Emissions of Greenhouse Gases in the United States 2003
Overview
| U.S. Anthropogenic Emissions of Greenhouse Gases, 1990-2003 Printer Friendly Version 
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U.S. emissions of greenhouse gases in 2003 totaled 6,935.7 million metric tons carbon dioxide equivalent, 0.7 percent more than in 2002 (6,890.9 million metric tons carbon dioxide equivalent). Although emissions of carbon dioxide and emissions of methane grew by 0.8 percent and 0.5 percent, respectively, those increases were partially balanced by reductions in emissions of nitrous oxide (-0.9 percent) and hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride (-0.3 percent). U.S. greenhouse gas emissions have averaged 1.0-percent annual growth since 1990.
The U.S. economy grew by 3.0 percent in 2003, which is equivalent to the average annual growth rate that has prevailed during the 1990-2003 period. Consequently, U.S. greenhouse gas intensity (greenhouse gas emissions per unit of real economic output) was 2.3 percent lower in 2003 than in 2002. From 1990 to 2003, U.S. greenhouse gas intensity has declined by 22.3 percent, or by an average of 1.9 percent per year (see discussion on "Trends in U.S. Carbon Intensity and Total Greenhouse Gas Intensity").
U.S. greenhouse gas emissions in 2003 were 13.4 percent higher than 1990 emissions (6,115.2 million metric tons carbon dioxide equivalent)—an average annual increase of 1.0 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 (1.0 percent) because of reductions in methane and nitrous oxide emissions since 1990.
Table ES1 shows trends in emissions of the principal greenhouse gases, measured in million metric tons of gas. In Table ES2, the value shown for each gas is weighted by its global warming potential (GWP), which is a measure of marginal radiative efficiency. 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 global warming potential of carbon dioxide.1 (See discussion on "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 0.6 percent higher when the GWPs from the Third Assessment Report are used; however, the trends in growth of greenhouse gas emissions are similar for both sets of GWP values.
During 2003, 83.0 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.
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Other 2003 U.S. greenhouse gas emissions include carbon dioxide from non-combustion sources (1.6 percent of total U.S. greenhouse gas emissions), methane (8.7 percent), nitrous oxide (4.6 percent), and other gases (2.1 percent) (Figure ES1). 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 hydrofluorocarbons (HFCs), used primarily as refrigerants; perfluorocarbons (PFCs), released as fugitive emissions from aluminum smelting and also used in semiconductor manufacture; and sulfur hexafluoride (SF6), used as an insulator in utility-scale electrical equipment.
This report, required by 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 monitoring.