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Greenhouse gas emissions from international aviation and allocation options

4 Aviation indicators and trends

A growing concern over emissions of greenhouse gases into the atmosphere has led governments to sign agreements on future reduction schemes [UNFCCC 1997]. Currently, the emissions from international air traffic are not included in these international commitments, but an increasing political focus on the sector internationally suggests that they might be in the future. In this respect it becomes relevant to assess the possible role of commercial civil air transport in a future greenhouse gas (GHG) reduction scheme.

4.1 Aviation's contribution to climate change

Air transport, being the fastest growing transportation mode, is currently a much smaller energy consumer than road transport, but may become a relatively large source in the future if the sector continues to grow at current rates. In 1990, road transport emitted around 75% of the CO₂ emissions from transport activities, while around 12% was attributable to air transport and 7% to international shipping and around 6% to rail and inland waterways [IPCC 1999]. Air transport is currently estimated to emit approximately 3% of the total CO₂ emissions associated with combustion of fossil fuels [IEA 2001]. According to the IEA, little more than half of the CO₂ emissions from air transport are related to international aviation, the rest being consumed in domestic aviation activities [IEA 2001]. However, it should be mentioned, that the distinction between domestic and international fuel consumption is relatively uncertain [Velzen 2000].

Aviation's contribution to climate change has been described by the IPCC in a comprehensive special assessment report "Aviation and the Global Atmosphere", requested by the International Civil Aviation Organisation (ICAO) and the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer [IPCC 1999].

The above mentioned IPCC report concluded that aircraft engine emissions at high altitudes are considered to change the atmospheric composition by altering the "concentration of atmospheric greenhouse gases, including carbon dioxide (CO₂), oxone (O₃) and methane (CH₄); trigger formation of condensation trails (contrails); and may increase cirrus cloudiness - all of which contribute to climate change" [IPCC 1999, p. 3]. Furthermore, according to the IPCC, the current knowledge about commercial civil air transport's overall contribution to climate change suggests that the total positive radiative forcing (warming) effect might be 2-4 times higher than that of CO₂ emissions from aircraft alone. This is because emissions of water vapour and NO_x act as greenhouse gases when emitted at cruising altitude (it should be noted that emissions of CO₂ do not contribute more to climate change when emitted at high altitude than when emitted at ground level). However, there is a high degree of uncertainty connected to this estimate, because the current knowledge about some of the atmospheric processes induced by high altitude aircraft engine emissions is relatively weak. Among the major uncertainties is the potential of persistent contrail formations to trigger the formation of cirrus clouds. The best estimate from the IPCC on the radiative forcing resulting from emissions from subsonic aircraft in 1992 is that they contribute with about 3,5% of the total radiative forcing caused by anthropogenic activities [IPCC 1999, pp 3- 10].

4.2 Demand growth and technical/operational improvements

Passenger air travel, measured in revenue passenger kilometres⁵ (RPKs), has grown continuously from year to year since 1960 except for two years, namely 1991 and 2001. In 1991, the war in the Persian Gulf pressed up the oil price leading to a general downturn in the economy and to some extent scared travellers from flying through fears of hijackings. Likewise, in 2001, the September 11 terrorist attack on World Trade Centre in New York lead to a decrease in the global demand for air traffic as compared to 2000. From 1960 to 1998 the number of RPKs increased more than 20-fold from around 131 billions to around 2888 billions, corresponding around 44 RPKs per capita globally in 1960 and almost 500 RPKs per capita in 1998 [Nielsen 2001]. Figure 2 illustrates the growth in the seat capacity of the World's scheduled airlines since the early days of commercial civil aviation.

Figure 2:
Seat capacity of the world's scheduled airlines (excluding the former Soviet Union). Source for data: ICAO Statistics taken from [DTI 1999].

While the demand for air travel and airfreight has grown in the last decades the fuel intensity of the aircraft fleet has been substantially reduced, mainly as a consequence of the use of less fuel intensive aircraft combined with an increase in the average load factor. However, these technical and operational improvements have not been substantial enough to reduce the total fuel use. Efficiency gains are overridden by volume growth.

This can be exemplified by an analysis of some overall developments for the American air carriers in the period 1973-1997, see Figure 3. Within that period the amount of revenue passenger kilometres (RPKs) and revenue freight tonne kilometres (RFTKs) grew by factors of 3,6 and 4,6 respectively, leading to an increase in the total amount of revenue tonne kilometres (RTKs) by a factor of 3,8. At the same time the specific average fuel consumption per revenue tonne kilometre was reduced by some 55% leading to an overall increase in fuel consumption by a factor of 1,7. Freight transport and passenger transport have grown at average yearly rates of around 6,5% and 5,4% since 1973. While the yearly growth rate in passenger air travel has slowed down in the second half of the period freight transport has grown faster in these later years than in the first half, see Figure 3.

