The USGCRP NCA3 finds that, "Climate change will, absent other changes, amplify some of the existing health threats the nation now faces.  Certain people and communities are especially vulnerable, including children, the elderly, the sick, the poor, and some communities of color."  According to the USGCRP NCA3, certain climate change related impacts -- including heat waves, degraded air quality, and extreme weather events -- have disproportionate effects on low-income and some communities of color, raising environmental justice concerns.  Existing health disparities and other inequities in these communities increase their vulnerability to the health effects of climate change.
The USGCRP NCA3 also finds that climate change, in addition to chronic stresses such as extreme poverty, is affecting indigenous peoples' health in the United States through impacts such as reduced access to traditional foods, decreased water quality, and increasing exposure to health and safety hazards.  The IPCC AR5 finds that climate change-induced warming in the Arctic and resultant changes in environment (e.g., permafrost thaw, effects on traditional food sources) have significant observed and projected impacts on the health and well-being of Arctic residents, especially indigenous peoples.  Small, remote, predominantly-indigenous communities are especially vulnerable given their "strong dependence on the environment for food, culture, and way of life; their political and economic marginalization; existing social, health, and poverty disparities; as well as their frequent close proximity to exposed locations along ocean, lake, or river shorelines."  In addition, increasing temperatures and loss of Arctic sea ice increases the risk of drowning for those engaged in traditional hunting and fishing.
 The USGCRP NCA3 concludes that children will suffer disproportionately from climate change given the unique physiological and developmental factors that occur during this lifestage.  Impacts on children are expected from heat waves, air pollution, infectious and waterborne illnesses, and mental health effects resulting from extreme weather events.  The IPCC AR5 indicates that children are among those especially susceptible to most allergic diseases, as well as health effects associated with heat waves, storms, and floods.  The IPCC finds that additional health concerns may arise in low income households, especially those with children, if climate change reduces food availability and increases prices, leading to food insecurity within households.
Both the USGCRP and IPCC conclude that climate change will increase health risks facing the elderly.  Older people are at much higher risk of mortality during extreme heat events.  Pre-existing health conditions also make older adults susceptible to cardiac and respiratory impacts of air pollution and to more severe consequences from infectious and waterborne diseases.  Limited mobility among older adults can also increase health risks associated with extreme weather and floods.
         7. More Recent Evidence that Greenhouse Gases Endanger Public Welfare
The EPA has carefully reviewed the recent scientific conclusions in the assessments regarding human-induced climate change impacts on public welfare.  The EPA finds that they are largely consistent with or strengthen the underlying science supporting the 2009 Endangerment Finding regarding public welfare effects on food production and agriculture; forestry; water resources; sea level rise and coastal areas; energy, infrastructure, and settlements; ecosystems and wildlife; and impacts on the U.S. population from climate change effects occurring outside of the United States.  These key findings are described briefly here.
Regarding agriculture, the assessment literature cited in the 2009 Endangerment Finding found potential for increased CO2 levels to benefit yields of certain crops in the short-term, but with considerable uncertainty.  The body of evidence pointed towards increasing risk of net adverse impacts on U.S. food production and agriculture over time, with the potential for significant disruptions and crop failure in the future.  The most recent assessments now have greater confidence that climate change will negatively affect U.S. agriculture over this century.  Specifically, the USGCRP NCA3 concludes, "While some U.S. regions and some types of agricultural production will be relatively resilient to climate change over the next 25 years or so, others will increasingly suffer from stresses due to extreme heat, drought, disease, and heavy downpours.  From mid-century on, climate change is projected to have more negative impacts on crops and livestock across the country."  The IPCC AR5 concludes, "Overall yields of major crops in North America are projected to decline modestly by mid-century and more steeply by 2100 among studies that do not consider adaptation (very high confidence)."  The IPCC AR5 notes that in the absence of extreme events, climate change may benefit certain regions and crops, but that in North America significant harvest losses have been observed due to recent extreme weather events.  In addition, the IPCC SREX assessment specifically notes that projected changes in extreme weather events will increase disaster risk in the agriculture sector.
Regarding forestry, the assessment literature cited in the 2009 Endangerment Finding found that near term benefits to forest growth and productivity in certain parts of the country from elevated CO2 concentrations and temperature increases to date are offset by longer term risks from wildfires and the spread of destructive pests and disease that present serious adverse risks for forest productivity.  The most recent assessments provide further support for this conclusion.  Both the USGCRP NCA3 and the IPCC AR5 conclude that climate change is increasing risks to forest health from fire, tree disease and insect infestations, and drought.  The IPCC AR5 also notes risks to forested ecosystems associated with changes in temperature, precipitation amount, and CO2 concentrations, which can affect species and ecological communities, leading to ecosystem disruption, reorganization, movement or loss.  The NRC Arctic assessment states that climate change is likely to have a large negative impact on forested ecosystems in the high northern latitudes due the effects of permafrost thaw and greater wildfire frequency, extent, and severity.  The NRC Climate Stabilization Targets assessment found that for an increase in global average temperature of 1 to 2°C above pre-industrial levels, the area burnt by wildfires in western North America will likely more than double.
Regarding water resources, the assessment literature cited in the 2009 Endangerment Finding concluded that increasing temperatures and increased variability in precipitation associated with climate change will impact water quality and quantity through changes in snowpack, increased risk of floods, drought, and other concerns such as water pollution.  Similarly, the new assessments further support projections of water resource impacts associated with increased floods and short-term drought in most U.S. regions.  The USGCRP NCA3 also finds that, "Climate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some areas."  The IPCC AR5 finds that in part of the western United States, "water supplies are projected to be further stressed by climate change, resulting in less water availability and increased drought conditions."  The IPCC AR5 also projects that climate change will degrade surface water quality, including the Great Lakes, and will negatively affect drinking water treatment/distribution and sewage collection systems.
The assessment literature cited in the 2009 Endangerment Finding found that the most serious potential adverse effects to coastal areas are the increased risk of storm surge and flooding in coastal areas from sea level rise and more intense storms.  Coastal areas also face other adverse impacts from sea level rise such as land loss due to inundation, erosion, wetland submergence, and habitat loss.  The most recent assessments provide further evidence in line with the science supporting the 2009 Endangerment Finding. The USGCRP NCA3 finds that, "Sea level rise, combined with coastal storms, has increased the risk of erosion, storm surge damage, and flooding for coastal communities, especially along the Gulf Coast, the Atlantic seaboard, and in Alaska."
The IPCC AR5, the USGCRP NCA3, and three of the new NRC assessments provide estimates of projected global sea level rise.  These estimates, while not always directly comparable as they assume different emissions scenarios and baselines, are at least 40 percent larger than, and in some cases more than twice as large as, the projected rise estimated in the IPCC AR4 assessment, which was referred to in the 2009 Endangerment Finding.  The NRC Sea Level Rise assessment projects a global sea level rise of 0.5 to 1.4 meters by 2100, which is sufficient to lead to a relative rise in sea level even around the northern coasts of Washington State, where the land is still rebounding from the disappearance of the great ice sheets.  The NRC National Security Implications assessment suggests that "the Department of the Navy should expect roughly 0.4 to 2 meters global average sea-level rise by 2100."  The NRC Climate Stabilization Targets assessment states that an increase of 3°C will lead to a sea level rise of 0.5 to 1 meter by 2100.  While these NRC and IPCC assessments continue to recognize and characterize the uncertainty inherent in accounting for ice sheet processes, these revised estimates are consistent with the assessments underlying the 2009 Endangerment Finding.
Regarding climate impacts on energy, infrastructure, and settlements, the 2009 Endangerment Finding cited the assessment literature's findings that temperature increases will change heating and cooling demand; that declining water quantity may adversely impact the availability of cooling water and hydropower in the energy sector; and that changes in extreme weather events will threaten energy, transportation, water, and other key societal infrastructure, particularly on the coast.  The most recent assessments provide further evidence in line with the science supporting the 2009 Endangerment Finding.  For example, the USGCRP NCA3 finds that, "Coastal infrastructure, including roads, rail lines, energy infrastructure, airports, port facilities, and military bases, are increasingly at risk from sea level rise and damaging storm surges."  The NRC Arctic assessment identifies threats to human infrastructure in the Arctic from increased flooding, erosion, and shoreline ice pile-up, or ivu, associated with summer sea ice loss and the increasing frequency and severity of storms.
Regarding ecosystems and wildlife, the assessment literature cited in the 2009 Endangerment Finding found that climate change will predominantly adversely impact both terrestrial and marine biodiversity and the ability of these ecosystems to provide goods and services.  The NRC Arctic assessment states that major marine and terrestrial biomes will likely shift pole ward, with significant implications for changing species composition, food web structures, and ecosystem function.  The NRC Climate Stabilization Targets assessment found that coral bleaching will increase due both to warming and ocean acidification.  The NRC Understanding Earth's Deep Past assessment notes four of the five major coral reef crises of the past 500 million years were caused by acidification and warming that followed GHG increases of similar magnitude to the emissions increases expected over the next hundred years.  Similarly, the NRC Ocean Acidification assessment finds that "[t]he chemistry of the ocean is changing at an unprecedented rate and magnitude due to anthropogenic CO2 emissions; the rate of change exceeds any known to have occurred for at least the past hundreds of thousands of years."  The assessment notes that the full range of consequences is still unknown, but the risks "threaten coral reefs, fisheries, protected species, and other natural resources of value to society."  The IPCC AR5 also projects biodiversity losses in marine ecosystems, especially in the Arctic and tropics.
 In general, climate change impacts related to public welfare are expected to be unevenly distributed across different regions of the United States and have a greater impact on certain populations, such as indigenous peoples and the poor.  The USGCRP NCA3 finds climate change impacts such as the rapid pace of temperature rise, coastal erosion and inundation related to sea level rise and storms, ice and snow melt, and permafrost thaw are affecting indigenous people in the United States.  Particularly in Alaska, critical infrastructure and traditional livelihoods are threatened by climate change and, "[i]n parts of Alaska, Louisiana, the Pacific Islands, and other coastal locations, climate change impacts (through erosion and inundation) are so severe that some communities are already relocating from historical homelands to which their traditions and cultural identities are tied."  The IPCC AR5 notes, "Climate-related hazards exacerbate other stressors, often with negative outcomes for livelihoods, especially for people living in poverty (high confidence).  Climate-related hazards affect poor people's lives directly through impacts on livelihoods, reductions in crop yields, or destruction of homes and indirectly through, for example, increased food prices and food insecurity."
In the 2009 Endangerment Finding, the Administrator considered impacts on the U.S. population from climate change effects occurring outside of the United States, such as national security concerns that may arise as a result of climate change impacts in other regions of the world.  The most recent assessments provide further evidence in line with the science supporting the 2009 Endangerment Finding.  The NRC Climate and Social Stress assessment found that it would be "prudent for security analysts to expect climate surprises in the coming decade . . . and for them to become progressively more serious and more frequent thereafter."  The NRC National Security Implications assessment recommends preparing for increased needs for humanitarian aid; responding to the effects of climate change in geopolitical hotspots, including possible mass migrations; and addressing changing security needs in the Arctic as sea ice retreats.
In addition, the NRC Abrupt Impacts report examines the potential for tipping points, thresholds beyond which major and rapid changes occur in the Earth's climate system, as well as in natural and human systems that are impacted by the changing climate.  The Abrupt Impacts report did find less cause for concern than some previous assessments regarding some abrupt events within the next century such as disruption of the oceanic Atlantic Meridional Overturning Circulation (AMOC) and sudden releases of high-latitude methane from hydrates and permafrost.  But, the same report found that the potential for abrupt changes in ecosystems, weather and climate extremes, and groundwater supplies critical for agriculture now seem more likely, severe, and imminent.  The assessment found that some abrupt changes were already underway (e.g., Arctic sea ice retreat and increases in extinction risk due to the speed of climate change), and cautioned that even abrupt changes such as the AMOC disruption that are not expected in this century can have severe impacts if/when they happen.
         8. Consideration of Other Climate Forcers
Both in the 2009 Endangerment Finding and in this action, the Administrator recognizes that there are other substances in addition to the six well-mixed GHGs that are emitted from human activities and affect Earth's climate (referred to as climate forcers). These can be grouped into two categories: (1) other substances with similar physical properties to the six well-mixed GHGs -- these include the ozone-depleting substances of chlorofluorocarbons, hydrochlorofluorocarbons, and halons, as well as nitrogen trifluoride and similar recently identified substances; and (2) short-lived substances -- tropospheric ozone and its precursor gases, water vapor, and aerosol particles and precursors.  Some of these short-lived substances  --  namely, water vapor; NOx; and aerosol particles including black carbon  -  have different, and often larger, climate effects when emitted at high altitudes.
As described in section III.B of the 2009 Endangerment Finding and in section IV.A.1 of this preamble, the primary reasons for defining the air pollution as the aggregate group of the six well-mixed GHGs include their common physical properties relevant to climate change (i.e., long-lived, well-mixed, directly emitted), the fact that these gases are considered the primary drivers of climate change, and that these gases remain the best understood drivers of anthropogenic climate change. After considering additional information in the new assessments regarding the climate-relevant substances outside the basket of the six well-mixed GHGs, it is the Administrator's view that the reasons originally stated for not including these substances in the scope of the GHG air pollution still apply at this time.  For example, nitrogen triflouride and other recently discovered substances are not as well understood as the six well-mixed GHGs, and tropospheric ozone is short-lived in the atmosphere and is not directly emitted (it forms from emissions of various precursor gases).
Regarding the short-lived substances with different climate effects when emitted at high altitudes, the Aircraft Petition (see section II of this preamble) mentions the effects of water vapor and NOx on clouds and atmospheric chemistry.  The major peer-reviewed scientific assessments of the IPCC and NRC provide the current state of scientific understanding of these effects; the USGCRP assessments have not dealt specifically with emissions at high altitude.  The EPA considered the following assessment reports to obtain the best estimates of these substances' net impact on the climate system, which is generally discussed in terms of radiative forcing: the IPCC AR5, the IPCC 2007 Fourth Assessment Report (AR4), the IPCC Special Report: Aviation and the Global Atmosphere (IPCC 1999), the NRC's Advancing the Science of Climate Change (NRC 2010), and the NRC's Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project (NRC 1999).  In addition to high altitude water vapor  and NOx, the literature indicates that aerosol particles, including black carbon, emitted at high altitudes have more interactions with clouds and therefore have different effects on the global energy balance than do particles emitted at the surface.
The state of the science as represented in the assessment literature highlights significant scientific uncertainties regarding the total net forcing effect of water vapor, NOx, and aerosol particles when emitted at high altitudes.  Given these uncertainties, the Agency is not including them in the proposed definition of air pollution for purposes of the endangerment finding under section 231 of the CAA.  The short-lived nature of these substances means that, unlike the long-lived GHGs, the climatic impact of the substance is dependent on a number of factors at the location and time of its emission.  The magnitude, and often the direction (positive/warming or negative/cooling), of the globally averaged climate impact will differ depending on the location of the emission due to the local atmospheric conditions (e.g., due to differing concentrations of other compounds with which the emissions can react, background humidity levels, or the presence or absence of clouds).  In addition, for emissions at any given location, the spatial and temporal pattern of the climate forcing will be heterogeneous, again often differing in direction (for example, in the case of NOx emissions, the near term effect in the hemisphere in which the emissions occur is usually warming due to increased ozone concentrations, but the longer term effects, and effects in the other hemisphere, are often cooling due to increased destruction of methane).  As the climatic effects of these substances when emitted at high altitudes were not addressed at length in the 2009 Endangerment Finding, the following subsections briefly summarize the findings of the major scientific assessments regarding these substances' climatic effects at altitude and the various sources of uncertainty surrounding these estimates.

