Greenhouse Gas
Emissions from the U.S.
Transportation Sector
/ 990-2003
United States
Environmental Protection
Agency
Office of Transportation and Air Quality (6401A)
EPA 420 R 06 003
March 2006
www.epa.gov
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Greenhouse Gas Emissions from the
U.S. Transportation Sector, 1990-2003
March 2006
Prepared by:
U.S. Environmental Protection Agency
Office of Transportation and Air Quality
With support from:
ICF Consulting
93 00 Lee Highway
Fairfax, Virginia 22031
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Transportation GHG Emissions Report
How to Obtain Copies
You can electronically download this document on the U.S. EPA's Greenhouse Gas Emissions
from Mobile Sources web page at: www.epa.gov/otaq/climate.htm.
For Further Information
This report was prepared by EPA's Office of Transportation and Air Quality (OTAQ). Specific
questions about the report content may be directed to John Davies at (202)564-9467, or
davies.john@epa.gov. For additional information about climate change and greenhouse gas
emissions, visit the EPA web site at: www.epa. gov/globalwarming.
Released for printing: March 2006
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Transportation GHG Emissions Report
Acknowledgements
ICF Consulting contributed significantly to this effort.
The Environmental Protection Agency would like to acknowledge the many individual and
organizational reviewers whose work has significantly improved this document. These include:
Leif Hockstad, EPA Office of Atmospheric Programs; Mark Schipper, EIA Office of Energy
Markets and End Use, Energy Information Administration; Michael Wang, Argonne National
Laboratory; Mark DeLucchi, Institute of Transportation Studies, University of California at
Davis; Clare Sierawski, USDOT Office of Safety, Energy and Environment; Maryalice Locke,
FAA Office of Environment and Energy; Lourdes Maurice, Federal Aviation Administration,
Office of Environment and Energy; and John Maples, EIA Office of Integrated Analysis and
Forecasting, Energy Information Administration.
Preface
The United States Environmental Protection Agency (EPA) prepares the Greenhouse Gas
Emissions from the U.S. Transportation Sector report to provide researchers, transportation and
environmental practitioners, policy makers, and the public with a more complete understanding of
greenhouse gas (GHG) emissions from the U.S. transportation sector. GHG emissions estimates
in this report are based on the official U.S. Inventory of Greenhouse Gas Emissions and Sinks. In
addition, this report highlights factors affecting emissions trends, projections, and emerging
issues that may affect emissions in the future. It also includes information on the full life-cycle
GHG emissions associated with transportation, GHG emissions from other mobile sources, and
uncertainties associated with emissions estimates.
11
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Transportation GHG Emissions Report
TABLE OF CONTENTS
1 INTRODUCTION 1
1.1 Background 1
1.2 Report Organization 1
2 OVERVIEW OF GREENHOUSE GAS EMISSIONS AND TRANSPORTATION 3
2.1 Background on Greenhouse Gases 3
2.2 Transportation in the Context of U.S. Greenhouse Gas Emissions 5
2.3 Transportation Sources 7
2.4 GHG Emissions Trends for Major Transportation Sources 7
3 LIGHT-DUTY VEHICLES - PASSENGER CARS, SUVs, MINIVANS, PICKUP TRUCKS,
AND MOTORCYCLES 10
3.1 Overview 10
3.2 Factors Affecting Light-Duty Vehicle Emissions 11
4 HEAVY-DUTY VEHICLES—FREIGHT TRUCKS AND BUSES 18
4.1 Heavy-Duty "Freight" Trucks 18
4.2 Buses 20
5 AIRCRAFT 21
5.1 Overview 21
5.2 Factors Affecting Aircraft Emissions 21
5.3 Other Considerations in Estimating Global Warming Impact 23
6 OTHER NON-ROAD TRANSPORTATION SOURCES 24
6.1 Boats/Ships 24
6.2 Rail 25
6.3 Pipelines 26
7 HFCs FROM MOBILE AIR CONDITIONERS AND REFRIGERATED TRANSPORT 27
8 NON-TRANSPORTATION MOBILE SOURCES—AGRICULTURAL AND CONSTRUCTION
EQUIPMENT, RECREATIONAL VEHICLES, AND OTHER 29
9 ESTIMATING TRANSPORTATION GHG EMISSIONS—METHODOLOGY AND
UNCERTAINTY 31
9.1 Carbon Dioxide Emissions 31
9.2 Methane and Nitrous Oxide Emissions 34
10 LIFECYCLE TRANSPORTATION EMISSIONS 36
10.1 Estimates of Transportation-Related CO2 Emissions 37
10.2 Other Issues/Next Steps 40
11 GHG EMISSIONS PROJECTIONS AND EMERGING ISSUES 41
11.1 Projected CO2 Emissions from Transportation 41
11.2 Emerging Issues Affecting Passenger Transportation 42
11.3 Emerging Issues Affecting Freight Transportation 44
11.4 Implications for the Future 45
in
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Transportation GHG Emissions Report
12 REFERENCES 46
13 APPENDIX A: SUMMARY OF GHG EMISSIONS FOR TRANSPORTATION AND MOBILE
SOURCES 49
14 APPENDIX B: CO2 EMISSIONS FROM VARIOUS COMPONENTS OF THE
TRANSPORTATION LIFECYCLE (PROPORTION RELATIVE TO DIRECT EMISSIONS) .... 55
15 ABBREVIATIONS, ACRONYMS, AND UNITS 59
IV
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Transportation GHG Emissions Report
LIST OF FIGURES
Figure 2-1. U.S. Greenhouse Gas Emissions by End-Use Economic Sector, 1990-2003 . 5
Figure 2-2. 2003 Transportation Greenhouse Gas Emissions, by Source 7
Figure 2-3. GHG Emissions by Modes of Transportation,211990-2003 8
Figure 3-1. GHG Emissions from Passenger Cars and Light-Duty Trucks, 1990-2003
(CO2Eq.) 10
Figure 3-2. Household Vehicle Fuel Consumption by Mode, 2001 11
Figure 3-3. Comparison of Percent Growth of U.S. Population, Households, Vehicle
Trips, and Vehicle Miles Traveled by Households, 1990-2001 12
Figure 3-4. Journey to Work Mode Choice, 1980, 1990,2000 13
Figure 3-5. Number of New Light-Duty Vehicles Sold, 1976-2003 14
Figure 3-6. New Light-Duty Vehicle Sales (Market Share) by Size Class, 1976, 1990, and
2003 15
Figure 3-7. VMT by Passenger Cars and Light-Duty Trucks, 1990-2003 15
Figure 3-8. Sales-Weighted Fuel Economy of New Light-Duty Vehicles (Combined Car
and Light-Truck Fleet) by Model Year, 1975-2004 16
Figure 5-1. Aircraft Passenger Load Factor, 1970-2003 22
Figure 5-2. Average Seat-Miles Traveled Per Gallon of Fuel Consumed, 1970-2003.... 23
Figure 6-1. Ton-Miles Shipped by Domestic Water Transportation, 1990-2003 25
Figure 7-1. HFC Emissions from Mobile Air Conditioners and Refrigerated Transport,
1990-2003 27
Figure 7-2. CFC and HCFC Emissions from Mobile Air Conditioners and Refrigerated
Transport, 1990-2003 28
Figure 8-1. Greenhouse Gas Emissions from Non-Transportation Mobile Sources, 1990
and 2003 29
Figure 11-1. EIA Projections of Transportation Energy Demand, High, Base, and Low
Economic Cases, 2003-2025 41
Figure 11-2. Historical and Projected VMT from Gasoline- and Diesel-Electric Hybrid
Vehicles, 2000-2025 43
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Transportation GHG Emissions Report
LIST OF TABLES
Table 4-1. 2003 Vehicle Registrations, Vehicle Miles Traveled, and Fuel Use for Heavy-
Duty Trucks 19
Table 9-1. Comparison of U.S. GHG Inventory Estimates and Bottom-Up Estimates of
CO2 for Selected Transportation Fuels and Sources 33
Table 10-1. Elements of the Transportation Lifecycle 37
Table 13-1. Total GHG Emissions from Transportation Sources (All Gases), 1990-2003
(TgC02Eq.) 49
Table 13-2. CO2 Emissions from Transportation Sources, 1990-2003 (Tg) 50
Table 13-3. Methane Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.)51
Table 13-4. Nitrous Oxide Emissions from Transportation Sources, 1990-2003 (Tg CO2
Eq.) 52
Table 13-5. HFC Emissions from Transportation Sources, 1990-2002 (Tg CO2 Eq.) 52
Table 13-6. GHG Emissions from Non-Transportation Mobile Sources (All Gases), 1990-
2003 (Tg CO2 Eq.) 53
Table 13-7. CO2 Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg) 53
Table 13-8. Methane Emissions from Non-Transportation Mobile Sources, 1990-2003
(TgC02Eq.) 54
Table 13-9. Nitrous Oxide Emissions from Non-Transportation Mobile Sources, 1990-
2003 (Tg CO2 Eq.) 54
Table 14-1. CO2 Emissions from Various Components of the Transportation Lifecycle
(Proportion Relative to Direct Emissions) 55
Table 14-2. Total CO2 Emissions from Various Components of the Transportation
Lifecycle (Tg) 57
VI
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Transportation GHG Emissions Report
1 Introduction
1.1 Background
Although transportation is a vital part of the economy and is essential for everyday activities, it is
also a significant source of greenhouse gas (GHG) emissions. In 2003, the transportation sector
accounted for about 27 percent of total U.S. GHG emissions, up from 24.8 percent in 1990.
Transportation GHG emissions increased by a larger amount than any other economic sector1
over this period, growing from 1509.3 Tg CO2 Eq. in 1990 to 1,866.7 Tg CO2 Eq. in 2003, an
increase of 24 percent.2 GHGs from all other sectors increased by a total of 9.5 percent over the
same timeframe. Looking forward, transportation GHGs are forecast to continue increasing
rapidly, reflecting the anticipated impact of factors such as economic growth, increased
movement of freight by trucks and aircraft, and continued growth in personal travel. According to
the U.S. Department of Energy (DOE), transportation energy use is expected to increase 48
percent between 2003 and 2025, despite modest improvements in the efficiency of vehicle
engines. This projected rise in energy consumption closely mirrors the expected growth in
transportation GHG emissions.3
This report was developed by the U.S. Environmental Protection Agency's (EPA) Office of
Transportation and Air Quality (OTAQ) to help transportation agencies, the transportation
industry, researchers, and the public better understand the connection between transportation and
GHG emissions in the United States. The GHG emissions estimates presented in this report are
taken from the official GHG Inventory produced by EPA, Inventory of U.S. Greenhouse Gas
Emissions and Sinks: 1990-2003 ("U.S. GHG Inventory"). As a complement to the U.S. GHG
Inventory, this report includes additional detail on GHG emissions from transportation and non-
transportation mobile sources. It also analyzes factors affecting emissions, uncertainty in the data,
and emerging issues.
1.2 Report Organization
The remainder of this report is organized in the following sections:
Section 2. Overview of Greenhouse Gas Emissions and Transportation—This section
provides a brief introduction to specific GHGs and their measurement. It also compares the
1 The "economic sectors" referred to in this report do not represent official Intergovernmental Panel on Climate
Change (IPCC) categories. EPA has found it useful to estimate emissions by sectoral categories that are commonly
used for policy analysis. One method allocates emissions to seven different "economic sectors," which include
Electricity Generation, and the non-electricity component of all six other sectors (Transportation, Industry, Agriculture,
Commercial, Residential and U.S. Territories). The second method distributes the emissions from Electricity
Generation to the remaining "end use" sectors. For purposes of simplicity, this report uses the second categorization
when referring to sectoral estimates.
2 U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003.
Washington, DC. The total transportation sector GHGs cited in this report are slightly lower than the transportation
sector totals reported in the published Inventory (approximately 0.7 to 0.9 Tg lower from 1990 to 2003). This small
increment represents "other" non-transportation mobile sources, such as lawn mowers and leaf blowers, which are
counted as transportation in the published Inventory but not this report. GHG emissions are typically reported in terms
of CO2 equivalent (CO2 Eq.) in order to provide a common unit of measure, and because CO2 is the most prevalent of
all GHGs.
3 U.S. Energy Information Administration, Annual Energy Outlook 2005 with Projections to 2025, Table A2. U.S.
Department of Energy, Energy Information Administration, Washington, DC.
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Transportation GHG Emissions Report
nation's transportation emissions to other sectors, and discusses variables contributing to the rise
transportation GHGs.
Section 3. Light-Duty Vehicles - Passenger Cars, SUVs, Minivans, Pickup Trucks, and
Motorcycles—This section describes GHG emissions from light-duty motor vehicles, currently
the largest sources of transportation GHGs.
Section 4. Heavy-Duty Vehicles—Freight Trucks and Buses—This section addresses
emissions and trends for heavy-duty vehicles.
Section 5. Aircraft—This section discusses emissions from aircraft, which are the largest source
of non-road transportation GHG emissions.
Section 6. Other Non-Road Transportation Sources—This section characterizes emissions
from boats and ships, rail, and pipelines.
Section 7. HFCs from Mobile Air Conditioners and Refrigerated Transport—This section
describes HFC emissions from transportation sources, which include mobile air conditioners and
refrigerated transportation units.
Section 8. Non-Transportation Mobile Sources—This section discusses GHG emissions from
non-transportation mobile sources, such as agricultural equipment, construction equipment, and
other utility equipment.
Section 9. Estimating Transportation GHG Emissions—Methodology and Uncertainty —
This section briefly describes methods that are used to estimate transportation GHG emissions
and explores uncertainties in the calculations.
Section 10. Lifecycle Transportation Emissions—This section examines GHG emissions from
a broader lifecycle perspective, including activities such as fuel processing and distribution,
vehicle manufacture and vehicle maintenance.
Section 11. GHG Emissions Projections and Emerging Issues—This section provides
forecasts of GHG emissions from transportation sources through 2025 and highlights some of the
issues affecting trends in GHG emissions from transportation during this time period.
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Global Warming Potentials
Transportation GHG Emissions Report
2 Overview of Greenhouse Gas Emissions and Transportation
2.1 Background on Greenhouse Gases
Greenhouse gases (GHGs) occur naturally in the Earth's atmosphere and help to keep the planet
hospitable to life by trapping some of the sun's natural heat. Without this "greenhouse effect," the
Earth's average surface temperature would be about 33 degrees Celsius cooler than it is
currently.4 The most important naturally occurring GHGs associated with this phenomenon are
water vapor (H2O), carbon dioxide (CO2), methane (CFL,), and nitrous oxide (N2O).
Human activities release GHG emissions and contribute to increasing concentrations of GHGs in
the atmosphere. CO2 is the predominant GHG emitted by human sources. Like most GHGs, CO2
is produced both by natural and human activities and can be removed from the atmosphere
through natural processes.5 However, increased production
of CO2 by human sources has caused total GHG emissions
to exceed natural absorption rates, resulting in increased * " ear Ime onzon>
atmospheric concentrations. Since the beginning of the
industrial revolution, atmospheric concentrations of CO2
have increased by nearly 30 percent, CH4 concentrations
have more than doubled, and N2O concentrations have
risen by approximately 15 percent. Human activities over
the past 70 years have also produced synthetic chemicals
that are greenhouse gases, including chlorofluorocarbons
(CFCs), hydrofluorocarbons (HFCs), perfluorocarbons
(PFCs), and sulfur hexafluoride (SF6). 6
Climate Change 2001. Geneva, Switzerland.
GHG emissions are typically reported in terms of CO2
equivalent (CO2 Eq.) to provide a common unit of measure, and because CO2 is the most
prevalent of all GHGs. Other GHGs are converted into CO2 equivalent on the basis of their global
warming potential (GWP), which is defined as the cumulative radiative forcing7 effects of a gas
over a specified time horizon in comparison to CO2 (see sidebar). For example, one kilogram of
CFL^is estimated to have the same radiative forcing effect as 21 kilograms of CO2 8
CO2 accounted for 85 percent of the radiative forcing effect of all human-produced GHGs in the
United States in 2003. This proportion is higher for transportation sources, with CO2 representing
about 96 percent of the sector's GWP-weighted emissions. The transportation sector is the largest
Greenhouse Gas GWP
CO2 1
N20 296
CH4 23
HFC-125 3,400
HFC-134a 1,300
HFC-143a 4,300
HFC-152a 120
Source: International Panel on Climate
Change, 2000. Third Assessment Report:
4 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific
Basis. Cambridge, UK.
5 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific
Basis. Cambridge, UK.
6 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific
Basis. Cambridge, UK.
7 Radiative forcing is the change in balance between radiation entering the Earth's atmosphere and radiation being
emitted back into space. A "positive radiative forcing effect" means that the ratio of incoming to outgoing radiation
increases, generally resulting in a warming of the Earth. Conversely, a "negative radiative forcing effect" generally
results in cooler Earth temperatures.
8 Note that the GWPs used in this report are those reported in IPCC's Second Assessment Report, which is consistent
with international inventory guidelines. The IPCC has published a Third Assessment Reportwith revised GWPs, which
are currently being considered for international inventory guidelines. This report presents estimates of CO2, N2O, CH4
and HFC emissions in teragrams or trillion grams of carbon dioxide equivalent (Tg CO2 Eq.) unless noted otherwise.
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Transportation GHG Emissions Report
source of domestic CO2 emissions, producing over 30 percent of the nation's total in 2003. The
vast majority of anthropogenic CO2 emissions come from the combustion of fossil fuels.9 CO2
production is related to the amount of fuel combusted and the fuel's carbon content.10 The U.S.
transportation sector derived all but 1 percent of its energy from fossil fuels in 2003, 97 percent of
which was petroleum.u
CH4 and N2O collectively represented 13 percent of all United States GHGs in 2003, but only
accounted for 2 percent of the transportation total. These gases are released during fossil fuel
consumption, although in much smaller quantities than CO2. They are also unlike CO2 in that
their emissions rates are affected by vehicle emissions control technologies.
A final category of GHGs comprises various families of synthetic chemicals. These include
compounds such as CFCs and hydrochlorofluorocarbons (HCFC) that result in stratospheric
ozone depletion and are controlled under the Montreal Protocol. Because of their required phase
out, ozone depleting substances are not included in official estimates of national GHGs.12
Compounds such as HFCs, perfluorocarbons (PFC), and SF6 have been identified as acceptable
alternatives to ozone depleting substances. Nonetheless, the replacement chemicals are also
potent greenhouse gases with very high global warming potential. While small quantities of these
chemicals are released, they accounted for approximately 2 percent of GWP-weighted GHGs
from all U.S. sectors in 2003. HFCs are the primary replacement chemicals associated with
transportation sources, replacing CFCs and HCFCs in vehicle air conditioning and refrigeration
systems. Leakage of HFCs was responsible for 2 percent of transportation GHGs in 2003.
Transportation sources emit several other compounds that are believed to have an indirect effect
on global warming but are not considered greenhouse gases. These substances include ozone,
carbon monoxide, (CO) and aerosols. Scientists have not yet been able to quantify their impact
with certainty, and these compounds are not included in the transportation GHG emissions
estimates.13
9 Approximately 95 percent in the U.S.
10 A lesser consideration is the fraction of the carbon oxidized, which is assumed to be 100 percent for emissions from
transportation. The formula for CO2 emissions from fossil fuels is Fuel Combusted X Carbon Content Coefficient X
Fraction Oxidized X (44/12).
11 Approximately 2.5 percent is in the form of natural gas, with less than 1 percent renewables (alcohol fuels blended
with gasoline to make gasohol) and electricity. Source: Oakridge National Laboratory, Transportation Energy Data
Book, Table 2.2. Citing, U.S. Department of Energy, Energy Information Administration, Monthly Energy Review.
