A Wedge Analysis of the U.S.
Transportation Sector
Greenhouse Gas Emissions from the Transportation Sector
The U.S. transportation sector represents approximately 10% of all energy-related
greenhouse gas emissions worldwide. Over the next 50 years, rising numbers and
use of vehicles could increase greenhouse gas emissions from U.S. transportation by
80% above current levels. However, many current and anticipated vehicle and fuel
technologies, as well as approaches for reducing vehicle use, could be deployed to
slow or halt the increase in accumulated greenhouse gasses in the atmosphere from
transportation. This fact sheet provides a summary of a number of these approaches
from a recent EPA study, "A Wedge Analysis of the U.S. Transportation Sector" .
www.epa.gov/otaq/climate/420r07007.pdf
Figure 1: Global-scale Stabilization Wedge
60,000
> 850 ppm C02
Stabilization Wedge Approach
EPA's analysis uses the "stabilization
wedge" concept first developed
by Rob Socolow and Stephen
Pacala at Princeton University.
This approaches focuses on
ways to reduce emissions from
a business-as-usual scenario in
which atmospheric carbon dioxide
(CO2) concentrations reach 850
parts per million (ppm) over the
next 50 years to a scenario that
stabilizes global concentrations at
550 ppm, which is less than twice
the preindustrial concentration of
atmospheric CO2 (see Figure I).1
The difference between these two
scenarios is the "stabilization triangle." Socolow and Pacala slice this triangle into seven
"stabilization wedges," each representing an activity that over the next 50 years could
< 550 ppm C02
2050 2060
1 Figure 1 is reproduced from Stephen Pacala and Robert Socolow (2004), "Stabilization Wedges: Solving the
Climate Problem for the Next 50 Years with Current Technologies." Science, 305, 968.
United States
Environmental Protection
Agency
Office of Transportation and Air Quality
EPA420-F-07-049
October 2007
-------
reduce 92,000 million metric tons (MMT) of CO2e on a global level.2 Eliminating all seven of
these wedges would allow global emissions to be flattened (or kept at today's level) over the next
50 years, helping to maintain an atmospheric CO2 concentration of 550 ppm.
Figure 2: U.S. Transportation Stabilization Wedge
5000
Q)
CM
o
o
4000
In the EPA study, the wedge
analysis approach was
scaled down and applied
to the U.S. transportation
sector. Figure 2 illustrates
the triangle necessary to
flatten emissions from the
U.S. transportation sector
from now until 2050. The
cumulative emissions
embodied by the upper
"stabilization triangle"
are approximately 45,000
MMT CO2e—roughly
half of one of Socolow and
Pacala's seven wedges. To
better evaluate the possible
strategies to reduce these
emissions, the EPA study divided the transportation-specific triangle into nine wedges of 5,000
MMT CO2e each. Elimination of all nine of these wedges would be needed to flatten the
sector's emissions by 2050.
£. 3000
to
o
« 2000
E
LII
O
O
1000
Wedges to
"Stabilization
Triangle"
Additional
Wedges
Options for Reducing Greenhouse Gas "Wedges" from the U.S.
Transportation Sector
There are three general approaches for reducing greenhouse gases in the transportation sector:
1) adopting advanced vehicle technologies, 2) switching to low-greenhouse gas fuels, and 3)
reducing vehicle miles traveled. The EPA wedge analysis examined numerous scenarios for
implementing these options separately and in combination.
The analysis focused only on passenger vehicles, which contribute approximately half of all U.S.
transportation sector emissions. Over the next 50 years, emissions growth from just passenger
vehicles is expected to contribute half of transportation emissions under a business-as-usual
scenario (the other emissions come from emissions growth of trucks, airplanes, rail, and other
modes of transportation). A number of different examples are shown that can reduce anywhere
from 4 to 9 wedges—just from passenger vehicles. Incorporating other transportation sources
2In their analysis, Scolow and Pacala measure emissions in billions of tons. This paper uses millions of metric tons (MMT).
One of the seven 92,000 MMT wedges described by Socolow and Pacala is equivalent to 25 billion tons. CO2e, or "CO2
equivalent" emissions, represent emissions of all greenhouse gasses converted to units of CO2 based on their relative global
warming potential.
