Emissions of Greenhouse Gases in the United States 2003

1. U.S. Emissions of Greenhouse Gases: Background and Context

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 eleventh annual update, covering national emissions over the period 1990-2002, with preliminary estimates of emissions for 2003. 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. Emissions estimates for carbon dioxide are reported in metric tons of carbon dioxide; estimates for other gases are reported in metric tons of gas (see “Units for Measuring Greenhouse Gases,” in the Executive Summary, page xi) and in carbon dioxide equivalent units where appropriate. Estimates of total national emissions in carbon dioxide equivalent units are shown in Table ES2 on page x.

This introductory chapter briefly summarizes some background information about global climate change and the greenhouse effect and reviews 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.

What’s New

Measures of Greenhouse Gas Intensity

This year, to be consistent with other EIA publications, we are using chain-weighted 2000 dollars for all calculations involving gross domestic product (GDP). This affects greenhouse gas (GHG) intensity calculations, because economic values expressed in 2000 dollars are 6.9 percent higher than those expressed in 1996 dollars, which were used in last year’s report. The change to chain-weighted 2000 dollars does not affect calculations related to the rate of change in emissions intensity over time.

Methane

The data sources and methodologies used to estimate methane emissions from mobile sources for 1990 through 2003 have changed, resulting in revised emissions estimates. Data used to calculate vehicle miles traveled (VMT) for light-duty vehicles (passenger cars and light-duty trucks) during the years 1990-2000 are now based on the most recent Polk data on vehicle stocks, with VMT modified by the Oak Ridge National Laboratory (ORNL). The data for 1996-2000 are further adjusted to incorporate fleet-vehicle data, and the methodology adjusts for the aging population of vehicles by applying survival curves. These changes also apply to nitrous oxide emissions. For the years 2001 through 2003, methane emissions from mobile sources are estimated using data from fleet vehicles and econometrically modeled VMT, while also adjusting for the aging population of vehicles. The data sources replace those taken historically from subscription publications such as Wards Automotive Yearbook.

Estimates have also been revised to reflect the inclusion of methane emissions from organic decomposition in industrial landfills. Emissions from industrial landfills are estimated to equal 7 percent of emissions from municipal solid waste landfills based on a methodology developed by the U.S. Environmental Protection Agency (EPA).¹ Industrial landfills have been included because the use of more rigorously gathered data on landfilled municipal solid waste reduces the likelihood that emissions from industrial landfills will be double counted in estimates of methane emissions from municipal solid waste.

Nitrous Oxide

The data sources and methodologies used to estimate nitrous oxide (N₂O) emissions from mobile sources for 1990 through 2003 have been changed in the same way as described above for methane emissions from mobile sources. For the years 2001 through 2003, N₂O emissions from mobile sources are estimated using data from fleet vehicles and econometrically modeled VMT, while also adjusting for the aging population of vehicles. The data sources replace those taken historically from subscription publications such as Wards Automotive Yearbook.

Other Gases: HFCs, PFCs, and SF₆

The data presented in Chapter 5 for other gases— hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆)—are provided by the EPA. Revisions in historical emissions estimates for specific sources are explained below:

• Electricity Transmission and Distribution. The changes in calculations of emissions from electricity transmission and distribution include both a revised methodology and updated activity data for 1990 through 1998. The revised methodology accounts for the quantity of SF₆ that is recaptured and/or released from operating electrical equipment, rather than assuming all SF₆ was released at the time of its initial installation into the equipment. The methodology assumes that only 22.5 percent of the SF₆ is emitted at the time of its installation and the remaining 77.5 percent is emitted over a 30-year period. The activity data were updated in two ways: first, with the results of a survey of SF₆ manufacturers conducted in 2002 by the RAND Corporation; and second, with revised data received from participants in the EPA’s SF₆ Emission Reduction Partnership for Electric Power Systems, which led to updated regression equations and extrapolations to non-reporting partners. These revisions resulted in a decrease in estimated SF₆ emissions from electric power systems of 9.0 percent for 1990, 18.2 percent for 1998, and 3.3 percent for 2001; and an increase in estimated emissions of 1.4 percent in 2000.
• Magnesium Production and Processing. The emissions estimates in this report were revised to reflect new historical data supplied by participants in EPA’s SF₆ Emission Reduction Partnership for the Magnesium Industry. This change resulted in a decrease of about 0.5 percent in estimated SF₆ emissions for 2001.
• Substitution of Ozone-Depleting Substances. The EPA updated assumptions for its Vintaging Model pertaining to market trends in chemicals and chemical substitutes. These changes resulted in an average annual increase of 9.1 percent in HFC and PFC emissions for the period 1990 through 2001.
• Aluminum Production. In cooperation with the EPA’s Voluntary Aluminum Industrial Partnership program, participants provided additional smelter-specific information on aluminum production and emission factors. The new information resulted in a decrease in PFC emissions of 1.0 percent for 1990 through 1994, an increase of 12.0 percent for 2000, and a decrease of 4.0 percent for 2001.
• Semiconductor Manufacture. The EPA updated activity data provided by participants in EPA’s PFC Reduction/Climate Partnership for the Semiconductor Industry, as well as its PFC Emissions Vintaging Model (PEVM), which estimates emissions from entities that do not participate in the partnership program. The EPA also revised its methodology for estimating the historical distribution of emissions among HFCs, PFCs, and SF₆, by incorporating market information on sales volumes that was obtained in surveys. These updates resulted in an average decrease in emissions estimates from semiconductor manufacture of 9 percent for 1995 through 2001.