   a. Changes in Clouds from High Altitude Emissions of Water Vapor and Particles
Aviation-induced cloudiness (sometimes called AIC) refers to all changes in cloudiness associated with aviation operations, which are primarily due to the effects of high altitude emissions of water vapor and particles.  Changes in cloudiness affect the climate by both reflecting solar radiation (cooling) and trapping outgoing longwave radiation (warming).  Unlike the warming effects associated with the six long-lived, well-mixed GHGs, the warming effects associated with changes in cloud cover are more regional and temporal in nature.  The three key components of aviation-induced cloudiness are persistent contrails, contrail-induced cirrus, and induced cirrus.
Aircraft engine emissions of water vapor at high altitudes during flight can lead to the formation of condensation trails, or contrails, under certain conditions such as ice-supersaturated air masses with specific humidity levels and temperature.  The NRC estimates that persistent contrails increased cloudiness above the United States by 2 percent between 1950 and 1988, with similar results reported over Europe.  As stated above, clouds can have both warming and cooling effects, and persistent contrails were once considered to have significant net warming effects.  However, more recent estimates suggest a smaller overall climate forcing effect of persistent contrails.  The IPCC AR5 best estimate for the global mean radiative forcing from contrails is 0.01 W/m[2] (medium confidence and with an uncertainty range of 0.005 to 0.03 W/m[2]).  To put this number into context, some examples of other IPCC AR5 best estimates for global mean radiative forcing include: 1.68 W/m[2] for CO2 (very high confidence and with an uncertainty range of 1.33 to 2.03 W/m[2]), 0.97 W/m[2] for methane (high confidence and with an uncertainty range of 0.74 to 1.20 W/m[2]), and 0.17 W/m[2] for nitrous oxide (very high confidence and with an uncertainty range of .013 to 0.21 W/m[2]).  In addition, the NRC (2010) assessment suggested that contrails may affect regional diurnal temperature differences, but this has been called into question by the recent findings presented in the IPCC AR5, which suggests that aviation contrails do not have an effect on mean or diurnal range of surface temperatures (medium confidence).
Persistent contrails also sometimes lose their linear form and develop into cirrus clouds, an effect referred to as contrail-induced cirrus.  Studies to date have been unable to isolate this climate forcing effect, but the IPCC AR5 provides a combined contrail and contrail-induced cirrus best estimate of 0.05 W/m[2] (low confidence and with an uncertainty range of 0.02 and 0.15 W/m[2]).
Particles emitted or formed in the atmosphere as a result of aircraft emissions may also act as ice nuclei and modify naturally forming cirrus clouds, an effect referred to as "induced cirrus."  The two primary aviation-induced particles are sulfates and black carbon, and their effects on cirrus cloud modification is an area of active research.  There are significant challenges in estimating the climatic impacts of induced cirrus; for example, the 2007 IPCC AR4 characterizes our knowledge of the natural freezing modes in cirrus conditions as "poor," and notes that cirrus cloud processes are not well represented in global models. Neither IPCC AR4 nor AR5 is able to provide global or regional estimates related to this forcing.
Given differences in scientific understanding of the three components of aviation-induced cloudiness, the more recent assessments have not been able to provide estimates of the net climate forcing effect of changes in clouds from high altitude emissions of water vapor and particles.  Going back to the 1999 IPCC assessment, the science is characterized as "very uncertain" with a range for the best estimate between 0 to 0.040 W/m[2].
   b. Direct Radiative Forcing Effects of High Altitude Particle Emissions
The 2009 Endangerment Finding noted that much of the uncertainty range surrounding the estimate of total net forcing due to all human activities was due to uncertainties about the cooling and warming effects of aerosols (though from all sources, not just aircraft).  The Finding noted that the magnitude of aerosol effects can vary immensely with location and season of emissions, and also discussed black carbon as a specific type of aerosol particle, noting that estimates of its total climate forcing effect have a large uncertainty range.  Here, we discuss the direct radiative forcing effects of high altitude emissions of the two primary aviation-induced particles, sulfates and black carbon.
Aircraft emit precursor gases that convert to sulfate particles in the atmosphere, such as sulfur dioxide.  Sulfate particles have direct effects on the climate by scattering solar radiation, which results in cooling.  The more recent assessments have not attempted to quantify this effect from aviation. Going back to the 1999 IPCC assessment, the direct effect of sulfate aerosols from aviation for the year 1992 is estimated at -0.003 W/m[2] with an uncertainty range between -0.001 and -0.009 W/m[2].
Black carbon emissions from aviation, which are produced by the incomplete combustion of jet fuel, primarily absorb solar radiation and heat the surrounding air, resulting in a warming effect.  The more recent assessments have not attempted to quantify this effect from aviation.  The 1999 IPCC assessment estimates the global mean radiative forcing of black carbon emissions to be 0.003 W/m[2] with uncertainty spanning 0.001 to 0.009 W/m[2].  The IPCC 1999 assessment suggests its estimates of radiative forcing are high because the stratospheric black carbon effect remains too uncertain to quantify.
   c. Changes in Atmospheric Chemistry from High Altitude Nitrogen Oxides Emissions
Emissions of NOx do not themselves have warming or cooling effects, but affect the climate through catalyzing changes in the chemical equilibrium of the atmosphere.  High altitude emissions of NOx increase the concentration of ozone, which has a warming effect in the short term.  This leads to decreases in the formation of methane, which has a cooling effect in the long-term.  This eventually contributes to decreases in ozone, which decreases the long-term net warming effect.  Thus, the net radiative impact of NOx emissions depends on the balance between the reductions in methane versus the production of ozone, which in turn depends on the time scale under consideration.  Quantifying these impacts is an area of active study with large uncertainties.  The quantification of the net global effect of NOx is difficult because the atmospheric chemistry effects are heavily dependent on highly localized atmospheric properties and mixing ratios.  Because the background atmospheric concentration of NOx is important for quantifying the impact of aviation NOx emissions on ozone and methane concentrations, the location of aircraft emissions would be an important additional factor.  In addition, NOx has different residence times in the atmosphere depending on the altitude at which it is emitted.  The residence time of NOx in the upper troposphere, or roughly the cruise altitude for jet aircraft, is on the order of several days.
The IPCC AR5 presents the impact of aviation NOx emissions using a different metric, global warming potential (GWP), which is a measure of the warming impact of a pulse of emissions of a given substance over 100 years relative to the same mass of CO2.  The AR5 presents a range from -21 to +75 for GWP of aviation NOx.  The uncertainty in sign indicates uncertainty whether the net effect is one of warming or cooling.  This report further suggests that at cruise altitude there is strong regional sensitivity of ozone and methane to NOx, particularly notable at low latitudes.  Going back to the IPCC 1999 assessment, the globally averaged radiative forcing estimates for aircraft emissions of NOx in 1992 were 0.023 W/m2 for O3-induced changes (uncertainty range of 0.011 to 0.046 W/m[2]), and -0.014 W/m[2] for methane-induced changes (uncertainty range of -0.005 to -0.042 W/m[2]).
The Administrator notes that NOx emissions are already regulated under the EPA's rules implementing CAA section 231, at 40 CFR part 87.  The prerequisite endangerment and cause or contribute findings that formed the basis for these standards, however, did not rely upon any conclusions regarding the climate forcing impacts of NOx, but rather the role of NOx emissions as a precursor to ozone formation in areas that did not meet the National Ambient Air Quality Standard (NAAQS) for ozone.  The continuing significant uncertainties regarding NOx as a climate forcer do not undermine the Agency's prior conclusion under CAA section 231 that emissions of NOx from aircraft engines cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare due to their contribution to ozone concentrations that exceed the NAAQS.
   a. Summary
Overall, the state of the science as represented in the assessment literature highlights significant scientific uncertainties regarding the total net forcing effect of water vapor, NOx, and aerosol particles, when emitted at high altitudes.  The dependence of the effects on where the substance is emitted, and the complex temporal and spatial patterns that result, mean that the current level of understanding regarding these short-lived substances is much lower than for the six long-lived, well-mixed GHGs.  Given these uncertainties, the Agency is not including these constituents in the proposed definition of air pollution for purposes of the endangerment finding under section 231 of the CAA.
Summary of the Administrator's Proposed Endangerment Finding under CAA Section 231
In sum, the Administrator proposes to find, for purposes of CAA section 231(a)(2)(A), that elevated atmospheric concentrations of the six well-mixed GHGs constitute air pollution that endangers both the public health and the public welfare of current and future generations.  In this proposed action under CAA section 231(a)(2)(A), the EPA relies primarily on the extensive scientific and technical evidence in the record supporting the 2009 Endangerment Finding, including the major, peer-reviewed scientific assessments regarding the question of whether GHGs in the atmosphere endanger public health and welfare, and on the analytical framework and conclusions upon which the EPA relied in making that finding.  This proposed finding under section 231 accounts for the EPA's careful consideration not only of the scientific and technical record for the 2009 Endangerment Finding, but also of new, major scientific assessments issued since closing the administrative record for the 2009 Endangerment Finding.  No more recent information or analyses suggest that it would be reasonable for the EPA to now reach a different or contrary conclusion for purposes of CAA section 231(a)(2)(A) than the Agency reached for purposes of section 202(a) in 2009.  In proposing this finding for purposes of section 231, we are not reopening or revisiting our 2009 Endangerment Finding.  To the contrary, in light of the recent judicial decisions upholding those findings, the EPA believes the 2009 Endangerment Finding is firmly established and well settled.  Moreover, there is no need for the EPA to reopen or revisit that finding for purposes of making an additional finding under section 231 of the CAA.  Therefore, public comments addressing this finding for purposes of section 231(a)(2)(A) should be limited to the section 231 context; the EPA will not consider or respond to comments on this proposal that seek a reevaluation of our 2009 Endangerment Finding for purposes of section 202(a).
The Proposed Cause or Contribute Finding for Greenhouse Gases under CAA Section 231
As noted above, the Administrator has proposed to define the air pollution for purposes of the endangerment finding under CAA section 231 to be the aggregate of six well-mixed GHGs in the atmosphere. The second step of the two-part endangerment test for this proposed finding is for the Administrator to determine whether the emission of any air pollutant from certain classes of aircraft engines causes or contributes to this air pollution. This is referred to as the cause or contribute finding, and is the second proposed finding by the Administrator in this action.
Section V.A of this proposal describes the Administrator's reasoning for using the same definition and scope of the GHG air pollutant that was used in the 2009 Endangerment Finding. Section V.B puts forth the Administrator's proposed finding that emissions of well-mixed GHGs from classes of aircraft engines used in covered aircraft contribute to the air pollution which endangers public health and welfare.
     A. The Air Pollutant
         1.  Proposed Definition of Air Pollutant
Under section 231, the Administrator is to determine whether emissions of any air pollutant from any class or classes of aircraft engines cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare.  As with the 2009 Endangerment Finding that the EPA conducted for purposes of CAA section 202(a), when making a cause or contribute finding under section 231(a)(2), the Administrator must first define the air pollutant being evaluated.  The Administrator has reasonably and logically considered the relationship between the GHG air pollution and air pollutant: while the air pollution is the concentration (e.g., stock) of the well-mixed GHGs in the atmosphere, the air pollutant is the same combined grouping of the well-mixed GHGs, the emissions of which are analyzed for contribution (e.g., the flow into the stock). See 74 FR at 66537 (similar discussion with respect to the finding for section 202). Thus, for purposes of section 231, the Administrator is proposing to use the same definition of the air pollutant that was used in the 2009 Endangerment Finding, namely, the aggregate group of the same six GHGs: CO2, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. See 74 FR at 66536-66537 (discussing the definition of the GHG air pollutant with respect to the finding for section 202).  That is, as for the 2009 Endangerment Finding, the Administrator is proposing to define a single air pollutant made up of these six GHGs.
To reiterate what the Agency has previously stated on this subject, this collective approach for the contribution test is consistent with the treatment of GHGs by those studying climate change science and policy, where it is common practice to evaluate GHGs on a collective, CO2-equivalent basis.  This collective approach to defining the air pollutant is not unique; grouping of many substances with common attributes as a single pollutant is common practice under the CAA, for example with particulate matter and volatile organic compounds (VOC).  As noted in section IV, these substances share common attributes that support their grouping as the air pollution for purposes of the endangerment finding.  These same common attributes also support the Administrator grouping the six GHGs for purposes of defining the air pollutant for the proposed cause or contribute finding under CAA section 231.
The Administrator recognizes that in this case, the aircraft engines covered by this notice emit two of the six gases, but not the other four gases.  Nonetheless, it is entirely appropriate, and in keeping with the 2009 Endangerment Finding and past EPA practice, for the Administrator to define the air pollutant in a manner that recognizes the shared relevant properties of all these six gases, even though they are not all emitted from the classes of sources before her.  For example, a source may emit only 20 of the possible 200-plus chemicals that meet the definition of VOC in the EPA's regulations, but that source is evaluated based on its emissions of VOC and not on its emissions of the 20 chemicals by name.  The fact that these six substances within the definition of GHGs share common, relevant attributes is true regardless of the type of sources being evaluated for contribution. By proposing to use the definition of the air pollutant as comprised of the six GHGs with common attributes, the Administrator is taking account of these shared attributes and how they are relevant to the air pollution that endangers public health and welfare.
         2. How the Definition of Air Pollutant in the Endangerment Determination Affects Section 231 Standards.
Under section 231(a), the Administrator is required to set "emission standards applicable to the emission of any air pollutant" from classes of aircraft engines that the Administrator determines causes or contributes to air pollution that endangers public health or welfare.  If the Administrator makes a final determination under section 231 that the emissions of the GHG air pollutant from certain classes of aircraft engines contribute to the air pollution that may reasonably be anticipated to endanger public health and welfare, then she is called on to set standards applicable to the emissions of this air pollutant.  The term "standards applicable to the emissions of any air pollutant" is not defined, and the Administrator has the discretion to interpret it in a reasonable manner to effectuate the purposes of section 231 to set standards that either control the emissions of the group of six well-mixed gases as a whole and/or control emissions of individual gases, as constituents of the class.  For example, it might be appropriate to set a standard that measures and controls the aggregate emissions of the group of GHGs, weighted by CO2 equivalent.  Depending on the circumstances, however, it may be appropriate to set standards for certain individual gases, or some combination of group and individual standards.  These and other similar approaches could appropriately be considered in setting a standard or standards applicable to the emissions of the group of GHGs that are defined as the air pollutant.  The Administrator would consider a variety of factors in determining what approach to take in setting the standard or standards; for example, she would consider the characteristics of the aircraft emissions, such as rate and variability, the kind and availability of control technology, and other matters relevant to setting standards under section 231.
Proposed Cause or Contribute Finding
         3. The Administrator's Approach in Making this Proposed Finding
As it did for the 2009 Endangerment Finding, and consistent with prior practice and current science, the EPA uses annual emissions as a reasonable proxy for contributions to the air pollution, i.e., elevated atmospheric concentrations of GHGs.  Cumulative emissions are responsible for the change in the stock of concentrations in the atmosphere (i.e., the fraction of a country's or an economic sector's cumulative emissions compared to the world's GHG emissions over a long time period will be roughly equal to the fraction of the change in concentrations attributable to that country or economic sector); likewise, annual emissions are a reasonable proxy for annual incremental changes in atmospheric concentrations.
There are a number of possible ways of assessing whether air pollutants cause or contribute to the air pollution which may reasonably be anticipated to endanger public health and welfare, and no single approach is required or has been used exclusively in previous determinations under the CAA.  Because the air pollution against which the contribution is being evaluated is the six well-mixed GHGs, the logical starting point for any contribution analysis is a comparison of the emissions of the air pollutant from the section 231 category to the total U.S. and total global emissions of the six GHGs.  The Administrator recognizes that there are other valid comparisons that can be considered in evaluating whether emissions of the air pollutant cause or contribute to the combined concentration of the six GHGs.  To inform the Administrator's assessment, section V.B.2 presents the following types of simple and straightforward comparisons of U.S. aircraft GHG emissions:
         * As a share of current total U.S. GHG emissions;
         * As a share of current U.S. transportation GHG emissions;
         * As a share of current total global GHG emissions; and
         * As a share of the current global transportation GHG emissions.
All annual GHG emissions data are reported on a CO2-equivalent (CO2eq) basis, which as described above is a commonly accepted metric for comparing different GHGs.  This approach is consistent with how EPA determined contribution for GHGs under section 202 of the CAA in 2009.
         4. Overview of Greenhouse Gas Emissions
Atmospheric concentrations of CO2 and other GHGs are now at essentially unprecedented levels compared to the distant and recent past.  This is the unambiguous result of human emissions of these gases.  Global emissions of well-mixed GHGs have been increasing, and are projected to continue increasing for the foreseeable future. According to IPCC AR5, total global (from all major emitting sources including forestry and other land use) emissions of GHGs in 2010 were about 49,000 teragrams of CO2 equivalent (TgCO2eq).  This represents an increase in global GHG emissions of about 29 percent since 1990 and 23 percent since 2000. In 2010, total U.S. GHG emissions were responsible for about 14 percent of global GHG emissions (and about 12 percent when factoring in the effect of carbon sinks from U.S. land use and forestry).
The Inventory of U.S. Greenhouse Gas Emissions and Sinks Report (hereinafter "U.S. Inventory"), in which 2012 is the most recent year for which data are available, indicates that total U.S. GHG emissions increased by 4.7 percent from 1990 to 2012 (or by about 2.7 percent when including the effects of carbon sinks), while emissions decreased from 2011 to 2012 by 3.4 percent.  This decrease was attributable to multiple factors including reduced carbon intensity of fuels for electricity generation, improvements in vehicle fuel efficiency with reductions in miles traveled, and a warmer winter leading to a decrease in heating requirements.  The U.S. Inventory also shows that while overall U.S. GHG emissions grew between 1990 and 2012, transportation GHG emissions grew at a significantly higher rate, 17 percent, more rapidly than any other U.S. sector.  Within the transportation sector, aircraft remain the single largest source of GHG emissions not yet subject to any GHG regulations.
 Because 2010 is the most recent year for which IPCC emissions data are available, we provide 2011 estimates from another widely used and recognized global dataset, the World Resources Institute's (WRI) Climate Analysis Indicators Tool (CAIT), for comparison.  According to WRI/CAIT, the total global GHG emissions in 2011 were 43,816 Tg of CO2eq, representing an increase in global GHG emissions of about 42 percent since 1990 and 30 percent since 2000 (excluding land use, land use change and forestry).  These estimates are generally consistent with those of IPCC.  In 2011, WRI/CAIT data indicate that total U.S. GHG emissions were responsible for almost 15.5 percent of global emissions, which is also generally in line with the percentages using IPCC's 2010 estimate described above.  According to WRI/CAIT, current U.S. GHG emissions rank only behind China's, which was responsible for 24 percent of total global GHG emissions.
Section V.B.2.a which follows describes U.S. aircraft GHG emissions within the domestic context, while section V.B.2.b describes these same GHG emissions in the global context.  Section V.B.2.c addresses future projections of aircraft GHG emissions.
      a. U.S. Aircraft GHG Emissions Relative to U.S. GHG Transportation and Total U.S. GHG Inventory
Relying on data from the U.S. Inventory, we compare U.S. aircraft GHG emissions to the transportation sector and to total U.S. GHG emissions as an indication of the role this source plays in the total domestic contribution to the air pollution that is causing climate change.  In 2012, total U.S. GHG emissions from all sources were 6,526 Tg CO2eq.  As stated above, total U.S. GHG emissions have increased by almost 5 percent between 1990 and 2012, while U.S. transportation GHG emissions from all categories have grown 17 percent since 1990. The U.S. transportation sector was the second largest GHG emitting sector (behind electricity generation), contributing 1,948 Tg CO2eq or about 30 percent of total U.S. GHG emissions in 2012.  This sectoral total includes emissions from combustion of U.S. international bunker fuels, which are fuels used for transport activities, from aviation (both commercial and military) and marine sources.  Consistent with IPCC guidelines for common and consistent accounting and reporting of GHGs under the UNFCCC, the "U.S. international aviation bunker fuels" category includes emissions from combustion of fuel purchased in and used by aircraft departing from the United States, regardless of whether they are a U.S. flagged carrier.  Total U.S. aircraft emissions constitute a significant part of the U.S. transportation sector's emissions, accounting for 212 Tg CO2eq or 11 percent of such emissions (see Table V.1.).  In 2012, emissions from aircraft (212 Tg CO2eq) were the third largest transportation source of GHGs within the United States, behind light-duty vehicles and medium- and heavy-duty trucks (1,535.6 Tg CO2eq).
For purposes of making this cause or contribute finding, the EPA is focused on, and proposes to include, a set of aircraft engine classes used in types of aircraft as described below, which corresponds to the scope of the international CO2 emissions standard contemplated by ICAO.
As mentioned earlier in section II.E traditionally the EPA (and FAA) participates at ICAO in the development of international standards, and then where appropriate, the EPA establishes domestic aircraft engine emission standards under CAA section 231 of at least equivalent stringency to ICAO's standards.  An international CO2 emissions standard is anticipated in February 2016, and provided that the EPA makes a positive endangerment finding and ICAO adopts an international CO2 emissions standard that is both consistent with CAA section 231 and appropriate for domestic needs, we would expect to proceed with promulgating a CO2 emissions standard (or GHG standard) of at least equivalent stringency domestically.  As described later in section VI.D, the thresholds of applicability for the international CO2 emissions standard  are based on gross weight as follows: for subsonic jet aircraft, a maximum takeoff mass (MTOM) greater than 5,700 kilograms; and for subsonic propeller driven (e.g., turboprop) aircraft, a MTOM greater than 8,618 kilograms.  Applying these gross weight thresholds, our proposed cause or contribute finding applies to GHG emissions from classes of engines used in covered aircraft.  Examples of covered aircraft  would include smaller jet aircraft such as the Cessna Citation II and the Embraer E170, up to the largest jet aircraft -- the Airbus A380 and the Boeing 747.  Other examples of covered aircraft would include larger turboprop aircraft, such as the ATR 72 and the Bombardier Q400.  Our intention is for the scope of the contribution finding to correspond to the aircraft engine GHG emissions that are from aircraft that match the applicability thresholds for the international aircraft CO2 standard. As such we have also identified aircraft that are not covered aircraft for purposes of our proposed contribution finding.  That includes aircraft that fall below the international applicability thresholds: smaller turboprop aircraft, such as the Beechcraft King Air, and smaller jet aircraft, such as the Cessna Citation I.  In addition, ICAO (with U.S. participation) has agreed to exclude "piston-engine aircraft,"  "helicopters," and "military aircraft" from the types of aircraft that would be covered by the anticipated ICAO standards. These aircraft would not be covered aircraft and consequently, we are also not including GHG emissions from classes of engines used in these types of aircraft in our proposed cause or contribute finding.
Thus, for the purposes of the cause or contribute finding, the EPA proposes to include GHG emissions from aircraft engines used in  covered aircraft in the scope of this proposed cause or contribute finding.  This is an equivalent scope of applicability as that contemplated by ICAO.      The majority of the GHG emissions from all classes of aircraft engines would be covered by this scope of applicability.  Below we describe the contribution of these U.S. covered aircraft GHG emissions to U.S. GHG emissions, and later in section V.B.2.b we discuss the contribution of these U.S. covered aircraft emissions to global GHG emissions.
In 2012, GHG emissions from U.S. covered aircraft (which includes U.S. international aviation bunker fuels in certain cases) comprised 90 percent (190 Tg CO2eq) of total U.S. aircraft GHG emissionsand 10 percent of total U.S. transportation sector GHG emissions (See Table V.1.).  Overall, U.S. covered aircraft comprised the third largest source of GHG emissions in the U.S. transportation sector behind only the light-duty vehicle and medium- and heavy-duty truck sectors, which is the same ranking as total U.S. aircraft. The U.S. covered aircraft also represent 3 percent of total U.S. GHG emissions, which is approximately equal to the contribution from total U.S. aircraft of 3.2 percent (Table V.1).
It is important to note that in regard to the six well-mixed GHGs (CO2, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride), only two of these gases -- CO2 and nitrous oxide -- are reported as non-zero emissions for total aircraft and covered aircraft.  CO2 represents 99 percent of all GHGs from both total aircraft (210 Tg CO2eq) and U.S. covered aircraft (188 Tg CO2eq), and nitrous oxide represents about one percent from total aircraft (2 Tg CO2eq) and covered aircraft (1.8 Tg CO2eq).  Modern aircraft do not emit methane, and hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are not products of aircraft engine combustion.
Table V.1
 COMPARISONS OF U.S. AIRCRAFT GHG EMISSIONS TO TOTAL U.S. TRANSPORTATION AND TOTAL U.S. GHG EMISSIONS

