Achieving targeted regional reduction in greenhouse gas (GHG) emissions from the transportation sector will require concentrated efforts to change travel behavior and reduce vehicle miles travelled in addition to advances in vehicle technology and fuels, according to two recent studies.
A paper by researchers at the University of Minnesota, published in the ACS journal Environmental Science & Technology, projects GHG mitigation strategies for Minnesota, which has adopted a strategic goal of 80% emissions reductions below 2005 levels by 2050. A paper by researchers at the Institute of Transportation Studies (ITS), University of California–Davis, to be published in the journal Transportation Research Part D: Transport and Environment, examines how California may reduce transportation greenhouse gas emissions 80% below 1990 levels by 2050 (“80in50”). (Earlier version of study, earlier post.)
Minnesota. The team from the Ecosystem Science and Sustainability Initiative, University of Minnesota examined the potential for policies and technologies to effect GHG reductions across all sectors in the state. The researchers used GHG mitigation categories developed by Pacala and Socolow in a variant of the Pacala and Socolow wedge framework, but took a very different approach to analyzing the strategies’ contributions to GHG mitigation.
[Pacala and Socolow] selected diverse strategies that could each contribute a set reduction of 1 Gt carbon emissions over 50 years, whereas we examined the maximum emissions reduction possible within Minnesota for the strategies considered and did not limit these wedges in terms of a contribution to a pre-determined amount of emissions reduction. In our study, wedge refers to the size of the GHG reduction that a specific technology or strategy could contribute by 2050 compared to a trajectory of emissions representing currently implemented reduction actions and historically observed economic impacts on energy consumption (we call this trajectory “business as usual” or BAU).—Olabisi et al. (2009)
The researchers used scenarios to evaluate potential GHG reductions from technologies determined to be feasible and available in Minnesota within 10-20 years, primarily in the transportation and electricity production sectors.
Under the business as usual (BAU) scenario, GHG emissions in Minnesota would rise to approximately 223 million metric tons of CO2 equivalent by 2050—an increase of 49% over 2005 emissions levels. Minnesota’s population is expected to grow by 30% during this time period, and real gross state product by 253%.
Transportation wedges (and resulting reductions from projected BAU 2050 emissions) include:
Vehicle miles driven cut to 50% of 2005 levels by 2050, with ethanol blending mandates (20% reduction from 2050 BAU levels);
Fleet vehicle efficiency increased to 55 miles per gallon (mpg) average by 2030, with current ethanol blending mandates (19% reduction);
Complete transition to switchgrass-based ethanol (18% reduction);
Complete fleet transition to PHEVs while retaining conventional electricity mix (9% reduction); and
Complete transition to corn-based ethanol for personal vehicles (7% reduction).
The wedges from all sectors could be combined in several ways (portfolios) to enable the state to meet the goal of reducing GHG emissions 80% below 2005 levels. One example given combined generating all of Minnesota’s electricity from poplar biomass and sequestering the carbon emitted at the generation plants; reducing vehicle miles driven by half; improving vehicle fleet fuel efficiency to 55 mpg; and reforesting 5% of Minnesota’s land area. This resulted in the “maximum reduction” portfolio of 95% below 2005 GHG emission levels.
The trajectory of GHG emissions in Minnesota is more sensitive to the number of miles Minnesotans drive than it is to the amount of electricity they use. This makes intuitive sense, as driving is a more carbon-intensive activity than electricity consumption in terms of daily emissions. By reducing vehicle miles traveled in the state to 1999 levels by 2050, Minnesota could restrict its GHG emissions growth to only 23% above 2005 levels, rather than 49% as predicted under the BAU projection. This would mitigate the need for Minnesota to adopt as many other carbon-saving technologies.
...Public acceptance to deploy the wedges will be related to cost and the degree to which they fit within embedded political interests and require a transformation of existing infrastructures—significant for switching to renewable electricity production or reducing vehicle miles driven; less significant for producing motor fuels from corn or improving the efficiency of electricity use. Reducing vehicle miles driven in Minnesota would likely induce spillover effects, such as increased bus traffic, which were not analyzed in this study.—Olabisi et al. (2009)
|Greenhouse gas emission reductions for Silver Bullet scenarios relative to Reference scenario for Instate emissions. Yang et al. (2009) Click to enlarge.|
California. The ITS study used a scenario approach looking across all transportation subsectors (light-duty vehicles, heavy-duty vehicles (buses and trucks), rail, aviation, marine, agriculture, and off-road) to explore options for reducing emissions in the transportation sector by 80%. Scenarios included strategies for reducing travel demand, improving efficiency and using advanced technologies with alternative fuels.
