|Aviation and the Global Atmosphere|
|Other reports in this collection|
This section compares CO2 emissions from various transport modes. Only CO2 emissions are discussed, primarily because other pollutant emissions are not comparable at all altitudes. Climate forcing from CO2 is independent of the altitude and geographical location of its release. Potential atmospheric effects of other gaseous particle emissions from aircraft will likely depend on where and when they occur. For example, the potential climate forcing and ozone perturbation effects of NOx emitted at altitude will be significantly different from the effects of NOx emitted by ground transport sources. Finally, substituting other transport modes for air transport can have environmental impacts on, for example, local air quality and noise exposure (which are outside the scope of this report).
Emissions of CO2 from all transport sectors currently account for about 22% of all global emissions of CO2 from fossil fuel use (IPCC, 1996a). In 1990, aviation was responsible for about 12% of CO2 emissions from the transport sector (see Figure 8-2) (Faiz et al., 1996; IPCC, 1996b; OECD, 1997a,b). Regional variations also occur, as shown in Figure 8-3 for North America. Consequently, aviation is currently responsible for about 2% of total global emissions of CO2 from the use of fossil fuels (Sprinkle and Macleod, 1993; WMO, 1995; Gardner et al., 1996).
Civil aviation can be divided into domestic and international flights, and their respective levels of CO2 emissions can be determined. Balashov and Smith (1992) provided an estimate of this breakdown (see Table 8-5).
Chapter 9 provides a detailed assessment of the future growth of the aviation industry, fleet scenarios, and projected fuel burn.
Rail and other forms of transport, such as bus or motorcar, have been suggested as substitutes for short-haul air travel (there is little alternative for longer distances). Flight stages of 800 km or less are estimated to represent only about 15-20% of all scheduled passenger operations (expressed in terms of available seat-km), according to an estimate provided by the ICAO Secretariat based on an analysis of 1996 airline schedules. Bearing in mind that a significant proportion of passenger journeys include more than one flight stage and that some short-haul flights bypass physical obstacles such as water, mountains, or inadequate ground infrastructure, the potential scope for replacing air transport with other modes seems unlikely to exceed about 10%.
Fare, trip time, and frequency of service rather than environmental considerations influence the choice a passenger makes for a particular mode of transport. A paper presented to a European Air Traffic Forecasting Forum (ECAC, 1996) estimates that "...even under favorable assumptions for rail, less than 10% of the European air passengers could be substituted by high-speed train."
Figure 8-4 compares CO2 emissions from major passenger transport modes. The wide ranges in CO2 intensity of passenger transport reflect many differences between countries and regions, including the availability of renewable and nuclear energy, the extent of road and rail infrastructure, and culture. For example, in the United States, greater importance is placed on aircraft and automobile modes of travel than on rail. The automobile mode serves mainly shorter urban and commuter needs, whereas aircraft serve longer intercity needs. This pattern is illustrated by the number of domestic flights-more than five times the number of international flights-which demonstrates greater U.S. dependence on short-haul aviation than in other geographic regions. In addition, U.S. cars and light trucks tend to be higher emitters because of increased vehicle and engine size (right of the range shown in Figure 8-4).
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 long-haul 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).
Figure 8-5 provides a more detailed analysis of CO2 emissions (g C per passenger-km) from a range of aircraft based on flight and fuel use data collected by British Airways and studies from Germany, Switzerland, The Netherlands, and the European Commission (Hofstetter and Meienberg, 1992; Prognos AG, 1995; Centre for Energy Conservation and Environmental Technology, 1997a,b; British Airways, 1998b; European Commission, 1998). New aircraft, particularly long-haul aircraft (e.g., B747-400), are significantly more fuel efficient and emit less CO2 per passenger-km than older aircraft (e.g., DC-10). Over short-haul routes, advanced turboprops (ATP) emit about 20% less CO2 (as carbon) per passenger-km than new jet aircraft (B737-400) and up to three-fold less CO2 than older jet aircraft (MD81/ F-100). Chapter 7 provides a detailed assessment of aircraft fuel efficiencies.
Figure 8-6 provides a comparison of CO2 emissions from major freight transport modes. In terms of CO2 (g C per tonne-km), aviation emits 1 to 2 orders of magnitude more carbon than other forms of transport. Cost and weight limitations do not allow aircraft to compete in the transport of heavy goods. For perishable freight and high-value goods, however, there may be no other suitable form of transportation.
Freight may be carried to improve capacity utilization on combination freight-passenger aircraft or passenger aircraft with appropriate space. This approach improves efficiencies, though such improvements are not necessarily reflected in all statistics.
The amount of CO2 emitted per passenger-km for different modes of transport is very dependent on the type of aircraft, train, or car and on the load factor. Typical CO2 emissions for air transport are in the range of 30 to 110 g C per passenger-km, which is comparable with passengers travelling by car or light truck. Emission of CO2 per passenger-km from bus or coach transport is significantly lower (< 20 g C per passenger-km). For rail travel, CO2 emissions per passenger-km depend on several factors, such as source of primary energy, type of locomotive, and load factor; emissions vary between < 5 and 50 g C per passenger-km.
Other reports in this collection