This is the second annual Energy Information Administration (EIA) report on U.S. emissions of greenhouse gases . This report presents estimates of U.S. anthropogenic (human-caused) emissions of carbon dioxide, methane , nitrous oxide, and several other greenhouse gases for the period 1987 to 1992. Estimates of 1993 carbon dioxide emissions are also provided, although complete 1993 estimates for other gases are not yet available (Table ES1).
Many of the estimates in this report have been revised from the first EIA report. Estimates of carbon dioxide emissions were revised upward by about 25 million metric tons of carbon dioxide (0.5 percent) for each year as a result of revised emissions coefficients and revised estimates of nonfuel use of fossil fuels . Methane emissions estimates were revised downward by about 2 million metric tons (6.6 percent) for each year as a result of changes in estimation methods.
In Table ES2, the emissions of each gas are weighted by its global warming potential (GWP), which is taken as a measure of radiative forcing . (E-1) This concept provides a measure of the comparative impacts of different greenhouse gases on global warming, with the effect of carbon dioxide being equal to 1. The global warming potentials are the same as those employed by the U.S. Government for the Climate Change Action Plan. (E-2) This method permits a comparison of the relative importance of different greenhouse gases.
Carbon dioxide accounts for 87 percent of U.S. GWP-weighted emissions of greenhouse gases. GWP-weighted emissions rose by 0.6 percent between 1990 and 1992. In Table ES1, carbon dioxide emissions are shown at full molecular weight. In Table ES2 they are shown in carbon equivalent units (see "Units for Measuring Greenhouse Gases" on the following page). Trends in the growth of emissions of the major greenhouse gases (Figure ES1) are described below.
Figure ES1. Indices of U.S. Emissions
of Greenhouse Gases, 1980-1993
Source: EIA estimates documented in this report.
Some 98.5 percent of U.S. anthropogenic carbon dioxide emissions are caused by the combustion of fossil fuels. The causes of changing carbon dioxide emissions can be found in the causes of changes in energy consumption and changes in the composition of fossil fuels burned to provide energy services. During the late 1980s, U.S. anthropogenic emissions of carbon dioxide flattened out, despite rising energy consumption. Rising hydroelectric power generation, the completion of nuclearpower plants commissioned in the early 1970s, and increased natural gas use all contributed to this outcome.
In 1990 and 1991, with the onset of economic recession and rising oil prices, both energy consumption and carbon emissions declined. However, the past 2 years have seen economic recovery, falling oil prices, rising energy consumption, and rising carbon dioxide emissions. Emissions rose by 1.5 percent in 1992 and by a further 2.0 percent in 1993, and they are now some 2.5 percent (34 million metric tons of carbon) higher than in 1990.
U.S. anthropogenic methane emissions have three principal sources: production and transportation of coal, natural gas, and oil; anaerobic decomposition of municipal waste in landfills; and raising livestock. Smaller sources include combustion of fossil fuels, rice cultivation, and industrial processes.
Methane emissions rose during the late 1980s. The principal cause of this trend appears to have been increasing production from a group of underground coal mines with very high rates of methane emissions. Emissions from municipal landfills appear to have been stable, because growth in the volumes of solid waste landfilled were offset by growing volumes of waste burned for energy recovery and increased recovery of methane.
Since 1990, overall methane emissions appear to have been almost unchanged. However, the estimation techniques for methane emissions are much more uncertain than those for carbon emissions.
In reporting on emissions of greenhouse gases, the Energy Information Administration endeavors to link information and methods from two different communities, which use different units of measure. Scientific research on emissions sources and global climate change is conducted largely by scientists using metric units. Many of the metric units customary in scientific discourse on global climate change are unfamiliar and are not commonly used even in European industry. The Intergovernmental Panel on Climate Change, for example, describes emissions and energy consumption in terms of "gigagrams," "petagrams," and "exajoules." U.S. energy and industrial statistics are collected and reported almost entirely in English units.
In this report, the EIA has elected to place familiarity above consistency, and to try to report information in forms that are most likely to be intuitively familiar to users of the document. Therefore, energy and industrial data are reported in their native units (usually English units). Oil production is reported in thousand barrels per day, and energy production and sales in (higher heating value) quadrillion British thermal units (Btu).