                                     1990
                                     2000
                                     2005
                                     2010
                                     2011
                                     2012
Total U.S. Aircraft GHG emissions (Tg CO2eq)
Share of U.S. Transportation
Share of total U.S. Inventory
                                                                            228
                                                                            14%
                                                                           3.7%
                                                                            262
                                                                            12%
                                                                           3.7%
                                                                            254
                                                                            12%
                                                                           3.5%
                                                                            216
                                                                            11%
                                                                           3.1%
                                                                            215
                                                                            11%
                                                                           3.2%
                                                                            212
                                                                            11%
                                                                           3.2%
U.S. Covered Aircraft GHG emissions (Tg CO2eq)
Share of U.S. aircraft GHG emissions
Share of U.S. Transportation
Share of total U.S. Inventory
                                                                            171
                                                                            75%
                                                                            10%
                                                                           2.7%
                                                                            223
                                                                            85%
                                                                            10%
                                                                           3.1%
                                                                            218
                                                                            86%
                                                                            10%
                                                                             3%
                                                                            191
                                                                            88%
                                                                           9.5%
                                                                           2.8%
                                                                            193
                                                                            90%
                                                                           9.8%
                                                                             3%
                                                                            190
                                                                            90%
                                                                           9.7%
                                                                             3%
Transportation Sector emissions
(Tg CO2eq)
Share of total U.S. Inventory
                                                                          1,661
                                                                            27%
                                                                          2,136
                                                                            30%
                                                                          2,136
                                                                            30%
                                                                          1,999
                                                                            29%
                                                                          1,969
                                                                            29%
                                                                          1,948
                                                                            30%

