The Energy Information Administration (EIA) is required by the Energy Policy Act of 1992 to prepare a report on
aggregate U.S. national emissions of greenhouse gases for the period 1987-1990, with annual updates thereafter. This
report is the fifth annual update, covering national emissions over the period 1989-1995, with preliminary estimates
of emissions for 1996. The estimates contained in this report have been revised from those in last year's report (see
"What's New in This Report" ). Emissions estimates for carbon dioxide are reported in metric tons of carbon;
estimates for other gases are reported in metric tons of gas (see "Units for Measuring Greenhouse Gases").
Chapter 1 of this report briefly recapitulates some background information about global climate change and the
greenhouse effect and discusses important recent developments in global climate change activities. Chapters 2 through
6 cover emissions of carbon dioxide, methane, nitrous oxide,
halocarbons, and criteria pollutants, respectively.
Chapter 7 describes potential sequestration and emissions of greenhouse gases as a result of land use changes.
Five appendixes are included with this report. Appendix A provides a detailed discussion of emissions sources,
estimation methods, and data requirements and sources. Appendix B describes the derivation of the carbon emissions
coefficients used for the inventory. Appendix C describes uncertainties in the emissions estimates.
Appendix D describes known emissions sources omitted from the main report because they are not deemed to be "anthropogenic,"
or because of extreme uncertainty. Appendix E provides some convenient conversion factors.
The Greenhouse Effect and Global Climate Change
The Earth is warmed by radiant energy from the Sun. Over time, the amount of energy transmitted to the Earth's
surface is equal to the amount of energy re-radiated back into space in the form of infrared radia-tion, and the
temperature of the Earth's surface stays roughly constant. However, the temperature of the Earth is strongly influenced
by the existence, density, and composition of the Earth's atmosphere. Many gases in the Earth's atmosphere absorb
infrared radiation re-radiated from the Earth's surface, trapping heat in the lower atmosphere. Without the natural
greenhouse effect, it is likely that the average temperature of the Earth's surface would be on the order of -19o Celsius,
rather than the +15o Celsius actually observed.(5) The gases that help trap the Sun's heat close to the Earth's surface are
referred to as "greenhouse gases." All greenhouse gases absorb infrared radiation (heat) at particular sets of
wavelengths.
The main greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and a host of engineered chemicals, such as chlorofluorocarbons (CFCs). Most greenhouse gases occur naturally. Water vapor is by far the most common, with an atmospheric concentration of nearly 1 percent, compared with less than 0.04 percent for carbon dioxide. Concentrations of other greenhouse gases are a fraction of that for carbon dioxide (Table 1).
It was recognized in the early 1960s that concentrations of carbon dioxide in the Earth's atmosphere were increasing
every year. Subsequently, it was discovered that atmospheric concentrations of methane, nitrous oxide, and many
engineered chemicals were also rising. As current concentrations of greenhouse gases keep the Earth at its present
temperature, scientists began to postulate that increasing concentrations of greenhouse gases would make the Earth
get even warmer.
In computer-based simulation models, rising concentrations of greenhouse gases nearly always produce an increase
in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather and in the level
of the oceans that might prove disruptive to current patterns of land use and human settlement, as well as to existing
ecosystems. To date, it has proved difficult to detect firm evidence of actual temperature changes, in part because the
normal temporal and spatial variations in temperature are far larger than the predicted change in the global average
temperature. Even when temperature changes are identified, it is not possible to be certain whether they are random
fluctuations that will reverse themselves or are the beginning of a trend. The possible effects of rising temperatures on
weather patterns are even more uncertain.
The most recent report of the Intergovernmental Panel on Climate Change (IPCC), an international assemblage of
scientists commissioned by the United Nations to study this matter, concluded that:
Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitudes and patterns of long-term variability and the time-evolving pattern of forcing by, and response to, changes in concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggests that there is a discernable human influence on global climate. (6)
While both the existence and consequences of human-induced global climate change remain uncertain, the threat of
climate change has put in train an array of efforts by the U.S. Government and governments abroad to find some
mechanism for limiting the risk of climate change and ameliorating possible consequences. To date, efforts have
focused on identifying levels and sources of emissions of greenhouse gases and on possible mechanisms for reducing
emissions or increasing absorption of greenhouse gases.
