Definition of Air Pollution in the 2009 Endangerment Finding
    The Administrator defined the scope and nature of the relevant air
pollution as the aggregate group of six key, well-mixed GHGs:
CO2, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride.\79\ The Administrator
considered five primary reasons for focusing on this aggregate group as
the air pollution in the 2009 Endangerment Finding: (1) They share
common physical properties that influence their climate effects; (2) on
the basis of these common physical properties, they have been
determined to be the primary cause of human-induced climate change, are
the best-understood driver of climate change, and are expected to
remain the key driver of future climate change; (3) they are the common
focus of climate change science research and policy analyses and
discussions; (4) using the combined mix of these gases as the
definition (versus an individual gas-by-gas approach) is consistent
with the science, because risks and impacts associated with GHG-induced
climate change are not assessed on an individual gas-by-gas basis; and
(5) using the combined mix of these gases is consistent with past EPA
practice, where separate substances from different sources, but with
common properties, may be treated as a class (e.g., oxides of nitrogen,
particulate matter, volatile organic compounds).\80\
---------------------------------------------------------------------------

    \79\ 74 FR 66516, December 15, 2009.
    \80\ 74 FR 66517 to 66519, December 15, 2009.
---------------------------------------------------------------------------

    The common physical properties these six GHGs share that are
relevant to the climate change problem include the following: All are
long-lived in the atmosphere; \81\ all become globally well mixed in
the atmosphere, resulting in similar GHG concentrations around the
globe regardless of geographic location of emissions; all trap outgoing
heat that would otherwise escape to space; and all are directly emitted
as GHGs rather than becoming a GHG in the atmosphere after emission of
a precursor gas. The Administrator acknowledged that other
anthropogenic climate forcers also play a role in climate change but
for various scientific and policy reasons, these substances were not
included in the air pollution definition.\82\
---------------------------------------------------------------------------

    \81\ We use ``long-lived'' here to mean that the gas has a
lifetime in the atmosphere sufficient to become globally well-mixed
throughout the entire atmosphere, which requires a minimum
atmospheric lifetime of about one year. IPCC also refers to these
six greenhouse gases as long-lived. According to the most recent
IPCC Fifth Assessment Report (2014), methane has an atmospheric
lifetime of about 12 years. One of the most commonly used
hydrofluorocarbons (HFC-134a) has a lifetime of about 13 years.
Nitrous oxide has a lifetime of around 130 years; sulfur
hexafluoride over 3,000 years; and some perfluorocarbons up to
10,000 to 50,000 years. CO2 is sometimes approximated as
having a lifetime of roughly 100 years, but for a given amount of
CO2 emitted a better description is that some fraction of
the atmospheric increase in concentration is quickly absorbed by the
oceans and terrestrial vegetation, some fraction of the atmospheric
increase will only slowly decrease over a number of years, and a
small portion of the increase will remain for many centuries or
more.
    \82\ 74 FR 66519 to 66521, December 15, 2009.
---------------------------------------------------------------------------

    As explained in more detail in the 2009 Endangerment Finding, the
EPA made the judgment that the scientific evidence is compelling that
elevated concentrations of 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.\83\ The first line of evidence arises from our basic physical
understanding of the effects of changing concentrations of GHGs,
natural factors, and other human impacts on the climate system. The
second line of evidence arises from indirect, historical estimates of
past climate changes that suggest that the changes in global surface
temperature over the last several decades are unusual. The third line
of evidence arises from the use of computer-based climate models to
simulate the likely patterns of response of the climate system to
different forcing mechanisms (both natural and anthropogenic).
---------------------------------------------------------------------------

    \83\ 74 FR 66518, December 15, 2009.
---------------------------------------------------------------------------

2. Public Health Impacts Detailed in the 2009 Endangerment Finding
    Climate change resulting from anthropogenic GHG emissions threatens
multiple aspects of public health.\84\ In determining that the well-
mixed GHG air pollution is reasonably anticipated to endanger public
health for current and future generations, the Administrator noted her
view that climate change can increase the risk of morbidity and
mortality.\85\ In making that public health finding, the Administrator
considered direct temperature effects, 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 was unlikely to be compensated for by decreases in cold-related
mortality (74 FR 66525, December 15, 2009). With regard to air quality
effects, climate change is expected to increase ozone pollution over
broad areas of the country, including large metropolitan population
centers, and thereby increase the risks of respiratory infection,
aggravation of asthma, and premature death. Other public health threats
stem from the potential for increased deaths, injuries, infectious and
waterborne diseases, stress-related disorders, and other adverse
effects associated with increased hurricane intensity and increased
frequency of intense storms and heavy precipitation associated with
climate change. In addition, climate change is expected to be
associated with an increase in the spread of food-, water-, and vector-
borne diseases in susceptible populations. Climate change also has the
potential to change aeroallergen production (for example, through
lengthening the growing season for allergen-producing plants), and
subsequent human exposures could increase allergenic illnesses.
Children, the elderly, and the poor are among the most vulnerable to
climate-related health effects.
---------------------------------------------------------------------------

    \84\ 74 FR 66524 to 66530, December 15, 2009.
    \85\ 74 FR 66524, December 15, 2009.
---------------------------------------------------------------------------

3. Public Welfare Impacts Detailed in the 2009 Endangerment Finding
    Climate change resulting from anthropogenic GHG emissions also
threatens multiple aspects of public welfare.\86\ In determining that
the well-

mixed GHG air pollution is reasonably anticipated to endanger public
welfare for current and future generations, the Administrator
considered the multiple pathways by which GHG air pollution and
resultant climate change affect public welfare by evaluating the
numerous and far-ranging risks to food production and agriculture;
forestry; water resources; sea level rise and coastal areas; energy,
infrastructure, and settlements; and ecosystems and wildlife. The
Administrator also considered impacts on the U.S. population from
climate change effects occurring outside of the United States. As
explained in more detail in the 2009 Endangerment Finding, the
potential serious adverse impacts of extreme events, such as wildfires,
flooding, drought, and extreme weather conditions provided strong
support for the determination. Climate change is expected to place
large areas of the country at serious risk of reduced water supplies,
increased water pollution, and increased occurrence of extreme events
such as floods and droughts. Coastal areas are expected to face
increased risks from storm and flooding damage to property, as well as
adverse impacts from rising sea level such as land loss due to
inundation, erosion, wetland submergence and habitat loss. Climate
change is expected to result in an increase in peak electricity demand,
and extreme weather from climate change threatens energy,
transportation, and water resource infrastructure. Climate change may
exacerbate existing environmental pressures in certain settlements,
particularly in Alaskan indigenous communities. Climate change is also
very likely to fundamentally change U.S. ecosystems over the 21st
century and to lead to predominantly negative consequences for
biodiversity, ecosystem goods and services, and wildlife. Though there
may be some benefits for agriculture and forestry in the next few
decades, the body of evidence points towards increasing risks of net
adverse impacts on U.S. food production, agriculture and forest
productivity as average temperature continues to rise. Looking across
all sectors discussed above, the risk and the severity of adverse
impacts on public welfare are expected to increase over time. Lastly,
these impacts are global and may exacerbate problems outside the United
States that raise humanitarian, trade, and national security issues for
the United States.
---------------------------------------------------------------------------

    \86\ 74 FR 66530 to 66536, December 15, 2009.
---------------------------------------------------------------------------

4. The Science Upon Which the Agency Relied
    As outlined in section III.A of the 2009 Endangerment Finding,\87\
the EPA's approach to providing the technical and scientific
information to inform the Administrator's judgment regarding the
question of whether GHGs endanger public health and welfare was to rely
primarily upon the recent, major assessments by the USGCRP, the IPCC,
and the NRC. These assessments addressed the scientific issues that the
EPA was required to examine, were comprehensive in their coverage of
the GHG and climate change issues, and underwent rigorous and exacting
peer review by the expert community, as well as rigorous levels of U.S.
government review, in which the EPA took part. Primary reliance on the
major scientific assessments provided assurance that the Administrator
was basing her judgment on the best available, well-vetted science that
reflected the consensus of the climate science research community. The
major findings of the USGCRP, IPCC, and NRC assessments supported the
Administrator's determination that elevated concentrations of GHGs in
the atmosphere may reasonably be anticipated to endanger the public
health and welfare of current and future generations. The EPA presented
this scientific support at length in the comprehensive record for the
2009 Endangerment Finding. Relevant sections of documents from the 2009
Endangerment Finding record have been placed in the docket for this
proposed finding under CAA section 231.
---------------------------------------------------------------------------

    \87\ 74 FR 66510 to 66512, December 15, 2009.
---------------------------------------------------------------------------

    The EPA then reviewed ten administrative petitions for
reconsideration of the Endangerment Finding in 2010.\88\ In the
Reconsideration Denial, the Administrator denied those petitions on the
basis of the Petitioners' failure to provide substantial support for
their argument that the EPA should revise the Endangerment Finding and
their objections' lack of ``central relevance'' to the Finding. The EPA
prepared an accompanying three-volume Response to Petitions document to
provide additional information, often more technical in nature, in
response to the arguments, claims, and assertions by the Petitioners to
reconsider the Endangerment Finding.\89\
---------------------------------------------------------------------------

    \88\ Administrative petitions are available from http://www.epa.gov/climatechange/endangerment/petitions.html (last accessed
May 12, 2015), and in the docket for the 2009 Endangerment Finding:
EPA-HQ-OAR-2009-017.
    \89\ U.S. EPA, 2010: Denial of the Petitions to Reconsider the
Endangerment and Cause or Contribute Findings for Greenhouse Gases
Under section 202(a) of the Clean Air Act, 75 FR 49557 (August 13,
2010) (``Reconsideration Denial''). In that notice, the EPA
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.
---------------------------------------------------------------------------

    The 2009 Endangerment Finding and the 2010 Reconsideration Denial
were challenged in a lawsuit before the U.S. Court of Appeals for the
D.C. Circuit.\90\ On June 26, 2012, the Court upheld the Endangerment
Finding and the Reconsideration Denial, ruling that the Finding
(including the Reconsideration Denial) was not arbitrary or capricious,
was consistent with the U.S. Supreme Court's decision in Massachusetts
v. EPA (which affirmed the EPA's authority to regulate greenhouse
gases) \91\ and the text and structure of the CAA, and was adequately
supported by the administrative record.\92\ The Court also agreed with
the EPA that the Petitioners had ``not provided substantial support for
their argument that the Endangerment Finding should be revised.'' \93\
The Court found that the EPA had based its decision on ``substantial
scientific evidence,'' observing that ``EPA's scientific evidence of
record included support for the proposition that greenhouse gases trap
heat on earth that would otherwise dissipate into space; that this
`greenhouse effect' warms the climate; that human activity is
contributing to increased atmospheric levels of greenhouse gases; and
that the climate system is warming,'' as well as providing extensive
scientific evidence for EPA's determination that anthropogenically
induced climate change threatens both public health and welfare.\94\
The court further noted that the EPA's reliance on assessments was
consistent with the methods decision-makers often use to make a
science-based judgment.\95\ Moreover, the Court supported the EPA's
reliance on the major scientific assessment reports conducted by
USGCRP, IPCC, and NRC and found:
---------------------------------------------------------------------------

    \90\ Coalition for Responsible Regulation, Inc. v. Environmental
Protection Agency, 684 F.3d 102 (D.C. Cir. 2012),), reh'g en banc
denied, 2012 U.S. App. LEXIS 25997, 26313, 26315 (D.C. Cir. 2012)
(CRR).
    \91\ 549 U.S. 497 (2007).
    \92\ CRR, 684 F.3d at 117-27.
    \93\ Id. at 125
    \94\ Id. at 120-121.
    \95\ 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.\96\
---------------------------------------------------------------------------

    \96\ Id. at 120.