Land Use Issues

The U.S. Forest Service provides the data for net carbon dioxide fluxes due to changes in carbon stocks in forests, urban trees, agricultural soil, and landfilled yard trimmings. This year, updates are provided on treatment of land use issues under current climate change negotiations, scientific research detailing uncertainty in the ability of forest soils to store carbon, and the status of carbon dioxide capture and geologic storage technologies.

U.S. Emissions in a Global Perspective

It is estimated that U.S. energy-related carbon dioxide emissions in 2001 totaled 5,692 million metric tons.² To put U.S. emissions in a global perspective, total energy-related carbon dioxide emissions for the world in 2001 are estimated at 23,899 million metric tons, making U.S. emissions about 24 percent of the world total (Table 1). Emissions for the industrialized world (North America, Western Europe, and Industrialized Asia) in 2001 are estimated at 11,634 million metric tons, or about 49 percent of the world total, implying that U.S. emissions represent about 49 percent of the energy-related carbon dioxide emissions from the industrialized world. The remaining 51 percent of 2001 worldwide energy-related carbon dioxide emissions come from developing countries (9,118 million metric tons) and the former Soviet Union and Eastern Europe (3,148 million metric tons). By 2025, however, the U.S. share of total world emissions is projected to fall to 22 percent (8,142 million metric tons out of a global total of 37,124 million metric tons). The reason for the expected decline in the U.S. share is that energy-related carbon dioxide emissions from the developing countries are projected to increase at an annual rate of 2.7 percent, while emissions from the industrialized countries increase by 1.2 percent annually.

From an economic perspective, U.S. GDP in 2001 was $9,394 billion (in constant 1997 dollars), compared with an estimated global total GDP of $32,354 billion. Thus, the U.S. share of world economic output in 2001 was about 29 percent. In 2025, when the world’s total GDP is projected to be $65,574 billion, U.S. share is projected to be $18,881 billion, or still about 29 percent. In other words, the United States is projected to maintain its share of economic output while its share of global emissions shrinks.

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 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 -19^o Celsius, rather than the +14^o Celsius actually observed.³ 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 (human-made) 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 (UNFCCC) or the Kyoto Protocol.⁴ Concentrations of other greenhouse gases, such as methane and nitrous oxide, are a fraction of that for carbon dioxide (Table 2).

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 greenhouse gas 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, however, it has proven difficult to disentangle the human impact on climate from normal temporal and spatial variations in temperature on both a global scale and geologic timeframe. The most recent report of the Intergovernmental Panel on Climate Change (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, estimates that the global average surface temperature has increased by 0.6 ± 0.2^oC since the late 19th century.⁵ The IPCC goes on to conclude that: “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”⁶

In the aftermath of the IPCC report, the Domestic Policy Council, in May 2001 as part of its review of U.S. policy on climate change, requested that the National Academy of Sciences identify areas of uncertainty in the science of climate change, as well as review the IPCC report and summaries.⁷ The National Academy of Sciences commissioned the National Research Council to carry out this review. The National Research Council in issuing its findings appeared to agree with some of the IPCC conclusions, but also seemed to suggest that further work needs to be done in identifying the impacts of natural climatic variability and reducing the uncertainty inherent in climate change modeling. Among the National Research Council findings are the following:⁸

Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.