                                                       Total U.S. GHG emissions

                                                                          6,233

                                                                          7,107

                                                                          7,254

                                                                          6,875

                                                                          6,753

                                                                          6,526

      b. U.S. Aircraft GHG Emissions Relative to Global Aircraft GHG Inventory and the Total Global GHG Inventory
For background information and context, we first provide information on the contribution of GHG emissions from global aircraft and the global transportation sector to total global GHG emissions, and describe how this compares to the emissions from aircraft that would be covered by the anticipated ICAO CO2 standard.  We then compare U.S. aircraft GHG emissions to the global aircraft sector, to the global transport sector, and to total global GHG emissions as an indication of the role this source plays in the total global contribution to the air pollution that is causing climate change.  As in the preceding section, we present comparisons from both total U.S. aircraft and U.S. covered aircraft.
According to IPCC AR5, global aircraft GHG emissions in 2010 were 11 percent of global transport GHG emissions and 2 percent of total global GHG emissions.  Data from ICAO's 2013 Environmental Report indicate that the vast majority of global emissions from the aircraft sector are emitted by the types of aircraft that would be covered by the anticipated ICAO CO2 standard ("ICAO covered aircraft").  When compared to global data from IPCC AR5, worldwide GHG emissions from ICAO covered aircraft represented about 93 percent (688 Tg CO2eq) of global aircraft GHG emissions, 10 percent of global transport GHG emissions, and 1.5 percent of  total global GHG emissions in 2010.
Comparing data from the U.S. Inventory to IPCC AR5, we find that total U.S. aircraft GHG emissions represented about 29 percent of global aircraft GHG emissions, about 3.1 percent of global transport GHG emissions, and about 0.5 percent of total global GHG emissions in 2010 (see Table V.2).   For U.S. covered aircraft in 2010 GHG emissions represented about 26 percent of global aircraft GHG emissions, 2.7 percent of global transport GHG emissions, and 0.5 percent of total global GHG emissions (see Table V.2).  Because 2010 is the most recent year for which IPCC emissions data are available, we also made comparisons using 2011 estimates from WRI/CAIT and the International Energy Agency (IEA) and found that they yield very similar results.
Table V.2
COMPARISONS OF U.S.AIRCRAFT GHG EMISSIONS TO TOTAL GLOBAL GREENHOUSE GAS EMISSIONS IN 2010

                                     2010
                                  (Tg CO2 eq)
                             Total U.S. Aircraft
                                   Share (%)
                             U.S. Covered Aircraft
                                   Share (%)
                           Global Aircraft Share (%)
Global Aircraft GHG emissions
                                      743
                                      29
                                      26
                                      --
Global Transport GHG emissions
                                     7,000
                                      3.1
                                      2.7
                                      11
Total Global GHG emissions
                                    49,000
                                      0.5
                                      0.5
                                       2

