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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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About This Report 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 sixth annual update, covering national emissions over the period 1990-1996, with preliminary estimates of emissions for 1997. The estimates contained in this report have been revised from those in last year's report (see "What's New in This Report," page 3). 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," page 6) but can be converted to carbon equivalents using the factors provided in this report. 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, especially the third Conference of the Parties to the Framework Convention on Climate Change, which was held in December 1997 in Kyoto, Japan. Chapters 2 through 5 cover emissions of carbon dioxide, methane, nitrous oxide, halocarbons and related gases, respectively. Chapter 6 describes potential sequestration and emissions of greenhouse gases as a result of land use changes. Six 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 excessive uncertainty. Appendix E provides a historical time series of U.S. carbon emissions. Appendix F 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 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 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.(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 a host of engineered chemicals, such as hydrofluorocarbons (HFCs). 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). 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 Intergovernmental Panel on Climate Change (IPCC), an international assemblage of scientists commissioned by the United Nations to study this matter, concluded that:
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 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 absorption of greenhouse gases. Global Sources 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 (human-made) 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 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.(3) 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).(4) 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 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.(5) 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 4 million metric tons. Halocarbons and Other Gases. In the twentieth century, human ingenuity has 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 of these 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 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. Beyond the halocarbons (CFCs, HFCs, and HCFCs) there are a range of engineered chemicals, produced in relatively small quantities, that also have direct radiative forcing effects. These include halocarbons such as perfluorocarbons (PFCs), which include CF4, C2F6, and C3F8. PFCs are emitted as byproducts of aluminum smelting. Other chemicals such as sulfur hexafluoride (SF6), used as an insulator in utility-scale electrical equipment, are not halocarbons but are potent greenhouse gases. Possibly, other chemicals not yet identified could exhibit similar radiative properties. All of these chemicals are potent greenhouse gases, because they combine relative scarcity in the atmosphere with very long atmospheric lifetimes. Finally, 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. Criteria Pollutants That Affect Climate. There are three gases, emitted primarily as byproducts of combustion (both of fossil fuels and of biomass), that have indirect effects on global warming: carbon monoxide, nitrogen oxides, and nonmethane volatile organic compounds (NMVOCs). These compounds, regulated in the United States 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. 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.(6) 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 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. The criteria pollutants are included in this report for completeness. Aerosols. Finally, there is a class of gases that probably exert a net cooling effect on the climate. These compounds create 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. Units for Measuring Greenhouse Gases In this publication, the EIA has elected to report information in forms that are most likely to be familiar to users of the document. Therefore, energy and industrial data are reported in their native units (usually international units). Oil production is reported in thousand barrels per day, and energy production and sales are reported in (higher heating value) British thermal units (Btu). Emissions data are reported in metric units. We have attempted to bridge the gap between users of metric units and international units by using the familiar "million metric tons" common in European industry instead of the "gigagrams" favored by the scientific community. Emissions of most greenhouse gases are reported here in terms of the full molecular weight of the gas (as in Table ES1). In Table ES2, however, and subsequently throughout the report, carbon dioxide is reported in carbon units, defined as the weight of the carbon content of carbon dioxide (i.e., just the "C" in CO2). Carbon dioxide units at full molecular weight can be converted into carbon units by dividing by 44/12, or 3.67. This approach has been adopted for two reasons:
While carbon dioxide emissions can be measured in tons of carbon, emissions of other gases (such as methane) can also be measured in "carbon dioxide equivalent" units by multiplying their emissions (in metric tons) by their global warming potentials. For comparability, carbon dioxide equivalent units can be converted to "carbon equivalent" by multiplying by 12/44 (as in Table ES2) to provide a measure of the relative effects of various gases on climate. 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.(7)It would be useful to determine the precise relative effectiveness of various greenhouse gases in affecting the Earth's climate. That 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. 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 1994(8) and updated in Climate Change 1995.(9) 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. 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 the net GWPs for CFCs and HCFCs are considerably lower than their direct GWPs.(10) Global Climate Change Policy 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 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 adoption, by more than 160 countries, including the United States, of the Framework Convention on Climate Change, opened for signature at the "Earth Summit" in Rio de Janeiro, Brazil on June 4, 1992.(11) From the Framework Convention to the Kyoto Protocol The objective of the Framework Convention is stated as follows:
