U.S. Anthropogenic Emissions of Greenhouse Gases, 1990-2004  printer friendly version   Carbon Dioxide Equivalent  Estimated 2004 Emissions (Million Metric Tons)  7,122.1  Change Compared to 2003 (Million Metric Tons)  138.8  Change from 2003 (Percent)  2.0%  Change Compared to 1990 (Million Metric Tons)  973.3  Change from 1990 (Percent)  15.8%  Average Annual Increase, 1990-2004 (Percent)  1.1%

U.S. emissions of greenhouse gases in 2004 totaled 7,122.1 million metric tons carbon dioxide equivalent (MMTCO₂e), 2.0 percent more than in 2003 (6,983.2 MMTCO₂e). The 2004 increase in total greenhouse gas emissions is attributable primarily to a 1.7-percent increase in emissions of carbon dioxide, along with increases in emissions of nitrous oxide (5.5 percent) and methane (0.9 percent). Emissions of engineered gases— hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆)—also increased, by 9.6 percent. The U.S. economy grew by 4.2 percent in 2004, the highest rate of growth since 1999. Consequently, U.S. greenhouse gas intensity (greenhouse gas emissions per unit of real economic output) was 2.1 percent lower in 2004 than in 2003. From 1990 to 2004, U.S. greenhouse gas intensity declined by 23 percent, or by an average of 1.9 percent per year (see "Trends in U.S. carbon Intensity and total Greenhouse Gas Intensity). 

U.S. greenhouse gas emissions in 2004 were 16 percent higher than the 1990 emissions level of 6,148.8 MMTCO₂e—an average annual increase of 1.1 percent over the period. Since 1990, U.S. emissions have increased more slowly than the average annual growth in population (1.2 percent), primary energy consumption (1.2 percent), electric power generation (1.9 percent), or gross domestic product (3.0 percent). While the annual growth rate in carbon dioxide emissions since 1990 (1.3 percent) has closely tracked annual growth in population and energy consumption, the average annual rate of growth in total greenhouse gas emissions has been lower (1.1 percent) because of reductions in methane emissions and low growth in nitrous oxide emissions (0.7 percent) since 1990. 

Table ES1 shows trends in emissions of the principal greenhouse gases, measured in million metric tons of native gas. In Table ES2, emissions trends are shown in carbon dioxide equivalents, which are derived by multiplying the volumes of native gas by the global warming potential (GWP) of each gas. The GWPs provide a measure of marginal radiative efficiency for each of the native gases. The GWP concept, developed by the Intergovernmental Panel on Climate Change (IPCC), provides a comparative measure of the impacts of added units of different greenhouse gases on global warming relative to the impact of carbon dioxide.¹ This allows for the normalization of different greenhouse gases, which have different warming impacts, into a common unit of measure—carbon dioxide equivalents—which can be aggregated across gases (see "Units for Measuring Greenhouse Gases"). 

In 2001, the IPCC Working Group I released its Third Assessment Report, Climate Change 2001: The Scientific Basis.² Among other things, the Third Assessment Report updated a number of the GWP estimates that appeared in the IPCC’s Second Assessment Report.³ 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 0.7 percent higher when the GWPs from the Third Assessment Report are used; however, the trends in growth of greenhouse gas emissions are similar for both sets of GWP values. 

During 2004, 82.4 percent of total U.S. greenhouse gas emissions consisted of carbon dioxide from the combustion of fossil fuels such as coal, petroleum, and natural gas (after adjustments for U.S. Territories and international bunker fuels). U.S. emissions trends are driven largely by trends in fossil energy consumption. In recent years, national energy consumption, like emissions, has grown relatively slowly, with year-to-year deviations from trend growth caused by weather-related phenomena, fluctuations in business cycles, changes in the fuel mix for electric power generation, and developments in domestic and international energy markets.

Other 2004 U.S. greenhouse gas emissions (Figure ES1) include carbon dioxide from non-combustion sources (1.5 percent of total U.S. greenhouse gas emissions), methane (9.0 percent), nitrous oxide (5.0 percent), and other gases (2.2 percent). Methane and nitrous oxide emissions are caused by the biological decomposition of various waste streams and fertilizer; fugitive emissions from chemical processes; fossil fuel production, transmission, and combustion; and many smaller sources. The other gases include HFCs, used primarily as refrigerants; PFCs, released as fugitive emissions from aluminum smelting and used in semiconductor manufacture; and SF₆, used as an insulator in utility-scale electrical equipment. 

This report, in accordance with Section 1605(a) of the Energy Policy Act of 1992, provides estimates of U.S. emissions of greenhouse gases. The estimates are based on activity data and applied emissions factors, not on measured or metered emissions.

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