12 Transportation GHG estimates reflect an accounting issue related to the phase-in of HFCs, primarily as a
replacement for CFCs in vehicle air conditioners. Transportation HFCs increased from virtually zero in 1990 to over 40
Tg CO2 Eq. in 2003, at which point they represented about 2 percent of total transportation GHGs. CFCs emissions
have declined over the same period, but they are not reported in official GHG inventories because of their required
phase-out under the Montreal Protocol. As a result, the official transportation GHG estimates do not reflect the net
impact of increasing HFCs and declining CFCs. On balance, the introduction of HFCs has reduced GWP-weighted
GHG emissions because these substances have lower global warming potential than CFCs.
13 Ozone traps heat in the atmosphere and prevents a breakdown of CH4, but its lifetime in the atmosphere varies from
weeks to months, making it difficult to estimate net radiative forcing effects. CO indirectly affects global warming by
reacting with atmospheric constituents that would otherwise destroy CH4 and ozone. Aerosols are small airborne
participles or liquid droplets that have both direct and indirect effects on global warming. The most prominent aerosols
are sulfates and black carbon, or soot. Sulfate aerosols also have some cooling effect by reflecting light back into space.
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Transportation GHG Emissions Report
2.2 Transportation in the Context of U.S. Greenhouse Gas Emissions
Although the United States accounts for approximately 5 percent of the world's population, it
produces an estimated 21 percent14 of the world's GHG emissions, amounting to 6,900 Tg CO2
Eq. in 2003.15 Transportation sources were responsible for about 27 percent of total U.S. GHG
emissions in 2003 (1,866.7 Tg CO2 Eq.).16 Non-transportation mobile sources, such as equipment
used for construction and agriculture, accounted for an additional 2.1 percent of the total U.S.
GHG emissions (144.8 Tg CO2 Eq.). These estimates are primarily representative of "tailpipe"
GHGs that result from the use of energy to power vehicles.17 They do not include "lifecycle"
emissions from processes such as the extraction of crude oil and manufacture of vehicles.
(Lifecycle issues are discussed in Chapter 10.)
Figure 2-1. U.S. Greenhouse Gas Emissions by End-Use Economic Sector, 1990-2003
2,500
O
o
2,000
1,500 -
1,000 -
500 -
Industry
Transportation
Commercial
Residential
Agriculture
1990
1992
1994
1996
1998
2000
2002
Source: Derived from U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003.
Washington, DC, Table 2-16.
Note: GHG emissions from electricity generation are distributed to economic sectors. Also, territories are excluded even though they are
reported in the U.S. inventory. Territories comprise less than 1 percent of national emissions.
Total U.S. production of greenhouse gases in 2003 was 13 percent greater than in 1990. By
comparison, transportation GHGs grew almost 24 percent over the same period. GHG emissions
14 Based on 2000 data reported by the Climate Analysis Indicators Tool (CAIT), World Resources Institute,
http://cait.wri.org/cait.php. Does not adjust for land-use and forestry change.
15 U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003.
Washington, DC, Table 2-16.
16 Based on global warming potential of all gases emitted.
17 There are two notable exceptions. Included in the transportation estimates are pipelines, which are used as a means
of transporting petroleum and natural gas. Pipeline GHGs include emissions from natural gas used to operate pumps,
motors, engines, and compressors, but not electricity used in the operation of pipelines. This is consistent with the
energy accounting procedures used by the U.S. Energy Information Administration. Second, the transportation sector
includes the emission of HFCs from vehicle air conditioning and refrigerated transport. This process occurs as a result
of leakage during equipment operation, servicing, and disposal. Pipeline and HFC emissions collectively accounted for
slightly less than 3.5 percent of total transportation GHGs.
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Transportation GHG Emissions Report
from the transportation sector increased more in absolute terms than any other sector,18 growing
by 357.4 Tg CO2 Eq. from 1990 to 2003 (Figure 2-1). The growth rate of transportation GHGs
was equal to the residential sector (also 24 percent), slightly above the commercial sector (22
percent), and considerably greater than agriculture (3 percent) and industry (which decreased by 2
percent).
The overall rise in U.S. GHGs primarily reflects increased emissions of CO2 as a result of
increasing fossil fuel combustion. Transportation petroleum use grew by 23 percent from 1990 to
2003 and accounted for 93 percent of the increase in total U.S. petroleum consumption over this
period. Considering only CO2, transportation sources emitted 1780.7 Tg CO2 in 2003, an increase
of 319.0 Tg (or 22 percent) from 1990. The combined emissions of CUt and N2O decreased by
4.0 Tg CO2 Eq. over the same period, due largely to the introduction of control technologies
designed to reduce criteria pollutant emissions.19 Meanwhile, HFC emissions from mobile air
conditioners and refrigerated transport increased from virtually no emissions in 1990 to 42.7 Tg
CO2 Eq. in 2003 as these chemicals were phased in as substitutes for ozone depleting substances.
Categorizing U.S. Greenhouse Gas Emissions: The Difference Between
"Economic Sector" and "End Use Economic Sector" Estimates
U.S. Greenhouse Gas Emissions by Economic Sector, 1990-2003
2500
2000
The "economic" sectors referred to in this report do not represent official Intergovernmental Panel on Climate
Change (IPCC) categories. IPCC guidelines allocate emissions into the following sectors: Energy, Industrial
Processes, Solvent and Other Product Use, Agriculture, Land Use Change and Forestry, and Waste. However,
EPA has also found it useful to allocate emissions into "economic sector" categories that are commonly used for
policy analysis. The
Inventory of U.S.
Greenhouse Gas Emissions
and Sinks classifies
"economic sector" estimates
in two different ways. The
first categorization identifies
the emissions from
Electricity Generation, as
well as the non-electricity
components of
Transportation, Industry,
Agriculture, Commercial,
Residential, and U.S.
Territories (note the
adjacent chart). The second
categorization distributes
the emissions from
Electricity Generation to the
remaining "end use
economic sectors" in which
electricity is consumed.
57
LU
6
o
ra
1500
1000
Electricity Generation
Transportation
Industry
.-Agriculture
f^
-Commercial
-Residential
1990 1992 1994 1996 1998 2000 2003
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003
Note: Territories are excluded even though they are reported in the U.S. inventory Territories comprise less
than 1 percent of national emissions.
The sectoral estimates in
this report are based on the second categorization. However, if electrical generation is considered a separate
sector, it accounts for 33 percent of all GHG emissions, and is larger than any of the five sectors discussed in this
report. Also, emissions from Electricity grew by 24.7 percent from 1990 to 2003, representing the largest
percentage increase of any economic sector. See the Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2003 Section 2.2 for additional discussion of the two approaches.
18 Based on "end use economic sector" estimates, in which emissions from Electricity Generation are allocated to
economic sectors in which electricity is consumed.
19 The decline in CFC emissions is not captured in the official transportation estimates. See footnote 9 above.
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Transportation GHG Emissions Report
2.3 Transportation Sources
In 2003, about 81 percent of transportation GHG emissions in the United States came from "on-
road" vehicles, including passenger cars, sport-utility vehicles (SUVs), vans, motorcycles, and
medium- and heavy-duty trucks and buses (Figure 2-2). "Light-duty" vehicles, which are used
primarily for personal transport, accounted for 62 percent of total transportation emissions. This
category consists of passenger cars, (35 percent of the transportation total), "light-duty trucks,"
including SUVs, minivans and pickup trucks (27 percent), and motorcycles (less than 1 percent).
Heavy-duty vehicles, which include trucks and buses, were responsible for 19 percent of total
transportation emissions.
Non-road transportation sources produced 16 percent of all transportation GHG emissions in
2003. Aircraft were the largest non-road source, producing 9 percent of total transportation
GHGs. Other non-road sources include boats and ships (3 percent), rail (2 percent), and pipelines
(2 percent).20
Finally, the transportation sector estimates include emissions from sources that are classified as
neither on-road nor non-road. Approximately 2 percent of total transportation emissions in 2003
consisted of HFCs from vehicle air conditioning and refrigerated transport. Another 1 percent
came from lubricants, consisting mainly of oil used in motor vehicle engine combustion.
Figure 2-2. 2003 Transportation Greenhouse Gas Emissions, by Source
Passenger
Cars
35% ^^^*\
Light Trucks
... . , 27%
Lubricants
1%
Pipelines
2% ~~~
Locomotives / / Heavy-Duty
2% _ ' Vehicles
Aircraft 19%
9%
Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington,
DC, Table 2-9.
2.4 GHG Emissions Trends for Major Transportation Sources
The increase in transportation emissions from 1990 to 2003 reflects continued growth in
passenger and freight travel, which has substantially exceeded improvements in the energy
efficiency of most major transport modes. GHG emissions from on-road vehicles increased by
20 Estimated pipeline GHGs include emissions from natural gas used to operate pumps, motors, engines, and
compressors, but not electricity used in the operation of pipelines. This is consistent with the energy accounting
procedures used by the U.S. Energy Information Administration.
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Transportation GHG Emissions Report
308.6 Tg CO2 Eq., or 26 percent, from 1990 to 2003. Meanwhile, GHG emissions from non-road
transportation sources increased by 7.8 Tg CO2 Eq., or 3 percent (Figure 2-3).21
Figure 2-3. GHG Emissions by Modes of Transportation,31990-2003
Share of Transportation GHG Emissions
2,000] 100%
80%
60%
Aircraft
ri •
Heavy-Duty Vehicles
40%
20%
Road
1990 1992 1994 1996 1998 2000 2002
1990 2003
Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington,
DC, Table 2-9.
Note: "Other Non-Road' includes boats and ships, rail, pipelines, and lubricants.
a Emissions of MFCs from refrigerated transport and mobile air conditioners are not included in this chart. To view HFC emission trends,
please refer to Table 13-5.
GHG emissions from light-duty vehicles (passenger cars and light-duty trucks) grew 19 percent
from 1990 to 2003. The overall rise can be broadly explained by a 34 percent increase in light-
duty vehicle miles traveled (VMT) over the period, which outweighed a small improvement in
overall light-duty fuel economy. However, it is worth noting that the improvement in vehicle
energy efficiency was due primarily to the replacement of less fuel-efficient vehicles from the
1970s and early-1980s. Since 1988, the average fuel economy of new light-duty vehicles sold has
declined as a result of increasing light-duty truck sales. In 2002, sales of new light-duty trucks
overtook passenger cars. As one primary result, GHGs from light-duty trucks increased by 51
percent from 1990 to 2003, compared with a 2 percent increase from passenger cars.
GHG emissions from heavy-duty vehicles (predominantly freight trucks) grew by 57 percent
from 1990 to 2003—more than twice the rate of light-duty vehicles. An increase in truck freight
haulage22 caused heavy-duty truck VMT to rise 48 percent over the same period.23 Meanwhile,
21 This does not include lubricants, which are used for all modes. Lubricant GHG emissions decreased by 1.6 Tg CO2
Eq. from 1990 to 2003.
22 According to data from the Commodity Flow Survey and additional estimates compiled by the Bureau of
Transportation Statistics, the value of goods transported by truck domestically increased by 42 percent, and ton-miles
increased 56 percent between 1993 and 2002 (survey data are not available for 1990 or 2003). These figures do not
include "multimodal combinations." Source: U.S. Department of Transportation, Bureau of Transportation Statistics.
Freight Shipments in America: Preliminary Highlights from the 2002 Commodity Flew Survey Plus Additional Data.
Table 1.
23 Refers to combination trucks and single-unit trucks, excluding two-axle, four-tire trucks. Federal Highway
Administration, 2004. Highway Statistics 2003. Washington, DC.
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Transportation GHG Emissions Report
overall heavy-duty truck fuel economy declined from 6.0 to 5.7 miles per gallon,24 although the
average vehicle size has increased slightly and data for this mode is less certain.
In contrast to on-road vehicles, aircraft GHG emissions decreased by 3 percent from 1990 to
2003. GHGs from military aircraft declined significantly over the period, while other sources of
aviation GHGs increased moderately. The largest source of aviation GHGs are commercial
aircraft, which produced 4.7 percent more greenhouse gases in 2003 than 1990. However, the rise
in commercial aircraft GHG emissions was significantly less than the growth in air travel, with
aircraft passenger miles increasing 48 percent over the same timeframe. Emissions per passenger
mile decreased by 24 percent from 1990 to 2003, representing the most significant improvement
in emissions intensity of any major mode. Most of the improvement reflected the increasing fuel
efficiency of aircraft and increased numbers of occupied seats.
Among other non-road sources, GHG emissions from rail increased 18 percent from 1990 to
2003. Water-based transportation GHGs appear to have increased similarly (17 percent), although
the data show much more fluctuation and have a higher degree of uncertainty. (See Section 9.1.2
for a discussion of uncertainty in GHG emissions estimates.) Pipeline emissions were virtually
unchanged between 1990 and 2003.
Impact of Freight Transportation
Freight GHG emissions25 increased by 46 percent between 1990 to 2003, while GHGs from
passenger modes increased by 20 percent.26 Collectively, freight sources emitted 13 percent more
GHGs per ton-mile in 2003 than in 1990. Most of the increase in GHG intensity resulted from a
shift to energy-intensive freight modes. Rail is typically the least energy-intensive freight mode.
Measured in BTUs per ton-mile, rail used 90 percent less energy than trucks and 80 percent less
than ships.27 While the share of freight carried by rail has remained roughly constant, trucks'
share of freight ton-miles increased from 26 percent in 1993 to 32 percent in 2002,28 accounting
for most of the overall increase in freight GHG output and intensity.29
24 According to FHWA's estimates of average fuel economy, based on estimates of fuel consumption and VMT by
vehicle type. Federal Highway Administration, 1997. Highway Statistics: Summary to 1995. Washington, DC, Table
VM-201A, and Highway Statistics 2003, Table VM-1.
25 Freight modes are those used to ship materials and goods, and include heavy-duty trucks, freight rail, freight vessels,
and pipelines. Emissions from refrigerated transport are also freight-related and so are allocated to freight
transportation.
26 The U.S. GHG Inventory does not explicitly calculate aircraft emissions resulting from shipping materials and
goods. These emissions are generally included in overall estimates for "commercial aircraft," which are categorized as
passenger transport.
27 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend
Data. Available at http://intensityindicators.pnl.gov/data.html.
28 Data not available for 1990 or 2002. U.S. Department of Transportation, Bureau of Transportation Statistics, and
U.S. Department of Commerce, U.S. Census Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow
Survey. Washington, DC.
29 Air shipments required approximately 82 times more energy per ton-mile than rail in 2001, according to the DOE
estimates referenced above. While air was the fastest-growing mode of freight transport, its share of total shipments
remained below 1 percent.
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Transportation GHG Emissions Report
3 Light-Duty Vehicles - Passenger Cars, SUVs, Minivans, Pickup
Trucks, and Motorcycles
3.1 Overview
Light-duty vehicles30 in the U.S. produced 1152.6 Tg CO2 Eq. in 2003, representing 77 percent of on-road
vehicle GHG emissions and 62 percent of total transportation emissions.31 GHG emissions from light-
duty vehicles increased 19 percent between 1990 and 2003. CO2 emissions increased 20 percent, or 187.8
Tg, mirroring the growth in light-duty vehicle fuel consumption. Meanwhile, emissions of CIL, and N2O
from light-duty fleet vehicles decreased by 2.1 and 2.9 Tg CO2 Eq., due to the introduction of vehicle
emissions control technologies in newer vehicles.
A growing portion of new vehicle sales from 1990 to 2003 consisted of light-duty trucks, which include
pickup trucks, SUVs, and vans. GHG emissions from light-duty trucks increased 51 percent from 1990 to
2003, while emissions from passenger cars increased about 2 percent (Figure 3-1). In 2003, light-duty
trucks produced 43 percent of light-duty vehicle GHG emissions, up from 34 percent in 1990.
Motorcycles make up a very small proportion of light-duty GHG emissions (less than 0.1 percent in
2003), and this share has remained relatively constant.
Figure 3-1. GHG Emissions from Passenger Cars and Light-Duty Trucks, 1990-2003 (CO2 Eq.)
1,200
I QQQ • • • •
100%
Share of On-Road
GHG Emissions
s 80°
8 600 f
O)
1990 1992 1994 1996 1998 2000 2002
1990 2003
Source: Derived from U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington,
DC, Table 2-9, with adjustments.
Light-duty vehicles are primarily used for personal transportation, although some are used for business
purposes, or are maintained as part of public sector or private sector fleets.32 Among vehicles owned by
households, light-duty trucks account for an even higher share of light-duty GHG emissions. Nearly half
of household vehicle fuel consumption was by light-duty trucks in 2001, including vans (22 percent),
SUVs (17 percent), and pickup trucks (11 percent).33 Passenger cars consumed about 49 percent of the
30 Light-duty vehicles are defined as vehicles with a gross vehicle weight rating (GVWR) of less than 8,500 Ibs.
31 This figure does not include motorcycle emissions.
32 Such uses include rental cars, taxis, police vehicles, and government vehicles.
33 National Household Travel Survey, 2001.
10
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Transportation GHG Emissions Report
motor vehicle fuel used by households, and the remaining 1 percent was used by other light-duty trucks,
recreational vehicles (RVs), and motorcycles.34
Figure 3-2. Household Vehicle Fuel Consumption by Mode, 2001
Other Light-Duty
Vehicles, RVs,
and Motorcycles
1%
Passenger Cars
49%
Pickup Trucks
11%
Source: Energy Information Administration, 2005. Household Vehicles Energy Use: Latest Data & Trends. Washington, DC. Table A1, p. 54. Available
online at http://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/tablefiles/table-a01 .pdf.
3.2 Factors Affecting Light-Duty Vehicle Emissions
The increase in GHG emissions from light-duty vehicles reflects 1.) growth in vehicle travel and 2.)
limited improvement in vehicle fuel economy, largely associated with the increased sales and use of light-
duty trucks. Light-duty vehicle fuel consumption increased 22 percent between 1990 and 2003,35
resulting in a 20 percent increase in CO2 emissions. CIL, and N2O emissions also have been influenced by
the increase in travel, although their potential growth has been mitigated by technologies in newer
vehicles that have reduced GHG emissions.
3.2.1 Increasing VMT
Nationally, travel by light-duty vehicles rose 34 percent between 1990 and 2003. VMT has grown more
than twice as fast as population, with economic, social, and land use factors spurring increased vehicle
trip making and VMT per person. According to data from the National Household Travel Survey
(NHTS),36 VMT by households (which are a subset of all vehicle users) increased by 35 percent between
1990 and 2001, while the total number of households in the United States increased by only 15 percent
(Figure 3-3).37
34 Energy Information Administration, 2005. Household Vehicles Energy Use: Latest Data & Trends. Washington, DC. Table
Al, p. 54. Available online athttp://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/tablefiles/table-a01.pdf.
35 U.S. Department of Energy, 2004, Transportation Energy Data Book, Edition 24. Washington, DC.
36 The predecessor survey of the NHTS is the Nationwide Personal Travel Survey (NPTS).
37 Federal Highway Administration, 2001 National Household Travel Survey. Washington, DC. "Summary Statistics on
Demographic Characteristics and Total Travel 1969, 1977, 1983, 1990, and 1995 NPTS, and 2001 NHTS" and "Americans and
Their Vehicles," presentation by Pat Hu, Director, Center for Transportation Analysis, Oak Ridge National Laboratory. Available
online at http://nhts.oml.gov/2001/html_files/trends_ver6.shtml and
http: //nhts. oml. go v/2001 /presentations/american Vehicles/index, shtml.
11
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Transportation GHG Emissions Report
Figure 3-3. Comparison of Percent Growth of U.S. Population, Households, Vehicle Trips, and
Vehicle Miles Traveled by Households, 1990-2001
40%
oco/
35%
30%
25%
20%
15%
10%
5%
no/_-
35%
21%
15% 16%
Households U.S. Population Vehicle Trips Vehicle Miles Traveled
Source: Federal Highway Administration, 2001 National Household Travel Survey. Washington, DC. "Summary Statistics on Demographic
Characteristics and Total Travel 1969, 1977, 1983, 1990, and 1995 NPTS."