For additional background information and context, we used 2011 WRI/CAIT and IEA data to make comparisons between the aircraft sector and the emissions inventories of entire countries and regions.  When compared to entire countries, total global aircraft GHG emissions in 2011 ranked 9[th] overall, behind only China, United States, India, Russian Federation, Japan, Brazil, Germany, and Indonesia, and ahead of about 175 other countries.  Total U.S. aircraft GHG emissions have historically been and continue to be by far the largest contributor to global aircraft GHG emissions.  Total U.S. aircraft GHG emissions are about 7 times higher than aircraft GHG emissions from China, which globally is the second ranked country for aircraft GHG emissions, and about 5 times higher than aircraft GHG emissions from all of Asia.  U.S. covered aircraft GHG emissions are about 6 times more than aircraft GHG emissions from China, and about 4 times more than aircraft GHG emissions from all of Asia.  If U.S. covered aircraft emissions of GHGs were ranked against total GHG emissions for entire countries, these covered aircraft emissions would rank ahead of Belgium, Czech Republic, Ireland, Sweden and about 150 other countries in the world.
      c. Aircraft GHG Emissions Are Projected to Increase in the Future
While overall GHG emissions from U.S. covered aircraft increased by about 12 percent from 1990 to 2010, the portion attributable to U.S. international aviation bunker fuels increased by about 100 percent.  During this same time period, global aircraft GHG emissions grew by about 40 percent, and the portion attributable to global international aviation bunker fuels increased by 80 percent.[,]  Notwithstanding the substantial growth in GHG emissions from U.S. international aviation bunker fuels, U.S. covered aircraft emissions have not increased as much as global aircraft emissions primarily because the U.S. aviation market was relatively mature compared to the markets in Europe and other emergent markets, and because during this time period the U.S. commercial air carriers suffered several major shocks that reduced demand for air travel.[,]  After consolidation and restructuring in recent years, the U.S. commercial air carriers have regained profitability and are forecasted by the FAA to grow more over the next 20 to 30 years.  With regard to global aircraft GHG emissions, the aviation markets in Asia/Pacific, Europe (where airline deregulation has stimulated significant new demands in this period), and the Middle East (and other emerging markets) have been growing rapidly, and the global market is expected to continue to grow significantly over the next 20 to 30 years.
Recent studies estimate that both ICAO covered aircraft and U.S. covered aircraft will experience substantial growth over the next 20 to 30 years in their absolute fuel burn, and that this will translate into increased GHG emissions.  ICAO estimates that the global fuel burn from ICAO covered aircraft is estimated to increase by about 120 percent from 2010 to 2030 and by about 210 percent from 2010 to 2040 (for a scenario with moderate technology and operational improvements).  The FAA projects that the fuel consumption from U.S. air carriers and general aviation aircraft operating on jet fuel will grow by 47 percent from 2010 to 2034, corresponding to an average annual increase rate in fuel consumption of 1.9 percent.  These aircraft groups (U.S. air carriers and general aviation aircraft operating on jet fuel) are of similar scope to the U.S. covered aircraft whose engine GHG emissions are the subject of this proposed finding.  Using fuel burn growth rates provided above as a scaling factor for growth in GHG emissions (globally and nationally), it is estimated that GHG emissions from ICAO covered aircraft and U.S. covered aircraft would increase at a similar rate as the fuel burn by 2030, 2034, and 2040.
         1. Proposed Contribution Finding for the Single Air Pollutant Comprised of the of Six Well-Mixed Greenhouse Gases
Taking into consideration the data summarized in section 2 above, the Administrator proposes to find that GHG emissions from classes of engines used in U.S. covered aircraft, which are subsonic jet aircraft with a maximum takeoff mass (MTOM) greater than 5,700 kilograms and subsonic propeller driven (e.g., turboprop) aircraft with a MTOM greater than 8,618 kilograms, contribute to the air pollution that endangers public health and welfare.  The Administrator is not at this time proposing a contribution finding for GHG emissions from engines not used in covered aircraft (i.e., those used in smaller turboprops, smaller  jet aircraft, piston-engine aircraft, helicopters and military aircraft).  We solicit comment on the scope of the proposed contribution finding, whether a broader contribution finding (e.g., including all engines used in aircraft certified by the FAA) would be appropriate, and the extent to which EPA has discretion to establish standards pursuant to a contribution finding that do not impose requirements on every engine or class of engines within the scope of that finding.
It is the Administrator's judgment that the collective GHG emissions from the classes of engines used in U.S. covered aircraft are significant, whether the comparison is domestic (10 percent of all U.S. transportation GHG emissions, representing 3 percent of total U.S. emissions) or global (26 percent of total global aircraft GHG emissions representing 2.7 percent of total global transportation emissions and 0.5 percent of all global GHG emissions).  The proposed scope of GHG emissions from engines used in U.S. covered aircraft under this cause or contribute finding would result in the vast majority (90 percent) of U.S. aircraft GHG emissions being included in this determination.  The Administrator believes that consideration of the global context is important for the cause or contribute test, but that the analysis should not solely consider the global context.  GHG emissions from engines used in U.S. covered aircraft will become globally well-mixed in the atmosphere, and thus will have an effect not only on the U.S. regional climate but also on the global climate as a whole, for years and indeed many decades to come.  It is the Administrator's view that the cause or contribute test used here under CAA section 231 can follow the same reasoning that was used in the 2009 GHG cause or contribute finding under CAA section 202; that is, the Administrator believes a positive cause or contribute finding for GHG emissions from engines used in U.S. covered aircraft is justified whether only the domestic context is considered, only the global context is considered, or both the domestic and global GHG emissions comparisons are viewed in combination.
As was the case in 2009, no single GHG source category dominates on the global scale, and many (if not all) individual GHG 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 Administrator continues to believe that these unique, global aspects of the climate change problem -- including that from a percentage perspective there are no dominating sources emitting GHGs and fewer sources that would even be considered to be close to dominating -- tend to support consideration of contribution to the air pollution at lower percentage levels than EPA typically encounters when analyzing contribution towards a more localized air pollution problem.  Thus, the Administrator, similar to the approach taken in the 2009 GHG cause or contribute finding under CAA section 202, is placing significant weight on the fact that engines used in U.S. covered aircraft contribute 3 percent of total U.S. GHG emissions for the proposed contribution finding and comprise the single largest transportation source in the United States that has not yet been regulated for GHG emissions.
         2. Additional Considerations
The Administrator is also concerned that reasonable estimates of GHG emissions from engines used in U.S. covered aircraft are projected to grow over the next 20 to 30 years.  Given the projected growth in aircraft emissions compared to other sectors, it is reasonable for the Administrator to consider future emissions projections as adding additional weight to her primary reliance on annual emissions.  Recent projections reveal that by 2034 GHG emissions from all aircraft and U.S. covered aircraft engines are likely to increase by almost 50 percent.  By contrast, it is estimated that by 2034 the light duty vehicle sector will see a 30 percent reduction in GHG emissions from the 2010 baseline, while the heavy duty vehicle sector will experience a 35 percent increase in GHG emissions from the 2010 baseline (this projected increase does not reflect the impact of GHG reductions anticipated from the Phase 2 heavy duty GHG standards that have not yet been promulgated).  In addition, by 2034 the rail sector is projected to experience a 6 percent reduction in GHG emissions from 2010 baseline. Because the projected growth in aircraft engine GHG emissions from U.S. covered aircraft appears to be greater in percentage terms than other transportation sources, this future consideration adds weight to the Administrator's proposed positive contribution finding.
Advance Notice of Proposed Rulemaking:  Discussion of Ongoing International Proceedings to Develop Aircraft CO2 Emissions Standard and Request for Comment
For more than four years, the EPA and FAA have been engaged with the ICAO's Committee on Aviation Environmental Protection (ICAO/CAEP) to establish an international CO2 emissions standard which the EPA could then consider proposing for adoption under its section 231 authority of the CAA.  This section of today's notice serves as an ANPR to discuss the key issues of the ongoing international proceedings prior to February 2016, when ICAO/CAEP is expected to finalize an international aircraft CO2 standard.  An ANPR is intended to solicit comments and/or information from the public prior to an agency determining whether to propose a rulemaking. As such, an ANPR does not propose or impose any regulatory requirements. The EPA may choose to develop an ANPR for actions (such as the promulgation of standards pursuant to CAA section 231 to implement an international aircraft CO2 standard domestically) which are still in the early stages of development and for which public input may be particularly helpful.  This also helps ensure transparency, while assisting the EPA in obtaining input from a wide range of stakeholders as we continue work within CAEP to establish an international CO2 aircraft standard.   The EPA is seeking comments on a variety of issues related to setting an international CO2 standard for aircraft, including whether such standards should apply to in-production aircraft instead of new aircraft types only, the appropriate effective dates for the potential international CO2 standard, as well as the appropriate stringency levels.
  CAEP met an important milestone at its 9[th] meeting (CAEP/9) in 2013 in reaching an agreement on the appropriate metric to be used in assessing fuel efficiency (or CO2 emissions) of an engine/aircraft combination.  They also reached agreement on a mature certification requirement to evaluate CO2 emissions for new aircraft types and also agreed on certain aspects of the scope of applicability of the CO2 emissions standard; however, work on applicability options for in-production aircraft continues.
At the CAEP Steering Group meeting in November 2013, there was agreement on a set of stringency options to be used for the cost-effectiveness analysis, and at the Steering Group meeting in September 2014 there was a decision on the associated inputs for costs and technology responses to be utilized in the cost-effectiveness analysis of these stringency options.  This analysis, and work on the applicability of the standard to in-production aircraft and the certification requirement are scheduled to be completed prior to the 10[th] CAEP meeting (CAEP/10) in February 2016. As described in section II. E, the EPA and the FAA traditionally work within the ICAO/CAEP standard-setting process to establish international emission standards and related requirements. Under this approach, international emission standards have first been adopted by ICAO, and subsequently the EPA has initiated rulemakings under CAA section 231 to establish domestic standards that are of at least equal stringency as ICAO's standards.  This approach has been affirmed as reasonable by the U.S. Court of Appeals for the D.C. Circuit.  Provided the EPA makes a positive endangerment finding under CAA section 231 and ICAO adopts an international aircraft CO2 standard that is consistent with CAA section 231 and U.S. domestic needs, we would expect to proceed with a similar approach promulgating a CO2 emissions standard (or GHG standard) of at least equivalent stringency domestically.
     B. Purpose of the International Standard
At the CAEP Steering Group meeting in 2011, the U.S provided a paper recommending that CAEP agree to the purpose of the international CO2 emissions standard to be "to achieve  CO2 emissions reductions from the aviation sector beyond expected `business as usual'  -  i.e., a standard that achieves CO2 emissions reductions from the aviation sector beyond what would be achieved in the absence of a standard.  This would be analyzed using ICAO criteria of technical feasibility, environmental benefit, cost effectiveness, and impacts of interdependencies." The Steering Group accepted the U.S. proposal for the purpose of the international CO2 standard, and it is expected to be included in the standard setting process.  The metric system, stringency options, costs, technology responses (inputs to be utilized in the cost-effectiveness analysis), and applicability ultimately chosen will all have an effect on whether the international CO2 emissions standard adheres to this stated purpose of the standard.  The U.S. continues to support the adoption of an international CO2 emissions standard that meets this stated purpose, and the EPA requests comment on this continued support.  The EPA requests comment on how to achieve the purpose of the standard.
Applicability of the International CO2 Emissions Standard
We are requesting comment on whether the aircraft CO2 standard should apply to in-production aircraft, including aircraft with any engineered fuel efficiency improvements (e.g.,  different engines, redesigned wings, or engine performance improvement packages, etc.) or whether the aircraft CO2 standard should apply only to completely new aircraft type designs.
In-production aircraft and new aircraft types are defined as follows:
 In-production aircraft: those aircraft types which have already received a Type Certificate, and for which manufacturers either have existing undelivered sales orders or would be willing and able to accept new sales orders.[,][,]

