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Greenhouse Gas Estimates for Selected Industry Sectors

The most detailed source of information on U.S. greenhouse gas (GHG) emissions is the Inventory of U.S. Greenhouse Gas Emissions and Sinks, issued by the EPA Office of Atmospheric Programs. This source provides detailed emissions data, broken down by industrial process, for about a dozen processes whose contributions to total GHG emissions are of particular significance.

In a few cases (cement manufacture, for example), the Inventory’s breakdown by process corresponds to breakdown by NAICS code sector. However, in general, it is not possible to derive sector-specific information directly from the process data provided in the Inventory.

A process-specific breakdown is very useful for gaining an understanding of GHG emissions from a material flow standpoint. However, there are circumstances under which a sector-specific breakdown may be desirable. For example, when designing programs for encouraging the deployment of resources to decrease overall GHG emissions, it may be most effective to consider groups of companies that are under similar economic constraints in order to evaluate the most cost-effective approaches. Companies in the same sector often must operate under similar market conditions, and can be conveniently considered as a unit. It would therefore be useful for such purposes to develop a sector-specific analysis, to help quantify the potential for improvement that might be expected to result from programs targeted to specific sectors.

The approach adopted for this analysis is based on the fact that, for many sectors, the total contribution of the sector processes to overall GHG emissions is dominated by the carbon dioxide generated from fossil fuel combustion. Information on energy consumption for specific fossil fuel types, broken down by sector, is available from the Energy Information Administration (EIA) of the U. S. Department of Energy. The carbon content of each fossil fuel type is known. Since virtually all of the carbon consumed when fuel is burned is released to the atmosphere in the form of carbon dioxide, it is possible to convert fuel usage figures into GHG emissions for each sector covered by the EIA energy usage information.

It should be noted that EIA also provides estimates of annual U. S. GHG emissions.  However, just as in the case of the EPA Inventory, the EIA estimates for GHG emissions do not provide data down to the level of individual NAICS codes.  It is still useful to compare the EIA with the EPA numbers.  Such comparisons provide consistency checks for conversion factors, and for the respective assumptions on which the separate data sources are based.  For example, the total greenhouse gas emissions in 2000 are given in the EIA document Emissions of Greenhouse Gases in the United States:  2000 Summary as 1,906.3 million metric tons carbon equivalent.  This may be compared with the total for 2000 listed in the EPA Inventory document, given as 7,001.2 Tg CO₂ equivalent.  One teragram (Tg) is the same as one million metric tons (10⁶ metric tons X 10⁶ grams per metric tons = 10¹² grams = 1 teragram.)  Using the factor 44/12 to convert from carbon (atomic weight 12) to carbon dioxide (molecular weight 44) changes the EIA total, expressed as Tg CO₂ equivalent, to 6,989.8, in reasonable agreement with the EPA number.  (EIA data were used in part to derive the Inventory estimates, so this degree of agreement is not surprising, but it is still a useful check on the conversion factors used here.)

Sectors covered by EIA data

The table below lists the sectors of interest for this analysis, together with the corresponding sectors and NAICS codes for which EIA fossil fuel consumption data are available.  In some cases, the mapping is not exact, but a reasonably close correspondence has been selected.

Table 1.  Mapping between sectors of interest and NAICS codes for sectors with EIA energy consumption data

Those sectors for which no EIA data are available are, for the most part, less significant contributors to GHG emissions, and will be omitted from consideration below.  An exception is the utility sector, which is treated separately.

Conversion to carbon dioxide equivalent units

Turning to the EIA fossil fuel consumption data, the first task is to convert the numbers (given both in physical units such as tons, barrels, and cubic feet, and in energy equivalents expressed in Btu) into units suitable for GHG emission comparisons.  The EPA units, Tg CO₂ equivalents, will be used in what follows.

Carbon content values for various fuel types, in terms of Tg carbon per quadrillion Btu, are provided in Annex A of the Inventory, in Tables A-14 and A-15, pp. 249-250.  These values are converted to Tg CO₂ per quadrillion Btu with the factor of 44/12, as in the previous section.

The result of applying these conversion factors to the sectors of interest covered by the EIA data is summarized in the table below.

Table 2.  Impacts of energy consumed by sectors of interest on greenhouse gas emissions, 1998 EIA data, converted to Tg CO₂ equivalent.  (See Table 3 below for additional detail on fuel types and conversion factors.)