section also summarizes the uncertainty about the potential indirect
effects on public health associated with changes due to increases in
UV-B radiation exposure, such as UV-B radiation-related skin cancers,
that may be associated with reductions in ambient levels of ground-
level [ozone], as discussed in chapter 10 of the Criteria Document and
chapter 3 of the Staff Paper.'' 72 FR 37818, 37827. See also, 72 FR
37837 (``* * * the Criteria Document also assesses the potential
indirect effects related to the presence of [ozone] in the ambient air
by considering the role of ground-level [ozone] in mediating human
health effects that may be directly attributable to exposure to solar
ultraviolet radiation (UV-B).'')
    Thus, EPA's approach to UV-B related health effects clearly shows
the Agency has treated indirect health effects not as welfare effects,
as commenters suggest, but as human health effects that need to be
evaluated when setting the public health based primary NAAQS. In this
ozone NAAQS rulemaking, EPA did not draw a line between direct and
indirect health effects for purposes of evaluating UV-B related health
effects and the public health based primary NAAQS.

    Similarly, the NOX/SOX criteria document does
not establish a precedent that indirect human health effects are
welfare effects. Toxic algal blooms themselves are a welfare effect, so
it is not surprising a discussion of algal blooms appears in sections
dealing with welfare effects. The more relevant question is how EPA
evaluated information regarding human health risks resulting from algal
blooms. In the case of the Criteria Document, the role of nitrogen in
causing algal blooms was unclear. As a result, the Agency did not have
occasion to evaluate any resulting human health effects and the
Criteria Document does not support the view that EPA treats indirect
health effects as anything other than a public health issue.
    Finally, EPA disagrees that its action here is at odds with the
listing of municipal solid waste landfills under CAA section 111. In
the landfills New Source Performance Standard (NSPS) EPA did not
consider health effects resulting from climate change much less draw
any conclusions about health effects from climate change being health
or welfare effects. If anything, the landfills NSPS is consistent with
EPA's approach. In the proposed rule, EPA stated: ``The EPA has
documented many cases of acute injury and death caused by explosions
and fires related to municipal landfill gas emissions. In addition to
these health effects, the associated property damage is a welfare
effect'' (56 FR 24474). EPA considered injury and death from fires
resulting from landfill gasses to be health effects. Yet the injury did
not result from direct exposure to the pollutant (landfill gas).
Instead, the injury resulted from the combustion of the pollutant--the
injury is essentially an indirect effect of the pollutant. Yet, as with
this action, EPA considered the injury as a human health effect.
    Case law. Several commenters argue that EPA's proposed endangerment
finding was inconsistent with NRDC v. EPA, 902 F.2d 962 (DC Cir 1990).
Commenters argue that in rejecting the argument that EPA must consider
the health effects of increased unemployment that could result from a
more stringent primary NAAQS standard, the DC Circuit explained that,
``[i]t is only the health effects relating to pollutants in the air
that EPA may consider.'' Id. at 973. Several commenters further argue
that EPA later relied on that holding to defend its decision to set a
primary NAAQS for ozone based solely on direct health effects of ozone.
Citing, EPA Pet'n for Rehearing, Am. Trucking Ass'n v. EPA, No. 97-1440
(DC Cir. June 28, 1999) (``ATA I'') (arguing that the primary NAAQS
should be set through consideration of only ``direct adverse effects on
public health, and not indirect, allegedly beneficial effects.'')
    The NRDC case is not contrary to EPA's endangerment finding. In
NRDC, petitioner American Iron and Steel Institute argued that EPA had
to consider the costs of health consequences that might arise from
increased unemployment. The court ruled that, ``[c]onsideration of
costs associated with alleged health risks from unemployment would be
flatly inconsistent with the statute, legislative history and case law
on this point.'' 902 F.2d at 973. The cases cited by the court in
support of its decision all hold that EPA may not consider economic or
technological feasibility in establishing a NAAQS. The NRDC decision
does not establish a precedent that the CAA prohibits EPA from
considering indirect health effects as a public health issue rather
than a public welfare issue.
    EPA also believes reliance on the Agency's petition for rehearing
in noted above is misplaced. In that case, EPA did not argue that
indirect beneficial health effects were not public health issues.
Instead EPA argued that under the CAA, it did not have to consider such
indirect beneficial health effects of an air pollutant when setting the
health based primary NAAQS. EPA was interpreting the NAAQS standard
setting provisions of CAA section 109, and argued that they were
intended to address only certain public health impacts, those that were
adverse, and were not intended to address indirect, beneficial public
health impacts. The issue in the case was not whether indirect health
effects are relevant for purposes of making an endangerment decision
concerning public health, but rather whether EPA must consider such
beneficial health effects in establishing a primary NAAQS under CAA
section 109. EPA's interpretation of CAA section 109 was rejected in
ATA v. EPA, 175 F.3d at 1027 (1999) reh'g granted in part and denied in
part, 195 F.3d at 4 (DC Cir. 1999). The court made it clear that the
potential indirect beneficial impact of ambient ozone on public health
from screening UVB rays needed to be considered when setting the NAAQS
to protect public health. As discussed above, EPA has done just that as
noted above in the UV-B context. Moreover, as discussed in Section II
of these Findings, EPA is doing that here as well (e.g., considering
any benefits from reduced cold weather related deaths).
ii. EPA's Treatment and Balancing of Heat- vs. Cold-Related Public
Health Risks Was Reasonable
    A number of public commenters maintain that the risk of heat waves
in the future will be modulated by adaptive measures. The Administrator
is aware of the potential benefits of adaptation in reducing heat-
related morbidity and mortality and recognizes most heat-related deaths
are preventable. Nonetheless, the Administrator notes the assessment
literature \30\ indicates heat is the leading weather-related killer in
the United States even though countermeasures have been employed in
many vulnerable areas. Given projections for heat waves of greater
frequency, magnitude, and duration coupled with a growing population of
older adults (among the most vulnerable groups to this hazard), the
risk of adverse health outcomes from heat waves is expected to
increase. Intervention and response measures could certainly reduce the
risk, but as we have noted, the need to adapt supports an increase in
risk or endangerment. For a general discussion about EPA's treatment of
adaptation see Section III.C of these Findings.
---------------------------------------------------------------------------

    \30\ Karl et al. (2009).
---------------------------------------------------------------------------

    Several commenters also suggest cold-related mortality will
decrease more than heat-related mortality will increase, which
indicates a net reduction in temperature-related mortality. Some
commenters point to research suggesting migration to warmer climates
has contributed to the increased longevity of some Americans, implying
climate warming will have benefits for health. The Administrator is
very clear that the exact balance of how heat- versus cold-related
mortality will change in the future is uncertain; however, the
assessment literature points to evidence suggesting that the increased
risk from heat would exceed the decreased risk from cold in a warming
climate. The Administrator does not dispute research indicating the
benefits of migration to a warmer climate and nor that average climate
warming may indeed provide health benefits in some areas. These points
are reflected in the TSD's statement projecting less cold-related
health effects. The Administrator considers these potential warming
benefits independent of the potential negative effects of extreme heat
events which are projected to increase under future climate change
scenarios affecting vulnerable groups and communities.

iii. EPA Was Reasonable To Find That the Air Quality Impacts of Climate
Change Contribute to the Endangerment of Public Health
    Several commenters suggest that air quality effects of climate
change will be addressed through the CAA's NAAQS process, as
implemented by the State Implementation Plans (SIP) and national
regulatory programs. According to these commenters, these programs will
ensure no adverse impact on public health due to climate change. Though
climate change may cause certain air pollutant ambient concentrations
to increase, States will continue to be compelled to meet the
standards. So, while additional measures may be necessary, and result
in increased costs, these commenters assert that, ultimately, public
health will be protected by the continued existence of the NAAQS and
therefore no endangerment with respect to this particular climate
change-related impact will occur. One commenter states that EPA
inappropriately assigns air quality risk to climate change that will be
addressed through other programs. The CAA provides a mechanism to meet
the standards and additional control measures consistent with the CAA
will be adopted in the future, keeping pollution below unhealthy
levels. The commenters state that the fact that NAAQS are in place that
require EPA to fulfill its legal obligation to prevent this particular
form of endangerment to public health.
    EPA does have in place NAAQS for ozone, which are premised on the
harmfulness of ozone to public health and welfare. These standards and
their accompanying regulatory regime have helped to reduce the dangers
from ozone in the United States. However, substantial challenges remain
with respect to achieving the air quality protection promised by the
NAAQS for ozone. It is the Administrator's view that these challenges
will be exacerbated by climate change.
    In addition, the control measures to achieve attainment with a
NAAQS are a mitigation measure aimed at reducing emissions of ozone
precursors. As discussed in Section III.C of these Findings, EPA is not
considering the impacts of mitigation with respect to future reductions
in emissions of greenhouse gases. For the same reasons, EPA is
reasonably not considering mitigation in the form of the control
measures that will need to be adopted in the future to reduce emissions
of ozone precursors and thereby address the increased ambient ozone
levels that can occur because of climate change.
    It is important to note that controls to meet the NAAQS are
typically put in place only after air quality concentrations exceeding
the standard are detected. Furthermore, implementation of controls to
reduce ambient concentrations of pollutants occurs over an extended
time period, ranging from three years to more than twenty years
depending on the pollutant and the seriousness of the nonattainment
problem. Thus, while the CAA provides mechanisms for addressing adverse
health effects and the underlying air quality exacerbation over time,
it will not prevent the adverse impacts in the interim. Given the
serious nature of the health effects at issue--including respiratory
and cardiovascular disease leading to hospital admissions, emergency
department visits, and premature mortality--this increase in adverse
impacts during the time before additional controls can be implemented
is a serious public health concern. Historically, a large segment of
the U.S. population has lived in areas exceeding the NAAQS, despite the
CAA and its implementation efforts. Half of all Americans, 158 million
people, live in counties where air pollution exceeds national health
standards.\31\ Where attainment of the NAAQS is especially difficult,
leading to delays in meeting attainment deadlines, the health effects
of increased ozone due to climate change may be substantial.
---------------------------------------------------------------------------

    \31\ U.S. EPA (2008) National Air Quality: Status and Trends
Through 2007. EPA-454/R-08-006, November 2008.
---------------------------------------------------------------------------

    It is also important to note that it may not be possible for States
and Tribes to plan accurately for the impacts of climate change in
developing control strategies for nonattainment areas. As noted in the
TSD and EPA's 2009 Interim Assessment report (IA), climate change is
projected to lead to an increase in the variability of weather, and
this may increase peak pollution events including increases in ozone
exceedances. While the modeling studies in the IA all show significant
future changes in meteorological quantities, there is also significant
variability across the simulations in the spatial patterns of these
future changes, making it difficult to select a set of future
meteorological data for planning purposes. At this time, models used to
develop plans to attain the NAAQS do not take potential changes in
future meteorology into consideration. Inability to predict the
frequency and magnitude of such events could lead to an underestimation
of the controls needed to bring areas into attainment, and a prolonged
period during which adverse health impacts continue to occur.
    Even in areas that meet the NAAQS currently, air quality may
deteriorate sufficiently to cause adverse health effects for some
individuals. Some at-risk individuals, for example those with
preexisting health conditions or other characteristics which increase
their risk for adverse effects upon exposure to PM or ozone, may
experience health effects at levels below the standard. Current
evidence suggests that there is no threshold for PM or ozone
concentrations below which no effects can be observed. Therefore,
increases in ozone or PM in locations that currently meet the standards
would likely result in additional adverse health effects for some
individuals, even though the pollution increase might not be sufficient
to cause the area to be designated nonattainment. While the NAAQS is
set to protect public health with an adequate margin of safety, it is
recognized that in attainment areas there may be individuals who remain
at greater risk from an increase in ozone levels. The clear risk to the
public from ozone increases in nonattainment areas, in combination with
the risk to some individuals in attainment areas, supports the finding
that overall the public health is endangered by increases in ozone
resulting from climate change.
    Finally, it is also important to note that not all air pollution
events are subject to CAA controls under the NAAQS implementation
provisions. ``Exceptional events'' are events for which the normal
planning and regulatory process established by the CAA is not
appropriate (72 FR 13561). Emissions from some events, including some
wildfires, are not reasonably controllable or preventable. Such
emissions, however, can adversely impact public health and welfare and
are expected to increase due to climate change. As described in the
TSD, PM emissions from wildfires can contribute to acute and chronic
illnesses of the respiratory system, particularly in children,
including pneumonia, upper respiratory diseases, asthma and chronic
obstructive pulmonary disease. The IPCC (Field et al., 2007) reported
with very high confidence that in North America, disturbances like
wildfires are increasing and are likely to intensify in a warmer future
with drier soils and longer growing seasons.
2. The Air Pollution Is Reasonably Anticipated to Endanger Public
Welfare
    The Administrator also finds that the well-mixed greenhouse gas air
pollution may reasonably be anticipated to

endanger public welfare, both for current and future generations.
    As with public health, the Administrator considered the multiple
pathways in which the greenhouse gas air pollution and resultant
climate change affect climate-sensitive sectors, and the impact this
may have on public welfare. These sectors include food production and
agriculture; forestry; water resources; sea level rise and coastal
areas; energy, infrastructure, and settlements; and ecosystems and
wildlife. The Administrator also considered impacts on the U.S.
population from climate change effects occurring outside of the United
States, such as national security concerns for the United States that
may arise as a result of climate change impacts in other regions of the
world. The Administrator examined each climate-sensitive sector
individually, informed by the summary of the scientific assessments
contained in the TSD, and the full record before EPA, and weighed the
extent to which the risks and impacts within each sector support or do
not support a positive endangerment finding in her judgment. The
Administer then viewed the full weight of evidence looking across all
sectors to reach her decision regarding endangerment to public welfare.
a. Food Production and Agriculture
    Food production and agriculture within the United States is a
sector that will be affected by the combined effects of elevated carbon
dioxide concentrations and associated climate change. The Administrator
considered how these effects, both adverse and beneficial, are
affecting the agricultural sector now and in the future, and over
different regions of the United States, taking into account that
different regions of the country specialize in different agricultural
products with varying degrees of sensitivity and vulnerability to
elevated carbon dioxide levels and associated climate change.
    Elevated carbon dioxide concentrations can have a stimulatory
effect on grain and oilseed crop yield, as may modest temperature
increases and a longer growing season that results. A report under the
USGCRP concluded that, with increased carbon dioxide and temperature,
the life cycle of grain and oilseed crops will likely progress more
rapidly. However, such beneficial influences need to be considered in
light of various other effects. For example, the literature indicates
that elevated carbon dioxide concentrations may also enhance pest and
weed growth. Pests and weeds can reduce crop yields, cause economic
losses to farmers, and require management control options. How climate
change (elevated carbon dioxide, increased temperatures, altered
precipitation patterns, and changes in the frequency and intensity of
extreme events) may affect the prevalence of pests and weeds is an
issue of concern for food production and the agricultural sector.
Research on the combined effects of elevated carbon dioxide and climate
change on pests, weeds, and disease is still limited. In addition,
higher temperature increases, changing precipitation patterns and
variability, and any increases in ground-level ozone induced by higher
temperatures, can work to counteract any direct stimulatory carbon
dioxide effect, as well as lead to their own adverse impacts. There may
be large regional variability in the response of food production and
agriculture to climate change.
    For grain and oilseed crop yields, there is support for the view
that in the near term climate change may have a beneficial effect,
largely through increased temperature and increased carbon dioxide
levels. However there are also factors noted above, some of which are
less well studied and understood, which would tend to offset any near
term benefit, leaving significant uncertainty about the actual
magnitude of any overall benefit. The USGCRP report also concluded that
as temperature rises, these crops will increasingly begin to experience
failure, especially if climate variability increases and precipitation
lessens or becomes more variable.
    A key uncertainty is how human-induced climate change may affect
the intensity and frequency of extreme weather events such as droughts
and heavy storms. These events have the potential to have serious
negative impact on U.S. food production and agriculture, but are not
always taken into account in studies that examine how average
conditions may change as a result of carbon dioxide and temperature
increases. Changing precipitation patterns, in addition to increasing
temperatures and longer growing seasons, can change the demand for
irrigation requirements, potentially increasing irrigation demand.
    Another key uncertainty concerns the many horticultural crops
(e.g., tomatoes, onions, fruits), which make up roughly 40 percent of
total crop value in the United States. There is relatively little
information on their response to carbon dioxide, and few crop
simulation models, but according to the literature, they are very
likely to be more sensitive to the various effects of climate change
than grain and oilseed crops.
    With respect to livestock, higher temperatures will very likely
reduce livestock production during the summer season in some areas, but
these losses will very likely be partially offset by warmer
temperatures during the winter season. The impact on livestock
productivity due to increased variability in weather patterns will
likely be far greater than effects associated with the average change
in climatic conditions. Cold-water fisheries will likely be negatively
affected; warm-water fisheries will generally benefit; and the results
for cool-water fisheries will be mixed, with gains in the northern and
losses in the southern portions of ranges.
    Finally, with respect to irrigation requirements, the adverse
impacts of climate change on irrigation water requirements may be
significant.
    There is support for the view that there may be a benefit in the
near term in the crop yield for certain crops. This potential benefit
is subject to significant uncertainty, however, given the offsetting
impact on the yield of these crops from a variety of other climate
change impacts that are less well understood and more variable. Any
potential net benefit is expected to change to a disbenefit in the
longer term. In addition, there is clear risk that the sensitivity of a
major segment of the total crop market, the horticultural sector, may
lead to adverse affects from climate change. With respect to livestock
production and irrigation requirements, climate change is likely to
have adverse effects in both the near and long terms. The impact on
fisheries varies, and would appear to be best viewed as neutral
overall.
    There is a potential for a net benefit in the near term for certain
crops, but there is significant uncertainty about whether this benefit
will be achieved given the various potential adverse impacts of climate
change on crop yield, such as the increasing risk of extreme weather
events. Other aspects of this sector are expected to be adversely
affected by climate change, including livestock management and
irrigation requirements, and there is a risk of adverse effect on a
large segment of the total crop market. For the near term, the concern
over the potential for adverse effects in certain parts of the
agriculture sector appears generally comparable to the potential for
benefits for certain crops.
    However, considering the trend over near- and long-term future
conditions, the Administrator finds that the body of evidence points
towards increasing risk

of net adverse impacts on U.S. food production and agriculture, with
the potential for significant disruptions and crop failure in the
future.
b. Forestry
    The factors that the Administrator considered for the U.S. forest
sector are similar to those for food production and agriculture. There
is the potential for beneficial effects due to elevated concentrations
of carbon dioxide and increased temperature, as well as the potential
for adverse effects from increasing temperatures, changing
precipitation patterns, increased insects and disease, and the
potential for more frequent and severe extreme weather events. The
potential beneficial effects are better understood and studied, and are
limited to certain areas of the country and types of forests. The
adverse effects are less certain, more variable, and also include some
of the most serious adverse effects such as increased wildfire,
drought, and major losses from insects and disease. As with food
production and agriculture, the judgment to be made is largely a
qualitative one, balancing impacts that vary in certainty and
magnitude, with the end result being a judgment as to the overall
direction and general level of concern.
    According to the underlying science assessment reports, climate
change has very likely increased the size and number of wildfires,
insect outbreaks, and tree mortality in the Interior West, the
Southwest, and Alaska, and will continue to do so. Rising atmospheric
carbon dioxide levels will very likely increase photosynthesis for
forests, but the increased photosynthesis will likely only increase
wood production in young forests on fertile soils. Nitrogen deposition
and warmer temperatures have very likely increased forest growth where
water is not limiting and will continue to do so in the near future.
    An increased frequency of disturbance (such as drought, storms,
insect-outbreaks, and wildfire) is at least as important to forest
ecosystem function as incremental changes in temperature,
precipitation, atmospheric carbon dioxide, nitrogen deposition, and
ozone pollution. Disturbances partially or completely change forest
ecosystem structure and species composition, cause short-term
productivity and carbon storage loss, allow better opportunities for
invasive alien species to become established, and command more public
and management attention and resources. The combined effects of
expected increased temperature, carbon dioxide, nitrogen deposition,
ozone, and forest disturbance on soil processes and soil carbon storage
remain unclear.
    Precipitation and weather extremes are key to many forestry
impacts, accounting for part of the regional variability in forest
response. If existing trends in precipitation continue, it is expected
that forest productivity will likely decrease in the Interior West, the
Southwest, eastern portions of the Southeast, and Alaska, and that
forest productivity will likely increase in the northeastern United
States, the Lake States, and in western portions of the Southeast. An
increase in drought events will very likely reduce forest productivity
wherever such events occur.
    Changes in disturbance patterns are expected to have a substantial
impact on overall gains or losses. More prevalent wildfire disturbances
have recently been observed in the United States. Wildfires and
droughts, among other extreme events (e.g., hurricanes) that can cause
forest damage, pose the largest threats over time to forest ecosystems.
    For the near term, the Administrator believes the beneficial impact
on forest growth and productivity in certain parts of the country from
climate change to be more than offset by the clear risk from the more
significant and serious adverse effects from the observed increases in
wildfires, combined with the adverse impacts on growth and productivity
in other areas of the country and the serious risks from the spread of
destructive pests and disease. Increased wildfires can also increase
particulate matter and thus create public health concerns as well. For
the longer term, the Administrator views the risk from adverse effects
to increase over time, such that overall climate change presents
serious adverse risks for forest productivity. The Administrator
therefore finds there is compelling reason to find that the greenhouse
gas air pollution endangers U.S. forestry in both the near and long
term, with the support for a positive endangerment finding only
increasing as one considers expected future conditions in which
temperatures continue to rise.
c. Water Resources
    The sensitivity of water resources to climate change is very
important given the increasing demand for adequate water supplies and
services for agricultural, municipal, and energy and industrial uses,
and the current strains on this resource in many parts of the country.
    According to the assessment literature, climate change has already
altered, and will likely continue to alter, the water cycle, affecting
where, when, and how much water is available for all uses. With higher
temperatures, the water-holding capacity of the atmosphere and
evaporation into the atmosphere increase, and this favors increased
climate variability, with more intense precipitation and more droughts.
    Climate change is causing and will increasingly cause shrinking
snowpack induced by increasing temperature. In the western United
States, there is already well-documented evidence of shrinking snowpack
due to warming. Earlier meltings, with increased runoff in the winter
and early spring, increase flood concerns and also result in
substantially decreased summer flows. This pattern of reduced snowpack
and changes to the flow regime pose very serious risks to major
population regions, such as California, that rely on snowmelt-dominated
watersheds for their water supply. While increased precipitation is
expected to increase water flow levels in some eastern areas, this may
be tempered by increased variability in the precipitation and the
accompanying increased risk of floods and other concerns such as water
pollution.
    Warmer temperatures and decreasing precipitation in other parts of
the country, such as the Southwest, can sustain and amplify drought
impacts. Although drought has been more frequent and intense in the
western part of the United States, the East is also vulnerable to
droughts and attendant reductions in water supply, changes in water
quality and ecosystem function, and challenges in allocation. The
stress on water supplies on islands is expected to increase.
    The impact of climate change on groundwater as a water supply is
regionally variable; efforts to offset declining surface water
availability due to increasing precipitation variability may be
hampered by the fact that groundwater recharge will decrease
considerably in some already water-stressed regions. In coastal areas,
the increased salinization from intrusion of salt water is projected to
have negative effects on the supply of fresh water.
    Climate change is expected to have adverse effects on water
quality. The IPCC concluded with high confidence that higher water
temperatures, increased precipitation intensity, and longer periods of
low flows exacerbate many forms of water pollution and can impact
ecosystems, human health, and water system reliability and operating
costs. These changes will also exacerbate many forms of water
pollution, potentially making attainment of water quality goals more

