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U.S. Emissions of Greenhouse Gases in Perspective About This
Report Global Climate Change International Developments 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 seventh annual update, covering national emissions over the period 1990-1998, with preliminary estimates of emissions for 1999. The methods used by EIA to estimate national emissions of greenhouse gases are subject to continuing review. As better methods and information become available, EIA revises both current and historical emissions estimates (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”). Total national emissions estimates measured in carbon equivalents are shown in Table ES2. Chapter 1 of this report briefly summarizes some background information about global climate change and the greenhouse effect and discusses important recent developments in global climate change activities. Chapters 2 through 4 cover emissions of carbon dioxide, methane, and nitrous oxide, respectively. Chapter 5 focuses on emissions of engineered gases, including hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. Chapter 6 describes potential sequestration and emissions of greenhouse gases as a result of land use changes. 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 approximately equal to the amount of energy re-radiated back into space in the form of infrared radiation, 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 its atmosphere. Many gases in the Earth’s atmosphere absorb infrared radiation re-radiated from the 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.1 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 wavelengths. The most important greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several engineered gases, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). 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. The effect of human activity on global water vapor concentrations is considered negligible, however, and anthropogenic emissions of water vapor are not factored into national greenhouse gas emission inventories for the purposes of meeting the requirements of the United Nations Framework Convention on Climate Change (FCCC) or the Kyoto Protocol.2 Concentrations of other greenhouse gases, such as methane and nitrous oxide, are a fraction of that for carbon dioxide (Table 1). Table 1. Global Atmospheric Concentrations of Selected Greenhouse Gases Scientists recognized in the early 1960s that concentrations of carbon dioxide in the Earth’s atmosphere were increasing every year. Subsequently, they discovered that atmospheric concentrations of methane, nitrous oxide, and many engineered chemicals also were rising. Because 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 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 the beginning of a trend. The possible effects of rising temperatures on weather patterns are even more uncertain. The most recent report of the IPCC, an international assemblage of scientists commissioned by the United Nations to assess the scientific, technical, and socioeconomic information relevant for the understanding of the risk of human-induced climate change, concluded that:
While both the existence and consequences of human-induced global climate change remain uncertain, the threat of climate change has put in motion an array of efforts by the United States and other governments 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 sequestration of greenhouse gases. Global Sources of Greenhouse Gases Most greenhouse gases have natural sources in addition to human-made sources, and there are 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 (human-made) and natural emissions and absorption of the principal greenhouse gases. Table 2. Global Natural and Anthropogenic Sources and Absorption of 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 is returned to Earth in the form of rain and snow. Water vapor is so plentiful in the atmosphere 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.4 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 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).5 Known anthropogenic sources (including deforestation) were estimated to account for about 7 billion metric tons of carbon per year in the early 1990s. The principal anthropogenic source is the combustion of fossil fuels, which accounts for about three-quarters of total anthropogenic emissions of carbon dioxide worldwide. Natural processes—primarily, uptake by the ocean and photosynthesis—absorb substantially all 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.6 Methane. Methane is also a common compound. The methane cycle is less well understood than the carbon cycle. Natural 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 processes 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 are much more speculative than those for other greenhouse gases. The principal natural sources are thought to be bacterial breakdown of nitrogen compounds 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 existence of significant unidentified sources of nitrous oxide has set off a search for new sources, one consequence of which has been recent revisions to the IPCC emissions estimations methods for nitrous oxide from nitrogen fertilization of soils. 