Because there is considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, current estimates of the magnitude of future warming should be regarded as tentative and subject to future adjustments (either upward or downward).

The committee generally agrees with the assessment of human-caused climate change presented in the IPCC Working Group I (WGI) scientific report, but seeks here to articulate more clearly the level of confidence that can be ascribed to those assessments and the caveats that need to be attached to them.

Greenhouse Gas Sources and Sinks

Most greenhouse gases have both natural and human-made emission sources. There are, however, significant natural mechanisms (land-based or ocean-based sinks) for removing them from the atmosphere. However, increased levels of anthropogenic emissions have pushed the total level of greenhouse gas emissions (both natural and anthropogenic) above the natural absorption rates for these gases. This positive imbalance between emissions and absorption has resulted in the continuing growth in atmospheric concentrations of these gases. Table 3 illustrates the relationship between anthropogenic and natural emissions and absorption of the principal greenhouse gases on an annual average basis during the 1990s.

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. The recent IPCC report, however, cites a possible positive feedback from increased water vapor formation due to increased warming caused by rising atmospheric CO₂ concentrations.⁹ Elevated atmospheric temperatures increase the water-holding capability of the atmosphere. According to some of the IPCC emission scenarios, higher water vapor content could double the predicted atmospheric warming above what it would be if water vapor concentration stayed constant. These scenarios, however, have an element of uncertainty due to the possible countervailing effect of increased cloud formation, which can act to cool the planet by absorbing and reflecting solar radiation or warm the planet through the emission of long-wave radiation. According to the IPCC, increases in atmospheric temperatures would not necessarily result in increased concentrations of water vapor, because most of the atmosphere today is undersaturated.

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. Because of its relative abundance, total carbon dioxide in the atmosphere has a radiative forcing value of 1.46 watts per square meter.¹⁰ (See page 11 for a discussion of radiative forcing.) According to the IPCC, before 1750, atmospheric carbon dioxide concentration was around 280 ± 10 parts per million for several thousand years. The IPCC goes on to say that the present carbon dioxide concentration has not been exceeded during the past 420,000 years, and likely not during the past 20 million years.¹¹

The most important natural sources of carbon dioxide are releases from the oceans (330 billion metric tons of carbon dioxide per year) and land (440 billion metric tons of carbon dioxide annually), including 220 billion metric tons of carbon dioxide from plant respiration, 202 billion metric tons of carbon dioxide from non-plant respiration (bacteria, fungi, and herbivores), and 15 billion metric tons of carbon dioxide from combustion of natural and human-made fires.¹² Known anthropogenic sources (including deforestation) were estimated to account for about 29 billion metric tons of carbon dioxide per year during the 1989 to 1998 time period.¹³ The principal anthropogenic source is the combustion of fossil fuels, which accounts for about 80 percent 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 11.4 to 12.1 billion metric tons.¹⁴ Recent evidence indicates there has been an increase in the rate of growth in atmospheric concentrations of carbon dioxide in the northern hemisphere. The text box on page 8 discusses factors that may have influenced this situation.

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 in wetlands, by the digestive tracts of termites in the tropics, by the ocean, and by leakage from methane hydrate deposits. 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. Anthropogenic sources are estimated to be 60 percent of total methane emissions.¹⁵ The main sources of absorption are thought to be tropospheric reactions with hydroxyl (OH) radicals that break down methane into the methyl radical CH₃ and water vapor (506 million metric tons), stratospheric reactions with hydroxyl radicals and chlorine (40 million metric tons), and decomposition by bacteria in soils (30 million metric tons). Known and unknown sources of methane are estimated to total 598 million metric tons annually; known sinks (i.e., absorption by natural processes) total about 576 million metric tons. The annual increase in methane concentration in the atmosphere accounts for the difference of 22 million metric tons.¹⁶ The radiative forcing of methane is 0.48 watts per square meter, about one-third that of carbon dioxide.¹⁷

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, fluxes from ocean upwellings, and stratospheric photo dissociation and reaction with electronically excited oxygen atoms. The primary human-made sources are enhancement of natural processes through application of nitrogen fertilizers, combustion of fuels (in fossil-fueled power plants and from the catalytic converters in automobiles), certain industrial processes (nylon and nitric acid production), biomass burning, and cattle and feedlots. Worldwide, estimated known sources of nitrous oxide total 16.4 million metric tons annually (6.9 million metric tons from anthropogenic sources), and known sinks total 12.6 million metric tons. The annual increase in concentrations in the atmosphere is thought to total 3.8 million metric tons.¹⁸ The radiative forcing of nitrous oxide is 0.15 watts per square meter, about one-tenth that of carbon dioxide.¹⁹

