Appendix D. Emissions Sources Excluded

Certain sources of greenhouse gas emissions are not included in the estimates presented in this report. The omissions have been made on the basis of lack of essential data, highly speculative estimation methods, or classification as "natural" sources.

The carbon found in biofuels is the result of atmospheric uptake. During the combustion of biofuels, there is an immediate release of the carbon in the form of carbon dioxide. As part of the natural carbon cycle, however, these carbon emissions are reabsorbed over time. Since they produce no net change in the overall carbon budget, such emissions are not included in this report. If the initial flux had been counted, annual carbon dioxide emissions estimates would have been approximately 75 million metric tons higher than reported in Chapter 2 (Table D1).

Emissions are estimated by multiplying Energy Information Administration (EIA) energy consumption data for biofuels by the applicable emissions factors. The EIA data for municipal solid waste include methane recovery from landfills, but since the methane is not used as a biofuel, it has been subtracted for these calculations. Carbon dioxide emissions factors for combustion of wood fuels and municipal solid waste are taken from the EIA report, Electric Power Annual 1995.⁽²³⁶⁾ The emissions coefficient for alcohol fuels, 17.99 million metric tons of carbon per quadrillion Btu, was derived specifically for use in this report.

Domestic military energy consumption is incorporated into U.S. energy statistics; however, energy consumption for overseas operations is a more complex issue. The data can either be reported in the national energy statistics of the host country or included in U.S. export statistics if domestic oil is transported to ships and other facilities. In some circumstances, the oil consumption may go unreported.

Estimating, even roughly, the quantity of oil consumed for overseas military operations is an uncertain procedure. The Defense Fuel Supply Center reports that petroleum sales for fiscal year 1996 totaled 120 million barrels.⁽²³⁷⁾ Of that, approximately 77 percent was acquired domestically and is assumed to be included in U.S. statistics. A reasonable estimate of military oil consumption not reported elsewhere would, therefore, be 23 percent of total military consumption of jet fuel, middle distillates, and residual oil.⁽²³⁸⁾ By this method, emissions for 1996 are estimated at 3.6 million metric tons of carbon (Table D2).

Forest fires are known to create greenhouse gas fluxes within the atmosphere over extensive time periods. Specifically, forest fires produce carbon dioxide, methane, and nitrous oxide. Considering that carbon uptake occurs with subsequent regrowth (assumed to balance out the initial carbon flux), and because emissions from natural forest fires cannot be distinguished from those from human-induced fires, estimates from this source are not included in this report.

The carbon dioxide content of natural gas varies by reservoir. Marketed natural gas, however, must consist largely of methane. When natural gas direct from the reservoir does not meet marketable standards, it is treated in a gas processing plant. During treatment, impurities and heavy hydrocarbons are removed from the gas. Excess carbon dioxide, which is one of the separated compounds, is either used industrially (estimates provided in Chapter 2) or vented to the atmosphere.

The EIA reports that 389 billion cubic feet of nonhydrocarbon gases were removed from total natural gas production in 1995. This figure does not take into account the 11 (out of 33) gas-producing States that do not report annual amounts of nonhydrocarbon gases removed.⁽²³⁹⁾ In its reporting, Texas quantifies commercial carbon dioxide recovery from gas plants. From those data, the EIA is able to make rough estimates of carbon dioxide recovery in other States (Table D3).

In Texas, the amount of nonhydrocarbon gas removed is approximately 2 percent of gross natural gas production. This same ratio is applied to gross production for the nonreporting States. Similarly, the ratio of commercial carbon dioxide production to nonhydrocarbon gas removal in Texas (roughly 90 percent) is applied to other States to determine the total amount of carbon dioxide removed from gas plants in the United States. In order to avoid double counting with industrial carbon dioxide production, the commercial recovery in Texas is deducted from the total. Any commercial recovery in other States is double counted; however, without additional data, this is currently unavoidable.

In 1995, carbon dioxide coproduction at natural gas plants was estimated at 5 million metric tons of carbon. Because of its speculative nature, this estimate is not included in the report. First, there is no basis to support the assumption that the ratios applicable to Texas are generalizable to other States. Second, there is no basis for determining the precise amount of carbon dioxide recovered for commercial use in States other than Texas.

In previous years, the EIA has included in its estimated emissions a category called "unmetered natural gas." In those years, U.S. natural gas producers consistently reported selling about 3 percent more natural gas than U.S. consumers reported buying. In EIA natural gas statistics, this "missing" gas is described as "the balancing item" or "unaccounted for gas." The balancing item can be viewed as the sum of leakage, measurement errors, data collection problems, and undetected over- and underreporting, as well as undetected nonreporting. Only a fraction of this amount can credibly be attributed to leakage from transmissions systems. Evidence from the electric utility sector--where transmissions companies report gas sales and electric utilities report gas purchases--suggests that there may be undercounting of natural gas consumption.