To achieve this objective, 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.(13) The Convention requires Annex I parties to the convention to undertake "policies and measures" to limit emissions of greenhouse gases, but there are no compulsory targets and no compulsory measures. It does require Annex I signatories to prepare national emissions inventories and to report to the Secretariat on the actions taken "with the aim of returning . . . to their 1990 levels these anthropogenic emissions of carbon dioxide and other greenhouse gases" (Article 4.2b). 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 quantitative limitations on emissions for the parties, to be signed at COP-3, which was to be held in Kyoto, Japan, in December 1997. The Kyoto Protocol The Kyoto Protocol to the Framework Convention, adopted on December 11th, is the most ambitious and far-reaching international environmental agreement ever attempted.(14) Its most fundamental feature 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 these countries taken as a group. Developing countries signatories do not have quantified targets. Some of the key features of the Protocol are summarized below: Differentiated Targets. Each Annex I signatory has a "quantified emissions reduction limitation commitment," which limits the signatory to some fraction, ranging from 90 to 110 percent, of its 1990 greenhouse gas emissions. Both the European Union (EU) and the individual members of the EU signed the Protocol and are both individually and collectively responsible for meeting their commitments. A Commitment Period. Each target is defined as the average of the signatory's emissions over the 5-year period 2008-2012, called "the commitment period." Six Gases. Participants are to limit their emissions of carbon dioxide, methane, nitrous oxide, HFCs, PFCs, and sulfur hexafluoride, weighted by the global warming potential of each gas. HFCs and PFCs are actually classes of gases with multiple members, but the term "six gases" has stuck. Participants may use 1995 as the baseline for HFCs, PFCs, and SF6, instead of 1990. Demonstrable Progress. Annex I countries are required to have made "demonstrable progress" toward achieving their commitments by 2005. Land Use and Forestry. The Protocol includes complicated provisions on forestry, the implication being that some emissions and sequestration arising from changes in land use and forestry since 1990 can be counted against the target. Flexibility Mechanisms. The Protocol includes an array of flexibility mechanisms described at greater length below: "emissions trading," "joint implementation," "joint fulfillment," and the "Clean Development Mechanism." Entry into Force. The Protocol enters into force when 55 countries and Annex I signatories with carbon dioxide emissions totaling 55 percent of total Annex I emissions "have deposited their instruments of ratification, acceptance, approval, or accession." As of August 25, 1998, 50 countries (including 25 Annex I countries) had signed (not ratified) the Protocol. No country has ratified the Protocol to date. The Kyoto Protocol and the United States The U.S. Government, in the form of the Executive Branch, has "agreed to" (but not yet signed) the Kyoto Protocol and is actively negotiating the various subsidiary arrangements necessary to fully define the agreement.(15) The Senate, with a constitutional duty to "advise and consent" to the international undertakings of the United States, placed certain conditions (in advance) on the kind of instrument to which it would be prepared to consent. Those conditions were embodied in the Byrd-Hagel Resolution (S.Res. 98) a "sense of the Senate" resolution, which was passed by a vote of 95 to 0 on July 25, 1997. The Byrd-Hagel resolution specifies that:
The Protocol has not yet been sent to the Senate. U.S. emissions of greenhouse gases have increased at a compounded annual rate of 1.4 percent since 1990. In the context of a growing economy and low fossil fuel prices, similar increases are expected over the next 10 years. The EIA, in its "baseline" study of U.S. energy trends, projects that energy-related carbon dioxide emissions can be expected to increase at a 1.6-percent annual rate between 1996 and 2010, reaching a level of 1,803 million metric tons of carbon by 2010, some 550 million metric tons (34 percent) above the Kyoto target.(16) The same projection envisages a 2010 U.S. population some 23 percent larger than in 1996 (0.8-percent annual growth) and a gross domestic product some 47 percent greater (1.9-percent annual growth) than in 1996. The nature and cost of possible policy interventions that would help the United States meet the Kyoto targets are uncertain and controversial. The President's Council of Economic Advisors estimates that the costs of compliance would be lower than suggested by other analyses of purely domestic costs, because of the domestic and international "flexibility" provisions of the Kyoto Protocol.(17) The key flexibility provisions are:
