estimates and can therefore help guide policies towards higher global
net benefits for GHG reductions.\78\ Furthermore, international effects
of climate change may also affect domestic benefits directly and
indirectly to the extent U.S. citizens value international impacts
(e.g., for tourism reasons, concerns for the existence of ecosystems,
and/or concern for others); U.S. international interests are affected
(e.g., risks to U.S. national security, or the U.S. economy from
potential disruptions in other nations); and/or domestic mitigation
decisions affect the level of mitigation and emissions changes in
general in other countries (i.e, the benefits realized in the U.S. will
depend on emissions changes in the U.S. and internationally). The
economics literature also suggests that policies based on direct
domestic benefits will result in little appreciable

reduction in global GHGs (e.g., Nordhaus, 1995).\79\
---------------------------------------------------------------------------

    \77\ Nordhaus, W., 2006, ``Paul Samuelson and Global Public
Goods,'' in M. Szenberg, L. Ramrattan, and A. Gottesman (eds),
Samuelsonian Economics, Oxford.
    \78\ Both the United Kingdom and the European Commission
following these economic principles in consideration of the global
social cost of carbon (SCC) for valuing the benefits of GHG emission
reductions in regulatory impact assessments and cost-benefit
analyses (Watkiss et al, 2006).
    \79\ Nordhaus, William D. (1995). ``Locational Competition and
the Environment: Should Countries Harmonize Their Environmental
Policies?'' in Locational Competition in the World Economy,
Symposium 1994, ed., Horst Siebert, J. C. B. Mohr (Paul Siebeck),
Tuebingen, 1995.
---------------------------------------------------------------------------

    These economic principles suggest that global benefits should also
be considered when evaluating alternative GHG reduction policies.\80\
In the literature, there are a variety of global marginal benefits
estimates (see the Tol, 2005, and Tol, 2007, meta analyses).\81\ A
marginal benefit is the estimated monetary benefit for each additional
unit of carbon dioxide emissions reduced in a particular year.\82\
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    \80\ Recently, the National Highway Traffic Safety
Administration (NHTSA) proposed a new rulemaking for average fuel
economy standards for passenger cars and light trucks that is based
on domestic marginal benefit estimates for carbon dioxide
reductions. See section V.A.7.l.(iii) ``Economic value of reductions
in CO2 emissions'' (p. 24413) of Vol. 73 of the Federal
Registry. Department of Transportation, National Highway Traffic
Safety Administration, 49 CFR Parts 523, 531, 533, 534, 536 and 537
[Docket No. NHTSA-2008 -0089], RIN 2127-AK29, Average Fuel Economy
Standards: Passenger Cars and Light Trucks, Model Years 2011-2015,
http://www.regulations.gov/fdmspublic/component/
main?main=DocumentDetail&;o=0900006480541adc.
    \81\ Tol, Richard, 2005. The marginal damage costs of carbon
dioxide emissions: an assessment of the uncertainties. Energy Policy
33: 2064-2074. Tol, Richard, 2007. The Social Cost of Carbon:
Trends, Outliers and Catastrophes. Economics Discussion Papers
Discussion Paper 2007-44, September 19, 2007. Tol (2007) has been
published on-line with peer review comments (http://www.economics-
ejournal.org/economics/discussionpapers/2007-44).
    \82\ This is sometimes referred to as the social cost of carbon,
which specifically is defined as the net present value of the change
in climate change impacts over the atmospheric life of the
greenhouse gas and the resulting climate inertia associated with one
additional net global metric ton of carbon emitted to the atmosphere
at a particular point in time.
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    Based on the characteristics of GHGs and the economic principles
that follow, EPA developed ranges of global and U.S. marginal benefits
estimates. The estimates were developed as part of the work evaluating
potential GHG emission reductions from motor vehicles and their fuels
under Executive Order 13432. However, it is important to note at the
outset that the estimates are incomplete since current methods are only
able to reflect a partial accounting of the climate change impacts
identified by the IPCC (discussed more below). Also, as noted above,
domestic estimates omit potential impacts on the United States (e.g.,
economic or national security impacts) resulting from climate change
impacts in other countries. The global estimates were developed from a
survey analysis of the peer reviewed literature (i.e. meta analysis).
U.S. estimates, and a consistent set of global estimates, were
developed from a single model and are highly preliminary, under
evaluation, and likely to be revised.
    The range of estimates is wide due to the uncertainties described
above relating to socio-economic futures, climate responsiveness,
impacts modeling, as well as the choice of discount rate. For instance,
for 2007 emission reductions and a 2% discount rate the global meta
analysis estimates range from $-3 to $159/tCO2, while the
U.S. estimates range from $0 to $16/tCO2. For 2007 emission
reductions and a 3% discount rate, the global meta-estimates range from
$-4 to $106/tCO2, and the U.S. estimates range from $0 to
$5/tCO2.\83\ The global meta analysis mean values for 2007
emission reductions are $68 and $40/tCO2 for discount rates
of 2% and 3% respectively (in 2006 real dollars) while the domestic
mean value from a single model are $4 and $1/tCO2 for the
same discount rates. The estimates for future year emission changes
will be higher as future marginal emissions increases are expected to
produce larger incremental damages as physical and economic systems
become more stressed as the magnitude of climate change increases.\84\
---------------------------------------------------------------------------

    \83\ See the Technical Support Document on Benefits of Reducing
GHG Emissions for global estimates consistent with the U.S.
estimates in the text and for a comparison to the Tol (2005) meta
analysis peer reviewed estimates. Tol (2005) estimates were cited in
NHTSA's proposed rule and by the 9th U.S. Circuit Court (Center for
Biodiversity v. NHTSA, F. 3d. 9th Cir., Nov. 15, 2007).
    \84\ Note that, except for illustrative purposes, marginal
benefits estimates in the peer reviewed literature do not use
consumption discount rates as high as 7%.
---------------------------------------------------------------------------

    The current state-of-the-art for estimating benefits is also
important to consider when evaluating policies. There are significant
partially unquantified and omitted impact categories not captured in
the estimates provided above. The IPCC WGII (2007) concluded that
current estimates are ``very likely'' to be underestimated because they
do not include significant impacts that have yet to be monetized.\85\
Current estimates do not capture many of the main reasons for concern
about climate change, including non-market damages (e.g., species
existence value and the value of having the option for future use), the
effects of climate variability, risks of potential extreme weather
(e.g., droughts, heavy rains and wind), socially contingent effects
(such as violent conflict or humanitarian crisis), and potential long-
term catastrophic events. Underestimation is even more likely when one
considers that the current trajectory for GHG emissions is higher than
typically modeled, which when combined with current regional population
and income trajectories that are more asymmetric than typically
modeled, imply greater climate change and vulnerability to climate
change.
    Finally, with projected increasing changes in climate, some types
of potential climate change impacts may occur suddenly or begin to
increase at a much faster rate, rather than increasing gradually or
smoothly. In this case, there are likely to be jumps in the functioning
of species and ecosystems, the frequency and intensity of extreme
conditions (e.g., heavy rains, forest fires), and the occurrence of
catastrophic events (e.g., collapse of the West Antarctic Ice Sheet).
As a result, different approaches are necessary for quantifying the
benefits of ``small'' (incremental) versus ``large'' (non-incremental)
reductions in global GHGs. Marginal benefits estimates, like those
presented above, can be useful for estimating benefits for small
changes in emissions. However, for large changes in emissions, a more
comprehensive assessment of impacts would be needed to capture changes
in economic and biophysical dynamics and feedbacks in response to the
policy. Even small reductions in global GHG emissions are expected to
reduce climate change risks, including catastrophic risks.
---------------------------------------------------------------------------

    \85\ IPCC WGII, 2007. In the IPCC report, ``very likely'' was
defined as a greater than 90% likelihood based on expert judgment.
---------------------------------------------------------------------------

    EPA solicits comment on the appropriateness of using U.S. and
global values in quantifying the benefits of GHG reductions and the
appropriate application of benefits estimates given the state of the
art and overall uncertainties. We also seek comment on our estimates of
the global and U.S. marginal benefits of GHG emissions reductions that
EPA has developed, including the scientific and economic foundations,
the methods employed in developing the estimates, the discount rates
considered, current and proposed future consideration of uncertainty in
the estimates, marginal benefits estimates for non-CO2 GHG
emissions reductions, and potential opportunities for improving the
estimates. We are also interested in comments on methods for
quantifying benefits for non-incremental reductions in global GHG
emissions.
5. Energy Security
    In recent actions, both EPA and NHTSA have considered other
benefits of a regulatory program that, though not directly
environmental, can result from compliance with the program and may

be quantified.\86\ One of these potential benefits, related to the
transportation sector, is increased energy security due to reduced oil
imports. It is clear that both financial and strategic risks can result
within the U.S. economy if there is a sudden disruption in the supply
or a spike in the costs of petroleum. Conversely, actions that promote
development of lower carbon fuels that can substitute for petroleum or
technologies that more efficiently combust petroleum during operation
can result in reduced U.S. oil imports, and can therefore reduce these
financial and strategic risks. This reduction in risks is a measure of
improved energy security and represents a benefit to the U.S. As the
Agency evaluates potential actions to reduce GHGs from the U.S.
economy, it intends to also consider the energy security impacts
associated with these actions.
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    \86\ The EPA has worked with Oak Ridge National Laboratory to
develop a methodology that quantifies energy security benefits
associated with the reduction of imported oil. This methodology was
used to support the EPA's 2007 Renewable Fuels Standards Rulemaking
and NHTSA's 2008 proposed Average Fuel Economy Standards for
Passenger Cars and Light Trucks Rulemaking for Model Years 2001--
2015.
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6. Interactions With Other Policies
    Climate change and GHG mitigation policies will likely affect most
biophysical and economic systems, and will therefore affect policies
related to these systems. For example, as previously mentioned, climate
change will affect air quality and GHG mitigation will affect criteria
pollutant emissions. These effects will need to be evaluated, both in
the context of economic costs and benefits, as well as policy design in
order to exploit synergies and avoid inefficiencies across policies.
Non-climate policies, whether focused on traditional air pollutants,
energy, transportation, or other areas, can also affect baselines and
mitigation opportunities for climate policies. For instance, energy
policies can change baseline GHG emissions and the development path of
particular energy technologies, potentially affecting the GHG
mitigation objectives of climate policies as well as changing the
relative costs of mitigation technologies. EPA seeks comment on
important policy interactions.
7. Integrating Economic and Noneconomic Considerations
    While economics can answer questions about the cost effectiveness
and efficiency of policies, judgments about the appropriate mitigation
policy, potential climate change impacts, and even the discount rate
can be informed by economics and science but also involve important
policy, legal, and ethical questions. The ultimate choice of a global
climate stabilization target may be a policy choice that incorporates
both economic and non-economic factors, while the choice of specific
implementation strategies may be based on effectiveness criteria.
Furthermore, other quantitative analyses are generally used to support
the development of regulations. Distributional analyses, environmental
justice analyses, and other analyses can be informative. For example,
to the extent that climate change affects the distribution of wealth or
the distribution of environmental damages, then climate change
mitigation policies may have significant distributional impacts, which
may in some cases be more important than overall efficiency or net
benefits. EPA seeks comment on how to adequately inform economic
choices, as well as the broader policy choices, associated with GHG
mitigation policies.

IV. Clean Air Act Authorities and Programs

    In developing a response to the Massachusetts decision, EPA
conducted a thorough review of the CAA to identify and assess all of
the Act's provisions that might be applied to GHG emissions. Although
the Massachusetts decision addresses only CAA section 202(a)(1), which
authorizes new motor vehicle emission standards, the Act contains a
number of provisions that could conceivably be applied to GHGs
emissions. EPA's review of these provisions and their interconnections
indicated that a decision to regulate GHGs under section 202(a) or
another CAA provision could or would lead to regulation under other CAA
provisions. This section of the notice provides an overview of the CAA
and examines the various interconnections among CAA provisions that
could lead to broad regulation of GHG emission sources under the Act.