Most greenhouse gases have substantial natural sources in addition to human-made sources, and there are powerful natural mechanisms for removing them from the atmosphere. However, the continuing growth in atmospheric concentrations establishes that, for each of the major greenhouse gases, more gas is being emitted than is being absorbed each year: that is, the natural absorption mechanisms are lagging behind. Table 2 illustrates the relationship between anthropogenic and natural emissions and absorption of the principal greenhouse gases.
Water Vapor. Water vapor, as noted above, is the most common greenhouse gas present in the atmosphere. It is
emitted into the atmosphere in enormous volumes, through natural evaporation from oceans, lakes, and soils, and
returned to Earth in the form of rain andsnow. Water vapor is so plentiful in the atmosphere already that additional
emissions are unlikely to absorb any significant amount of infrared radiation. It is also likely that the amount of water
vapor held in the atmosphere is generally in equilibrium, and that increasing emissions of water vapor would not
increase atmospheric concentrations.(7) According to currently available information, anthropogenic water vapor
emissions at the Earth's surface are unlikely to be an important element in either causing or ameliorating climate
change.
Carbon Dioxide. Carbon is a common element on the planet, and immense quantities can be found in the atmosphere,
in soils, in carbonate rocks, and dissolved in ocean water. All life on Earth participates in the "carbon cycle," by which
carbon dioxide (CO2) is extracted from the air by plants and decomposed into carbon and oxygen, with the carbon
being incorporated into plant biomass and the oxygen released to the atmosphere. Plant biomass, in turn, ultimately
decays (oxidizes), releasing carbon dioxide back into the atmosphere, or storing organic carbon in soil or rock. There
are vast exchanges of carbon dioxide between the ocean and the atmosphere, with the ocean absorbing carbon from
the atmosphere and plant life in the ocean absorbing carbon from water, dying, and spreading organic carbon on the
sea bottom, where it is eventually incorporated into carbonate rocks such as limestone.
Records from Antarctic ice cores indicate that the carbon cycle has been in a state of imbalance for the past 200 years,
with emissions of carbon dioxide to the atmosphere exceeding absorption. Consequently, carbon dioxide
concentrations in the atmosphere have been steadily rising.
The most important natural sources of carbon dioxide are releases from the oceans (90 billion metric tons per year),
aerobic decay of vegetation (30 billion metric tons), and plant and animal respiration (30 billion metric tons).(8) Known
anthropogenic sources account for 7 billion metric tons of carbon per year. The principal anthropogenic source is the
combustion of fossil fuels, which accounts for about three-quarters of total anthropogenic emissions of carbon
worldwide. Natural processes--primarily, uptake by the ocean and photosynthesis--absorb substantially all of the
naturally produced carbon dioxide and some of the anthropogenic carbon dioxide, leading to an annual net increase
in carbon dioxide in the atmosphere of 3.1 to 3.5 billion metric tons.(9)
Methane. Methane (CH4) is also a common compound. The methane cycle is understood less well than is the carbon
cycle. Methane is released primarily by anaerobic decay of vegetation, by the digestive tracts of termites in the tropics,
and by several other lesser sources. The principal anthropogenic sources are leakages from the production of fossil
fuels, human-promoted anaerobic decay in landfills, and the digestive tracts of domestic animals. The main sources
of absorption are thought to be decomposition (into carbon dioxide) in the atmosphere and decomposition by bacteria
in soil. Known and unknown sources of methane are estimated to total about 600 million metric tons annually; known
sinks (i.e., absorption by natural processes) total about 560 million metric tons. The annual increase in methane
concentration in the atmosphere accounts for the difference of 35 to 40 million metric tons.