A Wedge Analysis of the U.S. Transportation Sector
-------
into the analysis would yield many more possible approaches for reducing emissions and could
yield much larger reductions than those from passenger vehicles alone.
What if We Just Changed One Thing? Scenarios Focused on Stand-alone
Changes to the Transportation Sector
EPA analyzed the effect of a number of stand-alone changes to vehicle technologies, fuels, or
vehicle use that would reduce greenhouse gasses. The scenarios focused on changing vehicle
technology examined what would happen if various types of vehicles reached a market share
in 2025 that was 30 percentage points above their current market share and then maintained
the new market share until 2050. A vehicle that currently has 10% of the market would, for
example, have 40% of the market from 2025 to 2050. Figure 3 illustrates that such a boost in
market share for gasoline-hybrid electric vehicles (HEV) would reduce emissions by one wedge.
The results of other vehicle technology scenarios and their wedge counts are shown in the
figure's inset box. (The ranges of GHG reductions shown account for low and high estimates
for fuel-cycle emissions depending, for example, on what technologies are used to generate
electricity for plug-in hybrid electric vehicles or to produce ethanol.)
Figure 3: Potential Emissions Reductions from Vehicle Technology Scenarios
3000
CD
co
2500 -
2000 -
1500 -
if] 1000
CD
X
° 500 H
0
Example "What-iP Scenario:
Gasoline HEVs, increase in market share in 2010 to 30%
greater market share by 2025 (1 wedge)
A wedge count
for other vehicle "S
technologies
(same what-if scenarios)
Currently Available Technology
(Adv. Gasoline, or Adv. Diesels,
or Gasoline HEVs)
Each can 0.8 - 1.0 wedges
achieve: __^- *m^^-\
13-16 -a Blodles
Diesel HEVs ^**°**s\ Dlesel
_ ,. . . 1.3-2.7 .-< Cellulo
Optimized ^-' i
Optimized E85 1 -8 - 2,9--^H
Advanced ICE ^f^^^t
11-22
Plug-In HEVs -•''^t
ei(B20) Electric Vehicles* ' ,,--''
><-- 0.6-2.8 .--
Fuel Cell Vehicles* ,,-''
1990 2000 2010 2020 2030 2040 2050
Figure notes: HEV=hybrid electric vehicle; ICE=internal combustion engine. *For electric vehicle and
fuel cell vehicles, the "what-if market share reaches 30% five years later (2030) due to large technical
hurdles remaining.
EPA also analyzed other stand-alone changes to the transportation sector that would reduce
greenhouse gas emissions. Some of these changes involved switching to low greenhouse gas
fuels while others reduced vehicle miles traveled (VMT). For fuels, the analysis revealed that
substituting 60 billion gallons of ethanol for gasoline by 2050 (25% from corn ethanol and 75% from
cellulosic ethanol and no changes in vehicle technology) would achieve 1.4 wedges. An alternative
A Wedge Analysis of the U.S. Transportation Sector
-------
case, involving 90 billion gallons of ethanol, would achieve 2.3 wedges. Reducing vehicle miles
traveled by 5%, 10%, and 15% by 2050 through land use planning, shifting travel modes, and
other approaches would achieve 0.5, 0.9, and 1.3 wedges respectively.
What if We Changed Many Things? Scenarios That Use a "Systems Approach"
to Analyze Multiple Inter-related Changes to the Transportation Sector
By far, the greatest reductions in greenhouse gasses come from a "systems approach" that
accounts for the important relationships among strategies for advanced vehicle technologies,
low greenhouse gas fuels, and/or reducing VMT. Figure 4 shows the result of a deployment of
a "systems approach" that includes both advanced vehicles and low-GHG fuels with a strong
emphasis on hybrid electric vehicles. This scenario results in 5 wedges (the inset shows how
the components of this systems scenario add up to the total). A large variety of other scenarios
is possible. Three of these are described in Table 1, and additional scenarios are described in
the full paper. The most aggressive scenario—achieving up to 9 wedges—involves extensive
deployment of near-zero emission vehicle technologies and fuels (e.g. cellulosic ethanol and
electricity from nuclear or renewable sources), as well as travel demand management approaches
for reducing VMT (see Sceanrio 3 in Table 1, "Emphasis on Near-zero Emissions Vehicles and
Fuels and Aggressive Travel Demand Management").
