A rn* United Slates
Environmwilfll Protection
^1 Agency
Endangerment and Cause or
Contribute Findings for
Greenhouse Gases Under Section
202(a) of the Clean Air Act:
EPA's Response to Public
Comments
Volume 10: The Cause or
Contribute Finding
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The Cause or Contribute Finding
U. S. Environmental Protection Agency
Office of Atmospheric Programs
Climate Change Division
Washington, D.C.
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FOREWORD
This document provides responses to public comments on the U.S. Environmental Protection Agency's
(EPA's) Proposed Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section
202(a) of the Clean Air Act, published at 74 FR 18886 (April 24, 2009). EPA received comments on
these Proposed Findings via mail, e-mail, and facsimile, and at two public hearings held in Arlington,
Virginia, and Seattle, Washington, in May 2009. Copies of all comment letters submitted and transcripts
of the public hearings are available at the EPA Docket Center Public Reading Room, or electronically
through htto://www.regulations.gov by searching Docket ID EPA-HQ-OAR-2009-0171.
This document accompanies the Administrator's final Endangerment and Cause or Contribute Findings
for Greenhouse Gases Under Section 202(a) of the Clean Air Act (Findings) and the Technical Support
Document (TSD), which contains the underlying science and greenhouse gas emissions data.
EPA prepared this document in multiple volumes, with each volume focusing on a different broad
category of comments on the Proposed Findings. This volume of the document provides responses to
public comments regarding the cause or contribute finding.
In light of the very large number of comments received and the significant overlap between many
comments, this document does not respond to each comment individually. Rather, EPA summarized and
provided a single response to each significant argument, assertion, and question contained within the
totality of comments. Within each comment summary, EPA provides in parentheses one or more lists of
Docket ID numbers for commenters who raised particular issues; however, these lists are not meant to be
exhaustive and EPA does not individually identify each and every commenter who made a certain point in
all instances, particularly in cases where multiple commenters expressed essentially identical arguments.
Several commenters provided additional scientific literature to support their arguments. EPA's general
approach for taking such literature into consideration is described in Volume 1, Section 1.1, of this
Response to Comments document. As with the comments, there was overlap in the literature received.
EPA identified the relevant literature related to the significant comments, and responded to the significant
issues raised in the literature. EPA does not individually identify each and every piece of literature
(submitted or incorporated by reference) that made a certain point in all instances.
Throughout this document, we provide a list of references at the end of each volume for additional
literature cited by EPA in our responses; however, we do not repeat the full citations of literature cited in
the TSD.
EPA's responses to comments are generally provided immediately following each comment summary. In
some cases, EPA has discussed responses to specific comments or groups of similar comments in the
Findings. In such cases, EPA references the Findings rather than repeating those responses in this
document.
Comments were assigned to specific volumes of this Response to Comments document based on an
assessment of the principal subject of the comment; however, some comments inevitably overlap multiple
subject areas. For this reason, EPA encourages the public to read the other volumes of this document
relevant to their interests.
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TABLE OF CONTENTS
10.0 The Cause or Contribute Finding
10.1 Definition of "Air Pollutant"
10.2 Assessment of Contribution
10.3 Level of Contribution
10.3.1 Single Air Pollutant Composed of the Well-Mixed Greenhouse Gases
10.3.2 Each GHG as an Individual Air Pollutant
Acronyms and Abbreviations
CAA
Clean Air Act
CAFE
Corporate Average Fuel Economy
CAIT
Climate Analysis Indicators Tool
CCSP
U.S. Climate Change Science Program
ch4
methane
C02
carbon dioxide
C02e
carbon dioxide equivalent
GHG
greenhouse gases
GWP
global warming potential
HFC
hydrofluorocarbon
IPCC
Intergovernmental Panel on Climate Change
LULUCF
land use, land-use change, and forestry
N20
nitrous oxide
ODS
ozone-depleting substance
PFC
perfluorocarbon
PM
particulate matter
PSD
Prevention of Significant Deterioration
sf6
sulfur hexafluoride
Tg
teragram
TSD
Technical Support Document
UNFCCC
United Nations Framework Convention on Climate Change
USGCRP
U.S. Global Change Research Program
VOC
volatile organic compound
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10.0 The Cause or Contribute Finding
10.1 Definition of "Air Pollutant"
Comment (10-1):
Some commenters (e.g., 3435.1, 3449.1, 4173, 10922) state that Section 202(a) of the Clean Air Act
(CAA) addressed an "air pollutant" in the singular, and therefore each greenhouse gas (GHG) should be
addressed individually (both in the Findings and in the science).
Response (10-1):
EPA addresses why the six long-lived and well-mixed GHGs as a group are included in both the
definition of "air pollution" and the definition of "air pollutant" in the Findings. See Section V of the
Findings for a discussion of the Administrator's rationale for including all six GHGs in the definition of
air pollutant. The statutory language demonstrates that EPA has ample discretion to combine air pollution
agents with shared attributes when defining an air pollutant.
Section 202 (a) (1) of the CAA states that:
The Administrator shall by regulation prescribe (and from time to time revise) standards
applicable to the emission of any air pollutant from any class or classes of new motor
vehicles or new motor vehicle engines, which in [her] judgment cause, or contribute to,
air pollution which may reasonably be anticipated to endanger public health or welfare.
(Emphasis added). Thus, the first step in analyzing whether emissions of any air pollutant from new
motor vehicles cause or contribute to air pollution which endangers is to define the term "air pollutant."
Section 302(g) states that as used in the CAA,
[t]he term "air pollutant" means any air pollution agent or combination of such agents,
including any physical, chemical, biological, radioactive (including source material,
special nuclear material, and byproduct material) substance or matter which is emitted
into or otherwise enters the ambient air. Such term includes any precursors to the
formation of any air pollutant, to the extent the Administrator has identified such
precursor or precursors for the particular purpose for which the term "air pollutant" is
used.
