Aviation and the Global Atmosphere



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9.4.4. World Wide Fund for Nature Long-Term Scenario

Table 9-20: Results of MIT reference scenario-passenger travel and carbon emissions.
	1990 (10¹² pkm^a)	2050 (10¹² pkm^a)	1990 (10¹² mt C)	2050 (10¹² mt C)
Industrialized
High-Speed	1.5	32.7	32.7	0.66
Total	12.4	44.4	0.52	1.12
Reforming
High-Speed	0.3	2.1	0.02	0.04
Total	2.3	7.1	0.07	0.20
Developing
High-Speed	0.4	7.2	0.02	0.14
Total	8.6	53.8	0.18	1.29
World
High-Speed	2.2	42.0	0.13	0.84
Total	23.3	105.3	0.77	2.61
*Source:* Schafer and Victor (1997); additional data supplied by David Victor (June 1998). apkm=passenger kilometers.

Figure 9-23: Flight tracks above 13-km altitude for a
fleet of 500 high-speed civil transports (Baughcum and
Henderson, 1998).

Figure 9-24: Altitude distribution of fuel burned-with
and without HSCT fleet-based on IS92a scenario
(Fa1,2).

A study by WWF addresses future aviation demand by analyzing load factors and capacity constraints, particularly in the freight market (Barrett, 1994). Analysis of historical data shows that increases in the number of seats per aircraft have begun to level off. The study examines the effects of pollution control strategies such as phasing out of air freight and policies to encourage intermodal shifts to road and rail. Technological options for reducing the environmental impact of aviation (such as operational improvements, changes in cruise altitude and alternative fuel sources) are examined. In particular, these models consider the feasibility that increases in load factors (percentage of total passenger seats that are occupied) could increase fuel efficiency per seat-km for aviation. The model evaluates a wide range of policy and operational choices, including a 100% load factor and a 100% fuel tax.

The model includes explicit assumptions of fixed growth rates in leisure travel, business travel, average trip length for passenger and freight traffic, and freight tonnage. It assumes that passenger load factors rise to 75% by 2020 in the base case. Constant rates of improvement are assumed for aircraft size, airframe efficiency, and EI(NOx).

With an annual growth rate of 5.2%, demand rises by a factor of more than 12 between 1991 and 2041 in the "business-as-usual" case. Proposed policies, including changes in load factor, and technological improvements result in a forecast for demand increase of about a factor of 3 in the "demand management" case. Carbon emissions in 2041 constitute 550 Tg C, and aviation's share of global carbon emissions rises to 15% by 2041.

9.4.5. Massachusetts Institute of Technology Long-Term Scenarios

A study of the long-term future mobility of the world population has been undertaken at MIT. This study constructed scenarios based on the simple yet powerful assumption that time spent and share of expenditures on travel remain constant (Zahavi, 1981), on average, over time and across regions of the globe (Schafer and Victor, 1997). Stability of average time budgets for travel (motorized and nonmotorized) is substantiated by a considerable amount of aggregate historical data. Although there is some variability in travel budgets from poorer to richer nations, within each society travel budgets have generally followed a predictable pattern-rising with income and motorization and stabilizing at 10-15%.

Using the constant travel budget hypothesis, Schafer and Victor (1997) produced global passenger mobility scenarios for 11 world regions and four transport modes for the period 1990-2050. Adding estimates of changes in the energy intensity of transportation modes, they also generated scenarios of CO2 emissions from passenger transport (see Table 9-20).

The high-speed travel category includes aviation, but the aviation portion of high-speed travel is not explicitly characterized. Results of this model projection therefore cannot be used directly in evaluations of the effect of aviation on the atmosphere, nor can they be directly compared to other long-term projections of emissions from aviation.

Table 9-21: Results of substitution of 1,000-unit parametric HSCT fleet in 2050.
Scenario	Fuel (Tg)	CO2 (Tg as C)	% Change (Fuel)	NOx (Tg as NO2)	% Change (NOx)	Fleet EI(NOx)
Fa1-All Subsonic Fa1H-With 1,000 active HSCTs Fe1-All Subsonic Fe1H-With 1,000 active HSCTs	471 557 744 831	405 479 641 715	Base +18 Base +12	7.2 7.0 11.4 11.3	Base -2 Base -1	15.2 12.6 15.3 13.6

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