Abstract:The use of energy accounts for a major fraction of all anthropogenic emissions of greenhouse gases, and in most industrialized countries the use of transportation fuels and electricity accounts for a major fraction of all energy-related emissions. In the transportation sector alone, emissions of carbon dioxide (CO₂) from the production and use of motor-vehicle fuels account for as much as 30% of CO₂ emissions from the use of all fossil fuels. The production and use of fuels for transportation and electricity also results in emissions of other greenhouse gases, including methane (CH₄) and nitrous oxide (N₂O). In light of this, and in the face of growing concern about global climate change, analysts have started to evaluate energy strategies for their potential impact on global climate. The "Climate Change Action Plan" proposed by President Clinton and Vice President Gore in 1993 calls on the "National Economic Council, the Office on Environmental Policy, and the Office of Science and Technology Policy to co-chair a process...to develop measures to significantly reduce greenhouse gas emissions from personal motor vehicles, including cars and light trucks".

It is a complex task to evaluate energy strategies for their potential impact on global climate. In the first place, there are many primary energy resources (e.g., fossil fuels, nuclear power, biomass, hydropower, wind power, and direct solar energy), many energy production technologies (e.g., oil refining, biomass gasification and synthesis into liquid fuels, and photovoltaic electricity production), and, in the case of transportation, many energy end-use technologies (e.g., otto-cycle spark-ignition engines, diesel-cycle compression-ignition engines, gas turbines, and electric motors) to consider. In each energy pathway, from the recovery of the primary energy source through energy production to energy end use, there are many sources of greenhouse gases: the use of auxiliary or "process" energy to recover, transport, and produce primary energy and end-use fuels; leaks or releases of greenhouse gases from production fields, pipelines, and soils; and end-use combustion of fuels. Each source can produce several kinds of greenhouse gases: CH₄, N₂O, ozone (O₃) precursors (carbon monoxide [CO], nonmethane hydrocarbons [NMHCs], and nitrogen oxides [NOx]), and chlorofluorocarbons (CFCs). Finally, each greenhouse gas must be "weighted" according to its relative expected contribution to global warming or the economic damage therefrom.

The intent of this paper is to present and analyze much of the information needed to evaluate the impact of greenhouse-gases other than CO₂ (hereafter referred to as "non-CO₂ greenhouse gases"), for various transportation-fuel and electricity options. Although there are general inventories of global or U.S. emissions of non-CO₂ greenhouse gases, recent detailed inventories of national emissions of a single non-CO₂ greenhouse gas, and workbooks for estimating emissions of non-CO₂ GHGs, there is no published detailed analysis of emissions of all major non-CO₂ greenhouse gases from all emission sources in the production and use of traditional and alternative fuels for transportation fuels and electricity generation. This paper presents such an analysis. It also shows the contribution of all individual greenhouse gases, including CO₂, to total, fuelcycle, CO₂-equivalent emissions.

The paper focuses on non-CO₂ greenhouse gases because emissions of CO₂ from fuel combustion are easy to estimate: they can be approximated as the carbon content of the fuel multiplied by 3.667 (the ratio of the molecular mass of CO₂ to the molecular mass of carbon), on the assumption that virtually all of the carbon in fuel oxidizes to CO₂ (for data and discussions pertaining to estimating CO₂ emissions from energy use see EIA, 1995b; Grubb, 1989; lEA, 1991; Marland and Pippin, 1990; OECD, 1991). In contrast, combustion emissions of all the other greenhouse gases are a function of many complex aspects of combustion dynamics (such as temperature, pressure, and air-to-fuel ratio) and of the type of emission control systems used, and hence cannot be derived from one or two basic characteristics of a fuel. Instead, one must use published emission factors for each combination of fuel, end-use technology, combustion conditions, and emission control system. Likewise, non-combustion emissions of greenhouse gases (for example, gas flared at oil fields, or N₂O produced and emitted from fertilized soils), which can contribute significantly to the overall global warming impact of an energy cycle, also cannot be derived from basic fuel properties, and instead must be measured and estimated source-by-source and gas-by-gas.

The analysis presented here includes a wide range of fuels, feedstocks, and energy-conversion technologies (Table 1). The boundaries of the analysis are drawn widely, to include emissions from the recovery and transport of primary energy feedstocks, the production of fuels from feedstocks, the distribution of fuels to end users, and the end use of fuels in vehicles or power plants. We refer to all these stages together as a "fuel cycle".

To estimate total fuel-cycle emissions of greenhouse gases, we use CO₂equivalency factors (CEFs) to convert mass emissions of the non-CO₂ greenhouse gases into the mass amount of CO₂ that would have an equivalent climatic or economic impact. Converted emissions plus actual CO₂ emissions are equal to total fuel-cycle CO₂-equivalent emissions. It is important to note that the equivalency factors, while quite useful, also are very uncertain, and may be revised in the future, perhaps substantially. (The implications of uncertainty about the CEFs are discussed more below). The paper is organized by greenhouse gas, beginning with methane.