(Emphasis added). This language clearly contemplates that EPA may combine two or more air pollution
agents into one air pollutant. Once one thinks of GHGs as air pollution agents, rather than air pollutants
themselves, the concept of combining several for purposes of defining a single air pollutant under Section
302(g) becomes clearer.
The language of CAA Section 302(g) is quite broad, providing EPA ample discretion to determine what
combination of air pollution agents are a reasonable definition of air pollutant. As the Supreme Court
noted in Massachusetts v. EPA, 549 U.S. 497 (2007), this is a "sweeping" and "capacious" definition, and
GHGs are "unquestionably 'agents' of air pollution." 549 U.S. at 528, 532, 529 n.26. Although the Court
did not interpret the term "combination of' air pollution agents, there is no reason this phrase would be
interpreted any less broadly than the definition as a whole. Congress used the term "any," which is
typically given an expansive meaning, and did not qualify the kind of combinations that the agency could
define as a single air pollutant.
Because Congress provided EPA broad discretion to determine an appropriate combination of air
pollutant agents that should be treated as a single air pollutant, we then turn to the reasons for combining
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the GHGs in the definition of air pollutant. As noted in the Findings, the important common attributes are
(as described more fully in the Findings):
They are all directly emitted (not formed by secondary processes in the atmosphere)
They are all sufficiently long-lived in the atmosphere after they are emitted that they become
"well-mixed," which means that their concentration is essentially uniform in the atmosphere (as
opposed to having significant local/regional variation)
They have well understood atmospheric properties (e.g., radiative forcing)
Grouping them is consistent with the focus of climate science—their effects are considered as a
group in the Intergovernmental Panel on Climate Change (IPCC) reports (i.e., the IPCC reports
assess the climate change effects on health, society, and the environment as a result of human-
induced climate change driven by the group of GHGs)
Grouping them is consistent with the focus of climate policy—the United Nations Framework
Convention on Climate Change (UNFCCC) requires reporting of these six gases; the
commitments under the UNFCCC and Kyoto Protocol are based on the combined emissions of
these six gases
It is standard practice to compute the "carbon dioxide (C02) equivalency" of aggregate emissions
using global warming potentials (GWPs)
All of these common attributes are relevant to the air pollution for which GHGs are agents—in other
words, they are all related to increased atmospheric concentrations of the mix of six GHGs and climate
change science and policy. Thus, they are a reasonable basis upon which to decide that it is appropriate to
define the air pollutant as the combination of these six GHGs that share these attributes. Importantly, it is
the commonality of these attributes that provides the reasonable basis for grouping several air pollution
agents together, not whether they are all emitted from any given source category that may be before the
Agency.1
Indeed, HFCs are already widely considered to be a "single" GHG, despite being a group of a large
number of substances with a shared chemical attribute. The same is true for perfluorocarbons (PFCs). Yet
those advocating a definition of "air pollutant" that is composed of either each GHG individually, or the
group of four GHGs emitted by motor vehicles (see below), do not advocate inclusion of only those
hydrofluorocarbons (HFCs) emitted by motor vehicles. Thus, even these alternative approaches recognize
the reasonableness of grouping like substances into a single air pollutant, even if not all members of that
group are emitted by the source category before the Agency.
EPA has exercised that discretion here by defining the air pollutant for purposes of this Section 202(a)
determination as the specified group of we 11-mixed GHGs. The reasons for this are provided in the
Findings and discussed above, and demonstrate the reasonableness of this definition.
Comment (10-2):
A number of commenters (e.g., 3217.1, 3327.1, 3347.1, 3388, 3473.1, 3498.1, 3559.1, 3568.1, 3605.1,
3628.1, 3747.1, 4173) question the inclusion of sulfur hexafluoride (SF6) and PFCs in an endangerment
finding arising from Section 202(a) as they are not emitted from motor vehicles. Numerous commenters
(e.g., 3347.1, 3605.1, 3628.1, 3747.1, 4173) question EPA's definition of "air pollutant" and its
relationship to the definition of "air pollution" (i.e., question why air pollution is much broader in scope
1 We note that there are potential flexibilities presented by a definition that recognizes the shared attributes of the
six GHGs and groups them as a single air pollutant. See, e.g., Proposed Prevention of Significant Deterioration and
Title V GHG Tailoring Rule, 74 FR 55292, 55327-29 (Oct. 27, 2009).
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and the result of multiple sources beyond Section 202[a] sources), as well as the relevance of the fact that
motor vehicles do not emit two of the six GHGs in both definitions. They argue that by improperly
defining "air pollutant" to include PFCs and SF6—substances that are not present in motor vehicle
emissions—the Agency has exceeded its statutory authority under CAA Section 202(a). One commenter
(4173) states that "treating the gases as a group leads to the irrational regulatory conclusion that emissions
of PFCs, SF6 and HFCs from onroad vehicles 'cause or contribute to' a danger to the public health or
welfare." Commenters (e.g., 3347.1, 3605.1, 3747.1) contend that past endangerment findings under
Section 202(a) demonstrate EPA's consistent approach of defining "air pollutant(s)" in accordance with
the Act's clear direction, to include only those emitted from the relevant source category (citing Notice of
Proposed Rulemaking for Heavy-Duty Engine and Vehicle Standards finding that "emissions of NOx,
VOCs, SOx, and PM from heavy-duty trucks can reasonably be anticipated to endanger the public health
or welfare" (U.S. EPA, 2006: 65 Fed. Reg. 35436, June 2, 2000). Commenters (e.g., 3498.1, 4173) argue
that EPA itself is inconsistent in the Proposal, sometimes referring to "air pollutant" as the group of six
GHGs, and other times falling back on four GHGs emitted by motor vehicles. Some commenters (e.g.,
0914, 1924) question the decision to include SF6 or PFCs in the endangerment decision while excluding
water vapor.
Response (10-2):
EPA addresses why the six long-lived and well-mixed GHGs, including PFCs and SF6, are included in
both the definition of "air pollution" and the definition of "air pollutant" in the Findings. See Section V of
the Findings for a discussion of the Administrator's rationale for including all six GHGs in the definition
of air pollutant. We elaborate further below. Note that, compared to the Proposed Findings, the Final
Findings now refer to "well-mixed GHGs" when referring to the group air pollutant to consistently mean
the group of all six well-mixed GHGs.