A. Overview of the Clean Air Act

    The CAA provides broad authority to combat air pollution. Cars,
trucks, construction equipment, airplanes, and ships, as well as a
broad range of electric generation, industrial, commercial and other
facilities, are subject to various CAA programs. Implementation of the
Act over the past four decades has resulted in significant reductions
in air pollution at the same time the nation's economy has grown.
    As more fully examined in Section VII of this notice, the CAA
provides three main pathways for regulating stationary sources of air
pollutants. They include, in order of their appearance in the Act,
national ambient air quality standards (NAAQS) and state plans for
implementing those standards (SIPs); performance standards for new and
existing stationary sources; and hazardous air pollutant standards for
stationary sources. In addition, the Prevention of Significant
Deterioration (PSD) program requires preconstruction permitting and
emission controls for certain new and modified major stationary
sources, and the Title V program requires operating permits for all
major stationary sources.
    Section 108 of the CAA authorizes EPA to list air pollutants that
are emitted by many sources and that cause or contribute to air
pollution problems such as ozone (smog) and particulate matter (soot).
For every pollutant listed, EPA is required by section 109 to set NAAQS
that are ``requisite'' to protect public health and welfare. EPA may
not consider the costs of meeting the NAAQS in setting the standards.
Under section 110, every state develops and implements plans for
meeting the NAAQS by applying enforceable emission control measures to
sources within the state. The Act's requirements for SIPs are more
detailed and stringent for areas not meeting the standards
(nonattainment areas) than for areas meeting the standards (attainment
areas). Costs may be considered in implementing the standards. States
are aided in their efforts to meet the NAAQS by federal emissions
standards for mobile sources and major categories of stationary sources
issued under other sections of the Act.
    Under CAA section 111, EPA establishes emissions performance
standards for new stationary sources and modifications of existing
sources for categories of sources that contribute significantly to
harmful air pollution. These new source performance standards (NSPS)
reduce emissions of air pollutants addressed by NAAQS, but can be
issued regardless of whether there is a NAAQS for the pollutants being
regulated. NSPS requirements for new sources help ensure that when
large sources of air pollutants are built or modified, they apply
available emission control technologies and strategies.
    When EPA establishes a NSPS for a pollutant, section 111(d) calls
upon states to issue a standard for existing sources in the regulated
source category except in two circumstances. First, section 111(d)
prohibits regulation of a NAAQS pollutant. Second, ``where a source
category is being regulated under section 112, a section 111(d)
standard of performance cannot be established to

address any HAP listed under section 112(b) that may be emitted from
that particular source category.''\87\ In effect, existing source NSPS
provides a ``regulatory safety net'' for pollutants not otherwise
subject to major regulatory programs under the CAA. Section 111
provides EPA and states with significant discretion concerning the
sources to be regulated and the stringency of the standards, and allows
consideration of costs in setting NSPS.
---------------------------------------------------------------------------

    \87\ See 70 FR 15994, 16029-32 (Mar. 29, 2005).
---------------------------------------------------------------------------

    CAA section 112 provides EPA with authority to list and issue
national emissions standards for hazardous air pollutants (HAPs) from
stationary sources. HAPs are broadly defined as pollutants that
present, or may present, a threat of adverse human or environmental
effects. HAPs include substances which are, or may reasonably be
anticipated to be, carcinogenic, mutagenic, neurotoxic or acutely or
chronically toxic. Section 112 contains low emissions thresholds for
regulation in view of its focus on toxic pollutants, and requires
regulation of all major sources of HAPs. Section 112 also provides for
``maximum achievable control technology'' (MACT) standards for major
sources, limiting consideration of cost.
    The PSD program under Part C of Title I of the Act is triggered by
regulation of a pollutant under any other section of the Act except for
sections 112 and 211(o). As mentioned previously in this notice, under
this program, new major stationary sources and modifications at
existing major stationary sources undergo a preconstruction permitting
process and install best available control technology (BACT) for each
regulated pollutant. These basic requirements apply regardless of
whether a NAAQS exists for the pollutant; additional PSD requirements
apply in the event of a NAAQS. The PSD program's control requirements
help prevent large new and modified sources of air pollutants from
significantly degrading the air quality in clean air areas. A similar
program, called ``new source review,'' ensures that new or modified
large sources in areas not meeting the NAAQS do not make it more
difficult for the areas to eventually attain the air quality standards.
    Title II of the CAA provides comprehensive authority for regulating
mobile sources of air pollutants. As more fully described in Section VI
of this notice, Title II authorizes EPA to address all categories of
mobile sources and take an integrated approach to regulation by
considering the unique aspects of each category, including passenger
vehicles, trucks and nonroad vehicles, as well as the fuels that power
them. Title II requires EPA to consider technological feasibility,
costs, safety and other factors in setting standards, and gives EPA
discretion to set technology-forcing standards as appropriate. In
addition, section 211(o) of the Act establishes the renewable fuel
standard (RFS) program, which was recently strengthened by EISA to
require substantial increases in the use of renewable fuels, including
renewable fuels with significantly lower lifecycle GHG emissions than
the fossil fuel-based fuels they replace.\88\ The CAA's mobile source
authorities work in tandem with the Act's stationary source authorities
to help protect public health and the environment from air pollution.
---------------------------------------------------------------------------

    \88\ As explained further below, EISA provides that regulation
of renewable fuels based on lifecycle GHG emissions does not trigger
any other regulation of GHGs under the CAA.
---------------------------------------------------------------------------

    Title VI of the CAA authorizes EPA to take various actions to
protect stratospheric ozone, a layer of ozone high in the atmosphere
that helps protect the Earth from harmful UVB radiation. As discussed
in Section VIII of this notice, section 615 provides broad authority to
regulate any substance, practice, process or activity that may
reasonably be anticipated to affect the stratosphere and that effect
may reasonably be anticipated to endanger public health or welfare.

B. Interconnections Among Clean Air Act Provisions

    The provisions of the CAA are interconnected in multiple ways such
that a decision to regulate one source category of GHGs could or would
lead to regulation of other source categories of GHGs. As described in
detail below, there are several provisions in the CAA that contain
similar endangerment language. An endangerment finding for GHGs under
one provision of the Act could thus have ramifications under other
provisions of the Act. In addition, CAA standards applicable to GHGs
for one category of sources could trigger PSD requirements for other
categories of sources that emit GHGs. How a term is interpreted for one
part of the Act could also affect other provisions using the same term.
    These CAA interconnections are by design. As described above, the
Act combats air pollutants in several ways that reflect the nature and
effects of the particular air pollutant being addressed. The Act's
approaches are in many cases complementary and reinforcing, ensuring
that air pollutants emitted by various types of emission sources are
reduced in a manner and to an extent that reflects the relative
contribution of particular categories of sources. The CAA's authorities
are intended to work together to achieve air quality that protects
public health and welfare.
    For GHGs, the CAA's interconnections mean that careful attention
needs to be paid to the consequences and specifics of decisions
regarding endangerment and regulation of any particular category of GHG
sources under the Act. In the case of traditional air pollutants, EPA
and States have generally regulated pollutants incrementally over time,
adding source categories or program elements as evolving circumstances
make appropriate. In light of the broad variety and large number of GHG
sources, any decision to regulate under the Act could lead, relatively
quickly, to more comprehensive regulation of GHG sources under the Act.
A key issue to consider in examining the Act's provisions and their
interconnections is the extent to which EPA may choose among and/or
tailor the CAA's authorities to implement a regulatory program that
makes sense for GHGs, given the unique challenges and opportunities
that regulating them would present.
    This section of the notice explores these interconnections, and
later sections explain how each CAA provision might apply to GHGs.
1. Similar Endangerment Language Is Found in Numerous Sections of the
Clean Air Act
    The Supreme Court's decision in Massachusetts v. EPA requires EPA
to address whether GHG emissions from new motor vehicles meet the
endangerment test of CAA section 202(a)(1). That section states:

    [t]he Administrator shall by regulation prescribe (and from time
to time revise) * * * standards applicable to the emissions of any
air pollutant from any class or classes of new motor vehicles or new
motor vehicle engines, which in his judgment cause, or contribute
to, air pollution which may reasonably be anticipated to endanger
public health or welfare.

CAA section 202(a)(1). If the Administrator makes a positive
endangerment determination for GHG emissions from new motor vehicles,
he must regulate those GHG emissions under section 202(a) of the Act.
    Similar endangerment language is found in numerous sections of the
CAA, including sections 108, 111, 112, 115, 211, 213, 231 and 615. For
example, CAA section 108(a)(1) (regarding listing pollutants to be
regulated by NAAQS)

states, ``[T]he Administrator shall * * * publish, and shall from time
to time thereafter revise, a list which includes each air pollutant (A)
emissions of which, in his judgment, cause or contribute to air
pollution which may reasonably be anticipated to endanger public health
or welfare * * *'' CAA section 111(b)(1)(A) (regarding listing source
categories to be regulated by NSPS) states: ``[The Administrator] shall
include a category of sources in such list if in his judgment it
causes, or contributes significantly to, air pollution which may
reasonably be anticipated to endanger public health or welfare.''\89\
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    \89\ Other CAA endangerment provisions read as follows:
    CAA section 115 (regarding international air pollution) states:
``Whenever the Administrator, upon receipt of reports, surveys or
studies from any duly constituted international agency has reason to
believe that any air pollutant or pollutants emitted in the United
States cause or contribute to air pollution which may reasonably be
anticipated to endanger public health or welfare in a foreign
country or whenever the Secretary of State requests him to do so
with respect to such pollution which the Secretary of State alleges
is of such a nature, the Administrator shall give formal
notification thereof to the Governor of the State in which such
emissions originate.''
    CAA section 211(c)(1) (regarding regulating fuels and fuel
additives) states: ``The Administrator may, * * * [regulate fuels or
fuel additives] (A) if in the judgment of the Administrator any
emission product of such fuel or fuel additive causes, or
contributes, to air pollution which may reasonably be anticipated to
endanger public health or welfare, (B) * * *''
    CAA section 213(a)(4) (regarding regulating nonroad engines)
states: ``If the Administrator determines that any emissions not
referred to in paragraph 2 [regarding CO, NOX and VOC
emissions] from new nonroad engines or vehicles significantly
contribute to air pollution which may reasonably be anticipated to
endanger public health or welfare, the Administrator may promulgate
* * * standards applicable to emissions from those classes or
categories of new nonroad engines and new nonroad vehicles (other
than locomotives) which in the Administrator's judgment cause, or
contribute to, such air pollution, * * *''.
    CAA section 231 (regarding setting aircraft standards) states:
``The Administrator shall * * * issue proposed emissions standards
applicable to the emission of any air pollutant from any class or
classes of aircraft engines which in his judgment causes, or
contributes to, air pollution which may reasonably be anticipated to
endanger public health or welfare.''
    CAA section 615 (regarding protection of stratospheric ozone)
states: ``If, in the Administrator's judgment, any substance,
practice, process, or activity may reasonably be anticipated to
affect the stratosphere, especially ozone in the stratosphere, and
such effect may reasonably be anticipated to endanger public health
or welfare, the Administrator shall promptly promulgate regulations
respecting the control of such substance, practice, process, or
activity * * *''
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    While no two endangerment tests are precisely the same, they
generally call on the Administrator of EPA to exercise his or her
judgment regarding whether a particular air pollutant or source
category causes or contributes to air pollution which may reasonably be
anticipated to endanger public health or welfare. For provisions
containing endangerment language, a positive finding of endangerment is
a prerequisite for regulation under that provision.\90\ The precise
effect of a positive or negative finding depends on the specific terms
of the provision under which it is made. For some provisions, a
positive endangerment finding triggers an obligation to regulate (e.g.,
section 202(a)(1)), while for other provisions, a positive finding
allows the Agency to regulate in its discretion (e.g., section 213). In
some cases, other criteria must also be met to authorize or require
regulation (e.g., section 108). Each of these sections is discussed in
more detail later in this notice.
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    \90\ As defined by the CAA, ``air pollutant'' includes virtually
any substance or material emitted into the ambient air. Given the
breadth of that term, many CAA provisions require the Administrator
to determine whether a particular air pollutant causes or
contributes to an air pollution problem as a prerequisite to
regulating emissions of that pollutant.
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2. Potential Impact Cross the Clean Air Act From a Positive or Negative
Endangerment Finding or Regulation of GHGs Under the Act
a. Potential Impact on Sections Containing Similar Endangerment
Language
    One important issue is whether a positive or negative endangerment
finding under one section of the CAA (e.g., under section 202(a) in
response to the ICTA petition remand) would necessarily or
automatically lead to similar findings under other provisions of the
Act containing similar language. Even though CAA endangerment tests
vary to some extent, an endangerment finding under one provision could
have some bearing on whether endangerment could or should be found
under other CAA provisions, depending on their terms and the facts at
issue. EPA request comment on the extent to which an endangerment
finding under any section of the CAA would lead EPA to make a similar
endangerment finding under another provision.
    In discussing the implications of making a positive endangerment
finding under any CAA section, we use the actual elements of the
endangerment test in section 202(a) for new motor vehicles as an
example. The section 202(a) endangerment test asks two distinct
questions--
    (1) whether the air pollution at issue may reasonably be
anticipated to endanger public health or welfare, and
    (2) whether emissions from new motor vehicles cause or contribute
to that air pollution. The first question is generic and looks at
whether the type of air pollution at issue endangers public health or
welfare. The second question is specific to motor vehicles, and
considers the contribution of motor vehicle emissions to the particular
air pollution problem. EPA must answer both questions in the
affirmative for the Agency to regulate under section 202(a) of the Act.
    A finding of endangerment under one section of the Act would not by
itself constitute a complete finding of endangerment under any other
section of the CAA. How much of a precedent an endangerment finding
under one CAA provision would be for other CAA provisions would depend
on the basis for the finding, the statutory tests for making findings,
and the facts. For example, the two-part endangerment test in section
202(a) (motor vehicles) is similar to that in sections 211(c)(1)
(highway and nonroad fuels) and 231(a)(2) (aircraft). An affirmative
finding under section 202(a) on the first part of the test--whether the
air pollution at issue endangers public health or welfare--would appear
to satisfy the first part of the test for the other two provisions as
well. However, an affirmative finding on the second part of the test,
regarding the contribution of the particular source category to that
air pollution, would not satisfy the test for the other provisions,
which apply to different source categories. Still, a finding that a
particular source category's emissions cause or contribute to the air
pollution problem would likely establish some precedent for what
constitutes a sufficient contribution for purposes of making a positive
endangerment finding for other source categories.
    Other similarities and differences among endangerment tests are
also relevant. While the first part of the test in sections 213(a)(4)
(nonroad engines and vehicles) and 111(b) (NSPS) is similar to that in
other sections (i.e., whether the air pollution at issue endangers
public health or welfare), the second part of the test in sections
213(a)(4) and 111(b) requires a finding of ``significant''
contribution. In addition, the test under section 111(b) applies to
source categories, not to a particular air pollutant.\91\ Sections 112
and 615 have somewhat different tests.
---------------------------------------------------------------------------