Halocarbons and Other Gases. During the 20th century, human ingenuity created an array of “engineered” chemicals, not normally found in nature, whose special characteristics render them particularly useful. One family of engineered gases is the halocarbons. A halocarbon is a compound containing either chlorine, bromine, or fluorine and carbon. Halocarbons are powerful greenhouse gases. Halocarbons that contain bromine or chlorine also deplete the Earth’s ozone layer. One of the best-known groups of halocarbons is 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 direct global warming potentials hundreds or thousands of times greater, gram-per-gram, than that of carbon dioxide. However, because of their relatively small concentrations in the atmosphere, their current levels of radiative forcing are low (see page 11 for a discussion of global warming and radiative forcing).

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 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 longer time scale. The ozone-depleting substances with the most potential to influence climate, CFC-11, CFC-12 and CFC-113, are beginning to show reduced growth rates in atmospheric concentrations in the aftermath of the Montreal Protocol. The present radiative forcing of CFC-11 is about 0.065 watts per square meter, and that of CFC-12 is around 0.2 watts per square meter.²⁰

HFCs have no chlorine and consequently have no effect on the ozone layer, but they are powerful greenhouse gases. The three most prominent HFCs in the atmosphere today are HFC-23, HFC-134a, and HFC-152a.

HFC-23 is formed as a byproduct of HCFC-22 production, which is being phased out under the Montreal Protocol. Although HFC-23 is very long-lived (260 years), the growth rate in its atmospheric concentration has begun to level off in accordance with reductions in HCFC-22 production. HFC-134a production was 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. HFC-134a has a lifetime of 13.8 years, and emissions have grown rapidly from near zero in 1990 to 0.034 million metric tons in 2002.²¹ HFC-152a emissions have risen steadily since about 1995, but its short lifetime of 1.4 years has kept concentration levels below 1 part per trillion.

Another new class of engineered halocarbons is the perfluorocarbons (PFCs), which include perfluoromethane (CF₄) and perfluoroethane (C₂F₆). PFCs are emitted as byproducts of aluminum smelting and are increasingly being used in the manufacture of semiconductors. They are powerful greenhouse gases and extremely long-lived. Perfluoromethane has a 100-year global warming potential (GWP) of 5,700 and a lifetime in excess of 50,000 years. Perfluoroethane has a GWP of 11,900 and a lifetime of 10,000 years. Perfluoromethane is a naturally occurring compound in fluorites, and emissions from this source create a natural abundance of 40 parts per trillion in the atmosphere. Increases in anthropogenic emissions, growing at about 1.3 percent annually, have raised atmospheric concentrations to 80 parts per trillion.²² Perfluoroethane does not occur naturally in the atmosphere, and current concentrations (3.0 parts per trillion) are attributable to anthropogenic emissions, which are growing by 3.2 percent annually. Sinks for PFCs are photolysis and ion reactions in the mesosphere.²³

Sulfur hexafluoride (SF₆) is used as an insulator in large-scale electrical equipment and as a cover gas in magnesium smelting. It is not a halocarbon, but it is a powerful greenhouse gas. SF₆ has a 100-year GWP of 22,200 and a lifetime of 3,200 years. Like perfluoromethane, SF₆ occurs naturally in fluorites, which produce a natural abundance of 0.01 parts per trillion in the atmosphere. Current atmospheric concentrations (3.0 parts per trillion) can be traced to anthropogenic emissions, which grew by approximately 7 percent annually during the 1980s and 1990s. Also like PFCs, sinks for SF₆ are photolysis and ion reactions in the mesosphere.

There may be other chemicals not yet identified that exhibit radiative properties similar to those of the halocarbons and other gases described above. One recent discovery identified trifluoromethyl sulfur pentafluoride (SF₅CF₃) as a new anthropogenic greenhouse gas in the atmosphere.²⁴ It is believed that SF₅CF₃ is created by the breakdown of SF₆ in high-voltage equipment, which produces CF₃ that reacts with SF₅ radicals resulting from high-voltage discharges. Its atmospheric concentration has grown from near zero in 1960 to 0.12 parts per trillion in 1999. To date, SF₅CF₃ has the largest radiative forcing on a per-molecule basis of any gas found in the atmosphere.²⁵ The UNFCCC does not yet specifically address this gas.

