1. U.S. Emissions of Greenhouse Gases in Perspective
1. U.S. Emissions of Greenhouse Gases in PerspectiveThis is the second annual Energy Information Administration (EIA) report on U.S. emissions of greenhouse gases . It presents estimates of U.S. anthropogenic (human caused) emissions of carbon dioxide, methane , nitrous oxide , and a number of other greenhouse gases for the period 1987 to 1992. Estimates of emissions for 1993 have also been provided where possible, although it has not been feasible to provide 1993 estimates for all sources of emissions. Many of the estimates have been revised from the first report.
Chapter 1 of this report briefly recapitulates some background information about global climate change and the greenhouse effect , discusses important recent developments in global climate change activities, and places U.S. emissions of greenhouse gases in an international perspective. Chapters 2 through 6 cover emissions of carbon dioxide, methane, nitrous oxide, halocarbons, and criteria pollutants , respectively. Chapter 7 describes potential sequestration and emissions of greenhouse gases as a result of land use changes.
In addition, the report contains five appendices. Appendix A describes the derivation of carbon emissions coefficients used in this report. Appendix B describes uncertainties in emissions estimates. Appendix C contains tables extending some of the principal emissions estimates back to 1982, as well as some of the background data used to derive emissions estimates. Appendix D describes known emissions sources omitted from the main report due to definitions of "anthropogenic" or due to excessive uncertainty. Appendix E provides some convenient conversion factors.
The composition of the Earth's atmosphere is a primary determinant of the planet's temperature, which in turn establishes the conditions and limits for all life on Earth. Without the heat-trapping properties of so-called "greenhouse gases ," which make up no more than 1 or 2 percent of the Earth's atmosphere, the average surface temperature of the Earth would be similar to that of Mars: 60 degrees Fahrenheit (16 degrees Celsius).
The main greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (such as CFC11 and CFC12). With the exception of halocarbons, most greenhouse gases occur naturally. Water vapor is by far the most common, with an atmospheric concentration of nearly 1 percent. Carbon dioxide concentration is less than 0.04 percent. Concentrations of other greenhouse gases are a fraction of that for carbon dioxide (Table 1).
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. To date, scientists have been unable to verify model predictions of rising temperatures with actual measurements of temperature change. Detecting the expected temperature change has been difficult, in part because normal temporal and spatial variations in temperature are far larger than the predicted change in the global average temperature. Even when changes are identified, it is not possible to be certain whether one is observing a random fluctuation that will reverse itself or the beginning of a trend. The possible effects of rising temperatures on weather patterns are even more uncertain.
Most greenhouse gases have substantial natural sources in addition to human- made sources, and there are powerful natural mechanisms for removing them from the atmosphere. However, the continuing growth in atmospheric concentrations establishes that for each of the major greenhouse gases, more gas is being emitted than is being absorbed each year: that is, the natural absorption mechanisms are lagging behind. Table 2 illustrates the relationship between anthropogenic and natural emissions and absorption of the principal greenhouse gases.
The principal natural sources of carbon dioxide are the release of carbon dioxide by oceans (100 billion metric tons per year), the aerobic decay of vegetation (30 billion metric tons), and plant and animal respiration (30 billion metric tons). The principal anthropogenic source is the combustion of fossil fuels, which accounts for about three-quarters of total anthropogenic emissions of carbon worldwide. Natural processes, known and unknown, absorb substantially all of the naturally produced carbon dioxide and some of the anthropogenic carbon dioxide, leading to an annual increase in carbon dioxide in the atmosphere of about 3.2 to 3.6 billion metric tons.
Methane is released primarily by anaerobic decay of vegetation, by the digestive tracts of termites in the tropics, and by several other lesser sources. The principal anthropogenic sources are leakages from the production of fossil fuels , human- promoted anaerobic decay in landfills, and the digestive tracts of domestic animals. The main sources of absorption are thought to be decay (into carbon dioxide) in the atmosphere and decomposition by bacteria in the soil.
The sources and absorption of nitrous oxide are much more speculative than those for other greenhouse gases. The principal sources are thought to be bacterial breakdown of nitrogen in soils, particularly forest soils. The primary human-made sources are enhancement of natural processes through application of nitrogen fertilizers, combustion of fuels, and certain industrial processes. The annual increase in concentration in the atmosphere is thought to be on the order of 4 million metric tons.
Some greenhouse gases are more potent at affecting climate change than are others. As a result, comparable increases in the concentrations of different greenhouse gases can have vastly different heat-trapping effects. Among those identified, carbon dioxide is least effective as a greenhouse gas. Considering only heat-absorption potential, one molecule of methane can have 22 times the effect on climate that one molecule of carbon dioxide has (1) .
It would be useful to determine the precise relative effectiveness of various greenhouse gases in affecting the Earth's climate. This information would help policymakers know whether it would be more effective to concentrate effort on reducing the very small emissions of powerful greenhouse gases, such as carbon tetrafluoride, or whether they should bend their efforts to controlling the very large emissions of relatively ineffective gases, such as carbon dioxide.
There has been extensive study of the relative effectiveness of various greenhouse gases in trapping the Earth's heat. This research has led to the development of the concept of a "global warming potential ," or GWP. The GWP is intended to demonstrate the relative impacts on global warming of various gases, compared with carbon dioxide. Over the past few years, the Intergovernmental Panel on Climate Change has conducted an extensive research program aimed at summarizing the effects of various greenhouse gases through a set of GWPs. The results of this work are expected to be published in the near future. The work has established that the effects of various gases on global warming are too complex to permit them to be summarized as a single number. The complexity takes three forms:
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" is already captured, and the effects of an additional unit of emissions decline. Therefore, the effect of an additional unit of emissions of a gas that is relatively plentiful in the atmosphere, such as carbon dioxide, tends to be less than that of a rare gas, such as sulfur hexafluoride .