Nitrous Oxide. The sources and absorption of nitrous oxide (N2O) are much more speculative than those for other
greenhouse gases. The principal sources are thought to be bacterial breakdown of nitrogen compounds in soils,
particularly forest soils, and fluxesfrom ocean upwellings. The primary human-made sources are enhancement of
natural processes through application of nitrogen fertilizers, combustion of fuels, and certain industrial processes. The
most important sink is thought to be decomposition in the stratosphere. Worldwide estimated known sources of
nitrous oxide total 13 to 20 million metric tons annually, and known sinks total 10 to 17 million metric tons. The annual
increase in concentrations in the atmosphere is thought to total about 4 million metric tons.
Halocarbons and Other Chemicals. In the twentieth century, human ingenuity has produced an array of "engineered"
chemicals, not normally found in nature, whose special characteristics render them particularly useful. Some
engineered chemicals are also greenhouse gases. The best known class of greenhouse chemicals are the
chlorofluorocarbons (CFCs), particularly CFC-12, often known by its trade name, "freon-12." CFCs have many desirable
features: they are relatively simple to manufacture, inert, nontoxic, and nonflammable. Because CFCs are chemically
stable, once emitted, they remain in the atmosphere for hundreds or thousands of years. Because they are not found
in nature, these molecules absorb reflected infrared radiation at wavelengths that otherwise would be largely
unabsorbed, and they are potent greenhouse gases, with a direct radiative forcing effect hundreds or thousands of
times greater, gram-per-gram, than that of carbon dioxide.
Because of their long atmospheric lives, a portion of the CFCs emitted into the atmosphere eventually find their way into the stratosphere, where they can be destroyed by sunlight. This reaction, however, releases free chlorine atoms into the stratosphere, and the free chlorine atoms tend to destroy stratospheric ozone, which protects the surface of the Earth from certain wavelengths of potentially damaging solar ultraviolet radiation (ultraviolet radiation, for example, is one cause of human and animal skin cancers). The destruction of stratospheric ozone, notwithstanding its potential damage to living organisms, exerts a net cooling effect on the surface of the planet, making the net effects of CFCs on radiative forcing ambiguous.
The threat posed by CFCs to the ozone layer has caused the United States and many other countries to commit
themselves to phasing out the production of CFCs and their chemical cousins, hydrochlorofluorocarbons (HCFCs)
pursuant to an international treaty, the 1987 Montreal Protocol. As use of CFCs has declined, many related chemicals
have emerged as alternatives, including HCFCs and hydrofluorocarbons (HFCs). HCFCs are similar to CFCs, but they
are more reactive and consequently have shorter atmospheric lives, with less effect on the ozone layer and smaller
direct global warming effects. HCFCs are also being phased out, but over a much longer time scale.
HFCs have no chlorine, and consequently have no effect on the ozone layer, but they have potentially powerful direct
effects on climate. HFCs were rare before 1990, but in 1994 HFC-134a was adopted as the standard motor vehicle air
conditioning refrigerant in virtually all new cars made in America. Consequently, HFC emissions are now rising
rapidly, though from a negligible base.
Beyond the halocarbons (CFCs, HFCs, HCFCs, and PFCs) there are a range of engineered chemicals, produced in
relatively small quantities, which also have direct radiative forcing effects. These include the perfluorocarbons (CF4,
C2F6, and C3F8) emitted as byproducts of aluminum smelting; some industrial solvents such as carbon tetrachloride,
methyl chloroform, and methylene chloride; and other more obscure chemicals such as sulfur hexafluoride (SF6) and,
possibly, other chemicals not yet identified. Some of these compounds are regulated in the United States as ozone
depleters, or for toxicity, or both.
Criteria Pollutants. There are three gases, emitted primarily as byproducts of combustion (both of fossil fuels and of
biomass), which have an indirect effect on global warming: carbon monoxide, nitrogen oxides, and nonmethane volatile
organic compounds (NMVOCs). These compounds, regulated in the United Sates pursuant to the Clean Air Act, are
often referred to (along with particulates, lead, and sulfur dioxide) as "criteria pollutants." The criteria pollutants are
reactive compounds, and they tend to remain in the atmosphere for only hours or days. The sequence of reactions that
removes them from the atmosphere, however, tends to promote the formation of ozone (O3), a reactive and unstable
molecular form of oxygen. While ozone in the stratosphere protects life on Earth from ultraviolet radiation, ozone at
ground level at high concentrations causes respiratory distress in people and animals and also is, itself, a potent
(though short-lived) greenhouse gas.(10)
It has not proved possible to make a general determination of the contribution of criteria pollutants to global warming.