difficult. Water pollutants of concern that are particularly relevant
to climate change effects include sediment, nutrients, organic matter,
pathogens, pesticides, salt, and thermal pollution. As waters become
warmer, the aquatic life they now support will be replaced by other
species better adapted to warmer water. In the long term, warmer water,
changing flows, and decreased water quality may result in deterioration
of aquatic ecosystems.
    Climate change will likely further constrain already over-allocated
water resources in some regions of the United States, increasing
competition among agricultural, municipal, industrial, and ecological
uses. Although water management practices in the United States are
generally advanced, particularly in the West, the reliance on past
conditions as the basis for current and future planning may no longer
be appropriate, as climate change increasingly creates conditions well
outside of historical observations. Increased incidence of extreme
weather and floods may also overwhelm or damage water treatment and
management systems, resulting in water quality impairments. In the
Great Lakes and major river systems, lower water levels are likely to
exacerbate challenges relating to water quality, navigation,
recreation, hydropower generation, water transfers, and bi-national
relationships.
    The Administrator finds that the total scientific literature
provides compelling support for finding that greenhouse gas air
pollution endangers the water resources important for public welfare in
the United States, both for current and future generations. The
adequacy of water supplies across large areas of the country is at
serious risk from climate change. Even areas of the country where an
increase in water flow is projected could face water resource problems
from the variability of the supply and water quality problems
associated with precipitation variability, and could face the serious
adverse effects from risks from floods and drought. Climate change is
expected to adversely affect water quality. There is an increased risk
of serious adverse effects from extreme events of flooding and drought.
The severity of risks and impacts may only increase over time with
accumulating greenhouse gas concentrations and associated temperature
increases and precipitation changes.
d. Sea Level Rise and Coastal Areas
    A large percentage of the U.S. population lives in coastal areas,
which are particularly vulnerable to the risks posed by climate change.
The most vulnerable areas are the Atlantic and Gulf Coasts, the Pacific
Islands, and parts of Alaska.
    According to the assessment literature, sea level is rising along
much of the U.S. coast, and the rate of change will very likely
increase in the future, exacerbating the impacts of progressive
inundation, storm-surge flooding, and shoreline erosion. Cities such as
New Orleans, Miami, and New York are particularly at risk, and could
have difficulty coping with the sea level rise projected by the end of
the century under a higher emissions scenario. Population growth and
the rising value of infrastructure increases the vulnerability to
climate variability and future climate change in coastal areas. Adverse
impacts on islands present concerns for Hawaii and the U.S.
territories. Reductions in Arctic sea ice increases extreme coastal
erosion in Alaska, due to the increased exposure of the coastline to
strong wave action. In the Great Lakes, where sea level rise is not a
concern, both extremely high and low water levels resulting from
changes to the hydrological cycle have been damaging and disruptive to
shoreline communities.
    Coastal wetland loss is being observed in the United States where
these ecosystems are squeezed between natural and artificial landward
boundaries and rising sea levels. Up to 21 percent of the remaining
coastal wetlands in the U.S. mid-Atlantic region are potentially at
risk of inundation between 2000 and 2100. Coastal habitats will likely
be increasingly stressed by climate change impacts interacting with
development and pollution.
    Although increases in mean sea level over the 21st century and
beyond will inundate unprotected, low-lying areas, the most devastating
impacts are likely to be associated with storm surge. Superimposed on
expected rates of sea level rise, projected storm intensity, wave
height, and storm surge suggest more severe coastal flooding and
erosion hazards. Higher sea level provides an elevated base for storm
surges to build upon and diminishes the rate at which low-lying areas
drain, thereby increasing the risk of flooding from rainstorms. In New
York City and Long Island, flooding from a combination of sea level
rise and storm surge could be several meters deep. Projections suggest
that the return period of a 100-year flood event in this area might be
reduced to 19-68 years, on average, by the 2050s, and to 4-60 years by
the 2080s. Additionally, some major urban centers in the United States,
such as areas of New Orleans are situated in low-lying flood plains,
presenting increased risk from storm surges.
    The Administrator finds that the most serious risk of adverse
effects is presented by the increased risk of storm surge and flooding
in coastal areas from sea level rise. Current observations of sea level
rise are now contributing to increased risk of storm surge and flooding
in coastal areas, and there is reason to find that these areas are now
endangered by human-induced climate change. The conclusion in the
assessment literature that there is the potential for hurricanes to
become more intense with increasing temperatures (and even some
evidence that Atlantic hurricanes have already become more intense)
reinforces the judgment that coastal communities are now endangered by
human-induced climate change, and may face substantially greater risk
in the future. The Administrator has concluded that even if there is a
low probability of raising the destructive power of hurricanes, this
threat is enough to support a finding that coastal communities are
endangered by greenhouse gas air pollution.
    In addition, coastal areas face other adverse impacts from sea
level rise such as shoreline retreat, erosion, wetland loss and other
effects. The increased risk associated with these adverse impacts also
endangers the welfare of current and future generations, with an
increasing risk of greater adverse impacts in the future.
    Overall, the evidence on risk of adverse impacts for coastal areas
from sea level rise provides clear support for finding that greenhouse
gas air pollution endangers the welfare of current and future
generations.
e. Energy, Infrastructure and Settlements
    The Administrator also considered the impacts of climate change on
energy consumption and production, and on key climate-sensitive aspects
of the nation's infrastructure and settlements.
    For the energy sector, the Administrator finds clear evidence that
temperature increases will change heating and cooling demand, and to
varying degrees across the country; however, under current conditions
it is unclear whether or not net demand will increase or decrease.
While the impacts on net energy demand may be viewed as generally
neutral for purposes of making an endangerment determination, climate
change is expected to call for an increase in electricity production,
especially supply for peak demand. The U.S. energy sector, which relies
heavily on water for cooling capacity and

hydropower, may be adversely impacted by changes to water supply in
reservoirs and other water bodies.
    With respect to infrastructure, climate change vulnerabilities of
industry, settlement and society are mainly related to extreme weather
events rather than to gradual climate change. The significance of
gradual climate change, e.g., increases in the mean temperature, lies
mainly in changes in the intensity and frequency of extreme events.
Extreme weather events could threaten U.S. energy infrastructure
(transmission and distribution), transportation infrastructure (roads,
bridges, airports and seaports), water infrastructure, and other built
aspects of human settlements. Moreover, soil subsidence caused by the
melting of permafrost in the Arctic region is a risk to gas and oil
pipelines, electrical transmission towers, roads, and water systems.
Vulnerabilities for industry, infrastructures, settlements, and society
to climate change are generally greater in certain high-risk locations,
particularly coastal and riverine areas, and areas whose economies are
closely linked with climate-sensitive resources. Additionally,
infrastructures are often connected, meaning that an impact on one can
also affect others.
    A significant fraction of U.S. infrastructure is located in coastal
areas. In these locations, rising sea levels are likely to lead to
direct losses (e.g., equipment damage from flooding) as well as
indirect effects such as the costs associated with raising vulnerable
assets to higher levels. Water infrastructure, including drinking water
and wastewater treatment plants, and sewer and storm water management
systems, may be at greater risk of flooding, sea level rise and storm
surge, low flows, saltwater intrusion, and other factors that could
impair performance and damage costly investments.
    Within settlements experiencing climate change stressors, certain
parts of the population may be especially vulnerable based on their
circumstances. These include the poor, the elderly, the very young,
those already in poor health, the disabled, those living alone, and/or
indigenous populations dependent on one or a few resources. In Alaska,
indigenous communities are likely to experience disruptive impacts,
including shifts in the range or abundance of wild species crucial to
their livelihoods and well-being.
    Overall, the evidence strongly supports the view that climate
change presents risks of serious adverse impacts on public welfare from
the risk to energy production and distribution as well as risks to
infrastructure and settlements.
f. Ecosystems and Wildlife
    The Administrator considered the impacts of climate change on
ecosystems and wildlife and the services they provide. The
Administrator finds clear evidence that climate change is exerting
major influences on natural environments and biodiversity, and these
influences are generally expected to grow with increased warming.
Observed changes in the life cycles of plants and animals include
shifts in habitat ranges, timing of migration patterns, and changes in
reproductive timing and behavior.
    The underlying assessment literature finds with high confidence
that substantial changes in the structure and functioning of
terrestrial ecosystems are very likely to occur with a global warming
greater than 2 to 3 [deg]C above pre-industrial levels, with
predominantly negative consequences for biodiversity and the
provisioning of ecosystem goods and services. With global average
temperature changes above 2 [deg]C, many terrestrial, freshwater, and
marine species (particularly endemic species) are at a far greater risk
of extinction than in the geological past. Climate change and ocean
acidification will likely impair a wide range of planktonic and other
marine calcifiers such as corals. Even without ocean acidification
effects, increases in sea surface temperature of about 1-3 [deg]C are
projected to result in more frequent coral bleaching events and
widespread mortality. In the Arctic, wildlife faces great challenges
from the effects of climatic warming, as projected reductions in sea
ice will drastically shrink marine habitat for polar bears, ice-
inhabiting seals, and other animals.
    Some common forest types are projected to expand, such as oak-
hickory, while others are projected to contract, such as maple-beech-
birch. Still others, such as spruce-fir, are likely to disappear from
the contiguous United States. Changes in plant species composition in
response to climate change can increase ecosystem vulnerability to
other disturbances, including wildfires and biological invasion.
Disturbances such as wildfires and insect outbreaks are increasing in
the United States and are likely to intensify in a warmer future with
warmer winters, drier soils and longer growing seasons. The areal
extent of drought-limited ecosystems is projected to increase 11
percent per [deg]C warming in the United States. In California,
temperature increases greater than 2 [deg]C may lead to conversion of
shrubland into desert and grassland ecosystems and evergreen conifer
forests into mixed deciduous forests. Greater intensity of extreme
events may alter disturbance regimes in coastal ecosystems leading to
changes in diversity and ecosystem functioning. Species inhabiting salt
marshes, mangroves, and coral reefs are likely to be particularly
vulnerable to these effects.
    The Administrator finds that the total scientific record provides
compelling support for finding that the greenhouse gas air pollution
leads to predominantly negative consequences for biodiversity and the
provisioning of ecosystem goods and services for ecosystems and
wildlife important for public welfare in the U.S., both for current and
future generations. The severity of risks and impacts may only increase
over time with accumulating greenhouse gas concentrations and
associated temperature increases and precipitation changes.
g. Summary of the Administrator's Finding of Endangerment to Public
Welfare
    The Administrator has considered how elevated concentrations of the
well-mixed greenhouse gases and associated climate change affect public
welfare by evaluating numerous and far-ranging risks to food production
and agriculture, forestry, water resources, sea level rise and coastal
areas, energy, infrastructure, and settlements, and ecosystems and
wildlife. For each of these sectors, the evidence provides support for
a finding of endangerment to public welfare. The evidence concerning
adverse impacts in the areas of water resources and sea level rise and
coastal areas provide the clearest and strongest support for an
endangerment finding, both for current and future generations. Strong
support is also found in the evidence concerning infrastructure and
settlements, as well ecosystems and wildlife. Across the sectors, the
potential serious adverse impacts of extreme events, such as wildfires,
flooding, drought, and extreme weather conditions provide strong
support for such a finding.
    Water resources across large areas of the country are at serious
risk from climate change, with effects on water supplies, water
quality, and adverse effects from extreme events such as floods and
droughts. Even areas of the country where an increase in water flow is
projected could face water resource problems from the supply and water
quality problems associated with temperature increases and
precipitation variability, and could face the increased risk of serious
adverse effects from extreme events, such as floods and