Figure 3:
Some main developments for the US air carriers 1973-1997. Source: [Davis 1995 and 1999]

In the next decades, aviation fuel consumption is expected by the Intergovernmental Panel on Climate Change (IPCC) to continue growing. The IPCC describes and compares several long-term scenarios for global air traffic demand and associated fuel use and emissions until the middle of this century. These scenarios consider different combinations of developments in the demand for passenger air travel and airfreight and the specific fuel consumption and associated emissions of NO_x. In the scenarios the demand for air traffic, measured in Revenue Passenger Kilometres (RPK), is assumed to grow by between 360 percent and 2140 percent by 2050 as compared to 1990 leading to increases in fuel consumption of between 160 and 1600 percent and increases in emissions of NO_x of between 160 and 810 percent. A central IPCC estimate for the next fifteen years projects air traffic and fuel use to grow by 5 percent and 3 percent per year respectively [IPCC 1999, p. 5 and p. 329]. It should be noted that these scenario calculations were presented prior to the 11 September 2001 terrorist attacks on World Trade Centre in New York. However, even though the magnitude seems uncertain, aviation's future contribution to climate change seems likely to grow in the next decades.

4.3 The significance of airfreight

On a global scale air freight accounts for some 30% of the weight transported by commercial airlines while passengers and their baggage accounts for the residual 70% (see Appendix H) [Nielsen 2001]. Therefore, the contribution of freight transport to the overall environmental load of air transport should not be neglected. In the last decade freight transport by air has grown stronger than passenger transport and the importance of airfreight is therefore growing. The tendency is exemplified by the main developments for the US air carriers illustrated in Figure 3. Some airfreight is carried in dedicated freighter aircraft, but the major share is carried as belly-hold in passenger aircraft. On short- to medium distance passenger flights freight typically only accounts for a rather insignificant share of the weight transported, but on long-distance flights freight often account for more than 40% of the revenue weight [Nielsen 2001].

4.4 Distributional issues and prospects for growth

Figure 4 illustrates, in overall terms, the flows of passengers and freight within and between World regions. Most of the freight tonne kilometres (a freight tonne kilometre describes transport of one tonne of freight over one kilometre) are transported between North America, Europe and Asia and within North America. We note that the passenger traffic within North America alone represents around 29% of the revenue passenger kilometres performed by scheduled airlines on a global scale.

Figure 4:
Major Traffic flows between regions of the world 1999. Scheduled services performed by IATA member airlines. Source: [IATA 2000].

People living in highly industrialised countries generate the bulk of passenger air travel and airfreight. In some industrialised countries people travel several thousand kilometres per year on average while people living in developing countries generally fly less than 100 kilometres per year, and in some countries less than 10 kilometres per year. For example, globally, people travel less than 500 kilometres by air per year on average, but average European and American citizens travel around 1200 and 3400 kilometres per year respectively. This does not only have implications for distributional concerns, but also exemplifies the growth potential represented by developing countries that may be on their way towards adapting Western consumption patterns. For example, if people currently living in China and India begin flying as much per capita each year as Europeans currently do on average, they would alone generate almost as much air traffic per year as is currently generated globally.

4.5 Determinants of air transport growth

Some important economic, physical, social and political determinants of passenger air travel growth are illustrated in the diagram in Figure 5. The circle in Figure 5 illustrates the size of passenger air travel demand. The arrows pointing out from the circle represents elements that currently seems to drive passenger air travel growth, while the arrows pointing towards the circle centre are meant to represent current and potential impeders. Note that many of the current drivers could become impeders in the future, i.e. the current drivers are not necessarily per se going to continue increasing the demand for air travel in the future.

Figure 5:
Some main determinants of air travel growth. Source: [Nielsen 2001].

Air transport growth is furthered by constantly enlarging the physical capacity of commercial civil air transport's socio-technical system and by improving its productivity while cutting real costs. Improved airline productivity brings reduced real airfares, and increasing income allows a higher number of people to fly. Economic growth in general as well as globalisation of economies, companies, markets, political systems and personal relations leads to the drive for travelling more often and over longer distances. Increasing migration, marriages across national borders and population growth are further aspects.

People are basically restricted from air travel by financial and time constraints as well as technology and geography. Financial constraints are mainly connected to airfares and personal incomes. Technology is an important constraint in the sense that aircraft speed, range and capacity limit the distance people are able to fly within the time available. Geographical characteristics also play an important part in the sense that the earth is a limited geographical area, and unless space-flight becomes available for a broad part of the population, there seems to be upper limits as to how far each person might want to travel in a year. Current impeders to passenger air travel growth are congested airports and airspace. In the future new environmental policies might emerge, and on the longer term a reduction or a saturation of world economic- and population growth could reduce air travel growth.

Figure 5 illustrates in broad terms that there are numerous driving forces generating air transport growth. It is beyond the scope of this report to give an in depth description of all these drivers. However, Figure 5 is intended to illustrate the broad range of factors that influences the demand for air travel to emphasize that potential policies aimed at reducing the future growth in air transport could, in principle, be directed towards changing any of these driving forces.

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