NOTES: Highway includes passenger cars, buses, light-duty trucks, and other trucks. Total 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.
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: 1990–2002 (Washington, DC: April 2004), table 2-9, available at http://www.epa.gov, as of May 2004.