As stated in the Findings, all six GHGs share a set of common attributes. We disagree with the
commenters who claim that the definition of air pollutant must strictly depend on the specific compounds
emitted by a source category, which in this case are on-road vehicles covered by Section 202(a) of the
CAA. It is not necessarily the source category being evaluated that determines, by itself, the
reasonableness of defining a group air pollutant. As discussed above, the statutory language demonstrates
that EPA has ample discretion to combine air pollution agents with shared attributes when defining an air
pollutant. Using the broad discretion provided by the definition of "air pollutant" to define the air
pollutant in this way, without limiting it to just the individual gases emitted by the specific source
category, makes sense because it gives effect to the shared attributes (discussed above and in the
Findings) and how they are relevant to the air pollution to which they contribute. An approach that
combines some air pollution agents with shared attributes together, but not others simply because the
particular source category before EPA may not emit those other agents, essentially ignores or devalues the
reasons that make combining the air pollution agents together reasonable. The fact that these six air
pollution agents share these common, relevant attributes is true regardless of the source category being
evaluated for contribution. Grouping these six substances as one air pollutant is reasonable regardless of
whether a contribution analysis is undertaken for CAA Section 202(a) sources that emit one subset of the
six substances (e.g., C02, methane [CH4], nitrous oxide [N20], and HFCs, but not PFCs and SF6), or for
another category of sources that may emit another subset. For example, electronics manufacturers that
may emit N20, PFCs, HFCs, SF6, and other fluorinated compounds, but not C02 or CH4 unless there is
on-site fuel combustion; power plants emit C02, CH4, and N20, but not HFCs.
Defining the group air pollutant in this manner is also consistent with the requirement in Section 202(a)
that we determine whether "emissions of the air pollutant" contributes to the air pollution that endangers.
The emissions of the single air pollutant are reasonably and readily considered to be the combination of
emissions of the individual gases that make up the combination air pollutant. As with other air pollutants
that are composed of one or more air pollution agents (e.g., particulate matter [PM]), EPA sums the
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emissions of whichever of the GHG air pollution agents motor vehicles emit to determine the total
emissions of the GHG air pollutant.
That emissions of well-mixed GHGs from the transportation sources covered under Section 202(a) of the
Act contribute to the total GHG air pollution does not imply that SF6 is emitted from those sources, any
more than a statement that volatile organic compounds (VOCs) emitted from a given source contribute to
ozone air pollution would imply that every single possible VOC is emitted from that source. Likewise,
saying that motor vehicles emit HFCs does not imply they emit every chemical compound that is included
in the group of compounds recognized as HFCs. Motor vehicles commonly emit only certain HFCs, those
that are used as refrigerants in motor vehicles. The statement on contribution implies no more than what
EPA did—EPA determined the contribution of the emissions of the well-mixed GHGs from new motor
vehicles by determining the contribution of the actual emissions of the members of well-mixed GHG air
pollutant class emitted by those vehicles. Any possible confusion on what this means is removed by the
discussion in the Findings and this Response to Comments document.
Commenters mischaracterize the decision on contribution in the Heavy Duty Engine and Vehicle
Standards rulemaking. EPA considered the emissions of those air pollutants, commonly recognized
groups of pollutants, but did not discuss the question of whether it was required to limit its consideration
to only those compounds emitted by this category of vehicles and engines. Implicitly EPA rejected any
such approach, as EPA did not determine, and did not need to determine, whether these vehicles and
engines emitted every one of the chemical compounds included in the large groupings of chemicals
covered, for example, by VOCs and PM. EPA also did not consider whether all of the various oxides of
nitrogen were in fact emitted by these vehicles and engines. Thus this past practice, as well as other
similar examples, shows that EPA has routinely evaluated emissions of a group air pollutant from a
source category by considering those emissions from the source category within the group, without
considering whether the group does or does not include any compounds that are not emitted by the source
category. EPA has followed the same approach in this case.
The well-mixed GHGs clearly share the key common attribute for climate change discussed above. The
GWP calculation for each GHG gas is analogous to the stratospheric ozone depletion calculation for the
many different Montreal Protocol ozone-depleting substances, or the tropospheric ozone production
efficiency for VOCs. In each case, the various different substances can be compared based on their
contribution to a common kind of air pollution problem. For these and the other reasons discussed in the
Findings, it is reasonable to include the six GHGs within a single group for the definition of "air
pollutant."
The reasoning for the inclusion of the fluorinated gases while excluding water vapor is discussed in
Volume 9, "Other Substances," of the Response to Comments document and in Section IV.A.6 of the
Findings, regarding the definition of air pollution. The same arguments apply for excluding water vapor
from the definition of air pollutant.
Comment (10-3):
Several commenters (e.g., 2977.1, 3579, 3579.1) argue that GHGs are not "air pollutants" under the CAA.
In particular, at least one commenter (3579.1) contends that EPA must show how GHGs impact or
materially change "ambient air" when defining air pollutant and making the endangerment finding. This
commenter argues that because C02 is a naturally occurring and necessary element in the atmosphere, it
cannot be considered to materially change air.
Response (10-3):
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These arguments have already been rejected by the Supreme Court, and are specifically addressed in
Section V of the Findings.
Comment (10-4):
Several commenters (e.g., 3435.1, 3704.2, 3702.1) disagree with EPA's proposed definition of a single air
pollutant composed of the six GHGs as a class. They argue that the analogy to VOCs is misplaced
because VOCs are all part of a defined group of chemicals, for which there are established quantification
procedures, and for which there were extensive data showing that the group of compounds had
demonstrated and quantifiable effects on ambient air and human health and welfare, and for which
verifiable dispersion models existed. They contend this is in stark contrast to the entirely diverse set of
organic and inorganic compounds EPA has lumped together for purposes of the Proposed Findings, and
for which no model can accurately predict or quantify the actual impact or improvement resulting from
controlling the compounds. Moreover, they argue that the gases EPA is proposing to list together as one
pollutant are all generated by different processes and, if regulated, would require different types of
controls; the four gases emitted by mobile sources can generally be limited only by using controls that are
specific to each.
