traveled would require information about the number of passengers per vehicle—and would only change the final result if more passengers on average travelled on vehicles powered by a particular fuel. Presentation of results per gallon of fuel makes for difficult comparisons since different fuels have different energy contents per gallon.
In this report, impacts are assessed nationally using detailed models for the overall activities. Using a VMT basis for the transportation emissions estimates includes not only the differences in the impacts for different fuels, but also includes differences in the size and weights of vehicles that constitute the national vehicle fleet.
Carbon dioxide (CO2) emissions from energy production and use are expressed in tons (short tons) or metric tons (tonnes) of CO2-equivalent (CO2-eq). Although CO2 is the principal greenhouse gas associated with energy use, other gases such as methane, nitrous oxide, black carbon, and SF6, also make some contributions to warming potential. These other contributions are converted to an equivalent amount of CO2 with a similar effect and the total is therefore expressed as tonnes of CO2-equivalent. The United States emits about 7 billion tonnes of CO2-equivalent per year, about 6 billion of which is CO2 arising primarily from energy production and use. Average annual CO2 emissions in the United States are about 20 tonnes per person. [Note: Sometimes greenhouse gas emissions are reported in terms of tonnes of “carbon.” One tonne of carbon emissions equals 3.7 tonnes of CO2 emissions, since the weight of CO2 also includes the weight of the oxygen in the molecule.]