Transportation GHG Emissions and Trends
Carbon dioxide (CO2) from fossil fuel combustion is responsible for almost all greenhouse gas (GHG) emissions from transportation sources. As shown in the following table, fossil fuel CO2 accounts for 95 percent of mobile source GHG emissions:
U.S. Greenhouse Gas Emissions from Transportation and Mobile Sources (Tg CO2 Eq.)
NA = Not Applicable, as there were no HFC emissions allocated to the transport sector in 1990, and thus a growth rate cannot be calculated.
CH4 and N2O emissions also result from fuel combustion. HFC emissions are associated with motor vehicle air conditioners, cooling systems used on passenger trains and buses, and refrigerants used in transporting certain types of freight
U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.)
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006, April 2008
As one can calculate from the above table, transportation sector emissions have grown at an average rate of about 2.0 percent annually since 1990, having grown considerably faster than those of other sectors except the electric power industry.
CO2 emissions are calculated based on the carbon content and total domestic consumption of different transportation fuels (e.g., gasoline, diesel fuel, aviation gasoline, jet fuel, residual fuel oil). Subsequent calculations are performed to estimate the share of emissions attributable to different vehicle types. Results are summarized in the following table, which aggregates emissions of each of the above gases.
U.S. Greenhouse Gas Emissions from Transportation and Mobile Sources, by Vehicle Type (Tg CO2 Eq.)
Emissions from passenger cars have grown slower than those from other highway vehicles and locomotives, and emissions from aircraft (domestic flights) have grown relatively slowly.
U.S. Greenhouse Gas Emissions by Mode and Gas
Note: Totals may not sum due to independent rounding.
U.S. GHG Inventory Structure
Under Article 4 of the United Nations Framework Convention on Climate Change (UNFCCC), the U.S. and other signatories have committed to the development, periodic update, and publication of national inventories of sources and sinks (e.g., forests) of greenhouse gas (GHG) emissions resulting from human activities, consistent with calculation methodologies agreed upon by the Conference of Parties to the UNFCCC.
The U.S. Environmental Protection Agency (EPA) prepares the nation's inventory of greenhouse gas emissions and sinks. To ensure that the U.S. emissions inventory is comparable to those of other UNFCCC signatory countries, EPA uses methodologies consistent with those recommended by the Intergovernmental Panel on Climate Change (IPCC). These guidelines utilize the major source and sink categories summarized below:
IPCC Greenhouse Gas Source/Sink Categories*
*Nonmethane volatile organic compounds.
Under these guidelines, the majority of transportation-related emissions, which result from the combustion of transportation fuels, are reported as a subset of the energy source category, which is by far the largest overall source category in the inventory. In addition to reporting using these IPCC source categories, the U.S. inventory also provides summary information on GHGs allocated to economic sector. Inventory economic sectors include transportation, agriculture, commercial, industry, and residential.
Forecasts of Transportation GHG Emissions
Transportation GHG forecasts are projections of possible future GHG emissions from the transportation sector. Forecasts take into account variables that may significantly affect change in emissions over time. Forecasts are often based on current GHG inventory reports. As forecasts are developed, they can also be used to estimate the impacts of the mitigation strategies. GHG forecasts, along with GHG inventories, assist in the ongoing development of approaches to measure and mitigate GHG emissions from the transportation sector.
More complete estimates of greenhouse gases are available for the U.S. than for many other countries. Although this complicates international comparisons, the U.S. also estimates carbon emissions from fossil fuel use in all countries. Because fossil fuel use accounts for the majority of greenhouse gas emissions, this is a useful starting point for comparison. As shown below, the U.S. is responsible for more greenhouse gas emissions than any other single country, consistent with its large population and relative level of economic development:
World Carbon Emissions from Fossil Fuel Use
However, this table also reveals that emissions have been growing much more rapidly in some countries than in the U.S. (On the other hand, emissions in Europe have largely remained the same since 1980, and emissions have declined in Eurasia since 1990.) Emissions have been growing particularly quickly in some developing countries, such as China. Depending upon future population and economic trends, emissions from such countries could overtake those of the U.S. and other developed countries in the foreseeable future.
In EIA's 2005 emissions inventory report, total U.S. energy-related carbon dioxide emissions in 2004 (including nonfuel uses of fossil fuels) were estimated at 5,923.2 MMT (one million metric ton is equal to one teragram, or1012 grams). With the 2004 world total for energy-related carbon dioxide emissions estimated at 26,922 MMT, U.S. emissions were about 22 percent of the world total (see table below).
World Energy-Related Carbon Dioxide Emissions by Region, 1990-2030 (Million Metric Tons Carbon Dioxide)
Carbon dioxide emissions related to energy use in the mature economies of countries that are members of the Organization for Economic Cooperation and Development (OECD)—including OECD North America, OECD Europe, Japan, and Australia/New Zealand—are estimated at 13,457 MMT, or about one-half of the world total. With the remaining 50 percent of worldwide energy-related carbon dioxide emissions (13,465 MMT) coming from non-OECD countries, 2004 marked the first year in which global emissions were split evenly between the OECD and non-OECD economies.