drought. The severity of risks and impacts is likely to increase over
time with accumulating greenhouse gas concentrations and associated
temperature increases and precipitation changes.
    Overall, the evidence on risk of adverse impacts for coastal areas
provides clear support for a finding that greenhouse gas air pollution
endangers the welfare of current and future generations. The most
serious potential adverse effects are the increased risk of storm surge
and flooding in coastal areas from sea level rise and more intense
storms. Observed sea level rise is already increasing the risk of storm
surge and flooding in some coastal areas. The conclusion in the
assessment literature that there is the potential for hurricanes to
become more intense (and even some evidence that Atlantic hurricanes
have already become more intense) reinforces the judgment that coastal
communities are now endangered by human-induced climate change, and may
face substantially greater risk in the future. Even if there is a low
probability of increasing the destructive power of hurricanes, this
threat is enough to support a finding that coastal communities are
endangered by greenhouse gas air pollution. In addition, coastal areas
face other adverse impacts from sea level rise such as land loss due to
inundation, erosion, wetland submergence, and habitat loss. The
increased risk associated with these adverse impacts also endangers
public welfare, with an increasing risk of greater adverse impacts in
the future.
    Strong support for an endangerment finding is also found in the
evidence concerning energy, infrastructure, and settlements, as well
ecosystems and wildlife. While the impacts on net energy demand may be
viewed as generally neutral for purposes of making an endangerment
determination, climate change is expected to result in an increase in
electricity production, especially to meet peak demand. This increase
may be exacerbated by the potential for adverse impacts from climate
change on hydropower resources as well as the potential risk of serious
adverse effects on energy infrastructure from extreme events. Changes
in extreme weather events threaten energy, transportation, and water
resource infrastructure. Vulnerabilities of industry, infrastructure,
and settlements to climate change are generally greater in high-risk
locations, particularly coastal and riverine areas, and areas whose
economies are closely linked with climate-sensitive resources. Climate
change will likely interact with and possibly exacerbate ongoing
environmental change and environmental pressures in settlements,
particularly in Alaska where indigenous communities are facing major
environmental and cultural impacts on their historic lifestyles. Over
the 21st century, changes in climate will cause some species to shift
north and to higher elevations and fundamentally rearrange U.S.
ecosystems. Differential capacities for range shifts and constraints
from development, habitat fragmentation, invasive species, and broken
ecological connections will likely alter ecosystem structure, function,
and services, leading to predominantly negative consequences for
biodiversity and the provision of ecosystem goods and services.
    With respect to food production and agriculture, there is a
potential for a net benefit in the near term for certain crops, but
there is significant uncertainty about whether this benefit will be
achieved given the various potential adverse impacts of climate change
on crop yield, such as the increasing risk of extreme weather events.
Other aspects of this sector may be adversely affected by climate
change, including livestock management and irrigation requirements, and
there is a risk of adverse effect on a large segment of the total crop
market. For the near term, the concern over the potential for adverse
effects in certain parts of the agriculture sector appears generally
comparable to the potential for benefits for certain crops. However,
the body of evidence points towards increasing risk of net adverse
impacts on U.S. food production and agriculture over time, with the
potential for significant disruptions and crop failure in the future.
    For the near term, the Administrator finds the beneficial impact on
forest growth and productivity in certain parts of the country from
elevated carbon dioxide concentrations and temperature increases to
date is offset by the clear risk from the observed increases in
wildfires, combined with risks from the spread of destructive pests and
disease. For the longer term, the risk from adverse effects increases
over time, such that overall climate change presents serious adverse
risks for forest productivity. There is compelling reason to find that
the support for a positive endangerment finding increases as one
considers expected future conditions where temperatures continue to
rise.
    Looking across all of the sectors discussed above, the evidence
provides compelling support for finding that greenhouse gas air
pollution endangers the public welfare of both current and future
generations. The risk and the severity of adverse impacts on public
welfare are expected to increase over time.
h. Impacts in Other World Regions That Can Affect the U.S Population
    While the finding of endangerment to public health and welfare
discussed above is based on impacts in the United States, the
Administrator also considered how human-induced climate change in other
regions of the world may in turn affect public welfare in the United
States. According to the USGCRP report of June 2009 and other sources,
climate change impacts in certain regions of the world may exacerbate
problems that raise humanitarian, trade, and national security issues
for the United States.\32\ The IPCC identifies the most vulnerable
world regions as the Arctic, because of the effects of high rates of
projected warming on natural systems; Africa, especially the sub-
Saharan region, because of current low adaptive capacity as well as
climate change; small islands, due to high exposure of population and
infrastructure to risk of sea-level rise and increased storm surge; and
Asian mega-deltas, such as the Ganges-Brahmaputra and the Zhujiang, due
to large populations and high exposure to sea level rise, storm surge,
and river flooding. Climate change has been described as a potential
threat multiplier with regard to national security issues.
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    \32\ ``In an increasingly interdependent world, U.S.
vulnerability to climate change is linked to the fates of other
nations. For example, conflicts or mass migrations of people
resulting from food scarcity and other resource limits, health
impacts or environmental stresses in other parts of the world could
threaten U.S. national security.'' (Karl et al., 2009).
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    The Administrator acknowledges these kinds of risks do not readily
lend themselves to precise analyses or future projections. However,
given the unavoidable global nature of the climate change problem, it
is appropriate and prudent to consider how impacts in other world
regions may present risks to the U.S. population. Because human-induced
climate change has the potential to aggravate natural resource, trade,
and humanitarian issues in other world regions, which in turn may
contribute to the endangerment of public welfare in the United States,
this provides additional support for the Administrator's finding that
the greenhouse gas air pollution is reasonably anticipated to endanger
the public welfare of current and future

generations of the United States population.
i. Summary of Key Public Comments on Endangerment to Public Welfare
    Several public commenters point out the anticipated benefits that
increasing carbon dioxide levels and temperatures will have on
agricultural crops. In addition, commenters note how U.S. agricultural
productivity, in particular, has been steadily rising over the last 100
years. Responses to major comments are found here and more detailed
responses are found in the Response to Comments document.
    The Administrator acknowledges that plants including agricultural
crops respond to carbon dioxide positively based on numerous well-
documented studies. However, previous assessments of food production
and agriculture have been modified to highlight increasing
vulnerability, stress, and adverse impacts from climate change over
time, based on improvements in the understanding of plant physiology,
concern over impacts on plant pests and pathogens, and the implications
of changes in average temperatures for temperature extremes and for
changes in the patterns of precipitation and evaporation. While it is
still the case today and for the next few years that climate change
benefits agriculture in some places and harms them in others, the
Administrator considers that the far larger temperature increases
expected over coming decades and beyond on the ``business as usual''
trajectory will put significant stresses on agriculture and land
resources in all regions of the United States. The Administrator
prudently considers increased climate variability associated with a
warming climate, which may overwhelm the positive plant responses from
elevated carbon dioxide over time. Further, the effects of climate
change on weeds, insect pests, and pathogens are recognized as key
factors in determining plant damage in future decades. The
Administrator also notes that scientific literature clearly supports
the finding that drought frequency and severity are projected to
increase in the future over much of the United States, which will
likely reduce crop yields because of excesses or deficits of water.
Vulnerability to extended drought, according to IPCC, has been
documented as already increasing across North America. Further, based
on review of the assessment literature, the Administrator considers
multiple stresses, such as limited availability of water resources,
loss of biodiversity, and air pollution, which are likely to increase
sensitivity and reduce resilience in the agricultural sector to climate
change over time.
    Similar to food production and agriculture, public commenters often
noted that forest productivity is projected to increase in the coming
years due to the direct stimulatory effect of carbon dioxide on plant
growth combined with warmer temperatures and thus extended growing
seasons. The Administrator notes this phenomenon has been well
documented by numerous studies but recognizes that increased
productivity will be associated with significant variation at local and
regional scales. The Administrator considers that climate strongly
influences forest productivity and composition, and the frequency and
magnitude of disturbances that impact forests. Based on the most recent
IPCC assessment of the scientific literature, several recent studies
confirm previous findings that temperature and precipitation changes in
future decades will modify, and often limit, direct carbon dioxide
effects on plants. For example, increased temperatures may reduce
carbon dioxide effects indirectly, by increasing water demand. The
Administrator also considers that new research more firmly establishes
the negative impacts of increased climate variability. Projected
changes in the frequency and severity of extreme climate events have
significant consequences for forestry production and amplify existing
stresses to land resources in the future.
    Several public commenters maintain that wildfires are primarily the
result of natural climatic factors and not climate change and dispute
that they are or will increase in the future. The Administrator notes
the scientific literature and assessment reports provide several lines
of evidence that suggest wildfires will likely increase in frequency
over the next several decades because of climate warming. Wildfires and
droughts, among other extreme events (e.g., hurricanes) that cause
forest damage, pose the largest threats over time to forest ecosystems.
The assessment literature suggests that large, stand-replacing
wildfires will likely increase in frequency over the next several
decades because of climate warming and general climate warming
encourages wildfires by extending the summer period that dries fuels,
promoting easier ignition and faster spread. Furthermore, current
climate modeling studies suggest that increased temperatures and longer
growing seasons will elevate wildfire risk in connection with increased
aridity.

V. The Administrator's Finding That Emissions of Greenhouse Gases From
CAA Section 202(a) Sources Cause or Contribute to the Endangerment of
Public Health and Welfare

    As discussed in Section IV.A of these Findings, the Administrator
is defining the air pollution for purposes of the endangerment finding
to be the elevated concentration of well-mixed greenhouse gases in the
atmosphere. The second step of the two-part endangerment test is for
the Administrator to determine whether the emission of any air
pollutant emitted from new motor vehicles cause or contribute to this
air pollution. This is referred to as the cause or contribute finding,
and is the second finding by the Administrator in this action.
    Section V.A of these Findings describes the Administrator's
definition and scope of the air pollutant ``well-mixed greenhouse
gases.'' Section V.B of these Findings puts forth the Administrator's
finding that emissions of well-mixed greenhouse gases from new motor
vehicles contribute to the air pollution which is reasonably
anticipated to endanger public health and welfare. Section V.C of these
Findings provides responses to some of the key comments on these
issues. See Response to Comments document Volume 10 for responses to
other significant comments on the cause or contribute finding. More
detailed emissions data summarized in the discussion below can be found
in Appendix B of the TSD.

