Personal transportation is unduly targeted for emission reductions at the onset as an accident of how we account for greenhouse gas emissions. We're frequently told that the transportation sector is the largest contributor of greenhouse gas emissions in the United States, but this is a misleadingly statement. The U.S. Environmental Protection Agency (EPA) inventories emissions in six main categories-electricity generation, transportation, industry, agriculture, commercial and residential. When inventoried in those categories, electricity generation is the largest source of greenhouse gas emissions (34 percent in 2006), followed by transportation (28 percent), industry, agriculture, commercial, through to the smallest sector, residential (5 percent). Electrical power is an industrial product "consumed" by the other economic sectors, however, so the next step EPA reports is a calculation of emissions with electricity generation allocated among the five smaller sectors. After these emissions are distributed, "industry" accounts for the largest share of greenhouse gas emissions (29 percent in 2006), followed by transportation (28 percent), commercial, residential and agriculture (8 percent).
This simple example shows how the categories in which emissions are reported in has a large influence over how each economic sector is perceived. Distributing emissions from the electrical power industry dramatically changes the respective contribution of the other sectors, with the exception of transportation, where a nearly insignificant amount of rail transportation contributes a small amount of electricity to the mix. Transportation is "consumed" by the other sectors, just as electricity is, but the accounting method just lumps together passenger cars, light-duty trucks, sport utility vehicles, commercial trucks, domestic aviation, military aircraft, commercial and recreational boats and emissions from all other modes of motorized transport.
Even with commercial, personal and other kinds of transportation included in one sector, it still doesn't add up to the "largest share" of total U.S. greenhouse gas emissions. Rather, it is the largest share of carbon dioxide (CO2)-the dominant greenhouse gas-from fossil fuel combustion-the main source of anthropogenic CO2. Weighted for global warming potential over a 100-year time horizon, EPA reports that CO2 from fossil fuel combustion accounted for approximately 80 percent of U.S. greenhouse gas emissions in 2006. The other 20 percent, including CO2 from industrial processes, methane from livestock and landfills, nitrous oxide from agriculture, and some chemical solvents and propellants, should not be overlooked in greenhouse gas reduction strategies-in fact, these categories provide some of the quickest and least expensive opportunities for emission reductions. Household vehicle use currently is responsible for a much smaller portion of U.S. greenhouse gas emissions, 16-18 percent, than is commonly believed.
What is vastly more important than the relative amount of greenhouse gas emissions from this sector, however, is the utility provided by passenger cars and light-duty trucks and the opportunities that lie ahead for reducing the greenhouse gas intensity of personal motorized transport while maximizing utility.
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