    In addition, the EPA's reliance on the major assessments to inform
the Administrator's judgment allowed for full and explicit recognition
of scientific uncertainty regarding the endangerment posed by the
atmospheric buildup of GHGs. The Administrator considered the fact that
``some aspects of climate change science and the projected impacts are
more certain than others.'' \97\ The D.C. Circuit subsequently noted
that ``the existence of some uncertainty does not, without more,
warrant invalidation of an endangerment finding.'' \98\
---------------------------------------------------------------------------

    \97\ 74 FR at 66524, December 15, 2009.
    \98\ 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.'' \99\ Thus, the Supreme
Court did not disturb the D.C. Circuit's holding that affirmed the 2009
Endangerment Finding.
---------------------------------------------------------------------------

    \99\ Utility Air Reg. Group v. EPA, 134 S. Ct. 2427, 2438 (2014)
(internal marks and citations omitted). See also Virginia v. EPA,
134 S. Ct. 418 (2013), Pac. Legal Found. v. EPA, 134 S. Ct. 418
(2013), and CRR, 134 S. Ct. 468 (2013) (all denying cert.).
---------------------------------------------------------------------------

B. Recent Science Further Supports the Administrator's Judgment That
the Six Well-Mixed Greenhouse Gases Endanger Public Health and Welfare

    Since the closure of the administrative record concerning the 2009
Endangerment Finding (including the denial of petitions for
reconsideration), a number of new major, 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 propose a different interpretation of, or place more or less
weight on, the major findings reflected in the previous assessment
reports that underpinned the Administrator's judgment that the six
well-mixed GHGs endanger public health and welfare. From its review,
the EPA finds that these new assessments are largely consistent with,
and in many cases strengthen and add to, the already compelling and
comprehensive scientific evidence detailing the role of the six well-
mixed GHGs in driving climate change, detailed in the 2009 Endangerment
Finding. Therefore, the new scientific assessments do not provide any
reasonable basis on which to propose under CAA section 231(a)(2)(A) a
different conclusion than the one the EPA reached in 2009 under CAA
section 202(a). Rather, they provide further support for this proposed
finding under section 231. In particular, the new assessments discussed
in this preamble provide additional detail regarding public health
impacts, particularly on groups and people at certain lifestages
especially vulnerable to climate change including children, the
elderly, low-income communities and individuals, indigenous groups, and
communities of color.
    The subsections below present brief summaries of the relevant key
findings from the new major peer-reviewed scientific assessments, which
include the following:
     IPCC's 2013-2014 Fifth Assessment Report (AR5) \100\
---------------------------------------------------------------------------

    \100\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, 1535 pp, doi:10.1017/CBO9781107415324; IPCC, 2014:
Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A:
Global and Sectoral Aspects. Contribution of Working Group II to the
Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D.
Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C.
Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R.
Mastrandrea, and L.L. White (eds.)]. Cambridge University Press,
1132 pp; IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, 688 pp; and IPCC, 2014: Climate Change 2014:
Mitigation of Climate Change. Contribution of Working Group III to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E.
Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P.
Eickemeier, B. Kriemann, J. Savolainen, S. 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) \101\
---------------------------------------------------------------------------

    \101\ IPCC, 2012: Managing the Risks of Extreme Events and
Disasters to Advance Climate Change Adaptation. A Special Report of
Working Groups I and II of the Intergovernmental Panel on Climate
Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken,
K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen,
M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, 582
pp.
---------------------------------------------------------------------------

     USGCRP's 2014 ``Climate Change Impacts in the United
States: the Third National Climate Assessment'' (NCA3) \102\
---------------------------------------------------------------------------

    \102\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, 841 pp.
---------------------------------------------------------------------------

     NRC's 2010 ``Ocean Acidification: A National Strategy to
Meet the Challenges of a Changing Ocean'' (Ocean Acidification) \103\
---------------------------------------------------------------------------

    \103\ 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) \104\
---------------------------------------------------------------------------

    \104\ 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) \105\
---------------------------------------------------------------------------

    \105\ 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) \106\
---------------------------------------------------------------------------

    \106\ 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) \107\
---------------------------------------------------------------------------

    \107\ 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)
\108\
---------------------------------------------------------------------------

    \108\ 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) \109\
---------------------------------------------------------------------------

    \109\ 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) \110\
---------------------------------------------------------------------------

    \110\ 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) \111\.
---------------------------------------------------------------------------

    \111\ NRC, 2014: The Arctic in the Anthropocene: Emerging
Research Questions. The National Academies Press, 220 pp.
---------------------------------------------------------------------------

1. More Recent Evidence That Elevated Atmospheric Concentrations of the
Six Greenhouse Gases Are the Root Cause of Observed Climate Change
    The EPA has carefully reviewed the recent assessments regarding
elevated concentrations of the six well-mixed GHGs in the atmosphere.
The EPA finds that the new assessments of the IPCC, USGCRP, and NRC
support and strengthen the science underlying the 2009 Endangerment
Finding that the six well-mixed GHGs are the root cause of recently
observed climate change. Key findings are described briefly here.
    According to the IPCC AR5, observations of the Earth's globally
averaged combined land and ocean surface temperature over the period
1880 to 2012 show a warming of 0.85 [0.65 to 1.06] degrees Celsius or
1.53 [1.17 to 1.91] degrees Fahrenheit.\112\ The IPCC AR5 concludes
that the global average net effect of the increase in atmospheric GHG
concentrations, plus other human activities (e.g., land use change and
aerosol emissions), on the global energy balance since 1750 has been
one of warming. This total net heating effect, referred to as
``forcing,'' is estimated to be 2.3 Watts per square meter (W/m2),
which has increased from the previous 2007 IPCC Fourth Assessment
Report (AR4) total net estimate of 1.6 Watts per square meter (W/m2)
that was referred to in the record for the 2009 Endangerment Finding.
The reasons for this increase include continued increases in GHG
concentrations, as well as reductions in the estimated negative forcing
due to aerosols. The IPCC AR5 rates the level of confidence \113\ in
their radiative forcing estimates as ``high'' for methane and ``very
high'' for CO2 and nitrous oxide.
---------------------------------------------------------------------------

    \112\ ``IPCC, 2013: Summary for Policymakers. In: Climate Change
2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor,
S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, 29 pp.
    \113\ The IPCC expresses levels of confidence using five
qualifiers: very low, low, medium, high, and very high. These levels
are based on a qualitative evaluation of the robustness of the
evidence (considering the type, amount, quality, and consistency of
evidence such as data, mechanistic understanding, theory, models,
and expert judgment) and the degree of agreement among the findings.
---------------------------------------------------------------------------

    The new assessments also have greater confidence in attributing
recent warming to human causes. The IPCC AR5 stated that it is
extremely likely (>95 percent likelihood) that human influences have
been the dominant cause of warming since the mid-20th century, which is
a stronger statement than the AR4 conclusion that it is very likely
(>90 percent likelihood) that most of the increase in temperature since
the mid-20th century was due to the increase in GHG concentrations. The
AR4 conclusion was referred to in the record for the 2009 Endangerment
Finding. In addition, the IPCC AR5 found that concentrations of
CO2 and several other of the major GHGs are higher than they
have been in at least 800,000 years. This is an increase from what was
reported in IPCC AR4, which found higher concentrations than in at
least 650,000 years.
    The USGCRP NCA3 states that there is very high confidence \114\
that the global climate change of the past 50 years is primarily due to
human activities. Human activities are affecting climate through
increasing atmospheric levels of heat-trapping gases, through changing
levels of various particles that can have either a heating or cooling
influence on the atmosphere, and through activities such as land use
changes that alter the reflectivity of the Earth's surface and cause
climatic warming and cooling effects. The USGCRP concludes that
``considering all known natural and human drivers of climate since
1750, a strong net warming from long-lived greenhouse gases produced by
human activities dominates the recent climate record.'' \115\
---------------------------------------------------------------------------

    \114\ The NCA expresses levels of confidence using four
qualifiers: low, medium, high, and very high. These levels are based
on the strength and consistency of the observed evidence; the skill,
range, and consistency of model projections; and insights from peer-
reviewed sources.
    \115\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 741
---------------------------------------------------------------------------

    These recent and strong conclusions attributing recent observed
global warming to human influence have been made despite what some have
termed a warming slowdown or ``hiatus'' over the past 15 years or so.
The IPCC AR5 notes that global mean surface temperature exhibits
substantial natural decadal and interannual variability, such that
trends based on short records are very sensitive to the beginning and
end dates and do not in general reflect long-term climate trends. As an
example, the IPCC AR5 notes that the rate of warming over the 15 year
period from 1998-2012 was less than that over the period 1951-2012.
This short term variability does not alter the long-term climate trend
that the IPCC AR5 finds after its review of independently verified
observational records: ``Each of the past three decades has been
successively warmer at the Earth's surface than all the previous
decades in the instrumental record, and the first decade of the 21st
century has been the warmest.'' 116 117
---------------------------------------------------------------------------

    \116\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, p. 161.
    \117\ Furthermore, we would note that according to both NOAA and
NASA, 2014 was the warmest year in the modern instrumental record
for globally averaged surface temperature, and that the ten warmest
years, with the exception of 1998, have now occurred since 2000.
Available at http://www.giss.nasa.gov/research/news/20150116/ (last
accessed May 12, 2015).
---------------------------------------------------------------------------

    The NRC Climate Stabilization Targets assessment concludes that
CO2 emissions are currently altering the atmosphere's
composition and will continue to alter Earth's climate for thousands of
years. The NRC Understanding Earth's Deep Past assessment finds that
``the magnitude and rate of the present greenhouse gas increase place
the climate system in what could be one of the most severe increases in
radiative forcing of the global climate system in Earth history.''
\118\ This assessment finds that if no emissions reductions are made
CO2 concentrations by the end of the century are projected
to increase to levels that Earth has not experienced for more than 30
million years.
---------------------------------------------------------------------------

    \118\ NRC, 2011: Understanding Earth's Deep Past: Lessons for
Our Climate Future. The National Academies Press, p. 138.
---------------------------------------------------------------------------

2. More Recent Evidence That Greenhouse Gases Endanger Public Health
    The EPA has carefully reviewed the key conclusions in the recent
assessments regarding human-induced climate change risks and impacts on
public health. The EPA finds that the new assessments are consistent
with or strengthen the underlying science considered in the 2009
Endangerment Finding regarding public health effects from changes in
temperature, air quality, extreme weather, and climate-sensitive
diseases and aeroallergens. These key findings are described briefly
here.