A. The Administrator's Definition of the ``Air Pollutant''

    As discussed in the Proposed Findings, to help appreciate the
distinction between air pollution and air pollutant, the air pollution
can be thought of as the total, cumulative stock in the atmosphere,
while the air pollutant, can be thought of as the flow that changes the
size of the total stock. Given this relationship, it is not surprising
that the Administrator is defining the air pollutant similar to the air
pollution; while the air pollution is the concentration (e.g., stock)
of the well-mixed greenhouse gases in the atmosphere, the air pollutant
is the same combined grouping of the well-mixed greenhouse gases, the
emissions of which are analyzed for contribution (e.g., the flow into
the stock).
    Thus, the Administrator is defining the air pollutant as the
aggregate group of the same six long-lived and directly-emitted
greenhouse gases: Carbon dioxide, methane, nitrous oxide,
hydrofluorocarbons, perfluorocarbons,

and sulfur hexafluoride. As noted above, this definition of a single
air pollutant made up of these well-mixed greenhouse gases is similar
to definitions of other air pollutants that are comprised of substances
that share common attributes with similar effects on public health or
welfare (e.g., particulate matter and volatile organic compounds).
    The common attributes shared by these six greenhouse gases are
discussed in detail in Section IV.A of these Findings, where the
Administrator defined the ``air pollution'' for purposes of the
endangerment finding. These same common attributes support the
Administrator grouping these six greenhouse gases for purposes of
defining a single air pollutant as well. These attributes include the
fact that they are all greenhouse gases that are directly emitted
(i.e., they are not formed through secondary processes in the
atmosphere from precursor emissions); they are sufficiently long-lived
in the atmosphere such that, once emitted, concentrations of each gas
become well mixed throughout the entire global atmosphere; and they
exert a climate warming effect by trapping outgoing, infrared heat that
would otherwise escape to space. Moreover, the radiative forcing effect
of these six greenhouse gases is well understood.
    Furthermore, these six greenhouse gases are currently the common
focus of climate science and policy. For example, the UNFCCC, signed
and ratified by the U.S. in 1992, requires its signatories to
``develop, periodically update, publish and make available * * *
national inventories of anthropogenic emissions by sources and removals
by sinks of all greenhouse gases not controlled by the Montreal
Protocol \33\, using comparable methodologies * * * '' \34\ To date,
the focus of UNFCCC actions and discussions has been on the six
greenhouse gases that are the same focus of these findings. As a Party
to the UNFCCC, EPA annually submits the Inventory of U.S. Greenhouse
Gas Emissions and Sinks to the Convention, which reports on national
emissions of anthropogenic emissions of the well-mixed greenhouse
gases. International discussions about a post-Kyoto agreement also
focus on the well-mixed greenhouse gases.
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    \33\ The Montreal Protocol covers ozone-depleting substances
which may also share physical attributes of the six key greenhouse
gases in this action, but they do not share other attributes such as
being the focus of climate science and policy. See section * * *.
    \34\ UNFCCC Art. 4.1(b).
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    As noted above, grouping of many substances with common attributes
as a single pollutant is common practice under the CAA. Thus, doing so
here is not novel. Indeed CAA section 302(g) defines air pollutant as
``any air pollutant agent or combination of such agents, * * * '' CAA
Sec.  302(g) (emphasis added). Thus, it is clear that the term ``air
pollutant'' is not limited to individual chemical compounds. In
determining that greenhouse gases are within the scope of this
definition, the Supreme Court described section 302(g) as a
``sweeping'' and ``capacious'' definition that unambiguously included
greenhouse gases, that are ``unquestionably `agents' of air
pollution.'' Massachusetts v. EPA, 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. Congress used the term ``any'', and did not qualify
the kind of combinations that the agency could define as a single air
pollutant. Congress provided EPA broad discretion to determine
appropriate combinations of compounds that should be treated as a singe
air pollutant.\35\
---------------------------------------------------------------------------

    \35\ Indeed, the greenhouse gases hydrofluorocarbons and
perfluorocarbons each are already a combination of multiple
compounds.
---------------------------------------------------------------------------

    For the same reasons discussed in Section IV.A above, at this time,
only carbon dioxide, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride share all of these common
attributes and thus they are the only substances that the Administrator
finds to meet the definition of ``well-mixed greenhouse gas'' at this
time.\36\ Also as noted above, if in the future other substances are
shown to meet the same criteria they may be added to the definition of
this single air pollutant.
---------------------------------------------------------------------------

    \36\ The term ``well-mixed greenhouse gases'' is based on one of
the shared attributes discussed above--these greenhouse gases are
sufficiently long-lived in the atmosphere such that, once emitted,
concentrations of each gas become well mixed throughout the entire
global atmosphere. Defining the air pollutant to be the combination
of these six well-mixed greenhouse gases is based in part on this
attribute--after the gases are emitted, they are sufficiently long-
lived in the atmosphere to become well mixed as part of the air
pollution.
---------------------------------------------------------------------------

    The Administrator is aware that CAA section 202(a) source
categories do not emit all of the substances meeting the definition of
well-mixed greenhouse gases. But that does not change the fact that all
of these greenhouse gases share the attributes that make grouping them
as a single air pollutant reasonable. As discussed further below, the
reasonableness of this grouping does not turn on the particular source
category being evaluated in a contribution finding.

B. The Administrator's Finding Regarding Whether Emissions of the Air
Pollutant From Section 202(a) Source Categories Cause or Contribute to
the Air Pollution That May Be Reasonably Anticipated To Endanger Public
Health and Welfare

    The Administrator finds that emissions of the well-mixed greenhouse
gases from new motor vehicles contribute to the air pollution that may
reasonably be anticipated to endanger public health and welfare. This
contribution finding is for all of the CAA section 202(a) source
categories and the Administrator considered emissions from all of these
source categories. The relevant mobile sources under CAA section 202
(a)(1) are ``any class or classes of new motor vehicles or new motor
vehicle engines, * * *.'' CAA section 202(a)(1) (emphasis added). The
new motor vehicles and new motor vehicle engines (hereinafter ``CAA
section 202(a) source categories'') addressed are: Passenger cars,
light-duty trucks, motorcycles, buses, and medium and heavy-duty
trucks. Detailed combined greenhouse gas emissions data for CAA section
202(a) source categories are presented in Appendix B of the TSD.\37\
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    \37\ For section 202(a) source categories, only the
hydrofluorocarbon emissions related to passenger compartment cooling
are included. Emissions from refrigeration units that may be
attached to trucks are considered emissions from nonroad engines
under CAA section 213.
---------------------------------------------------------------------------

    The Administrator reached her decision after reviewing emissions
data on the contribution of CAA section 202(a) source categories
relative to both global greenhouse gas emissions and U.S. greenhouse
gas emissions. Given that CAA section 202(a) source categories are
responsible for about 4 percent of total global greenhouse gas
emissions, and for just over 23 percent of total U.S. greenhouse gas
emissions, the Administrator finds that both of these comparisons,
independently and together, support a finding that CAA section 202(a)
source categories contribute to the air pollution that may be
reasonably anticipated to endanger public health and welfare. The
Administrator is not placing primary weight on either approach; rather
she finds that both approaches clearly establish that emissions of the
well-mixed greenhouse gases from section 202(a) source categories
contribute to air pollution with may reasonably be anticipated to
endanger public health and welfare. As the Supreme Court noted,
``[j]udged by any standard, U.S.

motor-vehicle emissions make a meaningful contribution to greenhouse
gas concentrations and hence, * * * to global warming.'' Massachusetts
v. EPA, 549 U.S. at 525.\38\
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    \38\ Because the Administrator is defining the air pollutant as
the combination of well-mixed greenhouse gases, she is not issuing a
final contribution finding based on the alternative definition
discussed in the proposed findings (e.g., each greenhouse gas as an
individual air pollutant).
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1. Administrator's Approach in Making This Finding
    Section 202(a) of the CAA source categories consist of passenger
cars, light-duty trucks, motorcycles, buses, and heavy- and medium-duty
trucks. As noted in the Proposed Findings, in the past the requisite
contribution findings have been proposed concurrently with proposing
emission standards for the relevant mobile source category. Thus, prior
contribution findings often focused on a subset of the CAA section
202(a) (or other section) source categories. This final cause or
contribute finding, however, is for all of the CAA section 202(a)
source categories. The Administrator is considering emissions from all
of these source categories in the determination.
    Section 202(a) source categories emit the following well-mixed
greenhouse gases: carbon dioxide, methane, nitrous oxide, and
hydrofluorocarbons. As the basis for the Administrator's determination,
EPA analyzed historical data of emissions of the well-mixed greenhouse
gases for motor vehicles and motor vehicle engines in the United States
from 1990 to 2007.
    The Proposed Findings discussed a number of possible ways of
assessing cause or contribute and the point was made that no single
approach is required by the statute or has been used exclusively in
previous determinations under the CAA. The Administrator also discussed
how, consistent with prior cause or contribute findings and the
science, she is using emissions as a proxy for contributions to
atmospheric concentrations. This approach is reasonable for the well-
mixed greenhouse gases, because cumulative emissions are responsible
for the cumulative change in the concentrations in the atmosphere.
Similarly, annual emissions are a perfectly reasonable proxy for annual
incremental changes in atmospheric concentrations.
    In making a judgment about the contribution of emissions from CAA
section 202(a) source categories, the Administrator focused on making a
reasoned overall comparison of emissions from the CAA section 202(a)
source categories to emissions from other sources of greenhouse gases.
This allows a determination of how the CAA section 202(a) source
categories compare to all of the other sources that together as a group
make up the total emissions contributors to the air pollution problem.
The relative importance of the CAA section 202(a) source categories is
central to making the contribution determination. Both the magnitude of
these emissions and the comparison of these emissions to other sources
provide the basis to determine whether the CAA section 202(a) source
categories may reasonably be judged as contributing to the air
pollution problem.
    In many cases EPA makes this kind of comparison of source
categories by a simple percentage calculation that compares the
emissions from the source category at issue to a larger total group of
emissions. Depending on the circumstances, a larger percentage often
means a greater relative impact from that source category compared to
the other sources that make up the total of emissions, and vice versa.
However, the actual numerical percentages may have little meaning when
viewed in isolation. The context of the comparison is needed to ensure
the information is useful in evaluating the relative impact of one
source compared to others. For example, the number of sources involved
and the distribution of emissions across all of the sources can make a
significant difference when evaluating the results of a percentage
calculation. In some cases a certain percentage might mean almost all
other sources are larger or much larger than the source at issue, while
in other circumstances the same percentage could mean that the source
at issue is in fact one of the larger contributors to the total.
    The Administrator therefore considered the totality of the
circumstances in order to best understand the role played by CAA
section 202(a) source categories. This is consistent with Congress'
intention for EPA to consider the cumulative impact of all sources of
pollution. In that context, the global nature of the air pollution
problem and the breadth of countries and sources emitting greenhouse
gases means that no single country and no single source category
dominate or are even close to dominating on a global scale. For
example, the United States as a country is the second largest emitter
of greenhouse gases, and emits approximately 18 percent of the world's
total greenhouse gases. The total emissions of greenhouse gases
worldwide are from numerous sources and countries, with each country
and each source category contributing a relatively small percentage of
the total emissions. That means that the relative ranking of countries
or sources is not at all obvious from the magnitude of the percentage
by itself. A country or a source may be a large contributor, in
comparison to other countries or sources, even though its percentage
contribution may appear relatively small.
    In this situation, addressing a global air pollution problem may
call for many different sources and countries to address emissions even
if none by itself dominates or comes close to dominating the global
inventory. A somewhat analogous situation can be found in the ozone air
pollution problem in the United States. Emissions of NOx and volatile
organic compounds (VOCs) often come from numerous small sources, as
well as certain large source categories. We have learned that
successful ozone control strategies often need to take this into
account, and address both the larger sources of NOx and VOCs as well as
the many smaller sources, given the breadth of sources that as a group
lead to the total inventory of VOCs and NOx.
    The global aspects of the greenhouse gas air pollution problem
amplify this kind of situation many times over, where no single country
or source category dominates or comes close to dominating the global
inventory of greenhouse gas emissions. These unique, global aspects of
the climate change problem tend to support consideration of
contribution at lower percentage levels of emissions than might
otherwise be considered appropriate when addressing a more typical
local or regional air pollution problem. In this situation it is quite
reasonable to consider emissions from source categories that are more
important in relation to other sources, even if their absolute
contribution initially may appear to be small.
    In addition, the Administrator is aware of the fact that the United
States is the second largest emitter of well-mixed greenhouse gases in
the world. As the United States evaluates how to address climate
change, the Administrator will analyze the various sources of emissions
and the source's share of U.S. emissions. Thus, when analyzing whether
a source category that emits well-mixed greenhouse gases in the United
States contributes to the global problem, it is appropriate for the
Administrator to consider how that source category fits into the larger
picture of U.S. emissions. This ranking process within the United
States allows the importance of the source category to

