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). In 2001, the IPCC Working Group I released its Third Assessment Report, Climate Change 2001: The Scientific Basis. Among other things, the Third Assessment Report updated a number of the GWP estimates that appeared in the IPCC’s Second Assessment Report. The GWPs published in the Third Assessment Report were used for the calculation of carbon dioxide equivalent emissions for this report. Generally, the level of total U.S. carbon dioxide equivalent emissions is 0.6 percent higher when the GWPs from the Third Assessment Report are used; however, the trends in growth of greenhouse gas emissions are similar for the two sets of GWP values. GWPs from the Second Assessment Report still are used for comparisons among countries. 

^aIntergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis (Cambridge, UK: Cambridge University Press, 2001). 

^bIntergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change (Cambridge, UK: Cambridge University Press, 1996).

Greenhouse Gas Emissions in the U.S. Economy 

The diagram illustrates the flow of U.S. greenhouse gas emissions in 2005, 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 in the power sector. 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 (5,945 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 (6,051 MMTCO₂e). The energy component of unadjusted emissions can be divided into direct fuel use (3,570 MMTCO₂e) and fuel converted to electricity (2,375 MMTCO₂e). 

Non-CO₂ Gases. Methane (612 MMTCO₂e) and nitrous oxide (367 MMTCO₂e) sources include emissions related to energy, agriculture, waste management, and industrial processes. Other gases (160 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 (59 MMTCO₂e) are added to the U.S. total, and CO₂ emissions from fuels used for international transport (both oceangoing vessels and airplanes) (101 MMTCO₂e) are subtracted to derive total U.S. greenhouse gas emissions (7,147 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 (1,284 MMTCO₂e) include energy-related CO₂ emissions (1,266 MMTCO₂e); and non-CO₂ emissions (18 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. 

Emissions in the commercial sector (1,301 MMTCO₂e) include both energy-related CO₂ emissions (1,061 MMTCO₂e) and non-CO₂ emissions (240 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 (2,562 MMTCO₂e) include CO₂ emissions (1,804 MMTCO₂e)—which can be broken down between combustion (1,699 MMTCO₂e) and process emissions (105 MMTCO₂e)—and non-CO₂ emissions (757 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 (2,000 MMTCO₂e) include energy-related CO₂ emissions from mobile source combustion (1,877 MMTCO₂e); and non-CO₂ emissions (123 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.

Distribution of Total U.S. Greenhouse Gas Emissions by End-Use Sector, 2005