The researchers used a spreadsheet model, the Long-term Evaluation of Vehicle Emission Reduction Strategies (LEVERS) model, which is built around a transportation-variant of the Kaya identity, to analyze GHG emissions. The Kaya identity decomposes CO2 emissions into the product of several important parameters. While the original Kaya identity defines activity in terms of GDP, the ITS variation of the Kaya equation focuses on transport intensity as the main activity driver in the transportation sector. Transportation CO2 emissions are decomposed into four main drivers: population, travel demand, vehicle fuel consumption, and fuel carbon intensity.
Three of the Kaya parameters correspond with three main “levers” for reducing emissions: reducing transport intensity (T), energy intensity (E) and fuel carbon intensity (C). Population is not considered in this analysis as a means of reducing emissions; California’s population is expected to double between 1990 and 2050. Important considerations for determining how effective a strategy will be in reducing emissions include the following: what mitigation options are used, how broadly they are applied, and the degree of improvement they provide. Some mitigation options cannot be implemented in all subsectors.
...Transport intensity may be reduced by several means. Better land-use planning, higher-density developments, telecommuting and increased co-location of jobs and housing can reduce travel demand even while maintaining or improving the ability of people to access their desired destinations. Another method is mode-switching from private cars to mass transit (buses, trains, etc.), which has the capacity to carry a large number of people at a given time. Changes in consumer and industrial purchasing behavior can reduce activity in the freight sector.—Yang et al. (2009)
The ITS researchers examined seven scenarios: one reference and six “Silver Bullet” scenarios. Silver Bullet (SB) scenarios describe futures in which one mitigation option, such as an advanced vehicle technology or alternative fuel, is employed to the maximum feasible extent from a technology perspective in 2050.
Reference scenario. Doubling of population, modest increase (21%) in transport intensity, slight efficiency improvement (35%) and similar carbon intensity relative to 1990.
Moderate efficiency SB. No breakthrough technological advances, but applies all advances in conventional technologies towards improving vehicle efficiency to achieve doubling of average vehicle efficiency from 1990. Same carbon intensity as Reference, except for some electrified rail.
High efficiency SB. Significant breakthroughs in conventional technologies to achieve nearly triple (265%) vehicle efficiency from 1990. Same carbon intensity as Reference, except for some electrified rail.
Hydrogen-intensive SB. Applies FCV and low-carbon hydrogen fuels (9.5 gCO2e/MJ) aggressively across most subsectors, except aviation, and provides 58% of all transport miles. Fleet market share of on-road H2 vehicles is limited to 60% in 2050. Assumes that the obstacles to use of hydrogen in heavy-duty trucks are overcome.
Electricity-intensive SB. Electric vehicles (BEVs and PHEVs) and very low-carbon electricity are applied across many subsectors except marine and aviation, providing 77% of all transport miles. Electricity carbon intensity (6.5 gCO2e/MJ) is 94% below the 1990 value.
Biofuel-intensive SB. Low-carbon biofuels (16.3 gCO2e/MJ) are the primary fuels used in conventional vehicles (low efficiency) in all transport subsectors, providing 59% of all transport miles. Biofuels are limited to 15–20% of future US supply.
Passenger Miles Travelled (PMT) SB. About 25–50% reductions in passenger travel demand for LDVs and aviation relative to Reference scenario, through better land use, smart growth, transit and high-speed rail. No alternative fuels; same carbon intensity as Reference. Improved energy intensities due to increased vehicle load factors.
The Silver Bullet scenarios show that no mitigation option can singlehandedly meet the target goal because travel demand is expected to increase significantly by 2050 and advanced technologies and fuels may not be suitable for use in all subsectors or may be limited in availability. The 80in50 scenarios illustrate that the 80% reduction goal could potentially be met in multiple ways. The Efficient Biofuels 80in50 and Electric-drive 80in50 scenarios show that if vehicle and fuels technologies become clean enough, California can preserve its current levels of mobility. The former requires more primary energy and relies heavily on biomass, while the latter uses fuel more efficiently and has the potential for a significantly more diverse resource mix. The Actor-based 80in50 scenario shows that large shifts in social and travel behavior are valuable mitigation options, especially if technology is not as successful. This scenario has the lowest energy resource requirements.—Yang et al. (2009)
Laura Schmitt Olabisi, Peter B. Reich, Kris A. Johnson, Anne R. Kapuscinski, Sangwon Suh and Elizabeth J. Wilson (2009) Reducing Greenhouse Gas Emissions for Climate Stabilization: Framing Regional Options. Environ. Sci. Technol., doi: 10.1021/es801171a
Yang, Christopher, David L. McCollum, Ryan W. McCarthy, Wayne Leighty (2009) Meeting an 80% Reduction in Greenhouse Gas Emissions from Transportation by 2050: A Case Study in California. Transportation Research Part D (article in press) doi: 10.1016/j.trd.2008.11.010