be seen compared to other U.S. sources, informing the judgment of the
importance of emissions from this source category in any overall
national strategy to address greenhouse gas emissions.
    It is in this broader context that EPA considered the contribution
of CAA section 202(a) sources. This provides useful information in
determining the importance that should be attached to the emissions
from the CAA section 202(a) sources.
    In reaching her determination, the Administrator used two simple
and straightforward comparisons to assess cause or contribute for CAA
section 202(a) source categories: (1) As a share of total current
global aggregate emissions of the well-mixed greenhouse gases; and (2)
as a share of total current U.S. aggregate emissions of the well-mixed
greenhouse gases.
    Total well-mixed greenhouse gas emissions from CAA section 202(a)
source categories were compared to total global emissions of the well-
mixed greenhouse gases. The total air pollution problem, as already
discussed, is the elevated and climbing levels of the six greenhouse
gas concentrations in the atmosphere, which are global in nature
because these concentrations are globally well mixed (whether they are
emitted from CAA section 202(a) source categories or any other source
within or outside the United States). In addition, comparisons were
also made to U.S. total well-mixed greenhouse gases emissions to
appreciate how CAA section 202(a) source categories fit into the larger
U.S. contribution to the global problem. It is typical for the
Administrator to consider these kinds of comparisons of emissions of a
pollutant in evaluating contribution to air pollution, such as the
concentrations of that same pollutant in the atmosphere (e.g., the
Administrator analyzes PM2.5 emissions to determine if a
source category contributes to PM2.5 air pollution). When
viewed in the circumstances discussed above, both of these comparisons
provide useful information in determining whether these source
categories should be judged as contributing to the total air pollution
problem.
a. Section 202(a) of the CAA--Share of Global Aggregate Emissions of
the Well-Mixed Greenhouse Gases
    Global emissions of well-mixed greenhouse gases have been
increasing, and are projected to continue increasing unless the major
emitters take action to reduce emissions. Total global emissions of
well-mixed greenhouse gases in 2005 (the most recent year for which
data for all countries and all greenhouse gases are available) \39\
were 38,726 teragrams of CO2-equivlant (TgCO2eq.)
\40\ This represents an increase in global greenhouse gas emissions of
about 26 percent since 1990 (excluding land use, land use change and
forestry). In 2005, total U.S. emissions of well-mixed greenhouse gases
were responsible for 18 percent of global emissions, ranking only
behind China, which was responsible for 19 percent of global emissions
of well-mixed greenhouse gases.
---------------------------------------------------------------------------

    \39\ The source of global greenhouse gas emissions data, against
which comparisons are made, is the Climate Analysis Indicators Tool
of the World Resources Institute (WRI) (2007). Note that for global
comparisons, all emissions are from the year 2005, the most recent
year for which data for all greenhouse gas emissions and all
countries are available. WRI (2007) Climate Analysis Indicators Tool
(CAIT). Available at http://cait.wri.org. Accessed August 5, 2009.
    \40\ One teragram (Tg) = 1 million metric tons. 1 metric ton =
1,000 kg = 1.102 short tons = 2,205 lbs. Long-lived greenhouse gases
are compared and summed together on a CO2 equivalent
basis by multiplying each gas by its Global Warming Potential
(GWPs), as estimated by IPCC. In accordance with UNFCCC reporting
procedures, the U.S. quantifies greenhouse gas emissions using the
100-year time frame values for GWPs established in the IPCC Second
Assessment Report.
---------------------------------------------------------------------------

    In 2005 emissions of the well-mixed greenhouse gas pollutant from
CAA section 202(a) source categories represented 4.3 percent of total
global well-mixed greenhouse gas emissions and 28 percent of global
transport well-mixed greenhouse gas emissions (Table 1 of these
Findings). If CAA section 202(a) source categories' emissions of well-
mixed greenhouse gas were ranked against total well-mixed greenhouse
gas emissions for entire countries, CAA section 202(a) source category
emissions would rank behind only China, the United States as a whole,
Russia, and India, and would rank ahead of Japan, Brazil, Germany and
every other country in the world. Indeed, countries with lower
emissions than the CAA section 202(a) source categories are members of
the 17 ``major economies'' ``that meet to advance the exploration of
concrete initiatives and joint ventures that increase the supply of
clean energy while cutting greenhouse gas emissions.'' See http://
www.state.gov/g/oes/climate/mem/. It would be anomalous, to say the
least, to consider Japan and these other countries as major players in
the global climate change community and an integral part of the
solution, but not find that CAA section 202(a) source category
emissions contribute to the global problem. Thus, the Administrator
finds that emission of well-mixed greenhouse gases from CAA section
202(a) source categories contribute to the air pollution of well-mixed
greenhouse gases.

 Table 1--Comparison to Global Greenhouse Gas (GHG) Emissions (Tg CO2e)
------------------------------------------------------------------------
                                                        Sec 202(a) share
                                           2005            (percent)
------------------------------------------------------------------------
All U.S. GHG emissions............              7,109               23.5
Global transport GHG emissions....              5,968               28.0
All global GHG emissions..........             38,726                4.3
------------------------------------------------------------------------

b. Section 202(a) of the CAA--Share of U.S. Aggregate Emissions of the
Well-Mixed Greenhouse Gases
    The Administrator considered compared total emissions of the well-
mixed greenhouse gases from CAA section 202(a) source categories to
total U.S. emissions of the well-mixed greenhouse gases as an
indication of the role these sources play in the total U.S.
contribution to the air pollution problem causing climate change.\41\
---------------------------------------------------------------------------

    \41\ Greenhouse gas emissions data for the United States in this
section have been updated since the Proposed Findings to reflect
EPA's most up-to-date information, which includes data for the year
2007. The source of the U.S. greenhouse gas emissions data is the
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007,
published in 2009 (hereinafter ``U.S. Inventory'').
---------------------------------------------------------------------------

    In 2007, U.S. well-mixed greenhouse gas emissions were 7,150
TgCO2eq. The dominant gas emitted was carbon dioxide, mostly
from fossil fuel combustion. Methane was the second largest well-mixed
greenhouse gas, followed by N2O, and the fluorinated gases
(HFCs, PFCs, and SF6). Electricity generation was the
largest emitting sector (2,445 TgCO2eq or 34 percent of

total U.S. greenhouse gas emissions), followed by transportation (1,995
TgCO2eq or 28 percent) and industry (1,386
TgCO2eq or 19 percent). Emissions from the CAA section
202(a) source categories constitute the major part of the
transportation sector. Land use, land use change, and forestry offset
almost 15 percent of total U.S. emissions through net sequestration.
Total U.S. well-mixed greenhouse gas emissions have increased by over
17 percent between 1990 and 2007. The electricity generation and
transportation sectors have contributed the most to this increase.
    In 2007 emissions of well-mixed greenhouse gases from CAA section
202(a) source categories collectively were the second largest emitter
of well-mixed greenhouse gases within the United States (behind the
electricity generating sector), emitting 1,663 TgCO2eq and
representing 23 percent of total U.S. emissions of well-mixed
greenhouse gases (Table 2 of these Findings). The Administrator is
keenly aware that the United States is the second largest emitter of
well-mixed greenhouse gases. Part of analyzing whether a sector within
the United States contributes to the global problem is to see how those
emissions fit into the contribution from the United States as a whole.
This informs her judgment as to the importance of emissions from this
source category in any overall national strategy to address greenhouse
gas emissions. Thus, it is relevant that CAA section 202(a) source
categories are the second largest emitter of well-mixed greenhouse
gases in the country. This is part of the Administrator looking at the
totality of the circumstances. Based on this the Administrator finds
that emission of well-mixed greenhouse gases from CAA section 202(a)
source categories contribute to the air pollution of well-mixed
greenhouse gases.

               Table 2--Sectoral Comparison to Total U.S. Greenhouse Gas (GHG) Emissions (Tg CO2e)
----------------------------------------------------------------------------------------------------------------
          U.S. emissions                1990         1995         2000         2005         2006         2007
----------------------------------------------------------------------------------------------------------------
Section 202(a) GHG emissions......       1231.9       1364.4       1568.1       1670.5       1665.7       1663.1
Share of U.S. (%).................        20.2%        21.1%        22.4%        23.5%        23.6%        23.3%
Electricity Sector emissions......       1859.1       1989.0       2329.3       2429.4       2375.5       2445.1
Share of U.S. (%).................        30.5%        30.8%        33.2%        34.2%        33.7%        34.2%
Industrial Sector emissions.......       1496.0       1524.5       1467.5       1364.9       1388.4       1386.3
Share of U.S. (%).................        24.5%        23.6%        20.9%        19.2%        19.7%        19.4%
                                   -----------------------------------------------------------------------------
    Total U.S. GHG emissions......       6098.7       6463.3       7008.2       7108.6       7051.1       7150.1
----------------------------------------------------------------------------------------------------------------

C. Response to Key Comments on the Administrator's Cause or Contribute
Finding

    EPA received numerous public comments regarding the Administrator's
proposed cause or contribute finding. Below is a brief discussion of
some of the key comments. Responses to comments on this issue are also
contained in the Response to Comments document, Volume 10.
1. The Administrator Reasonably Defined the ``Air Pollutant'' for the
Cause or Contribute Analysis
a. The Supreme Court Held that Greenhouse Gases Fit Within the
Definition of ``Air Pollutant'' in the CAA
    Several commenters reiterate arguments already rejected by the
Supreme Court, arguing that greenhouse gases do not fit into the
definition of ``air pollutant'' under the CAA. In particular, at least
one commenter contends that EPA must show how greenhouse gases impact
or materially change ``ambient air'' when defining air pollutant and
making the endangerment finding. This commenter argues that because
carbon dioxide is a naturally occurring and necessary element in the
atmosphere, it cannot be considered to materially change air.
    These and similar arguments were already rejected by the Supreme
Court in Massachusetts v. EPA, 549 U.S. 497 (2007). Briefs before the
Supreme Court also argued that carbon dioxide is an essential role for
life on earth and therefore cannot be considered an air pollutant, and
that the concentrations of greenhouse gases that are a potential
problem are not in the ``ambient air'' that people breathe.
    The Court rejected all of these and other arguments, noting that
the statutory text forecloses these arguments. ``The Clean Air Act's
sweeping definition of `air pollutant' includes `any air pollution
agent or combination of such agents, including any physical, chemical *
* * substance or matter which is emitted into or otherwise enters the
ambient air . * * *' Sec.  7602(g) (emphasis added). On its face, the
definition embraces all airborne compounds of whatever stripe, and
underscores that intent through the repeated use of the word `any.'
Carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons are
without a doubt `physical [and] chemical * * * substance[s] which [are]
emitted into * * * the ambient air.' The statute is unambiguous.''
    547 U.S. at 529-30 (footnotes omitted); see also id. at 530, n26
(the distinction regarding ambient air, however, finds no support in
the text of the statute, which uses the phrase ``the ambient air''
without distinguishing between atmospheric layer.). Thus, the question
of whether greenhouse gases fit within the definition of air pollutant
under the CAA has been decided by the Supreme Court and is not being
revisited here.
b. The Definition of Air Pollutant May Include Substances Not Emitted
by CAA Section 202(a) Sources
    Many commenters argue that the definition of ``air pollutant''--
here well-mixed greenhouse gases--cannot include PFCs and SF6 because
they are not emitted by CAA section 202(a) motor vehicles and hence,
cannot be part of any ``air pollutant'' emitted by such sources. They
argue that by improperly defining ``air pollutant'' to include
substances that are not present in motor vehicle emissions, the Agency
has exceeded its statutory authority under CAA section 202(a).
Commenters contend that past endangerment findings under CAA section
202(a) demonstrate EPA's consistent approach of defining ``air
pollutant(s)'' in accordance with the CAA's clear direction, to include
only those pollutants 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.'' (65 FR 35436,
June 2, 2000). Commenters argue that EPA itself is inconsistent in the
Proposed Findings, sometimes referring