    Regarding temperature effects, the conclusions of the assessment
literature cited in the 2009 Endangerment Finding were uncertain with
respect to the exact balance of how heat- versus cold-related mortality
will change in the future, but noted that the available evidence
suggested that the increased risk from heat would exceed the decreased
risk from cold in a warming climate. The most recent assessments now
have greater confidence that increases in heat-related mortality will
be larger than the decreases in cold-related mortality. The USGCRP NCA3
concludes that, ``While deaths and injuries related to extreme cold
events are projected to decline due to climate change, these reductions
are not expected to compensate for the increase in heat-related
deaths.'' \119\ The IPCC AR5 also notes a potential benefit of climate
change could include ``modest reductions in cold-related mortality and
morbidity in some areas due to fewer cold extremes (low confidence),''
\120\ but that, ``[o]verall, we conclude that the increase in heat-
related mortality by mid-century will outweigh gains due to fewer cold
periods.'' \121\
---------------------------------------------------------------------------

    \119\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 224.
    \120\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros,
D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee,
K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N.
Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, p. 713.
    \121\ 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. The USGCRP NCA3 similarly concludes that, ``Climate change is
projected to harm human health by increasing ground-level ozone and/or
particulate matter air pollution in some locations. . . . There is less
certainty in the responses of airborne particles to climate change than
there is about the response of ozone.'' \122\ The IPCC AR5 finds that
ozone and particulate matter have been associated with adverse health
effects in many locations in North America, and that ozone
concentrations could increase under future climate change scenarios if
emissions of precursors were held constant. For particulate matter,
both the USGCRP NCA3 and IPCC AR5 discuss increasing wildfire risk
under climate change, and explain that wildfire smoke exposure can lead
to various respiratory and cardiovascular impacts. The NRC Indoor
Environment assessment identifies potential adverse health risks
associated with climate-change induced alterations in the indoor
environment, including possible exposure to air pollutants like ozone
via changes in outdoor air quality. Other risks include potential for
alterations in indoor 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.
---------------------------------------------------------------------------

    \122\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 222.
---------------------------------------------------------------------------

    Regarding extreme weather events (e.g., storms, heavy
precipitation, and, in some regions of the United States, floods and
droughts), the conclusions of the assessment literature cited in the
2009 Endangerment Finding found potential for increased deaths,
injuries, infectious and waterborne diseases, and stress-related
disorders. Similarly, the USGCRP NCA3 discusses elevated waterborne
disease outbreaks and the potential for mold contamination and degraded
indoor air quality following heavy precipitation. Other impacts include
mortality associated with flooding and impacts on mental health, such
as anxiety and post-traumatic stress disorder. The IPCC AR5 also
discusses death and injury in coastal zones and regions vulnerable to
inland flooding. The USGCRP NCA3 and the IPCC AR5 both find that
climate change may increase exposure to and health risks associated
with drought conditions, which includes impacts from wildfires, dust
storms, extreme heat events, flash flooding, degraded water quality,
reduced water quantity, and water-related diseases. The IPCC SREX
assessment projects further increases in some extreme weather and
climate events during this century, and specifically notes that changes
in extreme weather events have implications for disaster risk in the
health sector.
    The effects of climate change on climate-sensitive diseases were
also cited in the 2009 Endangerment Finding, including a likely
increase in the spread of several food and water-borne pathogens among
susceptible populations, and the potential for range expansion of some
zoonotic disease carriers such as the Lyme disease-carrying tick. The
new assessment literature similarly focuses on increased exposure risk
for some diseases under climate change, finding that increasing
temperatures may expand or shift the ranges of some disease vectors
like mosquitoes, ticks, and rodents. The IPCC AR5 notes that climate
change may influence the ``growth, survival, persistence, transmission,
or virulence of pathogens'' \123\ that cause food and water-borne
disease. The USGCRP NCA3 notes that uncertainty remains regarding
future projections of increased human burden of vector-borne disease,
given complex interacting factors such as ``local, small-scale
differences in weather, human modification of the landscape, the
diversity of animal hosts, and human behavior that affects vector-human
contact, among other factors.'' \124\
---------------------------------------------------------------------------

    \123\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros,
D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee,
K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N.
Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, p. 726.
    \124\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 225.
---------------------------------------------------------------------------

    Regarding aeroallergens, the assessment literature cited in the
2009 Endangerment Finding found potential for climate change to affect
the prevalence and severity of allergy symptoms, but that definitive
data or conclusions were lacking on how climate change might impact
aeroallergens in the United States. The most recent assessments now
express greater confidence that climate change will influence
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, ``warming will
lead

to further changes in the seasonal timing of pollen release (high
confidence).'' \125\
---------------------------------------------------------------------------

    \125\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, Cambridge, p. 1465-1466.
---------------------------------------------------------------------------

    The assessment literature cited in the 2009 Endangerment Finding
concluded that certain populations, including children, the elderly,
and the poor, are most vulnerable to climate-related health effects.
The 2009 Endangerment Finding also described climate change impacts
facing indigenous peoples in the United States, particularly Alaska
Natives. The new assessment literature strengthens these conclusions by
providing more detailed findings regarding these populations'
vulnerabilities and 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 that, ``Climate change will, absent other
changes, amplify some of the existing health threats the nation now
faces. Certain people and communities are especially vulnerable,
including children, the elderly, the sick, the poor, and some
communities of color.'' \126\ Limited resources make low-income
populations more vulnerable to ongoing climate-related threats, less
able to adapt to anticipated changes, and less able to recover from
climate change impacts. Low-income populations also face higher
prevalence of chronic health conditions than higher income groups,
which increases their vulnerability to the health effects of climate
change.
---------------------------------------------------------------------------

    \126\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 221.
---------------------------------------------------------------------------

    According to the USGCRP NCA3 and IPCC AR5, some populations defined
jointly by ethnic/racial characteristics and geographic location are
more vulnerable to certain health effects of climate change due to
factors such as existing health disparities (e.g., higher prevalence of
chronic health conditions), increased exposure to health stresses, and
social factors that affect local resilience and ability to recover from
impacts.
    The USGCRP NCA3 also finds that climate change, in addition to
chronic stresses such as extreme poverty, is affecting indigenous
peoples' health in the United States through impacts such as reduced
access to traditional foods, decreased water quality, and increasing
exposure to health and safety hazards. The IPCC AR5 finds that climate
change-induced warming in the Arctic and resultant changes in
environment (e.g., permafrost thaw, effects on traditional food
sources) have significant observed and projected impacts on the health
and well-being of Arctic residents, especially indigenous peoples.
Small, remote, predominantly-indigenous communities are especially
vulnerable given their ``strong dependence on the environment for food,
culture, and way of life; their political and economic marginalization;
existing social, health, and poverty disparities; as well as their
frequent close proximity to exposed locations along ocean, lake, or
river shorelines.'' \127\ In addition, increasing temperatures and loss
of Arctic sea ice increases the risk of drowning for those engaged in
traditional hunting and fishing.
---------------------------------------------------------------------------

    \127\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, Cambridge, p. 1581.
---------------------------------------------------------------------------

    The USGCRP NCA3 concludes that children will suffer
disproportionately from climate change given the unique physiological
and developmental factors that occur during this lifestage. Impacts on
children are expected from heat waves, air pollution, infectious and
waterborne illnesses, and mental health effects resulting from extreme
weather events. The IPCC AR5 indicates that children are among those
especially susceptible to most allergic diseases, as well as health
effects associated with heat waves, storms, and floods.
    Both the USGCRP and IPCC conclude that climate change will increase
health risks facing the elderly. Older people are at much higher risk
of mortality during extreme heat events. Pre-existing health conditions
also make older adults susceptible to cardiac and respiratory impacts
of air pollution and to more severe consequences from infectious and
waterborne diseases. Limited mobility among older adults can also
increase health risks associated with extreme weather and floods.
3. More Recent Evidence That Greenhouse Gases Endanger Public Welfare
    The EPA has carefully reviewed the recent scientific conclusions in
the assessments regarding human-induced climate change impacts on
public welfare.\128\ The EPA finds that they are largely consistent
with or strengthen the underlying science supporting the 2009
Endangerment Finding regarding public welfare effects on food
production and agriculture; forestry; water resources; sea level rise
and coastal areas; energy, infrastructure, and settlements; ecosystems
and wildlife; and impacts on the U.S. population from climate change
effects occurring outside of the United States. These key findings are
described briefly here.
    Regarding agriculture, the assessment literature cited in the 2009
Endangerment Finding found potential for increased CO2
levels to benefit yields of certain crops in the short-term, but with
considerable uncertainty. The body of evidence pointed towards
increasing risk of net adverse impacts on U.S. food production and
agriculture over time, with the potential for significant disruptions
and crop failure in the future. The most recent assessments now have
greater confidence that climate change will negatively affect U.S.
agriculture over this century. Specifically, the USGCRP NCA3 concludes,
``While some U.S. regions and some types of agricultural production
will be relatively resilient to climate change over the next 25 years
or so, others will increasingly suffer from stresses due to extreme
heat, drought, disease, and heavy downpours. From mid-century on,
climate change is projected to have more negative impacts on crops and
livestock across the country.'' \129\ The IPCC AR5 concludes, ``Overall
yields of major crops in North America are projected to decline

modestly by mid-century and more steeply by 2100 among studies that do
not consider adaptation (very high confidence).'' \130\ The IPCC AR5
notes that in the absence of extreme events, climate change may benefit
certain regions and crops, but that in North America significant
harvest losses have been observed due to recent extreme weather events.
In addition, the IPCC SREX assessment specifically notes that projected
changes in extreme weather events will increase disaster risk in the
agriculture sector.
---------------------------------------------------------------------------