The rapid increase in VMT by households has been influenced in part by the increasing ownership of
personal vehicles. The proportion of households without a motor vehicle dropped from 9.2 percent in
1990 to 7.9 percent in 2001, continuing a longer term pattern of increasing vehicle ownership. In 1969,
about 20.6 percent of households owned no vehicles. By 2001, more households owned four or more
vehicles (8.5 percent) than owned no vehicles.38 The most substantial changes in vehicle ownership
occurred during the late 1960s through 1990, a period when a significant number of women entered the
workforce, the number of licensed drivers increased rapidly, and disposable income grew. The average
household in 1969 had 3.16 persons and 1.16 vehicles. Average household size in 1990 dropped to 2.56
persons, while the number of vehicles increased to 1.77 per household, exceeding the number of licensed
drivers per household. In 2001, there were about 203.9 million household vehicles serving 190.3 million
licensed drivers.39
As vehicle ownership has increased, average vehicle occupancy has declined and the number of vehicle
trips has grown. According to the Census Bureau's Journey to Work Survey, the proportion of commute
trips taken by single-occupant vehicle increased from 73.2 percent in 1990 to 75.7 percent in 2000, while
carpooling, transit, and walking mode shares declined. These trends reflect longer-term changes in
commuting patterns. In 1980, 64.4 percent of commuters drove to work alone, while nearly 20 percent
carpooled, about 6 percent used public transit, and nearly 6 percent walked. By 2000, the proportion using
carpools had fallen to 12 percent, transit to about 5 percent, and walking to 3 percent (Figure 3-4).40
Shared use of vehicles also has declined for other forms of personal travel, due in part to smaller
household sizes and increased vehicle availability. Across all trip purposes, the average number of
occupants per vehicle in 2000 was 1.6 persons, down from 1.9 in 1977.
38 Federal Highway Administration, 1999. Summary of Travel Trends: 1995 Nationwide Personal Transportation Survey.
Washington, DC, Table 16, p. 28; and Federal Highway Administration. 2001 National Household Travel Survey online tool.
Available online at http://nhts.oml.gov/2001/index.shtml.
39 Household vehicles are a subset of all light-duty vehicles and do not include business fleets or vehicles maintained by persons
living in institutions, such as colleges and military bases. Federal Highway Administration, 2001 National Household Travel
Survey. Washington, DC. "Summary Statistics on Demographic Characteristics and Total Travel 1969, 1977, 1983, 1990, and
1995 NPTS, and 2001 NHTS." Available online athttp://nhts.ornl.gov/2001/html_files/trends_ver6.shtml.
40 Although work trips make up only about one-quarter of total vehicle trips, they are important because work trips tend to
involve longer distances than trips for other purposes and often are conducted as part of a chain of trips.
12
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Transportation GHG Emissions Report
73%
76%
Figure 3-4. Journey to Work Mode Choice, 1980,1990, 2000
80%
70%
60%
50%
40%
30%
20%
D1980
• 1990
D2000
20%
4% 3% 3% 4% 4%
Drove Alone Carpooled Public Transit Walk/Bicycle
Work at
Home/Other
Source: U.S. Decennial Census, Supplemental Survey: Journey-to-Work, various census years, 1960 to 2000, as tabulated by Alan Pisarski and
reported in A. Pisarski, Commuting in America III. Washington, DC: Eno Transportation Foundation, 2003.
The average household in 2001 took more than 100 additional vehicle trips per year compared to 1990.41
In addition to the increase in trip-making, the average household trip length also increased from 8.7 miles
in 1990 to 9.7 miles in 2001.42 Consequently, the average household traveled over 3,000 more vehicle
miles in 2001 (21,253 vehicle miles) compared to 1990 (18,161 vehicle miles).
3.2.2 Changes in Vehicle Fleet Composition and Limited Improvements in Fuel
Economy
Over the past 25 years, consumer preferences for new vehicles have shifted notably, with an increasing
share of buyers opting to purchase light-duty trucks instead of passenger cars (Figure 3-5). In 1976,
approximately four new passenger cars were sold for each new light-duty truck. By 1990, the ratio had
shifted to two-to-one (67.1 percent passenger cars and 32.9 percent light-duty trucks), and in 2002, sales
of light-duty trucks surpassed those of passenger cars.
41 Federal Highway Administration, 2004. Summary of Travel Trends: 2001 National Household Travel Survey. Table 6.
42 However, the trend before 1990 is inconsistent. Calculated using Federal Highway Administration, 2001. 2001 National
Household Travel Survey. Washington, DC. Summary Statistics on Demographic Characteristics and Total Travel 1969, 1977,
1983, 1990, and 1995 NPTS, and 2001 NHTS. Available online athttp://nhts.ornl.gov/2001/html_files/trends_ver6.shtml.
13
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Transportation GHG Emissions Report
Figure 3-5. Number of New Light-Duty Vehicles Sold, 1976-2003
2 10
G) O c -
^ — D
°I
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Transportation GHG Emissions Report
Figure 3-6. New Light-Duty Vehicle Sales (Market Share) by Size Class, 1976,1990, and 2003
si ivs n 5%
Vans, 5.0% SUVs, 6.8%
Pickup Trucks, Vans> 9-7% SUVs, 27.0%
14.5%
Pickup Trucks,
16.4%
Vans, 8.5%
Pickup Trucks,
17.3%
Cars, 80.1%
Cars, 67.2%
Cars, 47.2%
1976
1990
2003
Source: U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Washington, DC, Table 4.9. Note: Original source percentage
shares do not equal exactly 100%.
The growing number of light-duty trucks on the road has corresponded with an increase in light-duty
truck travel. Light-duty truck VMT increased by 74 percent from 1990 to 2003, while passenger car VMT
increased by 18 percent (
Figure 3-7). In 2003, light-duty trucks comprised about 37 percent of all VMT by light-duty vehicles, up
from 29 percent in 1990.45 The increasing share of VMT by light-duty trucks is significant because those
vehicles tend to be less fuel efficient than their passenger car counterparts.
Figure 3-7. VMT by Passenger Cars and Light-Duty Trucks, 1990-2003
1!
> 9]
ns — 2-
> 10
5 E
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Transportation GHG Emissions Report
The Impact of Corporate Average Fuel Economy Standards
Since 1978, Congress has set standards for the average fleet fuel economy of new cars through the
Corporate Average Fuel Economy (CAFE) program. These standards initially required a fleet average of
at least 18 miles per gallon (mpg) for new passenger cars. The required average was subsequently
increased nearly every year until 1990, when it reached 27.5 mpg. The passenger car requirement does
not cover light-duty trucks, which must meet their own CAFE standards. The light-duty truck fuel
economy requirement is lower than the passenger car standard and has been increased more slowly, from
17.5 mpg in 1982 to 20.7 mpg in 1996.46 The reported fuel economy of both passenger cars and light-
duty trucks has closely mirrored CAFE standards. The most significant improvements in passenger car
fuel economy were reported between 1978 and 1985, and there were moderate increases in new light-duty
truck fuel economy through the late 1980s. Since then, the fuel economy of both new passenger cars and
new light-duty trucks has been relatively flat.
As a result of the increasing market share of light-duty trucks, the sales-weighted fuel economy of new
light-duty vehicles has steadily declined from its peak of 22.1 mpg in 1987 and 1988 (Figure 3-8). By
model year 2004, the new light-duty vehicle average had declined to 20.8 mpg.
Figure 3-8. Sales-Weighted Fuel Economy of New Light-Duty Vehicles (Combined Car and Light-
Truck Fleet) by Model Year, 1975-2004
zu
c
o
nj -ic
O 15 "
Q_
«-i n
s
n
r-i
r-|
Pe
ak
9
1-1
J
a
-
t;
>;
^.1
m
n
it
/
~
jnc
j 1
98
8
~
n
-
r-i
_
p.
1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003
Model Year
Source: U.S. Environmental Protection Agency, 2005. Light-Duty Automotive Technology and Fuel Economy Trends, 1975 through 2004. Washington,
DC.
Note: This graph represents the estimated sales-weighted fuel economy of new vehicles, based on EPA's adjusted estimates for 55/45 combined
city/highway driving.
46
Standards for light trucks were initially set in 1978, with separate standards for two-wheel drive and four-wheel drive vehicles.
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Transportation GHG Emissions Report
While EPA-rated new light-duty vehicle fuel economy
has declined since the late 1980s, average fuel
economy for the in-use fleet of all light-duty vehicles
has increased. Most of the gain occurred in the early
1990s, reflecting the retirement of a large number of
less fuel efficient vehicles built in the late 1970s and
early 1980s. Overall fuel economy of the in-use fleet
fuel increased from 18.9 mpg in 1990 to 19.6 mpg in
1991, then fluctuated and rose slowly to 20.3 mpg in
2003, representing a 7 percent improvement from
1990 levels.47'48 However, the increase in vehicle
energy efficiency was offset by a 34 percent increase
in light-duty VMT from 1990 to 2003, accounting for
the overall growth in light-duty CO2 emissions.
Effect of Light-duty Vehicle Technologies on
Criteria Air Pollutants
Criteria pollutants are not included in transportation
GHG totals but are considered to be indirect
greenhouse gases and are reported to the United
Nations Framework Convention on Climate Change
(UNFCCC). Increasingly stringent vehicle emissions
controls have dramatically reduced criteria pollutants
from light-duty motor vehicles. Between 1970 and
2002, during a time when light-duty VMT increased
by 32 percent, light-duty vehicle emissions of CO
dropped by 38 percent, volatile organic compounds
(VOCs) dropped by 48 percent, and oxides of
nitrogen (NOX) fell by 33 percent.
Source: U.S. Environmental Protection Agency, Emission
Factors and Inventory Group, Office of Air Quality Planning and
Standards, National Emission Inventory (NEI) Air Pollutant
Emission Trends web site.
3.2.3 Improvements in Vehicle Technologies and Emission Controls
Changes in light-duty vehicle technologies have not significantly impacted CO2 emissions. For the most
part, these technologies have been used to improve vehicle power, safety, and driving performance, rather
than to increase vehicle fuel economy.49 By contrast, vehicle emissions control technologies have reduced
emissions of CFLt and N2O between 1990 and 2003, although the two gasses have been affected
differently by various generations of these technologies.
Emissions control technologies were primarily designed to reduce emissions of criteria air pollutants
under EPA emissions standards. Beginning in the 1970s, auto manufacturers switched from non-catalyst
control systems to early oxidation catalysts and then to Tier 0-compliant technologies in the mid-1980s.
Tier 0 technologies were intended to control emissions of the hydrocarbons CO and NOX, but had the co-
benefit of reducing CHt (which is also a hydrocarbon). However, they also increased the amount of N2O
emitted per mile, and caused overall N2O emissions from light-duty vehicles to grow 27 percent between
1990 and 1998.
Newer generation technologies have lowered N2O emission rates. These include Tier 1-compliant
technologies, introduced in 1994, and low emissions vehicles (LEVs), which entered the fleet in 1996.
Nonetheless, these emissions rates are still higher than with the non-catalyst control systems. Since these
newer vehicles entered the fleet, light-duty emissions of N2O have declined to nearly 8 percent below
their 1990 level. These new vehicle technologies have had a larger impact on CH^ emissions rates from
light-duty vehicles, which decreased by 52.5 percent from 1990 to 2003 despite a significant increase in
VMT.
47 Federal Highway Administration, 1997. Highway Statistics: Summary to 1995. Washington, DC, Table VM-201A, and
Highway Statistics (annual editions), Table VM-1. FHWA estimates average fuel economy for the in-use fleet from fuel
consumption data (based on state fuel tax receipts) and estimates of VMT (based on travel monitoring data from states).
48 For instance, studies of 1984 model vehicles found maximum fuel efficiency achieved at steady-state speeds of 35 to 40 mph,
while newer studies of 1997 model vehicles showed peak fuel efficiency at steady-state speeds of 50 to 55 mph, and less fuel
economy loss above 55 mph (U.S. Department of Energy, Transportation Energy Data Book. Edition 23, Table 4.24).
49 The introduction and growing market share of gas-electric hybrid vehicles is increasing the number of fuel-efficient vehicles
on the road. A hybrid vehicle uses an electric motor in combination with a traditional combustion engine to power the vehicle
using less fuel. For instance, according to EPA fuel economy estimates, a 2003 Honda Civic Hybrid gets 47 combined mpg,
compared to 34 combined mpg with the traditional Honda Civic. The 2005 Ford Escape SUV hybrid gets 33 MPG, while the
traditional gasoline engine Escape achieves 23 mpg.
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Transportation GHG Emissions Report
4 Heavy-Duty Vehicles—Freight Trucks and Buses
Heavy-duty vehicles are the second-largest source of U.S. transportation GHGs,50 accounting for 19
percent of the transportation total in 2003, or 343 Tg CO2 Eq. Heavy-duty vehicles consist of medium-
and heavy-duty trucks used primarily for freight haulage (97 percent of heavy-duty emissions) and buses
(3 percent). Emissions from heavy-duty sources grew by 57 percent between 1990 and 2003, the largest
increase of any major transportation source. By comparison, emissions from passenger "light-duty"
vehicles increased 19 percent. Heavy-duty vehicles accounted for 23 percent of the on-road total GHG
emissions in 2003, up from 18 percent in 1990.
The majority of heavy-duty trucks run on diesel, while a smaller percentage run on motor gasoline.51
(Light-duty vehicles generally use motor gasoline.) It should be noted that the U.S. GHG Inventory's CO2
calculations for heavy-duty sources are based on diesel estimates from the Energy Information
Administration, which are lower than estimates compiled from Federal Highway Administration (FHWA)
and industry sources. As a result, the U.S. GHG Inventory may underestimate the CO2 emissions from
heavy-duty vehicles. (See Section 9 for discussion of uncertainty.)
4.1 Heavv-Dutv "Freight" Trucks
Data for heavy-duty trucks are less certain than that of light-duty vehicles, but generally indicate a
significant increase in activity and constant or slightly declining fleetwide fuel economy. Heavy-duty
truck ton-miles increased 55.5 percent between 1993 and 2002 to 1.45 trillion ton-miles.52 VMT grew by
a more modest 48 percent between 1990 and 2003,53 most likely reflecting improvements in distributional
efficiency. Nevertheless, overall GHG intensity of truck shipments increased 1 percent from 1990 to 2003
as total fleet fuel economy edged downward over the period. In 2001, trucks required 11 times more
energy to carry a ton-mile than rail, and 2.2 times more than ships.54 The relative GHG intensity of truck
haulage has significantly impacted total freight GHG emissions, especially as trucks' share of total ton-
miles increased from 19 percent in 1980 to 26 percent in 1993 and 32 percent in 2002.55
50 Heavy-duty vehicles are defined by EPA as vehicles weighted over 8,500 pounds. Vehicles weighing between 8,500 and
10,000 pounds are sometimes considered to be medium-duty trucks, but for simplicity, this category is referred to simply as
"heavy-duty vehicles." In addition, to freight trucks, heavy-duty trucks encompass larger commercial vehicles that are not used to
transport goods, such as some utility, service, and construction vehicles in addition to those used by some households.
51 A very small portion, consisting mainly urban delivery vehicles, run on alternative fuels.
52 U.S. Department of Transportation, Bureau of Transportation Statistics, and U.S. Department of Commerce, U.S. Census
Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey. Washington, DC.
53 Refers to combination trucks and single-unit trucks, excluding two-axle, four-tire trucks. Federal Highway Administration,
2004. Highway Statistics 2003. Washington, DC.
54 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend Data
Available online at http://intensityindicators.pnl.gov/data.html.
55 U.S. Department of Transportation, Bureau of Transportation Statistics, and U.S. Department of Commerce, U.S. Census
Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey. Washington, DC.
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Transportation GHG Emissions Report
4.1.1 Factors Underlying Increase in Trucking Activity
A number of factors contributed to increased freight movement, including the growth in domestic
consumption and global trade, as well as declining oil costs. Trucks carried a larger share of the
increasing domestic freight load for several reasons:56
Changing composition of shipments. The United States and world economies shifted toward more high-
value, low-weight products, such as electronic, electrical, office equipment, pharmaceuticals, and food
products, which are more conducive to haulage by trucks than by rail or ships.
Just-in-time inventory practices. Manufacturers that employ just-in-time systems strive to minimize on-
site inventory by coordinating their supply deliveries with production schedules. This requires smaller,
more frequent, and more reliable inbound shipments—characteristics that typically favor trucking over
rail and also may diminish the loads of smaller trucks.
Declining labor costs. The costs of truck freight transport have decreased relative to other shipping
modes, in part because of stiff price competition that followed trucking deregulation with the 1980 Motor
Carrier Act.
Manufacturing and warehouse location patterns. Manufacturing and warehousing have migrated from
urban areas to suburban or rural locations that often provide greater highway access and cheaper land and
labor. Longer hauls by truck carriers are required to connect more distant supply, production, and
consumption facilities. At the same time, these facilities are increasingly inaccessible by rail.
4.1.2 Fuels, Energy Efficiency, and GHG Emissions
Overall fuel economy for heavy-duty trucks fell from 6.0 mpg in 1990 to 5.7 mpg in 2003.57 These
calculations are somewhat uncertain and may reflect changes in the average size and weight of trucks or
other factors. According to the 1992 Truck Inventory and Use Survey (TIUS) and 1997 Vehicle Inventory
and Use Survey (VIUS), there was a small increase in mean fuel economy for most sizes of heavy-duty
trucks between 1992 and 1997.
Although single-unit trucks outnumber combination trucks by more than two-to-one, combination trucks
account for 64 percent of heavy-duty truck VMT and 72 percent of truck fuel use (Table 4-1).
Combination trucks are typically much larger than single-unit models. They are also used for much longer
hauls and require more fuel per mile. Diesel fuel is generally used in combination trucks, while both
gasoline and diesel are commonly used by single-unit vehicles.
Table 4-1. 2003 Vehicle Registrations, Vehicle Miles Traveled, and Fuel Use for Heavy-Duty Trucks
Type of Truck
Single-Unit
Combination
# of Vehicles
5,666,933
2,245,085
%
72%
28%
VMT (millions)
77,562
138,322
%
35%
64%
Fuel Use
(million gallons)
10,690
26,895
%
28%
72%
Source: Federal Highway Administration, U.S. Department of Transportation, 2004. Highway Statistics 2003. Washington, DC. Table VM-1.
Overall, the share of heavy-duty vehicles using diesel has increased over the period 1990 to 2003. During
this time period, VMT by diesel-powered heavy-duty trucks increased 60 percent, while VMT by
gasoline-powered heavy-duty vehicles increased 7 percent.58
56 For a discussion of these issues, see A Guidebook for Forecasting Freight Transportation Demand, NCHRP Report 388,
Transportation Research Board, 1997.
57 Federal Highway Administration, U.S. Department of Transportation, 2004. Highway Statistics 2003. Washington, DC. Based
on estimates of fuel consumption and VMT by vehicle type
58 Vehicle miles of travel by alternative fuel heavy-duty vehicles increased by 82 percent for trucks but still comprise a very
small portion (about 1 percent) of total VMT.
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Transportation GHG Emissions Report
4.2 Buses
Buses produced approximately 0.5 percent of total transportation GHGs and 0.6 percent of on-road
emissions in 2003. Bus GHGs increased about 15 percent from 1990. Best estimates suggest that transit
buses produced about 46 percent of total bus GHGs, followed by schoolbuses at 38 percent, and intercity
buses at 16 percent.59
Transit bus VMT60 increased 45 percent between 1990 and 2002, growing from 1.67 billion to 2.43
billion vehicle miles.61 The number of schoolbuses in service is estimated to have risen by 21 percent
over the same timeframe, increasing from approximately 508,000 to 617,067.62 Intercity buses
passenger-miles and energy use also increased over this period.63
Most buses run on diesel, while a small portion run on gasoline and alternative fuels. Alternative fuels are
playing an increasingly significant role in bus travel. Between 1990 and 2003, VMT for buses running on
alternative fuels increased by 273 percent. Alternative fuels include biodiesel, ethanol, methanol,
compressed natural gas, and liquefied natural gas.