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 3 to 5 million metric tons. Halocarbons and Other Gases. During the twentieth century, human ingenuity created 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 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 combine with stratospheric ozone, which protects the surface of the Earth from certain wavelengths of potentially damaging solar ultraviolet radiation (ultraviolet radiation, for example, causes 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. Another new class of engineered halocarbons are the perfluorocarbons (PFCs), which include CF4, C2F6, and C3F8. PFCs are emitted as byproducts of aluminum smelting and are increasingly being used in the manufacture of semiconductors. Sulfur hexafluoride (SF6) is used as an insulator in utility-scale electrical equipment and as a cover gas in magnesium smelting. It is not a halocarbon, but it is a powerful greenhouse gas. Other chemicals not yet identified may exhibit similar radiative properties. All of these chemicals are potent greenhouse gases because they combine high radiative properties with very long atmospheric lifetimes. The solvents carbon tetrachloride, methyl chloroform, and methylene chloride are regulated in the United States both as ozone depleters and for toxicity. All these gases have direct radiative forcing effects, which are offset to some degree by their ozone-depleting effects. With the advent of the Framework Convention and the Kyoto Protocol, the halocarbon and other industrial chemicals can be grouped into two categories:
The “Kyoto Protocol gases” are deemed to “count” for climate change policy analysis. The ozone depleters generally are excluded, because they are dealt with elsewhere. Radiatively Important Gases. In addition there are a set of man-made pollutants, emitted primarily as byproducts of combustion (both of fossil fuels and of biomass), that have indirect effects on global warming: carbon monoxide, nitrogen oxides, nonmethane volatile organic compounds (NMVOCs), and sulfur dioxide. These compounds, regulated in the United States pursuant to the Clean Air Act, are often referred to (along with particulates and lead) 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 carbon monoxide, nitrogen oxides, and NMVOCs from the atmosphere, however, tends to promote the formation of ozone (O3), a reactive and unstable molecular form of oxygen. Ozone in the stratosphere protects life on Earth from ultraviolet radiation, but ozone at ground level in high concentrations causes respiratory distress in people and animals and, also, is itself a potent (though short-lived) greenhouse gas.7 It has not proved possible to make a general determination of the contribution of ozone precursors to global warming. The reactions that produce ozone are strongly affected by the relative concentrations of various pollutants, the ambient temperature, and local weather conditions. 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. Sulfur dioxide, on the other hand, probably exerts a net cooling effect on the climate. Sulfur dioxide creates 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 their concentrations have declined considerably in recent years. Particulate emissions are also likely to exert a net cooling effect by promoting cloud formation. Relative Forcing Effects of Various Gases 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.8 There has been extensive study of the relative effectiveness of various greenhouse gases in trapping the Earth’s heat. Such 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 19949 and updated in Climate Change 1995.10
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. For the purposes of calculating “carbon dioxide equivalent” units for this report, 100-year GWPs are used. Table 3. Numerical Estimates of Global Warming Potentials Compared With Carbon Dioxide The IPCC has also devoted effort to the study of indirect and interactive effects of various gases—particularly the indirect effects of CFCs and 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. Certain chemicals (halon-1301 and carbon tetrachloride, for example) are now believed to exert a net cooling influence—i.e., to have a negative GWP. All the net GWPs for CFCs and HCFCs are considerably lower than their direct GWPs.11 Global Climate Change International Developments Rising concentrations of carbon dioxide in the atmosphere were first detected in the late 1950s, and observations of atmospheric concentrations of methane, nitrous oxide, and other gases began in the late 1970s. Concern about the effects of rising atmospheric concentrations of greenhouse gases remained largely the province of atmospheric scientists and climatologists, however, 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 Organization (WMO). The IPCC was established under the auspices of the United Nations Environment Program and the WMO 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 (INC) for a Framework Convention on Climate Change. Beginning in 1991, the INC hosted a series of negotiating sessions that culminated in the adoption, by more than 160 countries, including the United States, of the Framework Convention on Climate Change (FCCC), opened for signature at the “Earth Summit” in Rio de Janeiro, Brazil on June 4, 1992.12 From the Framework Convention to the Kyoto Protocol The objective of the Framework Convention is stated as follows:
The Framework Convention divided its signatories into two groups: the countries listed in Annex I to the Protocol, and all others. The Annex I countries are developed industrial states: the United States, Eastern and Western Europe, Russia and the Ukraine, Japan, Australia, New Zealand, and Canada.14 The Convention requires all parties to undertake “policies and measures” to limit emissions of greenhouse gases, and to provide national inventories of emissions of greenhouse gases (Article 4.1a and b). Annex I parties are further required to take actions “with the aim of returning . . . to their 1990 levels these anthropogenic emissions of carbon dioxide and other greenhouse gases” (Article 4.2a and b). The signatories subsequently agreed that Annex I parties should provide annual inventories of greenhouse gas emissions. In April 1993, President Clinton committed to stabilizing U.S. emissions of greenhouse gases at the 1990 level by 2000, using an array of voluntary measures. In the following years, however, greenhouse gas emissions in