    \128\ The CAA states that ``[a]ll language referring to effects
on welfare includes, but is not limited to, effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility, and climate, damage to and deterioration of property,
and hazards to transportation, as well as effects on economic values
and on personal comfort and well-being, whether caused by
transformation, conversion, or combination with other air
pollutants.'' CAA section 302(h). This language is quite broad.
Importantly, it is not an exclusive list due to the use of the term
``includes, but is not limited to, . . . .'' Effects other than
those listed here may also be considered effects on welfare.
    \129\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 16.
    \130\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, Cambridge, p. 1462.
---------------------------------------------------------------------------

    Regarding forestry, the assessment literature cited in the 2009
Endangerment Finding found that near term benefits to forest growth and
productivity in certain parts of the country from elevated
CO2 concentrations and temperature increases to date are
offset by longer term risks from wildfires and the spread of
destructive pests and disease that present serious adverse risks for
forest productivity. The most recent assessments provide further
support for this conclusion. Both the USGCRP NCA3 and the IPCC AR5
conclude that climate change is increasing risks to forest health from
fire, tree disease and insect infestations, and drought. The IPCC AR5
also notes risks to forested ecosystems associated with changes in
temperature, precipitation amount, and CO2 concentrations,
which can affect species and ecological communities, leading to
ecosystem disruption, reorganization, movement or loss. The NRC Arctic
assessment states that climate change is likely to have a large
negative impact on forested ecosystems in the high northern latitudes
due to the effects of permafrost thaw and greater wildfire frequency,
extent, and severity. The NRC Climate Stabilization Targets assessment
found that for an increase in global average temperature of 1 to
2[deg]C above pre-industrial levels, the area burnt by wildfires in
western North America will likely more than double.
    Regarding water resources, the assessment literature cited in the
2009 Endangerment Finding concluded that increasing temperatures and
increased variability in precipitation associated with climate change
will impact water quality and quantity through changes in snowpack,
increased risk of floods, drought, and other concerns such as water
pollution. Similarly, the new assessments further support projections
of water resource impacts associated with increased floods and short-
term drought in most U.S. regions. The USGCRP NCA3 also finds that,
``[c]limate change is expected to affect water demand, groundwater
withdrawals, and aquifer recharge, reducing groundwater availability in
some areas.'' \131\ The IPCC AR5 finds that in part of the western
United States, ``water supplies are projected to be further stressed by
climate change, resulting in less water availability and increased
drought conditions.'' \132\ The IPCC AR5 also projects that climate
change will degrade surface water quality, including the Great Lakes,
and will negatively affect drinking water treatment/distribution and
sewage collection systems.
---------------------------------------------------------------------------

    \131\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 70.
    \132\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of Working
Group II to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D.
Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O.
Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S.
MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge
University Press, Cambridge, p. 1456-1457.
---------------------------------------------------------------------------

    The assessment literature cited in the 2009 Endangerment Finding
found that the most serious potential adverse effects to coastal areas
are the increased risk of storm surge and flooding in coastal areas
from sea level rise and more intense storms. Coastal areas also face
other adverse impacts from sea level rise such as land loss due to
inundation, erosion, wetland submergence, and habitat loss. The most
recent assessments provide further evidence in line with the science
supporting the 2009 Endangerment Finding. The USGCRP NCA3 finds that,
``Sea level rise, combined with coastal storms, has increased the risk
of erosion, storm surge damage, and flooding for coastal communities,
especially along the Gulf Coast, the Atlantic seaboard, and in
Alaska.'' \133\
---------------------------------------------------------------------------

    \133\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 9.
---------------------------------------------------------------------------

    The IPCC AR5, the USGCRP NCA3, and three of the new NRC assessments
provide estimates of projected global sea level rise. These estimates,
while not always directly comparable as they assume different emissions
scenarios and baselines, are at least 40 percent larger than, and in
some cases more than twice as large as, the projected rise estimated in
the IPCC AR4 assessment, which was referred to in the 2009 Endangerment
Finding.\134\ The NRC Sea Level Rise assessment projects a global sea
level rise of 0.5 to 1.4 meters by 2100, which is sufficient to lead to
a relative rise in sea level even around the northern coasts of
Washington State, where the land is still rebounding from the
disappearance of the great ice sheets. The NRC National Security
Implications assessment suggests that ``the Department of the Navy
should expect roughly 0.4 to 2 meters global average sea-level rise by
2100.'' \135\ The NRC Climate Stabilization Targets assessment states
that an increase of 3[deg]C will lead to a sea level rise of 0.5 to 1
meter by 2100. While these NRC and IPCC assessments continue to
recognize and characterize the uncertainty inherent in accounting for
ice sheet processes, these revised estimates are consistent with the
assessments underlying the 2009 Endangerment Finding.
---------------------------------------------------------------------------

    \134\ The 2007 IPCC AR4 assessment cited in 2009 Endangerment
Finding estimated a projected sea level rise of between 0.18 and
0.59 meters by the end of the century, relative to 1990. It should
be noted that in 2007, the IPCC stated that including poorly
understood ice sheet processes could lead to an increase in the
projections.
    \135\ NRC, 2011: National Security Implications of Climate
Change for U.S. Naval Forces. The National Academies Press, p. 28.
---------------------------------------------------------------------------

    Regarding climate impacts on energy, infrastructure, and
settlements, the 2009 Endangerment Finding cited the assessment
literature's findings that temperature increases will change heating
and cooling demand; that declining water quantity may adversely impact
the availability of cooling water and hydropower in the energy sector;
and that changes in extreme weather events will threaten energy,
transportation, water, and other key societal infrastructure,
particularly on the coast. The most recent assessments provide further
evidence in line with the science supporting the 2009 Endangerment
Finding. For example, the USGCRP NCA3 finds that, ``Coastal
infrastructure, including roads, rail lines, energy infrastructure,
airports, port facilities, and military bases, are increasingly at risk
from sea level rise and damaging storm surges.'' \136\ The NRC Arctic
assessment identifies threats to human infrastructure in the Arctic
from increased flooding, erosion, and shoreline ice pile-up, or ivu,
associated

with summer sea ice loss and the increasing frequency and severity of
storms.
---------------------------------------------------------------------------

    \136\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 9.
---------------------------------------------------------------------------

    Regarding ecosystems and wildlife, the assessment literature cited
in the 2009 Endangerment Finding found that climate change will
predominantly adversely impact both terrestrial and marine biodiversity
and the ability of these ecosystems to provide goods and services. The
NRC Arctic assessment states that major marine and terrestrial biomes
will likely shift pole ward, with significant implications for changing
species composition, food web structures, and ecosystem function. The
NRC Climate Stabilization Targets assessment found that coral bleaching
will increase due both to warming and ocean acidification. The NRC
Understanding Earth's Deep Past assessment notes four of the five major
coral reef crises of the past 500 million years were caused by
acidification and warming that followed GHG increases of similar
magnitude to the emissions increases expected over the next hundred
years. Similarly, the NRC Ocean Acidification assessment finds that
``[t]he chemistry of the ocean is changing at an unprecedented rate and
magnitude due to anthropogenic CO2 emissions; the rate of
change exceeds any known to have occurred for at least the past
hundreds of thousands of years.'' \137\ The assessment notes that the
full range of consequences is still unknown, but the risks ``threaten
coral reefs, fisheries, protected species, and other natural resources
of value to society.'' \138\ The IPCC AR5 also projects biodiversity
losses in marine ecosystems, especially in the Arctic and tropics.
---------------------------------------------------------------------------

    \137\ NRC, 2010: Ocean Acidification: A National Strategy to
Meet the Challenges of a Changing Ocean. The National Academies
Press, p. 5.
    \138\ Ibid.
---------------------------------------------------------------------------

    In general, climate change impacts related to public welfare are
expected to be unevenly distributed across different regions of the
United States and have a greater impact on certain populations, such as
indigenous peoples and the poor. The USGCRP NCA3 finds climate change
impacts such as the rapid pace of temperature rise, coastal erosion and
inundation related to sea level rise and storms, ice and snow melt, and
permafrost thaw are affecting indigenous people in the United States.
Particularly in Alaska, critical infrastructure and traditional
livelihoods are threatened by climate change and, ``[i]n parts of
Alaska, Louisiana, the Pacific Islands, and other coastal locations,
climate change impacts (through erosion and inundation) are so severe
that some communities are already relocating from historical homelands
to which their traditions and cultural identities are tied.'' \139\ The
IPCC AR5 notes, ``Climate-related hazards exacerbate other stressors,
often with negative outcomes for livelihoods, especially for people
living in poverty (high confidence). Climate-related hazards affect
poor people's lives directly through impacts on livelihoods, reductions
in crop yields, or destruction of homes and indirectly through, for
example, increased food prices and food insecurity.'' \140\
---------------------------------------------------------------------------

    \139\ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W.
Yohe, Eds., 2014: Climate Change Impacts in the United States: The
Third National Climate Assessment. U.S. Global Change Research
Program, p. 17.
    \140\ IPCC, 2014: Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part A: Global and Sectoral Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros,
D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee,
K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N.
Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, p. 796.
---------------------------------------------------------------------------

    In the 2009 Endangerment Finding, the Administrator considered
impacts on the U.S. population from climate change effects occurring
outside of the United States, such as national security concerns that
may arise as a result of climate change impacts in other regions of the
world. The most recent assessments provide further evidence in line
with the science supporting the 2009 Endangerment Finding. The NRC
Climate and Social Stress assessment found that it would be ``prudent
for security analysts to expect climate surprises in the coming decade
. . . and for them to become progressively more serious and more
frequent thereafter.'' \141\ The NRC National Security Implications
assessment recommends preparing for increased needs for humanitarian
aid; responding to the effects of climate change in geopolitical
hotspots, including possible mass migrations; and addressing changing
security needs in the Arctic as sea ice retreats.
---------------------------------------------------------------------------