to ``air pollutant'' as the group of six greenhouse gases, and other
times falling back on the four greenhouse gases emitted by motor
vehicles.
    EPA acknowledges that the Proposed Findings could have been clearer
regarding the proposed definition of air pollutant, and how it was
being applied to CAA section 202(a) sources, which emit only four of
the six substances that meet the definition of well-mixed greenhouse
gases. However, our interpretation does not exceed EPA's authority
under CAA section 202(a). It is reasonable to define the air pollutant
under CAA section 202(a) to include substances that have similar
attributes (as discussed above), even if not all of the substances that
meet that definition are emitted by motor vehicles. For example, as
commenters note, EPA has heavy duty truck standards applicable to VOCs
and PM, but it is highly unlikely that heavy duty trucks emit every
substance that is included in the group defined as VOC or PM. See 40
CFR 51.100(s) (defining volatile organic compound (VOC) as ``any
compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic
acid, metallic carbides or carbonates, and ammonium carbonate, which
participates in atmospheric photochemical reactions'', a list of
exemptions are also included in the definition); 40 CFR 51.100(oo)
(defining particulate matter (PM) as ``any airborne finely divided
solid or liquid material with an aerodynamic diameter smaller than 100
micrometers'').
    In this circumstance the number of substances included in the
definition of well-mixed greenhouse gases is much smaller than other
``group'' air pollutants (e.g., six greenhouse gases versus hundreds of
VOCs), and CAA section 202(a) sources emit an easily discernible number
of these six substances. However, this does not mean that the
definition of the well-mixed greenhouse gases as the air pollutant is
unreasonable. By defining well-mixed greenhouse gases as a single air
pollutant comprised of six substances with common attributes, the
Administrator is giving effect to these shared attributes and how they
are relevant to the air pollution to which they contribute. The fact
that these six substances 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., carbon dioxide, CH4, 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 N2O, PFCs, HFCs, SF6
and other fluorinated compounds, but not carbon dioxide or
CH4 unless there is on-site fuel combustion. In other words,
it is not necessarily the source category being evaluated for
contribution that determines the reasonableness of defining a group air
pollutant based on the shared attributes of the group.
    Even if EPA agreed with commenters, and defined the air pollutant
as the group of four compounds emitted by CAA section 202(a) sources,
it would not change the result. The Administrator would make the same
contribution finding as it would have no material effect on the
emissions comparisons discussed above.
c. It Was Reasonable for the Administrator To Define the Single Air
Pollutant as the Group of Substances With Common Attributes
    Several commenters disagree with EPA's proposed definition of a
single air pollutant composed of the six well-mixed greenhouse gases as
a class. Commenters 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.
    At least one commenter argues that EPA cannot combine greenhouse
gases into one pollutant because their common attribute is not a
``physical, chemical, biological or radioactive property'' (quoting
from CAA section 302(g)), but rather their effect or impacts on the
environment. They say this differs from VOCs, which share the common
attribute of volatility, or PM which shares the physical property of
being particles.
    As discussed above, the well-mixed greenhouse gases share physical
attributes, as well as attributes based on sound policy considerations.
The definition of ``air pollutant'' in CAA section 302(g) does not
limit consideration of common attributes to those that are ``physical,
chemical, biological or radioactive property'' as one commenter claims.
Rather, the definition's use of the adjectives ``physical, chemical,
biological or radioactive'' refer to the different types of substance
or matter that is emitted. It is not a limitation on what
characteristics the Administrator may consider when deciding how to
group similar substances when defining a single air pollutant.
    The common attributes that the Administrator considered when
defining the well-mixed greenhouse gases are reasonable. While these
six substances may originate from different processes, and require
different control strategies, that does not detract from the fact that
they are all long-lived, well-mixed in the atmosphere, directly
emitted, of well-known radiative forcing, and generally grouped and
considered together in climate change scientific and policy forums.
Indeed, other group pollutants also originate from a variety of
processes and a result may require different control technologies. For
example, both a power plant and a dirt road can result in PM emissions,
and the method to control such emissions at each source would be
different. But these differences in origin or control do not undermine
the reasonableness of considering PM as a single air pollutant. The
fact that there are differences, as well as similarities, among the
well-mixed greenhouse gases does not render the decision to group them
together as one air pollutant unreasonable.
2. The Administrator's Cause or Contribute Analysis Was Reasonable
a. The Administrator Does Not Need To Find Significant Contribution, or
Establish a Bright Line
    Many commenters essentially argue that EPA must establish a bright
line below which it would never find contribution regardless of the air
pollutant, air pollution, and other factors before the Agency. For
example, some commenters 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).

Commenters take issue with EPA's statement that 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.''
Commenters argue that EPA cannot act arbitrarily by determining that a
constituent contributing a certain percent to endangerment in one
instance is de minimis and in another is contributing to endangerment
of public health and welfare. They request that EPA revise the 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 argue that EPA 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.
    The commenters that argue that the air pollution EPA must analyze
to determine endangerment is limited to the air pollution resulting
from new motor vehicles also argue that as a result, the contribution
of emissions from new motor vehicles must be significant. They
essentially contend that the endangerment and cause or contribute tests
are inter-related and the universe of both tests is the same. In
support of their argument, commenters argue that because the clause
``cause, or contribute to, air pollution'' is in plural form, it must
be referring back to ``any class or classes of new motor vehicles or
new motor vehicle engines,'' demonstrating that EPA must consider only
the emissions from new motor vehicles which emit the air pollution
which endangers.
    Since the Administrator issued the Proposed Findings, the DC
Circuit issued another opinion discussing the concept of contribution.
See Catawba County v. EPA, 571 F.3d 20 (DC Cir. 2009). This decision,
along with others, supports the Administrator's interpretation that the
level of contribution under CAA section 202(a) does not need to be
significant. The Administrator is not required to establish a bright
line below which she would never find contribution under any
circumstances. Finally, it is reasonable for the Administrator to apply
a ``totality-of-the-circumstances test to implement a statute that
confers broad discretionary authority, even if the test lacks a
definite `threshold' or `clear line of demarcation to define an open-
ended term.'' Id. at 39 (citations omitted).
    In upholding EPA's PM2.5 attainment and nonattainment
designation decisions, the DC Circuit analyzed CAA section 107(d),
which requires EPA to designate an area as nonattainment if it
``contributes to ambient air quality in a nearby area'' not attaining
the national ambient air quality standards. Id. at 35. The court noted
that it had previously held that the term ``contributes'' is ambiguous
in the context of CAA language. See EDF v. EPA, 82 F.3d 451, 459 (DC
Cir. 1996). ``[A]mbiguities in statutes within an agency's jurisdiction
to administer are delegations of authority to the agency to fill the
statutory gap in reasonable fashion.'' 571 F.3d at 35 (citing Nat's
Cable & Telecomms. Ass'c v. Brand X Internet Servs, 545 U.S. 967, 980
(2005)).
    The court then proceeded to consider and reject petitioners'
argument that the verb ``contributes'' in CAA section 107(d)
necessarily connotes a significant causal relationship. Specifically,
the DC Circuit again noted that the term is ambiguous, leaving it to
EPA to interpret in a reasonable manner. In the context of this
discussion, the court noted that ``a contribution may simply exacerbate
a problem rather than cause it * * * '' 571 F.3d at 39. This is
consistent with the DC Circuit's decision in Bluewater Network v. EPA,
370 F.3d 1 (DC Cir. 2004), in which the court noted that the term
contribute in CAA section 213(a)(3) ``[s]tanding alone, * * * 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.'' 370 F.3d at 13. The court found that the
bare ``contribute'' language invests the Administrator with discretion
to exercise judgment regarding what constitutes a sufficient
contribution for the purpose of making an endangerment finding. Id. at
14.
    Finally, in Catawba County, the DC Circuit also rejected
``petitioners' argument that EPA violated the statute by failing to
articulate a quantified amount of contribution that would trigger'' the
regulatory action. 571 F.3d at 39. Although petitioners preferred that
EPA establish a bright-line test, the court recognized that the statute
did not require that EPA ``quantify a uniform amount of contribution.''
Id.
    Given this context, it is entirely reasonable for the Administrator
to interpret CAA section 202(a) to require some level of contribution
that, while more than de minimis or trivial, does not rise to the level
of significance. Moreover, the approach suggested by at least one
commenter collapses the two prongs of the test by requiring that
contribution must be significant because any climate change impacts
upon which an endangerment determination is made result solely from the
greenhouse gas emissions of motor vehicles. It essentially eliminates
the ``contribute'' part of the ``cause or contribute'' portion of the
test. This approach was clearly rejected by the en banc court in Ethyl.
541 F.2d at 29 (rejecting the argument that the emissions of the fuel
additive to be regulated must ``in and of itself, i.e. considered in
isolation, endanger[ ] public health.''); see also Catawba County, 571
F.3d at 39 (noting that even if the test required significant
contribution it would be reasonable for EPA to find a county's addition
of PM2.5 is significant even though the problem would
persist in its absence). It is the commenter, not EPA that is ignoring
the statutory language. Whether or not the clause ``cause, or
contribute to, air pollution'' refers back to ``any class or classes of
new motor vehicles or new motor vehicle engines,'' or to ``emission of
any air pollutant,'' the language of CAA section 202(a) clearly
contemplates that emission of an air pollutant from any class or
classes may merely contribute to, versus cause, the air pollution which
endangers.
    It is also reasonable for EPA to decline to establish a ``bright-
line `objective' test of contribution.'' 571 F.3d at 39. As noted in
the Proposed Findings, when exercising her judgment, the Administrator
not only considers the cumulative impact, but also looks at the
totality of the circumstances (e.g., the air pollutant, the air
pollution, the nature of the endangerment, the type of source category,
the number of sources in the source category, and the number and type
of other source categories that may emit the air pollutant) when
determining whether the emissions justify regulation under the CAA. Id.
(It is reasonable for an agency to adopt a totality-of-the-
circumstances test).
    Even if EPA agreed that a level of significance was required to
find contribution, for the reasons discussed above, EPA would find that
the contribution from CAA section 202(a) source categories is
significant. Their emissions are larger than the great majority of
emitting countries, larger than several major emitting countries, and
they constitute one of the largest parts of the U.S. emissions
inventory.
b. The Unique Global Aspects of Climate Change Are an Appropriate
Consideration in the Contribution Analysis
    Some commenters disagree with statements in the Proposed Findings
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 CAA or
existing EPA policy for this position, and that it reveals an apparent
effort to expand EPA's authority to 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 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 like CAA 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.
    As discussed above, the term ``contribute'' is ambiguous and
subject to the Administrator's reasonable interpretation. It is
entirely appropriate for the Administrator to look at the totality of
the circumstances when making a finding of contribution. In this case,
the Administrator believes that the global nature of the problem
justifies looking at contribution in a way that takes account of these
circumstances. More specifically, because climate change is a global
problem that results from global greenhouse gas emissions, there are
more sources emitting greenhouse gases (in terms both of absolute
numbers of sources and types of sources) than EPA typically encounters
when analyzing contribution towards a more localized air pollution
problem. From a percentage perspective, there are no dominating sources
and fewer sources that would even be considered to be close to
dominating. The global problem is much more the result of numerous and
varied sources each of which emit what might seem to be smaller
percentage amounts when compared to the total. The Administrator's
approach recognizes this reality, and focuses on evaluating the
relative importance of the CAA section 202(a) source categories
compared to other sources when viewed in this context.
    This recognition of the unique totality of the circumstances before
the Administrator now as compared to previous contribution decisions is
entirely appropriate. It is not an attempt by the Administrator to
regulate ``truly trivial or de minimis'' sources, or to regulate
sources based on their global effects. The Administrator is determining
whether greenhouse gas emissions from CAA section 202(a) sources
contribute to an air pollution problem is endangering U.S. public
health and welfare. As discussed in the Proposed Findings, no single
greenhouse gas source category dominates on the global scale, and many
(if not all) individual greenhouse gas source categories could appear
small in comparison to the total, when, in fact, they could be very
important contributors in terms of both absolute emissions or in
comparison to other source categories, globally or within the United
States. If the United States and the rest of the world are to combat
the risks associated with global climate change, contributors must do
their part even if their contributions to the global problem, measured
in terms of percentage, are smaller than typically encountered when
tackling solely regional or local environmental issues. The commenters'
approach, if used globally, would effectively lead to a tragedy of the
commons, whereby no country or source category would be accountable for
contributing to the global problem of climate change, and nobody would
take action as the problem persists and worsens. The Administrator's
approach, on the contrary, avoids this kind of approach, and is a
reasonable exercise of her discretion to determine contribution in the
global context in which this issue arises.
    Importantly, as discussed above, the contribution from CAA section
202(a) sources is anything but trivial or de minimis under any
interpretation of contribution. See, Massachusetts v. EPA, 549 U.S. at
1457-58 (``Judged by any standard, U.S. motor-vehicle emissions make a
meaningful contribution to greenhouse gas concentrations and hence, * *
* to global warming'').
c. The Administrator Reasonably Relied on Comparisons of Emissions From
Existing CAA Section 202(a) Source Categories
i. It Was Reasonable To Use Existing Emissions From Existing CAA
Section 202(a) Source Categories Instead of Projecting Future Emissions
From New CAA Section 202(a) Source Categories
    Many commenters argue that EPA improperly evaluated the emissions
from the entire motor vehicle fleet, and it is required to limit its
calculation to just emissions from new motor vehicles. Thus the
emissions that EPA should consider in the cause or contribute
determination is far less than the 4.3 percent of U.S. greenhouse gas
emissions attributed to motor vehicles in the Proposed Findings,
because this number includes both new and existing motor vehicles. One
commenter calculated the emissions from new motor vehicles as being 1.8
percent of global emissions, assuming approximately one year of new
motor vehicle production in the United States (11 million vehicles) in
a total global count currently of approximately 600 million motor
vehicles.
    In the Proposed Findings, EPA determined the emissions from the
entire fleet of motor vehicles in the United States for a certain
calendar year. EPA explained that, consistent with its traditional
practice, it used the recent motor vehicle emissions inventory for the
entire fleet as a surrogate for estimates of emissions for just new
motor vehicles and engines. This was appropriate because future
projected emissions are uncertain and current emissions data are a
reasonable proxy for near-term emissions.
    In effect, EPA is using the inventory for the current fleet of
motor vehicles as a reasonable surrogate for a projection of the
inventory from new motor vehicles over the upcoming years. New motor
vehicles are produced year in and year out, and over time the fleet
changes over to a fleet composed of such vehicles. This occurs in a
relatively short time frame, compared to the time period at issue for
endangerment. Because new motor vehicles are produced each year, and
continue to emit over their entire life, over a relatively short period
of time the emission from the entire fleet is from vehicles produced
after a certain date. In addition, the emissions from new motor
vehicles are not limited to the emissions that occur only during the
one year when they are new, but are emissions over the entire life of
the vehicle.
    In such cases, EPA has traditionally used the recent emissions from
the entire current fleet of motor vehicles as a reasonable surrogate
for such a projection instead of trying to project and model those
emissions. While this introduces some limited degree of uncertainty,
the difference between recent actual emissions from the fleet and
projected future emissions from the fleet is not expected to differ in
any way that would substantively change the decision made concerning
cause or contribution. There is not a specific numerical bright line
that must be achieved, and the numerical percentages are not treated
and do not need to be treated as precise values. This approach provides
a reasonable and clear indication of the relative magnitudes involved,
and EPA does not believe that attempting to make future