Other commenters (e.g., 4173) argue that while the definition of "air pollutant" is flexible enough to
allow certain mixtures of polluting "agents" to be considered together, it expressly references the term
"air pollutant" in the singular and does not suggest that an agent that already has been designated as
individual pollutant may be captured and possibly counted a second time as part of a broadly defined
group of unrelated agents comprising a single "pollutant." Rather, they continue, common sense suggests
that Congress could not have intended for EPA to adopt the "kitchen sink" interpretation of "pollutant" it
has proposed, because the six GHGs are discrete, unrelated agents that are generated by a variety of
disparate sources and activities.
Response (10-4):
The rationale for including the six long-lived and we 11-mixed GHGs in both the definition of "air
pollution" and "air pollutant" is provided in the Findings, Sections IV.A and V, respectively; some
additional arguments are provided here. EPA disagrees with these arguments against the definition of
GHGs as a class of air pollutant. The six GHGs share common physical attributes that are central to the
climate change problem (e.g., relative atmospheric lifetimes and heat-trapping capability), so it is
inaccurate to state that these gases are unrelated. For example, the common physical attributes of these six
gases have enabled the quantification and common use of global warming potentials (GWPs) to allow for
estimates of C02 equivalent units among these six gases. This has been the common practice for the past
15 years or so, as employed by EPA and the entire climate change research and policy community.
Furthermore, EPA disagrees that there are no quantification procedures for the six GHGs. The Inventory
of U.S. Greenhouse Gases and Sinks published annually by EPA, for example, demonstrates the use of
emission estimation methods consistent with IPCC's National Guidelines for Greenhouse Gas Inventories
(Eggelston, 2006). These gases are represented in climate models with known heat-trapping properties
and atmospheric lifetimes, such that the climate effect of these gases can be assessed over time.
EPA also disagrees with the argument that because the six GHGs are emitted from a variety of sources or
may require a variety of different controls, that this is a reason not to group the six GHGs together. As
explained in the Findings, it is not necessarily the source category being evaluated that determines the
reasonableness of defining a group air pollutant. In this case the most important aspect of the compounds
is the shared attributes of the group. In other words, it is more important what the climate "sees," as these
GHGs will essentially have the same effect on the climate regardless of the type (or location) of the
source.
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This approach is indeed analogous to how VOCs are a defined group of chemicals; although VOCs
include a variety of chemicals that share the common attribute of photochemical reactivity, these
chemicals also have attributes that differ, are produced by a large number of different sources, and may be
controlled with a variety of different control technology and work practices. VOCs are considered as a
class in a definition of "air pollutant," and yet not all VOCs are regulated each time a VOC standard is
promulgated. Therefore, we do not find that the definition of GHGs as an air pollutant is in anyway
inconsistent with precedents already set under the Act.
It is not clear which of the six GHGs that combined are the definition of air pollutant in the Findings the
commenter above believes is already an individual air pollutant, which they argue precludes it being part
of the well-mixed GHG air pollutant. Nonetheless, as discussed above, EPA has broad authority to define
"air pollutant" and determine what grouping of air pollution agents is reasonable. The statutory language
does not prohibit EPA from grouping an air pollution agent in more than one definition of air pollutant, as
long as the reason for including that air pollution agent in both definitions is reasonable.
Comment (10-5):
Some commenters (e.g., 4004.1, 4036.5) agree with EPA's definition of "air pollutant," stating that EPA
has used a similar approach to other pollutants, including VOCs and PM, which have health effects that
are both similar to others in their classification and different. At least one commenter (e.g., 2889.1) states
that EPA appropriately proposed to treat GHGs as a single air pollutant, based on appropriate GWP
weighting.
They also agree that EPA should use AR4 GWPs, adjusted over time as the science evolves, to weigh the
various GHGs. Nonetheless, these commenters also suggest that EPA should carefully balance the
advantages of promulgating a single C02e-weighted GHG air pollutant with the possible needs to regulate
individual GHG classes separately, based on the differing nature, emission sources, and markets for each
GHG class. After summarizing EPA's 1996 proposed rule regarding an ozone-depleting substance (ODS)
significance level in the Prevention of Significant Deterioration (PSD) program, the commenter noted
many strong policy reasons to adopt an aggregation approach with respect to individual classes of GHGs,
such as HFCs not specially created for societal use: 1) it is far easier from an administrative standpoint to
regulate and track emissions of a class of compounds rather than basing determinations on their GWP; 2)
the pollutants should be aggregated based upon their environmental impact, and not their use by sources;
3) aggregating GHGs within classes allows users maximum flexibility, and allows the PSD program to be
as transparent as possible.
Response (10-5):
We agree with the commenters that the concept of aggregating the six GHGs has a clear precedent in
EPA's approach to other pollutants. The use of GWPs for comparing emissions of gases in terms of C02
equivalents is consistent with UNFCCC, IPCC, U.S. Climate Change Science Program (CCSP), U.S.
Global Change Research Program (USGCRP), and standard scientific practice. EPA clarifies, however,
that the definition of air pollutant does not itself include the concept of C02 equivalency. The air pollutant
is the group of these six gases. The fact that a C02 equivalency metric exists demonstrates the common
attributes of these six gases with respect to climate change. C02 equivalency is considered when
determining whether the emissions from new motor vehicles of the gases that make up this air pollutant
contribute to the air pollution. The use of C02 equivalency is brought in to help evaluate whether the
emissions of the air pollutant meet the contribution criteria of Section 202(a). However, the definition of
air pollutant does not itself include a term of C02 equivalency. Whether to use C02 equivalency to
characterize the emissions of the air pollutant is best considered as an issue to address in the kind of
decision-making or other action that is being taken with respect to the emissions of the air pollutant.