    \91\ As discussed below, EPA has already listed a very wide
variety of source categories under section 111(b)(1)(A).
---------------------------------------------------------------------------

    The extent to which an endangerment finding would set precedent
would also depend on the pollutants at issue. For example, the ICTA
petition to regulate motor vehicles under section 202(a)

addresses CO2, CH4 , N2O, and HFCs,
while the petitions to regulate GHGs from other mobile source
categories collectively address water vapor, NOX and black
carbon, as well as CO2, CH4, and N2O.
As further discussed below, the differences in the GHGs emitted by
different types of sources may be relevant to the issue of how to
define ``air pollutant'' for purposes of applying the endangerment
tests.
    In addition, some CAA sections require EPA to act following a
positive endangerment finding, while others do not. In the case of
section 202(a)(1), if we make a positive endangerment finding, we are
required to issue standards applicable to motor vehicle emissions of
the GHGs covered by the finding. Section 231(a) (aircraft) uses similar
mandatory language, while sections 211(c)(1) (highway and nonroad fuel)
and 213(a)(4) (nonroad engines and vehicles) authorize but do not
require the issuance of regulations. Section 108 (NAAQS pollutants)
requires that EPA list a pollutant under that section if a positive
endangerment finding is made and two other criteria are met.
    In sum, a positive or negative endangerment finding for GHG
emissions under one provision of the Act could have a significant and
direct impact on decisions under other CAA sections containing similar
endangerment language. EPA requests comment on the interconnections
between the CAA endangerment tests and the impact that a finding under
one provision of the Act would have for other CAA provisions.
b. Potential Impact on PSD Program
    Another important issue is the potential for a decision to regulate
GHGs for mobile or stationary sources to automatically trigger
additional permitting requirements for stationary sources under the PSD
program. As explained previously and in detail in Section VII of this
notice, the main element of the PSD program under Part C of Title I of
the Act is the requirement that a PSD permit be obtained prior to
construction of any new major source or any major modification at an
existing major source. Such a permit must contain emissions limitations
based on BACT for each pollutant subject to regulation under the Act.
EPA does not interpret the PSD program provisions to apply to GHG at
this time, but any requirement to control CO2 or other GHGs
promulgated by EPA under other provisions of the CAA would make parts
of the PSD program applicable to any additional air pollutant(s) that
EPA regulates in this manner.
    The PSD program applies to each air pollutant (other than a HAP)
that is ``subject to regulation under the Act'' within the meaning of
sections 165(a)(4) and 169(3) of the Clean Air Act and EPA's
regulations.\92\ As a practical matter, the identification of
pollutants subject to the PSD program is driven by the BACT requirement
because this requirement applies to the broadest range of pollutants.
Under EPA's PSD program regulations, BACT is required for ``each
regulated NSR pollutant.'' 40 CFR 52.21(j)(2)-(3). EPA has defined this
term to include pollutants that are regulated under a NAAQS or NSPS, a
class I or II substance under Title VI of the Act, or ``[a]ny pollutant
otherwise subject to regulation under the Act.'' See 52.21(b)(50).\93\
Similarly, the determination of whether a source is a major source
subject to PSD is based on whether the source emits more than 100 or
250 tons per year (depending on the type of source) of one or more
regulated pollutants.\94\
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    \92\ Section 112(b)(6) precludes listed HAPs from the PSD
program. Section 210(b) of EISA provides that nothing in section
211(o) of the Act, or regulations issued pursuant to that
subsection, ``shall affect or be construed to affect the regulatory
status of carbon dioxide or any other greenhouse gas, or to expand
or limit regulatory authority regarding carbon dioxide or any other
greenhouse gas, for purposes of other provisions (including section
165) of this Act.''
    \93\ This definition reflects EPA's interpretation of the phrase
``each pollutant subject to regulation under the Act'' that is used
in the provisions in the Clean Air Act that establish the BACT
requirement. Since this statutory language (as implemented in the
definition of ``regulated NSR pollutant'') can apply to additional
pollutants that are not also subject to a NAAQS, the scope of the
BACT requirement determines the overall range of pollutants that are
subject to the PSD permitting program.
    \94\ Under the relevant regulations, a major stationary source
is determined by its emissions of ``any regulated NSR pollutant.''
See 40 CFR 52.21(b)(1)(i). Thus, the emissions that are considered
in identifying a major source are determined on the basis of the
same definition that controls the applicability of the BACT.
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    EPA has historically interpreted the phrase ``subject to regulation
under the Act'' to describe air pollutants subject to CAA statutory
provisions or regulations that require actual control of emissions of
that pollutant.\95\ PSD permits have not been required to contain BACT
emissions limit for GHGs because GHGs (and CO2 in
particular) have not been subject to any CAA provisions or EPA
regulations issued under the Act that require actual control of
emissions.\96\ Although CAA section 211(o) now targets GHG emissions,
EISA provides that neither it nor implementing regulations affect the
regulatory status of GHGs under the CAA. In the absence of statutory or
regulatory requirements to control GHG emissions under the Act, a
stationary source need not consider those emissions when determining
its major source status.
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    \95\ 43 FR 26388, 26397 (June 19, 1978); Gerald E. Emison,
Director, Office of Air Quality Planning and Standards,
Implementation of North County Resource Recovery PSD Remand (Sept.
22, 1987) (footnote on the first page).
    \96\ See briefs filed before the Environmental Appeal Board on
behalf of specific EPA offices in challenges to the PSD permits for
Deseret Power Electric Cooperative (PSD Appeal No. 07-03) and
Christian County Generation LLC (PSD Appeal No. 07-01), as well as
the Response to Public Comments on Draft Air Pollution Control
Prevention of Significant Deterioration (PSD) Permit to Construct
[for Deseret Power Electric Cooperative], Permit No. PSD-OU-0002-
04.00 (August 30, 2007), at 5-6, available at http://www.epa.gov/
region8/air/permitting/deseret.html. EPA has not previously
interpreted the BACT requirement to apply to air pollutants that are
only subject to requirements to monitor and report emissions. See,
67 FR 80186, 80240 (Dec. 31, 2002); 61FR 38250, 38310 (July 31,
1996); In Re Kawaihae Cogeneration Project 7 E.A.D. 107, 132 (EAB
1997); Inter-power of New York, 5 E.A.D. 130, 151 (EAB 1994);
Memorandum from Jonathan Z. Cannon, General Counsel to Carol M.
Browner, Administrator, entitled EPA's Authority to Regulate
Pollutants Emitted by Electric Power Generation Sources (April 10,
1998) (emphasis added); Memorandum from Lydia N. Wegman, Deputy
Director, Office of Air Quality Planning and Standards, entitled
Definition of Regulated Air Pollutant for Purposes of Title V, at 5
(April 26, 1993).
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    The Supreme Court's conclusion that GHGs are ``air pollutants''
under the CAA did not automatically make these pollutants subject to
the PSD program. A substance may be an ``air pollutant'' under the Act
without being regulated under the Act. The Supreme Court directed the
EPA Administrator to determine whether GHG emissions from motor
vehicles meet the endangerment test of CAA section 202(a). A positive
finding of endangerment would require the Administrator to then set
standards applicable to GHG emissions from motor vehicles under the
Act. The positive finding itself would not constitute a regulation
requiring actual control of emissions. GHGs would become regulated
pollutants under the Act if and when EPA subjects GHGs to control
requirements under a CAA provision other than sections 112 and 211(o).
c. Definition of ``Air Pollutant''
    Another way in which a decision to regulate GHGs under one section
of the Act could impact other sections of the Act involves how the term
``air pollutant'' is defined as part of the endangerment analysis. As
described above, many of the Act's endangerment tests require a two-
part analysis: Whether the air pollution at issue may reasonably be
anticipated to endanger public health or welfare, and whether emissions
of particular air pollutants cause or contribute to that air pollution.

As discussed in more detail in the following sections, what GHGs might
be defined as an ``air pollutant'' and whether those GHGs are treated
individually or as a group could impact EPA's flexibility to define the
GHGs as air pollutants elsewhere in the CAA.
    For example, as noted above, how EPA defines GHGs as air pollutants
in making any positive endangerment finding could carry over into
implementation of the PSD program. If EPA defines each individual GHG
as a separate air pollutant in making a positive endangerment finding,
then each GHG would be considered individually as a ``regulated NSR
pollutant'' in the PSD program. On the other hand, if EPA defines the
group of GHGs as an air pollutant, then the PSD program would need to
treat the GHGs in the same manner--as a group. As discussed in more
detail below, there are flexibilities and considerations under various
approaches. One question is whether we could or should define GHGs as
an ``air pollutant'' one way under one section of the Act (e.g.,
section 202) and another way under another section (e.g., section 231).
See, e.g., Environmental Defense v. Duke Energy Corp., 127 S.Ct. 1423,
1432 (2007) (explaining that the general presumption that the same term
has the same meaning is not rigid and readily gives way to context).
Another question is whether having different definitions of ``air
pollutant'' would result in both definitions applying to the PSD
program, and whether that result would mean that any flexibilities
gained under one definition would be lost with the application of the
second.
    Another consideration, noted above, is that different source
categories emit different GHGs. This fact could impact the definition
of ``air pollutant'' more broadly. EPA requests comment on the issues
raised in this section, to assist the Agency as it considers the
implications of how to define a GHG ``air pollutant'' for the first
time under any section of the Act.
2. Relationships Among Various Stationary Source Programs
    As a result of other interactions among various CAA sections, a
decision to act under one part of the CAA may preclude action under
another part of the Act. These interactions reflect the Act's different
regulatory treatment of pollutants meeting different criteria, and
prevent duplicative regulation. For instance, listing a pollutant under
section 108(a), which leads to setting a NAAQS and developing SIPs for
the pollutant, generally precludes listing the same air pollutant as a
HAP under section 112(b), which leads to every major source of a listed
HAP having to comply with MACT standards for the HAP. CAA section
112(b)(2).\97\ Listing an air pollutant under section 108(a) also
preludes regulation of that air pollutant from existing sources under
section 111(d), which is intended to provide for regulation of air
pollutants not otherwise subject to the major regulatory programs under
the Act. CAA section 111(d)(1)(A).
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    \97\ ``No air pollutant which is listed under section 108(a) may
be added to the list under this section, except that the prohibition
of this sentence shall not apply to any pollutant which
independently meets the listing criteria of this paragraph and is a
precursor to a pollutant which is listed under section 108(a) or to
any pollutant which is in a class of pollutants listed under such
section.''
---------------------------------------------------------------------------