A number of chemical solvents are also strong greenhouse gases. The solvents carbon tetrachloride (GWP of 1,800 and lifetime of 35 years) and methyl chloroform (GWP of 140 and lifetime of 4.8 years), however, are regulated in the United States for the purposes of both ozone depletion and 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 United Nations Framework Convention and the Kyoto Protocol, the halocarbon and other industrial chemicals can be grouped into two categories:

• Ozone-depleting chemicals regulated under the Montreal Protocol but excluded from the Framework Convention (CFCs, HCFCs, and others)
• “Kyoto gases” (HFCs, PFCs, and SF₆).

The “Kyoto gases” are deemed to “count” for the purposes of meeting national obligations under the Framework Convention. The ozone depleters, however, are excluded from the Framework Convention because they are regulated by the Montreal Protocol.

Other Important Radiative Gases. There are a number of additional gases and particles, resulting in part from human sources, that produce radiative forcing of the Earth’s climate but are not included under the Framework Convention or the Montreal Protocol. In general, these gases are short-lived, they have only indirect climate effects, or there is a fair amount of uncertainty about their climatic impacts. They can be broken down into three general classes: (1) ozone, both tropospheric and stratospheric; (2) criteria pollutants that are indirect greenhouse gases; and (3) aerosols, including sulfates and black soot.

Ozone (O₃) is present in both the troposphere and the stratosphere. Tropospheric ozone is not directly emitted into the atmosphere but instead forms through the photochemical reactions of various ozone precursors (primarily, nitrogen oxides and volatile organic compounds). In the troposphere, ozone acts as a direct greenhouse gas. The lifetime of ozone in the atmosphere varies from weeks to months, which imparts an element of uncertainty in estimating tropospheric ozone’s radiative forcing effects. The IPCC estimates that the radiative forcing of tropospheric ozone is 0.35 ± 0.2 watts per square meter.²⁶ The depletion of stratospheric ozone due to the emission of halocarbons, on the other hand, has tended to cool the planet. The IPCC estimates that the cooling due to stratospheric ozone depletion is on the order of -0.15 ± 0.1 watts per square meter.²⁷ As the ozone layer recovers, however, due to the impacts of the Montreal Protocol, it is expected that stratospheric ozone will exert a positive radiative forcing effect on the Earth’s climate.

There are also a number of compounds (carbon monoxide, nitrogen oxides, and volatile organic compounds) that are indirect greenhouse gases. These gases are regulated in the United States pursuant to the Clean Air Act, and they are often referred to as “criteria pollutants.” They are emitted primarily as byproducts of combustion (both of fossil fuels and of biomass), and they influence climate indirectly through the formation of ozone and their effects on the lifetime of methane emissions in the atmosphere. Carbon monoxide, through its effects on hydroxyl radicals, can help promote the abundance of methane in the atmosphere, as well as increase ozone formation. Some IPCC model calculations indicate that 100 metric tons of carbon monoxide emissions is equivalent to the emissions of about 5 metric tons of methane.²⁸

Nitrogen oxides, including NO and NO₂, influence climate by their impacts on other greenhouse gases. Nitrogen oxides not only promote ozone formation, they also impact (negatively) methane and HFC concentrations in the atmosphere. The deposition of nitrogen oxides could also reduce atmospheric carbon dioxide concentrations by fertilizing the biosphere.²⁹

Volatile organic compounds (VOCs), although they have some short-lived direct radiative-forcing properties, primarily influence climate indirectly via their promotion of ozone formation and production of organic aerosols. The main sources of global VOC emissions are vegetation (primarily tropical) (377 million metric tons carbon equivalent), fossil fuels (161 million metric tons carbon equivalent), and biomass burning (33 million metric ton carbon equivalent).³⁰

Aerosols, which are small airborne particles or droplets, also affect the Earth’s climate. Aerosols have both direct effects, through their ability to absorb and scatter solar and thermal radiation, and indirect effects, through their ability to modify the physical properties and amount of clouds. In terms of climate change, the most prominent aerosols are sulfates, fossil fuel black carbon aerosols (sometimes called “black soot”), fossil fuel organic carbon aerosols, and biomass-burning aerosols.