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10.2 Assessment of Contribution
Comment (10-6):
Several commenters (e.g., 3325.1, 3252.1, 33941.1) note that although EPA looks at emissions from all
motor vehicles regulated under Section 202(a) in its contribution analysis, the May Presidential
announcement (Office of the Press Secretary, 2009) indicated that EPA was planning to regulate only a
subset of 202(a) sources. Thus, they question whether the correct contribution analysis should look only
at the emissions from that subset and not all of 202(a) sources.
Response (10-6):
This comment is addressed in Section V.C of the Findings.
Comment (10-7):
Commenters (e.g., 3252.1, 3286.1, 3320.1, 3379.1, 3482.1, 3705.1) argue that because emission standards
will not eliminate all GHG emissions from motor vehicles, the comparison should compare the amount of
GHG emissions reduced by those standards to the global GHG emissions. They also contend that the cost
of the new standards will cause individual consumers, businesses, and other vehicle purchasers to hold on
to their existing vehicles to a greater extent, thereby decreasing the amount of emissions reductions
attributable to the standard and appropriately considered in the contribution analysis. At least one
commenter (e.g., 2892.1) goes further and contends that EPA can only include that incremental reduction
that the EPA regulations will achieve beyond any reductions resulting from Corporate Average Fuel
Economy (CAFE) standards that the National Highway Traffic Safety Administration will set.
Response (10-7):
See Section V.C of the Findings on why the Administrator reasonably looks at current emissions of
Section 202(a) source categories for the contribution analysis. As discussed in the Findings, there is no
basis in the statutory language for the argument that when evaluating contribution, the Administrator must
look not at current emissions from Section 202(a) source categories, but the reduction of those emissions
that will occur from the standards that follow such a contribution finding and then only the incremental
reduction that occurs beyond those projected to result from any CAFE standards.
Comment (10-8):
Many commenters (e.g., 3325.1, 3394.1, 4037.1) argue that EPA cannot compare emissions from the
current fleet of 202(a) motor vehicles for the contribution analysis. Rather, they contend that EPA must
compare emissions from the subset of 202(a) vehicles subject to any attendant GHG regulations—that is,
only "new" motor vehicles, which they continue is by definition a smaller number than from the entire
existing 202 inventory. More specifically, one commenter (3347.1) estimates that there are over 600
million passenger cars on the roads today worldwide, that about 50 million are new motor vehicles, and
around 11 million new motor vehicles are manufactured in the U.S. annually. This commenter continues
that new motor vehicles manufactured in the U.S. make up around 1.8% of the global motor vehicle
sectors, and that emissions from new motor vehicles are most likely lower per vehicle than the existing
fleet due to fuel economy improvements. Thus, some commenters conclude the source category of new
motor vehicles in the U.S. represent a minimal contribution to the global stock of GHG emissions, and do
not justify regulation under Section 202(a).
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At least one commenter (3553.1) appears to believe that EPA is considering the total GHG contribution
from the entire transportation sector, citing the Proposed Findings. This comment appears to be similar to
that above, arguing that EPA is asserting it has no choice but to regulate new car emissions under the
Clean Air Act because it cannot regulate old car emissions.
Response (10-8):
These comments are addressed in Section V.C.2.C of the Findings. Note that, as explained in the Findings,
the Administrator considered the emissions from Section 202(a) source categories, not from all
transportation sources.
Comment (10-9):
Several commenters (e.g., 2889.1, 3436.1, and 4036.5) agree with EPA's contribution analysis, and its
proposed findings that emissions of the single air pollutant from Section 202(a) sources contribute to the
air pollution which endangers. They also agree with EPA's discussion that if the substances were looked
at as individual air pollutants, the emissions from Section 202(a) sources would still contribute.
Response (10-9):
We agree with the commenters that the well-mixed GHG emissions from Section 202(a) source categories
contribute to the air pollution which endangers.
Comment (10-10):
At least one commenter (3217.1) argues that the EPA significantly overstates the certainty of current
climate science and knowledge. According to the commenter, these documents also fail to include
pertinent information about GHG emissions and global warming. Specially, the commenter claims that
the rate of growth of C02 emissions globally will swamp U.S. emissions and that the EPA does not
provide context or a complete picture of the global emissions of C02 because it fails to describe how
quickly emissions are increasing from the developing world. The commenter states that, according to data
from the Global Carbon Project, from 2000 through 2007, China's carbon dioxide emissions increased
98%, India's increased 36%, the global total increased 26%, Russia's increased 10%, the U.S.'s increased
3%. The commenter argues that Figure 2.3 in the TSD should be accompanied with context for the rate of
growth of GHG emissions from developing countries such as China. The commenter states that the EPA
fails to note that if the U.S. reduced C02 emissions from the transportation sector to zero, the rest of the
world would replace those emissions in less than 2 years at the current rate of growth. Furthermore, if the
U.S. were to completely cease using fossil fuels, the increase from the rest of the world would, according
to the commenter, replace U.S. carbon dioxide emissions in less than eight years.
Response (10-10):
EPA is aware of how the rate of GHG emissions growth varies from nation to nation, but EPA disagrees
that these effects are either completely missing in EPA's analyses, and disagrees that some of the issues,
if more explicitly accounted for, would alter the Findings. First, it is EPA's view that, for the contribution
finding comparisons, the primary use of current (either data year 2007 or 2005) GHG emissions data is
also a reasonable proxy for emissions over the last few years and for the near term. The fact that China,
the United States, India, and Russia are now the world's major GHG emitters is not something that is
expected to fundamentally change for the foreseeable future. Likewise, given the magnitude of Section
202(a) source category GHG emissions—currently one of the largest sources in the world—it is EPA's
view that the finding that the group air pollutant contributes to air pollution would not change if EPA
attempted to project future emissions, where Section 202(a) sources may then be a smaller percentage of
the world's total emissions but nevertheless remain one of the world's largest sources.