Estimates of carbon dioxide emissions from this source were included in previous reports, on the grounds that there was an element of systematic underreporting of consumption in the balancing item. In the past 2 years, however, the sign of the balancing item has changed, and reported consumption now exceeds reported production. This development reduces the credibility of the undercounting theory, and consequently this report no longer carries "unmetered natural gas" consumption as a source of emissions. It is possible, however, that there is an element of undercounting of natural gas consumption, which may be on the order of 1 percent (3 million metric tons) of natural gas emissions.

During the fermentation process, complex organic compounds are split through a variety of chemical reactions. The most common is the anaerobic conversion of sugar into carbon dioxide and alcohol. Fermentation does not create a net flux of emissions, however, because the carbon dioxide produced is reused in the process.

As a replacement for natural gas, carbon dioxide is being injected into reservoirs for the purpose of retrieving additional oil. Over time, the carbon dioxide seeps into the producing well, creating a mixture of oil, natural gas, and carbon dioxide. If the energy content is sufficiently high, the gaseous portion of this mix will probably be sent to a gas plant. If the energy content is low, the gas is likely to be vented or flared. At this time, there is no basis for the EIA to estimate the quantity of added carbon dioxide that is vented or flared. The annual amount of carbon dioxide used for enhanced oil recovery is probably on the order of 12 million metric tons,⁽²⁴⁰⁾ and emissions would be some fraction of that figure.

Smelting of lead includes a stage in which limestone undergoes calcination. As described in Chapter 2, carbon dioxide is released as a byproduct of the calcination reaction. Emissions estimates cannot be calculated for this report because there are no known statistics regarding the amount of limestone used in lead smelting. The EIA is currently researching alternative data sources in an effort to include estimates of these emissions in future reports.

Measurements taken from 20 abandoned coal mines in 1994 showed a total of some 25,000 metric tons of methane emissions for that year.⁽²⁴¹⁾ Extrapolating from this small sample to a national-level emissions estimate requires assumptions about the total number of abandoned mines, their emissions profiles during operation, their current condition (flooded or not), and other variables. Past extrapolation efforts have led to national-level estimates of emissions from abandoned coal mines of about 280,000 metric tons annually. However, EPA's Atmospheric Pollution Prevention Division now believes that the total may be as high as 384,000 metric tons.

Wetlands are a known source of methane. Environments low in oxygen, combined with abundant organic matter, are conducive to the creation of methane, and wetlands meet both criteria. Wetlands cover approximately 274 million acres of land in the United States and are a potentially important source of atmospheric methane.

The stock of natural wetlands in the United States has diminished considerably over the past two centuries, which should, in principle, have reduced methane emissions from wetlands (the EIA is unaware of research proving or disproving this principle). A recent study of wetland losses concluded that the United States had lost approximately 30 percent of its wetlands between colonial times and the mid-1980s. Almost all of this loss has occurred in the lower 48 States, which have lost 53 percent of their original wetlands.⁽²⁴²⁾ Ten States--Arkansas, California, Connecticut, Illinois, Indiana, Iowa, Kentucky, Maryland, Missouri, and Ohio--have lost 70 percent or more of their original wetland acreage. By the mid-1980s, a total of approximately 119 million acres had been lost from the original U.S. total.

An update of the wetlands study indicates that 654,000 acres were converted from wetlands to other uses between 1982 and 1987, and that an additional 431,000 acres were converted between 1987 and 1991.⁽²⁴³⁾ Extrapolating from these data, it is estimated that wetlands in the United States are currently destroyed at a rate of approximately 86,000 acres per year. Wetlands, also known as swamps and marshes, have historically been drained or filled in for agriculture, land development, and mosquito control, although it is currently illegal to drain or fill a wetland without a permit from the U.S. Army Corps of Engineers. It is difficult to find information on the conversion of other land categories to wetlands. It is assumed that the number and extent of wetland creations is small enough to leave the above loss estimates essentially unchanged.

Estimates of global methane fluxes from wetlands suggest that methane emissions from temperate-zone wetlands are minimal--typically between 5 and 10 million metric tons of methane per year for worldwide temperate-zone wetlands (which include U.S. wetlands)--when compared with estimated global wetlands emissions of 110 million metric tons.⁽²⁴⁴⁾ The U.S. share of all temperate-zone wetlands is about 57 percent, and temperate-zone wetlands lost during the 1980s accounted for about 0.5 percent of U.S. wetlands at the beginning of the period. Consequently, the reduction in natural methane emissions from wetlands lost might be on the order of 0.57 × 0.005 × 5 to 10 million metric tons of methane, or from 10,000 to 20,000 metric tons of methane annually over the decade.

