Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Care must be taken in interpreting such data to appreciate underlying assumptions and statistics from which they have been drawn. The load factor of transportation
modes is critical to the analysis. Car occupancy, in particular, can vary between 1 and 4. For example, in Europe the average is 1.65 (Centre for Energy Conservation
and Environmental Technology, 1997b), but in the United States this value is generally less than 1.2 (Institute of the Association for Commuter Transportation, 1997)which
implies a significant margin in specific emissions (per passenger-km) relative to average occupancy. Occupancy levels for air, rail, and bus also vary
significantly, but because of commercial pressures they are more likely to operate at higher levels than private road vehicles. European scheduled airlines typically
operate at a load factor of about 70% (AEA, 1997) and charter airlines at about 90%. These figures are comparable to those in the United States, where in 1996 the
average passenger load factor ranged from 48.6 to 75.4% for various passenger aircraft types and was 69.4% for all air carrier aircraft types (Bureau of Transportation
Statistics, 1997).
Energy consumption and CO2 emissions from electrically powered vehicles, particularly trains, are very dependent on the mode of electrical power generation. In
countries that have a large dependency on hydroelectric or nuclear power generation, emission of CO2 per passenger-km by rail may be very low (Figure 8-4).
Conversely, emissions of CO2 per passenger-km from high-speed locomotives with power derived from coal-fired electricity are considerably higher. In the case of
aviation, flight distance is very important. On a short flight (250 km), energy consumption and CO2 emissions are significantly higher than they are for medium- or longhaul
flights (see Figures 8-4 and 8-5), because a greater proportion of the flight is at take-off power (with a relatively higher fuel consumption). Also, available data do
not differentiate between aviation fuel used for passenger transport and that for freight. The Organisation for Economic Cooperation and Development (OECD) has
calculated that passengers roughly account for 71% of the load carried (OECD, 1997a), although on short-haul routes freight may account for less than 10% of the
weight (Centre for Energy Conservation and Environmental Technology, 1997b).
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Figure 8-6: CO 2 intensity of freight (Whitelegg, 1993; IPCC, 1996a; OECD, 1997a).