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 1987-1992, with annual updates thereafter. This is the third annual update report, covering national emissions over the period 1987-1993, with preliminary estimates of U.S. carbon dioxide and halocarbon emissions for 1994.
Calculating national aggregate emissions (or "national inventories") of greenhouse gases is a recently developed form of intellectual endeavor. Greenhouse gas emissions are rarely measured directly or reported to statistical agencies. Thus, to prepare emissions inventories usually requires inferring emissions indirectly from information collected for other purposes. Both the available information and the inferences drawn may be of varying reliability.
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 appendices are included with this report. Appendix A describes the derivation of the carbon emissions coefficients used for the inventory. Appendix B describes uncertainties in emissions estimates. Appendix C contains tables extending some of the principal emissions estimates back to 1983, as well as some of the background data used to derive emissions estimates. Appendix D describes known emissions sources omitted from the main report due to definitions of "anthropogenic" or due to excessive uncertainty. Appendix E provides some convenient conversion factors.
Greenhouse gas emissions inventories were first prepared in the late 1970s by or for atmospheric scientists, who sought to determine and analyze the human contribution to rising atmospheric concentrations of greenhouse gases. Emissions were assigned to particular countries primarily as a matter of convenience, because underlying energy and industrial production data were organized at the national level. The accuracy of early estimates did not need to be particularly high, nor was there any requirement for detail. However, as climate change has shifted from being a matter of scientific debate to a topic of policy concern, both the methods of computing inventories and the level of detail that is desirable have changed.
The United Nations' Intergovernmental Panel on Climate Change (IPCC) has been asked to develop a methodology for national inventories that will provide comparable estimates across nations. The IPCC developed an initial draft methodology for preparing national inventories in 1991: a final document was published this year.(Note 1) The IPCC document itself is aimed at a range of countries with widely differing national statistical systems and widely varying capacities: consequently, the IPCC methods are designed to be applicable to countries with varying levels of sophistication. This report follows the IPCC guidelines to the extent that they are consistent with available U.S. data and national emissions sources.
All emissions inventories have inherent limitations in their accuracy and comparability. The first problem is the absence of any directly measured or reported information on greenhouse gas emissions, and the consequent necessity to infer emissions from available information. In the case of carbon dioxide, emissions are calculated by multiplying reported energy consumption by the estimated carbon content of fossil fuels. This is a fairly reliable estimate; both energy statistics and estimates of carbon content are probably accurate within a few percent. For methane and nitrous oxide, however, emissions are generally inferred by extrapolating experiments conducted on small number of samples across a large, national, sometimes heterogenous population. As a result, methane and nitrous oxide emissions estimates are much more uncertain than carbon dioxide emissions estimates, and they are more prone to large revisions as additional information becomes available. The reliability of emissions estimates for halocarbons and other gases varies considerably, depending in part on the degree of commercial and regulatory importance of each gas.
A second source of difficulty in preparing national emissions estimates lies in definitional questions. The international community has devoted considerable resources to developing common definitions and rules for the computation of economic statistics (particularly, trade and national accounts statistics), but the definition of common rules for emissions inventories has only just begun. In this report, emissions estimates that are affected by definitional questions include the following:
The composition of the Earth's atmosphere is a primary determinant of the planet's temperature, which in turn establishes the conditions and limits for all life on Earth. Without the heat-trapping properties of so-called "greenhouse gases," which make up no more than 1 or 2 percent of the Earth's atmosphere, the average surface temperature of the Earth would be similar to that of Mars: -60 degrees Fahrenheit (-16 degrees Celsius).
The main greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halocarbons (such as CFC-11 and CFC-12). With the exception of halocarbons, 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).
Table 1. Global Atmospheric Concentrations of Greenhouse Gases
Source: M. Prather et al., "Other Trace Gases and Atmospheric Chemistry," in Intergovernmental Panel on Climate Change, Climate Change 1994: Radiative Forcing of Climate Change (Cambridge, UK: Cambridge University Press, 1995), p. 80.
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 proven difficult to detect hard evidence of actual temperature changes, in part, because 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 the beginning of a trend. The possible effects of rising temperatures on weather patterns are even more uncertain.
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.
Table 2. Global Natural and Anthropogenic Sources and Absorption of Greenhouse Gases
Source: Summarized from ranges appearing in Intergovernmental Panel on Climate Change, Climate Change 1994: Radiative Forcing of Climate Change (Cambridge, UK: Cambridge University Press, 1995), pp. 41, 51, 86, and 90.