The scientific literature suggests that both grasslands and forest lands are weak natural sinks for methane and weak natural sources for nitrous oxide. Natural soils apparently serve as methane sinks: well-aerated soils contain a class of bacteria called "methanotrophs" that use methane as food and oxidize it into carbon dioxide. Experiments indicate that cultivation reduces methane uptake by soils and increases nitrous oxide emissions.

One report indicates that methane uptake in temperate evergreen and deciduous forests in the United States ranges from 0.19 to 3.17 milligrams (measured in carbon units) per square meter per day, equivalent to the uptake of 36.8 to 624.4 metric tons of methane per million acres per year. The range is larger for agricultural lands: 0.2 to 6.3 milligrams per square meter per day. Estimates for methane uptake resulting from the abandonment of farmland range from 0.6 to 6.1 milligrams per square meter per day. While all of these ranges are wide, the total amount of methane in question is less than 1 percent of methane emissions from anthropogenic sources.

Of all the greenhouse gases discussed in Chapter 7, the least amount of data is available for nitrous oxide. It is known that conversion of forests and grasslands to cropland accelerates nitrogen cycling and increases nitrous oxide emissions from the soil. It is not known with certainty by how much.⁽²⁴⁵⁾ Some estimates have been made of the difference between fertilized and unfertilized soils. According to one study, unfertilized soils produce emissions of 0.25 to 0.35 milligrams (measured in nitrogen units) per square meter per day, while emissions from fertilized soils range from 0.6 to 1.65 milligrams per square meter per day.⁽²⁴⁶⁾ Thus, abandoning fertilization should reduce nitrous oxide emissions by 0.35 to 1.3 milligrams per square meter per day--the equivalent of 86 to 321 metric tons of nitrous oxide per million acres per year.

Applying this figure to the 35 million acres of cropland idled between 1982 and 1992 implies a reduction in nitrous oxide emissions ranging from 3,010 to 11,235 metric tons annually. In principle, however, about three-quarters of the reduction in emissions from this source should be captured by reduced application of nitrogen fertilizers; thus, reporting emissions reductions using this method would result in significant double counting of units already included in the agriculture statistics in Chapter 4.

If such estimates are to be applied to emissions inventories, a problem of crediting the uptakes applies. Removing an acre of farmland from production in a particular year creates a permanent annual methane sink that will absorb small additional amounts of methane each year thereafter, or at least until the use of the land changes. The method that should be used to credit such permanent reductions to a particular year is not obvious.

As noted in the Executive Summary, emissions of nitrous oxide were only 2.2 percent of U.S. GWP-weighted emissions of greenhouse gases in 1994. Of those emissions, it is difficult to say what percentage resulted from land uses, primarily because of the wide range of estimates concerning the magnitude of nitrous oxide emissions from nitrogen fertilizer use (which in any case should be included in agriculture statistics instead of land use statistics--see Chapter 4) and the lack of research data on nitrous oxide emissions from forest and grassland soils.

One report, based on experimental data, indicates that methane uptake in temperate evergreen and deciduous forests in the United States ranges from 0.19 to 3.17 milligrams (measured in carbon units) per square meter per day, equivalent to the uptake of 36.8 to 624.4 metric tons of methane per million acres per year.⁽²⁴⁷⁾ Thus (assuming no methane uptake at all from previous use), adding 6 million acres of forest land from 1987 to 1992 should increase methane absorption by 221 to 3,746 metric tons per year. Using the same numbers, all forest land in the United States may remove from 27,122 to 460,183 metric tons of methane per year. Comparing this figure with U.S. anthropogenic methane emissions of about 31 million metric tons per year, as estimated in this report, indicates that the magnitude of methane uptake by natural soils is not great.

Another report in the scientific literature indicates that some sample plots of pastureland in the United States have methane uptake rates of 4.1 milligrams (measured in carbon units) per square meter per day (for fertilized pasture) and 6.3 milligrams per square meter per day (for unfertilized pasture), with uptake from fertilized wheat and maize fields ranging from 0.2 to 0.9 milligrams per square meter.⁽²⁴⁸⁾ Accordingly, an additional 0.6 to 6.1 milligrams of methane per square meter per day would be absorbed by abandoned farmland, equivalent to 118.1 to 1,201.1 grams per acre per year. Applying these figures to the 35 million acres of cropland idled between 1982 and 1992 implies an increase in methane uptake of 4,133 to 42,038 metric tons per year from this source.

TO:
Appendix E. Common Conversion Factors

GG97RPT Home Page

File last modified: 10/21/97
Energy Information Administration/Emissions of Greenhouse Gases in the United States 1996
URL: http://www.eia.doe.gov/oiaf/1605/gg97rpt/appd.html

If you having technical problems with this site, please contact the EIA Webmaster at wmaster@eia.doe.gov