    Similarly, regulation of a substance under Title VI precludes
listing that substance as a HAP under section 112(b) based solely on
the adverse effects on the environment of that air pollutant. CAA
section 112(b)(2). Moreover, listing an air pollutant as a HAP under
section 112(b) generally precludes regulation of that air pollutant
from existing sources under section 111(d). CAA section
111(d)(1)(A).\98\ Finally, section 112(b)(6) provides that the
provisions of the PSD program ``shall not apply to pollutants listed
under [section 112].'' CAA section 112(b)(6), 42 U.S.C. 7412(b)(6)
---------------------------------------------------------------------------

    \98\ However, see 70 FR 15994, 16029-32 (2005) (explaining EPA's
interpretation of the conflicting amendments to section 111(d)
regarding HAPs).
---------------------------------------------------------------------------

V. Endangerment Analysis and Issues

    In this section, we present our work to date on an endangerment
analysis in response to the Supreme Court's decision in Massachusetts
v. EPA. As explained previously, the Supreme Court remanded EPA's
denial of the ICTA petition and ruled that EPA must either decide
whether GHG emissions from new motor vehicles cause or contribute to
air pollution which may reasonably be anticipated to endanger public
health or welfare, or explain why scientific uncertainty is so profound
that it prevents making a reasoned judgment on such a determination.
    In response to the remand, EPA analyzed synthesis reports and
studies on how elevated concentrations of GHGs in the atmosphere, and
other factors, contribute to climate change, and how climate change is
affecting, and may affect in the future, human health and welfare,
primarily within the United States. We also analyzed direct GHG effects
on human health and welfare, i.e., those effects from elevated
concentrations of GHGs that do not occur via climate change. This
information, summarized briefly below, is contained in the Endangerment
Technical Support Document found in the docket for today's notice. In
addition, we compiled information concerning motor vehicle GHG
emissions to assess whether motor vehicles cause or contribute to
elevated concentrations of GHGs in the atmosphere. Information on motor
vehicle emissions is contained in the Section 202 Technical Support
Document, also found in the docket.
    As discussed above, making an endangerment finding under one
section of the CAA has implications for other sections of the Act. In
this ANPR, we consider, and seek comment on these implications and
other questions relevant to making an endangerment finding regarding
GHG emissions.
    This section is organized as follows. Section A discusses the legal
framework for the endangerment analysis. Section B provides information
on how ``air pollution'' could be defined for purposes of the
endangerment analysis, as well as a summary of the science regarding
GHGs and climate change and their effects on health and welfare.
Section C uses the information on emissions of GHGs from the mobile
source categories relevant to the ICTA Petition to frame a discussion
about whether GHGs as ``air pollutants'' ``cause or contribute'' to
``air pollution'' which may reasonably be anticipated to endanger
public health or welfare.

A. Legal Framework

    The endangerment language relevant to the ICTA petition is
contained in section 202(a) of the CAA. As explained previously, it is
similar to endangerment language in many other provisions of the Act
and establishes a two-part test. First, the Administrator must decide
if, in his judgment, air pollution may reasonably be anticipated to
endanger public health or welfare. Second, the Administrator must
decide whether, in his judgment, emissions of any air pollutant from
new motor vehicles or engines cause or contribute to this air
pollution.
1. Origin of Current Endangerment and Cause or Contribute Language
    The endangerment language in section 202(a) and other provisions of
the CAA share a common legislative history that sheds light on the
meaning of this language. As part of the 1977 amendments to the CAA,
Congress added or revised endangerment language in various sections of
the Act. The legislative history of those amendments, particularly the
report by the House Committee on Interstate and Foreign Commerce,
provides important information regarding Congress' intent

when it revised this language. See H.R. Rep. 95-294 (1977), as
reprinted in 4 A Legislative History of the Clean Air Act Amendments of
1977 at 2465 (hereinafter ``LH'').
a. Ethyl Corp. v. EPA
    In revising the endangerment language, Congress relied heavily on
the approach discussed in a federal appeals court opinion interpreting
the pre-1977 version of CAA section 211. In Ethyl Corp v. EPA, 541 F.2d
1 (D.C. Cir. 1976), the en banc (i.e. full) court reversed a 3-judge
panel decision regarding an EPA rule restricting the content of lead in
leaded gasoline.\99\ The en banc court began its opinion by stating:
---------------------------------------------------------------------------

    \99\ At the time of the 1973 rules requiring the reduction of
lead in gasoline, section 211(c)(1)(A) of the CAA stated that the
Administrator may promulgate regulations that control or prohibit
the manufacture, introduction into commerce, offering for sale, or
sale of any fuel or fuel additive for use in a motor vehicle or
motor vehicle engine (A) if any emissions product of such fuel or
fuel additive will endanger the public health or welfare * * * .
    CAA section 211(c)(1)(A) (1970) (emphasis added). The italicized
language in the above quote is the relevant language revised by the
1977 amendments.

    Man's ability to alter his environment has developed far more
rapidly than his ability to foresee with certainty the effects of
---------------------------------------------------------------------------
his alterations.

541 F.2d at 6. After reviewing the relevant facts and law, the full-
court evaluated the statutory language at issue to see what level of
``certainty [was] required by the Clean Air Act before EPA may act.''
Id.
    By a 2-1 vote, the 3-judge panel had held that the statutory
language ``will endanger'' required proof of actual harm, and that the
actual harm had to come from fuels ``in and of themselves.'' Id. at 12.
The en banc court rejected this approach, finding that the term
``endanger'' allowed the Administrator to act when harm is threatened,
and did not require proof of actual harm. Id. at 13. ``A statute
allowing for regulation in the face of danger is, necessarily, a
precautionary statute.'' Id. Optimally, the court held, regulatory
action would not only precede, but prevent, a perceived threat. Id.
    The court also rejected petitioners' argument that any threatened
harm must be ``probable'' before regulation was authorized.
Specifically, the court recognized that danger ``is set not by a fixed
probability of harm, but rather is composed of reciprocal elements of
risk and harm, or probability or severity.'' Id. at 18. Next, the court
held that EPA's evaluation of risk is necessarily an exercise of
judgment, and that the statute did not require a factual finding. Id.
at 24. Thus, ultimately, the Administrator must ``act, in part on
`factual issues,' but largely on choices of policy, on an assessment of
risks, [and] on predictions dealing with matters on the frontiers of
scientific knowledge * * * .'' Id. at 29 (citations omitted). Finally,
the en banc court agreed with EPA that even without the language in
section 202 regarding ``cause or contribute to,'' section 211
authorized EPA to consider the cumulative impact of lead from numerous
sources, not just the fuels being regulated under section 211. Id. at
29-31.
b. The 1977 Clean Air Act Amendments
    The dissent in the original Ethyl Corp decision and the en banc
opinion were of ``critical importance'' to the House Committee which
proposed the revisions to the endangerment language in the 1977
amendments to the CAA. H.R. Rep. 95-294 at 48, 4 LH at 2515. In
particular, the Committee believed the Ethyl Corp decision posed
several ``crucial policy questions'' regarding the protection of public
health and welfare.'' Id.\100\ The Committee addressed those questions
with the endangerment language that now appears in section 202(a) and
several other CAA provisions--``which in [the Administrator's] judgment
cause, or contribute to, air pollution which may reasonably be
anticipated to endanger public health or welfare.''
---------------------------------------------------------------------------

    \100\ The Supreme Court recognized that the current language in
section 202(a)(1) is ``more-protective'' than the 1970 version that
was similar to the section 211 language before the D.C. Circuit in
Ethyl Corp. 127 S.Ct. at 1447, fn 1.
---------------------------------------------------------------------------

    The Committee intended the language to serve several purposes
consistent with the en banc decision in Ethyl Corp.\101\ First, the
phrases ``in his judgment'' and ``in the judgment of the
Administrator'' call for the Administrator to make comparative
assessment of risks and projections of future possibilities, consider
uncertainties, and extrapolate from limited data. Thus, the
Administrator must balance the likelihood of effects with the severity
of the effects in reaching his judgment. The Committee emphasized that
``judgment'' is different from a factual ``finding.'' Importantly,
projections, assessments and estimates must be reasonable, and cannot
be based on a ``crystal ball inquiry.'' Moreover, procedural safeguards
apply (e.g., CAA 307(d)) to the exercise of judgment, and final
decisions are subject to judicial review. Also, the phrase ``in his
judgment'' modifies both phrases ``cause and contribute'' and ``may
reasonably be anticipated'' discussed below. H.R. Rep. 95-294 at 50-51,
4 LH at 2517-18.
---------------------------------------------------------------------------

    \101\ Specifically, the language (1) emphasizes the
precautionary or preventive purpose of the CAA; (2) authorizes the
Administrator to reasonably project into the future and weigh risks;
(3) requires the consideration of the cumulative impact of all
sources; (4) instructs that the health of susceptible individuals,
as well as healthy adults, should be part of the analysis; and (5)
indicates an awareness of the uncertainties and limitations in
information available to the Administrator. H.R. Rep. 95-294 at 49-
50, 4 LH at 2516-17. Congress also wanted to standardize this
language across the various sections of the CAA which address
emissions from both stationary and mobile sources which may
reasonably be anticipated to endanger public health or welfare. H.R.
Rep. 95-294 at 50, 4 LH at 2517; Section 401 of CAA Amendments of
1977.
---------------------------------------------------------------------------

    As the Committee further explained, the phrase ``may reasonably be
anticipated'' builds upon the precautionary and preventative goals
already provided in the use of the term ``endanger.'' Thus, the
Administrator is to assess current and future risks rather than wait
for proof of actual harm. This phrase is also intended to instruct the
Administrator to consider the limitations and difficulties inherent in
information on public health and welfare. H.R. Rep. 95-294 at 51, 4 LH
at 2518.
    Finally, the phrase ``cause or contribute'' ensures that all
sources of the contaminant which contribute to air pollution be
considered in the endangerment analysis (e.g., not a single source or
category of sources). It is also intended to require the Administrator
to consider all sources of exposure to a pollutant (e.g., food, water,
air) when determining risk. Id.
3. Additional Considerations for the ``Cause or Contribute'' Analysis
    While the legislative history sheds light on what should be
considered in making an endangerment finding, it is not clear regarding
what constitutes a sufficient ``contribution'' for purposes of making a
finding. The CAA does not define the concept ``cause or contribute''
and instead requires that the Administrator exercise his judgment when
determining whether emissions of air pollutants cause or contribute to
air pollution. As a result, the Administrator has the discretion to
interpret ``cause or contribute'' in a reasonable manner when applying
it to the circumstances before him.
    The D.C. Circuit has discussed the concept of ``contribution'' in
the context of a CAA section 213 rule for nonroad vehicles. In
Bluewater Network v. EPA, 370 F.3d 1 (2004), industry argued that
section 213(a)(3) requires a finding of a significant contribution
before EPA could regulate, but EPA argued that the CAA requires a
finding only of ``contribution.'' \102\ Id. at 13. The court

looked at the ``ordinary meaning of `contribute''' when upholding EPA's
reading. After referencing dictionary definitions of contribute,\103\
the court also noted that ``[s]tanding alone, the term has no inherent
connotation as to the magnitude or importance of the relevant `share'
in the effect; certainly it does not incorporate any `significance'
requirement.'' Id.\104\ The court also found relevant the fact that
section 213(a) uses the term ``significant contributor'' in some places
and the term ``contribute'' elsewhere, suggesting that the
``contribute'' language invests the Administrator with discretion to
exercise his judgment regarding what constitutes a sufficient
contribution for the purpose of making an endangerment finding. Id. at
14
---------------------------------------------------------------------------