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Though it is true that emissions in developing countries are rising faster than those of the United States,
the commenters fail to mention that, given the long atmospheric lifetimes of these well-mixed GHGs,
historic emissions are also largely responsible for the current and near-term concentrations of GHGs in
the atmosphere. For that reason, the United States' share of current atmospheric concentrations would be
somewhat larger than the percentage of current U.S. emissions compared to current global emissions. The
future emission projections underlying future projections of climate change do indeed take into account
different scenarios of how different regions of the world are developing, and therefore how GHG
emission growth rates will vary by region.
How emissions may be addressed and reduced from Section 202(a) sources is not part of the
Administrator's cause or contribute test. Furthermore, the commenters put forth highly hypothetical
scenarios, in that it would be very unrealistic for the United States to undertake such significant emission
reduction action with zero implications for the rest of the world.
10.3 Level of Contribution
10.3.1 Single Air Pollutant Composed of the Well-Mixed Greenhouse Gases
Comment (10-11):
Some commenters (e.g., 3217.1, 3394.1, 3449.1) argue that EPA must provide some basis for determining
de minimis amounts that fall below the threshold of "contributing" to the endangerment of public health
and welfare under CAA Section 202(a). For example, commenters (e.g., 3285.1, 3449.1, 3722) note that
water vapor represents the GHG with the greatest impact on global warming—all other GHGs represent
only approximately 5% of all contribution to the climate change phenomenon (74 Fed. Reg. at 18897).
They question whether this small percentage is de minimis since EPA is not including water vapor in the
endangerment finding at this time. Commenters (e.g., 3449.1, 3702.1) also argue that EPA cannot act
arbitrarily by determining that a constituent contributing less than 5% to endangerment in one instance is
de minimis and another contributing the same percentage is endangering public health and welfare. They
request that EPA revise preamble language to make clear that the regulated community can rely on its
past determinations with respect to "contribution" determinations to predict future agency action and
should promulgate guidance on how it determines whether a contribution exceeds a de minimis level for
purposes of CAA Section 202(a) before finalizing the Proposal.
At least one commenter (3394.1) takes issue with EPA's statement it "need not determine at this time the
circumstances in which emissions would be trivial or de minimis and would not warrant a finding of
contribution." The commenter argues that by also stating that the emissions of methane would contribute,
when methane emissions from Section 202(a) source categories were less than 0.01% of total global GHG
emissions in 2005, EPA is implicitly determining that a share of total emissions of less than one one-
hundredth of one percent is not "trivial or de minimis." This determination, they continue, is inconsistent
with EPA's suggestion that it would apply a de minimis exception under Section 202(a)(1). More
fundamentally, they argue, that determination reflects an incoherent and internally inconsistent approach
by EPA in this proceeding to the question of what constitutes a statutorily cognizable "contribution."
Response (10-11):
We disagree with the assessment that water vapor accounts for the overwhelming share of the global
warming or climate change problem, and thus to endangerment. The role of water vapor is discussed in
Section 4(a) of the TSD, and in Volumes 2 and 9 of the Response to Comments document. The claimed
"95% contribution" of water vapor is also irrelevant in that what constitutes the natural greenhouse effect
is not at stake here; rather, it is the change or intensification of the natural greenhouse effect, and thus
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heating effect, due to changes in GHGs that causes endangerment. Anthropogenic changes in GHG
concentrations contribute the vast majority of positive radiative forcing in the recent past and in the
projected future scenarios. As explained in Section IV.A.6 of the Findings, water vapor is excluded from
the definition of air pollution precisely because direct human emissions of water vapor are not a
significant driver of human-induced climate change.
The issue of determining a bright line or defining a de minimis threshold is discussed in Section V.C.2 of
the Findings.
Comment (10-12):
At least one commenter (3347.1) argues that EPA overestimated the percent emissions from the 202
source category, noting that while EPA estimates it to be 4.3% of global GHG emissions, the IPCC data
suggests the number is only 3.4%—EPA's estimate is 20% too high.
Response (10-12):
For global and national GHG emissions data, EPA uses the World Resource Institute's (WRI's) Climate
Analysis Indicators Tool, or CAIT (WRI, 2009) and not the more aggregated emissions data bases used
by the IPCC's Fourth Assessment Report. This is not meant to be a reflection on the quality of the IPCC's
emission data. EPA elected to use CAIT for our analysis because it is the most up to date tool which
contains gross emission estimates for all gases, all countries, and all sources (2005). Much of the analysis
conducted in the Findings involves comparing emissions from the Section 202(a) source category against
the emissions of other nations and the total amount of emissions from the global transportation sector.
The IPCC's Fourth Assessment report distributes GHG emissions to regions of the globe, but not to
specific countries as the CAIT tool does.
EPA's view is that CAIT is an appropriate tool to use for this analysis because it contains national level
data submitted by Parties to the UNFCCC, and other independent and peer-reviewed data sets, such as
those from the International Energy Agency. These emission inventories are derived from bottom-up
methodologies that use data on specific emission-generating activities. The major difference between the
aggregated global emission totals provided in CAIT and in the IPCC's Fourth Assessment Report is that
the IPCC's Fourth Assessment report contains C02 emissions and C02 uptake from land-use change,
whereas these emissions are not included in CAIT. It is EPA's view that determining the percentage of
GHG emissions from Section 202(a) source categories without considering land-use change is an
appropriate analysis for a number of reasons. First, "gross" emissions (i.e., without netting out any carbon
uptake from terrestrial carbon sinks) from CAIT is more appropriate than "net" emissions (i.e.,
accounting for carbon uptake from terrestrial carbon sinks) from IPCC because the global top-down (i.e.,
derived from atmospheric measurements and not on-the-ground activities) data from the IPCC Fourth
Assessment Report does not precisely identify anthropogenic versus natural land-use emissions by
country. Second, including land-use changes means including removals (sinks) as well as emissions,
which creates a like-to-like comparison problem because there are no such carbon sinks covered under
Section 202(a) of the Act. Third, this approach of using gross emissions follows international convention
under the UNFCCC, where member nations report their GHG emissions separately, both with and
without land-use change; Parties to the UNFCCC want separate reporting of gross and net emissions
because use of the net numbers only can obscure emission trends. If we were to use the U.S. domestic net
emissions number (with land-use) as the denominator, then the actual share of U.S. emissions from
Section 202(a) would go up dramatically because the U.S. land-use change and forestry sector is a big net
sink, though all gross emission sectors would be affected equally, such that Section 202(a) sources would
still rank as the second largest GHG-emitting sector behind electricity generation.
