Emissions of Greenhouse Gases in the United States 2003: Executive Summary
Trends in U.S. Carbon Intensity and Total Greenhouse Gas Intensity
| Historical Growth Rates for U.S. Carbon Intensity Printer Friendly Version 
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From 2002 to 2003, the greenhouse gas intensity of the U.S. economy fell from 684 to 668 metric tons per million 2000 dollars of GDP (2.3 percent), continuing a trend of decreases in both carbon intensity (see figure at right) and total greenhouse gas intensity. As shown in the table below, declines in carbon intensity by decade have ranged from a low of 3.3 percent in the 1960s to 25.9 percent in the 1980s. From 1990 to 2003, total U.S. greenhouse gas intensity fell by 22.3 percent, at an average rate of 1.9 percent per year.
The carbon intensity and greenhouse gas intensity of the U.S. economy move in lockstep, because carbon dioxide emissions make up most of the total for U.S. greenhouse gas emissions. Energy-related carbon dioxide emissions represent approximately 83 percent of total U.S. greenhouse gas emissions. As such, trends in energy-related carbon dioxide emissions have a significant impact on trends in total greenhouse gas emissions. Historical trends in U.S. carbon intensity (energy-related carbon dioxide emissions per unit of economic output) are described below.
The carbon intensity of the economy can largely be decomposed into two basic elements: (1) energy intensity, defined as the amount of energy consumed per dollar of economic activity; and (2) carbon intensity of energy supply, defined as the amount of carbon emitted per unit of energy. As illustrated by the formulas below, the multiplication of the two elements produces a numerical value for U.S. carbon intensity, defined as the amount of carbon dioxide emitted per dollar of economic activity:
Energy Intensity x Carbon Intensity of Energy Supply = Carbon Intensity of the Economy ,
or, algebraically,
(Energy/GDP) x (Carbon Emissions/Energy) =
(Carbon Emissions/GDP) .
Components of Energy Intensity. Since World War II the U.S. economy has been moving away from traditional “smokestack” industries towards 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 units, because 1 metric ton is only about 10 percent heavier than 1 English short ton.
Emissions of most greenhouse gases are reported here in terms of the full molecular weight of the gas (as in Table ES1). In Table ES2, however, carbon dioxide and other greenhouse gases are reported 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).
The Methane to Markets Partnership
On July 28, 2004, President Bush announced the Methane to Markets Partnership. Under this program, developed countries, developing countries, and countries with economies in transition will collaborate to recover and cost-effectively use methane from landfills, coal mines, and the natural gas and petroleum systems that otherwise would have been emitted to the atmosphere. On November 16, 2004, representatives from Argentina, Australia, Brazil, China, Colombia, India, Italy, Japan, Mexico, Nigeria, Russia, Ukraine, and the United Kingdom, joined the United States in signing Terms of Reference that formally created the Partnership.
The United States intends to commit up to $53 million over the next 5 years to facilitate the development and implementation of methane projects in developing countries and countries with economies in transition. The EPA will have the lead U.S. role in the Partnership, coordinating efforts with the Department of State, DOE, the U.S. Agency for International Development and the U.S. Trade and Development Agency. According to the EPA, the Partnership has the potential to generate reductions of as much as 50 million metric tons of carbon equivalent annually through 2015, the equivalent of removing 33 million cars from the road for one year, or planting 55 million acres of trees. The energy embodied in the methane recovered, equal to about 500 billion cubic feet of natural gas, could heat approximately 7.2 million households for one year.