small amounts of nitrous oxide (N2O) and methane (CH4). Aircraft operating at high altitudes emit not only nitrogen oxides (NOx) [which increases the rate of ozone production by speeding the oxidation of carbon monoxide (CO) and CH4] but also water vapor [which generates contrails that, depending on the time of day they are produced, either reflect solar radiation back into space (daytime) or trap it (nighttime)] (IPCC 1999; see also Stuber et al. 2006).

Transport activity is also associated with two additional categories of emissions: (a) those produced in the extraction, production, and distribution of transport fuels and (b) those produced in the manufacture, distribution, and disposal of transport vehicles.2 A rough idea of the relative significance of these additional categories of emissions can be obtained from life-cycle studies that attempt to track all emissions related to a vehicle and its fuel. One of the best known of these studies estimates that the life-cycle CO2 emissions generated by a 1996-vintage midsize U.S. passenger car using gasoline as its fuel total 263 g/km, of which the vehicle manufacturing cycle (including disposal) accounts for 18 g/km (6.8 percent); the fuel cycle, 49 g/km (18.7 percent); and fuel combustion, 196 g/km (74.5 percent) (Weiss et al. 2000, 5–8).3

In this appendix, the committee attempts to provide as comprehensive a picture as possible of transport-related GHG emissions. It was not feasible to include emissions from each life-cycle stage or emissions of each GHG gas; we do, however, take care to identify which emissions are included in the data presented.


According to the 2005 edition of the International Energy Agency (IEA) publication CO2Emissions from Fuel Combustion, worldwide CO2 emis-

potential. Nearly all the remainder was accounted for by N2O (U.S. Energy Information Administration 2004, 31, 49, 62). This publication provides no information on aerosols produced by transport activity, but these are believed to be relatively insignificant.


 The second of these categories is of concern only with respect to road vehicles. The number of nonroad vehicles (locomotives, ships, and aircraft) is so small that the GHG emissions related to their manufacture, distribution, and disposal are minimal.


 The report assumes 95 percent recycling of metals and 50 percent recycling of plastics. In the report, the emissions figures are stated in grams of carbon per kilometer. For consistency with the other emissions data in this appendix, the figures have been converted here to grams of CO2 per kilometer.

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