Environmental Stewardship

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.