One of the primary precursors of sulfates 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. The major source of anthropogenic black soot and organic carbon aerosols is the burning of fossil fuels, primarily coal and diesel fuels. Biomass-burning aerosols are formed by the incomplete combustion of forest products. The IPCC estimates the direct radiative forcing for aerosols as follows: sulfates, -0.4 watts per square meter; black soot, +0.2 watts per square meter; fossil fuel organic carbon, -0.1 watts per square meter; and biomass-burning aerosols, -0.2 watts per square meter.³¹ Although the indirect climate effects of aerosols are uncertain, some preliminary evidence points to an indirect cooling effect due to cloud formation.³²

Relative Forcing Effects of Various Gases

The ability of a greenhouse gas to affect global temperatures depends not only on its radiative or heat-trapping properties but also on its lifetime or stability in the atmosphere. Because the radiative properties and lifetimes of greenhouse gases vary greatly, comparable increases in the concentrations of different greenhouse gases can have vastly different heat-trapping effects. The cumulative effect (radiative forcing—measured in watts per square meter) can vary substantially from the marginal impact of a gas. For example, among the “Kyoto gases,” carbon dioxide is the most prominent in terms of emissions, atmospheric concentration, and radiative forcing (1.46 watts per square meter), but it is among the least effective as a greenhouse gas in terms of the marginal impact of each additional gram of gas added to the atmosphere. Other compounds, on a gram-per-gram basis, appear to have much greater marginal effects.

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 an additional unit of a given gas relative to carbon dioxide over a specific time horizon. 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 originally released in 1995 in an IPCC report, Climate Change 1994,³³ and subsequently updated in Climate Change 1995³⁴ and Climate Change 2001.³⁵ The box on page 12 provides details on the differences in emission calculations using the GWP values from the two assessments.

The calculation of a GWP is based on the radiative efficiency (heat-absorbing ability) of the gas relative to the radiative efficiency of the reference gas (carbon dioxide), as well as the removal process (or decay rate) for the gas relative to the reference gas over a specified time horizon. The IPCC, however, has pointed out that there are elements of uncertainty in calculating GWPs.³⁶ The uncertainty takes several forms:

• The radiative efficiencies of greenhouse gases do not necessarily stay constant over time (as calculated in GWPs), particularly if the abundance of a gas in the atmosphere increases. Each gas absorbs radiation in a particular set of wavelengths, or “window,” in the spectrum. In some cases, where concentrations of the gas are low and no other gases block radiation in the same window, small emissions of the gas will have a disproportionate absorptive effect. However, if concentrations of the gas rise over time, a larger and larger portion of the total light passing through the “window” will already have been captured, and the marginal effects of additional emissions will not be as large. Therefore, the effect of an additional unit of emission of a gas that is relatively plentiful in the atmosphere, such as water vapor or carbon dioxide, tends to be less than that of a rare gas, such as sulfur hexafluoride. This “diminishing return” effect implies that increasing the concentration of a particular gas reduces the impact of additional quantities of that gas. Thus, the relative impacts of various gases will change as their relative concentrations in the atmosphere change.
• The lifetime of a greenhouse gas (used in GWP calculations), particularly carbon dioxide, is also subject to uncertainty. Various natural processes cause many greenhouse gases to decompose into other gases or to be absorbed by the ocean or ground. These processes can be summarized in terms of the “atmospheric lifetime” of a particular gas, or the period of time it would take for natural processes to remove a unit of emissions from the atmosphere. Some gases, such as CFCs, have very long atmospheric lifetimes, in the hundreds of years. Others, such as carbon monoxide, have lives measured in hours or days. Methane, which decays into carbon dioxide over a period of a few years, has a much larger short-run effect on global warming than does an equivalent amount of carbon dioxide; however, over longer and longer periods—from 10 years to 100 years to 500 years, for example—the differences between the GWPs of methane and carbon dioxide become less significant, because carbon dioxide has a longer atmospheric lifetime than methane.

Table 4 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 “CO₂ equivalent” units for this report, 100-year GWPs are used.