59 Total GHG estimates for buses and the breakdown of emissions into these subcategories are somewhat uncertain, given
different estimates of bus fuel consumption from different sources and limited data on fuel consumption by schoolbuses. For
instance, estimates of fuel consumption by transit buses from the American Public Transportation Association (APTA) and fuel
consumption from schoolbuses and intercity buses from the Eno Transportation Foundation systematically exceed the estimates
of total bus fuel consumption reported by FFFWA for the years 1990 to 2003. The estimates in this report and the U.S. GHG
inventory rely on the FHWA data, and thus the GHG estimates reported here are lower than estimates that would result using the
APTA and Eno data directly.
60 Includes trolley buses.
61 No data available for 2003 at time pf publication. U.S. Department of Transportation, Bureau of Transportation Statistics,
2005. National Transportation Statistics 2004. Washington, DC, Table 1-32.
62 Federal Highway Administration, 2004. Highway Statistics 2003. Washington, DC, Table MV-10.
63 U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Table 5.13.
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Transportation GHG Emissions Report
5 Aircraft
5.1 Overview
Aircraft produced about 9 percent of U.S. transportation greenhouse gas emissions in 2003 (173.1 Tg CO2
Eq.) and were the largest source of non-road transportation GHGs. In total, aircraft GHG emissions
decreased approximately 3 percent from 1990 to 2003. GHGs from military aircraft declined by 40
percent over the period, and commercial aircraft GHGs increased moderately.
Commercial aircraft64 produced 72 percent of U.S. aircraft GHGs in 2003 (124.0 Tg CO2 Eq.), which was
4.7 percent greater than in 1990. The moderate increase reflected a 20 percent rise in GHGs from 1990 to
2001, followed by a substantial decline following the terrorist attacks of September 11, 2001, and a small
increase in 2003. Passenger travel rose much more rapidly than the level of GHG emissions, due to a
higher number of occupied seats per plane and improved aircraft fuel efficiency. Consequently, GHG
emissions per passenger-mile decreased 24 percent from 1990 to 2003, the largest improvement of any
transportation mode.
The remainder of aircraft GHG emissions in 2003 came from military aircraft (12 percent), general
aviation65 aircraft (7 percent), and "other"66 aircraft (10 percent). In the U.S. GHG Inventory, aircraft
emissions are based on domestic travel only, and exclude international travel to and from U.S. cities.67
Commercial and military aircraft rely almost exclusively on jet fuel, while about one-quarter of the fuel
used for general aviation is aviation gasoline.68 GHG emissions from aircraft in 2003 were 99 percent
CO2, about 1 percent N2O, and less than 1 percent CH4.
5.2 Factors Affecting Aircraft Emissions
Aircraft emissions have risen due to increased air travel activity by both passengers and freight, but this
has been offset to a large degree by the increased efficiency of aircraft and their operations. Between 1990
and 2003, passenger-miles traveled on certificated domestic services increased by 48 percent, from 345.9
billion to 505.2 billion passenger-miles. (In comparison, light-duty vehicle passenger-miles increased 31
percent over the same timeframe.) The increase in air travel would likely have been greater if not for the
terrorist attacks of September 11, 2001. From 1990 to 2000, commercial aircraft passenger-miles
increased by an average of 4.1 percent annually; passenger miles dropped by 6.6 percent between 2000
and 2002, and then increased by 4.7 percent in 2003.69
Although air cargo accounted for less than 1 percent of total United States freight ton-miles in 2002,
aviation was the fastest growing mode of freight transportation. Air ton-miles increased 63 percent from
1993 to 2002. The value of air freight shipments nearly doubled over the same period, increasing from
$395 billion in 1993 to more than $770 billion in 2002, at which point it represented 7 percent of the total
64 Represents any aircraft used in "common carriage." These are generally certificated air carriers (aircraft holding a certificate
issued by the Federal Aviation Administration to conduct scheduled and/or non-scheduled (charter) services) and may carry
passengers and/or freight.
65 These are non-certificated civil aviation operations, typically operated for personal or business use. The types of aircraft used
in general aviation range from corporate multiengine jet aircraft piloted by professional crews to amateur-built single-engine
acrobatic planes to balloons and dirigibles.
66 The balance of aircraft emissions are from "other" aircraft, which may include other uses of jet fuel such as heating oil.
67 GHGs associated with international travel are reported in the Inventory under bunker fuel estimates.
68 In total, aviation gasoline makes up about 1 percent of total aircraft fuel use.
69 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004.
Washington, DC,Table 1-11.
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Transportation GHG Emissions Report
goods transported domestically. (Freight truck ton-miles increased 24 percent from 1993 to 2002, with the
value of their cargo increasing 45 percent.) Based on the energy used per ton-mile, aviation is the most
energy intensive mode of freight haulage. In 2001, the energy required to move a ton-mile of air cargo
was 7.5 times greater than heavy-duty trucks, over 17 times that of ships, and 83 times greater than rail.70
Using an energy intensity metric based on the monetary value of goods moved (such as Btu per dollar
value shipped), air cargo is closer to other modes. However, it is also important to note that almost all air
cargo shipments begin and end their journey by truck, meaning that the growth in air freight has increased
demand for truck and intermodal services.71
The energy intensity of passenger air travel has declined substantially, in part because of increased
occupancy of aircraft. The average passenger load factor (percent of available seats that are occupied) on
certificated air carriers' domestic operations increased from 60.4 percent in 1990 to 72.4 percent in 2002,
continuing a longer term pattern of increasing passenger loads (Figure 5-1). As a result, aircraft passenger
miles grew faster than aircraft miles traveled between 1990 and 2000 (49 percent versus 43 percent).72
Figure 5-1. Aircraft Passenger Load Factor, 1970-2003
80%-
70%-
60%-
o 50%-
"5
" 40%-
TZ
30%-
20%-
10%-
0%
71.0%
,<>/„ 72.4%
65.4%
54.6%
58.0%
60.7% 60.4%
48.9%
n
1970 1975 1980 1985 1990 1995 2000 2003
Source: U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004. Washington, DC. Table
4-21.
The reduced energy intensity of commercial aviation also reflects improvements in aircraft fuel
efficiency.73 For new production aircraft, the fuel economy improvements have averaged 1 to 2 percent
per year since the 1950s.74 These developments have been market-driven, as airlines have improved
airframe and propulsion technology in order to reduce fuel costs.
70U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend Data
http://intensityindicators.pnl.gov/data.html.
71 For more discussion, see: Bureau of Transportation Statistics, 2004. Freight Shipments in America: Preliminary Highlights
from the Commodity Flow Survey Plus Additional Data.
72 Aircraft miles traveled increased from 3.96 billion to 5.66 billion by certified carriers. Source: Bureau of Transportation
Statistics, 2005. National Transportation Statistics 2004. Table 1-32.
73 Measured in fuel consumed per aircraft-mile traveled.
74 Intergovernmental Panel on Climate Change, September 1999. Aviation and the Global Atmosphere.
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Transportation GHG Emissions Report
One measure of fuel efficiency is the number of aircraft seat-miles per gallon of fuel consumed. The
measure, aircraft seat-miles, is calculated by multiplying the total air mileage traveled by the total number
of seats available.75 Available aircraft seat-miles per gallon increased by about 15 percent between 1990
and 2003 (from 46 to 53 seat-miles per gallon), although about half of this gain occurred since 2001 as
airlines reduced the number of flights. Nevertheless, the overall increases indicate the impact of longer-
term improvements in aircraft fuel efficiency, as well as the retirement of older, less fuel-efficient aircraft
(Figure 5-2).76
Figure 5-2. Average Seat-Miles Traveled Per Gallon of Fuel Consumed, 1970-2003
60
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6 Other Non-Road Transportation Sources
6.1 Boats/Ships
Boats and other marine vessels produced 3 percent of U.S. transportation GHG emissions in 2003 (58.0
Tg CO2 Eq.), a 17 percent increase from 1990. However, these figures reflect wide fluctuations in year-to-
year estimates of residual fuel consumption and are subject to a high degree of uncertainty. Much of this
uncertainty results from the challenge of separating the domestic and international components of fuel
consumption estimates. According to the UNFCCC reporting guidelines, national totals of GHG
emissions should reflect only domestic transport, including the domestic leg of shipments bound for
foreign markets. (The international component is represented by bunker fuel estimates). However,
differentiating domestic and international fuel consumption is often difficult, resulting in significant year-
to-year variations in the official estimates.
Domestic water shipments77 declined by 27.3 percent from 1990 to 2003 (Figure 6-1). As a share of total
freight movement, waterborne haulage declined from 24 percent of domestic ton-miles in 1993 to 16
percent in 2002.78 Part of the decrease reflected shifts to trucks for goods movement, especially in the
transport of lighter weight, time-sensitive commodities. Water shipments were also impacted by
maintenance needs on the lock-and-dam systems, environmental constraints for river channel dredging
and dam-controlled water levels, and a reduction in crude oil shipment from Alaska.79 In contrast to
domestic water shipments, tons of waterborne imports and exports increased between 1990 and 2003, as
the nation's international trade grew.
77 Measured in ton-miles. Waterborne commerce is comprised of several major elements: tugs and barges on the major rivers and
inland waterways, oil tankers carrying Alaskan crude to California, ocean-going ships carrying various cargo between the
continental United States, Hawaii, Alaska, and Puerto Rico, and large bulk vessels carrying coal, grain, and iron ore on the Great
Lakes.
78 U.S. Department of Commerce, U.S. Census Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey.
Washington, DC.
79 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004.
Washington, DC, Table 1-52.
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Transportation GHG Emissions Report
Figure 6-1. Ton-Miles Shipped by Domestic Water Transportation, 1990-2003
1,000,000
1990
1992
1994
1996
1998
2000
2002
Source: U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2005. Washington, DC, Table
1-46.
Recreational boats make up about one-fifth of boat and ship GHG emissions. Recreational boat GHGs
have increased steadily, growing from about 9.5 Tg CO2 Eq. in 1990 to 11.2 Tg CO2 Eq. in 2003, most
likely the result of increased activity and a growing number of crafts. Between 1990 and 2002, the
number of recreational boats registered in the United States increased by 17 percent from nearly 11.0
million to 12.9 million.80
6.2 Rail
Rail produced 2 percent of total transportation GHG emissions in 2003, or 43.2 Tg CO2 Eq. This was an
increase of 18 percent from 1990. About 89 percent of rail GHGs were from freight haulage, with the
remainder coming from passenger sources such as urban transit rail,81 commuter rail, and intercity rail
(Amtrak).82 GHG emissions from both freight and passenger rail increased. The majority of rail GHG
emissions are from the combustion of diesel fuel (92 percent), with electricity use comprising the balance.
6.2.1 Freight Rail
Between 1990 and 2003, total ton-miles shipped via rail increased by 50 percent, an average annual
growth rate of about 3.2 percent.83 Several factors contributed to the growth in ton-miles of rail
shipments, including the economic expansion through much of the 1990s and steady growth in coal
shipments. Demand also has grown for other bulk commodities that rely heavily on rail transport,
including chemicals, lumber and wood products, and farm products.
80 U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Table 9.7.
81 Especially the electricity used for heavy- and light-rail systems.
82 The Energy Information Administration's estimates of transportation electricity consumption, which are used to calculate CO2
emissions for the U.S. GHG inventory, only account for electricity used by urban transit rail, not for intercity rail (Amtrak). As a
result, the GHG electricity figures for rail are likely underestimated. Moreover, industry estimates of diesel fuel consumption by
railroads exceeds the estimate calculated for the U.S. GHG Inventory, based on the apportionment of diesel fuel consumption to
individual transportation sources (see Chapter 9 for a discussion of uncertainty).
83 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2003.
Washington, DC, Table 1-46.
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Rail fuel economy has improved steadily from 1990 to 2003.84 Calculations by the Association of
American Railroads (AAR) show that revenue ton-miles per gallon for Class I railroads85 has been
increasing at a rate of about 1.6 percent annually over the last 12 years. In 2003, this measure was 405
revenue ton-miles per gallon, up from 332 in 1990.86 This increase can be attributed to a number of
factors, including the introduction of more efficient locomotives and lighter weight railcars. Railroads
have also taken steps to improve the efficiency of their operations, by minimizing the movement of empty
railcars and short trains. For example, railroads in the Great Plains states have closed many of their
smaller spur lines and now carry grain shipments on 100-car "unit trains" that operate only on the
railroads' major trunk lines.
6.2.2 Passenger Rail
The increase in GHG emissions from passenger rail reflects a significant growth in passenger rail
services, with a number of light-rail and commuter rail lines coming into service or expanding operations.
Between 1990 to 2003, the number of vehicle miles of rail transit operations87 increased by 21.6 percent,
from 560.9 million to 682 million vehicle miles. Meanwhile, commuter rail operations increased by 33
percent, from 212.7 to 284 million vehicle miles. Amtrak train-miles and energy use also increased,
although overall ridership declined from 1990 to 2003. Meanwhile, passenger-miles traveled on urban
transit and commuter rail increased at an even higher rate than vehicle miles and fuel consumption.
6.3 Pipelines
While pipelines are not technically "mobile," they are generally classified as part of the transportation
sector because they serve an important purpose in transporting energy products domestically. (The U.S.
GHG Inventory includes pipelines in its transportation sector estimates, but excludes them from the
mobile source section.) More than 1.4 million miles of pipelines are used to transport natural gas, and
almost 177,000 miles are used to transport crude oil and petroleum products in the United States. While
there are far fewer miles of pipelines dedicated to transporting petroleum products, they account for two-
thirds of domestic petroleum transport. Pipelines also are used to transport coal slurry and water.
According to the Commodity Flow Survey, pipelines accounted for about 17 percent of total ton-miles of
all raw and finished products transported in 2002, similar to their share in 1993.88
The two primary "fuels" used to operate pipelines are natural gas and electricity, which are used in
pumps, motors, engines, and compressors. Natural gas used to power pipelines produced an estimated 35
Tg CO2 in 2003, or about 2 percent of total GHG emissions from transportation, a figure that has stayed
fairly constant since 1990.89
84 Locomotive fuel efficiency is generally reported in ton-miles per gallon. Gross ton-miles are based on the movement of the
entire train, including locomotives, railcars, and freight. Revenue ton-miles are based on the movement of freight for which the
railroad collects revenue—roughly half of gross ton-miles.
85 Class I railroads have operating revenues of more than $50 million.
86 Association of American Railroads, 2004. Railroad Facts. Washington, DC, p. 40.
87 Includes heavy and light rail.
88 U.S. Department of Transportation, Bureau of Transportation Statistics, 2004. Freight Shipments in America: Preliminary
Highlights from the Commodity Flow Survey Plus Additional Data.
89 The reported estimates do not include CO2 emissions associated with electricity use, although electricity is a major power
source for pipelines. According to estimates in the Transportation Energy Data Book, electricity used to power pipelines
consumed about 3.0 billion kilowatthours (kWh) in 2002 (most recent year available). At the average rate of CO2 emitted per
kWh, based on fuel mix in the United States, this level of electricity use equates to approximately 1.8 Tg CO2.
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Transportation GHG Emissions Report
7 HFCs from Mobile Air Conditioners and Refrigerated Transport
Hydrofluorocarbon (HFC) emissions from mobile air conditioners and transport refrigeration produced
about 2 percent of total transportation GHG emissions in 2003. HFCs are used as a replacement for
chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFCs), which deplete ozone and are
required to be phased out through international agreement under the Montreal Protocol. (Official GHG
Inventory estimates do not include CFC and HCFC estimates in national totals because of their mandated
retirement.) As HFCs have replaced CFCs and HCFCs, their emissions have risen steadily from nearly
zero emissions in 1990 to approximately 42.7 Tg CO2 Eq. in 2003 (Figure 7-1). While estimating the net
effect of this transition is difficult, the introduction of HFCs has lowered GWP-weighted emissions
because the replacement gases generally have lower GWPs than their predecessors. For instance, the
standard automobile air conditioner refrigerant, HFC-134a, has a GWP of 1,300, compared with its
predecessor's net effect of between 7,300 and 9,900.90
Figure 7-1. HFC Emissions from Mobile Air Conditioners and Refrigerated Transport, 1990-2003
50
o-
N 30
O
O
E? 20
10
1990 1992 1994 1996 1998 2000 2002
Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington, DC.
Approximately 71 percent of HFC emissions in 2003 were associated with mobile air conditioners, which
are used to cool the passenger compartments of on-road vehicles. The remaining HFC emissions came
from refrigerated transport. HFCs are commonly used to refrigerate perishable food and temperature-
sensitive items during transport on reefer ships, refrigerated freight trains, and insulated trucks and trailers
with self-contained refrigeration units.
HFC-134a was introduced in some automobiles beginning in 1992 and became the standard automobile
refrigerant in 1994. Some vehicles with CFC air conditioners were retrofitted with HFC-134a or a
refrigerant blend. Regulations and industry practices established during the CFC phase-out, such as better
training of technicians, certification requirements to purchase CFC refrigerant, and requirements to use
recovery equipment and not vent refrigerant during equipment service, have reduced leakage and other
"unnecessary" emissions. Some of these practices and requirements have also been implemented with
newer HFC-equipped vehicles, which has helped limit the growth of HFC emissions. Manufacturers have
also become increasingly sensitive to the environmental effects of refrigerants. Many have responded by
reducing refrigerant charge sizes and leak rates, increasing reliability of their equipment, and
investigating refrigerants with even lower GWPs.
90 CFC-12 was the predecessor to HFC-134a. Although CFC-12 has an estimated direct GWP of 10,600, it also destroys
stratospheric ozone, thus lowering its net effect.
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Transportation GHG Emissions Report
Other vehicle air conditioning systems also are transitioning away from ozone depleting chemicals.91
Most buses and many passenger trains currently use HCFC-22, which is an ozone depleting chemical that
will be phased out under the Montreal Protocol. HCFC-22 has an ozone depleting impact estimated to be
5.5 percent that of CFC-12. While production is currently allowed under the Montreal Protocol in all
countries, many nations have accelerated phase-out through regulations and other measures. Some new
buses and trains have utilized alternatives to HCFC-22, most notably HFC-134a, R-407C (a blend of
HFC-32, HFC-125, and HFC-134a), and R-410A (a blend of HFC-32 and HFC-125).
CFC and HCFC emissions are not included in official U.S. GHG inventory estimates. Due to the required
phase-out of these gases, their emissions have been steadily declining (Figure 7-2). The use of HFCs as a
replacement to CFCs and HCFCs has resulted in lower GWP-weighted emissions, although there is no
official estimate of the net change in GWP-weighted emissions.
Figure 7-2. CFC and HCFC Emissions from Mobile Air Conditioners and Refrigerated Transport,
1990-2003
on
In
E fin
o
.c
ii- /in
on
n -
1 — 1
,— i
—
—
—
—
i —
Hn
1990 1992 1994 1996 1998 2000 2002
NOTE: CFC and HCFC emissions are not included in the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
Source: U.S. Environmental Protection Agency, 2004. U.S. EPAVintaging Model. Version VM IOfile_11-08-04.
91 In the past, transportation-related air conditioning and refrigeration systems have used a wide variety of CFCs and HCFCs.
Most notable have been CFC-12, HCFC-22, and R-502 (a blend of CFC-115 and HCFC-22). These are being replaced primarily
by HFCs, although some non-fluorocarbon alternatives such as ammonia, hydrocarbons, and water also are used. CFC-12 and R-
502 have been replaced by HFC-134a, R-404A (a blend of HFC-125, HFC-143a, and HFC-134a) and R-507A (a blend of HFC-
125 and HFC-143a). HCFC-22 is still used but also is being replaced by these refrigerants as well as R-407C (a blend of HFC-32,
HFC-125 and HFC-134a) and R-410A (a blend of HFC-32 and HFC-125).
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Transportation GHG Emissions Report
8 Non-Transportation Mobile Sources—Agricultural and
Construction Equipment, Recreational Vehicles, and Other
This report focuses on GHG emissions from the transportation sector. For the most part, transportation
sources are associated with the movement of people and goods. There are several other mobile sources
that serve functions other than transportation, such as construction or shelter. These "non-transportation
mobile" sources were estimated to have produced 144.8 Tg CO2 Eq. in 2003.92
Non-transportation mobile sources include:
• Agricultural equipment—This category predominantly consists of tractors, mowers,
combines, balers, and other farm-related equipment that perform functions while moving.