The reactions that produce ozone are strongly affected by the relative concentrations of various pollutants, the ambient
temperature, and local weather. Emissions of criteria pollutants can create very high, though localized, ozone
concentrations under favorable conditions (for example, a warm, sunny day combined with still air and low humidity)
and negligible concentrations under unfavorable conditions. The criteria pollutants are included in this report for
completeness.
Aerosols. Finally, there is a class of gases which probably exert a net cooling effect on the climate. These compounds exert a cooling effect by creating tiny solid particles (aerosols) in the atmosphere, which, in turn, act as nuclei for collections of water droplets and stimulate cloud formation. The clouds, in turn, reflect sunlight back into space, cooling the planet. The most important such gas is sulfur dioxide (SO2), which is emitted largely as a byproduct from the combustion of sulfur-containing fossil fuels, particularly coal. Sulfur dioxide reacts in the air to form sulfate compounds that are effective in promoting cloud formation. Sulfur dioxide emissions are regulated in the United States under the Clean Air Act and have declined considerably in recent years. Particulate emissions are also likely to exert a net cooling effect by promoting cloud formation.
Some greenhouse gases are more potent in affecting global temperatures than are others. As a result, comparable
increases in the concentrations of different greenhouse gases can have vastly different heat-trapping effects. Among
those identified, carbon dioxide is among the least effective as a greenhouse gas. Other compounds, on a gram-per-gram basis, appear to have much greater effects.(11)
It would be useful to determine the precise relative effectiveness of various greenhouse gases in affecting the Earth's
climate. This information would help policymakers know whether it would be more effective to concentrate effort on
reducing the very small emissions of powerful greenhouse gases, such as HFC-134a, or whether they should direct their
efforts to controlling the very large emissions of relatively ineffective gases, such as carbon dioxide.
There has been extensive study of the relative effectiveness of various greenhouse gases in trapping the Earth's heat.
This research has led to the development of the concept of a "global warming potential," or GWP. The GWP is intended
to illustrate the relative impacts on global warming of various gases, compared with that of carbon dioxide. Over the
past few years, the IPCC has conducted an extensive research program aimed at summarizing the effects of various
greenhouse gases through a set of GWPs. The results of that work were released in 1995 in an IPCC report, Climate
Change 1994(12) and updated in Climate Change 1995.(13)
The IPCC's work has established that the effects of various gases on global warming are too complex to permit them
to be easily summarized as a single number. The complexity takes several forms:
Despite such complexity, the scientific community is working to develop GWP approximations. Table 3 summarizes the consensus results of the most recent studies by scientists working on behalf of the IPCC, showing estimates of atmospheric lifetimes and global warming potentials across various time scales.
The IPCC has also devoted effort to the study of indirect and interaction effects of various gases-- particularly the indirect effects of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) on the ozone layer--compared with their direct radiative forcing effects. The IPCC presented GWP estimates for the first time in 1996 that quantified the direct and indirect effects of certain CFCs and HCFCs (Table 4). Certain chemicals (halon-1301 and carbon tetrachloride, for example) are now believed to exert a net cooling influence--i.e., to have a negative global warming potential. All of the net global warming potentials for CFCs and HCFCs are considerably lower than their direct GWPs. The authors of the IPCC report believe that the relative magnitudes of the net GWPs are fairly reliable, but that the absolute levels have an uncertainty of ±50 percent.(14)
Global Climate Change Policy Developments
Rising concentrations of carbon dioxide in the atmosphere were first detected in the late 1950s. Observations of
atmospheric concentrations of methane, nitrous oxide, and other gases began in the late 1970s. However, concern about
the effects of rising atmospheric concentrations of greenhouse gases remained largely the province of atmospheric
scientists and climatologists until the mid-1980s, when a series of internationalscientific workshops and conferences
began to move the topic onto the agenda of United Nations specialized agencies, particularly, the World Meteorological
Office.