Flexibility Provisions: Other Gases In the Kyoto Protocol, not all reductions in greenhouse gas emissions must be in terms of energy-related carbon dioxide. Unlike carbon dioxide, the other greenhouse gases are often fugitive emissions, not necessarily central to the workings of modern industrial economies. Nitrous oxide and methane are naturally occurring substances, ubiquitous in nature, and rarely regulated. The engineered chemicals are usually inert, nontoxic, and also unregulated. In some cases, emissions of other gases are correlated with energy-related carbon dioxide emissions, and limiting energy-related carbon dioxide emissions would also tend to limit emissions of the other gases. In an environment in which carbon dioxide emissions are significantly constrained, it may be possible to reduce emissions of the other gases. In the case of the United States, however, the other gases collectively account for only about 17 percent of total emissions. Thus, even large reductions in emissions of other gases would have a limited impact on total emissions, in the range of a few percent. Flexibility Provisions: Land Use and Forestry The forestry sections of the Kyoto Protocol are complex and difficult to understand. At the recent Bonn Meeting of the Subsidiary Body on Scientific and Technical Advice, the negotiators decided to commission the IPCC to prepare a special study with recommendations to the Conference of the Parties.(18) In general, the Kyoto Protocol provides that net "sinks" of carbon arising from land use changes and forestry in the 2008-2012 period (including those in the United States) can count as changes in carbon stocks (net sequestration), so long as the changes are "measurable and verifiable" and are limited to changes caused by anthropogenic "afforestation, deforestation, and reforestation since 1990."(19) As discussed in Chapter 6 of this report, much of the carbon sequestered through forestry and land use in the United States comes not from sequestering carbon in the form of trees (relatively easy to measure) but from sequestering carbon in roots and soil (much more uncertain). Table 4 offers a first approximation of the distinction, using the work of Birdsey and Heath.(20) According to this "business-as-usual" projection, the carbon stored in relatively easily measurable trees would account for 69 million metric tons of carbon, equivalent to about 5 percent of U.S. emissions. On the other hand, the harder-to-measure carbon stored in roots, soil, and forest floor is much larger: 123 million metric tons. Only a fraction of this pool would "count" toward the Kyoto target, however, because of the "afforestation, deforestation, and reforestation since 1990" clause of the treaty. In principle, the Protocol requires partitioning forest land into two categories: "subject to afforestation, deforestation, and reforestation since 1990," and undisturbed since 1990. Carbon accumulated on land in the "since 1990" category would count, but carbon in the "undisturbed" category would not. The relative sizes of the two categories would be difficult to derive from current forest statistics. As a first approximation, however, about 4 million acres (0.6 percent) of U.S. forest are harvested annually, and another 4 million acres are lost to wildfire. Thus, whether wildfires "count" as "anthropogenic deforestation" turns out to be an important question. Assuming that wildfires do not count, at most about 12 percent of U.S. forest land will have been harvested and replanted between 1990 and 2010. Carbon accumulation is not evenly distributed across U.S. forest lands, as Table 4 also illustrates. When projected sequestration is considered by ownership category, most of the harvested land will be in the "forest industry" category, although some portions of national forests are also harvested and replanted. Net sequestration in land owned by the forest industry is low precisely because the land is regularly harvested and reforested. Land in the "other public" and "other private" categories is probably rarely or never harvested and, consequently, might not "count" under the protocol, even if the land in these categories sequesters substantial amounts of carbon. Flexibility Provisions: Emissions Trading Among Annex I Countries The key flexibility provision of the Kyoto Protocol is the ability of Annex I countries to trade emissions among themselves, which would permit Annex I countries to meet their commitment amounts by buying and selling tons of greenhouse gas emissions. A crude calculation of the possible surpluses and deficits of the Annex I countries before any policy interventions occur can be made by using the projections of carbon dioxide emissions in the EIA's International Energy Outlook 1998,(21) and comparing them with the Kyoto targets (Table 5). Table 5 indicates that several countries of the former Soviet Union (Russia, Ukraine, and the Baltic Republics) might have targets in excess of their expected 2008-2012 emissions, even if they do nothing to limit future emissions. Such excess tons have been called "hot air." The size and ultimate disposition of these tons is one of biggest analytical complexities in determining the consequences of the Kyoto Protocol. Flexibility Provisions: Joint Fulfillment Under Article 4.1 of the Protocol, signatories may agree to fulfill their commitments jointly. This mechanism was designed for the European Union, but it is open to any group of Annex I countries that wish to use it. The group would have to commit itself in advance, at the time that the participants ratify the Protocol, and would have to develop a scheme for allocating the group target among the participants. In the case of the EU, the allocation was based on negotiation between EU member governments. Flexibility Provisions: Transfers of Project Reductions "Joint implementation" is a term of art, whose meaning has evolved with the passage of time. As originally intended, joint implementation was a mechanism by which discrete emissions reduction projects could be undertaken by private parties or governments outside their home countries. The U.S. Government hoped that joint implementation would evolve into a program in which emissions reductions "credits" would be exchanged for project financing. In the Kyoto Protocol, two parallel mechanisms implement the concept of joint implementation:
Flexibility Provisions: The Clean Development Mechanism The CDM would create a flow of "emissions reductions units" from projects executed outside the Annex I countries, thus allowing these countries, as a group, to make smaller reductions in their total emissions than would otherwise be the case. Unlike joint implementation, CDM projects would be able to generate tons as early as 2000, with cumulative reductions accrued between 2000 and 2007 applied to the 2008-2012 commitment period. Article 12.3(b) specifies that Annex I countries may use CDM tons ". . . to contribute to compliance with part of their . . . commitments . . .." Several restrictions apply:
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