• This year's report contains additional information on the forces that shape year-to-year fluctuations in carbon dioxide emissions, including discussions of weather-adjusted emissions and the interaction between national economic growth and industrial energy emissions.

• International bunker fuel emissions are defined as emissions from fuel consumed by ships and aircraft moving internationally. The guidelines of the International Panel on Climate Change (IPCC) for preparing national emissions inventories specify that bunker fuel emissions be deducted from national emissions totals. This year's bunker fuel emissions estimate uses (for the first time) unpublished data on international marine bunker and international aviation fuel consumption collected by the U.S. Department of Commerce and intended for use as a component in U.S. balance-of-payments statistics. With these data, EIA can for the first time include international aviation bunker emissions from airlines operating in the United States and can replace inferred international marine bunker emissions with actual data. (The U.S. Department of Defense has indicated that it is willing, in principle, to provide estimates of military bunker fuel consumption but has not yet done so as of the date of publication.) The new information increases EIA's estimate of national bunker fuel emissions by about 10 million metric tons of carbon (0.6 percent of emissions), which in turn lower national total emissions by a comparable amount for all years.

• Chapter 4, "Nitrous Oxide Emissions," contains estimates of nitrous oxide emissions from a new source, biological fixation of nitrogen in agricultural soils. The revision increases nitrous oxide emissions by 0.2 million metric tons of gas, or 1 percent of U.S. greenhouse gas emissions in 1998. A text box in Chapter 4 discusses the revision to the estimation method for nitrous oxide emissions and its impact on the estimates for 1990 through 1998 in this report.

• Chapter 4 now includes estimates of nitrous oxide emissions from the consumption of wood in the electric utility, residential, and industrial sectors. This modification to the estimation method results in an average increase of 0.009 million metric tons (20 percent) of nitrous oxide emissions from stationary combustion sources for each year from 1990 through 1998.

• In Chapter 2, "Carbon Dioxide Emissions," consumption of ethanol blended at the pump has been estimated back to 1981, thus slightly reducing emissions for the years before 1993.

• There is a text box in Chapter 2 analyzing the results of EIA's 1994 Manufacturing Energy Consumption Survey in terms of carbon emissions from the largest emitting industries.

• In Chapter 2, estimates of carbon dioxide emissions associated with the consumption of limestone in flue gas desulfurization units have been revised to reflect modifications to the estimation methodology. The modifications were based on a May 1999 report obtained from the U.S. Geological Survey, which characterized limestone utilization in fluidized-bed combustion systems and in flue gas desulfurization units (for details on the revisions to the estimation method, see Appendix A).

In this publication, EIA reports information in forms that are most likely to be familiar to users of the document. Therefore, energy and industrial data are reported in their native units. For example, oil production is reported in thousand barrels per day, and energy production and sales are reported in (higher heating value) British thermal units (Btu). Higher heating value Btu can be a relatively intuitive unit for metric users, because a lower heating value exajoule is only 1 to 6 percent larger than a higher heating value quadrillion Btu.

Emissions data are reported in metric units. This report uses the familiar "million metric tons" common in European industry instead of the "gigagrams" favored by the scientific community. Metric tons are also relatively intuitive for users of English units, because a metric ton is only about 10 percent heavier than an English short ton.

Emissions of most greenhouse gases are reported here in terms of the full molecular weight of the gas (as in Table ES1). In Table ES2, however, and subsequently throughout the report, carbon dioxide is reported in carbon units, defined as the weight of the carbon content of carbon dioxide (i.e., just the "C" in CO₂). Carbon dioxide units at full molecular weight can be converted into carbon units by dividing by 44/12, or 3.6667. This approach has been adopted for two reasons: