Components of Energy Intensity. Since World War II the U.S. economy has been moving away from traditional “smokestack” industries toward more service-based or information-based enterprises. This has meant that over the second half of the 20th century economic growth was less tied to growth in energy demand than it was during the period of industrialization in the 19th and early 20th century. Other factors contributing to decreases in energy intensity include: 

• Improvements in the energy efficiency of industrial equipment as new materials and methods improved performance in terms of energy inputs versus outputs 
• Increased efficiency of transportation equipment as lighter materials and more efficient engines entered the marketplace 
• Improvements in commercial and residential lighting, refrigeration, and heating and cooling equipment 
• Developments in new electricity generating technologies, such as combined-cycle turbines. 

Further reductions in energy intensity, which are projected to continue, will among other things promote deeper reductions in U.S. carbon intensity. 

Components of the Carbon Intensity of Energy Supply. Changes in the carbon intensity of energy supply have been less dramatic than changes in energy intensity. There was a slow but steady decline from 1980 until about the mid-1990s, after which it has remained relatively unchanged. The primary reason for the decline has been the development of nuclear power, which is carbon-free and therefore weights the fuel mix toward lower carbon intensity. Other factors that can decrease the carbon intensity of the energy supply include: 

• Development of new renewable resources, such as wind power, for electricity generation 
• Substitution of natural gas for coal and oil in power generation 
• Transportation fuels with a higher biogenic component, such as ethanol.

Units for Measuring Greenhouse Gases 

Emissions data are reported here in metric units, as favored by the international scientific community. Metric tons are relatively intuitive for users of English measurement units, because 1 metric ton is only about 10 percent heavier than 1 English short ton. 

Table ES1 shows emissions of greenhouse gases in terms of the full molecular weights of the native gases. In Table ES2, and subsequently throughout this report, emissions of carbon dioxide and other greenhouse gases are given in carbon dioxide equivalents. In the case of carbon dioxide, emissions denominated in the molecular weight of the gas or in carbon dioxide equivalents are the same. Carbon dioxide equivalent data can be converted to carbon equivalents by multiplying by 12/44. 

Emissions of other greenhouse gases (such as methane) can also be measured in carbon dioxide equivalent units by multiplying their emissions (in metric tons) by their global warming potentials (GWPs). Carbon dioxide equivalents are the amount of carbon dioxide by weight emitted into the atmosphere that would produce the same estimated radiative forcing as a given weight of another radiatively active gas. 

Carbon dioxide equivalents are computed by multiplying the weight of the gas being measured (for example, methane) by its estimated GWP (which is 23 for methane).

Greenhouse Gas Emissions in the U.S. Economy 

The diagram on page 10 illustrates the flow of U.S. greenhouse gas emissions in 2004, from their sources to their distribution across the U.S end-use sectors. The left side shows gases and quantities; the right side shows their distribution by sector. The center of the diagram indicates the split between emissions from direct fuel combustion and electricity conversion. Adjustments indicated at the top of the diagram for U.S. territories and international bunker fuels correspond to greenhouse gas reporting requirements developed by the UNFCCC. 

CO₂. CO₂ emission sources include energy-related emissions (primarily from fossil fuel combustion) and emissions from industrial processes. The energy subtotal (5,900 MMTCO₂e) includes petroleum, coal, and natural gas consumption and smaller amounts from renewable sources, including municipal solid waste and geothermal power generation. The energy subtotal also includes emissions from nonfuel uses of fossil fuels, mainly as inputs to other products. Industrial process emissions (105 MMTCO₂e) include cement manufacture, limestone and dolomite calcination, soda ash manufacture and consumption, carbon dioxide manufacture, and aluminum production. The sum of the energy subtotal and industrial processes equals unadjusted CO₂ emissions (6,005 MMTCO₂e). The energy component of unadjusted emissions can be divided into direct fuel use (3,601 MMTCO₂e) and fuel converted to electricity (2,299 MMTCO₂e). 

Non-CO₂Gases. Methane (639 MMTCO₂e) and nitrous oxide (354 MMTCO₂e) sources include emissions related to energy, agriculture, waste management, and industrial processes. Other gases (156 MMTCO₂e) include HFCs, PFCs, and SF₆. These gases have a variety of uses in the U.S. economy, including refrigerants, insulators, solvents, and aerosols; as etching, cleaning, and firefighting agents; and as cover gases in various manufacturing processes. 

Adjustments. In keeping with the UNFCCC, CO₂ emissions from U.S. Territories (62 MMTCO₂e) are added to the U.S. total, and CO₂ emissions from fuels used for international transport (both oceangoing vessels and airplanes) (94 MMTCO₂e) are subtracted to derive total U.S. greenhouse gas emissions (7,122 MMTCO₂e). 

Emissions by End-Use Sector. CO₂ emissions by end-use sectors are based on EIA’s estimates of energy consumption (direct fuel use and purchased electricity) by sector and on the attribution of industrial process  emissions by sector. CO₂ emissions from purchased electricity are allocated to the end-use sectors based on their shares of total electricity sales. Non-CO₂ gases are allocated by direct emissions in those sectors plus emissions in the electric power sector that can be attributed to the end-use sectors based on electricity sales. 

Residential emissions (1,241 MMTCO₂e) include energy-related CO₂ emissions (1,225 MMTCO₂e); and non-CO₂ emissions (16 MMTCO₂e). The non-CO₂ sources include direct methane and nitrous oxide emissions from direct fuel use. Non-CO₂ indirect emissions attributable to purchased electricity, including methane and nitrous oxide emissions from electric power generation and SF₆ emissions related to electricity transmission and distribution, are also included. 

Commercial sector emissions (1,298 MMTCO₂e) include energy-related CO₂ emissions (1,035 MMTCO₂e); and non-CO₂ emissions (263 MMTCO₂e). The non-CO₂ emissions include direct emissions from landfills, wastewater treatment plants, commercial refrigerants, and stationary combustion emissions of methane and nitrous oxide. Non-CO₂ indirect emissions attributable to purchased electricity, including methane and nitrous oxide emissions from electric power generation and SF₆ emissions related to electricity transmission and distribution, are also included. 

Industrial emissions (2,599 MMTCO₂e) include CO₂ emissions (1,853 MMTCO₂e), which can be broken down between stationary source combustion (1,748 MMTCO₂e) and industrial emissions (105 MMTCO₂e); and non-CO₂ emissions (746 MMTCO₂e). The non-CO₂ direct emissions include emissions from agriculture (methane and nitrous oxide), coal mines (methane), petroleum and natural gas pipelines (methane), industrial process emissions (methane, nitrous oxide, HFCs, PFCs and SF₆), and direct stationary combustion emissions of methane and nitrous oxide. Non-CO₂ indirect emissions attributable to purchased electricity, including methane and nitrous oxide emissions from electric power generation and SF₆ emissions related to electricity transmission and distribution, are also included. 

Transportation emissions (1,984 MMTCO₂e) include energy-related CO₂ emissions from mobile source combustion (1,869 MMTCO₂e); and non-CO₂ emissions (124 MMTCO₂e). The non-CO₂ emissions include methane and nitrous oxide emissions from mobile source combustion and HFC emissions from the use of refrigerants for mobile source air-conditioning units.