The IPCC was established under the auspices of the United Nations in late 1988, to accumulate available scientific
research on climate change and to provide scientific advice to policymakers. A series of international conferences
provided impetus for an international treaty aimed at limiting the human impact on climate. In December 1990, the
United Nations established the Intergovernmental Negotiating Committee for a Framework Convention on Climate
Change (generally called the INC). Beginning in 1991, the INC hosted a series of negotiating sessions that culminated
in the signing, by more than 160 countries, including the United States, of the Framework Convention on Climate
Change in Rio de Janeiro on May 4, 1992.(15) The objective of the Framework Convention ("the Rio Treaty") was to:
. . . achieve . . . stabilization of the greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.(16)
The Framework Convention, as it emerged from the negotiations, was based on the concept of voluntary commitments
by signatories to take steps to implement the objectives of the Convention. These steps, as described in the treaty,
include national commitments to prepare and submit for review national action plans and periodic national emissions
inventories. Developed country signatories (including the United States), referred to in the language of the convention
as "Annexe I Countries," made additional commitments:(17)
Each of these Parties shall communicate . . . detailed information on its policies and measures . . . [to limit emissions of greenhouse gases] . . . with the aim of returning individually or jointly to their 1990 levels these anthropogenic emissions of carbon dioxide and other greenhouse gases not controlled by the Montreal Protocol. This information will be reviewed by the Conference of the Parties, at its first session and periodically thereafter . . . .(18)
The greenhouses gases "controlled by the Montreal Protocol" are CFCs and HCFCs, which are explicitly defined as
being outside the scope of the Framework Convention.
Pursuant to the requirement to "communicate . . . detailed information" on policies and measures, the outgoing Bush
Administration prepared a draft national action plan in December 1992.(19) On April 21, 1993 (Earth Day), President
Clinton committed the United States to stabilizing its emissions of greenhouse gases at 1990 levels by the year 2000.
The methods proposedby the Government to achieve this objective were described in the President's Climate Change
Action Plan, published in October 1993.(20) That document spells out a range of largely voluntary programs intended to
achieve the stabilization objective. More detail-oriented readers may wish to consult the Technical Supplement to the
Plan, published in early 1994, which spells out the assumptions underlying the Plan in greater detail.(21) Subsequent
developments are described in the U.S. Government's "national Communications" under the Framework Convention,
the most recent of which was released in July 1997.(22)
The Conference of the Parties is required to meet annually to discuss the implementation of the Framework
Convention, and to review countries' voluntary commitments to limit their emissions. The first such meeting was held
in Berlin in April 1995. By 1995, the signatories' focus had shifted from meeting emissions targets for the year 2000 to
the question of what steps would be taken beyond 2000.
At Berlin, the Conference of the Parties agreed on "the Berlin Mandate," which was an agreement "to begin a process
to enable it [the Conference of the Parties] to take appropriate action for the period beyond 2000 . . . through the
adoption of a protocol or another legal instrument"(23)--in other words, to negotiate a successor agreement to the
Framework Convention for the next decade. However, the Berlin Mandate accelerated the "two track" approach to
emissions limitation already in evidence in the Framework Convention. There were to be no new commitments for
developing countries: rather they were to be encouraged to implement their commitments under the Framework
Convention. Meanwhile, the Annexe I countries would move ahead with negotiating additional measures.