Various natural processes cause many greenhouse gases to decompose into other gases, or to be absorbed into the ocean or ground. These processes can be summarized in terms of the "atmospheric lifetime" of a particular gas: i.e., the period of time it would take for natural processes to remove a unit of emissions from the atmosphere. Some gases, such as chlorofluorocarbons (CFCs), have very long atmospheric lifetimes, in the hundreds of years,
while 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, as the time scale lengthens from 10 years to 100 years to 500 years, the differences between methane and carbon dioxide become less significant.
Many gases are chemically active, and they may react in the atmosphere in ways that promote the formation of greenhouse gases. For example, nitrogen oxides and carbon monoxide combine to promote the formation of ozone, which is a potent greenhouse gas, while chlorofluorocarbons tend to destroy atmospheric ozone, thus promoting global cooling. These indirect effects have proven impossible to summarize in terms of global warming potentials. In some cases, such as CFCs, it is unclear whether they contribute to a net warming or cooling of the Earth's atmosphere. In other cases, it is clear that they contribute to a net warming, but the magnitude of the indirect effect is unclear.
Table 3 summarizes the results of the most recent studies by scientists working on behalf of the Intergovernmental Panel on Climate Change. It summarizes estimates of atmospheric lifetimes, global warming potentials across various time scales, and the signs of any indirect effects. For gases with short atmospheric lifetimes, their effects are greatest when measured over a short time span. As the time span lengthens, their total effect diminishes. Thus, the estimated direct global warming potential of any particular gas is highest for time periods shorter than its atmospheric lifetime and lower for longer time periods.
Since the publication of last year's report, one of the most significant research developments has been an apparent slowing of the growth rate of atmospheric concentrations of carbon dioxide and methane . This slowing first began to appear in 1990 and has continued through 1993. In the early 1980s, carbon dioxide concentrations were growing by 0.4 percent per year (Figure 1). At present, the growth rate of atmospheric concentrations has slowed to less than 0.2 percent annually. While the level of energy-related carbon dioxide emissions has dropped slightly, the decline is far too small to account for the observed sharp drop in the growth of atmospheric concentrations. Consequently, either natural sources of emissions have also declined, or natural absorption of carbon dioxide and methane has increased.
Some scientists theorize that global cooling from the vast quantities of sulfate aerosols deposited in the atmosphere by the eruption of Mount Pinatubo, in the Philippines, has stimulated absorption mechanisms.(2) If the Pinatubo explanation is correct, then the growth rate will accelerate again over the next few years, as the Pinatubo materials are gradually washed from the atmosphere. However, no one really knows the cause of the slowing of growth in concentrations.
Figure 1. Percent Change in Atmospheric Concentrations of Carbon Dioxide and Methane, 1959-1993
As noted earlier, U.S. carbon emissions account for the bulk of U.S. emissions of greenhouse gases . The United States also emits more carbon dioxide than any other country, accounting for about 23 percent of world carbon emissions. The United States also has the world's highest carbon emissions per capita. In recent years, however, the growth in annual carbon emission rates in the United States has been small.
Before 1970, emissions grew rapidly in all regions of the world. The bulk of the emissions was accounted for by the United States, Western Europe, and the then-Soviet bloc (Figure 2). Since the mid-1970s, the growth of emissions in North America and Western Europe has slowed markedly. Emissions growth in the former Soviet bloc, however, continued at a rapid pace until the political collapse of communism. Since then, the economic collapse of the former Soviet Union and the Eastern bloc has had unprecedented effects on energy consumption and carbon emissions, with a drop of more than 20 percent between 1988 and 1992.
Figure 2. Energy-Related Carbon Emissions by Region, 1950-1992
The decline in emissions from the former Soviet bloc has been sufficient to reduce 1991 and 1992 world energy-related carbon emissions below their 1989 and 1990 levels, even though emissions in most of the rest of the world have continued to grow. There has been no cessation in the growth of fossil energy consumption (and hence emissions) in the developing world, particularly in the rapidly growing economies of East Asia.
Figure 3 provides some insight into the forces contributing to these shifting trends. The centrally planned economies were notable for their exceptionally high levels of energy consumption compared to the size of their economies. The process of reform should ultimately increase the efficiency with which these economies use all inputs, particularly fossil fuels .
Figure 3. Energy Consumption per Dollar of GNP Versus GNP per Capita for 83 Countries, 1990
The energy consumption experience of developing countries is diverse: in small countries with specialized economies, energy consumption patterns are dominated by the industries in which they specialize. Oil exporters, for example, tend to have high levels of energy consumption and carbon emissions due to their propensity toward low energy prices and specialization in energy-intensive industries. On the other hand, larger and more diversified economies tend to pass through three stages of energy consumption:
In the poorest countries, energy consumption shifts from noncommercial, biomass fuels to commercial (usually fossil) fuels as economic conditions improve. This trend tends to accelerate the growth of measured energy consumption and carbon emissions.
In middle-income countries, the populations acquire energy- consuming consumer durables, such as stoves, refrigerators, motor scooters, automobiles, and air conditioners. These products tend to contain large amounts of steel and rubber, and thus they are energyintensive to build. Their use sharply boosts commercial energy consumption. Thus, as living standards rise, there is a more-than proportional increase in energy consumption.
When the demand for consumer durables has been saturated, energy consumption growth tends to flatten out at high levels, as seen in Europe and North America since 1975. In this phase, economic growth tends to be far less energy-intensive, and the underlying growth rate of energy consumption is low enough to be partially or wholly offset by increasing energy efficiency.
Thus, the future growth of carbon emissions probably will be dominated by the changing consumption patterns and technology choices of middle- and lower-income developing countries.
