Environmental Stewardship

In the United States, transportation accounted for 28 percent of greenhouse gas (GHG) emissions in 2006.¹ Since 1995, GHG emissions related to transportation have grown 28 percent.² During the same period, vehicle-miles traveled (VMT) increased by 591,420 million miles (24 percent) (Table 2-3-1: U.S. Vehicle-Miles). However, the estimated VMT in 2008 shows a decline.³ Fuel efficiency has been relatively flat for the U.S. vehicle fleet over this period,⁴ increasing 1.3 miles per gallon (6 percent) for passenger cars and 0.7 miles per gallon (4 percent) for other two-axle, four-tire vehicles (Table 1-1-3: Average Fuel Efficiency of U.S. Passenger Cars and Light Trucks).

Transportation and Carbon Dioxide (CO₂) Emissions

In the United States, the predominant GHG emitted is CO₂, which accounted for 85 percent of the global warming potential (GWP) of all human-produced emissions. CO₂’s role in transportation of GHGs is higher still, accounting for about 96 percent of GWP emissions.⁵

Transportation Compared to Other Sectors’ CO₂ Emissions

The transportation sector now produces the most CO₂ emissions in the United States (Table 1-1-2: U.S. Energy-Related Carbon Dioxide Emissions by End-Use Sector). That share has grown from 32 percent in 1995 to 34 percent in 2007. Over that period, the total growth of CO₂ emissions from the transportation sector was 19 percent. In 1999, the transportation sector overtook the industrial sector as the largest source as industrial emissions remained comparatively flat. A recent decline has the industrial sector producing 5 percent less CO₂ in 2007 than in 1995. Increases for residential sources (20 percent) and commercial sources (28 percent) are comparable to the increase for the transportation sector (Table 1-1-2: U.S. Energy-Related Carbon Dioxide Emis sions by End-Use Sector).

CO₂ Emission by Mode of Transport

In 2006, passenger cars contributed about one-third (34 percent) of the transportation-induced CO₂ emissions (Table 1-1-1: Carbon Dioxide Emissions by Mode). Light-duty trucks (28 percent), medium- and heavy-duty trucks (21 percent), and aircraft (9 percent) made up the bulk of the remaining transportation-induced CO₂ emissions. Emissions from trucks of all sizes increased from 42 percent in 1995 to 49 percent of the total transportation CO₂ emissions in 2006. Serving as the backbone of many public transit systems, buses account for less than 1 percent of CO₂ emissions.

The growth in on-road transportation CO₂ between 1995 and 2006 is attributed mostly to light-duty trucks increasing 27 percent and medium- and heavy-duty trucks increasing 48 percent. Passenger cars CO₂ also increased, but at a much slower rate: 4.9 percent. Nonroad sources produced 16 percent of all transportation CO₂ emissions in 2006. The largest nonroad source, aircraft, produced almost 9 percent of total transportation CO₂. Aircraft CO₂ declined 2.3 percent during the 1995-2006 period.

Transportation and Other GHG Emissions

Two other naturally occurring GHGs, nitrous oxide (N₂O) and methane (CH₄), have seen growth in human-source emissions, although these two only account for 2 percent of all transportation sector GHGs. Other synthetic GHGs emitted from transportation sources include chloroflourocarbons (CFCs), hydrofluorocarbons (HFCs), and hydrochlorofluorocarbons (HCFCs), all principally used for air conditioning and refrigeration.⁶

¹ U.S. Environmental Protection Agency, Trends in Greenhouse Gas Emissions, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006, Washington, D.C.: April 2008, p. 2-21, available at http://www.epa.gov/climatechange/emissions/usgginventory.html as of July 30, 2008.

² U.S. Environmental Protection Agency, Trends in Greenhouse Gas Emissions, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006, Washington, D.C.: April 2008, p. 2-21, available at http://www.epa.gov/climatechange/emissions/usgginventory.html as of July 30, 2008.

³ U. S. Department of Transportation, Federal Highway Administration, Office of Highway Policy Information, Traffic Volume Trends, Washington, D.C.: August 2008, available at http://www.fhwa.dot.gov/ohim/tvtw/08augtvt/08augtvt.pdf as of Oct. 24, 2008.

⁴ U.S. Environmental Protection Agency, Trends in Greenhouse Gas Emissions, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006, Washington, D.C.: April 2008, p. 2-21, available at http://www.epa.gov/climatechange/emissions/usgginventory.html as of July 30, 2008.

⁵ U.S. Environmental Protection Agency, Greenhouse Gas Emissions from the U.S. Transportation Sector: 1990- 2003, Washington, D.C.: March 2006, p. 3, available at http://www.epa.gov/oms/climate/420r06003.pdf as of July 30, 2008.

⁶ U.S. Environmental Protection Agency, Greenhouse Gas Emissions from the U.S. Transportation Sector: 1990-2003, Washington, D.C.: March 2006, p. 4, available at http://www.epa.gov/oms/climate/420r06003.pdf as of July 30, 2008.