The past several years have been spent in considering the possible forms the successor agreement might take. The most
recent report of the IPCC has been cited by member governments, including the U.S. Government, as evidence that
motivates further measures to limit emissions of greenhouse gases. The second Conference of the Parties was held in
Geneva in July 1996. At that meeting, the Conference issued a "Ministerial Declaration" to the effect that the
Governments present would:
Instruct their representatives to accelerate negotiations on the text of a legally-binding protocol or other legal instrument to be completed in due time for adoption at the Third Session of the Conference of the Parties. The outcome should fully encompass the remit of the Berlin Mandate, in particular,
At the June 26, 1997, "Earth Summit+5" Conference held at the United Nations, President Clinton committed to three
initiatives aimed at curbing greenhouse gases: (1) The United States pledged $1 billion over a 5-year period to promote
energy efficiency and alternative energy sources in the developing world. (2) The Overseas Private Investment
Corporation will strengthen environmental guidelines in hopes of encouraging all U.S. companies to hold to more
stringent environmental standards. (3) In the United States, government will work with industry to install solar panels
on 1 million roofs by 2010.(25)
While the United States has yet to announce a specific target and timetable in anticipation of the Kyoto summit in
December, the European community has announced that it will support reductions that average 15 percent under 1990
levels by 2010, when the European countries are considered as a whole. The U.S. position stresses joint implementation
between developed and developing countries, emissions trading, and flexible approaches--positions that are not
popular with many developing and other developed countries. Thus, as of mid-year 1997, the United States and other
countries have yet to agree on binding protocols.Several factors inhibit quick progress toward agreement on fixed
stabilization commitments. One key area of concern relates to the prospective costs of pursuing a cap on carbon
emissions. Substitution away from carbon-based fuels will have economic costs that affect both energy-using and
energy-producing nations. There is considerable debate as to the magnitude of such costs.(26) Differences in views reflect
relative levels of optimism regarding opportunities for technological substitution; whether policy initiatives are
pursued in a national or international context; and whether and when all nations contributing to carbon emissions
pledge to pursue stabilization goals.
U.S. Emissions in an International Perspective
The United States is the world's largest single emitter of carbon dioxide, accounting for about 23 percent of energy-related carbon emissions worldwide. The U.S. share of methane and nitrous oxide emissions, although uncertain, is
likely to be much lower than its share of carbon dioxide emissions, as the principal sources of methane and nitrous
oxide emissions are more common outside than within the United States. In the case of halocarbons and other gases,
the U.S. share is likely to be considerably larger than 23 percent, because the use of cooling and refrigeration equipment
is probably much more pervasive in the United States than elsewhere in the world.
In recent decades, the carbon dioxide emissions of North America and Western Europe have been growing relatively
slowly. The worldwide growth in energy-related carbon dioxide emissions has come from rapid growth in the
developing world. The most striking development in the 1990s has been the rapid reduction in energy consumption
(and hence carbon emissions) in the countries of the former Soviet Union and Eastern Europe, where emissions
dropped by more than 20 percent between 1989 and 1992 and continued to decline through 1994. Emissions reductions
in former communist countries were sufficient to stabilize world energy-related carbon dioxide emissions at 1990 levels
through 1994, despite continuing rapid growth in the developing world.
The EIA provided a projection of worldwide carbon emissions estimates in its International Energy Outlook 1997.(27) That
projection suggests that the post-communist decline in energy consumption is a one-time phenomenon, and that energy
consumption in the former communist countries will "bottom out" in the next few years and begin to rise again. Since
the EIA also expects rapid growth in energy consumption in the developing world to continue, the prospect is for continued growth in world carbon emissions. As indicated in Figure 1, by 2015, if there is no change in policy, emissions
from China will equal or surpass those of the United States. Rapid growth in other developing countries will mean that
the total emissions, if unabated, will increase from 1990 levels of about 6 billion metric tons to almost 10 billion metric
tons by 2015. Of this increase, almost 3 billion metric tons is projected to come from the nonindustrialized world and
about 1 billion metric tons from the industrialized world.
The current situation has implications for possible reduction scenarios. There have been various proposals for
addressing greenhouse gas emissions in a global context. To date they focus on the industrialized world. Figure 2
presents total projected emissions worldwide for four scenarios: (1) business as usual; (2) 1990 stabilization by 2010;
(3) the European proposal of 15 percent below 1990 levels by 2010 (assumed for purposes of this analysis to be adopted
by all industrialized countries); and (4) the Alliance of Small Island States proposal of 20 percent below 1990 levels by
2005. All assume stabilization after the target year. None of the scenarios involves limits on emissions from developing
countries. Without such limits for the developing world, even in the most aggressive scenario--in which the
industrialized world would reduce its emissions by about 2 billion metric tons below the business as usual
projection--the total amount of carbon emitted annually into the atmosphere in 2015 would still be about 2 billion
metric tons higher than it was in 1990.
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