There are twice as many diesel vehicles as gasoline vehicles in this category.
• Construction equipment—Construction equipment includes cement and mortar mixers,
excavators, forklifts, loaders, bore drill rigs, and other equipment.
• Recreational vehicles—These vehicles include snowmobiles, off-road motorcycles, all-terrain
vehicles, and golf carts.
• Lawn and garden equipment—This equipment type includes lawnmowers, lawn and garden
tractors, snowblowers, leaf blowers, and other equipment used for residential and commercial
purposes.
• Other commercial and industrial equipment—This source includes equipment such as airport
ground service equipment, aerial lifts, sweepers/scrubbers, and other utility equipment.
Figure 8-1. Greenhouse Gas Emissions from Non-Transportation Mobile Sources, 1990 and 2003
59.1
Construction Farm
Equip. Equip.
Lawn and Recreational Ind/
Garden Equip. Comm.
Equip. Equip.
Source: Calculated by summing estimates of N2O, CH4, and CO2. N2O and CH4 estimates were taken from U.S. Environmental Protection Agency.
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington, DC, Tables 3-20 and 3-21. CO2 estimates were developed based
92 The U.S. GHG Inventory presents estimates of CH4 and N2O from non-transportation mobile sources but does not provide
estimates of CO2 emissions, since fuel consumption from these sources is not broken out as a separate data element in the Energy
Information Administration's fuel data. In the U.S. GHG Inventory, CO2 from these sources is encompassed in other sectors
(e.g., industrial) and is not included among transportation sources. As a result, this report includes CO2 estimates calculated for
these non-transportation mobile sources based on fuel consumption estimates from EPA's NONROAD Model, which are used in
the CH4 and N2O inventory calculations for mobile sources.
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Transportation GHG Emissions Report
on fuel consumption data contained in U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003.
Washington, DC, Annex 3, Table 3-13.
Although there is considerable uncertainty93 associated with these estimates, it appears likely that GHG
emissions from non-transportation mobile sources were nearly equal to the combined GHGs of boats and
ships, rail, and pipelines in 2003. Collectively, GHG emissions from non-transportation mobile sources
increased by 44 percent from 1990 (Figure 8-1). CO2 accounted for more than 99 percent of GHG
emissions, and N2O and QrU each produced less than 1 percent.
93 Activity and fuel consumption data for these sources are limited in comparison to transportation sources. There is no one data
source that currently has information on all the non-transportation mobile sources, and different publications report significantly
different estimates, complicating the estimation process. For instance, estimates derived from a 2004 analysis of various data
sources, including FHWA, EPA, and EIA by Oak Ridge National Laboratory (ORNL), were considerably higher than estimates
currently used in developing the U.S. GHG Inventory. Moreover, it is likely that the transportation and mobile sources estimates
of GHG emissions in the U.S. GHG Inventory are missing emissions associated with off-road use of trucks, and these emissions
may be captured under other sectors, especially the industrial sector.
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Transportation GHG Emissions Report
9 Estimating Transportation GHG Emissions—Methodology and
Uncertainty
All GHG emissions estimates presented in this report are from the official Inventory of U.S. Greenhouse
Gas Emissions and Sinks, published annually by EPA.94 These estimates are calculated using data such as
vehicle miles traveled, fuel consumption, and emissions factors. The quality and reliability of these data
sources significantly determine the reliability of GHG emissions estimates. This chapter briefly describes
the methods used to calculate transportation GHG emissions, and then discusses the uncertainty
associated with these methods and the supporting data sources.
9.1 Carbon Dioxide Emissions
9.1.1 Methodology
Carbon dioxide emissions are a direct product of fossil fuel combustion. They are calculated for each fuel
type as a simple product of the following factors:
• Fuel consumption (in Btus) - These estimates are based on data provided by EIA.
• The carbon coefficient of a particular fuel - These values represent the total amount of carbon
released when the fuel is burned (expressed in Tg carbon per Btu). The carbon coefficient
depends on the density, carbon content, and gross heat combustion of the fuel.
• The percent of fuel that is combusted - The 1996 IPCC assumes that oxidation is 99 percent
complete and that 1 percent of the carbon remains sequestered.95
In the U.S. GHG Inventory, CO2 emissions from the transportation sector are estimated using a multistage
process. First, total national CO2 emissions from fossil fuel combustion are calculated by accounting for
the factors described above. National-level estimates of fuel consumption by fuel type are multiplied by
the carbon content of each fuel and the percent of fuel that is oxidized, producing fuel-specific CO2
estimates. These fuel-specific CO2 estimates then are apportioned to economic sectors based on each
sector's contribution to total fuel consumption. Within each sector, emissions then are allocated to
individual sources (such as particular transportation modes) based on fuel consumption data.
9.1.2 Uncertainty
Since the vast majority of transportation GHG emissions are in the form of CO2, uncertainty in the CO2
estimates has a much greater effect on the transportation sector estimates than uncertainty associated with
N2O, CH4, or HFC emissions. EPA believes that the uncertainty in CO2 estimates for the United States as
a whole is relatively small.
As described in Chapter 3 of the U.S. GHG Inventory, the uncertainty associated with total CO2
emissions from fossil fuel combustion is a function of the uncertainty in primary input data: fuel
consumption, carbon content, and oxidation factors. Fuel-specific consumption data are obtained from
EIA, primarily from its Monthly Energy Review, as well as unpublished sources within the agency. EIA
also provides the carbon contents. Oxidation fractions are published by the IPCC.
94 Non-transportation mobile estimates for CO2 are not provided in the body of the Inventory report but are calculated in the
Inventory Annex. For this report, a minor correction was made to the diesel CO2 figures for non-transportation mobile sources, so
the figures reported differ slightly from those in the Inventory Annex. Some emissions from sources considered "non-
transportation mobile sources" in this report are counted as part of transportation sector emissions in the Inventory.
95 Note that the 2006 IPCC guidelines specify a 100 percent oxidation fraction.
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Transportation GHG Emissions Report
Fuel sales are tracked for many reasons, such as taxation and economic analyses. On an aggregate level,
fuel consumption data are believed to be relatively accurate. The carbon content of fuels and oxidation
fraction values are also relatively certain. As a result, the total estimate of CO2 emissions from fossil fuel
combustion reflects a small degree of presumed uncertainty, estimated to range from 1 percent below the
U.S. GHG Inventory's actual estimate to 6 percent above it, based on a 95 percent confidence interval.96
However, additional uncertainty is introduced when the national totals are allocated to individual sectors
(such as transportation) and sources within sectors (such as modes and vehicle types). The allocation
process is based on the relative consumption of fuel by each individual source. For example, if
transportation is estimated by EIA to comprise X percent of national gasoline consumption, then the
transportation sector is allocated X percent of CO2 emissions from gasoline. The CO2 emissions then are
allocated to individual modes and vehicle types by EPA based on fuel consumption data from a variety of
sources, including FHWA, American Public Transportation Association, AAR, and DOE.
The apportionment methodology used to develop CO2 estimates for the GHG Inventory represents a "top-
down" calculation approach. These values are somewhat different than estimates that would be calculated
"bottom-up" starting with primary data sources, such as FHWA's Highway Statistics. These differences
are a source of uncertainty in the transportation GHG estimates.
Differences in fuel consumption reported by EIA and other sources largely stem from different survey
methodologies, data collection processes, and allocations of fuel use. EIA's estimates of transportation
diesel fuel consumption, which are used in the official GHG inventory, are systematically 2.5 to 10.0
percent lower than estimates from various bottom-up sources for 1990 to 2003.97 EIA's estimates of
transportation motor gasoline fuel consumption for 1990 to 2003 also are systematically lower by a small
amount (ranging from 0.6 to 2.4 percent) than estimates compiled by EPA using FHWA's Highway
Statistics for on-road vehicles and EPA's NONROAD Model for recreational boats. On the other hand,
EIA's estimates of transportation jet fuel use are consistently higher (9.1 to 12.3 percent) than estimates
compiled by EPA for 1990 to 2003.
Using the "bottom-up" method described above, VMT were apportioned by fuel type and then allocated
to individual model years using temporal profiles of both the vehicle fleet by age and vehicle usage by
model year in the United States provided by EPA (2004b) and EPA (2000). Although the uncertainty
associated with total U.S. VMT is believed to be low, the uncertainty within individual source categories
was assumed to be higher, given uncertainties associated with apportioning total VMT into individual
vehicle categories, by technology type, and equipment age. The uncertainty of individual estimates was
assumed to relate to the magnitude of estimated VMT (i.e., it was assumed smaller sources had a greater
percentage of uncertainty). A further source of uncertainty occurs because FHWA and EPA use different
definitions of vehicle type, and estimates of VMT by vehicle type (provided by FHWA) are broken down
by fuel type using EPA vehicle categories. The bottom-up estimates are also subject to several possible
sources of error, such as unregistered vehicles, unreported fuel sales to avoid fuel taxes, differences in
achieved versus estimated fuel economy, and measurement and estimation errors.
Despite these issues, EPA believes that the uncertainty associated with CO2 from the transportation sector
is still relatively small. Depending on the fuel type, these values range from 6 percent below to 7 percent
above actual estimates based on a 95 percent confidence interval. However, it is likely that the uncertainty
for individual modes is higher. For instance, FHWA is recognized as being the preeminent data source for
96 Based on Monte Carlo simulations, which randomly generate values for uncertain variables repeatedly. These randomly
generated numbers are used by the simulation to estimate results. For a 95 percent confidence interval, estimated values fell
within the specified range for 19 out of 20 simulations.
97 These include FHWA's Highway Statistics for highway vehicles, EIA's Fuel Oil and Kerosene Sales Report for ships and
boats, and individual data sources for locomotives (AAR for Class I railroads, the Upper Great Plains Transportation Institute for
Class II and III railroads, and the Transportation Energy Data Book for commuter rail and Amtrak).
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on-road vehicle fuel use, and EIA's transportation sector totals are estimated using FHWA data for on-
road vehicles and other survey data for non-road transportation sources. As a result, uncertainties in non-
road fuel use are believed to be largely responsible for discrepancies in consumption of motor gasoline
and diesel fuel between EIA and other sources. Nonetheless, if CO2 estimates from EIA are lower than
estimates that would be developed using bottom-up data sources, CO2 emissions for all modes are
reduced proportionately using the current allocation method.
Table 9-1 provides a summary comparison of estimates of transportation CO2 emissions reported in the
U.S. GHG Inventory for 1990 and 2003 compared with estimates developed based on a bottom-up
approach.
Table 9-1. Comparison of U.S. GHG Inventory Estimates and Bottom-Up Estimates of CO2 for
Selected Transportation Fuels and Sources
Fuel Type/Vehicle Type
Gasoline
Automobiles
Light-Duty Trucks
Heavy-Duty Trucks
Buses
Motorcycles
Boats (Recreational)
Diesel Fuel
Automobiles
Light-Duty Trucks
Heavy-Duty Trucks
Buses
Locomotives
Ships and Boats
Electricity
Jet Fuel
Commercial Aircraft
Military Aircraft
General Aviation Aircraft
Other Aircraft
1990
Inventory Bottom-Up
Est. Est.
955.2 973.5
605.1 616.7
301.0 306.7
37.7 38.5
0.3 0.3
1.7 1.7
9.4 9.6
253.7 264.1
7.4 7.7
10.7 11.2
178.4 186.4
7.5 7.8
33.3 34.8
16.3 16.2
3.0 3.2
174.2 158.2
117.2 117.2
34.8 34.8
6.3 6.3
15.9
Difference
18.3
11.6
5.7
0.8
0.0
0.0
0.2
10.4
0.3
0.5
8.0
0.3
1.5
-0.1
0.2
-16.0
0.0
0.0
0.0
2003
Inventory Bottom-Up
Est. Est.
1,143.7 1,153.9
630.2 635.8
460.9 465.0
39.6 39.9
0.3 0.3
1.6 1.6
11.0 11.1
386.6 417.0
3.4 3.7
17.6 19.0
301.1 325.5
8.0 8.6
39.6 42.8
17.0 17.4
3.2 3.9
169.0 152.7
122.8 122.8
20.5 20.5
9.4 9.4
16.3
Difference
10.2
5.6
4.1
0.3
0.0
0.0
0.1
30.4
0.3
1.4
24.4
0.6
3.2
0.4
0.7
-16.3
0.0
0.0
0.0
As shown in this table, bottom-up estimates suggest that GHG emissions from transportation diesel fuel
use may be higher than reported in the U.S. GHG Inventory, particularly from diesel fuel used by heavy-
duty trucks. GHG emissions from motor gasoline also may be higher, although the percentage difference
in these estimates is smaller than those of diesel. Electricity fuel consumption in transportation also
appears higher using a bottom-up methodology and shows a higher rate of growth; much of this
difference is likely attributable to increased electrification of Amtrak service in the Northeast Corridor,
which is not reflected in the EIA estimates. The total GHG estimates presented in the U.S. GHG
Inventory are believed to account for all these emissions, but the transportation sector estimate may not.
Finally, a recent EPA study suggested that the fraction of fuel combusted for light-duty gasoline motor
vehicles is 100 percent. The revised estimate has been peer reviewed and may be incorporated into future
IPCC guidance. It also is possible that diesel and gasoline vehicles burn virtually 100 percent of their fuel,
and EPA will be conducting further research to examine these estimates for transportation and non-
transportation sources.
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9.2 Methane and Nitrous Oxide Emissions
9.2.1 Highway Vehicles
Unlike CO2 emissions, which are directly proportional to fuel consumption, CHt and N2O emissions from
highway vehicles are affected by vehicle emissions control technologies. Emissions are calculated based
on VMT and per-mile emissions factors, which vary by type of emissions control technology system. The
total VMT driven within each class of vehicles is distributed among various emissions control systems,
based on distributions of vehicles by model year, VMT by vehicle age, and control technologies in place
by model year.
Uncertainty in CH4 and N2O emissions from highway vehicles is a product of uncertainty in VMT
estimates, the distribution of VMT to control technology types, and emissions factors.
VMT estimates by vehicle type are taken from FHWA. These estimates are believed to be relatively
accurate at the national level but are subject to several possible sources of error. The VMT are
apportioned by fuel type and then allocated to individual model years using EPA temporal profiles of both
the vehicle fleet by age and vehicle usage by model year in the United States. Although the uncertainty
associated with total U.S. VMT is believed to be low, the uncertainty within individual vehicle categories
is assumed to be higher, given uncertainties associated with apportioning total VMT into individual
vehicle categories, by technology type, and equipment age.
The emissions factors for highway vehicles used in the U.S. GHG Inventory are based on laboratory
testing of vehicles. Although the controlled testing environment simulates actual driving conditions, the
results from such testing can only approximate real world conditions and emissions. For some vehicle and
control technology types the testing did not yield statistically significant results within the 95 percent
confidence interval, requiring reliance on expert judgment when developing the emissions factors. In
those cases, the emissions factors were developed based on comparisons of fuel consumption between
similar vehicle and control technology categories.
The U.S. GHG Inventory reports that the uncertainty range of CFU and N2O emissions is greater than that
of CO2 emissions. CH4 is estimated to be somewhere between 9 percent below and 4 percent above the
actual U.S. GHG Inventory total, based on a 95 percent confidence interval. The calculated value of N2O
emissions has even greater uncertainty, with uncertainty estimates ranging from 16 percent below to 26
percent above the U.S. GHG Inventory total, based on a 95 percent interval. However, the overall
significance of uncertainty in CH4 and N2O estimates is presumed to be relatively minor because these
emissions comprise a small portion of total highway vehicle GHG emissions.
9.2.2 Other Mobile Sources
The U.S. GHG Inventory calculates CH4 and N2O emissions for other mobile sources by applying an
emissions factor to the quantity of fuel consumed. The uncertainty of these calculations is a direct product
of uncertainties in these two inputs, which often are considered to be highly uncertain. For example, the
IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories
reports that CH^ emissions from aviation and marine sources may be uncertain by a factor of two, while
N2O emissions may be uncertain by an order of magnitude for marine sources and several orders of
magnitude for aviation. No information is provided on the uncertainty of emissions factors for other non-
road sources.
Fuel consumption data are drawn from a variety of sources. Consumption tracking for some modes,
particularly for the less significant modes, is not highly accurate. Fuel consumption for some modes is
estimated using EPA's NONROAD model and is not based on actual sales records for each year. Sales
data often cannot accurately reflect the actual end use of a given fuel. For instance, some gasoline
purchased by the marine sector may be used for operating heavy equipment or even generators, instead of
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Transportation GHG Emissions Report
being used entirely by ships and boats. This distinction between mobile and stationary fuel users is not
made by El A.
An even greater level of uncertainty is associated with the emissions factors themselves. The U.S. GHG
Inventory relies on emissions factors provided by the IPCC. In some cases these factors cover very broad
categories. For example, the same emissions factors are used for tractors, snowmobiles, riding
lawnmowers, and construction vehicles. It is likely that these various modes emit differing amounts of
non-CO2 gases per gallon of fuel consumed. As another example, a single emissions factor is applied to
all jet fuel consumed, even though emissions amounts vary depending on whether the aircraft is in the
landing/take-off cycle or the cruise portion of flight.
Despite the large degree of uncertainty associated with non-road modes, the significance of this
uncertainty is low given the relatively small quantity of GHGs released by these sources.
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10 Lifecycle Transportation Emissions
This report primarily addresses GHG emissions from energy that is used for powering vehicles.
Transportation depends on array of additional processes, such as the manufacture of vehicles and
extraction of crude oil. Within the U.S. GHG Inventory, these activities are accounted for in other
economic sectors—most notably the industrial sector. Nevertheless, they are still a part of the
transportation lifecycle and can offer a broader perspective on the GHG impact of transportation.98
A full lifecycle assessment (LCA) of transportation takes into account all emissions associated with the
vehicles, fuel, infrastructure, and associated activities that make up the nation's transportation system.
Emissions occur during three lifecycle stages:
1. Upstream Emissions—Upstream emissions are those that occur before a product is used,
including extraction of raw materials, processing, manufacturing, and assembly. Sources of
upstream emissions include any fuel combustion associated with these processes, as well as
"fugitive" emissions, such as venting and/or flaring of natural gas from oil wells or natural
gas plants.
2. Direct Emissions—Direct emissions occur during the operation and maintenance of vehicles.
3. Downstream Emissions—Downstream emissions occur at the end of the lifecycle and are
associated primarily with disposal. Sources of downstream emissions include fuel combustion
used during disposal, collection of municipal solid waste, and landfills.
An LCA of transportation also should take into account emissions from three key components of
transportation systems: fuels, vehicles, and infrastructure.
Table 10-1 provides examples of sources of emissions at each stage of life for each component.
Transportation fuel use is the focus of traditional analysis of transportation emissions. An LCA of
transportation fuels, often referred to as a fuel cycle analysis, includes upstream emissions associated with
drilling, exploration and production, crude oil transport, refining, fuel transport, storage, and product
retail, as well as downstream disposal or recycling of oil products.
An analysis of vehicle lifecycle emissions includes each stage of vehicle manufacturing (raw material
extraction, processing, and transport; manufacture of finished materials; assembly of parts and vehicles;
and distribution to retail locations), vehicle operation and maintenance, and vehicle disposal.
Finally, an LCA of infrastructure includes emissions associated with construction, operation and
maintenance, and disposal of all transportation-related infrastructure, such as roads, parking lots,
pipelines, railroad tracks, bridges, tolls, airports, train and bus stations, and fuel stations.
A lifecycle assessment can be useful in evaluating certain policy questions. This approach is increasingly
used in the transportation sector to compare emissions from different fuel types, especially when the
emissions generated in fuel production may vary significantly from the tailpipe emissions during
combustion.
98 Although official estimates of national GHG emissions do not usually take a lifecycle approach, there are some exceptions.