    \141\ NRC, 2013: Climate and Social Stress: Implications for
Security Analysis. The National Academies Press, p. 18.
---------------------------------------------------------------------------

    In addition, the NRC Abrupt Impacts report examines the potential
for tipping points, thresholds beyond which major and rapid changes
occur in the Earth's climate system, as well as in natural and human
systems that are impacted by the changing climate. The Abrupt Impacts
report did find less cause for concern than some previous assessments
regarding some abrupt events within the next century such as disruption
of the oceanic Atlantic Meridional Overturning Circulation (AMOC) and
sudden releases of high-latitude methane from hydrates and permafrost.
But, the same report found that the potential for abrupt changes in
ecosystems, weather and climate extremes, and groundwater supplies
critical for agriculture now seem more likely, severe, and imminent.
The assessment found that some abrupt changes were already underway
(e.g., Arctic sea ice retreat and increases in extinction risk due to
the speed of climate change), and cautioned that even abrupt changes
such as the AMOC disruption that are not expected in this century can
have severe impacts if/when they happen.
4. Consideration of Other Climate Forcers
    Both in the 2009 Endangerment Finding and in this action, the
Administrator recognizes that there are other substances in addition to
the six well-mixed GHGs that are emitted from human activities and
affect Earth's climate (referred to as climate forcers). These can be
grouped into two categories: (1) other substances with similar physical
properties to the six well-mixed GHGs--these include the ozone-
depleting substances of chlorofluorocarbons, hydrochlorofluorocarbons,
and halons, as well as nitrogen trifluoride and similar recently
identified substances; and (2) short-lived substances--tropospheric
ozone and its precursor gases, water vapor, and aerosol particles and
precursors. For some short-lived substances--namely, water vapor;
NOX; and aerosol particles including black carbon--their
physical properties result in these substances having different, and
often larger, climate effects when emitted at high altitudes. However,
the very properties that lead to differential climate effects depending
on the altitude of emission--properties that are different from those
of the six well-mixed, long-lived GHGs--lead to more uncertainty in the
scientific understanding of these short-lived substances' total effect
on Earth's climate. More detail is provided below.
    As described in section III.B of the 2009 Endangerment Finding and
in section IV.A.1 of this preamble, the primary reasons for defining
the air pollution as the aggregate group of the six well-mixed GHGs
include their common physical properties relevant to climate change
(i.e., long-lived, well-mixed, directly emitted), the fact that these
gases are considered the primary drivers of climate change, and the
fact that these gases remain the best

understood drivers of anthropogenic climate change. The common physical
properties of the six well-mixed GHGs not only support grouping them
together as a class, but also contribute to their higher degree of
scientific understanding related to climate change, relative to short-
lived substances that are not well-mixed, or substances that are formed
indirectly rather than being directly emitted. After considering
additional information in the new assessments regarding the climate-
relevant substances outside the basket of the six well-mixed GHGs, it
is the Administrator's view that the reasons originally stated for not
including these substances in the scope of the GHG air pollution still
apply at this time. For example, nitrogen triflouride and some other
recently discovered substances are not as well studied or understood as
the six well-mixed GHGs. Similarly, for tropospheric ozone--a short-
lived gas in the atmosphere that is not directly emitted (it forms from
emissions of various precursor gases)--the understanding and
quantification of the link between precursor emissions and climate
change is not as strong as for the six well-mixed GHGs.
    Regarding the short-lived substances with different climate effects
when emitted at high altitudes, the Aircraft Petition (see section II
of this preamble) mentions the effects of water vapor and
NOX on clouds and atmospheric chemistry. The major peer-
reviewed scientific assessments of the IPCC and NRC provide the current
state of scientific understanding of these effects; the USGCRP
assessments have not dealt specifically with emissions at high
altitude. The EPA considered the following assessment reports to obtain
the best estimates of these substances' net impact on the climate
system, which is generally discussed in terms of radiative forcing: the
IPCC AR5, the IPCC 2007 Fourth Assessment Report (AR4),\142\ the IPCC
Special Report: Aviation and the Global Atmosphere (IPCC 1999),\143\
the NRC's Advancing the Science of Climate Change (NRC 2010),\144\ and
the NRC's Atmospheric Effects of Aviation: A Review of NASA's Subsonic
Assessment Project (NRC 1999).\145\ In addition to high altitude water
vapor and NOX, the literature indicates that aerosol
particles, including black carbon, emitted at high altitudes have more
interactions with clouds and therefore have different effects on the
global energy balance than do particles emitted at the surface.
---------------------------------------------------------------------------

    \142\ IPCC, 2007: Climate Change 2007: The Physical Scientific
Basis. Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change [Solomon,
S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor
and H.L. Miller (eds.)] Cambridge University Press, 996 pp.
    \143\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J. Dokken, M. McFarland (eds.)]
Cambridge University Press, 373 pp.
    \144\ NRC, 2010: Advancing the Science of Climate Change. The
National Academies Press, 528 pp.
    \145\ NRC, 1999: Atmospheric Effects of Aviation: A Review of
NASA's Subsonic Assessment Project. The National Academies Press, 54
pp.
---------------------------------------------------------------------------

    The state of the science as represented in the assessment
literature highlights significant scientific uncertainties regarding
the total net forcing effect of water vapor, NOX, and
aerosol particles when emitted at high altitudes. Given these
uncertainties, the Agency is not including them in the proposed
definition of air pollution for purposes of the endangerment finding
under section 231 of the CAA. The short-lived nature of these
substances means that, unlike the long-lived GHGs, the climatic impact
of the substance is dependent on a number of factors such as the
location and time of its emission. The magnitude, and often the
direction (positive/warming or negative/cooling), of the globally
averaged climate impact will differ depending on the location of the
emission due to the local atmospheric conditions (e.g., due to
differing concentrations of other compounds with which the emissions
can react, background humidity levels, or the presence or absence of
clouds). In addition, for emissions at any given location, the spatial
and temporal pattern of the climate forcing will be heterogeneous,
again often differing in direction (for example, in the case of
NOX emissions, the near term effect in the hemisphere in
which the emissions occur is usually warming due to increased ozone
concentrations, but the longer term effects, and effects in the other
hemisphere, are often cooling due to increased destruction of methane).
As the climatic effects of these substances when emitted at high
altitudes were not addressed at length in the 2009 Endangerment
Finding, the following subsections briefly summarize the findings of
the major scientific assessments regarding these substances' climatic
effects at altitude and the various sources of uncertainty surrounding
these estimates.
a. Changes in Clouds From High Altitude Emissions of Water Vapor and
Particles
    Aviation-induced cloudiness (sometimes called AIC) refers to all
changes in cloudiness associated with aviation operations, which are
primarily due to the effects of high altitude emissions of water vapor
and particles (primarily sulfates and black carbon). Changes in
cloudiness affect the climate by both reflecting solar radiation
(cooling) and trapping outgoing longwave radiation (warming). Unlike
the warming effects associated with the six long-lived, well-mixed
GHGs, the warming effects associated with changes in cloud cover are
more regional and temporal in nature. The three key components of
aviation-induced cloudiness are persistent contrails, contrail-induced
cirrus, and induced cirrus.
    Aircraft engine emissions of water vapor at high altitudes during
flight can lead to the formation of condensation trails, or contrails,
under certain conditions such as ice-supersaturated air masses with
specific humidity levels and temperature. The NRC estimates that
persistent contrails increased cloudiness above the United States by
two percent between 1950 and 1988, with similar results reported over
Europe.\146\ As stated above, clouds can have both warming and cooling
effects, and persistent contrails were once considered to have
significant net warming effects. However, more recent estimates suggest
a smaller overall 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\).\147\ To put this number 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\).\148\ In addition, the NRC (2010)
assessment suggested that contrails may affect regional diurnal
temperature differences, but this has been called into question by the
recent findings presented in the IPCC AR5, which suggests that aviation
contrails do not have an effect on mean or diurnal range of surface
temperatures (medium confidence).
---------------------------------------------------------------------------

    \146\ NRC, 1999: Atmospheric Effects of Aviation: A Review of
NASA's Subsonic Assessment Project. The National Academies Press, 54
pp.
    \147\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, 1535 pp.
    \148\ IPCC, 2013: Summary for Policymakers. In: Climate Change
2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor,
S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, 29 pp.
---------------------------------------------------------------------------

    Persistent contrails also sometimes lose their linear form and
develop into cirrus clouds, an effect referred to as contrail-induced
cirrus. Studies to date have been unable to isolate this climate
forcing effect, but the IPCC AR5 provides a combined contrail and
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\).\149\
---------------------------------------------------------------------------

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

    Particles emitted or formed in the atmosphere as a result of
aircraft emissions may also act as ice nuclei and modify naturally
forming cirrus clouds, an effect referred to as ``induced cirrus.'' The
two primary aviation-induced particles are sulfates and black carbon,
and their effects on cirrus cloud modification is an area of active
research. There are significant challenges in estimating the climatic
impacts of induced cirrus; for example, the 2007 IPCC AR4 characterizes
our knowledge of the natural freezing modes in cirrus conditions as
``poor,'' and notes that cirrus cloud processes are not well
represented in global models.\150\ Neither IPCC AR4 nor AR5 provided
global or regional estimates related to this forcing.
---------------------------------------------------------------------------

    \150\ IPCC, 2007: Climate Change 2007: The Physical Scientific
Basis. Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change [Solomon,
S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor
and H.L. Miller (eds.)] Cambridge University Press, 996 pp.
---------------------------------------------------------------------------

    Given differences in scientific understanding of the three
components of aviation-induced cloudiness, the more recent assessments
have not provided estimates of the net climate forcing effect of
changes in clouds from high altitude emissions of water vapor and
particles. Going back to the 1999 IPCC assessment, the science is
characterized as ``very uncertain'' with a range for the best estimate
between 0 to 0.040 W/m\2\.\151\
---------------------------------------------------------------------------

    \151\ IPCC, 1999: Aviation and the Global Atmosphere, Special
Report to the Intergovernmental Panel on Climate Change [Penner,
J.E., D.H. Lister, D.J. Griggs, D.J. Dokken, M. McFarland (eds.)]
Cambridge University Press, 373 pp.
---------------------------------------------------------------------------

b. Direct Radiative Forcing Effects of High Altitude Particle Emissions
    The 2009 Endangerment Finding noted that much of the uncertainty
range surrounding the estimate of total net forcing due to all human
activities was due to uncertainties about the cooling and warming
effects of aerosols \152\ (though from all sources, not just aircraft).
The Finding noted that the magnitude of aerosol effects can vary
immensely with location and season of emissions, and also discussed
black carbon as a specific type of aerosol particle, noting that
estimates of its total climate forcing effect have a large uncertainty
range.\153\ Here, we discuss the direct radiative forcing effects of
high altitude emissions of the two primary aviation-induced particles,
sulfates and black carbon.
---------------------------------------------------------------------------