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When the GHG emissions from land-use change are removed from the IPCC's Fourth Assessment
Report's emissions total, the total global GHG emission estimates from CAIT and the IPCC are in good
agreement (less than a 5% difference).
The commenter is correct that, had EPA used the IPCC Fourth Assessment Report in the Proposed
Findings (which estimated that the total global emissions of the six well-mixed GHGs were 49,000
teragrams (Tg) of C02 equivalent in 2004), then Section 202(a) source category emissions (1,665 Tg of
C02 equivalent for year 2006) would be 3.4% of the global total. However, considering these alternative
numbers, the Administrator would still find that Section 202(a) emissions contribute to the air pollution
that endangers, because this number would still represent one of the world's largest GHG sources.
Comment (10-13):
A commenter (3394.1) states that, because of its reliance on the annual U.S. inventory on GHGs for
projections, the TSD does not account for land use change and forestry emission and sinks. The
commenter objects to the fact that while the forestry sinks are included in the text, they are sometimes not
included in the figures. Also, the commenter notes that if other nations do include land use, land-use
change, and forestry (LULUCF) emissions, the U.S. emissions may appear larger in proportion than they
really are.
Response (10-13):
First, the 2005 inventories for all nations reported in the TSD are produced using the same
methodologies: none of these national emission estimates include LULUCF emissions, and it is therefore
appropriate to compare U.S. emissions to global emissions in 2005 on this basis. Indeed, as stated in the
caption for Figure 2.3, the data "[e]xcludes land use, land-use change and forestry, and international
bunker fuels. More recent emission data are available for the United States and other individual countries,
but 2000 is the most recent year for which data for all countries and all gases are available including
emissions from LULUCF."
However, we do address recent U.S. LULUCF emissions in the text: "Removals of carbon through land
use, land-use change and forestry activities are not included in Figure 2.2 but are significant; net
sequestration is estimated to be 1062.6 TgC02eq in 2007, offsetting 14.9% of total emissions (EPA,
2009b)." The caption for Figure 2.2 also reiterates the fact that LULUCF and international bunker fuel
emissions are not included in the data shown. Therefore, the TSD is clear and consistent in its discussion
of emissions and the inclusion or lack thereof of LULUCF emissions.
For comparison, using the CAIT (WRI, 2009) inventory for 2000 (the last year that land-use change and
forestry emissions are available globally through this tool), U.S. emissions are 21.17% of the global total
C02-equivalent emissions without LULUCF emissions, 15.41% including those emissions, and 15.35%
including those emissions and emissions from international bunker fuels. Please also refer to the response
immediately above regarding the use of the CAIT tool.
Comment (10-14):
Other commenters (e.g., 3394.1, 3597) disagree with EPA's proposed contribution findings, arguing that
U.S. EPA's own analysis of light-duty vehicles in the ANPR (Light Vehicle Technical Support document
Docket U.S. EPA-HQ-OAR-2008-0318-0084) determined that GHG emission standards for new vehicles
and new vehicle motors would produce a reduction of, at most, approximately 0.01 degree Celsius in
mean global temperature. They argue that this is a value that cannot be measured except by the most
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sensitive satellites.
Commenters (e.g., 3394, 3449.1, and 3747.1) argue that the "cause or contribute" prong of the Proposal's
endangerment analysis fails to satisfy the applicable legal standard, which requires more than a minimal
contribution to the "air pollution reasonably anticipated to endanger public health or welfare." They
contend that emissions representing approximately 4% of total global GHG emissions are a minimal
contribution to global GHG concentrations. These commenters disagree with statements in the Proposal
that the "unique, global aspects of the climate change problem tend to support a finding that lower levels
of emissions should be considered to contribute to the air pollution than might otherwise be appropriate
when considering contribution to a local or regional air pollution problem." They argue there is no basis
in the Act or existing EPA policy for this position, and that it reveals an apparent effort to expand EPA's
authority into the "truly trivial or de minimis" sources that are acknowledged to be outside the scope of
regulation, in that it expands EPA's authority to regulate pollutants to address global effects.
Commenters (e.g., 3347.1, 3747.1) also assert that contrary to EPA's position, lower contribution
numbers are appropriate when looking at local pollution, like nonattainment concerns—in other words, in
the context of a statutory provision (such as Section 213) specifically aimed at targeting small source
categories to help nonattainment areas meet air quality standards. However, they conclude this policy is
simply inapplicable in the context of global climate change.
EPA received many comments (e.g., 3252.1, 3325.1, 3347.1, 3379.1, 3394.1, and 4037) on the
appropriate comparison(s) for the contribution analysis. At least one commenter (3347.1) argues that in
order to get around the "problem" of basing an endangerment finding upon a source category that
contributes only 1.8% annually to global GHG emissions, EPA inappropriately also made comparisons to
total U.S. GHG emissions. This commenter argues that a comparison of 202(a) source emissions to U.S.
GHG emissions, versus global GHG emissions, is arbitrary for the purposes of the cause or contribute
analysis, because it conflicts with the Administrator's definition of "air pollution," as well as the nature of
global warming. They note that throughout the Proposal, the Administrator focuses on the global nature
of GHGs. Thus, they continue, while percentage share of motor vehicle emissions at the U.S. level may
be relevant for some purposes, it is irrelevant to a finding of whether these emissions contribute to air
pollution, which the Administrator has proposed to define on a global rather than a domestic basis.