Fuel ethanol derived from biomass is assumed to have net CO2 emissions of zero, as crops sequester carbon from the atmosphere
while they grow. Similarly, in some places in the U.S. Inventory, electricity emissions are accounted for in the transportation
sector, although they are all upstream emissions.
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Table 10-1. Elements of the Transportation Lifecycle
Vehicle Cycle
Fuel Cycle
Infrastructure Cycle
Upstream
Emissions
Upstream Vehicle Cycle
Raw material (e.g., ore for steel
or aluminum; petroleum for
plastics) extraction, processing,
production, and transport;
manufacture of finished
materials and components;
intermediate parts
transportation; assembly of
parts and vehicles; distribution
to retail locations
Upstream Fuel Cycle
Exploration, drilling, production,
and pumping; agricultural
activities for biomass;
production activities for other
energy sources; crude
oil/gas/material transport;
refining and processing into
motor fuel; product transport,
intermediate, wholesale, and
retail storage; retail product
sales and dispensing
Upstream Infrastructure
Cycle
Raw material production and
transport (e.g., asphalt,
cement, and steel);
desequestration (clear-
cutting) of land; construction
activities
Direct
(Operating)
Emissions
Direct Vehicle Cycle
Tire wear; engine oil and other
lubricant and fluid use; parts
replacement; other operations
and maintenance activities
Direct Fuel Cycle
Fuel combustion; fuel
evaporation [This element is
the only one covered under
traditional transportation
emissions analyses.]
Direct Infrastructure Cycle
Resurfacing; repainting and
striping; pothole repair;
plowing, street cleaning, other
operations and maintenance
activities
Downstream
Emissions
Downstream Vehicle Cycle
Disposal of vehicles, including
possible recycling of parts; tire
disposal and possible
incineration
Downstream Fuel Cycle
Disposal and possible recycling
of oil products
Downstream Infrastructure
Cycle
Disposal and possible
recycling of certain
infrastructure raw materials;
potential reclamation of land
(e.g., rails-to-trails)
10.1 Estimates of Transportation-Related CCb Emissions
A lifecycle analysis of CO2 emissions from the nation's transportation system was developed for this
report, examining upstream fuel cycle and vehicle cycle emissions. This analysis did not account for
emissions from the infrastructure cycle, although their potential impact is recognized to be potentially
significant. Nevertheless, the estimates in this section offer an initial perspective on some of the
additional GHG impacts associated with various vehicle types and modes, as well as the transportation
sector as a whole.
Special consideration needs to be given to potential areas of double-counting. For example, a
comprehensive LCA of the GHG impacts of passenger vehicles includes emissions from the transport of
crude oil and motor gasoline used by these vehicles, as well the original shipment of these vehicles from
the automotive manufacturing plant to the dealer. However, upstream transport-related emissions are
already represented in the direct emissions from other vehicle types, such as heavy-duty vehicles used to
transport new passenger cars. While attributing these transport-related emissions to passenger cars is
acceptable when examining the lifecycle impacts of these vehicles, it is not appropriate to include these
when considering the sector as a whole. Therefore, all upstream and downstream transport-related
emissions should be excluded when examining the entire sector.
Two leading transportation lifecycle models were used to assess GHG impacts for this analysis. The
Lifecycle Emissions Model (LEM) was developed by Mark A. Delucchi of the Institute of Transportation
Studies at the University of California-Irvine. The Greenhouse Gases, Regulated Emissions, and Energy
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Transportation GHG Emissions Report
Use in Transportation (GREET) model was developed by Argonne National Laboratory for DOE's Office
of Transportation Technologies. Each of these models is described briefly below."
10.1.1 Lifecycle Emissions Model (LEM)
The Lifecycle Emissions Model100 examines energy use, GHG emissions (CO2, CFL,, N2O, HFCs), and
criteria pollutant emissions associated with the full lifecycle of various transportation activities. This
model examines the following components:
• Fuel cycle—raw material production (e.g., crude oil), raw material transport, fuel production,
fuel distribution and storage, fuel dispensing, and end use;
• Material lifecycle—raw material recovery (e.g., iron ore), vehicle manufacture, and transport
of materials to end-users;
• Vehicle lifecycle—assembly, operations and maintenance, secondary fuel cycle; and
• Infrastructure lifecycle—energy use and materials production.
Lifecycle emissions for a number of vehicle types are calculated, including passenger cars, buses, and
medium- and heavy-duty trucks. No estimates regarding other vehicle types or any stage of infrastructure
lifecycle emissions have been included, as those estimates in LEM are still considered rudimentary.
10.1.2 GHGs, Regulated Emissions, and Energy Use in Transportation (GREET)
GREET 1.6101 estimates energy use, GHG emissions (CO2, CIL,, N2O), and criteria pollutant emissions
related to the fuel cycle of various vehicle and fuel combinations. The primary purpose of GREET is to
evaluate the energy and emissions impacts associated with alternative fueled vehicles and advanced
vehicle technologies in light-duty vehicles, for the purpose of assessing near- and long-term transportation
options. GREET examines more than 30 fuel-cycle pathways, and examines the following components:
• Feedstock production;
• Feedstock transportation;
• Feedstock storage;
• Fuel production,
• Fuel transportation and distribution;
• Fuel storage; and
• Vehicle operation (refueling, fuel combustion/conversion, fuel evaporation, tire/break wear).
10.1.3 Results
Both GREET and LEM provide estimates of upstream fuel cycle and vehicle cycle emissions for various
vehicle and fuel categories. While both models are capable of estimating emissions for alternative fuels,
this analysis focused on the upstream and direct emissions from gas and diesel vehicles, since they
comprise the majority of transportation emissions. For each lifecycle component, ranges of emissions for
99 Although the LEM model does not specifically account for downstream emissions, the GREET model does include emissions
resulting from the fuel cycle portion of the transportation lifecycle.
100 Delucchi, M. 2003a. Lifecycle Emissions Model (LEM), Mark A. Delucchi, Institute of Transportation Studies, University of
California, December.
101 U.S. Department of Energy, Center for Transportation Research, 2001. Greenhouse Gases, Regulated Emissions, and Energy
Use in Transportation (GREET) 1.6Model. Argonne National Laboratory, June 2001.
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each vehicle/fuel category were developed based on minimum and maximum values from both models.
Ranges were used to account for some of the assumptions and uncertainties behind these models.
Upstream Fuel Lifecycle Emissions
Elements of the upstream fuel cycle that were examined include extraction, shipment, refining, and
distribution of raw materials and finished products. Although the GREET model uses these categories, the
categories in LEM had to be mapped to this configuration. This mapping was performed using
information provided by the developer of LEM.102
From each model, CO2 emissions per million Btu (MMBtu) of fuel for each of these components were
obtained by mode, vehicle, and fuel type. Ratios of upstream emissions to direct emissions then were
determined for each component, as shown in Table 14-1.
Upstream Vehicle Lifecycle Emissions
Emissions from the upstream vehicle lifecycle were estimated only for highway vehicles. (Neither model
has been used to evaluate non-highway vehicle cycle emissions.). Ratios of vehicle lifecycle emissions to
direct fuel cycle emissions were obtained from LEM as inputs into this analysis.
The LEM model was used to specify the ratio of vehicle cycle emissions to direct fuel cycle emissions for
gasoline light-duty vehicles and for diesel heavy-duty. It was assumed that the vehicle cycle for gasoline
light-duty vehicles could be used as a proxy for diesel light-duty vehicles and that the vehicle cycle for
diesel heavy-duty vehicles could be used as a proxy for gasoline heavy-duty vehicles. This proportion of
total upstream vehicle cycle emissions then was disaggregated to transport- and non-transport-related
emissions. Ratios of upstream emissions to direct emissions also are shown in Table 14-1.
Total Lifecycle Emissions
To estimate total lifecycle emissions, emissions from the upstream fuel and vehicle cycles were summed
for each mode, vehicle type, and fuel type. These total estimates in Table 14-1 represent the ratio of
lifecycle emissions to direct emissions for each vehicle/fuel category.
These total estimates are valid when assessing the CO2 impact over the lifecycle of each vehicle and fuel
combination individually. However, it is important to recognize that some of the upstream emissions are
currently represented in the transportation totals of the U.S. GHG Inventory. In the upstream fuel cycle,
shipment and distribution of fuel fits in this category; in the upstream vehicle cycle, "transport" of
vehicles fits in this category. These components were subtracted out of the proportion of total emissions
to arrive at "total less transport." For example, GHG emissions associated with the lifecycle of passenger
cars running on conventional gasoline were estimated to be 1.35 to 1.43 times that of direct emissions,
when taking out transportation-related emissions that are counted elsewhere in the U.S. GHG Inventory.
The ratios of upstream fuel and vehicle cycle emissions shown in Table 14-1 then were applied to total
U.S. CO2 emissions from direct fuel combustion for each vehicle/fuel type to estimate total lifecycle
GHG emissions. These emissions are shown in Table 14-2.
Based on these results, total lifecycle emissions for the nation's transportation sector are estimated to be
27 to 37 percent higher than direct fuel combustion emissions. These estimates do not include some
important components of the transportation lifecycle, such as upstream vehicle cycle emissions for non-
highway vehicles, and emissions from the construction and maintenance of infrastructure. However, it
should also be noted that these estimates do include upstream emissions that take place outside the United
States, such as fuel produced and vehicles manufactured abroad that are used in the nation's
transportation system. As a result, the total GHG emissions presented in Table 14-2 reflect some
102 Delucchi, M. 2003b. Personal Communication between Mark Delucchi of the University of California and Bill Cowart of
ICF Consulting. June 14, 2003.
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emissions that are not included in the official U.S. GHG Inventory estimates, but rather are accounted for
in the estimates of other nations. While these emissions are not directly attributed to the United States,
they are nevertheless sizeable and important on a global scale.
10.2 Other Issues/Next Steps
The results of this analysis are presented to illustrate the potential impact of lifecycle emissions from the
transportation sector. A number of impacts still need to be addressed to present a more comprehensive
assessment of the transportation lifecycle. Some of these issues include:
• Impacts Not Quantified—While this analysis assesses many of the GHG impacts of the
transportation lifecycle, a significant number of impacts were not quantified. These include
fuel cycle emissions associated with alternative fuel vehicles (AFVs), and vehicle cycle
emissions associated with non-road transport. The analysis also did not assess infrastructure
lifecycle emissions or the land use impacts of transportation, such the removal of trees for
highway construction, parking lots, airports, and many other types of infrastructure.
Measuring the latter impacts is extremely challenging.
• Alternative Fuels and Vehicle Technologies—Resource limitations for this report prevented
analysis of these fuels and technologies. There is great variance in the lifecycle emissions
from alternative fuels, and substantial work has been done by others to quantify these
emissions. Although some of those fuels and vehicle technologies will likely be extremely
important in the future, their collective use is presently small enough for their contributions to
have a negligible effect on current lifecycle estimates. Future work should incorporate these
fuels and technologies because of the critical role they play in forward-looking policy
analyses.
• International Boundaries—Accounting for international boundaries could significantly
increase total transportations sector estimates. In 2001, approximately 55 percent of the
petroleum products consumed in the United States were derived from crude oil produced
abroad.103 Supplemental tables may be developed in the future to represent upstream
emissions occurring outside of the United States.
103 Energy Information Administration, 2004. International Energy Annual, 2002. Washington, DC.
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11 GHG Emissions Projections and Emerging Issues
11.1 Projected CCb Emissions from Transportation
CO2 from transportation is expected to remain a major source of total U.S. greenhouse gas emissions.
Estimates of transportation energy use are sensitive to factors such as fuel prices, economic growth, and
technology adoption. In its Annual Energy Outlook (AEO), EIA has developed various scenarios to
forecast the potential impact of these variables on future fuel consumption.104 In the reference (base) case,
transportation-related energy demand is projected to increase by 18 percent between 2003 and 2010, and
by 48 percent by 2025 (Figure 11-1).105 EIA's high- and low-economic cases show a similar trend (also
in Figure 11-1). Since CO2 emissions are very highly correlated with fuel consumption, transportation-
related emissions of GHSs are expected to increase at a similar rate.
Figure 11-1. EIA Projections of Transportation Energy Demand, High, Base, and Low Economic
Cases, 2003-2025
00
10
2003
2005
2010
2015
2020
2025
Source: Energy Information Administration, U.S. Department of Energy. Annual Energy Outlook 2005 with Projections to 2025. Washington, DC, Table
B2.
In the AEO reference case, motor gasoline use is projected to increase by 2.0 percent per year between
2003 to 2025, from 16.6 to 24.0 quadrillion Btu. Alternative fuels are projected to displace 2.2 percent of
light-duty vehicle fuel consumption by 2025. Gasoline's share of demand is nevertheless expected to be
sustained by low prices relative to the rate of inflation, and a slow increase in the fuel efficiency of
conventional cars, vans, pickup trucks, and SUVs. Industrial output is assumed to grow 2.3 percent per
year from 2003 to 2025, leading to continued growth in freight truck use and an annual increase of diesel
fuel consumption of 2.3 percent. Jet fuel consumption is expected to grow at 1.9 percent annually,
reflecting growth in passenger travel of 2.2 percent from 2003 to 2025.
An important assumption underlying the AEO and other forecasts is the continued growth in light-duty
travel, albeit at a decreasing rate. From 1980 to 2000, light-duty VMT increased at an average annual rate
of 2.99 percent. EIA forecasts that light-duty VMT will increase by 56 percent between 2003 and 2025, or
104 See Energy Information Administration, 2005, Annual Energy Outlook (AEO) 2005 with Projections to 2025.
105 Energy Information Administration, 2005, Annual Energy Outlook (AEO) 2005 with Projections to 2025. Washington, DC,
Table A2.
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2.0 percent annually. Meanwhile, EIA projects that the fuel economy of light-duty vehicles will improve
by about 10 percent over the same period,106 reflecting the planned increases in fuel economy standards
for light-duty trucks.107 CAFE currently requires that light trucks achieve a manufacturer average of 21.0
mpg in model year 2005, increasing to 22.2 mpg in 2007. The AEO reference forecast also assumes that
vehicle technology improvements will marginally improve light-duty fuel economy.
11.2 Emerging Issues Affecting Passenger Transportation
Increasing Vehicle Travel
A number of national-level travel forecasts suggest that the growth in passenger travel will decelerate.108
There are several reasons to believe that VMT growth could be lower in the future, including an increase
in the share of elderly drivers and the impact of highway congestion. These forecasts are nonetheless
speculative, and small variations from the projected annual growth rate of 2.0 percent could be significant
over time. Annual light-duty VMT growth of 2.5 percent would translate into a 72 percent increase in
light-duty VMT between 2003 and 2025, or over 1 trillion vehicle miles more in 2025 than the mileage
implied by a 2.0 percent annual growth rate.
Consumers' Vehicle Choice and the Impacts of Light-Duty Trucks
The growing representation of SUVs and other light-duty trucks in the vehicle fleet is expected to have a
continuing impact on average in-use vehicle fuel economy. Increased sales of light-duty trucks were
largely responsible for the decline in new vehicle fuel economy from its peak in the late 1980s. Although
long-term fuel price changes are uncertain, fuel prices historically have had an effect on vehicle purchase
decisions and on fuel consumption. Fuel prices have risen significantly since 2003, causing some
consumers to consider the purchase of vehicles with higher fuel efficiency. Continued price increases of
this magnitude would likely result in the purchase of more fuel-efficient vehicles. There is significant
evidence that people respond measurably to changes in fuel prices, with typical reported long-term motor
fuel price elasticities of-0.5 to -0.8. However, recent studies also suggest that consumers have become
less sensitive to fuel prices than they were in the past, due in large part to higher average incomes and
lower real fuel prices as a percentage of household expenses.109 As a result, the effects of fuel prices in
the near term are uncertain, but may likely be at the lower end of the above elasticity range. The long-
term sensitivity to fuel costs is even more uncertain, as are projections of consumers' future fuel costs.
Advanced Technology Vehicles
EIA projections show advanced technology vehicles accounting for 19 percent of light-duty sales in 2025.
Alcohol flexible-fuel vehicles are expected to comprise about 8 percent of new sales, hybrids about 6
percent, and turbo direct diesel vehicles about 4 percent. Travel in hybrids also is expected to grow
significantly from 2003 to 2025 (Figure 11-2), but would still represent less than 5 percent of total light-
duty miles in 2025.
1 °6 Energy Information Administration, 2005. Annual Energy Outlook 2005 with Projections to 2025. Washington, DC, Table
A7.
107 Includes all pickup trucks, vans, and SUVs with gross vehicle weight rating less than 8,500 pounds.
108 As noted above, EIA forecasts light-duty VMT to increase by 2.0 percent from 2003 to 2025; Highway Performance
Monitoring System (HPMS) forecasts are provided by state departments of transportation and forecast an average increase of
2.08 percent per year. See U.S. Department of Transportation, Status of the Nation's Highways, Bridges, and Transit: 2002
Conditions and Performance Report, Chapter 9. Available online at http://www.fhwa.dot.gov/policy/2002cpr/ch9.htm.
109 Several studies report on the elasticity of fuel consumption with respect to fuel prices. See, for example: Goodwin, Phil.
"Review of New Demand Elasticities," Journal of Transport Economics, May 1992. Hagler Bailly, "Potential for Fuel Taxes to
Reduce Greenhouse Gas Emissions from Transport," Transportation Table of the Canadian National Climate Change Process,
1999, and DOE's Policies and Measures for Reducing Energy-Related Greenhouse Gas Emissions.
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Increased sales of hybrid vehicles and other advanced technology vehicles offer the potential to reduce
fuel consumption. Nevertheless, manufacturers could also produce a greater number of less fuel-efficient
vehicles and remain compliant with current CAFE standards. Another possibility is that hybrid and other
advanced technology vehicles will be larger and equipped with more powerful engines, causing their fuel
economy to remain largely unchanged. In either case, the net impact on overall fleet fuel economy could
be negligible. Hydrogen is a potentially viable alternative to petroleum fuels in the long term. At present,
the production and storage costs of hydrogen are the major barriers to increased use of hydrogen in
vehicles. The Department of Energy is involved in several initiatives to increase the use of hydrogen in
automobiles.110
Figure 11-2. Historical and Projected VMT from Gasoline- and Diesel-Electric Hybrid Vehicles,
2000-2025
160,000
1 140,000
2
2 120,000
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Transportation GHG Emissions Report
In May 2005, New York Governor George Pataki proposed regulations similar to the California light-duty
standard. Beginning with model year 2009, all passenger vehicles registered in New York will be required
to meet fleet average standards for GHG emissions. This standard would become increasingly stringent
through 2016. Other states, including Connecticut, Massachusetts, Oregon, and Washington, are
considering similar regulations. These measures and other state-level actions have the potential to
influence the energy efficiency of vehicles sold throughout the United States.
Increasing Demand for Air Travel
Future GHG emissions from aviation will largely depend on the degree to which improvements in aircraft
energy efficiency keep pace with growing passenger travel demand. Passenger seat-miles available on
aircraft are expected to grow 46 percent between 2002 and 2015, and by 67 percent between 2002 and
2025. Fuel economy of commercial aircraft is expected to increase at a more modest rate, from 54.8 seat-
miles per gallon in 2002 to 63.3 in 2015 (an increase of 15.5 percent) and to 67.0 in 2025 (an increase of
22.3 percent). The result will likely be faster growth in fuel consumption and GHG emissions than
observed from 1990 to 2003.m
11.3 Emerging Issues Affecting Freight Transportation
Freight Trucks—Future Growth in Activity and Changes in Fuel Economy
GHGs from heavy-duty trucks increased faster than any other major source from 1990 to 2003. Much of
the growth resulted from rapid increases in freight haulage and vehicle travel, which overwhelmed a
nominal improvement in vehicle fuel efficiency. A number of trends suggest that similar growth in
activity is possible in the future. Average shipment sizes have been affected by developments such as the
growth in e-commerce and direct delivery to end users, which have tended to decrease vehicle loads and
increase VMT. Meanwhile, it is expected that truck fuel economy will improve marginally. According to
AEO estimates, the overall fuel efficiency of the freight truck fleet is expected to rise from 6 mpg in 2003
to 6.6 mpg in 2025.112 The AEO notes that freight companies are sensitive to the marginal costs of
implementing fuel-efficient strategies and technologies, but anticipates that numerous strategies should
still penetrate the industry. EIA forecasts that the penetration of these technologies in the freight industry
will increase new freight truck fuel efficiency from 6.1 mpg in 2003 to 6.8 mpg in 2025.113
Advanced Technology and Hybrid Vehicles
In coming years, gasoline- and diesel-electric hybrids will comprise a greater share of urban delivery
vehicles, although they will likely remain a small proportion in the near term. Hydraulic hybrids represent
a new technology that may significantly penetrate the heavy-duty vehicle market. Hydraulic hybrid
vehicles are similar to gasoline-electric hybrids, except that a hydraulic system replaces the battery and
electric motor. In a hydraulic hybrid, energy from regenerative braking is stored by compressing
hydraulic fluid in a reservoir. It is used later in a hydraulic pump to provide power to the wheels. Some
experts believe that larger vehicles, such as pickup trucks and delivery vans, may be able to incorporate
hydraulic hybrid technology at about the same cost as gasoline-electric systems.