    \152\ 74 FR at 66517, December 15, 2009.
    \153\ 74 FR at 66520, December 15, 2009.
---------------------------------------------------------------------------

    Aircraft emit precursor gases that convert to sulfate particles in
the atmosphere, such as sulfur dioxide. Sulfate particles have direct
effects on the climate by scattering solar radiation, which results in
cooling. The more recent assessments have not quantified this effect
from aviation. Going back to the 1999 IPCC assessment, the direct
effect of sulfate 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\.\154\
---------------------------------------------------------------------------

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

    Black carbon emissions from aviation, which are produced by the
incomplete combustion of jet fuel, primarily absorb solar radiation and
heat the surrounding air, resulting in a warming effect. The more
recent assessments have not quantified this effect from aviation. The
1999 IPCC assessment estimates the global mean radiative forcing of
black carbon emissions to be 0.003 W/m\2\ with uncertainty spanning
0.001 to 0.009 W/m\2\.\155\ The IPCC 1999 assessment suggests that
because the contribution of black carbon in the stratosphere (which
actually contribute to cooling of the surface rather than warming) was
not included in its calculations, its estimates of radiative forcing
were likely to be too high.
---------------------------------------------------------------------------

    \155\ Ibid.
---------------------------------------------------------------------------

c. Changes in Atmospheric Chemistry From High Altitude Nitrogen Oxides
Emissions
    Emissions of NOX do not themselves have warming or
cooling effects, but affect the climate through catalyzing changes in
the chemical equilibrium of the atmosphere. High altitude emissions of
NOX increase the concentration of ozone, which has a warming
effect in the short term. Elevated NOx concentrations also lead to an
increased rate of destruction of methane, which has a cooling effect in
the long-term. The reduced methane concentrations eventually contribute
to decreases in ozone, which also decreases the long-term net warming
effect. Thus, the net radiative impact of NOX emissions
depends on the balance between the reductions in methane versus the
production of ozone, which in turn depends on the time scale under
consideration. Quantifying these impacts is an area of active study
with large uncertainties. The quantification of the net global effect
of NOX is difficult because the atmospheric chemistry
effects are heavily dependent on highly localized atmospheric
properties and mixing ratios. Because the background atmospheric
concentration of NOX is important for quantifying the impact
of aviation NOX emissions on ozone and methane
concentrations, the location of aircraft emissions would be an
important additional factor. In addition, NOX has different
residence times in the atmosphere depending on the altitude at which it
is emitted. The residence time of NOX in the upper
troposphere, or roughly the cruise altitude for jet aircraft, is on the
order of several days. Going back to the IPCC 1999 assessment, the
globally averaged radiative forcing estimates for aircraft emissions of
NOX in 1992 were 0.023 W/m\2\ for O3-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\).\156\
---------------------------------------------------------------------------

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

    The IPCC AR5 presents the impact of aviation NOX
emissions using a different metric, global warming potential (GWP),
which is a measure of the warming impact of a pulse of

emissions of a given substance over 100 years relative to the same mass
of CO2. The AR5 presents a range from -21 to +75 for GWP of
aviation NOX.\157\ The uncertainty in sign indicates
uncertainty whether the net effect is one of warming or cooling. This
report further suggests that at cruise altitude there is strong
regional sensitivity of ozone and methane to NOX,
particularly notable at low latitudes.
---------------------------------------------------------------------------

    \157\ IPCC, 2013: Climate Change 2013: The Physical Science
Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Stocker,
T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge
University Press, 1535 pp.
---------------------------------------------------------------------------

    The Administrator notes that NOX emissions are already
regulated under the EPA's rules implementing CAA section 231, at 40 CFR
part 87. The prerequisite endangerment and cause or contribute findings
that formed the basis for these standards, however, did not rely upon
any conclusions regarding the climate forcing impacts of
NOX, but rather the role of NOX emissions as a
precursor to ozone formation in areas that did not meet the National
Ambient Air Quality Standard (NAAQS) for ozone.\158\ The continuing
significant uncertainties regarding NOX as a climate forcer
do not undermine the Agency's prior conclusion under CAA section 231
that emissions of NOX from aircraft engines cause or
contribute to air pollution which may reasonably be anticipated to
endanger public health or welfare due to their contribution to ozone
concentrations that exceed the NAAQS.
---------------------------------------------------------------------------

    \158\ U.S. EPA, ``Control of Air Pollution from Aircraft and
Aircraft Engines, Emission Standards and Test Procedures for
Aircraft.'' Final Rule, 38 FR 19088, July 17, 1973.
---------------------------------------------------------------------------

d. Summary
    Overall, the state of the science as represented in the assessment
literature highlights significant scientific uncertainties regarding
the total net forcing effect of water vapor, NOX, and
aerosol particles, when emitted at high altitudes. The dependence of
the effects on where the substance is emitted, and the complex temporal
and spatial patterns that result, mean that the current level of
understanding regarding these short-lived substances is much lower than
for the six long-lived, well-mixed GHGs. Given the aforementioned
scientific uncertainties at present, the Agency is not including these
constituents in the proposed definition of air pollution for purposes
of the endangerment finding under section 231 of the CAA.

C. Summary of the Administrator's Proposed Endangerment Finding Under
CAA Section 231

    In sum, the Administrator proposes to find, for purposes of CAA
section 231(a)(2)(A), that elevated atmospheric concentrations of the
six well-mixed GHGs constitute air pollution that endangers both the
public health and the public welfare of current and future generations.
In this proposed action under CAA section 231(a)(2)(A), the EPA relies
primarily on the extensive scientific and technical evidence in the
record supporting the 2009 Endangerment Finding, including the major,
peer-reviewed scientific assessments used to address the question of
whether GHGs in the atmosphere endanger public health and welfare, and
on the analytical framework and conclusions upon which the EPA relied
in making that finding. This proposed finding under section 231
accounts for the EPA's careful consideration not only of the scientific
and technical record for the 2009 Endangerment Finding, but also of
new, major scientific assessments issued since closing the
administrative record for the 2009 Endangerment Finding. No recent
information or analyses published since late 2009 suggest that it would
be reasonable for the EPA to now reach a different or contrary
conclusion for purposes of CAA section 231(a)(2)(A) than the Agency
reached for purposes of section 202(a). In proposing this finding for
purposes of section 231, we are not reopening or revisiting our 2009
Endangerment Finding. To the contrary, in light of the recent judicial
decisions upholding those findings, the EPA believes the 2009
Endangerment Finding is firmly established and well settled.\159\
Moreover, there is no need for the EPA to reopen or revisit that
finding for purposes of making an additional finding under section 231
of the CAA. Therefore, public comments addressing this finding for
purposes of section 231(a)(2)(A) should be limited to the section 231
context; the EPA will not consider or respond to comments on this
proposal that seek a reevaluation of our 2009 Endangerment Finding for
purposes of section 202(a).
---------------------------------------------------------------------------

    \159\ CRR, 684 F.3d at 117 (D.C. Cir. 2012), reh'g en banc
denied, 2012 U.S. App. LEXIS 25997, 26313, 26315 (D.C. Cir. 2012);
see also Utility Air Reg. Group v. EPA, 134 S. Ct. at 2438 (2014).
---------------------------------------------------------------------------

V. The Proposed Cause or Contribute Finding for Greenhouse Gases Under
CAA Section 231

    As noted above, the Administrator has proposed to define the air
pollution for purposes of the endangerment finding under CAA section
231 to be the aggregate of six well-mixed GHGs in the atmosphere. The
second step of the two-part endangerment test for this proposed finding
is for the Administrator to determine whether the emission of any air
pollutant from certain classes of aircraft engines causes or
contributes to this air pollution. This is referred to as the cause or
contribute finding, and is the second proposed finding by the
Administrator in this action.
    Section V.A of this proposal describes the Administrator's
reasoning for using the same definition and scope of the GHG air
pollutant that was used in the 2009 Endangerment Finding. Section V.0
puts forth the Administrator's proposed finding that emissions of well-
mixed GHGs from classes of aircraft engines used in covered aircraft
contribute to the air pollution which endangers public health and
welfare.

A. The Air Pollutant

1. Proposed Definition of Air Pollutant
    Under section 231, the Administrator is to determine whether
emissions of any air pollutant from any class or classes of aircraft
engines cause or contribute to air pollution which may reasonably be
anticipated to endanger public health or welfare. As with the 2009
Endangerment Finding that the EPA conducted for purposes of CAA section
202(a), when making a cause or contribute finding under section
231(a)(2), the Administrator must first define the air pollutant being
evaluated. The Administrator has reasonably and logically considered
the relationship between the GHG air pollution and air pollutant: while
the air pollution is the concentration (e.g., stock) of the well-mixed
GHGs in the atmosphere, the air pollutant is the same combined grouping
of the well-mixed GHGs, the emissions of which are analyzed for
contribution (e.g., the flow into the stock). See 74 FR at 66537,
(December 15, 2009), (similar discussion with respect to the finding
for section 202). Thus, for purposes of section 231, the Administrator
is proposing to use the same definition of the air pollutant that was
used in the 2009 Endangerment Finding, namely, the aggregate group of
the same six GHGs: CO2, methane, nitrous oxide,
hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. See 74
Federal Register at 66536-66537, (December 15, 2009), (discussing the
definition of the GHG air pollutant with respect to the finding for
section 202). That is, as for the 2009 Endangerment Finding, the
Administrator is proposing to define a

single air pollutant made up of these six GHGs.
    To reiterate what the Agency has previously stated on this subject,
this collective approach for the contribution test is consistent with
the treatment of GHGs by those studying climate change science and
policy, where it is common practice to evaluate GHGs on a collective,
CO2-equivalent basis.\160\ This collective approach to
defining the air pollutant is not unique; grouping of many substances
with common attributes as a single pollutant is common practice under
the CAA, for example with particulate matter and volatile organic
compounds (VOC). As noted in section IV, these substances share common
attributes that support their grouping as the air pollution for
purposes of the endangerment finding. These same common attributes also
support the Administrator grouping the six GHGs for purposes of
defining the air pollutant for the proposed cause or contribute finding
under CAA section 231.
---------------------------------------------------------------------------