projections (for both vehicles and the emissions value they are
compared to) would provide any greater degree of accuracy or precision
in developing such a relative comparison.
ii. The Administrator Did Not Have To Use a Subset or Reduced Emissions
Estimate From Existing CAA Section 202(a) Source Categories
    Several commenters note that although EPA looks at emissions from
all motor vehicles regulated under CAA section 202(a) in its
contribution analysis, the Presidential announcement in May 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 CAA
section 202(a) sources. Some commenters also argue that because
emission standards will not eliminate all greenhouse gas emissions from
motor vehicles, the comparison should compare the amount of greenhouse
gas emissions ``reduced'' by those standards to the global greenhouse
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.
Some commenters go further and contend that EPA also can only include
that incremental reduction that the EPA regulations will achieve beyond
any reductions resulting from CAFE standards that NHTSA will set.
    Although the May announcement and September proposed rule involved
only the light duty motor vehicle sector, the Administrator is making
this finding for all classes of new motor vehicles under CAA section
202(a). Thus, although the announcement and proposed rule involve light
duty vehicles, EPA is working to develop standards for the rest of the
classes of new motor vehicles under CAA section 202(a). As the Supreme
Court noted, EPA has ``significant latitude as to the manner, timing,
content, and coordination of its regulations with those of other
agencies. Massachusetts v. EPA, 549 U.S. at 533.
    The argument that the Administrator can only look at that portion
of emissions that will be reduced by any CAA section 202(a) standards,
and even then only the reduction beyond those attributable to CAFE
rules, finds no basis in the statutory language. The language in CAA
section 202(a) requires that the Administrator set ``standards
applicable to the emission of any air pollutant from [new motor
vehicles], which in [her] judgment cause, or contribute to, air
pollution which [endangers].'' It does not say set ``standards
applicable to the emission of any air pollutant from [new motor
vehicles], if in [her] judgment the emissions of that air pollutant as
reduced by that standard cause, or contribute to, air pollution which
[endangers].'' As discussed above, the decisions on cause or contribute
and endangerment are separate and distinct from the decisions on what
emissions standards to set under CAA section 202(a). The commenter's
approach would improperly integrate these separate decisions. Indeed,
because, as discussed above, the Administrator does not have to propose
standards concurrent with the endangerment and cause or contribute
findings, she would have to be prescient to know at the time of the
contribution finding exactly the amount of the reduction that would be
achieved by the standards to be set. As discussed above, for purposes
of these findings we look at what would be the emissions from new motor
vehicles if no action were taken. Current emissions from the existing
CAA section 202(a) vehicle fleet are an appropriate estimate.
d. The Administrator Reasonably Compared CAA Section 202(a) Source
Emissions to Both Global and Domestic Emissions of Well-Mixed
Greenhouse Gases
    EPA received many comments on the appropriate comparison(s) for the
contribution analysis. Several commenters argue that in order to get
around the ``problem'' of basing an endangerment finding upon a source
category that contributes only 1.8 percent annually to global
greenhouse gas emissions, EPA inappropriately also made comparisons to
total U.S. greenhouse gas emissions. These commenters argue that a
comparison of CAA section 202(a) source emissions to U.S. greenhouse
gas emissions, versus global emissions, is arbitrary for 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 Proposed Findings, the
Administrator focuses on the global nature of greenhouse gas. Thus,
they continue, while the 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 the air pollution,
which the Administrator has proposed to define on a global rather than
a domestic basis. Commenters 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).
    The language of CAA section 202(a) is silent regarding how the
Administrator is to make her contribution analysis. While it requires
that the Administrator assess whether emission of an air pollutant
contributes to air pollution which endangers, it does not limit how she
may undertake that assessment. It surely is reasonable that the
Administrator look at how CAA section 202(a) source category emissions
compare to global emissions on an absolute basis, by themselves. But
the United States as a nation is the second largest emitter of
greenhouse gases. It is entirely appropriate for the Administrator to
decide that part of understanding how a U.S. source category emitting
greenhouse gases fits into the bigger picture of global climate change
is to appreciate how that source category fits into the contribution
from the United States as a whole, where the United States as a country
is a major emitter of greenhouse gases. Knowing that CAA section 202(a)
source categories are the second largest emitter of well-mixed
greenhouse gases in the country is relevant to understanding what role
they play in the global problem and hence whether they ``contribute''
to the global problem. Moreover, the Administrator is not ``picking and
choosing'' when she applies a global or domestic approach in these
Findings. Rather, she is looking at both of these emissions comparisons
as appropriate under the applicable science, facts, and law.
e. The Amount of Well-Mixed Greenhouse Gas Emissions From CAA Section
202(a) Sources Reasonably Supports a Finding of Contribution
    Many commenters 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 four
percent of total global greenhouse gas emissions are a minimal
contribution to global greenhouse gas concentrations.
    EPA disagrees. As stated above, CAA section 202(a) source category
total emissions of well-mixed greenhouse gases are higher than most
countries in the world; countries that the U.S. and others believe play
a major role in the

global climate change problem. Moreover, the percent of global well-
mixed greenhouse gas emissions that CAA section 202(a) source
categories represent is higher than percentages that the EPA has found
contribute to air pollution problems. See Bluewater Network, 370 F.3d
at 15 (``For Fairbanks, this contribution was equivalent to 1.2 percent
of the total daily CO inventory for 2001.'') As noted above, there is
no bright line for assessing contribution, but as discussed in the
Proposed Findings and above, when looking at a global problem like
climate change, with many sources of emissions and no dominating
sources from a global perspective, it is reasonable to consider that
lower percentages contribute than one may consider when looking at a
local or regional problem involving fewer sources of emissions. The
Administrator agrees that ``[j]udged by any standard, U.S. motor-
vehicle emissions make a meaningful contribution to greenhouse gas
concentrations and hence, * * * to global warming.'' Massachusetts v.
EPA, 549 U.S. at 525.

VI. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under Executive Order (EO) 12866 (58 FR 51735, October 4, 1993),
this action is a ``significant regulatory action'' because it raises
novel policy issues. Accordingly, EPA submitted this action to the
Office of Management and Budget (OMB) for review under EO 12866 and any
changes made in response to Office of Management and Budget (OMB)
recommendations have been documented in the docket for this action.

B. Paperwork Reduction Act

    This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
Burden is defined at 5 CFR 1320.3(b). These Findings do not impose an
information collection request on any person.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this action on small
entities, small entity is defined as: (1) A small business as defined
by the Small Business Administration's (SBA) regulations at 13 CFR
121.201; (2) a small governmental jurisdiction that is a government of
a city, county, town, school district, or special district with a
population of less than 50,000; and (3) a small organization that is
any not-for-profit enterprise which is independently owned and operated
and is not dominant in its field.
    Because these Findings do not impose any requirements, the
Administrator certifies that this action will not have a significant
economic impact on a substantial number of small entities. This action
does not impose any requirements on small entities. The endangerment
and cause or contribute findings do not in-and-of-themselves impose any
new requirements but rather set forth the Administrator's determination
on whether greenhouse gases in the atmosphere may reasonably be
anticipated to endanger public health or welfare, and whether emissions
of greenhouse gases from new motor vehicles and engines contribute to
this air pollution. Accordingly, the action affords no opportunity for
EPA to fashion for small entities less burdensome compliance or
reporting requirements or timetables or exemptions from all or part of
the Findings.

D. Unfunded Mandates Reform Act

    This action contains no Federal mandates under the provisions of
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C.
1531-1538 for State, local, or tribal governments or the private
sector. The action imposes no enforceable duty on any State, local or
tribal governments or the private sector. Therefore, this action is not