Executive Summary

This report provides information on greenhouse gases (GHGs) as required by Section 503 a(4) and b(3) of the Energy Policy Act of 1992 (EPACT). Although EPACT is principally concerned with improving energy efficiency and curbing U.S. dependence on foreign oil, the requirement to estimate greenhouse gases reflects a desire that energy security not be promoted at the expense of the environment. EPACT was developed around the time (1992) that President Bush and other world leaders met in Rio de Janeiro to sign the “Framework Convention for Climate Change.” This document committed developed nations to stabilizing emissions of greenhouse gases at 1990 levels.

Section 503 requires the Secretary of Energy, in consultation with the Administrator of the Energy Information Administration (EIA), to estimate GHG emissions resulting from the use of the subset of replacement fuels [1] known as “alternative transportation fuels” [2]. Further, EPACT requires an examination of GHG emissions over the entire fuel cycle rather than only those produced from combustion. EIA has decided to use the “Delucchi” fuel cycle [3] approach because it offers the best framework for adhering to the EPACT requirements for estimating GHG emissions from alternative transportation fuels, using gasoline as a comparison [4]. For a discussion of the Delucchi approach and EIA's prior work in the area, consult Alternatives to Traditional Transportation Fuels: An Overview [5], Alternatives to Traditional Transportation Fuels 1993 [6], and Alternatives to Traditional Transportation Fuels 1994: Volume I [7].

The Earth's average temperature has been increasing for the past few centuries, leading to concern about a variety of related issues (e.g., average ocean levels). One possible cause of the temperature rise is the amount of “greenhouse gases” that mankind is emitting into the atmosphere. Basically, greenhouse gases are those that trap heat emanating from the Earth's surface. Without these gases, this heat would otherwise escape from the atmosphere. Some level of greenhouse gases is necessary; without them the Earth's average temperature would be nearly 0oF [8]. A major human activity responsible for increased greenhouse gases in the atmosphere is fuel combustion. Water vapor and carbon dioxide, both greenhouse gases, are the principal products produced from burning hydrocarbon fuel.

Concern about possible effects of greenhouse gases heightens when future trends in transportation fuel consumption are examined. Motor vehicle greenhouse gas emissions are projected to grow in the United States and throughout the world as a result of the increasing number of vehicles and vehicle miles traveled (VMT). VMT is expected to grow in the United States at double the rate of population growth and even faster in Africa and Asia. The transportation sector contributes about one-third of total carbon dioxide emissions in the United States and other countries that belong to the Organization for Economic Cooperation and Development.

Greenhouse gases considered by EIA are water vapor, carbon dioxide, methane, nonmethane hydrocarbons (NMHC), carbon monoxide, nitrous oxide, nitrogen oxides [9], and ozone. Water vapor is the most abundant greenhouse gas, varying from roughly 0 to 4 percent of the Earth's atmosphere. Currently, however, water vapor from fuel combustion is not believed to have a significant impact on atmospheric water vapor concentrations. Next is carbon dioxide, followed by ozone, nitrous oxide, carbon monoxide, and nitrogen dioxide. The amount of each gas, except NMHC and ozone [10], is reported for conventional gasoline, methanol, ethanol, compressed natural gas (CNG), and liquefied petroleum gas (LPG, i.e., propane) (Table ES1) [11]. GHG emissions are generally reported as moles per VMT throughout this report instead of the traditional grams per VMT [12]. The raw quantity of each greenhouse gas (“unweighted”) is reported, in terms of the amount emitted into the atmosphere per VMT. In addition, a “weighted” quantity is shown for each gas except water vapor. This represents the equivalent amount of carbon dioxide that would have to be emitted in order to have the same heat-absorbing capability as the unweighted amount of each gas.

Table ES1. Total Fuel Cycle Greenhouse Gas Emissions From Gasoline and Alternative Transportation Fuels
Greenhouse Gas	Gasoline	Methanol From Natural Gas	Ethanol From Corn	Compressed Natural Gas	Liquefied Petroleum Gas
Unweighted Quantities (millimoles per VMT)
Carbon Dioxide (CO2)	7,900	8,700	7,400	5,640	6,000
Water Vapor (H2O)	7,750	16,230	17,280	10,930	8,180
Methane (CH4)	22	34.6	39.3	91.3	17.2
Nitrous Oxide (N2O)	1.7	1.7	9.3	1.6	1.6
Nitrogen Oxides (NOx)	26.5	36.2	58.3	24.2	22.9
Carbon Monoxide (CO)	330.4	327.1	258.25	324.2	325
Weighted Quantities (moles CO2 equivalent per VMT)
Carbon Dioxide (CO2)	7.9	8.7	7.4	5.64	6
Water Vapor (H2O)a	NA	NA	NA	NA	NA
Methane (CH4)	0.22	0.35	0.39	0.91	0.17
Nitrous Oxide (N2O)	0.54	0.54	2.98	0.54	0.54
Nitrogen Oxides (NOx)	1.06	1.45	2.33	0.97	0.92
Carbon Monoxide (CO)	0.99	0.98	0.78	0.97	0.98
Totalb	10.71	12.02	13.88	9.03	8.61
aGlobal warming potential value for water vapor has not been determined by the Intergovernmental Panel on Climate Change. bTotal weighted greenhouse gas (GHG) emissions do not include contributions from water vapor. CO2 = Carbon dioxide. NA = Not applicable because the Intergovernmental Panel on Climate Change has not issued a global warming potential for water vapor. VMT = Vehicle mile traveled. For notes and sources, see Table 1.

Total “weighted” emissions of alternative transportation fuels do not vary by more than 20 percent from those of gasoline. Because of the uncertainties in estimating the “weighting factors” (i.e., the global warming potential (GWP) [13] of each gas, compared with carbon dioxide), the fuel-specific variations shown in weighted emissions may not be significant. Alcohol fuels show greater weighted emissions than conventional gasoline, while the gaseous fuels show less. Weighted estimates for water vapor are not presented because the Intergovernmental Panel on Climate Change has not established a definitive GWP factor for water vapor.

These results represent emissions from the total fuel cycle--from resource recovery through energy transformation (e.g., refining) through end-use consumption. Emission estimates for each portion of the fuel cycle are shown in the report. Notable facts involving total fuel cycle emissions include the following:

• CNG produces the lowest level of carbon dioxide emission across the total fuel cycle, followed by LPG and ethanol from corn.
• CNG produces the largest methane emissions.
• Ethanol from corn produces the largest nitrous oxide emissions across the total fuel cycle.

Except for methanol, the “vehicle” (end-use) portion of the fuel cycle accounts for at least 80 percent of total fuel cycle carbon dioxide emissions (Figure ES1). This suggests that examining actions to reduce greenhouse gases as a direct result of vehicle use is justified.

D

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