|Aviation and the Global Atmosphere|
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9.2. Factors Affecting Aircraft Emissions
9.2.1. Demand for Air Travel
In the past 50 years, the air transport industry has experienced rapid expansion as the world economy has grown and the technology of air transport has developed to its present state. The result has been a steady decline in costs and fares, which has further stimulated traffic growth. As an example of this growth, the output of the industry (measured in terms of tonne-km performed) has increased by a factor of 23 since 1960; total GDP, which is the broadest available measure of world output, increased by a factor of 3.8 over the same period (ICAO, 1997a).
Although growth in world air traffic has been much greater than world economic growth, economic theory and analytical studies indicate that there is a high correlation between the two, and most forecasts of aviation demand are based on the premise that the demand for air transport is determined primarily by economic development. Statistical analyses have shown that growth in GDP now explains about two-thirds of air travel growth, reflecting increasing commercial and business activity and increasing personal income and propensity to travel. Demand for air freight service is also primarily a function of economic growth. Air travel growth in excess of GDP growth is usually explained by other economic and structural factors:
Changes in technology and in the regulatory environment have also had great effects on the growth in air travel demand. The modern era of air transportation began in the 1960s, driven by the replacement of piston-engined aircraft with jet aircraft that increased the speed, reliability, and comfort of air travel while reducing the cost of operation. The continuing trend of declining fares (as measured in constant dollars) began in this period. In real terms, fares have declined by almost 2% per year since 1960. Deregulation of airline services in the United States in 1978 allowed airlines to improve services by expanding theirroute systems and reduce average costs by greatly increasing the efficiency of scheduling and aircraft use. Trends toward liberalization of airline services in Europe and elsewhere will continue to increase airline efficiency.
Sharp increases in oil prices have had important (though temporary) effects on traffic demand. In addition to an adverse effect on the world economy, the 10-fold increase in crude oil prices in 1973-74 and further escalation in 1979-81 (since ameliorated) greatly increased aviation fuel prices. Air fares increased in response to higher costs, with a resulting decline in demand growth rates.Figure 9-1 provides evidence of the relationship between the economy and traffic demand by illustrating fluctuations in the rate of growth of each from 1960 to the present. The economic recessions of 1974-75, 1979-82 (largely caused by the increase in oil prices), and 1990-91 (the Gulf War) and their impact on air traffic are clearly visible.
The growth rate in global passenger demand over the past 35 years is shown in Figure 9-2. Freight traffic, approximately 80% of which is carried in the bellies of passenger airplanes, has also grown over the same time period. The declining trend in the rate of growth as the size of the industry has increased by more than 20-fold is a natural result of the total size of the industry (it is difficult to sustain an "infant industry" growth rate as size increases) and a maturing of certain markets-primarily those in the developed world-that dominate the statistics.Changes in demand in regional markets are given in Table 9-1 for the period 1970-95. Over this period, global traffic measured in revenue passenger kilometers (RPK) increased by a factor of 4.6 (Boeing, 1996). Table 9-1 is ordered by 1995 regional RPK value.
The trend in fuel efficiency of jet aircraft over time has been one of almost continuous improvement; fuel burned per seat in today's new aircraft is 70% less than that of early jets. About 40% of the improvement has come from engine efficiency improvements and 30% from airframe efficiency improvements (Figure 9-3, after Figure III-A-1 in Albritton et. al, 1997).
The growth rate of fuel consumed by aviation therefore has been lower than the growth in demand. Improvement in engine fuel efficiency has come mainly from the increasing use of modern high-bypass engine technology that relies on increasing engine pressure ratios and higher temperature combustors as a means to increase engine efficiency. These trends have resulted in drastic decreases in emissions of carbon moNOxide (CO) and unburned hydrocarbons (HC), though they tend to increase emissions of oxides of nitrogen (NOx). As a result, total NOx emissions from aircraft are growing faster than fuel consumption (see Figure 9-4, from NASA emissions inventories discussed in Section 9.3). A discussion of the technology required to reduce NOx emissions while continuing to improve engine efficiency appears in Chapter 7.
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