the United States and many other Annex I countries continued to increase. The climate negotiators, continuing to meet as “the Conference of the Parties [to the Framework Convention]” (COP), took up the question of how to limit emissions in the post-2000 period, a topic on which the Framework Convention was silent. In 1995, COP-1, held in Berlin, Germany, agreed to begin negotiating a post-2000 regime. In 1996, COP-2, held in Geneva, Switzerland, agreed that the regime would encompass binding limitations on emissions for the parties, to be signed at COP-3, which was held in Kyoto, Japan, in December 1997. The most fundamental feature of the Kyoto Protocol to the Framework Convention, adopted on December 11, 1997, is a set of quantified greenhouse gas emissions targets for Annex I countries, which collectively are about 5 percent lower than the 1990 emissions of those countries taken as a group.15 Developing country signatories do not have quantified targets. Some of the key features of the Protocol are summarized below:
The Kyoto Protocol and the United States The U.S. Government formally signed the Kyoto Protocol on November 12, 1998. Under the U.S. Constitution, however, the Government may adhere to treaties only with the “advice and consent” of the Senate.17 The Protocol has not yet been sent to the Senate. The view of the Executive Branch was expressed by the White House Press Secretary:
Since the signing of the Kyoto Protocol, the signatories have continued to shape the “work in progress.” The Conference of the Parties met in November 1998 in Buenos Aires, Argentina (COP-4) and in October and November 1999 in Bonn, Germany (COP-5) and will meet again in November 2000 in The Hague, Netherlands (COP-6). The two “subsidiary bodies” of the Conference of the Parties, the Subsidiary Body for Implementation (SBI) and Subsidiary Body for Scientific and Technological Advice (SBSTA), meanwhile, met in advance of COP-6 in September 2000 in Lyon, France, to discuss such matters as land use changes, capacity building in developing countries, and “best practices” in policies. There are, however, still many implementation issues dividing Kyoto Protocol signatories—including “supplementarity,” the role of developing countries in meeting the Protocol’s goals, and what greenhouse gas mitigation credits should be granted for land use, land use changes, and forestry (LULUCF). The term “supplementarity” is derived from Article 17 of the Protocol, which in describing emissions trading says, “Any such trading shall be supplemental to domestic actions for the purpose of meeting quantified emission limitation and reduction commitments under that Article.” A similar sentence is included in Article 6 (Joint Implementation), and somewhat similar language (“. . . contribute to compliance . . .”) is included in Article 12 (Clean Development Mechanism). The European Union (EU) takes the position that “supplemental” in this context means that emissions reductions achieved through trading, joint implementation, and/or the Clean Development Mechanism should account for only a limited portion of national emissions reductions under the Protocol, and that this limit should be quantified and spelled out. The view of the U.S. Government is that no such limit was intended, and that imposing a “sublimit” would raise the cost of compliance without providing any environmental benefit. Another piece of unresolved business from Kyoto is the status of developing countries. Their status under the Kyoto Protocol itself is fairly clear: they do not have quantitative obligations, but they may participate in emissions reduction projects through the Clean Development Mechanism. The U.S. Government, however, has set “meaningful participation” by developing countries as a condition for submitting the Protocol to the Senate for ratification. At the Buenos Aires meeting, two developing countries, Argentina and Kazakhstan, agreed to take on voluntary emissions limitation targets. Bolivia has also subsequently agreed to take on voluntary targets. Kazakhstan, which applied for Annex I status in 1999, withdrew its application in 2000. Developing countries as a group, led by China and the so-called “Group of 77” (G77), have expressed opposition to the acceptance by any developing country of an emissions limitation target, even voluntarily, arguing that this is a violation of the principle of “common but differentiated” responsibilities articulated in the Framework Convention.19 Signatories also are divided as to the extent to which countries can meet their Protocol commitments through LULUCF projects. Because the extent to which a country can use LULUCF projects to meet its commitments will directly affect its compliance costs, the issue has engendered a division among signatory countries. Some countries with large carbon sinks in the form of forested lands, such as the United States, Australia, Canada, and Norway, strongly support the inclusion of LULUCF mechanisms for meeting national commitments. The EU, meanwhile, has attempted to limit the extent to which LULUCF can be used to meet the commitments, and the G77 countries have tried to limit LULUCF to human-induced activities. On August 1, 2000, the United States submitted its views on LULUCF to the FCCC in advance of COP-6, where further decisions on LULUCF will be made. The U.S. position proposes the inclusion of forest management, cropland management, and grazing land management as LULUCF mechanisms to capture carbon and meet future Kyoto commitments. The U.S. proposal includes a phase-in period during which LULUCF carbon removals would be discounted and only counted above a predetermined threshold level. If you would like to receive any information relating to any of our greenhouse gas reports via e-mail, click here and subscribe by entering your e-mail address.
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