| Carbon Dioxide | Methane | Nitrous Oxide | Halocarbons and Related Compounds | Criteria Pollutants | Land Use Issues |
In 1996, U.S. emissions of greenhouse gases increased by 3.4 percent over 1995 emissions, the highest rate of increase in recent years. Although U.S. emissions have been growing since 1991, their growth accelerated in 1996. Greenhouse gas emissions expanded more rapidly than U.S. energy consumption in 1996, and the growth of energy consumption (up 3.2 percent) exceeded the growth of the U.S. economy (up 2.4 percent).(1) Three principal sources contributed to the growth in U.S. greenhouse gas emissions:
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 as a measure of radiative forcing. This concept, developed by the Intergovernmental Panel on Climate Change (IPCC),(2) provides a comparative measure of the impacts of different greenhouse gases on global warming, with the effect of carbon dioxide being equal to 1 (see "Units for Measuring Greenhouse Gases"). The GWPs for other greenhouse gases are considerably higher (see discussion in Chapter 1). Overall, GWP-weighted emissions rose by 8.3 percent between 1990 and 1996 and by 3.4 percent between 1995 and 1996. On a GWP-weighted basis, carbon dioxide emissions account for 85 percent of U.S. greenhouse gas emissions (Figure ES1). While carbon dioxide emissions are growing, methane and nitrous oxide emissions have been roughly stable.
Table ES2 excludes several radiatively important gases: the criteria pollutants carbon monoxide, nitrogen oxides, and
particulates, as well as CFCs and hydrochlorofluorocarbons (HCFCs). These gases have ambiguous effects on climate,
which are difficult to quantify. In addition, CFCs and HCFCs are specifically excluded from coverage under the
international climate treaty, the Framework Convention on Climate Change. (See Chapters 1, 5, and 6 for discussion
related to the effects and emissions of these gases.)
The historical emissions estimates for the years 1989 through 1995 presented in this report are only slightly
revised from those in last year's report (see the box "What's New in This Report").
Some 98.5 percent of U.S. anthropogenic carbon dioxide emissions come from the combustion of fossil fuels. Changes
in carbon dioxide emissions can be traced to energy consumption trends and changes in the composition of fossil fuels
burned to provide energy services. During the 1980s and early 1990s, the energy intensity of the U.S. economy and the
carbon intensity of U.S. energy consumption steadily declined (Figure ES2).
Several unrelated factors caused the decline:
Currently, however, the growth in nuclear power generation has leveled off, and it is unlikely that future hydroelectric
generation will often match 1996 levels. World oil prices remain relatively low, and the U.S. economy is growing
rapidly.
Severe weather conditions in 1996 produced a series of anomalous results: residential and commercial natural gas
consumers used 7.8 percent more natural gas and 3.5 percent more electricity than in 1995, and natural gas prices
increased sharply. In response to the price signals, electric utilities reduced their gas consumption by 15 percent and
substituted coal. The result was a sharp increase in both total carbon emissions and emissions per kilowatthour for the
electric utility sector, accompanied by rapid increases in both direct (from natural gas and heating oil) and indirect
(from electricity) emissions from the residential and commercial sectors. Emissions from the industrial and
transportation sectors increased by a "more normal" 2.6 percent and 2.3 percent, respectively, in 1996 (Figure ES3).
Methane emissions estimates are more uncertain than those for carbon dioxide. 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. Estimated 1996 methane emissions are unchanged from emissions in 1995 (Figure ES4).
Methane emissions rose during the late 1980s. The principal cause appears to have been increased production from
a group of underground coal mines with very high rates of methane emissions. Meanwhile, emissions from municipal
landfills appear to have been stable, because growth in the volume of solid waste generated was offset by a growing
volume of waste burned for energy recovery and by increased recovery of methane at landfill sites.