    \160\ As detailed in the 2009 Endangerment Finding proposal (74
FR 18904 (April 24, 2009) and continuing today, the UNFCCC, the U.S.
and other Parties report their annual emissions of the six GHGs in
CO2-equivalent units. This facilitates comparisons of the
multiple GHGs from different sources and from different countries,
and provides a measure of the collective warming potential of
multiple GHGs. Emissions of different GHGs are compared using GWPs,
which as described in section IV.B of this preamble are measures of
the warming impact of a pulse of emissions of a given substance over
100 years relative to the same mass of CO2. Therefore,
GWP-weighted emissions are measured in teragrams of CO2
equivalent (Tg CO2eq). The EPA's Greenhouse Gas Reporting
Program (http://www.epa.gov/ghgreporting/index.html, (last accessed
May 12, 2015)) also reports GHG emissions on a CO2-
equivalent basis, recognizing the common and collective treatment of
the six GHGs.
---------------------------------------------------------------------------

    The Administrator recognizes that in this case, the aircraft
engines covered by this notice emit two of the six gases, but not the
other four gases. Nonetheless, it is entirely appropriate, and in
keeping with the 2009 Endangerment Finding and past EPA practice, for
the Administrator to define the air pollutant in a manner that
recognizes the shared relevant properties of all these six gases, even
though they are not all emitted from the classes of sources before
her.\161\ For example, a source may emit only 20 of the possible 200-
plus chemicals that meet the definition of VOC in the EPA's
regulations, but that source is evaluated based on its emissions of VOC
and not on its emissions of the 20 chemicals by name. The fact that
these six substances within the definition of GHGs share common,
relevant attributes is true regardless of the type of sources being
evaluated for contribution. By proposing to use the definition of the
air pollutant as comprised of the six GHGs with common attributes, the
Administrator is taking account of these shared attributes and how they
are relevant to the air pollution that endangers public health and
welfare.
---------------------------------------------------------------------------

    \161\ In the 2009 Endangerment Finding, the Administrator found
that four of the six gases that were included in the definition of
the air pollutant were emitted by section 202 sources. 74 FR 66496,
66537 (December 15, 2009).
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2. How the Definition of Air Pollutant in the Endangerment
Determination Affects Section 231 Standards
    Under section 231(a), the Administrator is required to set
``emission standards applicable to the emission of any air pollutant''
from classes of aircraft engines that the Administrator determines
causes or contributes to air pollution that endangers public health or
welfare. If the Administrator makes a final determination under section
231 that the emissions of the GHG air pollutant from certain classes of
aircraft engines contribute to the air pollution that may reasonably be
anticipated to endanger public health and welfare, then she is called
on to set standards applicable to the emissions of this air pollutant.
The term ``standards applicable to the emissions of any air pollutant''
is not defined, and the Administrator has the discretion to interpret
it in a reasonable manner to effectuate the purposes of section 231 to
set standards that either control the emissions of the group of six
well-mixed gases as a whole and/or control emissions of individual
gases, as constituents of the class. For example, it might be
appropriate to set a standard that measures and controls the aggregate
emissions of the group of GHGs, weighted by CO2 equivalent.
Depending on the circumstances, however, it may be appropriate to set
standards for certain individual gases, or some combination of group
and individual standards. These and other similar approaches could
appropriately be considered in setting a standard or standards
applicable to the emissions of the group of GHGs that are defined as
the air pollutant. The Administrator would consider a variety of
factors in determining what approach to take in setting the standard or
standards; for example, she would consider the characteristics of the
aircraft emissions, such as rate and variability, the kind and
availability of control technology, and other matters relevant to
setting standards under section 231.\162\
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    \162\ In setting GHG emissions standards for model years 2012-
2016 light-duty vehicles, the EPA set fleet-wide average
CO2 equivalent standards for cars and trucks based on a
technology assessment analysis which indicated that there was a wide
range of technologies available for manufacturers to use when
upgrading vehicles to reduce CO2 emissions and improve
fuel economy. The final standards were based on CO2
emissions-footprint curves, where each vehicle has a different
CO2 emissions compliance target depending on its
footprint value (related to the size of the vehicle). The EPA also
set standards to cap tailpipe nitrous oxide, methane emissions, and
provided compliance credits to manufacturers who improved air
conditioning systems, such as through reduced refrigerant leakage
(hydrofluorocarbons) and indirect CO2 emissions related
to the increased load on the engine. 75 FR 25324 (May 7, 2010).
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B. Proposed Cause or Contribute Finding

1. The Administrator's Approach in Making This Proposed Finding
    As it did for the 2009 Endangerment Finding, and consistent with
prior practice and current science, the EPA uses annual emissions as a
reasonable proxy for contributions to the air pollution, i.e., elevated
atmospheric concentrations of GHGs. Cumulative anthropogenic emissions
are primarily responsible for the observed change in concentrations in
the atmosphere (i.e., the fraction of a country's or an economic
sector's cumulative emissions compared to the world's GHG emissions
over a long time period will be roughly equal to the fraction of the
change in concentrations attributable to that country or economic
sector); likewise, annual emissions are a reasonable proxy for annual
incremental changes in atmospheric concentrations.
    There are a number of possible ways of assessing whether air
pollutants cause or contribute to the air pollution which may
reasonably be anticipated to endanger public health and welfare, and no
single approach is required or has been used exclusively in previous
determinations under the CAA. Because the air pollution against which
the contribution is being evaluated is the six well-mixed GHGs, the
logical starting point for any contribution analysis is a comparison of
the emissions of the air pollutant from the section 231 category to the
total U.S. and total global emissions of the six GHGs. The
Administrator recognizes that there are other valid comparisons that
can be considered in evaluating whether emissions of the air pollutant
cause or contribute to the combined concentration of the six GHGs. To
inform the Administrator's assessment, section V.B.2 presents the
following types of simple and straightforward comparisons of U.S.
aircraft GHG emissions:
     As a share of current total U.S. GHG emissions;
     As a share of current U.S. transportation GHG emissions;

     As a share of current total global GHG emissions; and
     As a share of the current global transportation GHG
emissions.
    All annual GHG emissions data are reported on a CO2-
equivalent (CO2eq) basis, which as described above is a
commonly accepted metric for comparing different GHGs. This approach is
consistent with how EPA determined contribution for GHGs under section
202 of the CAA in 2009.
2. Overview of Greenhouse Gas Emissions
    Atmospheric concentrations of CO2 and other GHGs are now
at essentially unprecedented levels compared to the distant and recent
past.\163\ This is the unambiguous result of human emissions of these
gases. Global emissions of well-mixed GHGs have been increasing, and
are projected to continue increasing for the foreseeable future.
According to IPCC AR5, total global (from all major emitting sources
including forestry and other land use) emissions of GHGs in 2010 were
about 49,000 teragrams \164\ of CO2 equivalent (Tg
CO2eq).\165\ This represents an increase in global GHG
emissions of about 29 percent since 1990 and 23 percent since 2000. In
2010, total U.S. GHG emissions were responsible for about 14 percent of
global GHG emissions (and about 12 percent when factoring in the effect
of carbon sinks from U.S. land use and forestry).
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    \163\ IPCC, 2013: Summary for Policymakers. In: Climate Change
2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor,
S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, p. 11.
    \164\ One teragram (Tg) = 1 million metric tons = 1 megatonne
(Mt). 1 metric ton = 1,000 kg = 1.102 short tons = 2,205 lbs.
    \165\ IPCC, 2014: Climate Change 2014: Mitigation of Climate
Change. Contribution of Working Group III to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change [Edenhofer,
O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth,
A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J.
Savolainen, S. Schl[ouml]mer, C. von Stechow, T. Zwickel and J.C.
Minx (eds.)]. Cambridge University Press, 1435 pp.
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    Because 2010 is the most recent year for which IPCC emissions data
are available, we provide 2011 estimates from another widely used and
recognized global dataset, the World Resources Institute's (WRI)
Climate Analysis Indicators Tool (CAIT),\166\ for comparison. According
to WRI/CAIT, the total global GHG emissions in 2011 were 43,816 Tg of
CO2eq, representing an increase in global GHG emissions of
about 42 percent since 1990 and 30 percent since 2000 (excluding land
use, land use change and forestry). These estimates are generally
consistent with those of IPCC. In 2011, WRI/CAIT data indicate that
total U.S. GHG emissions were responsible for about 16 percent of
global emissions, which is also generally in line with the percentages
using IPCC's 2010 estimate described above. According to WRI/CAIT,
current U.S. GHG emissions rank only behind China's, which was
responsible for 24 percent of total global GHG emissions.
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    \166\ World Resources Institute (WRI) Climate Analysis
Indicators Tool (CAIT) Data Explorer (Version 2.0). Available at
http://cait.wri.org (last accessed May 12, 2015).
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    The Inventory of U.S. Greenhouse Gas Emissions and Sinks Report
\167\ (hereinafter ``U.S. Inventory''), in which 2013 is the most
recent year for which data are available, indicates that total U.S. GHG
emissions increased by 5.7 percent from 1990 to 2013 (or by about 4.7
percent when including the effects of carbon sinks), and emissions
increased from 2012 to 2013 by 1.8 percent. This 2012 to 2013 increase
was attributable to multiple factors including an increase in carbon
intensity of fuels consumed for electricity generation, a small
increase in vehicle miles traveled and vehicle fuel use, and a colder
winter leading to an increase in heating requirements. The U.S.
Inventory also shows that while overall U.S. GHG emissions grew between
1990 and 2013, transportation GHG emissions grew at a significantly
higher rate, 15 percent, more rapidly than any other U.S. sector.
Within the transportation sector, aircraft remain the single largest
source of GHG emissions not yet subject to any GHG regulations.
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    \167\ U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions
and Sinks: 1990-2013, 564 pp. Available at http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last
accessed May 12, 2015).
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    Section V.B.2.a which follows describes U.S. aircraft GHG emissions
within the domestic context, while section V.B.2.b describes these same
GHG emissions in the global context. Section V.B.2.c addresses future
projections of aircraft GHG emissions.
a. U.S. Aircraft GHG Emissions Relative to U.S. GHG Transportation and
Total U.S. GHG Inventory
    Relying on data from the U.S. Inventory, we compare U.S. aircraft
GHG emissions to the transportation sector and to total U.S. GHG
emissions as an indication of the role this source plays in the total
domestic contribution to the air pollution that is causing climate
change. In 2013, total U.S. GHG emissions from all sources were 6,774
Tg CO2eq. As stated above, total U.S. GHG emissions have
increased by almost 6 percent between 1990 and 2013, while U.S.
transportation GHG emissions from all categories have grown 15 percent
since 1990. The U.S. transportation sector was the second largest GHG
emitting sector (behind electricity generation), contributing 1,911 Tg
CO2eq or about 30 percent of total U.S. GHG emissions in
2013. This sectoral total and the total U.S. GHG emissions include
emissions from combustion of U.S. international bunker fuels, which are
fuels used for transport activities, from aviation (both commercial and
military) and marine sources.\168\ Consistent with IPCC guidelines for
common and consistent accounting and reporting of GHGs under the
UNFCCC, the ``U.S. international aviation bunker fuels'' category
includes emissions from combustion of fuel purchased in and used by
aircraft departing from the United States, regardless of whether they
are a U.S. flagged carrier. Total U.S. aircraft emissions clearly
contribute to the U.S. transportation sector's emissions, accounting
for 216 Tg CO2eq or 11 percent of such emissions (see Table
V.1.). In 2013, emissions from aircraft (216 Tg CO2eq) were
the third largest transportation source of GHGs within the United
States, behind light-duty vehicles and medium- and heavy-duty trucks
(totaling 1,494 Tg CO2eq).
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    \168\ According to IPCC guidelines for common and consistent
accounting and reporting of GHGs under the UNFCCC, the total U.S.
GHG emissions from the U.S. Inventory that is reported to the UNFCCC
excludes international bunker fuel emissions (aviation and marine
international bunker fuel emissions) from the reported total
national GHG emissions. However, the total U.S. GHG emissions in
this proposed cause or contribute finding section of this action do
include international bunker fuel emissions because we want to
capture the full contribution of U.S. emissions, including those
from U.S. aircraft.
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    For purposes of making this cause or contribute finding, the EPA is
focused on, and proposes to include, a set of aircraft engine classes
used in types of aircraft as described below, which corresponds to the
scope of the international CO2 emissions standard
contemplated by ICAO.
    As mentioned earlier in section II.D, traditionally the EPA (and
FAA) participates at ICAO in the development of international
standards, and then where appropriate, the EPA establishes domestic
aircraft engine emission standards under CAA section 231 of at least
equivalent stringency to ICAO's standards. An international
CO2 emissions standard is anticipated in February 2016, and
provided that the EPA makes a positive endangerment finding and ICAO
adopts an