    \102\ The relevant language in section 213(a)(3) reads ``[i]f
the Administrator makes an affirmative determination under paragraph
(2) the Administrator shall, * * * promulgate (and from time to time
revise) regulations containing standards applicable to emissions
from those classes or categories of new nonroad engines and new
nonroad vehicles (other than locomotives or engines used in
locomotives) which in the Administrator's judgment cause, or
contribute to, such air pollution.'' Notably, CAA section 213(a)(2),
which is referenced in section 213(a)(3), requires that the
``Administrator shall determine * * * whether emissions of carbon
monoxide, oxides of nitrogen, and volatile organic compounds from
new and existing nonroad engines or nonroad vehicles (other than
locomotives or engines used in locomotives) are significant
contributors to ozone or carbon monoxide concentrations in more than
1 area which has failed to attain the national ambient air quality
standards for ozone or carbon monoxide'' (emphasis added).
    \103\ Specifically, the decision noted that `` `contribute'
means simply `to have a share in any act or effect,' Webster's Third
New International Dictionary 496 (1993), or `to have a part or share
in producing,' 3 Oxford English Dictionary 849 (2d ed. 1989).'' 370
F.3d at 13.
    \104\ The court explained, ``The repeated use of the term
`significant' to modify the contribution required for all nonroad
vehicles, coupled with the omission of this modifier from the
`cause, or contribute to' finding required for individual categories
of new nonroad vehicles, indicates that Congress did not intend to
require a finding of `significant contribution' for individual
vehicle categories.'' Id.
---------------------------------------------------------------------------

    In the past the Administrator has looked at emissions of air
pollutants in various ways to determine whether they ``cause or
contribute'' to the relevant air pollution. For instance, in some
mobile source rulemakings, the Administrator has looked at the percent
of emissions from the regulated mobile source category compared to the
total mobile source inventory for that air pollutant. See, e.g., 66 FR
5001 (2001) (heavy duty engine and diesel sulfur rule). In other
instances the Administrator has looked at the percent of emissions
compared to the total nonattainment area inventory of the air pollution
at issue. See, e.g., 67 FR 68,242 (2002) (snowmobile rule). EPA has
found that air pollutant emissions that amount to 1.2% of the total
inventory ``contribute.'' Bluewater Network, 370 F.3d at 15 (``For
Fairbanks, this contribution was equivalent to 1.2% of the total daily
CO inventory for 2001.'').
    We solicit comment on these prior precedents, including their
relevance to contribution findings EPA may be considering regarding GHG
emissions. Where appropriate, may the Administrator determine that
emissions at a certain level or percentage contribute to air pollution
in one instance, while also finding that the same level or percentage
of another air pollutant and involving different air pollution, and
different overall circumstances, does not contribute? When exercising
his judgment, is it appropriate for the Administrator to consider not
only the cumulative impact, but also the totality of the circumstances
(e.g., the air pollutant, the air pollution, the type of source
category, the number of sources in the source category, the number and
type of other source categories that may emit the air pollutant) when
determining whether the emissions ``justify regulation'' under the CAA?
See Ethyl Corp., 541 F.2d at 31, n62 (``Moreover, even under a
cumulative impact theory emissions must make more than a minimal
contribution to total exposure in order to justify regulation under
Sec.  211(c)(1)(A).'').

B. Is the Air Pollution at Issue Reasonably Anticipated to Endanger
Public Health or Welfare?

    This section discusses options for defining, with respect to GHGs,
the ``air pollution'' that may or may not be reasonably anticipated to
endanger public health or welfare, the first part of the two part
endangerment test. It also summarizes the state of the science on GHGs
and climate change, and relates that science to the endangerment
question. We solicit comment generally on the information and issues
discussed below.
1. What is the Air Pollution?
    As noted above, in applying the endangerment test in section 202(a)
or other sections of the Act to GHG emissions, the Administrator must
define the scope and nature of the relevant ``air pollution'' that may
or may not be reasonably anticipated to endanger public health or
welfare. The endangerment issue discussed in today's notice involves,
primarily, anthropogenic emissions of GHGs, the accumulation of GHGs in
the atmosphere, the resultant impacts including climate change, and the
risks and impacts to human health and welfare associated with those
impacts.
a. The Six Major GHGs of Concern
    The six major GHGs of concern are CO2, CH4,
N2O, HFCs, PFCs, and SF6. The IPCC focuses on
these six GHGs for both scientific assessments and emissions inventory
purposes because these are the six long-lived, well-mixed GHGs not
controlled by the Montreal Protocol on Substances that Deplete the
Ozone Layer. These six GHGs are directly emitted by human activities,
are reported annually in EPA's Inventory of U.S. Greenhouse Gas
Emissions and Sinks, and are the common focus of the climate change
research community. The ICTA petition addresses the first four of these
GHGs, and the President's Executive Orders 13423 and 13432 define GHGs
to include all six of these GHGs.
    Carbon dioxide is the most important GHG directly emitted by human
activities, and is the most significant driver of climate change. The
anthropogenic combined heating effect (referred to as forcing) of
CH4, N2O, HFCs, PFCs and SF6 is about
40% as large as the CO2 cumulative heating effect since pre-
industrial times, according to the Fourth Assessment Report of the
IPCC.
b. Emissions and Elevated Concentrations of the Six GHGs
    As mentioned previously, these six GHGs can remain in the
atmosphere for decades to centuries. Therefore, these GHGs, once
emitted, become well mixed throughout the global atmosphere regardless
of their emission origin, such that their average concentrations over
the U.S. are roughly the same as the global average. This also means
that current GHG concentrations are the cumulative result of both
historic and current emissions, and that future concentrations will be
the cumulative result of historic, current and future emissions.
    Greenhouse gases trap some of the Earth's heat that would otherwise
escape to space. The additional heating effect caused by the buildup of
anthropogenic GHGs in the atmosphere enhances the Earth's natural
greenhouse effect and causes global temperatures to increase, with
associated climatic changes (e.g., change in precipitation patterns,
rise in sea levels, and changes in the frequency and intensity of
extreme weather events). Current atmospheric concentrations of all of
these GHGs are significantly higher than pre-industrial (~1750) levels
as a result of human activities. Atmospheric concentrations of
CO2 and other GHGs

are projected to continue to climb over the next several decades.
    The scientific literature that assesses the potential risks and
end-point impacts of climate change (driven by the accumulation of
atmospheric concentrations of GHGs) does not assess these impacts on a
gas-by-gas basis. Observed climate change and associated effects are
driven by the buildup of all GHGs in the atmosphere, as well as other
natural and anthropogenic factors that influence the Earth's energy
balance. Likewise, the future projections of climate change that have
been done are driven by emission scenarios of all six GHGs, as well as
other pollutants, many of which are already regulated in the U.S. and
other countries.
    For these reasons, EPA is considering defining the ``air
pollution'' related to GHGs as the elevated combined current and
projected atmospheric concentration of the six GHGs. This approach is
consistent with other provisions of the CAA and previous EPA practice
under the CAA, where separate air pollutants from different sources but
with common properties may be treated as a class (e.g., Class I and
Class II substances under Title VI of the CAA). It also addresses the
cumulative effect that the elevated concentrations of the six GHGs have
on climate, and thus on different elements of health, society and the
environment. We seek comment on this potential approach, as well as
other alternative ways to define ``air pollution.'' One alternative
would be to define air pollution as the elevated concentration of an
individual GHG; however, in this case the Administrator may still have
to consider the impact of the individual GHG in combination with the
impacts caused by the elevated concentrations of the other GHGs.
c. Other Anthropogenic Factors That Have a Climatic Warming Effect
Beyond the Six Major GHGs
    There are other GHGs and aerosols that have climatic warming
effects: water vapor, chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs), halons, stratospheric and
tropospheric ozone (O3), and black carbon. Each of these is
discussed here. We seek comment on whether and how they should be
considered in the definition of ``air pollution'' for purposes of an
endangerment finding.
    Water vapor is the most abundant naturally occurring GHG and
therefore makes up a significant share of the natural, background
greenhouse effect. However, water vapor emissions from human activities
have only a negligible effect on atmospheric concentrations of water
vapor. Significant changes to global atmospheric concentrations of
water vapor occur indirectly through human-induced global warming,
which then increases the amount of water vapor in the atmosphere
because a warmer atmosphere can hold more moisture. Therefore, changes
in water vapor concentrations are not an initial driver of climate
change, but rather an effect of climate change which then acts as a
positive feedback that further enhances warming. For this reason, the
IPCC does not list direct emissions of water vapor as an anthropogenic
forcing agent of climate change, but does include this water vapor
feedback mechanism in response to human-induced warming in all modeling
scenarios of future climate change. Based on this recognition that
anthropogenic emissions of water vapor are not a significant driver of
anthropogenic climate change, EPA's annual Inventory of U.S. Greenhouse
Gas Emissions and Sinks does not include water vapor, and GHG inventory
reporting guidelines under the United Nations Framework Convention on
Climate Change (UNFCCC) do not require data on water vapor emissions.
    Water vapor emissions may be an issue for concern when they are
emitted by aircraft at high altitudes, where, under certain conditions,
they can lead to the formation of condensation trails, referred to as
contrails. Similar to high-altitude, thin clouds, contrails have a
warming effect. Extensive cirrus clouds can also develop from aviation
contrails, and increases in cirrus cloud cover would also have a
warming effect. The IPCC Fourth Assessment Report estimated a very
small positive radiative forcing effect for linear contrails, with a
low degree of scientific understanding. Unlike the warming effects
associated with the six long-lived, well-mixed GHGs, the warming
effects associated with contrails or contrail-induced cirrus cloud
cover are more regional and temporal in nature. Further discussion of
aviation contrails can be found in Section VI on mobile sources. EPA
invites input and comment on the scientific and policy issues related
to consideration of water vapor's association with aviation contrails
in an endangerment analysis.
    The CFCs, HCFCs, and halons are all strong anthropogenic GHGs that
are long-lived in the atmosphere and are adding to the global
anthropogenic heating effect. Therefore, these gases share common
climatic properties with the six GHGs discussed above. The production
and consumption of these substances (and hence their anthropogenic
emissions) are being controlled and phased out, not because of their
effects on climate change, but because they deplete stratospheric
O3, which protects against harmful ultraviolet B (UVB)
radiation. The control and phase-out of these substances in the U.S.
and globally is occurring under the Montreal Protocol on Substances
that Deplete the Ozone Layer, and in the U.S. under Title VI of the CAA
as well.\105\ Therefore, the climate change research and policy
community typically does not focus on these substances, precisely
because they are essentially already being 'taken care of' with non-
climate policy mechanisms. For example, the UNFCCC does not address
these substances, and instead defers their treatment to the Montreal
Protocol. As mentioned above, the President's Executive Orders 13423
and 13432 do not include these substances in the definition of GHGs.
For these reasons, EPA's preliminary conclusion is that we would not
include CFCs, HCFCs and halons in the definition of ``air pollution''
for purposes of an endangerment finding. We seek comment on this issue.
---------------------------------------------------------------------------

    \105\ Under the Montreal Protocol, production and consumption of
CFCs were phased out in developed countries in 1996 (with some
essential use exemptions) and are scheduled for phase-out by 2010 in
developing countries (with some essential use exemptions). For
halons the schedule was 1994 for phase out in developed countries
and 2010 for developing countries; HCFC production was frozen in
2004 in developed countries, and in 2016 production will be frozen
in developing countries; and HCFC consumption phase-out dates are
2030 for developed countries and 2040 in developing countries.
---------------------------------------------------------------------------

    The depletion of stratospheric O3 due to CFCs, HCFCs,
and other ozone-depleting substances has resulted in a small cooling
effect on the planet.
    Increased concentrations of tropospheric O3 are causing
a significant anthropogenic warming effect, but, unlike the long-lived
six GHGs, tropospheric O3 has a short atmospheric lifetime
(hours to weeks), and therefore its concentrations are more variable
over space and time. For these reasons, its global heating effect and
relevance to climate change tends to entail greater uncertainty
compared to the well-mixed, long-lived GHGs. More importantly,
tropospheric ozone is already listed as a NAAQS pollutant and is
regulated through SIPs and other measures under the CAA, due to its
direct health effects including increases in respiratory infection,
medicine use by asthmatics, emergency department visits and hospital
admissions, and its potential to contribute to premature death,
especially in susceptible populations such as asthmatics,

children and the elderly. Tropospheric O3 is not addressed
under the UNFCCC. For these reasons, EPA's preliminary conclusion is
that we would not include tropospheric O3 in the definition
of ``air pollution'' for purposes of an endangerment finding because,
as with CFCs, HCFCs and halons, it is already being addressed by
regulatory actions that control precursor emissions (NOX and
volatile organic compounds (VOCs)) from major U.S. sources. We invite
comment on this issue.
    Black carbon is an aerosol particle that results from incomplete
combustion of the carbon contained in fossil fuels, and it remains in
the atmosphere for about a week. Black carbon causes a warming effect
by absorbing incoming sunlight in the atmosphere (whereas GHGs cause
warming by trapping outgoing, infrared heat), and by darkening bright
surfaces such as snow and ice, which reduces reflectivity and increases
absorption of sunlight at the surface. Some recent research,\106\
published after the IPCC Fourth Assessment Report, has suggested that
black carbon may play a larger role in warming than previously thought.
Like other aerosols, black carbon can also alter the reflectivity and
lifetime of clouds, which in turn can have an additional climate
effect. How black carbon and other aerosols alter cloud properties is a
key source of uncertainty in climate change science. Given these
reasons, there is considerably more uncertainty associated with black
carbon's warming effect compared to the estimated warming effect of the
six long-lived GHGs.
---------------------------------------------------------------------------

