An important long-term threat posed by vehicle emissions is global climate change, which threatens to alter many natural systems in unpredictable ways. Carbon dioxide (CO2), which is produced during the combustion of gasoline, natural gas, and most other fuels, is one of the largest contributors to climate change.The US Department of Energy releases annual greenhouse gas emissions reports. The 2007 report shows that the majority of greenhouse emissions produced by vehicles are in the form of CO2. Non-CO2 emissions include methane and nitrous oxide emissions from mobile source combustion and hydrofluorocarbon (HFC-134a) emissions from vehicle air-conditioning units. The report notes that the transportation sector has led all sectors in the emission of CO2 since 1999. A general diagram of greenhouse gas emissions in the US economy are shown below.
The U.S. Environmental Protection Agency's MOVES model allows analysts to model carbon dioxide (CO2), methane (CH4), and other greenhouse gases as well as estimates for the total CO2 equivalent emissions. This modeling approach can produce the per-mile emission rates for these greenhouse gases for a base case and project alternative. Applying these per-mile emission rates to base case and project alternative travel forecasts produces total emissions for the base case and project alternative, the difference of which becomes the increase or decrease in emission output resulting from the transportation project.
Carbon dioxide (CO2) emissions can also be easily imputed from estimates of fuel consumption. This approach takes the difference in fuel consumption between base case and the project alternative to determine the change in carbon emissions. Fuel is converted into carbon emissions using the known carbon content of fuel, the ratio of the molecular weight of carbon dioxide to carbon, and an oxidation factor, for the proportion of the fuel that is oxidized into emissions during combustion (EPA, 2005).
The California Air Resources Board (CARB) is responsible for maintaining and updating California's Greenhouse Gas (GHG) Inventory. The GHG Inventory provides estimates of GHGs caused by human activities. CARB recently released a query tool for assessing the inventory values. The current GHG Inventory covers the years 1990 to 2004, and includes estimates for carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs) - the “six Kyoto gases.” The GHG inventory provided the basis for developing a 1990 statewide emissions level and 2020 emissions limit required by state legislation.
The UK Department for Environment Food and Rural Affairs (DEFRA) provides global warming potential factors for converting greenhouse gases into carbon dioxide equivalents. These factors could be used if methane or other greenhouse gas emissions need to be included in the benefit-cost analysis.
Various climate and economic models and climate change scenarios have been used to monetize greenhouse gases. Integrated assessment models for global climate change combine the geophysical modeling of climate change with models of economic growth and the future cost of the damage attributable to climate change to estimate the social cost of carbon.
In the United States, guidance was issued on the values for the social cost of carbon for use in federal regulatory impact assessments in March 2010. An interagency working group developed a total of four sets of estimates for the social cost of carbon based on the use of different discount rates or different climate change scenarios (United States Interagency Working Group, 2010). For the year 2010, the social cost of carbon from the guidance is $4.70, $21.40, $35.10, and $64.9 ($/tCO2, in 2007 dollars), reflecting discount rates of 5 percent, 3 percent, and 2.5 percent, respectively, and a higher-than-expected climate change scenario. Tol (2008) presents a literature review of 211 estimates of the cost per ton of carbon dioxide. This literature review found that the mean estimates of the social cost of CO2 were between $24 and $35 per metric ton of CO2 ($/tCO2, in 1995 dollars).
In the United Kingdom (UK), the government has required a Carbon Impact Assessment to be included in economic appraisals since 2003, as documented in the UK Treasury’s Appraisal and Evaluation in Central Government (or “Green Book”). In 2005, the UK Treasury sponsored an extensive review of the economics of climate change (the “Stern Review”). The UK Department for Environment Food and Rural Affairs (DEFRA) is tasked with valuing greenhouse gas emissions. With the help of AEA Technology, DEFRA developed an interim value using a social cost of carbon methodology. Since December 2007, DEFRA has adopted a more expansive approach based on the shadow price of carbon. The valuation reflects the full global cost of an incremental ton of CO2 equivalent (CO2e) emissions from the time of production to the damage it imposes over the whole of its time in the atmosphere. DEFRA has estimated future values, subjected the values to academic peer review, and published guidelines on the differences in the social cost and shadow prices as well as how to use the shadow price of carbon in policy appraisals (DEFRA 2007). DEFRA also maintains a website documenting all of its efforts to value greenhouse gas emissions. The DEFRA approach relies on a shadow price per metric ton of CO2e emitted in the Year 2000 and valued in 2000 dollars. Box 13.3 of the Stern Review shows that this price is $30 per metric ton of CO2e.
Over time, the social cost of carbon is expected to increase, reflecting the larger marginal cost per ton emitted due to accumulation of greenhouse gases in the atmosphere. The growth rate in the social cost of carbon is typically given as falling between 2 to 4 percent per year. The U.S. Federal guidance provides a schedule of the future values of the social cost of carbon. The DEFRA estimate of the shadow price per metric ton of CO2e is increased or “uprated” by two percent per year to reflect the increasing cumulative damage to the world environment each year.
United Kingdom Department for Environment, Food, and Rural Affairs, Economics Group (2007), The Social Cost of Carbon and the Shadow Price of Carbon: What They Are, and How to Use Them in Economic Appraisal in the UK.