international CO2 emissions standard that is both consistent
with CAA section 231 and appropriate for domestic needs, we would
expect to proceed with promulgating a CO2 emissions standard
(or GHG standard) of at least equivalent stringency domestically. As
described later in section VI.D, the thresholds of applicability for
the international CO2 emissions standard are based on gross
weight as follows: For subsonic jet aircraft, a maximum takeoff mass
(MTOM) greater than 5,700 kilograms; and for subsonic propeller driven
(e.g., turboprop) aircraft, a MTOM greater than 8,618 kilograms.\169\
Applying these gross weight thresholds, our proposed cause or
contribute finding applies to GHG emissions from classes of engines
used in covered aircraft. Examples of covered aircraft would include
smaller jet aircraft such as the Cessna Citation CJ2+ and the Embraer
E170, up to the largest commercial jet aircraft--the Airbus A380 and
the Boeing 747. Other examples of covered aircraft would include larger
turboprop aircraft, such as the ATR 72 and the Bombardier Q400. Our
intention is for the scope of the contribution finding to correspond to
the aircraft engine GHG emissions that are from aircraft that match the
applicability thresholds for the international aircraft CO2 standard.
As such we have also identified aircraft that are not covered aircraft
for purposes of our proposed contribution finding. That includes
aircraft that fall below the international applicability thresholds:
Smaller turboprop aircraft, such as the Beechcraft King Air 350i, and
smaller jet aircraft, such as the Cessna Citation M2. In addition, ICAO
(with U.S. participation) has agreed to exclude ``piston-engine
aircraft,'' ``helicopters,'' and ``military aircraft'' \170\ from the
types of aircraft that would be covered by the anticipated ICAO
standards.\171\ These aircraft would not be covered aircraft and
consequently, we are also not including GHG emissions from classes of
engines used in these types of aircraft in our proposed cause or
contribute finding.
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    \169\ ICAO, 2013: CAEP/9 Agreed Certification Requirement for
the Aeroplane CO2 Emissions Standard, Circular (Cir) 337, AN/192,
Available at http://www.icao.int/publications/ICAOProducts&Services2015catalogue/cat_2015en.pdf (last accessed May
12, 2015). The ICAO Circular 337 is found on page 85 of the ICAO
Products & Services 2015 catalog and is copyright protected; Order
No. CIR337.
    \170\ ICAO regulations only apply to civil aviation (aircraft
and aircraft engines), and consequently, ICAO regulations do not
apply to military aircraft.
    \171\ The applicability of the anticipated international
CO2 standard would be limited to subsonic aircraft, and
would not extend to supersonic aircraft. Since space vehicles (or
spacecraft) will be operated at supersonic speeds, space vehicles
would not be covered by the anticipated international CO2
standard.
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    Thus, for the purposes of the cause or contribute finding, the EPA
proposes to include GHG emissions from aircraft engines used in covered
aircraft in the scope of this proposed cause or contribute finding.
This is an equivalent scope of applicability as that contemplated by
ICAO. The majority of the GHG emissions from all classes of aircraft
engines would be covered by this scope of applicability. Below we
describe the contribution of these U.S. covered aircraft GHG emissions
to U.S. GHG emissions, and later in section V.B.2.b we discuss the
contribution of these U.S. covered aircraft emissions to global GHG
emissions.
    In 2013, GHG emissions from U.S. covered aircraft (which includes
U.S. international aviation bunker fuels in certain cases) comprised 90
percent (195 Tg CO2eq) of total U.S. aircraft GHG emissions
\172\ and 10 percent of total U.S. transportation sector GHG emissions
(See Table V.1.). Overall, U.S. covered aircraft comprised the third
largest source of GHG emissions in the U.S. transportation sector
behind only the light-duty vehicle and medium- and heavy-duty truck
sectors, which is the same ranking as total U.S. aircraft.\173\ The
U.S. covered aircraft also represent 3 percent of total U.S. GHG
emissions, which is approximately equal to the contribution from total
U.S. aircraft of 3.2 percent (Table V.1).\174\
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    \172\ Eastern Research Group, Incorporated (ERG), U.S. Jet Fuel
Use and CO2 Emissions Inventory for Aircraft Below ICAO CO2 Standard
Thresholds, Final Report, EPA Contract Number EP-D-11-006, May 7,
2015.
    \173\ Compared independently, total U.S. aircraft GHG emissions
and U.S. covered aircraft GHG emissions are both ranked the third
largest source in the U.S. transportations sector, behind only
light-duty vehicle and medium- and heavy-duty truck sectors.
    \174\ Total U.S. aircraft GHG emissions and U.S. covered
aircraft GHG emissions were from 12 to 32 percent greater in 2000
and 2005 than in 1990. These increases in aircraft GHG emissions are
primarily because aircraft operations (or number of flights) grew by
similar amounts during this time period. Also, total U.S. aircraft
GHG emissions and U.S. covered aircraft GHG emissions were from 10
to 17 percent greater in 2000 and 2005 than in 2013. These decreases
in aircraft GHG emissions are partly because aircraft operations
decreased by similar amounts during this time period. In addition,
the decreases in aircraft emissions are due in part to improved
operational efficiency that results in more direct flight routing,
improvements in aircraft and engine technologies to reduce fuel burn
and emissions, and the accelerated retirement of older, less fuel
efficient aircraft.
    Also, the U.S. transportation GHG emissions were changing at
similar rates as total U.S. aircraft GHG emissions and U.S. covered
aircraft GHG emissions for these same time periods, and thus, the
aircraft GHG emissions share of U.S. Transportation remains
approximately constant (over these time periods).
    (U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions and
Sinks: 1990-2013, 564 pp. Available at http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, Last
accessed May 12, 2015; U.S. FAA. 2015, APO Terminal Area Forecast
Summary Report--Forecast Issued January 2015, http://aspm.faa.gov/apowtaf/.).
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    It is important to note that in regard to the six well-mixed GHGs
(CO2, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride), only two of these gases--
CO2 and nitrous oxide--are reported as non-zero emissions
for total aircraft and covered aircraft.\175\ CO2 represents
99 percent of all GHGs from both total aircraft (214 Tg
CO2eq) and U.S. covered aircraft (193 Tg CO2eq),
and nitrous oxide represents about one percent from total aircraft (2
Tg CO2eq) and covered aircraft (1.8 Tg CO2eq).
Modern aircraft do not emit methane,\176\ and hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride are not products of aircraft
engine combustion.
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    \175\ U.S. EPA, 2015: Inventory of U.S. Greenhouse Gas Emissions
and Sinks: 1990-2013, 564 pp. Available at http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html#fullreport, (last
accessed May 12, 2015).
    \176\ Emissions of methane from jet fuels are no longer
considered to be emitted (based on the latest studies) across the
time series from aircraft gas turbine engines burning jet fuel A at
higher power settings (EPA, Recommended Best Practice for
Quantifying Speciated Organic Gas Emissions from Aircraft Equipped
with Turbofan, Turbojet and Turboprop Engines, EPA-420-R-09-901, May
27, 2009 (see http://www.epa.gov/otaq/regs/nonroad/aviation/420r09901.pdf (last accessed May 12, 2015)). Based on this data,
methane emissions factors for jet aircraft were reported as zero to
reflect the latest emissions testing data. Also, the 2006 IPCC
Guidelines indicate the following: ``Methane (CH4) may be
emitted by gas turbines during idle and by older technology engines,
but recent data suggest that little or no CH4 is emitted
by modern engines.'' (IPCC, 2006: IPCC Guidelines for National
Greenhouse Gas Inventories, The National Greenhouse Gas Inventories
Programme, The Intergovernmental Panel on Climate Change, H.S.
Eggleston, L. Buendia, K. Miwa, T Ngara, and K. Tanabe (eds.).
Hayama, Kanagawa, Japan.) The EPA uses an emissions factor of zero
to maintain consistency with the IPCC reporting guidelines, while
continuing to stay abreast of the evolving research in this area.