    \106\ Ramathan, V, and G. Carmichael (2008) Global and regional
climate changes due to black carbon. Nature Geoscience, 1: 221-227.
---------------------------------------------------------------------------

    Black carbon is also co-emitted with organic carbon, which tends to
have a cooling effect on climate because it reflects and scatters
incoming sunlight. The ratio of black carbon to organic carbon varies
by fuel type and by combustion efficiency. Diesel vehicles, for
example, emit a much greater portion of black carbon, whereas forest
fires tend to emit much more organic carbon. The net effect of black
carbon and organic carbon on climate should therefore be considered.
Also, black carbon is a subcomponent of particulate matter (PM), which
is regulated as a NAAQS pollutant under the CAA due to its direct
health effects caused by inhalation. Diesel vehicles are estimated to
be the largest source of black carbon in the U.S., but these emissions
are expected to decline substantially over the coming decades due to
recently promulgated EPA regulations targeting PM2.5
emissions from on-road and off-road diesel vehicles (the Highway Diesel
Rule and the Clean Air Nonroad Diesel Rule, the Locomotive and Marine
Compression Ignition Rule). Non-regulatory partnership programs such as
the National Clean Diesel Campaign and Smartway are reducing black
carbon as well. In sum, black carbon has different climate properties
compared to long-lived GHGs, and major U.S. sources of black carbon are
already being aggressively reduced through regulatory actions due to
health concerns. Nevertheless, EPA has recently received petitions
asking the Agency to reduce black carbon emissions from some mobile
source categories (see Section VI.). Therefore, EPA seeks comment on
how to treat black carbon (and co-emitted organic carbon) regarding the
definition of ``air pollution'' in the endangerment context.
2. Science Summary
    The following provides a summary of the underlying science that was
reviewed and utilized in the Endangerment Technical Support Document
for the endangerment discussion, which in turn relied heavily on the
IPCC Fourth Assessment Report. We seek comment on the best available
science for purposes of the endangerment discussion, and in particular
on the use of the more recent findings of the U.S. Climate Change
Science Program.
a. Observed Global Effects
    The global atmospheric CO2 concentration has increased about 35%
from pre-industrial levels to 2005, and almost all of the increase is
due to anthropogenic emissions. The global atmospheric concentration of
CH4 has increased by 148% since pre-industrial levels. Current
atmospheric concentrations of CO2 and CH4 far exceed the recorded
natural range of the last 650,000 years. The N2O concentration has
increased 18%. The observed concentration increase in these non-CO2
gases can also be attributed primarily to anthropogenic emissions. The
industrial fluorinated gases, HFCs, PFCs, and SF6, have relatively low
atmospheric concentrations but are increasing rapidly; these gases are
entirely anthropogenic in origin.
    Current ambient concentrations of CO2 and other GHGs remain well
below published thresholds for any direct adverse health effects, such
as respiratory or toxic effects.
    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 radiative forcing (a measure of the
heating effect caused by changing the Earth's energy balance) is
estimated to be +1.6 Watts per square meter (W/m\2\). The combined
radiative forcing due to the cumulative (i.e., 1750 to 2005) increase
in atmospheric concentrations of CO2, CH4, and N2O is +2.30 W/m\2\. The
rate of increase in positive radiative forcing due to these three GHGs
during the industrial era is very likely to have been unprecedented in
more than 10,000 years. The positive radiative forcing due to the
increase in CO2 concentrations is the largest (+1.66 W/m\2\). The
increase in CH4 concentrations is the second largest source of positive
radiative forcing (+0.48 W/m2). The increase in N2O has a positive
radiative forcing of +0.16 W/m\2\.
    Warming of the climate system is unequivocal, as is now evident
from observations of increases in global average air and ocean
temperatures, widespread melting of snow and ice, and rising global
average sea level. Global mean surface temperatures have risen by
0.74[deg]C (1.3[deg]F) over the last 100 years. The average rate of
warming over the last 50 years is almost double that over the last 100
years. Global mean surface temperature was higher during the last few
decades of the 20th century than during any comparable period during
the preceding four centuries.
    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. Global observed temperatures over the
last century can be reproduced only when model simulations include both
natural and anthropogenic forcings, i.e., simulations that remove
anthropogenic forcings are unable to reproduce observed temperature
changes. Thus, the warming cannot be explained by natural variability
alone.
    Observational evidence from all continents and most oceans shows
that many natural systems are being affected by regional climate
changes, particularly temperature increases. Observations show that
changes are occurring in the amount, intensity, frequency and type of
precipitation. There is strong evidence that global sea level gradually
rose in the 20th century and is currently rising at an increased rate.
Widespread changes in extreme temperatures have been observed in the
last 50 years. Globally, cold days, cold nights, and frost have become
less frequent, while hot days, hot nights, and heat waves have become
more frequent.

    The Endangerment Technical Support Document provides evidence that
the U.S. and the rest of the world are experiencing effects from
climate change now.
b. Observed U.S. Effects
    U.S. temperatures also warmed during the 20th and into the 21st
century. U.S. temperatures are now approximately 1.0 [deg]F warmer than
at the start of the 20th century, with an increased rate of warming
over the past 30 years. The past nine years have all been among the 25
warmest years on record for the contiguous U.S., a streak which is
unprecedented in the historical record. Like the average global
temperature increase, the observed temperature increase for North
America has been attributed to the global buildup of anthropogenic GHG
concentrations in the atmosphere.
    Widespread changes in extreme temperatures have been observed in
the last 50 years across all world regions including the U.S. Cold
days, cold nights, and frost have become less frequent, while hot days,
hot nights, and heat waves have become more frequent.
    Total annual precipitation has increased over the U.S. on average
over the last century (about 6%), and there is evidence of an increase
in heavy precipitation events. Nearly all of the Atlantic Ocean shows
sea level rise during the past decade with highest rate in areas that
include the U.S. east coast.
    Observations show that climate change is currently impacting the
nation's ecosystems and services in significant ways.
c. Projected Effects
    The Endangerment Technical Support Document, the IPCC Fourth
Assessment Report, and a report under the U.S. Climate Change Science
Program, provide projections of future ambient concentrations of GHGs,
future climate change, and future anticipated effects from climate
change under various scenarios. This section summarizes some of the key
global projections, such as changes in global temperature, as well as
those particular to North America and the United States.
    Overall risk to human health, society and the environment increases
with increases in both the rate and magnitude of climate change.
Climate warming may increase the possibility of large, abrupt, and
worrisome regional or global climatic events (e.g., disintegration of
the Greenland Ice Sheet or collapse of the West Antarctic Ice Sheet).
The majority of the climate change impacts literature assesses the
potential effects on health, society and the environment due to
projected changes in average conditions (e.g., temperature increase,
precipitation change, sea level rise) and do not take into account how
the frequency and severity of extreme events due to climate change may
cause certain additional impacts. Likewise, impact studies typically do
not account for large, abrupt climatic events, and generally consider
rates of warming that would result from climate sensitivities \107\
within the most likely range, not at the tails of the distribution. To
weigh the full range of risks and impacts, it is important to consider
these possible extreme outcomes, including those that are of low
probability.
---------------------------------------------------------------------------

    \107\ ``Climate sensitivity'' is a term used to describe how
much long-term global warming occurs if global atmospheric
concentrations of CO2 are doubled compared to their pre-
industrial levels. The IPCC Fourth Assessment Report states that
climate sensitivity is very likely greater than 1.5[deg]C (2.7
[deg]F) and likely to lie in the range of 2 [deg]C to 4.5 [deg]C
(3.6 [deg]F to 8.1 [deg]F), with a most likely value of about 3
[deg]C (5.4 [deg]F), and that a climate sensitivity higher than 4.5
[deg]C cannot be ruled out.
---------------------------------------------------------------------------

i. Global Effects
    The majority of future reference-case scenarios (assuming no
explicit GHG mitigation actions beyond those already enacted) project
an increase of global GHG emissions over the century, with climbing GHG
concentrations and associated increases in radiative forcing and
average global temperatures.
    Projected ambient concentrations of CO2 and other GHGs remain well
below published thresholds for any direct adverse health effects, such
as respiration or toxic effects.
    Through about 2030, the global warming rate is affected little by
different future scenario assumptions or different model sensitivities,
because there is already some degree of commitment to future warming
given past and present GHG emissions. By mid-century, the choice of
scenario becomes more important for the magnitude of the projected
warming because only about a third of that warming is projected to be
due to climate change that is already committed. By the end of the
century, projected average global warming (compared to average
temperature around 1990) varies significantly by emissions scenario,
with IPCC's best estimates ranging from 1.8 to 4.0 [deg]C (3.2 to 7.2
[deg]F), with a fuller likely range of 1.1 to 6.4 [deg]C (2.0 to 11.5
[deg]F), which takes into account a wider range of future emission
scenarios and a wider range of uncertainties.\108\
---------------------------------------------------------------------------

    \108\ The IPCC scenarios are also described in the Technical
Support Document and include a range of future global emission
scenarios and a range of climate sensitivities (which measure how
much global warming occurs for a given increase in global
CO2 concentrations).
---------------------------------------------------------------------------

    The IPCC identifies the most vulnerable world regions as the
Arctic, because of high rates of projected warming on natural systems;
Africa, especially the sub-Saharan region, because of current low
adaptive capacity; small islands, due to high exposure of population
and infrastructure to risk of sea-level rise and increased storm surge;
and Asian mega deltas, due to large populations and high exposure to
sea level rise, storm surge, and river flooding. Climate change impacts
in certain regions of the world may exacerbate problems that raise
humanitarian and national security issues for the U.S. Climate change
has been described as a potential threat multiplier regarding national
security issues.
ii. United States Effects
    Projected global warming is anticipated to lead to effects in the
U.S. For instance, all of the U.S. is very likely to warm during this
century, and most areas of the U.S. are expected to warm by more than
the global average. The U.S, along with the rest of the world, is
projected to see an increase in the intensity of precipitation events
and the risk of flooding, greater runoff and erosion, and thus the
potential for adverse water quality effects.
    Severe heat waves are projected to intensify in magnitude,
frequency, and duration over the portions of the U.S. where these
events already occur, with likely increases in mortality and morbidity,
especially among the elderly, young, and frail. Warmer temperatures can
also lead to fewer cold-related deaths. It is currently not possible to
quantify the balance between decreased cold-related deaths and
increased heat-related deaths attributable to climate change over time.
    The IPCC projects with virtual certainty (i.e., greater than 99%
likelihood) declining air quality in cities due to warmer days and
nights, and fewer cold days and nights, and/or more frequent hot days
and nights over most land areas, including the U.S. Climate change is
expected to lead to increases in regional ozone pollution, with
associated risks for respiratory infection, aggravation of asthma, and
potential premature death, especially for people in susceptible groups.
Climate change effects on ambient PM are currently less certain.
    Additional human health concerns include a change in the range of
vector-