Commenters (e.g., 3449.1) also accuse EPA of arbitrarily picking and choosing when it takes a global
approach (e.g., endangerment finding) and when it does not (e.g., contribution findings).
Response (10-14):
We disagree that the temperature or emissions reductions resulting from GHG standards are the
appropriate emissions to use for a "contribute" finding. The appropriate measure should be the emissions
from the sector as a whole, regardless of the reductions resulting from any set of proposed standards.
Section V.B of the Findings provides the Administrator's approach for making the cause or contribute
finding, and Section V.C contains responses to key comments.
Comments addressing the emissions contribution from projected emissions of new motor vehicles are
addressed in Section V.C.2.C of the Findings.
We disagree with the commenters that a 4% contribution to global GHG emissions is trivial. As stated in
the Findings, this 4% figure as a result of Section 202(a) source emissions represents a larger contribution
to global emissions than almost every single country in the world, with the exceptions of China, the U.S.,
Russia and India; this means the contribution from 202(a) sources alone is higher than countries as large
as Japan, Brazil and Germany. As stated in Section V.B.I of the Findings, no single GHG source category
dominates on the global scale—in this context, a source category that contributes 4% of GHG emissions
is quite significant. See Massachusetts v. EPA, 549 U.S. at 1457-58 ("Judged by any standard, U.S.
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motor-vehicle emissions make a meaningful contribution to greenhouse gas concentrations and hence, . . .
to global warming.").
Finally, we disagree with the comment that the cause or contribute finding is based inappropriately on a
comparison of Section 202(a) source category emissions to total U.S. emissions. Section V.B of the
Findings describes the Administrator's rationale for the cause or contribute finding, and explains why
both the U.S. and international comparisons were considered, and why these comparisons, independently
and together, support the cause or contribute finding.
10.3.2 Each GHG as an Individual Air Pollutant
Comment (10-15):
Commenter (3603.1) disagrees with the finding that methane as an individual air pollutant can be found to
contribute because methane emissions from existing Section 202(a) sources represent only 0.03% of U.S.
GHGs, and that number is overstated because the finding should refer to only new vehicles, not the entire
existing fleet. Commenter states that this 0.03% is below the threshold noted in the proposal and in other
proceedings such as interstate transport.
Response (10-15):
As explained in Section V of the Findings, the Administrator is not defining methane as an individual air
pollutant under Section 202(a), thus EPA is not responding directly to the argument that the
Administrator could not make a finding of contribution for methane as an individual air pollutant.
Note that methane is being included in both the definition of "air pollution" and the definition of "air
pollutant" because it shares all of the same common attributes with the other five well-mixed GHGs.
Importantly, methane is itself the second most important GHG directly emitted by human activities, in
terms of its anthropogenic heating effect on the climate. This is why methane is consistently a standard
part of climate change science analysis and policy discussions. Recognizing the important role that
methane plays in climate change, EPA now for several years has been running methane voluntary
programs to target cost-effective emission reductions in key sectors, within the U.S. and internationally.
The amount of emissions of methane, or comparisons based on those amounts, are not relevant for
determining whether emissions of the air pollutant—defined as the aggregate group of well-mixed
GHGs—contributes to the air pollution. The relevant amounts and comparisons in that case concern the
total emissions of the air pollutant, not a part of such emissions.
Comment (10-16):
A commenter (3377.1) notes that N20 is produced in extremely small amounts by motor vehicles—
almost certainly less than the estimate provided by automotive manufacturers. The commenter also
cautions EPA that CH4 regulations from motor vehicles might discourage compressed natural gas vehicles
that would otherwise provide a C02 reduction benefit, and therefore recommends no CH4 regulations.
Response (10-16):
The commenter did not provide a specific estimate or any documentation regarding their claim that N20
emissions from motor vehicles are extremely small. Further, the commenter is in error in implying that
the TSD relies upon estimates provided by automotive manufacturers. In fact, the values EPA used in
Section 2(a) of the TSD are taken from EPA's Inventory of Greenhouse Gas Emissions and Sinks 1990-
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2007 (U.S. EPA, 2009). The estimation methodologies and associated uncertainty are discussed in
Section 3.1 of the Inventory.
With regard to CH4 regulations mentioned by the commenter, these Findings do not put forth any policy
or regulatory proposals or strategies that would respond to the threat of climate change. Sections III.C and
F of the Findings define the appropriate scope of the endangerment analysis in this regard.
Comment (10-17):
One commenter (3427.1) objects to the reasoning behind the proposed Finding's statement that not
including HFCs "would be inconsistent with the U.S. practice of encouraging hydrofluorocarbon
emission reductions," claiming that this is irrelevant.
Response (10-17):
As explained in Section V of the Findings, the Administrator is not defining HFCs as an individual air
pollutant under Section 202(a), thus EPA is not responding directly to the argument that the
Administrator could not make a finding of contribution for HFCs as an individual air pollutant. The
reasoning for including HFCs (and the other five GHGs) together in both the definition of "air pollution"
and "air pollutant" is addressed in the Findings. As explained in the Findings, HFCs share common
properties with the other GHGs and are included in the common focus of climate change research, and
therefore it is appropriate and consistent with the science to include HFCs in the combined mix of these
gases.
Consistency with the practice of encouraging HFC emission reductions has not been included in the
reasoning of the current finding. The rationale for not considering mitigation measures in the Finding is
further discussed in Response to Comments, Volume 1.
References
Eggelston, S., L. Buendia, K. Miwa, T. Ngara, and K. Tanabe, eds. (2006). 2006IPCC Guidelines for
National Greenhouse Gas Inventories. Published for IPCC by the Institute for Global Environmental
Strategies, http://www.ipcc-nggip.iges.or.ip/public/2006gl/index.html.
Office of the Press Secretary (2009). President Obama announces national fuel efficiency policy. White
House press release. May 19.
U.S. EPA (2000). Control of air pollution from new motor vehicles: Proposed heavy-duty engine and
vehicle standards and highway diesel fuel sulfur control requirements. Fed. Reg. 65(107):35429-35559.
June 2.
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