111 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Table
7.
112 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Figure
57.
113 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Figure
58.
44
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Transportation GHG Emissions Report
Programs to Reduce Emissions from Heavy-Duty Vehicles
Many long-haul trucks idle for extended periods of time, using the engine to power cab amenities. This
idling is grossly inefficient, and a variety of technologies are available to provide cab heating, cooling,
and/or electrical supply while consuming less energy. These include direct-fire heaters, auxiliary power
units, and automatic engine idle systems. Truck stop electrification is another option for reducing truck
idling that many metropolitan areas are considering as a means to reduce air pollution. These strategies
would concurrently reduce vehicle fuel consumption and GHG emissions.
11.4 Implications for the Future
Forecasts indicate that transportation is likely to remain a major source of total U.S. GHGs, and may be a
primary contributor to the growth of national greenhouse gas emissions. The AEO 2005 reference case
scenario shows transportation accounting for the largest absolute increase in energy consumption of any
U.S. economic sector from 2003 to 2025. Transportation energy consumption is expected to be
responsible for more than 37 percent of the total increase in U.S. fuel consumption over this period,
representing an increase of 13.0 quadrillion Btu. While transportation GHGs will be influenced by factors
such as economic expansion and the cost of energy, a variety of measures may reduce the growth and
impact of these emissions. Broadly categorized, these measures could include efforts to encourage
energy-efficient vehicle technologies, promote efficient patterns of travel and land use, and develop
alternatives to petroleum-based fuels. The timing and implementation of such approaches will
significantly affect the future volume of greenhouse gases from U.S. transportation sources.
45
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Transportation GHG Emissions Report
12 References
American Association of State Highway and Transportation Officials. Freight-Rail Bottom Line Report,
Washington, DC.
American Trucking Associations (2004) U.S. Freight Transportation Forecast to 2014, Alexandria, VA.
Association of American Railroads (2004) Railroad Facts. Washington, DC.
Browning, L. (2003). "VMT Projections for Alternative Fueled and Advanced Technology Vehicles
through 2025," 13th CRC On-Road Vehicle Emissions Workshop, April 2003.
Delucchi, M (2003). Lifecycle Emissions Model (LEM), Mark A. Delucchi, Institute of Transportation
Studies, University of California, December 2003.
Delucchi, M (2003) Personal Communication between Mark Delucchi of the University of California and
Bill Cowart of ICF Consulting. June 14, 2003.
Center for Transportation Research (2001) Greenhouse Gases, Regulated Emissions, and Energy Use in
Transportation (GREET) 1.6Model. Argonne National Laboratory, U.S. Department of Energy,
Argonne, IL, June 2001.
DOE (2004) Transportation Energy Data Book, Edition 24. Center for Transportation Analysis,
Engineering Science and Technology Division, Oak Ridge National Laboratory, ORNL-6973.
Available online at http://www-cta.ornl.gov/data/Index.html.
DOE (2005) The Hydrogen Future. Energy Efficiency and Renewable Energy. Available online at
http://www.eere.energy.gov/hydrogenandfuelcells/future/barriers.html.
DOT (2005) Transportation Statistics Annual Report 2005. Bureau of Transportation Statistics, U.S.
Department of Transportation, Washington, DC. Available online
athttp://www.bts.gov/publications/transportation_statistics_annual_report/.
Transportation Statistics, U.S. Department of Transportation, Washington, DC. Available online at
http://www.bts.gov/publications/national_transportation_statistics/2004/index.html.
DOT (2004) National Transportation Statistics 2003. Bureau of Transportation Statistics, U.S.
Department of Transportation, Washington, DC. Available online at
http://www.bts.gov/publications/national_transportation_statistics/2003/index.html.
EIA (2005) Annual Energy Outlook 2005 with Projections to 2025. Energy Information Administration,
U.S. Department of Energy, Washington, DC. Available online at
http://www.eia.doe.gov/oiaf/archive/aeo05/index.html.
EIA (2004) Annual Energy Outlook 2004 with Projections to 2025. Energy Information Administration,
U.S. Department of Energy, Washington, DC. Available online at
http://www.eia.doe.gov/oiaf/archive/aeo04/index.html.
46
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Transportation GHG Emissions Report
EIA (2005) Household Vehicles Energy Use: Latest Data & Trends, Energy Information Administration,
U.S. Department of Energy, Washington, DC. Available online at
http://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/index.html.
EIA (2004) International Energy Annual, 2002 Edition. Energy Information Administration, U.S.
Department of Energy, Washington, DC. Available online at
http://www.eia.doe.gov/emeu/iea/contents.html.
EPA (2005) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. U.S. Environmental
Protection Agency, Washington, DC. Available online at
http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsGHGEmissio
nsUSEmissionsInventory2005 .html.
EPA (2005) Light-Duty Automotive Technology and Fuel Economy Trends, 1975 through 2005. Office of
Transportation and Air Quality, U.S. Environmental Protection Agency, Washington, DC.
Available online at http://www.epa.gov/otaq/fetrends.htm.
EPA (2004) U.S. EPA VintagingModel. Version VM IO file_l 1-08-04.
FHWA (2002 through 2004) Highway Statistics. Federal Highway Administration, U.S. Department of
Transportation, Washington, DC, report FHWA-PL-96-023-annual. Available online at
http://www.fhwa.dot.gov/policy/ohim/hs03/index.htm and
http://www.fhwa.dot.gov/ohim/hs01/index.htm.
FHWA (1997) Highway Statistics: Summary to 1995. Federal Highway Administration, U.S. Department
of Transportation, Washington, DC. Available online at
http://www.fhwa.dot.gov/ohim/summary95/index.html.
FHWA (2004) Summary of Travel Trends: 2001 Nationwide Household Transportation Survey. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC. Available online
at http://nhts.ornl.gov/2001/pub/STT.pdf
FHWA (1999) Summary of Travel Trends: 1995 Nationwide Personal Transportation Survey. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, report FHWA-
PLOO-006. Available online athttp://npts.ornl.gov/npts/1995/Doc/trends_report.pdf
First Fleet Corporation (2004) Fleet Managers' Survey, Ft. Lauderdale, FL. Spring. Available online at
http://ffcsurvey.starmark.com/FFC_Executive_Report.pdf?-
session=thisSession:96EC3B6CBA05324EEB617B369C2AF347.
Goodwin, Phil. Review of New Demand Elasticities, Journal of Transport Economics and Policy,
Volume 26, May 1992.
Hagler Bailly, Potential for Fuel Taxes to Reduce Greenhouse Gas Emissions from Transport,
Transportation Table of the Canadian National Climate Change Process, 1999.
IPCC (2001) Climate Change 2001: The Scientific Basis, Intergovernmental Panel on Climate Change;
J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg, and K. Maskell, eds.;
Cambridge University Press. Cambridge, U.K.
47
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Transportation GHG Emissions Report
U.S. Census Bureau (2003) Decennial Census, Supplemental Survey: Journey to Work. Department of
Commerce, Washington, DC. Reported in A. Pisarski, Commuting in America III. Eno River
Foundation.
48
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Transportation GHG Emissions Report
13 Appendix A: Summary of GHG Emissions for Transportation and Mobile Sources
This appendix contains summary tables with estimates of CO2, CH^, N2O, and HFCs from transportation and non-transportation sources.
Table 13-1. Total GHG Emissions from Transportation Sources (All Gases), 1990-2003 (Tg CO2 Eq.)
Fuel/Vehicle
Type
On-Road
Passenger Cars
Light-Duty
Trucks
Medium/Heavy-
Duty Trucks
Buses
Motorcycles
Aircraft
General
Aviation Aircraft
Commercial
Aircraft
Military Aircraft
Other Aircraft
Boats and
Ships
Locomotives
Pipelines
Lubricants
Mobile AC
Refrigerated
Transport
Total
1990
1,196.1
640.6
327.7
217.9
8.2
1.7
179.1
9.5
118.4
35.1
16.1
49.6
36.6
35.9
11.9
+
+
1,509.3
1991
1,190.8
605.1
359.5
216.8
7.7
1.7
171.2
8.5
110.5
34.9
17.3
44.2
35.0
32.7
10.6
+
+
1,484.5
1992
1,214.0
604.0
379.8
220.8
7.8
1.7
168.9
7.6
112.9
28.5
19.8
57.7
35.1
32.1
10.8
0.6
+
1,519.3
1993
1,246.2
613.2
395.0
227.9
8.3
1.8
169.9
6.8
114.7
27.9
20.5
54.7
35.2
33.9
11.0
2.0
0.3
1,553.2
1994
1,276.5
618.7
404.4
242.9
8.7
1.8
178.0
7.1
118.6
25.3
26.9
53.9
38.0
37.3
11.5
4.3
0.9
1,600.4
1995
1,304.7
621.6
419.9
252.9
8.6
1.8
173.6
8.1
121.3
24.4
19.8
55.7
39.4
38.0
11.3
6.6
2.3
1,631.8
1996
1 ,338.9
627.3
433.6
267.1
9.1
1.7
182.0
8.4
126.5
23.3
23.8
53.9
41.1
38.7
11.0
10.1
3.8
1,679.5
1997
1,363.2
624.1
446.6
281.4
9.4
1.8
180.9
8.9
129.8
21.2
20.9
39.1
40.4
40.9
11.6
13.8
5.5
1,695.4
1998
1,402.9
642.3
457.1
292.1
9.6
1.8
183.2
10.3
127.6
21.7
23.6
32.7
40.9
34.9
12.1
17.4
7.0
1,731.1
1999
1 ,442.3
650.0
473.9
305.9
10.6
1.8
188.7
12.0
137.9
20.8
18.0
42.7
42.1
35.3
12.3
20.8
8.5
1,792.5
2000
1,460.3
649.7
476.2
322.1
10.5
1.8
195.2
11.8
142.1
21.2
20.1
63.7
42.2
35.0
12.1
24.0
9.8
1,842.3
2001
1,468.7
650.2
477.7
329.2
9.9
1.7
185.3
11.6
134.2
23.1
16.6
43.2
43.3
33.4
11.1
26.7
10.8
1,822.4
2002
1 ,490.2
662.3
487.6
329.3
9.4
1.7
176.8
11.8
123.0
20.6
21.4
57.8
41.5
36.4
10.9
28.8
11.5
1,853.9
2003
1 ,504.8
654.6
496.3
343.0
9.1
1.7
173.1
11.8
124.0
20.8
16.5
58.0
43.2
34.8
10.2
30.3
12.3
1,866.7
Change
1990-
2003
+26%
+2%
+51%
+57%
+ 12%
-4%
-3%
+24%
+5%
-41%
+3%
+17%
+18%
-3%
-14%
NA
NA
+24%
- Less than 0.05 Tg CO2 Eq.
49
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Transportation GHG Emissions Report
Table 13-2. CO2 Emissions from Transportation Sources, 1990-2003 (Tg)
Mode /Vehicle
Type / Fuel
Type
Highway
Vehicles
Passenger
Cars
Light-Duty
Trucks
Medium/Heavy-
Duty Trucks
Buses
Motorcycles
Aircraft
General
Aviation Aircraft
Commercial
Aircraft
Military Aircraft
Other Aircraft
Boats and
Ships
Locomotives
Pipelines
Lubricants
TOTAL
1990
1,151.3
612.5
312.2
217.0
7.8
1.7
177.2
9.4
117.2
34.8
15.9
49.2
36.3
35.9
11.9
1,461.7
1991
1,143.5
577.6
341.3
215.6
7.3
1.6
169.4
8.3
109.4
34.5
17.1
43.7
34.7
32.7
10.6
1,434.7
1992
1,164.0
575.8
359.4
219.6
7.5
1.7
167.1
7.5
111.8
28.2
19.6
57.2
34.8
32.1
10.8
1,466.0
1993
1,194.3
584.8
373.0
226.7
8.0
1.7
168.1
6.7
113.5
27.6
20.3
54.2
34.8
33.9
11.0
1,496.4
1994
1,223.1
589.8
381.5
241.7
8.4
1.8
176.1
7.0
117.4
25.0
26.7
53.4
37.6
37.3
11.5
1,539.1
1995
1,250.4
592.5
396.2
251.6
8.3
1.7
171.8
8.0
120.1
24.1
19.6
55.2
39.1
38.0
11.3
1,565.8
1996
1,284.0
598.4
409.3
265.7
8.8
1.7
180.1
8.3
125.2
23.1
23.5
53.4
40.7
38.7
11.0
1,607.9
1997
1 ,307.8
595.5
421.6
279.9
9.1
1.7
179.0
8.8
128.5
21.0
20.6
38.7
40.0
40.9
11.6
1,618.0
1998
1,347.4
613.8
432.1
290.4
9.3
1.8
181.3
10.1
126.3
21.5
23.4
32.4
40.5
34.9
12.1
1,648.7
1999
1,388.0
622.4
449.2
304.3
10.4
1.8
186.7
11.8
136.4
20.6
17.8
42.3
41.7
35.3
12.3
1,706.2
2000
1,408.0
623.4
452.1
320.4
10.2
1.8
193.2
11.7
140.6
21.0
19.9
63.1
41.8
35.0
12.1
1,753.1
2001
1 ,420.7
625.7
456.2
327.5
9.6
1.6
183.4
11.4
132.8
22.8
16.4
42.7
42.8
33.4
11.1
1,734.2
2002
1 ,445.8
639.5
468.1
327.5
9.1
1.6
174.9
11.6
121.7
20.4
21.2
57.2
41.0
36.4
10.9
1,766.4
2003
1 ,464.2
633.7
478.8
341.2
8.9
1.6
171.3
11.6
122.8
20.5
16.3
57.5
42.8
34.8
10.2
1,780.7
Change
1990-
2003
+27%
+3%
+53%
+57%
+14%
-4%
-3%
+24%
+5%
-41%
+3%
+17%
+18%
-3%
-14%
+22%
50
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Transportation GHG Emissions Report
Table 13-3. Methane Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.)
Mode /Vehicle
Type / Fuel
Type
Highway
Vehicles
Passenger
Cars
Light-Duty
Trucks
Medium/Heavy-
Duty Trucks
Buses
Motorcycles
Aircraft
General
Aviation Aircraft
Commercial
Aircraft
Military Aircraft
Other Aircraft
Boats and
Ships
Locomotives
Total
1990
4.3
2.6
1.4
+
0.3
+
0.2
0.1
0.1
+
+
0.1
0.1
4.6
1991
4.3
2.4
1.5
+
0.3
+
0.1
0.1
0.1
+
+
0.1
0.1
4.5
1992
4.2
2.3
1.5
+
0.3
+
0.1
0.1
0.1
+
+
0.1
0.1
4.5
1993
4.1
2.3
1.5
+
0.3
+
0.1
+
0.1
+
+
0.1
0.1
4.4
1994
4.0
2.2
1.5
+
0.3
+
0.1
+
0.1
+
+
0.1
0.1
4.3
1995
3.9
2.1
1.4
+
0.3
+
0.1
0.1
0.1
+
+
0.1
0.1
4.2
1996
3.7
2.0
1.4
+
0.3
+
0.1
+
0.1
+
+
0.1
0.1
4.0
1997
3.6
1.9
1.3
+
0.3
+
0.2
0.1
0.1
+
+
0.1
0.1
3.8
1998
3.4
1.8
1.3
+
0.2
+
0.1
+
0.1
+
+
0.1
0.1
3.7
1999
3.1
1.7
1.1
+
0.2
+
0.2
0.1
0.1
+
+
0.1
0.1
3.4
2000
2.8
1.5
1.0
+
0.2
+
0.2
0.1
0.1
+
+
0.1
0.1
3.2
2001
2.6
1.4
0.9
+
0.2
+
0.1
+
0.1
+
+
0.1
0.1
2.9
2002
2.4
1.2
0.9
+
0.2
+
0.1
+
0.1
+
+
0.1
0.1
2.7
2003
2.1
1.1
0.8
+
0.2
+
0.1
+
0.1
+
+
0.1
0.1
2.4
Change
1990-
2003
-50%
-58%
-42%
+497%
-42%
-21%
-11%
-23%
+5%
-41%
+3%
+ 18%
+23%
-47%
+ Less than 0.05 Tg CO2 Eq.
51
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Transportation GHG Emissions Report
Table 13-4. Nitrous Oxide Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.)
Mode /Vehicle
Type / Fuel Type
Highway Vehicles
Passenger Cars
Light-Duty Trucks
Medium/Heavy-
Duty Trucks
Buses
Motorcycles
Aircraft
General Aviation
Aircraft
Commercial
Aircraft
Military Aircraft
Other Aircraft
Boats and Ships
Locomotives
Total
1990
40.6
25.5
14.1
0.9
+
+
1.7
0.1
1.1
0.3
0.2
0.4
0.3
43.0
1991
43.0
25.1
16.7
1.2
+
+
1.6
0.1
1.1
0.3
0.2
0.3
0.3
45.3
1992
45.8
25.8
18.8
1.1
+
+
1.6
0.1
1.1
0.3
0.2
0.5
0.3
48.1
1993
47.8
26.1
20.4
1.1
+
+
1.6
0.1
1.1
0.3
0.2
0.4
0.3
50.1
1994
49.3
26.7
21.4
1.2
+
+
1.7
0.1
1.2
0.2
0.3
0.4
0.3
51.8
1995
50.4
26.9
22.2
1.3
+
+
1.7
0.1
1.2
0.2
0.2
0.4
0.3
52.9
1996
51.2
26.9
22.9
1.4
+
+
1.8
0.1
1.2
0.2
0.2
0.4
0.3
53.7
1997
51.9
26.7
23.7
1.4
+
+
1.7
0.1
1.3
0.2
0.2
0.3
0.3
54.3
1998
52.1
26.7
23.7
1.6
+
+
1.8
0.1
1.2
0.2
0.2
0.3
0.3
54.4
1999
51.2
25.9
23.6
1.6
+
+
1.8
0.1
1.3
0.2
0.2
0.3
0.3
53.7
2000
49.5
24.7
23.0
1.7
+
+
1.9
0.1
1.4
0.2
0.2
0.5
0.3
52.2
2001
45.4
23.1
20.6
1.7
+
+
1.8
0.1
1.3
0.2
0.2
0.3
0.3
47.9
2002
42.0
21.6
18.6
1.8
+
+
1.7
0.1
1.2
0.2
0.2
0.5
0.3
44.5
2003
38.5
19.9
16.8
1.8
+
+
1.7
0.1
1.2
0.2
0.2
0.5
0.4
40.9
Change
1990-2003
-5%
-22%
+ 19%
+92%
+113%
-18%
-3%
+36%
+5%
-41%
+3%
+18%
+22%
-5%
- Less than 0.05 Tg CO2 Eq.
Table 13-5. HFC Emissions from Transportation Sources, 1990-2002 (Tg CO2 Eq.)
Tg CO2 Eq.