Carbon Dioxide. Carbon is an extremely 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, the carbon is incorporated into plant biomass, and the oxygen is 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 raining 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 (100 billion metric tons per year), aerobic decay of vegetation (30 billion metric tons), and plant and animal respiration (30 billion metric tons). 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, known and unknown, 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 3.2 to 3.6 billion metric tons.
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 sources and sinks of methane are estimated to total about 500 million metric tons each. The annual increase in methane concentrations in the atmosphere is estimated at 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 in soils, particularly forest soils, and fluxes from 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, 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 Miscellaneous 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." 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 occupy an infrared "window" that would otherwise be largely unoccupied, and they are potent greenhouse gases, with a direct radiative forcing effect hundreds or thousands of times that of carbon dioxide.
CFCs can be destroyed by sunlight. This reaction, however, releases free chlorine into the stratosphere, and the free chlorine tends to destroy stratospheric ozone, which protects the surface of the earth from solar ultraviolet radiation at wavelengths that are potentially damaging to plant and animal life (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. Recent research suggests that the negative indirect effects of CFCs during the 1980s may have been relatively small, and that the positive direct effect may have predominated.(Note 2)
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 pursuant to an international treaty, the 1987 Montreal Protocol. As emissions of CFCs have declined, many related chemicals have emerged as alternatives, including hydrochlorofluorocarbons (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. 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 HFCs) there are a range of engineered chemicals, produced in relatively small quantities, which also have direct radiative forcing effects. These include the perfluorocarbons (CF4 and C2F6) emitted as byproducts of aluminum smelting, some industrial solvents such as carbon tetrachloride, methyl chloroform, 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. Recent research suggests that some of the ozone-depleting solvents with low global warming potentials (such as methyl chloroform) may have had a net cooling effect on the global climate during the 1980s.(Note 3)
Criteria Pollutants. Finally, there are three gases, emitted primarily as a side effect 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 are regulated in the United Sates pursuant to the Clean Air Act, and are often referred to (along with particulates, lead, and sulfur dioxide) as "criteria pollutants." The criteria pollutants are highly 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.
It has not proven 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.
Some greenhouse gases are more potent at 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 least effective as a greenhouse gas. Considering only heat-absorption potential, one molecule of methane can have 24 times the effect on climate that one molecule of carbon dioxide has.(Note 4)
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 carbon tetrafluoride, or whether they should bend 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 demonstrate the relative impacts on global warming of various gases, compared with 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 this year in a new IPCC report, Climate Change 1994.(Note 5)
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 three forms:
Table 3. Numerical Estimates of Global Warming Potentials Relative to
Carbon Dioxide
(Carbon Dioxide = 1)
Lifetime Direct Effect for Time Horizons of
Gas (Years) 20 Years 100 Years 500 Years
Source: D.L. Albritton et al., "Trace Gas Radiative Forcing Indices," in J.T. Houghton et al., Climate Change 1994 (Cambridge, UK: Cambridge University Press, 1995), p. 222.
It is exceptionally difficult to determine unambiguously whether or not global warming is actually taking place, and it is even more difficult to determine the consequences of global warming for the Earth's climate. Finally, it is yet more difficult to determine how changes in climate may affect natural ecosystems and the human economy. The incontrovertible empirical observation is that global atmospheric concentrations of greenhouse gases are rising steadily, and have been for decades. Carbon dioxide concentrations in the atmosphere have been directly recorded, using consistent methods, since 1958, and concentrations of methane and nitrous oxide have been recorded for the past 15 years. Since the initial discovery that carbon dioxide concentrations in the atmosphere were increasing, scientists have exercised enormous ingenuity in pushing the record of atmospheric concentrations backward, using samples of "fossil air" trapped in ice cores from Greenland and the antarctic.
The long-run records indicate that both carbon dioxide and methane concentrations in the Earth's atmosphere stand at levels not previously attained (at least for any prolonged period) over the past 160,000 years. The growth in concentrations has occurred largely in the past 200 years, and especially since 1940.
Both the timing of the growth in concentrations, anomalous variations in observations between the northern and southern hemispheres, and observations of relative concentrations of isotopes in atmospheric carbon dioxide (fossil carbon has a different distribution of isotopes than carbon of contemporary biologic origin) all imply that the prime source for the growth in carbon dioxide concentrations is the combustion of fossil fuels in the northern hemisphere.