The GWPs discussed above are direct GWPs in that they consider only the direct impact of the emitted gas. The IPCC has also devoted effort to the study of indirect GWPs. Indirect GWPs are based on the climatic impacts of the atmospheric decomposition of a gas into other gases. A number of gases—including methane, carbon monoxide, halocarbons, and nitrogen oxides—are thought to have indirect climatic effects. Methane indirectly influences the climate through ozone formation and the production of carbon dioxide. Carbon monoxide can promote ozone formation and extend the lifetime of methane in the atmosphere, which results in a positive indirect GWP. Some CFCs and HCFCs produce an indirect cooling effect by removing ozone from the stratosphere. The indirect cooling effect leads to lower net GWPs in a number of cases, but in most cases their net GWPs are still positive. Nitrogen oxides promote the formation of tropospheric ozone and, thus, have a positive indirect GWP—on the order of 5 for surface emissions and 450 for aircraft emissions.³⁷

The Bush Administration is pursuing a broad range of strategies to address the issues of global climate change through the implementation of multiple new initiatives. Details of these initiatives were initially provided on February 14, 2002, when the President announced the Global Climate Change Initiative. This initiative sets a national goal for the United States to reduce its greenhouse gas intensity (total greenhouse gas emissions per unit of GDP) by 18 percent between 2002 and 2012 through voluntary measures (see discussion on "Trends in U.S. Carbon Intensity and Total Greenhouse Gas Intensity").

To meet this goal and encourage the development of strategies and technologies that can be used to limit greenhouse gas emissions both at home and abroad, the Administration has implemented a number of related initiatives, including the following:³⁸

• Climate Change Technology Program (CCTP): The CCTP is a multi-agency program to accelerate the development and deployment of key technologies that can achieve substantial greenhouse gas emissions reductions. CCTP includes climate change-related technology research, development, and deployment efforts as well as voluntary programs.
• Climate Change Science Program (CCSP): The CCSP is a Federal, multi-agency research program to investigate natural and human-induced changes in the Earth’s global environmental system; to monitor, understand, and predict global change; and to provide a sound scientific basis for national and international decisionmaking.
• International Cooperation: The United States is engaged in international efforts on climate change, both through multilateral and bilateral activities. Multilaterally, the United States is the largest funder of activities under the UNFCCC and the IPCC. Since June 2001, the United States has launched bilateral partnerships with numerous countries on issues ranging from climate change science to energy and sequestration technologies to policy approaches. As an example, a new international effort to curb methane emissions is detailed in the text box below. Bilateral partners of the United States include Australia, Brazil, Canada, China, Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama, the European Union, India, Italy, Japan, Mexico, New Zealand, Republic of Korea, the Russian Federation, and South Africa.
• Near-Term Greenhouse Gas Reduction Initiatives: The Federal Government administers a wide array of voluntary, regulatory, and incentive-based programs on energy efficiency, agricultural practices, and greenhouse gas reductions. Major initiatives announced by the Bush Administration include:
• Climate VISION Partnership: In February 2003, President Bush announced that twelve major industrial sectors and the membership of the Business Roundtable had committed to work with the EPA and three Federal departments (Energy, Transportation, and Agriculture) to reduce greenhouse gas emissions in the next decade. Participating industries include electric utilities; petroleum refiners and natural gas producers; automobile, iron and steel, chemical and magnesium manufacturers; forest and paper producers; railroads; and the cement, mining, aluminum, lime, and semiconductor industries.
• Climate Leaders: Announced in February 2002, Climate Leaders is an EPA partnership encouraging individual companies to develop long-term, comprehensive climate change strategies. Under this program, partners set corporate-wide GHG reduction goals and inventory their emissions to measure progress. More than 50 major companies are now participating, including General Motors, Alcoa, BP, Pfizer, Staples, International Paper, IBM, Miller Brewing, Eastman Kodak, and Target.
• Voluntary Greenhouse Gas Reporting Program: Responding to President Bush’s February 2002 Global Climate Change Initiative, the Secretaries of Energy, Commerce, and Agriculture, and the EPA Administrator provided the President with their initial recommendations for enhancing and improving the Department of Energy’s (DOE’s) greenhouse gas emissions reduction registry. The improvements are intended to enhance the accuracy, reliability, and verifiability of greenhouse gas reductions measurements. Revised guidelines were released for public comment in late 2003, and DOE held a public workshop in January 2004 to receive further input from the public and potential users of the program.

International Developments in Global Climate Change

The primary international agreement addressing climate change is the UNFCCC, which opened for signature at the “Earth Summit” in Rio de Janeiro, Brazil, in June 1992 and entered into force in March 1994.³⁹ The agreement currently has 185 signatories, including the United States. The objective of the Framework Convention is stated as follows:

The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.⁴⁰

The Framework Convention divided its signatories into three groups: the countries listed in Annex I; Annex II, which comprises the Annex I countries minus the countries with economies in transition; and non-Annex I countries, which include countries that ratified or acceded to the UNFCCC but are not included in Annex I. The Annex I countries include the 24 original members of the Organization for Economic Cooperation and Development (OECD) (including the United States), the European Union, and 14 countries with economies in transition (Russia, Ukraine, and Eastern Europe).⁴¹

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.