On the other hand, persons working with emissions data are most likely to be familiar with, and users of, metric units. Therefore, all emissions data are reported in metric units. We have attempted to bridge the gap between users of metric units and English units by using the familiar "million metric tons" common in European industry instead of the "gigagrams" favored by the scientific community. English unit users will probably remember that a metric ton is almost exactly 10 percent heavier than the familiar 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, and subsequently throughout the report, carbon dioxide is measured in carbon units, defined as the weight of the carbon content of carbon dioxide (i.e., just the "C" in CO2). Carbon dioxide units at full molecular weight can be converted into carbon units by dividing by 44/12, or 3.67. This approach has been adopted for two reasons:
While carbon dioxide emissions can be measured in tons of carbon, emissions of other gases (such as methane) can also be measured in "carbon equivalent" units by multiplying their emissions (in metric tons) by their global warming potential, and then multiplying by 12/44 (as in Table ES2). This method provides a measure of the relative effects of various gases on climate. Because scientific estimates of global warming potential are still evolving, however, this report gives emissions in carbon equivalent units for other gases only in Table ES2. No other data in the report are given in carbon equivalent units.
Nitrous oxide emissions are more uncertain than methane emissions: the principal sources are believed to be "excess" emissions from agricultural soils after the application of nitrogen fertilizers, industrial process emissions, and emissions from combustion of fossil fuels. Nitrous oxide emissions, estimated at 0.43 million metric tons in 1990, show a faint growth trend (up by 0.75 percent or 0.003 million metric tons from 1990 to 1992); but the uncertainty of the estimation methods makes it difficult to be confident of apparent trends.
Halocarbons and related compounds include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and some other halogenated compounds that defy simple categorization. These gases are powerful greenhouse gases with global warming potentials many times that of carbon.
CFCs are currently being phased out because they damage the stratospheric
ozone layer. Ozone, however,
is also a greenhouse gas, and gases that destroy tropospheric
ozone thus have indirect cooling effects that may offset their
direct warming effects as greenhouse gases. Chlorine-containing
chemicals (including CFCs and HCFCs) tend to react with
ozone; therefore, they have ambiguous effects as greenhouse gases.
HFCs and PFCs, which contain no chlorine, have no effect on ozone
and thus are unambiguously powerful greenhouse gases.
Information on production, sales, and emissions of these compounds
is fragmentary. Figure ES2 illustrates emissions trends for those
gases for which sufficient information is available to estimate
time series. At present, available data suggest that emissions
of CFCs-about 0.2 million metric tons in 1990-are declining. HCFC
emissions, estimated
at about 0.1 million metric tons in 1990, are rising-particularly
for the CFC substitute HCFC142b. HFC emissions are very low, perhaps
0.006 million metric tons in 1990. Emissions of HFC23, a byproduct
of HCFC22 production, appear to be growing. Emissions of the CFC
substitute HFC134a (not shown) probably have risen substantially
in the past 2 years, but the EIA has not yet been able to develop
a method for estimating emissions of this gas. PFC emissions are
low (0.003 million metric tons) and stable.
Figure ES2. Estimated U.S. Emissions of Halocarbons and Related Compounds, 1980-1992
Source: EIA estimates presented in Chapter 5 of this report.
Criteria pollutants (carbon monoxide, nitrogen oxides , and nonmethane volatile organic compounds ) are volatile, short-lived gases, which usually decay quickly in the atmosphere. They are not necessarily greenhouse gases in themselves, but they can promote atmospheric chemical reactions that create tropospheric ozone, which is a potent greenhouse gas. Because the precise ozone-creating effect of these gases varies with local atmospheric conditions, it is not possible to compute their impact directly. As they are precursors to urban "smog," their emissions are regulated under the Clean Air Act. The principal source of emissions of criteria pollutants is the combustion of fossil fuels, particularly in motor vehicles.
According to estimates from the U.S. Environmental Protection
Agency (EPA), national-level emissions of carbon monoxide have
been declining steadily since the late 1970s (Figure ES3). Emissions
of nitrogen oxides and nonmethane volatile organic compounds have
also declined, but at a much slower rate.
Figure ES3. Estimated U.S. Emissions of Criteria Pollutants, 1980-1992
Source: U.S. Environmental Protection Agency, Office of Air Quality
Planning and Standards, National Air
Pollutant Emission Trends, 1900-1992, EPA-454/R-93-032 (Research
Triangle Park, NC, October 1993), Tables A-1-A-11, pp. A-2-A-23.
While the combustion of fossil fuels accounts for most of the anthropogenic emissions of greenhouse gases in the United States, changes in land use can also have large, though difficult to quantify, effects on atmospheric concentrations. In the United States, the expansion of forest land and the growth of existing forests are responsible for removing large amounts of carbon from the atmosphere. Several studies of carbon sequestration by U.S. forests suggest that in the late 1980s and early 1990s, some 100 to 130 million metric tons of carbon was sequestered annually, equivalent to about 7 to 10 percent of U.S. anthropogenic carbon emissions. (E-3) However, considerable uncertainty is associated with this estimate-particularly, the amount of carbon sequestered in forest soils.
