NOTES: Highway includes passenger cars, buses, light-duty trucks, and other trucks. Total, all modes does not include international bunker fuels. International bunker fuel emissions result from the combustion of fuels purchased in the United States but used for international aviation and maritime transportation. Thus, aircraft and ships and boats data, which are included in U.S. total emissions, involve only domestic activities of these modes as do all other data shown. Aircraft emissions consist of emissions from all jet fuel (less bunker fuels) and aviation gas consumption. Alternative-fuel vehicle emissions are allocated to the specific vehicle types in which they were classified (i.e., passenger cars, light-duty trucks, and other trucks and buses).
The large annual variations in ships and boats data may result from methodological problems related to the domestic/international partition of maritime fuels. Economic factors may also contribute.
SOURCE: U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003 (Washington, DC: April 2005), table 2-17.