In the 1990s, several factors have tended to limit or reduce estimated methane emissions:
Nitrous oxide emissions estimates are more uncertain than estimates of methane emissions, and the uncertainty of the
estimation methods makes it difficult to be confident of apparent trends. The principal sources are believed to be
"excess" emissions from agricultural soils associated with fertilizer use, industrial process emissions, and emissions
from combustion of fossil fuels. Nitrous oxide emissions, estimated at 0.45 million metric tons in 1996, are essentially
unchanged since 1990. Declining agricultural emissions (due to reduced use of nitrogen fertilizers) have offset slowly
increasing energy-related and industrial emissions (Figure ES5).
Halocarbons and related compounds include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and other compounds that act as greenhouse gases. Halocarbons have many uses, but most emissions come from their use as refrigerants in cooling equipment, as solvents, or as blowing agents and from fugitive emissions in industrial processes.
CFCs are currently being phased out because they damage the stratospheric ozone layer. The warming effects of CFCs
and HCFCs are offset to some extent because they also destroy ozone, which is a potent greenhouse gas. Compounds
that contain no chlorine (such as HFCs and PFCs) do not affect ozone, and their effects on climate are therefore easier
to measure.
The available data suggest that emissions of CFCs-- about 0.2 million metric tons in 1990--are declining. Estimated
HCFC emissions (almost entirely HCFC-22, a popular refrigerant for home air conditioners) have been largely stable
since 1993. There is little information about emissions of "new" HCFCs, such as HCFC-141b and HCFC-142b, which
are CFC substitutes.
HFC emissions were very low--perhaps 0.006 million metric tons--in 1990. Emissions of HFC-23, a byproduct of HCFC-22 production, have also been roughly stable since 1993. Emissions of the CFC substitutes HFC-134a and HFC-152 have risen substantially in the past 3 years, from a base total of less than 0.001 million metric tons in 1990 (Figure ES6). HFC-134a became the standard automobile air conditioner refrigerant in 1994, and emissions will grow rapidly as CFCs are replaced throughout the automobile fleet. Consumption of HFC-152 is growing rapidly, but it has a relatively low global warming potential of 140.
The principal quantifiable source of PFCs is as a fugitive emission from aluminum smelting. Aluminum smelting
rebounded in 1996, after declining in the early 1990s, increasing the estimated emissions of PFCs. In recent years,
several PFCs have found markets in the semiconductor industry, but it has proved difficult to obtain reliable
information about sales and consumption, other than that the numbers are relatively small.
Another compound included in this category is sulfur hexafluoride, which is used primarily as an insulating gas in
electrical switchgear. The amounts used are uncertain but appear to be quite small (around 1,000 metric tons per year);
however, sulfur hexafluoride has an extremely high global warming potential (around 25,000), and, hence, even small
emissions have disproportionate consequences.
Criteria pollutants (carbon monoxide, nitrogen oxides, and nonmethane volatile organic compounds) are reactive gases
that usually decay quickly in the atmosphere. They are not necessarily greenhouse gases themselves, but they can
promote atmospheric chemical reactions that create tropospheric ozone, which is a potent greenhouse gas. Because
the ozone-creating effect of these gases varies with local atmospheric conditions,
it is not possible to compute their effects directly. As 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, national-level emissions of carbon monoxide
have been declining since the late 1970s (Figure ES7). Emissions of nonmethane volatile organic
compounds and nitrogen oxides have been essentially unchanged in recent years.
Changes in land use can also have large, though difficult to quantify, effects on atmospheric concentrations of greenhouse gases. 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 in U.S. forests suggest that in the late 1980s and early 1990s, some 111 to 238 million metric tons of carbon was sequestered annually, equivalent to about 8 to 17 percent of U.S. anthropogenic carbon emissions.(3) However, considerable uncertainty is associated with this estimate--particularly with the amount of carbon sequestered in forest soils.
The IPCC recommends including emissions and sequestration from land use changes in national inventories, and the U.S. "National Communication" for the Framework Convention follows this practice.(4) The EIA, however, has elected not to include carbon sequestration from forestry in its "total" estimate of U.S. emissions, for the following reasons:
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