Greenhouse Gas Emissions

U.S. greenhouse gas (GHG) emissions totaled 6,936 teragrams (trillion grams) of carbon dioxide equivalent (TgCO₂Eq) in 2001,¹ of which 1,867 TgCO₂Eq (27 percent) were emitted by transportation. Transportation emissions have grown 22 percent since 1990, while total U.S. emissions rose 13 percent. Carbon dioxide (CO₂), the predominant greenhouse gas, accounted for 84 percent of all U.S. emissions in 2001 [1]. Nearly all (97 percent) of CO₂ emissions are generated by the combustion of fossil fuels. Transportation was responsible for 1,780.9 TgCO₂Eq, or 31 percent of all CO₂ emissions. Transportation CO₂ emissions grew 24 percent between 1991 and 2001, an average annual change of 2 percent.

Highway vehicle emissions rose at an average annual rate of 2 percent between 1991 and 2001 (figure 75). At the same time, locomotive emissions grew at 3 percent and domestic aircraft emissions rose less than 1 percent. Domestic maritime emissions increased 2 percent but were volatile throughout the period. Under the United Nations Framework Convention on Climate Change reporting guidelines, only domestic aircraft and maritime emissions are included in the modal data. The balance of emissions, labeled international bunker fuels, declined 2 percent on an annual average basis between 1991 and 2001.

Highway vehicles emitted 79 percent of all transportation CO₂ emissions in 2001. Passenger cars and light-duty vehicles, which include pickup trucks, sport utility vehicles, and vans, were responsible for 78 percent of those highway emissions (figure 76). Over the period 1991 to 2001, emissions of all other trucks grew fastest, at 4 percent annually. The second highest average annual growth rate among highway vehicles was 3 percent for light-duty trucks.

Most air pollutants impact local or regional air quality. Greenhouse gases, on the other hand, could alter the earth’s climate on a regional and global scale. These potential changes include long-term fluctuations in temperature, wind, precipitation, and other perturbations of the Earth’s climate system. GHGs, including CO₂, methane, and nitrous oxide occur naturally and as a result of human activities.

See box for Greenhouse Gas Emissions

1. U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2001 (Washington, DC: April 2003), tables ES-3 and ES-8.

¹ Including sinks, net U.S. emissions totaled 6,098 TgCO₂Eq in 2001. A natural sink, according to the U.S. Environmental Protection Agency, is “A reservoir that uptakes a pollutant from another part of its cycle. Soil and trees tend to act as natural sinks for carbon.” Unnatural sinks are manmade depositories for pollutants (e.g., the Department of Energy is creating underground sinks into which CO₂ can be pumped).