Thus, it was particularly striking when, as noted in last year's report, there was a sudden slowing in the growth rate of atmospheric concentrations of both carbon dioxide and methane over the period 1990-1993 (Figure 1).(Note 7) This slowing first began to appear in 1990 and has continued through 1993. In the early 1980s, carbon dioxide concentrations were growing by 0.4 percent per year (Figure 1). During the period 1990-1993, the growth of atmospheric concentrations slowed to less than 0.2 percent annually. While the level of energy-related carbon dioxide emissions dropped slightly (due to sharply reduced energy consumption in the former Soviet Union and Eastern Europe), the decline is far too small to account for the observed sharp drop in the growth of atmospheric concentrations. Consequently, either natural sources of emissions have also declined, or natural absorption has increased.
Figure 1. Annual Percentage Change in Atmospheric Concentrations of Carbon Dioxide, Methane, and Nitrous Oxide, 1984-1994
Source: C.D. Keeling and T.P. Whorf (1993), "Atmospheric CO2 Records from Sites in the SIO Air Sampling Network," pp. 18-28, and E.J. Dlugokencky, P.M. Lang, K.A. Massarie, and L.P. Steel (1994), "Atmospheric CH4 Records from Sites in the NOAA/CMDL Air Sampling Network," pp. 50-126, in T.A. Boden, D.P. Kaiser, R.J. Stepanski, and F.W. Stoss (eds.), Trends '93: A Compendium of Data on Global Change, ORNL/CDIAC-65 (Oak Ridge, TN: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, in press, 1994).
In 1994, the growth in carbon dioxide concentrations resumed its previous trend. The growth rate of methane concentrations continued to be depressed, however, leaving behind a scientific puzzle. Some scientists theorize that global cooling from the vast quantities of sulfate aerosols deposited in the atmosphere by the eruption of Mount Pinatubo, in the Philippines, has stimulated absorption mechanisms.(Note 8) However, this remains at best an educated guess, and the most that can be said is that the carbon cycle contains forces that are as yet poorly understood.
In the case of methane, recent isotope studies suggest that the reduction is more likely to have been caused by a reduction in emissions than by an increase in absorption. Declining methane concentrations observed at stations in the high arctic suggest that a sharp reduction in methane leakage from pipeline systems in the former Soviet Union may be a contributing cause.(Note 9)
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 international scientific workshops and conferences began to move the topic onto the agenda of United Nations specialized agencies, particularly, the World Meteorological Office. When the Montreal Protocol to control ozone-depleting substances was signed, in late 1987, a large cadre of scientists, diplomats, policymakers, and members of environmental groups who had participated in the development of the Ozone Treaty turned their attention to the issue of global climate change.
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.(Note 10) 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."(Note 11)
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.
The outgoing Bush Administration prepared a draft national action plan in December 1992.(Note 12) 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 proposed by the Government to achieve this objective were described in the President's Climate Change Action Plan, published in October 1993.(Note 13) 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.(Note 14)
The United States is the world's largest single emitter of carbon dioxide, accounting for about 23 percent of energy-related carbon emissions. 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 (Figure 2). The worldwide growth in energy-related carbon dioxide emissions has come from rapid growth in the developing world and in the former centrally planned economies. The most striking development of recent years 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 have continued to decline through 1993. Emissions reductions in former communist countries have been sufficient to stabilize world energy-related carbon dioxide emissions at 1990 levels through 1993, despite continuing rapid growth in the developing world and slow growth in the United States and Europe.
Figure 2. Energy-Related Carbon Emissions by Region, 1950-1993
Note: Pre- and post-1970 emissions estimates differ in several minor ways: East Germany is included in "Former Soviet Union/Eastern Europe" prior to 1970, and in "Western Europe" after 1970. The two estimates may also use slightly different definitions of energy consumption, countries included in each region, and emissions coefficients. Pre-1970 estimates also include natural gas flaring and cement production, whereas post-1970 estimates exclude these sources.
Source: Emissions prior to 1970 from T.A. Boden, R.J. Stepanski, and F.W. Stoss, Trends '91: A Compendium of Data on Global Change, ORNL/CDIAC-46 (Oak Ridge, TN: Oak Ridge National Laboratory, December 1991), pp. 379-429. Emissions after 1970 were estimated by the Energy Information Administration, based on world energy consumption as reported in Energy Information Administration, International Energy Annual, DOE/EIA-0219 (Washington, DC, various years).
This year, the EIA released a projection of worldwide carbon emissions estimates in its International Energy Outlook 1995.(Note 15) That projection indicates that the post-communist decline in energy consumption is a one-time phenomenon, and that energy consumption in these 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 worldwide carbon emissions.