 New aircraft types: aircraft types that have applied for a Type Certificate after the effective date of a standard and that have never been manufactured prior to the effective date of a standard.
In addition, for context, out of production aircraft are those aircraft types which have already received a Type Certificate, but for which manufacturers either have no existing undelivered sales orders or would not be willing and able to accept new sales orders.  These aircraft are aircraft types that are no longer in active production.
As described earlier in section II.E, CAEP's Steering Group meeting in 2010 agreed that the scope of applicability for the international aircraft CO2 standard should be subsonic jets with an applicability weight threshold of maximum takeoff mass (MTOM) greater than 5,700kg (12,566lb) and turboprop aircraft with a MTOM greater than 8,618kg (19,000lb).  CAEP also agreed that the international CO2 standard should apply to new aircraft types, but not apply to out of production aircraft types, and that applying the standard  to in-production aircraft types should not be ruled out.
	It is important to further describe the difference between new aircraft types and in-production aircraft.  There are three categories of aircraft under consideration when describing a CO2 standard: new aircraft types submitted for certification (known as clean sheet designs), those with lesser levels of design change, such as a new series in an established type and model (considered to be significant partial redesigns), or an aircraft with incremental improvements.  New aircraft types or new type designs are significant investments for manufacturers and are used for new and significantly different designs (or also characterized as complete redesigns).    Significant partial redesigns may be characterized as a new or later series of an established model that may incorporate newly designed wings and give purchasers more choices of engines.  Incremental improvements are less extensive changes to an aircraft such as performance improvement packages that may be added to an aircraft or engine at some point during the production cycle..
New aircraft types or new type designs are infrequent.  The most recent new type designs introduced in service, such as the Airbus A380 in 2007, the Boeing 787 in 2011, and the original Boeing 777 in 1995,[,][,] indicate that it is unlikely a new type design will seek certification in the next 10 to 15 years.  Significant partial redesigns do not occur often, but  are slightly more frequent than new type designs.  For example, after the current significant partial redesign wave has passed (which includes the Boeing 747-8, Boeing 737 Max, Airbus 320 Neo, and Boeing 777-X), we do not currently have knowledge of many additional significant partial redesigns anticipated over the next decade (as the previous wave of significant partial redesigns included the Boeing 777-200LR in 2004, 777-300ER in 2006, 737NG in 1998, Airbus 319 in 1996, and Airbus 330-200 in 1998).  Incremental improvements will likely be frequent and occur in the near term.  One approach for CAEP would be to limit the applicability of any international CO2 standard to new type designs (or new aircraft types)..  Under this approach significant partial redesigned aircraft and incremental improvements would be characterized as changes made to in-production aircraft; thus, these categories of aircraft (or these changes) would need to meet the international CO2 standard only if it applied to in-production aircraft.
Another approach for applicability of the international CO2 standard that CAEP could adopt would be an approach based on criteria addressing significant changes to aircraft designs, which could be considered an applicability requirement different than that for new aircraft types only and in-production aircraft. This alternative approach could redefine a new aircraft type for CO2 purposes to include in-production aircraft that have a significant change in CO2 emissions, thus including in-production aircraft in the applicability of the CO2 standard.  The alternative approach could even cover significant partial redesigned aircraft, depending upon the definition.  CAEP's current mature certification requirement for the international CO2 standard provides further detail on technology changes to aircraft that would affect the aircraft's CO2 metric value.  A changed version of an aircraft could be defined as follows: an aircraft which incorporates changes in type design that may adversely affect its CO2 emissions.  This possible definition could also note the following: (1) where the proposed change in design, configuration, power or mass is so extensive that a substantially new investigation of compliance with the applicable airworthiness regulations is required, the aircraft should be considered to be a new type design rather than a changed version, and (2) "adversely" refers to an increase in CO2 emissions of more than an amount (or percentage) that has yet to be determined (this amount or criterion is still being considered by CAEP).  The EPA requests comment on this change-based criteria approach, including how to identify those changes that would result in treating in-production aircraft as new types subject to the standard.
If CAEP were to limit the scope of applicability to new aircraft types only (and without the significant change criteria approach described above), the international CO2 standard would not apply to later series aircraft with redesigned wings, aircraft that are available with different engines, or aircraft that undergo incremental improvements. Following are several examples that illustrate this situation. The re-engined Boeing 737 Max is an example of a significant partial redesigned aircraft that is expected to enter into service in 2017. This aircraft would fall under the original Boeing 737 Type Certificate that was issued in 1967 (and entered into service in 1968) -- or more specifically it would fall under an amended Type Certificate, and it would not be considered a new aircraft type as defined by CAEP.  The current 737s (Next Generation 737s or commonly abbreviated as 737 NGs) feature newer engines, have redesigned wings, and entered service in 1998 under the original 737 Type Certificate that was issued in 1967, and these also were not considered a new type aircraft when they were introduced in 1998.  Another example of an aircraft that does not qualify as a new type is the Boeing 747-8  aircraft, that entered into service in 2011, and which included a new wing, new engines, and a lengthened fuselage but fell under an amended Type Certificate for the original Boeing 747 that was certified in 1969 (and entered into service in 1969).  An example of incremental improvements to in-production aircraft, is the Boeing Next Generation 737 performance improvement package which was implemented between 2011 and 2013 and the Boeing 767-300 winglets that entered into service in 2008, both of which improve aircraft fuel efficiency.  There are many other examples that exist for different manufacturers and aircraft around the world as well, but for conciseness, we are limiting our discussion to these above examples.  These examples illustrate the typical certification for significant partial redesigns and incremental improvements by various aircraft certificating or certifying authorities (or national airworthiness authorities) around the world.
Using CAEP's current definition of new aircraft types (clean sheet designs, which are completely new aircraft) we cannot today identify the first aircraft to which that standard would apply..  As the examples above illustrate, new aircraft types are infrequent, and there are no currently announced new type designs that are expected to be introduced after the implementation dates being analyzed by CAEP  -  2020 and 2023.  Furthermore, based on provisions to which CAEP has already agreed, new aircraft types subject to the CO2 standard would be aircraft that submit an application for a Type Certificate after the implementation dates of 2020 and 2023 (dates for the stringency analysis) which would likely result in entry into service dates of about 2025 or 2028.  If the international CO2 standard is applied only to new aircraft types, then CO2 emissions would not be expected to begin to deviate from business-as-usual (in comparison to CO2 emissions reductions that would be achieved in the absence of a standard) before 2025.  Therefore, an international standard developed for only new aircraft types may not actually apply to any new aircraft for at least a decade.  Even if a few new type aircraft are introduced in this timeframe, it will take even longer for these aircraft to comprise any significant portion of the fleet.  Therefore, applying an international standard which applies only to new aircraft types will likely result in no additional CO2 reductions beyond what would have occurred absent a CO2 standard, either for the near- and mid-term, about 5 to 10 years from 2016, or even in the longer-term of 20 years plus.[,]
The EPA requests comments on the timeframes for introducing new aircraft types and their subsequent penetration into the fleet.  Are there any aircraft manufacturer announcements that we missed in regard to new aircraft types that will be introduced or apply for a Type Certificate after 2020 and 2023 (or new aircraft types that well be introduced or apply for a Type Certificate five years after these dates)?  If so, what are these new aircraft types?  How many new types are projected to enter the fleet in this timeframe and what portion of the fleet will they represent?
The alternative approach described earlier (addressing changes in design of in-production aircraft) may offer an opportunity to cover more aircraft in an earlier timeframe (including significant partial redesigns), but it is unclear what effect this approach would have on additional CO2 emissions reductions compared to a standard for only new aircraft types.  The EPA requests comments on the timeframe for CO2 emissions reductions under the alternative approach.
     Applying an emission standard to in-production aircraft (if the EPA makes a positive endangerment finding), means that all aircraft that are built (in-production) after the effective date of the regulation would need to certify and comply with the standard to remain in production.  This includes (as described earlier) in-production aircraft with incremental improvements (though we reiterate this would not include in-use aircraft).  As an example of in-production aircraft, the Gulfstream G650, which is currently in production and expected to remain so after 2020, would need to certify and comply with the new CO2 standard.  In the next section we discuss in more detail how applicability to in-production aircraft could work.
CAEP Discussion on In-Production Aircraft Applicability
At the request of the CAEP Steering Group meeting in November 2013, CAEP began work on defining potential options to implement applicability requirements for in-production aircraft.  Subsequently, based on the options provided to the 2014 Steering Group meeting, CAEP decided that it should continue to investigate potential in-production aircraft applicability options, and that these should be presented at the July 2015 Steering Group meeting, so that a decision can be taken at the 10th meeting of CAEP (CAEP/10) in February 2016 regarding whether the international CO2 standard will apply to in-production aircraft.  There are a wide range of options under consideration, including both mandatory and voluntary options for reporting and certification processes for in-production aircraft applicability, but the 2014 Steering Group meeting requested that CAEP focus on defining the mandatory options (in contrast to options such as voluntary reporting and certification).
         1. Applicability to In-Production Aircraft and Date of Implementation
At the 2014 Steering Group meeting, CAEP also agreed that 2023 represented the earliest possible date for an in-production aircraft standard to allow time to promulgate domestic regulations and process manufacturer certification applications.  CAEP did not rule out later dates though and could consider implementation dates for an in-production aircraft CO2 standard later than 2023 (five years following the new aircraft type applicability date, i.e. dates ranging from 2023 to 2028).
The EPA seeks comments on both a 2023 implementation date and on possible later implementation dates for an in-production domestic CO2 (or GHG) aircraft engine emissions standard that would be adopted under CAA section 231,  the impact the date of implementation might have on per-aircraft GHG or CO2 emissions rates and the ability of a domestic GHG or CO2 standard to achieve aircraft emission reductions beyond what would occur in the absence of such a standard.  Would the dates of applicability (e.g., 2023 to 2028) be consistent with the purpose of the standard as articulated by CAEP?
 As described in section VI.F.2, the technologies considered for the CAEP analyses are those technologies that will be widely used on in-production aircraft by 2016 or shortly thereafter.  The EPA requests comments regarding whether applying an international CO2 standard to in-production aircraft is consistent with the purpose of the standard as advised by the CAEP Steering Group meeting in 2011: "to achieve CO2 emission reductions from the aviation sector beyond expected `business as usual'... analyzed using ICAO criteria of technical feasibility, environmental benefit, cost effectiveness, and impacts of interdependencies."   The International Coalition for Sustainable Aviation (ICSA),  which is a CAEP Observer organization, submitted papers to CAEP that analyzed this issue.  Also, a  member of ICSA has developed similar analyses which indicate that applying the international standard only to new aircraft types would likely result in no additional CO2 reductions beyond what would have occurred absent a CO2 standard, either for the near- and mid-term, about 5 to 10 years from 2016, or even in the longer-term of 20 years plus.  This occurs, the ICCT states, because the development cycles for new aircraft are very lengthy and it is not unusual for new aircraft to take 8 to 10 years to develop from preliminary design to entry into service and once in service it takes significant time for new aircraft types to penetrate the fleet.[,] Another study funded by the EPA corroborates this analysis.  The EPA requests comments on whether applying the international CO2 aircraft standard only to new aircraft types would be consistent with the stated purpose of the international standard.  Lastly, the EPA requests comment on the appropriateness of following either of these approaches (applicability to only new aircraft types or both new aircraft types and in-production aircraft) in any future CAA section 231 aircraft engine CO2 or GHG emissions standard (if either approach is adopted by ICAO, and the EPA makes a positive endangerment finding).
Also, there have been concerns raised in CAEP about applying the international CO2 standard to in-production aircraft.  These concerns include (a) the added resource burden on certificating authorities to process manufacturers' certification applications, which will be more numerous compared to new aircraft types; and (b) the potential added costs to manufacturers to certify in-production aircraft.  The EPA requests comment on these two concerns, including providing supporting documentation on the extent of these concerns and any other issues the commenters may identify with applying the international CO2 standard to in-production aircraft.
         2. Reporting Requirement for New In-Production Aircraft
CAEP is working to define mandatory in-production aircraft options, and one possible option is a reporting requirement[,][,] for in-production aircraft CO2 emissions rates (measured according to the aircraft test procedure that was agreed upon at CAEP/9) as an alternative to establishing an aircraft CO2 standard for in-production aircraft.  Although a reporting requirement provides policy relevant information, it does not necessarily translate into specific emissions reductions.  The EPA recognizes that only a mandatory standard for in-production aircraft would ensure that the aircraft CO2 standard reduces per-aircraft CO2 emissions rates.  However, a reporting requirement could be an important component of an in-production aircraft CO2 standard, especially if it is implemented shortly after an in-production aircraft standard is adopted.  It would ensure that CO2 emissions rates data are gathered quickly prior to an effective date for the final standard (tracking CO2 emissions rates is beneficial for the reasons discussed later in this section and for potentially assisting with the assessment of a future CO2 standard).  The EPA requests comment on an aircraft manufacturer reporting requirement that is implemented soon after the adoption of an in-production international aircraft CO2 standard, as a component of the in-production aircraft CO2 standard.
In 2009 the EPA promulgated a final GHG reporting rule that applies to many sectors in the United States, including manufacturers of heavy-duty and offroad vehicles and engines, and manufacturers of aircraft engines.[,]  The EPA's experience with reporting programs indicates that the EPA and the public would be able to track CO2 emissions rates trends (i.e., trends of aircraft cruise fuel burn rates) from aircraft over time.  Requiring the reporting of aircraft CO2 emissions rates trends (e.g., trends of aircraft cruise fuel burn rates) from aircraft over time is appropriate and feasible.  Requiring aircraft manufacturers to report aircraft CO2 emissions rates shortly after an in-production international aircraft standard is adopted would enable and expedite the tracking and understanding of these emission trends.  In addition, reporting programs typically raise awareness of emissions and can improve the understanding of the factors that influence emission rates as well as the actions that can be taken to reduce emissions.  When similar methods for monitoring, measurement, and reporting are applied across an industry, it can lead to more consistent, accurate, and timely data to inform decision-making for individual manufacturers and the EPA (including a comparison of the CO2 emissions rates from aircraft of various manufacturers).  Thus, a reporting requirement could potentially contribute to efforts to identify and implement future aircraft CO2 emission reduction opportunities.
Independent of action that CAEP may or may not take in February 2016, the EPA could under its CAA section 114(a) authority pursue a reporting requirement for aircraft GHG or CO2 emissions rates (aircraft cruise GHG or CO2 emission rates)  -  to ensure we have GHG or CO2 emissions rates data on all in-production aircraft (and any new aircraft types that enter service).  The EPA could use the same metric agreed to at CAEP/9 (and in ICAO circular 337). This will be described in detail in VI.D.1 below.  In general, the EPA asks for comment on a mandatory reporting requirement for in-production aircraft GHG or CO2 emissions rates  -  either as part of the CAEP international standard or as an independent domestic requirement to be adopted by the EPA.  If the EPA were to pursue this requirement independently from CAEP, what lead time would be appropriate for manufacturers to report the GHG or CO2 emissions rates from all of their in-production aircraft (after we promulgate such a requirement)?  Additionally, if we were to pursue such an independent reporting requirement, should we require the annual reporting of the GHG or CO2 emissions rates from in-production aircraft (and any new type aircraft) to enable us to track any updates?  We are not at this time proposing to promulgate such a requirement in advance of ICAO's decision.  Due to the possibility of ICAO's adoption of a reporting requirement, we believe it is reasonable to await the outcome of that decision in order to determine whether to strictly follow ICAO's possible reporting requirement or make changes to it in the form of an additional U.S. domestic requirement, as appropriate.
Metric System, Applicability, and Certification Requirement
The CO2 metric system and mature certification procedure were agreed upon by CAEP in 2013.   This section describes the metric system that was developed, the scope of aircraft to be covered by the international CO2 standard, the certification test procedures that would be used to demonstrate compliance with the international CO2 standard, and CAEP's decision to focus on the entire aircraft for the international CO2 standard .
         3. CO2 Metric System
The metric system was developed to cover a wide range of aircraft types, designs, technology, and uses.  To do this, the metric system was designed to differentiate between generations of aircraft and to equitably capture improvements in aerospace technology (structural, propulsion, and aerodynamic) that contribute to a reduction in the airplane CO2 emissions. In addition, the metric system accommodates a wide range of technologies and designs which manufacturers may choose to implement to reduce CO2 emissions from their aircraft.
The metric system agreed to at CAEP uses multiple Specific Air Range (SAR) test points to represent cruise fuel burn.  SAR is a traditional measure of aircraft cruise performance which measures the distance an aircraft can travel for a unit of fuel. This is similar to the instantaneous "miles per gallon" readings in many cars today.  The SAR data are gathered at three gross weight points.  The three equally weighted points are used to represent a range of day to day aircraft operations.   The functional form of the metric system is provided below.
CO2 Metric Value= 1SARavgRGF0.24
Equation 1: CO2 Metric
(1/SAR)avg is calculated at 3 gross weight fractions of Maximum Takeoff Mass (MTOM):
High gross mass: 92% MTOM
Mid gross mass: Average of high gross mass and low gross mass
Low gross mass: (0.45 * MTOM) + (0.63 * (MTOM0.924))
The Reference Geometric Factor (RGF) is a measure of the fuselage size on a given aircraft. In analyzing various metric system options it was found that in some instances, namely stretch aircraft, changes in aircraft size, and thus capability, were not reflected in changes to the aircraft's gross weight (MTOM).  To account for these occurrences, and the variety of methods that manufacturers may use to make such a change, an adjustment factor was added (the RGF with a 0.24 exponent used in the metric system).
         4. Applicability
CAEP has decided to apply the international CO2 standard to subsonic jet and propeller-driven aircraft meeting the following criteria:
All subsonic jet aircraft over 12,566 lbs (5,700kg) MTOM.
All subsonic propeller driven (e.g. turboprop) aircraft over 19,000 lbs (8618 kg) MTOM, except amphibious airplanes and those designed and used for fire-fighting operations.
No military aircraft will be subject to this international standard.
         5. Certification requirement
CAEP has developed a mature certification requirement that would allow for the determination of an aircraft CO2 metric value for any aircraft meeting the applicability criteria set forth above. This certification requirement incudes the metric system and test procedure.  The test procedure was based upon industry's current best practices for establishing the cruise performance of their aircraft, and input from certification authorities. These procedures include specifications for aircraft conformity, weighing, fuel specifications, test condition stability criteria, required confidence intervals, measurement instrumentation required, and corrections to reference conditions.
These CO2 test procedures are based upon manufacturer's existing practices when certifying new aircraft. This means that there is a very heavy reliance on dedicated flight testing of the aircraft. This potentially poses challenges for the certification of in-production aircraft. Manufacturers have stated that there could be logistical challenges associated with the certification of aircraft for CO - 2 that have previously been type certificated (e.g. procuring and instrumenting an aircraft for flight testing). To address this, the EPA is currently working within CAEP to encourage the development of a modified or separate equivalent certification test procedure that would reduce this burden on manufacturers and allow for quicker/simpler certification of in-production types.
         6. Regulating the Entire Aircraft Instead of the Engine
The CO2 metric system intends to equitably reward improvements in aircraft technologies that reduce emissions, including advances in structures (aircraft weight), propulsion (engine specific fuel consumption), and aerodynamics.  These three factors are key to the overall aircraft CO2 emissions.  In addition, CAEP believes (as does the EPA) that it is best to consider the aircraft as a whole instead of only the aircraft engine technology in addressing factors that influence CO2 emissions, because of the effects and interaction these key factors have on the aircraft CO2 emissions from engines.  The three factors -- and technology categories that improve these factors -- are described as follows:
Structures: reducing basic aircraft weight to increase the commercial payload or extend range for the same amount of thrust and fuel burn;
Propulsion (thermodynamic and propulsion efficiency): advancing the overall specific performance of the engine, to reduce the fuel burn per unit of delivered thrust; and
Aerodynamics: advancing the aircraft aerodynamics, to reduce drag and its associate impacts on thrust.
Specific examples of technologies that affect these three factors help to further illustrate that it is best to consider the aircraft as a whole in addressing CO2 emissions.  For structural improvements, aircraft manufacturers have shown significant weight reduction results over time due to the progressive introduction of new technologies such as: advanced alloys and composite materials, improved and new manufacturing processes and techniques (including integration and global evaluation simulation ), and new systems ( e.g. fly-by-wire).
For propulsion improvements, technologies include enhanced compressors (e.g., intercooled compressors) and reduced hub-tip ratio fan.  As another example, manufacturers seek higher operating pressure ratios (OPR) to improve combustion and engine cycle refinements.
For aerodynamics, friction and lift-dependent drag are the biggest contributors to aerodynamic drag.   Advances in aerodynamics enable significant lift-dependent drag reduction by maximizing effective wing span extension.   For example, wing-tip devices can give an increase in the effective aerodynamic span of wings, particularly where wing lengths are limited by airport gate sizes.  Manufacturers are also looking at ways of decreasing the drag caused by skin friction. An example of a technology to improve aircraft local skin friction is to utilize riblets (which are micro-grooves on the surface) to maintain laminar flow via Natural Laminar Flow  and Hybrid Laminar Flow Control (HLFC) to reduce turbulent skin friction.  The first production example of a HLFC system went into service on the new Boeing 787-9 in 2014.
Stringency Options
At the Steering Group meeting in November 2013, CAEP agreed to analyze a range of CO2 stringency options that cover the full range of aircraft in-production and in-development around the world (within the applicable weight thresholds and categories), and this includes the wide range of technology that is currently in the aircraft fleet.  Generally, the stringency options that are being evaluated fall into three categories as follows: (1)  CO2 stringency levels that would impact only the oldest, least efficient aircraft in-production around the world, (2)  middle range CO2 stringency levels that would impact many aircraft currently in-production and comprising much of the current operational fleet, and (3) CO2  stringency levels that could impact aircraft that have either just entered production or are in final design phase but will be in-production by the time the international CO2 standard becomes effective. We are requesting comment on the level(s)  at which the CO2 stringency options should be set, what factors should be considered in establishing the stringency of the CO2 standard, and on their potential relationship to any future CAA section 231 standard.
The figures below are intended to show the range of stringency levels under consideration at CAEP and CO2 metric value levels of today's in-production and in-development aircraft.  The data shown were generated by the EPA using a commercially available aircraft modeling tool called PIANO.  This model contains non-OEM estimates of the performance of various aircraft. In contrast, CAEP is using manufacturer provided estimates of the aircraft metric value performance.
The stringency options under consideration at CAEP are upwards sloping and have a "kink" at 60,000 kilograms.  The "kink" was included in the stringency options as a technical approach to reflect the different behaviors observed between the larger and smaller aircraft.
The official stringency options under consideration at CAEP have not been cleared for release outside of the participating members since deliberations on the standard are still ongoing (proceedings are expected to be completed at CAEP/10 in February 2016).  To show the relative efficiency of the aircraft, Figure 1 and Figure 2 below show the aircraft metric values versus MTOM.  In place of the official stringency options under consideration, lines of constant technology are used to notionally show how the stringency options were set across the fleet. These lines notionally reflect the three ranges of options discussed above.  Figure 1 shows the makeup of the current production fleet and the in-development aircraft. This is what CAEP is using as the starting point for modeling the effect of the CO2 standard. Figure 2 shows what the EPA expects the market to look like in 2023, considering the publicly announced plans by industry to replace existing aircraft with new products.

