Text Boxes
- New global warming potentials. This year's Executive Summary incorporates the new consensus
global warming potentials for greenhouse gases published in the latest report by the
Intergovernmental Panel on Climate Change, Climate Change 1994. As shown in Table ES2, the
new coefficients indicate a somewhat greater effect of methane and nitrous oxide relative to that
of carbon dioxide.
- Revised annual emissions coefficients for gasoline, liquefied petroleum gas, jet fuel, and
crude oil. The composition of certain petroleum fuels has gradually shifted over time, and the
EIA has adopted annual coefficients for several fuels to reflect these shifts. In the United States,
changes in gasoline composition have been caused by the phasing out of leaded gasoline, the
growing popularity of premium gasoline, and the advent of oxygenated and reformulated gasoline
in 1994-1995. The composition of U.S. jet fuel changed in 1994 as the U.S. military began to
phase out naphtha-based jet fuels in favor of kerosene-based fuels. The jet fuel coefficient has
been revised downward by several percent to reflect more recent information, provided by the
Boeing Corporation, on the average density and composition of jet fuel. The net result of these
changes has been to reduce estimated carbon emissions by 0.4 to 1.5 million metric tons of carbon
(0.03 to 0.10 percent) in all years, with the smallest decline in 1990. The annualized coefficients
have caused emissions in the 1980s to be revised downward by greater amounts than emissions in
1990, resulting in a slightly higher historical growth rate for U.S. carbon emissions.
- Revised flare gas estimation methods. EIA's natural gas "venting and flaring" data are used to
estimate both carbon dioxide and methane emissions from flare gas. For this year's report, the
division between venting and flaring was recalculated on a State-by-State basis, rather than on a
national basis. These revisions increased annual methane emissions by about 0.2 million metric
tons (0.6 percent of total methane emissions).
- Revised historical estimates for methane emissions from coal mines and landfills. The EIA
extended the method used to account for methane emissions resulting from coal mine ventilation
to provide estimates from the early 1980s. The coal mine estimates were used to generate time
series estimates for coal mine methane back to 1980. In addition, methane emissions from
municipal solid waste landfilled during the period 1940-1960 have been estimated, slightly
increasing the estimates for landfill emissions in the 1990s, with larger increases for estimates in
earlier years.
- Expanded coverage of nitrous oxide combustion emissions. Following the method outlined in
the final report of the Intergovernmental Panel on Climate Change, the EIA expanded coverage of
nitrous oxide emissions from combustion of fossil fuels to cover all combustion, rather than just
some sources. This change, however, did not materially affect total nitrous oxide emissions
estimates.
- New data on methane and nitrous oxide emissions from agricultural burning. Estimates of
this minor emissions source have been added in this year's report, raising estimated methane
emissions by 0.1 million metric tons (0.3 percent of total methane emissions) and nitrous oxide
emissions by 0.005 million metric tons (1 percent of total nitrous oxide emissions).
- Revised 1993 energy data. Last year's report included estimated carbon dioxide emissions based
on preliminary 1993 energy data. Final energy consumption data for 1993 were released in early
1995, leading to a small downward revision in estimated 1993 carbon dioxide emissions. This
year's report includes preliminary 1994 carbon dioxide emissions estimates, which will be revised
next year.
In this report, the EIA has elected to report information in forms that are most likely to be intuitively 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 in (higher heating
value) quadrillion 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 measured 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:
- Carbon dioxide is most commonly measured in carbon units in the scientific community.
Scientists argue that not all carbon from combustion is, in fact, emitted in the form of carbon
dioxide. Because combustion is never perfect, some portion of the gases emitted are carbon
monoxide, methane, other volatile organic compounds, and particulates. These other gases
(particularly carbon monoxide) eventually decay into carbon dioxide, but it is not strictly accurate
to talk about "tons of carbon dioxide" emitted.
- Carbon units are more convenient for comparisons with data on fuel consumption and carbon
sequestration. Since most fossil fuels are 75 to 90 percent carbon by weight, it is easy and
convenient to compare the weight of carbon emissions (in carbon units) with the weight of the
fuel burned. Similarly, carbon sequestration in forests and soils is always measured in tons of
carbon, so that using carbon units makes it simple to compare sequestration with emissions.
While carbon dioxide emissions can be measured in tons of carbon, emissions of other gases (such as methane) can
also be measured in "carbon equivalent" units by multiplying their emissions (in metric tons) by their global
warming potential, and then multiplying by 12/44 (as in Table ES2). This method provides a measure of the relative
effects of various gases on climate. Because scientific estimates of global warming potential are still evolving,
however, this report gives emissions in carbon equivalent units for other gases only in Table ES2. No other data in
the report are given in carbon equivalent units.
The U.S. Environmental Protection Agency, the Gas Research Institute, the American Petroleum Institute,
and the American Gas Association are currently cooperating on a study to characterize more accurately
methane emissions from the domestic natural gas industry. The study focuses on producing a more
complete accounting of emissions sources and using improved measurement techniques for developing
emissions factors. Preliminary results indicate that 1992 emissions from the natural gas system may be
much larger than the estimates that appear in this report.* As the research is completed, the EIA will
review the data obtained and incorporate any new information into future reports.
_______________
*D. Kirchgessner, R.M. Cowgill, M. Harrison, and L.M. Campbell, "Methods for Estimating Methane Emissions from
the Domestic Natural Gas Industry," Presented at the 1995 Greenhouse Gas Emissions and Mitigation Research
Symposium (U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Air Pollution
Prevention Division, June 1995).
Carbon coefficients in upcoming years are likely to be affected by environmental regulation requiring the addition of blending
components during certain times of the year and/or in certain areas of the country. The Clean Air Act Amendments of 1990
mandated the addition of oxygenated blends to gasoline beginning in November 1992 and reformulated blends beginning in 1995 for
heavily polluted areas. Oxygenated mixtures reduce carbon monoxide emissions by increasing the oxygen content of gasoline, while
reformulated mixtures limit aromatic content, consequently reducing hydrocarbon emissions. Oxygenated and reformulated gasoline
are initially mandated only in pollution nonattainment areas during the winter months. Various studies suggest that factors such as
limited pump space and industry competition may push the supply of the new blends to anywhere from 25 percent of total market
sales to 100 percent by the end of the decade.* Data on oxygenated gasoline are just becoming available, and data on reformulated
gasoline should appear some time during 1996. The effect of Clean Air Act mandates on carbon emissions coefficients will depend
largely on how pervasive the new blends become outside nonattainment areas, and to what extent consumption patterns change in
response to the availability of the new fuels.**
Changes in carbon content due to the addition of oxygenated blending components may be negligible, but the effect of reformulated
gasoline appears more significant. According to DeLuchi, the carbon content of reformulated fuels may be as low as 83.3 percent.
The impact of the reduction in carbon content on an overall emissions coefficient is somewhat moderated by a simultaneous
reduction in energy content caused by the blending of reformulated gasoline. Thus, while each gallon of gasoline may have less
carbon, more gallons must be consumed to achieve the same level of vehicle miles traveled.
_______________
*R. Salthouse, Making Clean Gasoline (Logistics Management Institute, September 1992); and A.K. Rhodes, "U.S. Refiners
Scramble To Meet Reformulated Gasoline Mandate," Oil and Gas Journal (January 27, 1992).
**C. Dale et al., "First Oxygenated Gasoline Season Shakes Out Differently Than Expected," Oil and Gas Journal (October 25,
1993).