Mobile AC
Refrigerated
Transport
Total
1990
+
+
+
1991
+
+
+
1992
0.6
.
0.6
1993
2.0
0.3
2.3
1994
4.3
0.9
5.2
1995
6.6
2.3
8.9
1996
10.1
3.8
13.9
1997
13.8
5.5
19.4
1998
17.4
7.0
24.4
1999
20.8
8.5
29.3
2000
24.0
9.8
33.8
2001
26.7
10.8
37.4
2002
28.8
11.5
40.4
2003
30.3
12.3
42.7
Change
1990-2003
NA
NA
NA
+ Less than 0.05 Tg CO2 Eq.
52
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Transportation GHG Emissions Report
Table 13-6. GHG Emissions from Non-Transportation Mobile Sources (All Gases), 1990-2003 (Tg CO2 Eq.)
Fuel Type/Vehicle
Type
Farm Eq.
Construction Eq.
Industrial and
Commercial Eq.
Lawn and Garden Eq.
Recreational Eq.
Other*
Total
1990
30.5
39.0
9.8
12.1
6.6
2.6
100.7
1991
31.1
40.2
9.9
12.5
6.6
2.6
102.9
1992
32.3
41.4
10.0
12.9
6.6
2.6
105.8
1993
42.3
56.3
9.8
14.5
9.0
2.8
134.8
1994
35.0
44.2
9.0
13.6
6.6
2.7
111.0
1995
36.0
45.6
9.4
13.9
6.6
2.7
114.0
1996
36.8
47.0
9.5
14.6
6.5
2.7
117.1
1997
38.3
48.5
9.8
13.7
6.7
2.7
119.6
1998
38.5
49.3
10.3
13.7
6.9
2.7
121.3
1999
37.6
50.1
9.3
13.9
7.2
2.6
120.7
2000
38.0
51.6
9.6
14.1
7.5
2.7
123.6
2001
40.2
55.8
16.0
14.2
7.9
2.7
136.8
2002
41.3
57.4
16.6
14.2
8.4
2.8
140.6
2003
42.3
59.1
17.4
14.3
8.9
2.8
144.8
Change
1990-
2003
+39%
+51%
+77%
+18%
+34%
+7%
+44%
* "Other" includes logging equipment, railroad equipment, and airport equipment.
Table 13-7. CO2 Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg)
Fuel
Type/Vehicle
Type
Farm
Equipment
Construction
Equipment
Industrial and
Commercial
Eq.
Lawn and
Garden Eq.
Recreational
Eq.
Other*
Total
1990
30.18
38.69
9.71
12.03
6.56
2.59
99.76
1991
30.75
39.79
9.84
12.40
6.56
2.60
101.94
1992
31.91
41.03
9.93
12.77
6.57
2.61
104.82
1993
41.88
55.80
9.76
14.39
8.94
2.77
133.54
1994
34.61
43.76
8.93
13.47
6.56
2.63
109.96
1995
35.58
45.17
9.28
13.75
6.52
2.64
112.94
1996
36.39
46.55
9.46
14.48
6.48
2.65
116.01
1997
37.86
48.04
9.71
13.62
6.61
2.66
118.49
1998
38.06
48.82
10.21
13.56
6.81
2.67
120.15
1999
37.18
49.67
9.22
13.76
7.10
2.61
119.54
2000
37.64
51.14
9.49
14.02
7.47
2.69
122.43
2001
39.76
55.27
15.83
14.07
7.84
2.72
135.48
2002
40.84
56.88
16.42
14.11
8.31
2.74
139.31
2003
41.85
58.60
17.20
14.21
8.82
2.77
143.44
Change
1990-
2003
+39%
+51%
+77%
+18%
+34%
+7%
+44%
* "Other" includes logging equipment, railroad equipment, and airport equipment.
53
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Transportation GHG Emissions Report
Table 13-8. Methane Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg CO2 Eq.)
Fuel
Type/Vehicle
Type
Farm Equipment
Construction
Equipment
Industrial and
Commercial Eq.
Lawn and Garden
Eq.
Recreational Eq.
Other*
Total
1990
0.091
0.046
0.012
0.014
0.008
0.003
0.174
1991
0.092
0.048
0.012
0.015
0.008
0.003
0.178
1992
0.096
0.049
0.012
0.015
0.008
0.003
0.183
1993
0.126
0.067
0.012
0.017
0.011
0.003
0.236
1994
0.104
0.053
0.011
0.016
0.008
0.003
0.194
1995
0.107
0.054
0.011
0.017
0.008
0.003
0.200
1996
0.110
0.056
0.011
0.017
0.008
0.003
0.205
1997
0.114
0.058
0.012
0.016
0.008
0.003
0.211
1998
0.115
0.059
0.012
0.016
0.008
0.003
0.213
1999
0.112
0.060
0.011
0.017
0.009
0.003
0.211
2000
0.113
0.061
0.011
0.017
0.009
0.003
0.215
2001
0.120
0.066
0.019
0.017
0.009
0.003
0.235
2002
0.123
0.068
0.020
0.017
0.010
0.003
0.242
2003
0.126
0.070
0.021
0.017
0.011
0.003
0.248
Change
1990-
2003
+39%
+52%
+79%
+19%
+37%
+7%
+43%
* "Other" includes logging equipment, railroad equipment, and airport equipment.
Table 13-9. Nitrous Oxide Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg CO2 Eq.)
Fuel
Type/Vehicle
Type
Farm Equipment
Construction
Equipment
Industrial and
Commercial Eq.
Lawn and Garden
Eq.
Recreational Eq.
Other*
Total
1990
0.238
0.305
0.077
0.094
0.051
0.021
0.785
1991
0.242
0.314
0.078
0.097
0.051
0.021
0.802
1992
0.251
0.323
0.078
0.100
0.051
0.021
0.825
1993
0.330
0.440
0.077
0.112
0.070
0.022
1.051
1994
0.273
0.345
0.070
0.105
0.051
0.021
0.866
1995
0.281
0.356
0.073
0.108
0.051
0.021
0.892
1996
0.287
0.367
0.075
0.114
0.051
0.021
0.916
1997
0.299
0.379
0.077
0.107
0.052
0.021
0.936
1998
0.301
0.385
0.081
0.107
0.054
0.021
0.949
1999
0.294
0.392
0.073
0.109
0.056
0.021
0.944
2000
0.297
0.403
0.075
0.111
0.059
0.021
0.967
2001
0.314
0.436
0.126
0.111
0.062
0.022
1.071
2002
0.323
0.449
0.131
0.111
0.066
0.022
1.101
2003
0.331
0.462
0.137
0.112
0.070
0.022
1.134
Change
1990-
2003
+39%
+52%
+79%
+ 19%
+37%
+7%
+44%
* "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment, commercial equipment, and industrial equipment.
54
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Transportation GHG Emissions Report
14 Appendix B: CO2 Emissions from Various Components of the Transportation Lifecycle
(Proportion Relative to Direct Emissions)
Table 14-1. CO2 Emissions from Various Components of the Transportation Lifecycle (Proportion Relative to Direct Emissions)
Highway
Vehicles
Passenger
Cars
Light-Duty
Trucks
Medium/
Heavy- Duty
Trucks
Buses
Motorcycles
Conv
Gasb
US RFC
Diesel
AFVs
Conv
Gas
US RFC
Diesel
AFVs
Conv
Gas
US RFC
Diesel
AFVs
Conv
Gas
US RFC
Diesel
AFVs
Conv
Gas
US RFC
Direct
Direct
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
Fuel Cycle
Extraction
0.05 - 0.09
0.05-0.08
0.04 - 0.08
0.05 - 0.09
0.05-0.08
0.04 - 0.08
0.05 - 0.09
0.05-0.08
0.04 - 0.08
0.05 - 0.09
0.05-0.08
0.04 - 0.08
0.05 - 0.09
0.05-0.08
Shipment
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
Refining
0.18-
0.19
0.19
0.09-
0.13
0.18-
0.19
0.19
0.09-
0.13
0.18-
0.19
0.19
0.09-
0.13
0.18-
0.19
0.19
0.09-
0.13
0.18-
0.19
0.19
Distri-
bution
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
Sub-Total
0.24-0.31
0.25 - 0.30
0.15-0.25
0.24-0.31
0.25 - 0.30
0.15-0.25
0.24-0.31
0.25-0.3
0.15-0.25
0.24-0.31
0.25-0.3
0.15-0.25
0.24-0.31
0.25 - 0.30
Vehicle Manufacture Cycle
Non-
transport
0.12-
0.15
0.12-
0.15
0.11 -
0.17
0.12-
0.15
0.12-
0.15
0.11 -
0.17
0.05-
0.15
0.05-
0.15
0.04-
0.17
0.12-
0.15
0.12-
0.15
0.04-
0.17
0.00-
0.15
0.00-
0.15
Transport
0.02 - 0.03
0.02 - 0.03
0.03 - 0.04
0.02 - 0.03
0.02 - 0.03
0.03 - 0.04
0.01 -0.02
0.01 -0.02
0.02 - 0.03
0.02 - 0.03
0.02 - 0.03
0.02 - 0.03
0.00 - 0.02
0.00 - 0.02
Sub-
Total
0.14-
0.19
0.14-
0.18
0.14-
0.21
0.14-
0.19
0.14-
0.18
0.14-
0.21
0.06-
0.17
0.06-
0.17
0.06-
0.20
0.14-
0.19
0.14-
0.18
0.06-
0.20
0.00-
0.17
0.00-
0.17
Total Lifecycle
Direct
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
Indirect
0.38-
0.50
0.39-
0.47
0.29-
0.46
0.38-
0.50
0.39-
0.47
0.29-
0.46
0.30-
0.48
0.31 -
0.46
0.21 -
0.45
0.38-
0.50
0.39-
0.47
0.21 -
0.45
0.24-
0.48
0.25-
0.46
Total
1.38-
1.50
1.39-
1.47
1.29-
1.46
1.38-
1.50
1.39-
1.47
1.29-
1.46
1.30-
1.48
1.31 -
1.46
1.21 -
1.45
1.38-
1.50
1.39-
1.47
1.21 -
1.45
1.24-
1.48
1.25-
1.46
Total, Less
Transport
1 .35 - 1 .43
1.36- 1.41
1 .25 - 1 .39
1 .35 - 1 .43
1.36-1.41
1 .25 - 1 .39
1 .27 - 1 .43
1.28-1.41
1.18- 1.39
1 .35 - 1 .43
1.36- 1.41
1.18-1.39
1 .22 - 1 .43
1.23-1.41
55
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Transportation GHG Emissions Report
Aircraft
Boats and
Ships
Rail
Pipelines
Total
General
Aviation
Aircraft
Commercial
Aircraft
Military
Aircraft
Other Aircraft
International
(Bunkers)
Domestic
International
(Bunkers)
Jet Fuel
Aviation
Gasoline
Jet Fuel
Jet Fuel
Jet Fuel
Jet Fuel
Conv
Gas
Distillate
Fuel
Residual
Fuel
Distillate
Fuel
Residual
Fuel
Distillate
Fuel
Electricity
Natural
Gas
Electricity
Direct
Direct
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
Fuel Cycle
Extraction
0.05-0.10
0.05 - 0.08
0.05-0.10
0.05-0.10
0.05-0.10
0.05-0.10
0.05 - 0.09
0.05 - 0.09
0.04 - 0.08
0.05 - 0.09
0.04-0.08
0.05 - 0.09
0.03-0.04
0.03 - 0.04
0.03-0.04
Shipment
0.01 - 0.02
0.01 -0.02
0.01 - 0.02
0.01 - 0.02
0.01 - 0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.01 -0.02
0.00
0.00
0.00
Refining
0.11
0.19
0.11
0.11
0.11
0.11
0.18-
0.19
0.10-
0.11
0.05
0.10-
0.11
0.05
0.10-
0.11
1.00-
1.09
0.05-
0.11
1.00-
1.09
Distri-
bution
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.00
0.01 -
0.05
0.00
Sub-Total
0.17-0.24
0.25-0.30
0.17-0.24
0.17-0.24
0.17-0.24
0.17-0.24
0.24-0.31
0.16-0.23
0.10-0.16
0.16-0.23
0.10-0.16
0.16-0.23
1.03-1.12
0.09 - 0.20
1.03-1.12
Vehicle Manufacture Cycle
Non-
transport
Transport
Sub-
Total
Total Lifecycle
Direct
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.00
1.00
0.0
1.00
Indirect
0.17-
0.24
0.25-
0.30
0.17-
0.24
0.17-
0.24
0.17-
0.24
0.17-
0.24
0.24-
0.31
0.16-
0.23
0.10-
0.16
0.16-
0.23
0.10-
0.16
0.16-
0.23
1.03-
1.12
0.09-
0.20
1.03-
1.12
0.30 -
0.42
Total
1.17-
1.24
1.25-
1.3
1.17-
1.24
1.17-
1.24
1.17-
1.24
1.17-
1.24
1.24-
1.31
1.16-
1.23
1.10-
1.16
1.16-
1.23
1.10-
1.16
1.16-
1.23
1.03-
1.12
1.09-
1.20
1.03-
1.12
a
Total, Less
Transport
1.15-1.21
1 .23 - 1 .27
1.15- 1.21
1.15-1.21
1.15- 1.21
1.15-1.21
1 .22 - 1 .28
1.15- 1.20
1.09-1.13
1.15-1.20
1.09-1.13
1.15-1.20
1.03-1.12
1.08-1.15
1.03-1.12
1.27-1.37
a A "total" value is not calculated because it would be double-counting some transport emissions.
b Conv gas = convention gasoline; US RFG = reformulated gasoline; AFVs = alternative fuel vehicles
Note: The range in each cell is determined by the values provided by GREET and LEM. In some cases, GREET provided the lower values, while in other cases, LEM provided the lower values.
56
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Transportation GHG Emissions Report
Table 14-2. Total CO2 Emissions from Various Components of the Transportation Lifecycle (Tg)
Mode
On-Road
Aviation
Vehicle
Passenger
Cars
Light-Duty
Trucks
Medium/
Heavy Duty
Trucks
Buses
Motorcycles
General
Aviation
Aircraft
Commercial
Aircraft
Military Aircraft
Other Aircraft
International
(Bunkers)
Fuel
Conv Gasa
US RFC
Diesel
AFVs
Conv. Gas
US RFC
Diesel
AFVs
Conv. Gas
US RFC
Diesel
AFVs
Conv. Gas
US RFC
Diesel
AFVs
Conv. Gas
US RFC
Jet Fuel
Aviation
Gasoline
Jet Fuel
Jet Fuel
Jet Fuel
Jet Fuel
Direct
Emissions
395.2
235.1
3.4
+
289.0
171.9
17.6
0.3
24.8
14.8
301.1
0.5
0.2
0.1
8.0
0.6
1.0
0.6
9.4
2.2
122.8
20.5
16.3
59.6
Indirect
Emissions
151 -197
91.9-111.3
1 -1.6
— +
110.5-144.0
67.2-81.4
5.2-8.1
— +
7.4-12
4.5-6.8
63.2-134.4
+-
0.1
0.1
1.7-3.6
— +
0.2-0.5
0.1 -0.3
1.6-2.3
0.6-0.7
20.4-29.6
3.4-4.9
2.7-3.9
9.9-14.3
Total Lifecycle
Emissions
546.2-592.1
327.0 - 346.4
4.4-5.0
— +
399.4-433.0
239.1 -253.3
22.8-25.7
0.3
32.2-36.8
19.3-21.6
364.3-435.5
0.5
0.3
0.2
9.7-11.6
0.6
1.3-1.5
0.8-0.9
10.9-11.6
2.8-2.9
143.2-152.3
24-25.5
19.1 -20.3
69.5-73.9
Total Emissions,
Excluding Transport
532.3-565.4
318.6-331.5
4.3-4.8
— +
389.3-413.5
233-242.4
22 - 24.4
0.3
31.5-35.5
18.9-20.8
355-417.6
0.5
0.3
0.2
9.4-11.1
0.6
1.2-1.5
0.7-0.9
10.8-11.3
2.8-2.8
141.3-148.3
23.6-24.8
18.8-19.7
68.6-71.9
57
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Transportation GHG Emissions Report
Mode
Waterborne
Rail
Pipelines
Lubricants
TOTAL
Total %
Vehicle
Domestic
International
(Bunkers)
Locomotives
and Transit
Fuel
Conv. Gas
Distillate
Fuel
Residual
Fuel
Distillate
Fuel
Residual
Fuel
Distillate
Fuel
Electricity
Natural Gas
Electricity
All oils
Direct
Emissions
11.0
17.0
29.5
6.0
18.6
39.6
3.2
34.8
+
10.2
1,864.9
Indirect
Emissions
2.6-3.5
2.7-4
3.1 -4.8
1.0-1.4
1.9-3.1
6.4-9.2
+
3.1 -6.8
+
0-0
563.6-789.7
Total Lifecycle
Emissions
13.7-14.5
19.7-20.9
32.5-34.3
7-7.4
20.6-21.7
45.9-48.8
3.2
37.9-41.6
+
10.2-10.2
b
Total Emissions,
Excluding Transport
13.5-14.1
19.4-20.4
32.1 -33.4
6.9-7.2
20.3-21.1
45.3-47.6
3.2
37.6-39.9
— +
10.2-10.2
2,372.7 - 2,547.3
1.27% -1.37%
a Not estimated to avoid double-counting transport-related emissions
b Conv gas = convention gasoline; US RFG = reformulated gasoline; AFVs = alternative fuel vehicles
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15Abbreviations, Acronyms, and Units
AAR
AEO
AFV
APIA
Btu
°C
CAFE
CARS
CFC
CH4
CNG
CO
C02
CO2 Eq.
DOE
DOT
EIA
EPA
°F
FHWA
GHG
GREET
GWP
HCFC
HFC
HPMS
H2O
IPCC
Ibs
LC
LCA
LEM
LEV
LPG
mpg
MMBtu
mph
MTA
NEI
NHTS
NHTSA
N2O
Association of American Railroads
Annual Energy Outlook
Alternative fuel vehicle
American Public Transportation Association
British thermal unit
Degree Celsius
Corporate Average Fuel Economy
California Air Resources Board
Chi orofluorocarb on
Methane
Compressed Natural Gas
Carbon monoxide
Carbon dioxide
Carbon dioxide equivalent
U.S. Department of Energy
U.S. Department of Transportation
Energy Information Agency
U.S. Environmental Protection Agency
Degree Fahrenheit
Federal Highway Administration
Greenhouse gas
Greenhouse Gases, Regulated Emissions, and Energy Use in
Transportation
Global warming potential
Hy drochl orofluorocarb on
Hydrofluorocarbon
Highway Performance Monitoring System
Water
Intergovernmental Panel on Climate Change
Pounds
Lifecycle
Lifecycle assessment
Lifecycle Emissions Model
Low Emissions Vehicle
Liquefied petroleum gas
Miles per gallon
Million British thermal units
Miles per hour
Metropolitan Transportation Authority
National Emission Inventory
National Household Travel Survey
National Highway Traffic Safety Administration
Nitrous oxide
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NOX Oxides of nitrogen
NPTS Nationwide Personal Travel Survey
ORNL Oak Ridge National Laboratory
OTAQ Office of Transportation and Air Quality
PFC Perfluorocarbon
PPM Parts per million
RFG Reformulated gasoline
RV Recreational vehicle
SF6 Sulfur hexafluoride
SO2 Sulfur dioxide
SUV Sport utility vehicle
Tg CO2 Eq. Teragrams carbon dioxide equivalent
TIUS Truck Inventory and Use Survey
VIUS Vehicle Inventory and Use Survey
VMT Vehicle miles traveled
VOC Volatile organic compound
Conversions
1 Tg = 1 MMT (million metric ton)
1 Tg = IxlO12 grams
1 metric ton = 1,000 kilograms = 1.1023 short tons
1 pound = 0.454 kilograms
1 gallon = 3.785412 liters
1 mile = 1.609 kilometers
To convert degrees Fahrenheit to degrees Celsius, subtract 32 and multiply by 5/9.
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