borne diseases, and a likely trend towards more intense hurricanes
(even though any single hurricane event cannot be attributed to climate
change) and other extreme weather events. For many of these issues,
sensitive populations, such as the elderly, young, asthmatics, the
frail and the poor, are most vulnerable.
    Moderate climate change in the early decades of the century is
projected to increase aggregate yields of rainfed agriculture in the
United States by 5-20%. However, as temperatures continue to rise,
grain and oilseed crops will increasingly experience failure,
especially if climate variability increases and precipitation lessens
or becomes more variable. How climatic variability and extreme weather
events will continue to change under a changing climate is a key
uncertainty, and these events also have the potential to offset the
benefits of CO2 fertilization and a longer growing season.
    Climate change is projected to constrain over-allocated water
resources in the U.S., increasing competition among agricultural,
municipal, industrial, and ecological uses. Rising temperatures will
diminish snowpack and increase evaporation, affecting seasonal
availability of water.
    Disturbances like wildfire and insect outbreaks are increasing and
are likely to intensify in a warmer future with drier soils and longer
growing seasons. Overall forest growth in the U.S. will likely increase
by 10-20% as a result of extended growing seasons and elevated
CO2 over the next century, but with important spatial and
temporal variation. Although recent climate trends have increased
vegetation growth in parts of the United States, continuing increases
in disturbances are likely to limit carbon storage, facilitate invasive
species, and disrupt ecosystem services.
    The U.S. will be affected by global sea level rise, which is
expected to increase between 0.18 and 0.59 meters by the end of the
century relative to around 1990. These numbers represent the lowest and
highest projections of the 5 to 95% ranges for all scenarios considered
collectively and include neither uncertainty in carbon cycle feedbacks
nor rapid dynamical changes in ice sheet flow. U.S. coastal communities
and habitats will be increasingly stressed by climate change
interacting with development and pollution. Sea level is already rising
along much of the coast, and the rate of change is expected to increase
in the future, exacerbating the impacts of progressive inundation,
storm-surge flooding, and shoreline erosion.
    Climate change is likely to affect U.S. energy use (e.g., heating
and cooling requirements), and energy production (e.g., effects on
hydropower), physical infrastructures (including coastal roads,
railways, transit systems and runways) and institutional
infrastructures. Climate change will likely interact with and possibly
exacerbate ongoing environmental change and environmental pressures in
some settlements, particularly in Alaska where indigenous communities
are facing major environmental and cultural impacts.
3. Endangerment Discussion Regarding Air Pollution
    The Administrator must exercise his judgment in evaluating whether
the first part of the endangerment test is met, i.e., whether air
pollution (e.g., the elevated concentrations of GHGs) is reasonably
anticipated to endanger public health or welfare. As discussed above,
in exercising his judgment it is appropriate for the Administrator to
make comparative assessments of risk and projections of future
possibilities, consider uncertainties, and extrapolate from limited
data. The precautionary nature of the statutory language also means
that the Administrator should act to prevent harm rather than wait for
proof of actual harm.
    The scientific record shows there is compelling and robust evidence
that observed climate change can be attributed to the heating effect
caused by global anthropogenic GHG emissions. The evidence goes beyond
increases in global average temperature to include observed changes in
precipitation patterns, sea level rise, extreme hot and cold days, sea
ice, glaciers, ecosystem functioning and wildlife patterns. Global
warming trends over the last 50 years stand out as significant compared
to estimated global average temperatures for at least the last few
centuries. Some degree of future warming is now unavoidable given the
current buildup of atmospheric concentrations of GHGs, as the result of
past and present GHG emissions. Based on the scientific evidence, it is
reasonable to conclude that future climate change will result from
current and future emissions of GHGs. Future warming over the course of
the 21st century, even under scenarios of low emissions growth, is very
likely to be greater than observed warming over the past century.
    The range of potential impacts that can result from climate change
spans many elements of the global environment, and all regions of the
U.S. will be affected in some way. The U.S. has a long and populous
coastline. Sea level rise will continue and exacerbate storm-surge
flooding and shoreline erosion. In areas where heat waves already
occur, they are expected to become more intense, more frequent, and
longer lasting. Wildfires and the wildfire season are already
increasing and climate change is expected to continue to worsen
conditions that facilitate wildfires. Where water resources are already
scarce and over-allocated in the western U.S., climate change is
expected to put additional strain on these water management issues for
municipal, agricultural, energy and industrial uses. Climate change
also introduces an additional stress on ecosystems which are already
affected by development, habitat fragmentation, and broken ecological
dynamics. There is a wide range in the magnitude of these estimated
impacts, with there being more confidence in the occurrence of some
effects and less confidence in the occurrence of others.
    In addition to the effects from changes in climate, there are some
additional welfare effects that occur directly from the anthropogenic
GHG emissions themselves. For example, ocean acidification occurs
through elevated concentrations of CO2, and crop and other
vegetation growth can be enhanced through elevated CO2
concentrations as well.
    Current and projected levels of ambient concentrations of the six
GHGs are not expected to cause any direct adverse health effects, such
as respiratory or toxic effects, which would occur as a result of the
elevated GHG concentrations themselves rather than through the effects
of climate change. However, there are indirect human health risks
(e.g., heat-related mortality, exacerbated air quality, extreme events)
and benefits (e.g., less cold-related mortality) that occur due to
climate change. We seek comment on how these human health impacts
should be characterized under the CAA for purposes of an endangerment
analysis.
    Some elements of human health, society and the environment may
benefit from climate change (e.g., short-term increases in agricultural
yields, less cold-related mortality). We seek comment on how the
potential for some benefits should be viewed against the full weight of
evidence showing numerous risks and the potential for adverse impacts.
    Quantifying the exact nature and timing of impacts due to climate
change over the next few decades and beyond, and across all vulnerable
elements of U.S. health, society and the environment, is currently not
possible. However, the full weight of evidence as

summarized above and as documented in the Endangerment Technical
Support Document points towards the robust conclusion that expected
rates of climate change (driven by past, present and plausible future
GHG emissions) pose a number of serious risks to the U.S., even if the
exact nature of the risks is difficult to quantify with confidence. The
uncertainties in this context can also mean that future rates of
climate change are being underestimated, and that the potential for
associated and difficult-to-predict-and-quantify extreme events is not
adequately incorporated into impact assessments. The scientific
literature states that risk increases with increases in both the rate
and magnitude of climate change. We solicit comment on how these
uncertainties should be considered.
    We seek comment on whether, in light of the precautionary nature of
the statutory language, the Administrator needs to find that current
levels of GHG concentrations endanger public health or welfare now. As
noted above, the fact that GHGs remain in the atmosphere for decades to
centuries means that future concentrations are dependent not only on
tomorrow's emissions, but also on today's emissions. Should the
Administrator consider both current and projected future elevated
concentrations of GHGs, as well as the totality of the observed and
projected effects that result from current and projected
concentrations? Or should the Administrator focus on future projected
elevated concentrations of GHGs and their projected effects in the
United States because they are larger and of greater concern than
current GHG concentrations and observed effects?
    In sum, EPA invites comment on all issues relevant to making an
endangerment finding, including the scientific basis supporting a
finding that there is or is not endangerment under the CAA, as well as
the potential scope of the finding (i.e., public health, welfare, or
both).

C. Illustration for the ``Cause or Contribute'' Part of the
Endangerment Discussion: Do emissions of air pollutants from motor
vehicles or fuels cause or contribute to the air pollution that may
reasonably be anticipated to endanger public health or welfare in the
United States?

 1. What Is/Are the Air pollutant(s)?
a. Background and Context
    If the Administrator, in his judgment, finds that GHG ``air
pollution'' may reasonably be anticipated to endanger public health or
welfare, he must then define ``air pollutant(s)'' for purposes of
making the ``cause or contribute'' determination. The question is
whether the ``air pollutants'' to be evaluated for ``cause or
contribute'' should be the individual GHGs, or whether the ``air
pollutant'' is one or more classes of GHGs as a group.
    We recognize that the alternative definitions could have important
implications for how GHGs are treated under other provisions of the
Act. The Administrator seeks comment on these options, and is
particularly interested in views regarding the implications for the
potential future regulation of GHGs under other parts of the Act.
b. Defining ``Air Pollutant'' as Each Individual Greenhouse Gas
    Under this approach, the Administrator could define ``air
pollutant'' as each individual GHG rather than as GHGs as a collective
whole for the purposes of assessing ``cause or contribute.'' The
Administrator would evaluate each individual GHG to determine if it
causes, or contributes to, the elevated combined level of GHG
concentrations.
    This approach enables an evaluation of the unique characteristics
and properties of each GHG (e.g., radiative forcing, lifetimes, etc.),
as well as current and projected emissions. This facilitates a
customized approach accounting for these factors. This approach also is
consistent with the approach taken in several federal GHG programs
which target reductions of individual greenhouse gases. For example,
EPA manages a variety of partnership programs aimed at reducing
emissions of specific sources of methane and the fluorinated gases
(HFCs, PFCs and SF6).
c. Defining ``Air Pollutants'' Collectively as a Class of Greenhouse
Gases
    Under this approach, the Administrator could define the ``air
pollutant'' as (a) the collective group of the six GHGs discussed above
(CO2, CH4, N2O, HFCs, PFCs, and
SF6), (b) the collective group of the specific GHGs that are
emitted from the relevant source category at issue in the endangerment
finding (e.g., for section 202 sources it would be CO2,
CH4, N2O, and HFCs), or (c) other reasonable
groupings.
    There are several federal and state climate programs, such as EPA's
Climate Leaders program, DOE's 1605b program, and Multi-state Climate
Registry, that encourage firms to report (and reduce) emissions of all
six GHGs, recognizing that the non-CO2 GHG emissions are a
significant part of the atmospheric buildup of GHG concentrations and
thus radiative forcing. In addition, the President's recent 2007
Executive Orders (13423 and 13432) and his 2002-2012 intensity goal
both encompass the collective emissions of all six GHGs. Consideration
of a class of gases collectively takes into account the multiple
effects of mitigation options and technologies on each gas, thus
enabling a more coordinated approach in addressing emissions from a
source. For example, collection and combustion of fugitive methane will
lead to net increases in CO2 and possibly nitrous oxide
emissions, but this is nevertheless desirable from an overall
mitigation perspective given the lower total radiative forcing.
2. Discussion of ``Cause or Contribute''
    Once the ``air pollutant(s)'' is defined, the Administrator must
look at the emissions of the air pollutant from the relevant source
category in determining whether those emissions cause or contribute to
the air pollution he has determined may reasonably be anticipated to
endanger public health or welfare. There arguably are many possible
ways of assessing ``cause and contribute'' and different approaches
have been used in previous endangerment determinations under the CAA.
For example, EPA could consider how emissions from the relevant source
category would compare as a share of the following:
     Total global aggregated emissions of the 6 GHGs discussed
in the definition of ``air pollution'';
     Total aggregated U.S. emissions of the 6 GHGs;
     Total global emissions of the individual GHG in question;
     Total U.S. emissions of the individual GHG in question;
and
     Total global atmospheric concentrations of the GHG in
question.
    In the past, the smallest level or amount of emissions that the
Administrator determined ``contributed'' to the air pollution at issue
was just less than 1% (67 FR 68242 (2002)). We solicit comment on other
factors that may be relevant to a contribution determination for GHG
emissions. For example, given the global nature of the air pollution
being addressed in this rulemaking, one might expect that the
percentage contribution of specific GHGs and sectors would be much
smaller than for previous rulemakings when the nature of the air
pollution at issue was regional or local. On an absolute basis, a small
U.S. GHG source on a global scale may have emissions at the same level
as one of the largest sources in a single small to medium size country,
and given the

large size of the global denominator, even sectors with significant
emissions could be very small in percentage terms.
    In addition, EPA notes that the EPA promotes the reduction of
particular GHG emissions through a variety of voluntary programs (e.g.,
EPA's domestic CH4 partnership programs and the international Methane
to Markets Partnership (launched in 2004)). EPA requests comment on how
these and other efforts to encourage the voluntary reductions in even
small amounts of GHG emissions are relevant to decisions about what
level of ``contribution'' merits mandatory regulations.
    Below we use the section 202 source category to illustrate these
and other various ways to consider and compare source category GHG
emissions for the ``cause or contribute'' analysis. In keeping with the
discussion above regarding possible definitions of ``air pollutant,''
we provide the information on an individual GHG and collective GHG
basis. In addition, we raise various policy considerations that could
be relevant to a ``cause or contribute'' determination. EPA invites
comment on the various approaches, data, and policy considerations
discussed below.
a. Overview of Section 202 Source Categories
    The relevant mobile sources under section 202(a)(1) of the Clean
Air Act are ``any class or classes of new motor vehicles or new motor
vehicle engines, * * * '' CAA section 202(a)(1). To support this
illustrative assessment, EPA analyzed historical GHG emissions data for
motor vehicles and motor vehicle engines in the United States from 1990
to 2006.\109\
---------------------------------------------------------------------------

    \109\ The source of the emissions data is the Inventory of U.S.
Greenhouse Gas Emissions and Sinks: 1990-2006 (USEPA #430-R-08-005)
(hereinafter ``U.S. Inventory''). See the Emissions Technical
Support Document for a discussion on the correspondence between
Section 202 source categories and IPCC source categories. The most
recent year for which official EPA estimates are available is 2006.
---------------------------------------------------------------------------

    The motor vehicles and motor vehicle engines (hereinafter ``section
202 source categories'') addressed include passenger cars, light-duty
trucks, motorcycles, buses, medium/heavy-duty trucks, and cooling.\110\
Of the six primary GHGs, four are associated with section 202 source
categories: CO2, CH4, N2O, and HFCs.
---------------------------------------------------------------------------