Booz-Allen & Hamilton Inc. California Life-Cycle Benefit/Cost Analysis Model (Cal-B/C) Technical Supplement to User's Guide. California Department of Transportation (Caltrans), 1999. Available at: http://www.dot.ca.gov/hq/tpp/tools_files/tech_supp.pdf.
Mikhail Chester and Arpad Horvath (2008), Environmental Life-cycle Assessment of Passenger Transportation: A Detailed Methodology for Energy, Greenhouse Gas and Criteria Pollutant Inventories of Automobiles, Buses, Light Rail, Heavy Rail and Air v.2, Paper vwp-2008-2, UC Berkeley Center for Future Urban Transport (www.its.berkeley.edu/volvocenter). Available at: www.sustainable-transportation.com.
John Davies, Michael Grant, John Venezia and Joseph Aamidor (2007), “Greenhouse Gas Emissions of the U.S. Transportation Sector: Trends, Uncertainties, and Methodological Improvements,” Transportation Research Record 2017, TRB (www.trb.org), pp. 41-46; at http://trb.metapress.com/content/874k474474g5g767/?p=c4c8c51439f7453d9e494db833250bbb&pi=5.
DfT (2009), Transport Analysis Guidance: 3.3.5: The Greenhouse Gases Sub-Objective, Department for Transport (www.dft.gov.uk). Available at: www.dft.gov.uk/webtag/documents/expert/unit3.3.5.php.
EC (2005), ExternE: Externalities of Energy - Methodology 2005 Update, Directorate-General for Research Sustainable Energy Systems, European Commission (www.externe.info). Available at: www.externe.info/brussels/methup05a.pdf.
EDRG (2007), Monetary Valuation of Hard-to-Quantify Transportation Impacts: Valuing Environmental, Health/Safety & Economic Development Impacts, NCHRP 8-36-61, National Cooperative Highway Research Program (www.trb.org/nchrp). Available at: www.statewideplanning.org/_resources/63_NCHRP8-36-61.pdf.
Environmental Valuation Reference Inventory (www.evri.ca) is a searchable storehouse of empirical studies on the economic value of environmental benefits and human health effects.
EPA (2005). Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel. EPA420-F-05-001, February 2005. http://www.epa.gov/oms/climate/420f05001.htm
ITDP (2010), Manual for Calculating Greenhouse Gas Benefits of Global Environmental Facility Transportation Projects, Institute for Transportation and Development Policy, for the Scientific and Technical Advisory Panel of the Global Environment Facility (www.thegef.org). Available at:www.thegef.org/gef/GEF_C39_Inf.16_Manual_Greenhouse_Gas_Benefits.
Todd Litman (2009), “Evaluating Carbon Taxes As An Energy Conservation And Emission Reduction Strategy,” Transportation Research Record 2139, Transportation Research Board (www.trb.org), pp. 125-132; based on Carbon Taxes: Tax What You Burn, Not What You Earn, Victoria Transport Policy Institute (www.vtpi.org). Available at:www.vtpi.org/carbontax.pdf.
Todd Litman (2010), "Safety and Health Impacts," Transportation Cost and Benefit Analysis, Victoria Transport Policy Institute (www.vtpi.org). Available at www.vtpi.org/tca/tca0503.pdf.
M. Maibach, et al. (2008), Handbook on Estimation of External Cost in the Transport Sector, CE Delft (www.ce.nl). Available at: http://ec.europa.eu/transport/sustainable/doc/2008_costs_handbook.pdf.National Commission on Energy Policy, Allocating Allowances in a Greenhouse Gas Trading System, www.energycommision.org, accessed October 2, 2007.
Nadine Unger, et al. (2011), “Attribution Of Climate Forcing To Economic Sectors,” Proceedings of the National Academy of Sciences of the U.S. (www.pnas.org): at www.pnas.org/content/early/2010/02/02/0906548107.abstract.
Tol (2008). The Social Cost of Carbon: Trends, Outliers and Catastrophes. Economics: The Open-Access, Open-Assessment E-Journal. Vol. 2 2008-25, August 12, 2008.
United Kingdom Department for Environment, Food, and Rural Affairs, Economics Group (2007), How to Use the Shadow Price of Carbon in Policy Appraisal.
Urban Transportation Emissions Calculator (www.tc.gc.ca/UTEC) provides tools for estimating greenhouse gas (GHG) and criteria air pollution emissions from various types of vehicles.
Paul Watkiss and Thomas E. Downing (2008), The Social Cost of Carbon: Valuation Estimates and Their Use in UK Policy, The Integrated Assessment Journal Vol. 8, Iss. 1 (2008), (http://journals.sfu.ca/int_assess/index.php/iaj). Available at: http://journals.sfu.ca/int_assess/index.php/iaj/article/viewFile/272/236.
Anming Zhang, Anthony E. Boardman, David Gillen and W.G. Waters II (2005), Towards Estimating the Social and Environmental Costs of Transportation in Canada, Centre for Transportation Studies, University of British Columbia (www.sauder.ubc.ca/cts), for Transport Canada. Available at: www.sauder.ubc.ca/cts/docs/Full-TC-report-Updated-November05.pdf.