Study Concludes US LDV Fleet Needs to Reduce Carbon Emissions Per Mile By Up to 88% by 2050 to Meet 450ppm Stabilization Scenario; No Single Carbon Reduction Strategy Likely to Achieve This
Researchers at the University of Michigan (U-M) have calculated quantitative sustainable mobility targets for US light-duty vehicles (LDVs) to help stabilize atmospheric carbon dioxide concentrations at 450 or 550 ppm, based on the well-to-wheel carbon emissions per mile driven.
Average fleet-wide on-road light-duty vehicle (LDV) well-to-wheel carbon emissions must be reduced from 160 g carbon/mile (equivalent to 586.7 gCO2/mile) to as little as 20 gC/mile (equivalent to 73 gCO2/mile) under one scenario by 2050—an 88% reduction—to contribute to a goal of 450 ppm, according to according to Greg Keoleian, co-director of the U-M School of Natural Resources and Environment, and his co-authors, Hilary Grimes-Casey and Blair Willcox of the Center for Sustainable Systems.
The researchers defined an allowed global emissions budget using atmospheric carbon stabilization scenarios developed by Wigley et al. (known as WRE) for the IPCC Third Assessment Report (TAR) on climate change. They considered two scenarios for allocating emissions to LDVs: one in which LDVs maintain a relatively constant share of emissions, and one in which the share of US GHG emissions from LDV use grows due to easier reduction activities in other sectors.
|Required changes in fleet fuel economy to achieve 450 and 550 ppm with baseline conditions for VMT growth and fuel mix improvements. Credit: ACS. Click to enlarge.|
Improving vehicle fuel efficiency, the widespread use of low-carbon fuel, and reducing vehicle miles traveled (VMT) have all been suggested as ways to reduce carbon emissions in the LDV fleet. However, reaching the required carbon-reduction targets necessitates an aggressive combination of all strategies, the researchers write in a paper published in the 1 February edition of the ACS journal Environmental Science & Technology.
Attempting to reach the 450 ppm emissions-reduction targets by adopting just one of these approaches would require:
Fleetwide on-road fuel economy of 136 miles per gallon gasoline-equivalent (mpgge), given baseline conditions for VMT and projected fuel mix. This represents a 665% improvement over 2007 on-road average forecasted fuel economy. To stabilize emissions at 550 ppm, fleet fuel economy would have to reach over 94 mpgge.
Drivers would have to reduce their current driving activity by almost half, or their projected 2050 VMT by 75% to meet the 450 ppm target. For 550 ppm, current driving levels would have to be reduced 25% and projected 2050 levels would have to be reduced 37%.
Cellulosic (low-carbon) ethanol would have to capture almost 6% of the market by 2010, and almost 86% by 2050, in order to meet the 450 ppm sustainable mobility target for WTW carbon emissions.
In terms of the fuel mix, the authors modeled all petroleum-based fuel as gasoline (only 1.5% of the mix currently is diesel), and projected that total ethanol consumption will stabilize at 11% of LDV fuel energy by 2030. The study focused on spark ignition technology, and so excluded biodiesel and hydrogen.
In the U-M study, the authors assumed that grain-based ehtanol provides an 18% lifetime CO2-equivalent reduction on a per MJ basis, and that cellulosic ethanol provides an 88% reduction, also on that basis. These figures do not factor in land-use issues; the study thus “offers an optimistic scenario for the potential for biofuel use to reduce carbon dioxide emissions from LDVs.”
The results of “pushing” social and technological levers to reduce use phase LDV carbon emissions according to a global target demonstrate that any individual vehicle carbon reduction strategy is not likely to be successful over the long term. A policy (or technology) package that promotes low carbon vehicles, provides incentives to manufacturers to increase fuel efficiency, and manages LDV travel demand is needed.—Grimes-Casey et al. (2009)
The authors give plug-in hybrid electric vehicles (PHEVs) specific consideration in their discussion, noting that PHEV carbon intensity in CO2 equivalents per mile will depend on the carbon intensity of the future electricity generation mix.
Using baseline scenarios for energetic consumption of LDV fuels, if a 30 km-range (18.6-mile) PHEV was charged with the current US electricity generation mix, the PHEVs would have to replace 100% of gas powered vehicle driving by 2015 in order to meet the LDV target of 450 ppm. By 2017 this strategy, however, would be insufficient to meet LDV driving goals for vehicles powered by conventional liquid fuels. If future US electricity generation carbon intensity is high (130.2 g/mi assumption), PHEVs will not be sufficient to meet any of the analyzed targets by 2017.
This study demonstrates the need for a combination of strategies that significantly cut projected vehicle carbon emissions per mile according to the proposed emissions target pathways. Therefore, the development and market penetration of less-carbon-intensive vehicles and fuels must be accelerated in conjunction with a dramatic reduction in travel demand in order to achieve sustainable personal mobility...Even in the “best” or easiest to achieve case scenario, the LDV sector faces a substantial challenge in reducing CO2 emissions by 2050.—Grimes-Casey et al. (2009)
Hilary G. Grimes-Casey, Gregory A. Keoleian, and Blair Willcox (2009) Carbon Emission Targets for Driving Sustainable Mobility with US Light-Duty Vehicles. Environ. Sci. Technol., Article ASAP doi: 10.1021/es801032b