    \110\ Greenhouse gas emissions result from the use of HFCs in
cooling systems designed for passenger comfort, as well as auxiliary
systems for refrigeration.
---------------------------------------------------------------------------

    A summary of the section 202 emissions information is presented
here, and a more detailed description along with data tables is
contained in the Emissions Technical Support Document. All annual
emissions data are considered on a CO2 equivalent basis.
b. Carbon Dioxide Emissions From Section 202 Sources
    CO2 is emitted from motor vehicles and motor vehicle
engines during the fossil fuel combustion process. During combustion,
the carbon stored in the fuels is oxidized and emitted as
CO2 and smaller amounts of other carbon compounds.\111\
---------------------------------------------------------------------------

    \111\ Detailed CO2 emissions data from section 202
source categories are presented in the Emissions Technical Support
Document. Other carbon compounds emitted such as CO, and non-methane
volatile organic compounds oxidize in the atmosphere to form
CO2 in a period of hours to days.
---------------------------------------------------------------------------

    CO2 is the dominant GHG emitted from motor vehicles and
motor vehicle engines, and the dominant GHG emitted in the U.S. and
globally.\112\ CO2 emissions from section 202 sources grew
by 32% between 1990 and 2006, largely due to increased CO2
emissions from light-duty trucks (61% since 1990) and medium/heavy-duty
trucks (76%). Emissions of CO2 from section 202 sources, and
U.S. and global emissions are presented below in Table V-1.
---------------------------------------------------------------------------

    \112\ EPA typically uses current motor vehicle fleet emissions
information when making a contribution analysis under section 202.
We solicit comment on how or whether the reductions in
CO2 emissions expected by implementation of EISA, or any
other projected change in emissions from factors such as growth in
the fleet or vehicle miles traveled, would impact a contribution
analysis for CO2.

          Table V-1--Section 202 CO2, U.S. and Global Emissions
------------------------------------------------------------------------
                                                            Sec 202 CO2
             U.S. Emissions                    2006            share
                                                             (percent)
------------------------------------------------------------------------
Section 202 CO2.........................         1,564.6
All U.S. CO2............................          5983.1            26.2
U.S. emissions of Sec 202 GHG...........         1,665.4            93.9
All U.S. GHG emissions..................         7,054.2           22.2%
------------------------------------------------------------------------

                                                            Sec 202 CO2
                                                             share (in
            Global Emissions                   2000            2000)
                                                             (percent)
------------------------------------------------------------------------
All global CO2 emissions................        30,689.5             4.8
Global transport GHG emissions..........         5,315.2            27.5
All global GHG emissions................        36,727.9             4.0
------------------------------------------------------------------------

                                                           Share of U.S.
        Other Sources of U.S. CO2              2006        CO2 emissions
                                                             (percent)
------------------------------------------------------------------------
Electricity Sector CO2..................          2360.3            39.4
Industrial Sector CO2...................           984.1            16.4
------------------------------------------------------------------------

    Arguably, based on these data, if the Administrator did not find
that, for purposes of section 202, that CO2 emissions from
section 202 source categories contribute to the elevated combined level
of GHG concentrations, it is unlikely that he would find that the other
GHGs emitted by section 202 source categories contribute.
c. Methane Emissions From Section 202 Source Categories
    Methane (CH4) emissions from motor vehicles are a function of the
CH4 content of the motor fuel, the amount of

hydrocarbons passing uncombusted through the engine, and any post-
combustion control of hydrocarbon emissions (such as catalytic
converters). Methane emissions from these source categories decreased
by 58% between 1990 and 2006, largely due to decreased CH4 emissions
from passenger cars and light-duty trucks.\113\ Emissions of
CH4 from section 202 sources, and U.S. and global emissions
are presented below in Table V-2.
---------------------------------------------------------------------------

    \113\ Detailed methane emissions data for section 202 source
categories are presented in the Emissions Technical Support
Document.

          Table V-2--Section 202 CH4, U.S. and Global Emissions
------------------------------------------------------------------------
                                                            Sec 202 CH4
             U.S. Emissions                    2006            share
                                                             (percent)
------------------------------------------------------------------------
Section 202 CH4.........................            1.80
All U.S. CH4............................           555.3            0.32
U.S. emissions of Sec 202 GHG...........        1,665.40            0.11
All U.S. GHG emissions..................        7,054.20            0.03
------------------------------------------------------------------------

                                                            Sec 202 CH4
                                                             share (in
            Global Emissions                   2000            2000)
                                                             (percent)
------------------------------------------------------------------------
All global CH4 emissions................        5,854.90            0.05
Global transport GHG emissions..........        5,315.20            0.05
All global GHG emissions................       36,727.90            0.01
------------------------------------------------------------------------

                                                           Share of U.S.
        Other Sources of U.S. CH4              2006        CH4 emissions
                                                             (percent)
------------------------------------------------------------------------
Landfill CH4 emissions..................           125.7            22.6
Natural Gas CH4 emissions...............           102.4            18.4
------------------------------------------------------------------------

    EPA also notes that the EPA promotes the reduction of
CH4 and other non-CO2 GHG emissions, as
manifested in its domestic CH4 partnership programs and the
international Methane to Markets Partnership (launched in 2004), which
are not focused on the transportation sector. EPA requests comment on
how these and other efforts to encourage the voluntary reductions in
even small amounts of GHG emissions are relevant to decisions about
what level of ``contribution'' merits mandatory regulations.
d. Nitrous Oxide Emissions From Section 202 Source Categories
    Nitrous oxide (N2O) is a product of the reaction that
occurs between nitrogen and oxygen during fuel combustion.
N2O (and nitrogen oxide (NOX)) emissions from
motor vehicles and motor vehicle engines are closely related to fuel
characteristics, air-fuel mixes, combustion temperatures, and the use
of pollution control equipment.
    Nitrous oxide emissions from section 202 sources decreased by 27%
between 1990 and 2006, largely due to decreased emissions from
passenger cars and light-duty trucks.\114\ Earlier generation control
technologies initially resulted in higher N2O emissions,
causing a 24% increase in N2O emissions from motor vehicles
between 1990 and 1995. Improvements in later-generation emission
control technologies have reduced N2O output, resulting in a
41% decrease in N2O emissions from 1995 to 2006. Emissions
of N2O from section 202 sources, and U.S. and global
emissions are presented below in Table V-3.
---------------------------------------------------------------------------

    \114\ Detailed nitrous oxide emissions data for section 202
source categories are presented in the Emissions Technical Support
Document.

          Table V-3--Section 202 N2O, U.S. and Global Emissions
------------------------------------------------------------------------
                                                            Sec 202 N2O
             U.S. Emissions                    2006            share
                                                             (percent)
------------------------------------------------------------------------
Section 202 N2O.........................            29.5
All U.S. N2O............................           367.9             8.0
U.S. emissions of Sec 202 GHG...........          1665.4             1.8
All U.S. GHG emissions..................          7054.2             0.4
------------------------------------------------------------------------

                                                            Sec 202 N2O
                                                             share (in
            Global Emissions                   2000            2000)
                                                             (percent)
------------------------------------------------------------------------
All global N2O emissions................         3,113.8             1.6
Global transport GHG emissions..........         5,315.2             0.9
All global GHG emissions................        36,727.9             0.1
------------------------------------------------------------------------

                                                           Share of U.S.
        Other Sources of U.S. N2O              2006        N2O emissions
                                                             (percent)
------------------------------------------------------------------------
Agricultural Soil N2O emissions.........           265.0            72.0
Nitric Acid N2O emissions...............            15.6             4.3
------------------------------------------------------------------------

    Past experience has shown that substantial emissions reductions can
be made by small N2O sources. For example, the
N2O emissions from adipic acid production is smaller than
that of Section 202 sources, and this sector reduced its emission by
over 60 percent from 1990 to 2006 as a result of voluntary adoption of
N2O abatement technology by the three major U.S. adipic acid
plants.\115\
---------------------------------------------------------------------------

    \115\ Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2006 (USEPA #430-R-08-005), p.2-22.
---------------------------------------------------------------------------

e. Hydrofluorocarbons Emissions From Section 202 Source Categories
    Hydrofluorocarbons (a term which encompasses a group of eleven
related compounds) are progressively replacing CFCs and HCFCs in
section 202 cooling and refrigeration systems as they are being phased
out under the Montreal Protocol and Title VI of the CAA.\116\
---------------------------------------------------------------------------

    \116\ 2006 IPCC Guidelines, Volume 3, Chapter 7. Page 43.
---------------------------------------------------------------------------

    Hydrofluorocarbons were not used in motor vehicles or refrigerated
rail and marine transport in the U.S. in 1990, but by 2006 emissions
had increased to 70 Tg CO2e.\117\ Emissions of HFC from
section 202 sources, and U.S. and global emissions are presented below
in Table V-4.
---------------------------------------------------------------------------

    \117\ Detailed HFC emissions data for section 202 source
categories are presented in Tables in the Emissions Technical
Support Document.

          Table V-4--Section 202 HFC, U.S. and Global Emissions
------------------------------------------------------------------------
                                                            Sec 202 HFC
             U.S. Emissions                    2006            share
                                                             (percent)
------------------------------------------------------------------------
Section 202 HFC.........................            69.5
All U.S. HFC............................           124.5            55.8
U.S. emissions of Sec 202 GHG...........          1665.4             4.2
All U.S. GHG emissions..................          7054.2             1.0
------------------------------------------------------------------------

                                                            Sec 202 HFC
                                                             share (in
            Global Emissions                   2000            2000)
                                                             (percent)
------------------------------------------------------------------------
All global HFC emissions................           259.2            20.3
Global transport GHG emissions..........         5,315.2             1.0
All global GHG emissions................        36,727.9             0.1
------------------------------------------------------------------------

                                                           Share of U.S.
        Other Sources of U.S. HFC              2006        HFC emissions
                                                             (percent)
------------------------------------------------------------------------
HCFC-22 Production......................            13.8            11.1
Other ODS Substitutes...................            41.2            33.1
------------------------------------------------------------------------

    EPA notes that section 202 HFC emissions are the largest source of
HFC emissions in the United States, that these emissions increased by
274% from 1995 to 2006, and that section 202 sources are also the
largest source of emissions of high GWP gases (i.e., HFCs, PFCs or
SF6) in the U.S. Thus, a decision not to set standards for
HFCs under section 202 could be viewed as precedential with respect to
the likelihood of future regulatory actions for any of these three
gases.
f. Perfluorocarbons and Sulfur Hexafluoride
    Perfluorocarbons (PFCs) and sulfur hexafluoride (SF6)
are not emitted from motor vehicles or motor vehicle engines in the
United States.
g. Total GHG Emissions From Section 202 Source Categories
    We note if ``air pollutant'' were defined as the collective group
of four to six GHGs, the emissions of a single component (e.g.,
CO2) could theoretically support a positive contribution
finding. We also solicit comment on whether the fact that total GHG
emissions from section 202 source categories are approximately 4.3% of
total global GHG emissions would mean that adopting this definition of
``air pollutant'' would make it unnecessary to assess the individual
GHG emissions levels less than that amount. Table V-5 below presents
the contribution of individual GHGs to total GHG emissions from section
202 sources, and from all sources in the U.S.

              Table V-5--Contribution of Individual gases in 2006 to Section 202 and U.S. Total GHG
                                                  (In percent)
----------------------------------------------------------------------------------------------------------------
                                                   CO2        CH4        N2O        HFC        PFC        SF6
----------------------------------------------------------------------------------------------------------------
Section 202...................................       93.9        0.1        1.8        4.2

U.S. Total....................................       84.8        7.9        5.2        1.8        0.1        0.2
----------------------------------------------------------------------------------------------------------------

    Emissions of GHG from section 202 sources, and U.S. and global
emissions are presented below in Table V-6.

          Table V-6--Section 202 GHG, U.S. and Global Emissions
------------------------------------------------------------------------
                                                            Sec 202 GHG
             U.S. Emissions                    2006            share
                                                             (percent)
------------------------------------------------------------------------
Section 202 GHG.........................          1665.4
All U.S. GHG emissions..................          7054.2            23.6
------------------------------------------------------------------------

                                                            Sec 202 GHG
                                                             share (in
            Global Emissions                   2000            2000)
                                                             (percent)
------------------------------------------------------------------------
Global transport GHG emissions..........         5,315.2            29.5