Figure 1  -  Lines of constant technology level over the 2014 in-prodcution and in develpoment fleet

Figure 2  -  Lines of constant technology level over the predicted 2023 in-production fleet

A standard set near the upper constant technology level would affect a very modest number of aircraft, namely the oldest, least efficient types. Many of the aircraft that would be affected by such a stringency level are being produced in very limited numbers and may not be eligible to operate in U.S. air space.
Aircraft around the middle two constant technology levels reflect the performance of many aircraft that are currently in production and comprising much of the current operational fleet.  The current generation of single aisle aircraft from Boeing and Airbus are in this middle range.
Constant technology levels at the bottom of the charts reflect the most advanced aircraft currently for sale on the market.  These are aircraft that have either just entered production or are still in development but will be in-production by the effective date of the standard.  The replacement single aisle aircraft and new twin aisle aircraft from Boeing and Airbus are at the edge of the most stringent option.
Figure 3 and 4 show the margin of the lines of constant technology to the upper bound or worst performing technology bound under consideration. This is shown to further illustrate the range of technology present in the in-production and in development fleet.  Figure 3 provides the perspective from the current in-production and in-development fleet, and Figure 4 shows a viewpoint of the projected 2023 market.

Figure 3  -  Percent from least efficient line of constant technology for the 2014 in production and in development fleet

Figure 4 - Percent from least efficient line of constant technology for the predicted 2023 in - production fleet

