Sources of Greenhouse Gas Emissions
Transportation Sector Emissions
Total U.S. Greenhouse Gas Emissions by Economic Sector in 2011
The Transportation sector includes the movement of people and goods by cars, trucks, trains, ships, airplanes, and other vehicles. The majority of greenhouse gas emissions from transportation are CO2 emissions resulting from the combustion of petroleum-based products, like gasoline, in internal combustion engines. The largest sources of transportation-related greenhouse gas emissions include passenger cars and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans. These sources account for over half of the emissions from the sector. The remainder of greenhouse gas emissions comes from other modes of transportation, including freight trucks, commercial aircraft, ships, boats, and trains as well as pipelines and lubricants.
Relatively small amounts of methane (CH4) and nitrous oxide (N2O) are emitted during fuel combustion. In addition, a small amount of hydrofluorocarbon (HFC) emissions are included in the Transportation sector. These emissions result from the use of mobile air conditioners and refrigerated transport.
Emissions and Trends
In 2011, greenhouse gas emissions from transportation accounted for about 28% of total U.S. greenhouse gas emissions, making it the second largest contributor of U.S. greenhouse gas emissions after the Electricity sector. Greenhouse gas emissions from transportation have increased by about 18% since 1990. This historical increase is largely due to increased demand for travel and the stagnation of fuel efficiency across the U.S. vehicle fleet. The number of vehicle miles traveled by passenger cars and light-duty trucks increased 34% from 1990 to 2011. The increase in travel miles is attributed to several factors, including population growth, economic growth, urban sprawl, and low fuel prices over much of this period. Between 1990 and 2004, average fuel economy among new vehicles sold annually declined, as sales of light-duty trucks increased. However, new vehicle fuel economy began to improve in 2005, largely due to a lower light-duty truck market share and higher fuel economy standards.
To learn about projected greenhouse gas emissions to 2020, visit the U.S. Climate Action Report 2010 (PDF). (193 pp, 3.1 MB)
Greenhouse Gas Emissions from Transportation
Reducing Emissions from Transportation
There are a variety of opportunities to reduce greenhouse gas emissions associated with transportation. The table shown below categorizes these opportunities and provides examples. For a more comprehensive list, see Chapter 5 of the Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change .
|Type||How Emissions are Reduced||Examples|
|Fuel Switching||Using fuels that emit less CO2 than fuels currently being used. Alternative sources can include biofuels; hydrogen; electricity from renewable sources, such as wind and solar; or fossil fuels that are less CO2-intensive than the fuels that they replace.
Learn more about Alternative and Renewable Fuels.
|Improving Fuel Efficiency with Advanced Design, Materials, and Technologies||Using advanced technologies, design, and materials to develop more fuel-efficient vehicles.||
|Improving Operating Practices||Adopting practices that minimize fuel use.
Improving driving practices and vehicle maintenance.
Learn about how the freight transportation industry can reduce emissions through EPA's SmartWay Program.
|Reducing Travel Demand||
Employing urban planning to reduce the number of miles that people drive each day.
6,822 million metric tons of CO2 equivalent--what does that mean?
An Explanation of Units
A million metric tons is equal to about 2.2 billion pounds, or 1 trillion grams. For comparison, a small car is likely to weigh a little more than 1 metric ton. Thus, a million metric tons is roughly the same mass as 1 million small cars!
The U.S. Inventory uses metric units for consistency and comparability with other countries. For reference, a metric ton is a little bit larger (about 10%) than a U.S. "short" ton.
greenhouse gas emissions are often measured in carbon dioxide (CO2) equivalent. To convert emissions of a gas into CO2 equivalent, its emissions are multiplied by the gas's Global Warming Potential (GWP). The GWP takes into account the fact that many gases are more effective at warming Earth than CO2, per unit mass.
The GWP values appearing in the Emissions webpages reflect the values used in the U.S. Inventory, which are drawn from the IPCC's Second Assessment Report (SAR). For further discussion of GWPs and an estimate of greenhouse gas emissions using updated GWPs, see Annex 6 of the U.S. Inventory and the IPCC's discussion on GWPs.