The Kyoto Protocol

The Kyoto Protocol to the UNFCCC, negotiated in December 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.⁴² Developing country 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.⁴³ Among other Annex I countries that have signed the Protocol, both the European Union (EU) and the individual members of the EU have signed and are responsible for meeting their commitments.
• 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 GWP 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 sulfur hexafluoride, 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 methods by which Annex I countries can spread and reduce the cost of emissions limitations. The flexibility mechanisms include:
• Emissions Trading. Annex I countries can transfer portions of their quotas to one another.
• Joint Implementation. Annex I countries can undertake emissions reduction projects in other Annex I countries and receive a negotiated share of the emissions reductions generated by the projects.
• Joint Fulfillment. Like-minded Annex I countries may band together to reallocate national targets within the group, so long as the collective target is met. The European Union has such an agreement, with a collective goal of achieving an 8-percent reduction below 1990 levels. Individual countries  participating in the agreement have negotiated different percentage reduction goals.
• Clean Development Mechanism. Annex I countries may undertake emissions reduction projects in non-Annex I countries and receive credits countable against national targets.
• Entry into Force. The Protocol enters into force 90 days after 55 countries and Annex I signatories with carbon dioxide emissions totaling 55 percent of total Annex I emissions in 1990 “have deposited their instruments of ratification, acceptance, approval, or accession.” These conditions were set to be met in November 2004, following the Protocol’s formal acceptance by the Russian Parliament and President Putin’s signing of the ratifying legislation. Those actions bring the number of signatory countries to 118, representing 61.2 percent of total Annex I carbon dioxide emissions in 1990. The Protocol will enter into force in February 2005.

Under the U.S. Constitution, no international treaty (including the Kyoto Protocol) can be made binding on the United States without the “advice and consent” of the U.S. Senate, which requires a two-thirds vote of its members. Although the Clinton Administration negotiated and subsequently signed the Kyoto Protocol, it did not submit the treaty to the Senate for ratification. In 2001, President Bush stated that he did not intend to submit the Protocol to the Senate for ratification and that the United States would not agree to the Kyoto Protocol because “it exempts 80 percent of the world, including major population centers such as China and India, from compliance, and would cause serious harm to the U.S. economy.”⁴⁴

Beyond the Kyoto Protocol

Since the negotiation of the Kyoto Protocol in 1997, much of the work done at periodic meetings (usually annual) of the UNFCCC Conference of the Parties (COP) has been focused on filling in details related to the operation of the UNFCCC and the Protocol and their respective mechanisms. The Ninth Session of the Conference of the Parties (COP-9) was held in Milan, Italy, from December 1 to December 12, 2003.⁴⁵ Discussion continued on the Kyoto Protocol and the implementation of the UNFCCC. Although the Kyoto Protocol was not in force, some technical rules for the CDM were developed.

The most important decisions reached pertained to rules for carbon sink projects during the first commitment period. Two years before, at COP-7, parties agreed that afforestation and reforestation projects would be allowed under the CDM, but did not establish detailed rules for these projects. At COP-9, however, procedures were addressed in more detail. The difficulty with establishing rules for afforestation and reforestation projects is that forests are non-permanent. Prior to COP-9, parties had not decided who should be liable if a sink begins releasing its sequestered carbon dioxide into the atmosphere—the project developer, the host country, or the holder of emissions reductions credits for that project. At COP-9, parties decided to create temporary emissions reductions credits that are valid for only one commitment period, as well as long-term emissions reductions credits that can be renewed for 20-year periods. This accounting system assigns responsibility for maintaining sinks to the holder of emissions reductions credits and ensures that holders take credit only for current emission reductions.

COP-10, which is being held in Buenos Aires, Argentina, from December 6 through December 17, 2004, marks the 10th anniversary of the entry into force of the Framework Convention on Climate Change, which is a central theme for the meeting. In addition to the accomplishments of the past 10 years and future challenges, discussions at COP-10 highlight a range of climate-related issues, including the impacts of climate change and adaptation measures, mitigation policies and their impacts, and technology. Participants are also considering the entry into force of the Kyoto Protocol, which has been enabled by Russia’s recent ratification.

1. U.S. Emissions of Greenhouse Gases: Background and Context Tables

Notes and Sources

Released: December 2004