The EPA requests comment on a range of stringency options within the constant technology lines identified in Figure 1 and Figure 2, on their potential impact, and on their potential relationship to any future CAA section 231 standard.
CAEP is considering the possibility of adopting two separate CO2 stringency levels, one for new type aircraft and one for in-production aircraft. This would allow stringencies to be set for both new types and in-production aircraft at a level closer to what could be achieved by each aircraft type. Issues surrounding the potential for in-production standards are discussed in section 1.
 There is ongoing discussion on what appropriate levels of stringency may be for new type and in-production aircraft.  Any final decisions will have to wait until the full analysis has been conducted at CAEP.  As explained in sections B and 1, new types are infrequently developed and typically represent a step change in technology. It may be possible to set a level of stringency that is reasonable for in-production aircraft to meet, but at the same time provide an incentive for new type aircraft to improve. However, this is challenging to develop because of the significant efficiency improvements typically seen between in-production and new type aircraft. The EPA requests comment on the potential for developing a standard with two stringency levels at CAEP.
The development of a new aircraft type standard must take into consideration the standards potential effect on any future type designs.  Even the most stringent option under consideration at CAEP is still based on technology available today. Any new type aircraft that may be developed and certified 10 years or more from now would be expected to use more advanced fuel efficient technology that is not yet developed or tested.
The implications for an in-production standard are more significant in the near term for manufacturers. Aircraft currently in-production, and not meeting the level of an in-production standard, would need to be modified to meet the standard to remain in production; this would take time and resources from the manufacturers.  The full implications of this have not yet been resolved in CAEP.  However, we expect that the effect on aircraft CO2 emissions would be minimal for less stringent options. The aircraft with the highest CO2 metric values generally rely on older technology and were designed in the 1980's to early 1990's. Many of these aircraft are also expected to be replaced with updated versions in the near future, before a CO2 standard would be implemented and go into effect. The EPA requests comment on the levels at which in-production and new type standards might be set and on what factors should be considered in establishing the stringency.
Costs, Technology Responses for Stringency Options, and Cost-Effectiveness Analysis
The EPA has been involved in CAEP's effort to analyze the CO2 stringency options and the potential costs and environmental impacts that would result from both new type only CO2 standards and in-production international CO2 standards.  CAEP is still determining the best way to conduct portions of this analysis.   The inputs that have been developed by the CAEP include non-recurring costs data and technology responses for the various stringency options under consideration.  This section describes the development of these inputs.  The EPA requests comments on how the modeling should be conducted to differentiate in-production and new type scenarios.
         7. Non-Recurring Costs (engineering development costs)
CAEP developed a single cost estimate that could be used for all aircraft as a function of MTOM and percent metric value improvement required. Based on past practice, industry provided estimates for developing clean sheet designs and significant partial redesigns, only including high level information that has been made available to the public.  This was considered to be a top down estimate because it included all aircraft development costs (airworthiness certification, noise, etc.) not just those for CO2 improvements.  The EPA saw the need to supplement this dataset with an estimate of CO2 only changes, which was considered to be a bottom up estimate. The EPA contracted with ICF to develop an estimate of the cost to modify in production aircraft to comply with a CO2 standard.  ICF conducted a detailed literature search, conducted a number of interviews with industry leaders, and did its own modeling to estimate the cost of making modifications to in production aircraft.  The results from this peer-reviewed study were then combined with inputs from the industry and the rest of the CAEP to develop the CO2 technology response and cost estimation. For the cost estimation, the CAEP combined the two different methodologies to develop the final cost surface.
A top-down approach was used to model large changes to aircraft design, such as what would be seen in significant partial redesigns or new types.  For significant partial redesigns, that result in new series of an established model, these types of changes may include, redesigned wings, new engine options, longer fuselages, improved aerodynamics, or reduced weight.  When making significant changes to an aircraft many other changes and updates get wrapped into the process that do not have an effect on the CO2 emissions of the aircraft, and significant partial redesigns may not have been spurred by changes to fuel efficiency (CO2 reductions).  This confluence of changes led CAEP to agree that it was reasonable to use the full development cost for a new type (clean sheet) or significant partial redesign for major changes. Total costs for past projects were used to estimate non-recurring cost for the CAEP analysis.  This type of aircraft improvement/development program has historically ranged approximately from 1 to 15 billion Dollars (U.S.) depending on the size of the aircraft and scope of the improvements desired.
A bottom-up approach was used to model smaller incremental metric value changes to aircraft design.  The CAEP agreed that the above top-down approach would not be the best approach for minor changes or incremental improvements because the significant design efforts include many changes that would not be required for smaller CO2 reductions.  The EPA used the information gathered by ICF to provide input on the cost for individual technologies which were used to build up non-recurring costs for these incremental improvements (a bottom-up approach).   The technologies available to make incremental improvements to aircraft is wide ranging and aircraft specific. Some examples of technologies that could be integrated into an aircraft for incremental improvements include improved fan blade design or reduction in turbine clearances in the engine, reducing the gap between control surfaces, carbon brake pads, or advanced wing tip devices.  As an example, ICF estimated that depending on the additive nature of specific technologies and the magnitude improvement required, the cost to incrementally improve the Boeing 767 could range from approximately 230 million to 1.3 billion US dollars.
         8. Technology Responses
When CAEP started to develop the technology responses for the stringency options, a determination needed to be made on what level of technology could be considered as a response to the standard. At the outset, CAEP decided the international CO2 standard would be a technology following standard, rather than a technology forcing one.  This means that the international standard would reflect a level of emissions performance that is already achieved by some portion of current production aircraft.  Additionally, CAEP determined in 2012 that all technology responses would have to be based on technology that would be in common use by the time the standard was to be decided upon in 2016 or shortly thereafter, This generation of technology was defined within CAEP as a Technology Readiness Level (TRL) 8  -  an actual system completed and qualified through test and demonstration  -  by 2016 or shortly thereafter. This means that the technology responses considered for the future international CO2 standard, going into effect internationally in 2020 or 2023 for new types and potentially in 2023 or later for in-production, are based on what will be in operation by 2016 or shortly thereafter. EPA is requesting comment on the approach of establishing a new -type standard in 2020 or 2023 and in-production standard for 2023 and later based on proven 2016 technologies. In addition the EPA requests comment on how the purpose of the standard can be achieved considering the technology response assumptions agreed to at CAEP.
         9. Cost Effectiveness Analysis
CAEP is currently conducting the cost effectiveness analysis for new-type and in-production aircraft. With rare exceptions CAEP has historically developed new type only standards.  To model cost impacts of a new type standard, CAEP has historically used an assumption that the in-production aircraft will respond to the new type standard, even though the standard would not apply to them and has assumed that the aviation sector is competitive enough that market forces will drive manufacturers to voluntarily upgrade their fleet to meet any new type aircraft standard. This scenario is modeled no differently from a mandatory in-production standard. The EPA requests comment on modeling cost and environmental impacts of new-type standards based on the assumed attainment of such emissions levels by in-production aircraft.
Because CAEP has modeled all in-production aircraft as responding by the implementation date of the new-type standard, CAEP has by definition, performed an in-production analysis.   More stringent options for new-type aircraft may be restricted due to the assumed in-production impacts.
CAEP has recognized that its past methods for modeling a new-type only standard (by assuming in-production aircraft comply) may not be sufficient for the CO2 standard analysis.  Thus, CAEP developed new methods to model what cost and environmental impacts would result from only new types being regulated under a new-type emission standard.  CAEP is still determining the best way to conduct an analysis of impacts only on new types using the agreed upon technology responses and cost estimates.  The EPA requests comments on how to model cost impacts for only new types for the future international CO2 standard, if it were to apply only to new types. The EPA also requests comment on how the modeling should be conducted to differentiate in-production and new type scenarios.
Request for Comment on Potential for a Different EPA Standard than International CO2 Standard
As described earlier in section II.E, traditionally international emission standards for aircraft have first been adopted by ICAO, and subsequently the EPA has initiated rulemakings to establish domestic standards that are of at least equal stringency as ICAO's engine standards.  However, the Chicago Convention, which established ICAO, recognizes that ICAO member states may adopt their own unique standards that are more stringent than ICAO standards.  A participating member state (or nation) that adopts more stringent standards is obligated to notify ICAO of the differences between its standards and ICAO's standards.
Section 231(b) of the CAA requires that any emission standards "take effect after such period as the Administrator finds necessary (after consultation with the Secretary of Transportation) to permit the development and application of the requisite technology, giving appropriate consideration to the cost of compliance during such period." 42 U.S.C. 7571(b).  Section 231(a)(2)(B) provides that the Administrator shall consult with the Administrator of the FAA on standards, and "shall not change the aircraft engine emission standards if such change would significantly increase noise and adversely affect safety." 42 U.S.C. 7571(a)(2)(B).
As discussed in the 2005 rule (CAEP/4 aircraft engine NOx standard), the EPA needs to have a technical basis for expecting the standards will be achievable in a specific period of time. While the statutory language of section 231 is not identical to other provisions in title II of the CAA that direct the EPA to establish technology-based standards for various types of mobile sources, the EPA interprets its authority under section 231 to be similar to those provisions that grant us significant discretion to identify a reasonable balance of specified emissions reduction, cost, safety, noise, and other factors. See, e.g., Husqvarna AB v. EPA, 254 F.3d 195 (DC Cir. 2001) (upholding the EPA's promulgation of technology-based standards for small non-road engines under section 213(a)(3) of the CAA).  The EPA has significant discretion under section 231 in determining what standard is most reasonable for aircraft engine emissions.  In this regard, we note CAEP's stated policy for the purpose of the international CO2 standard, which is to achieve emissions rate reductions beyond that which would have occurred in the absence of a standard.
In ruling on a petition for judicial review of the 2005 rule, the U.S. Court of Appeals for the D.C. Circuit held that the EPA's approach in that action of tracking the ICAO standards was reasonable and permissible under the CAA.  NACAA v. EPA, 489 F.3d 1221, 1230-32 (D.C. Cir. 2007).  The Court also held that section 231 of the CAA confers a broad degree of discretion on the EPA to adopt aircraft emission standards as the Agency determines are reasonable.  Id.
The EPA requests comment on our traditional approach of establishing domestic standards of at least equivalent stringency to ICAO's standards.  Should we follow this same approach, as we have in the past?  Also, the EPA asks for comment on the possibility of the EPA adopting a different GHG standard than ICAO, provided ICAO/CAEP adopts a standard in 2016 and the EPA makes a positive endangerment finding.  For this possibility, the EPA requests that commenters consider the following factors (among others): the potential to reflect the GHG performance of products from U.S. manufacturers, competitive advantages and disadvantages for manufacturers, and certification reciprocity with certificating authorities of other nations.  In addition, the EPA asks for comment on what action the EPA should take if ICAO/CAEP fails to adopt a CO2 standard in 2016.  We also invite comment on any other topic in regard to the connection between the EPA and ICAO standards.
Statutory Authority and Executive Order Reviews
     A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review
This action is a significant regulatory action because it raises novel policy issues.  Accordingly, it was submitted to the Office of Management and Budget (OMB) for review. This action proposes a finding that GHG emissions from aircraft cause or contribute to air pollution that may be reasonably anticipated to endanger public health and welfare along with an ANPR which provides an overview of the international efforts to reduce GHG emissions, progress to date in establishing global aircraft standards that achieve meaningful CO2 reductions and, if the EPA finds that aircraft GHG emissions do cause or contribute to endangerment, the potential use of CAA section 231 to implement these standards domestically ensuring transparency and the opportunity for public comment.  Any changes made in response to OMB recommendations have been documented in the docket.
Paperwork Reduction Act (PRA)
This action does not impose an information collection burden under the PRA.  The proposed endangerment and cause or contribute findings under CAA section 231 do not contain any information collection activities.
Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA.  This action will not impose any requirements on small entities. The proposed endangerment and cause or contribute findings under CAA section 231 do not in-and-of-themselves impose any new requirements but rather set forth the Administrator's proposed determination that GHG emissions from certain classes of aircraft engines -- those used in U.S. covered aircraft -- cause or contribute to air pollution that may be reasonably anticipated to endanger public health and welfare.  Accordingly, this action affords no opportunity for the EPA to fashion for small entities less burdensome compliance or reporting requirements or timetables or exemptions from all or part of the proposal.
Unfunded Mandates Reform Act (UMRA)
This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531 - 1538, and does not significantly or uniquely affect small governments.  The action imposes no enforceable duty on any state, local or tribal governments or the private sector.
Executive Order 13132: Federalism
This action does not have federalism implications.  It will not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government.
Executive Order 13175: Consultation and Coordination with Indian Tribal Governments
This action does not have tribal implications as specified in Executive Order 13175. The proposed endangerment and cause or contribute findings under CAA section 231 do not in-and-of-themselves impose any new requirements but rather set forth the Administrator's proposed determination that GHG emissions from certain classes of aircraft engines -- those used in  U.S. covered aircraft -- cause or contribute to air pollution that may be reasonably anticipated to endanger public health and welfare.  Thus, Executive Order 13175 does not apply to this action.
Executive Order 13045: Protection of Children from Environmental Health Risks and Safety Risks
This action is not subject to Executive Order 13045 because it is not economically significant as defined in Executive Order 12866.  The Administrator considered climate change risks to children as part of this proposed endangerment finding under CAA section 231.  This action's discussion of climate change impacts on public health and welfare is found in section IV of this preamble.  Specific discussion with regard to children are contained in sections IV and I.D of the preamble titled "Children's Environmental Health."  A copy of all documents pertaining to the impacts on children's health from climate change have been placed in the public docket for this action.
Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution or Use