Figure 4: A "Systems" Scenario Assuming Predominantly Hybrid Electric Vehicles,
Including Plug-in Hybrids, by 2050
3000
^2500
E 2000
3 w
Q c
g 1500
to
1000
O
500
(a) Hybrid Electric Vehicle Focus
+ FFVs, Optimized E85 Vehicles,
Ethanol as 10% blend in gasoline
1990
2000
2010
2020
2030
2040
2050
A Wedge Analysis of the U.S. Transportation Sector
-------
Table 1: Summary of Selected "Systems" Scenarios for Reducing
Transportation-Related Greenhouse Gas Emissions
Scenario and Assumptions (Snapshot of Vehicle Sales, Fuels, and Travel
Demand Management in 2050)
1. Emphasis on Advanced Internal Combustion Technologies and Traditional
Fuels
n 80% market share in 2050 for advanced gasoline, advanced diesel, and gas-
hybrid vehicles
° 20% market share for optimized E85 vehicles
° Use of 50 billion gallons (bgal) per year of ethanol (15 bgal from corn, 35 bgal
from cellulose)
° No travel demand management
2. Emphasis on E85 Vehicles and Ethanol Fuels and Aggressive Travel
Demand Management
° 60% market share in 2050 for advanced gasoline and advanced diesel
vehicles
° 40% market share for optimized and advanced optimized E85 vehicles
° Use of 80 bgal per year of ethanol (15 bgal from corn, 65 bgal from cellulose)
° 15% reduction in vehicle miles traveled (VMT) from travel demand
management
3. Emphasis on Near-zero Emissions Vehicles and Fuels and Aggressive
Travel Demand Management
° 40% market share in 2050 for electric vehicles
° 30% market share for hydrogen fuel cell vehicles
° 30% market share for advanced optimized E85 vehicles
° Use of 40 bgal per year of ethanol (15 bgal from corn, 25 bgal from cellulose)
° 15% reduction in VMT from travel demand management
Wedge Count
4.0 wedges
6.0 wedges
5.2 to 9.0 wedges (depending
on the sources of electricity &
hydrogen for vehicles)
Conclusions
Overall, the analysis showed that with aggressive combined improvements in vehicle
technologies, fuels, and vehicle miles traveled, the future contribution of U.S. transportation
to accumulated greenhouse gasses in the atmosphere could be reduced or flattened. It also
suggested the following additional conclusions:
• By themselves, individual approaches incorporating vehicle technologies, fuels, or
transportation demand management (TDM) approaches could moderately reduce,
but not flatten, the nine transportation-related wedges from now until 2050.
"Systems approaches" that combine all three approaches, however, could yield the 4.3
wedges needed to flatten passenger vehicle emissions and even up to all nine wedges
under aggressive scenarios.
• If efforts are limited to only passenger vehicles, the task of achieving nine wedges will
be a challenging one. Incorporating solutions that involve commercial trucks, marine
vessels, railroads, airplanes, and non-road vehicle sources would yield much larger
possible reductions that could more easily eliminate nine or more wedges.
A Wedge Analysis of the U.S. Transportation Sector
-------
• Near'term vehicle technologies can have as much an impact in terms of GHG
reductions as future, longer-term technologies because their reductions begin to accrue
sooner. To achieve the most wedges, however, the reductions achievable only though
longer-term technologies are needed.
• Nearly all the approaches discussed also reduce petroleum use, which would have
benefits beyond GHG reductions. For example, achieving five wedges could result
in saving 7 to 8 million barrels of petroleum per day in 2050.
For More Information
You can access the full study on EPA's Office of Transportation and Air Quality (OTAQ)
Web site at:
www.epa.gov/otaq/greenhousegases.htm
References
Stephen Pacala and Robert Socolow (2004), "Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with
Current Technologies." Science, 305, 968.
Simon Mui, Jeff Alson, Benjamin Ellies, David Ganss (2007). "A Wedge Analysis of the U.S. Transportation Sector."
Transportation and Climate Division, Office of Transportation and Air Quality, U.S. Environmental Protection Agency.
A Wedge Analysis of the U.S. Transportation Sector
------- |