9.6. Evaluation and Assessment of Long-Term Subsonic Scenarios

9.6.1. Difficulties in Constructing Long-Term Scenarios

Long-term (beyond 20 years) projections of aviation traffic demand, fleet fuel burned, and fleet emissions are inevitably speculative. Difficulty in forecasting technological developments that might be appropriate for the long term, possible shifts in traffic demand, and myriad uncertainties resulting from human society's development over the period in question all conspire to make long-term projections unreliable-sometimes astoundingly so. Given the state of the aviation industry 50 years ago (in 1947), it is doubtful that either the technology or the scope of the industry in 1997 could have been forecast. However, because the transport aviation market and aviation technology seem to be maturing, a plausible way of making projections far into the future is to make reasonable extrapolations based on our knowledge of present trends in the world and in the aviation industry. These extrapolations are termed scenarios, rather than forecasts, as outlined in Section 9.1.

9.6.2. Structure and Assumptions

Before we review the outcomes of the scenario studies in the following section, we consider some differences and similarities between the models. This comparison is restricted to the EDF, DTI, and FESG models. Although the MIT model provides an interesting insight into future travel options based on the thesis of invariant travel time and travel expenditure budgets, it is excluded from this comparison because it provides only a highly aggregated scenario for the future mobility of total motorized passenger traffic; air traffic is only one-albeit important-portion of this picture, and the aircraft component cannot be identified. The WWF aviation scenario for 2041 provides aggregated fuel burned and CO2 emissions projections but does not provide regionally distributed NOx emissions estimates.

Of the long-term scenarios considered, the EDF, FESG, and DTI studies allow assessment of the impacts of CO2 from aviation. However, only the results from the DTI and the FESG models are suitable for use in chemical transport models for modeling other emissions (see Chapters 2 and 4) and their effects on radiative forcing (see Chapter 6) because they provide gridded data that include a consideration of the potential changes in the spatial distribution of emissions. Only the EDF study provides scenarios for demand from the aviation sector and subsequent global CO2 and NOx emissions to 2100. The EDF study provided 10 scenarios based on five different IPCC IS92 world scenarios for the long-term development of world economy and population and two air traffic demand scenarios (base case and high case). The FESG study calculated three air traffic demand scenarios based on the IPCC IS92a, IS92c, and IS92e world scenarios, which were combined with two engine technology scenarios to produce six different emissions inventories. The FESG scenarios of regional and global air traffic were based on a logistic regression model of traffic demand since 1960 using global GDP as a predictor. The FESG model used a combination of top-down and bottom-up approaches, in which global volumes of civil aircraft flight kilometers were predicted using the regression model for different GDP scenarios. All available information on regions, including regional variation in growth, was then used to disaggregate these global values in a consistent way over 45 traffic flows within and between the regions of the world by using a market share allocation model. Year 2050 values of fuel burned and NOx emissions for military traffic were estimated separately.

The EDF scenarios also were based on the use of logistic growth curves to model air traffic growth for business and personal travel (plus military and freight traffic). Model parameters were chosen through observation of historical traffic trends in the United States. Regional population was used as a predictor of personal passenger travel, and regional GNP was used as a predictor of business passenger travel and freight demand. Both the FESG and EDF models incorporate the underlying assumption that the chosen parameters are satisfactory predictors of aviation demand and that aviation markets eventually mature.

There are large differences between the EDF and FESG models with respect to the development of emissions scenarios. The EDF model uses a constant capacity logistic to describe fuel efficiency improvements, which extrapolates Greene's (1992) forecast to 2010 with varied rates for five geographic world regions and the military/freight aviation sector. For the trend in fleet EI(NOx) a single global logistic model extrapolates from the 1990 and 2015 values. The FESG scenarios are based on two engine technology scenarios developed by ICCAIA for ICAO/FESG and IPCC (see Chapter 7). These scenarios represent an industry perspective on likely future developments in fuel efficiency and NOx reduction technologies, as well as further potentials and limitations. The fuel efficiency technology element of the DTI scenario was similar in this respect, but a NOx technology scenario appropriate to stricter emissions regulations was assumed, in which subsonic engine research programs would deliver emissions levels similar to those targeted in the NASA HSCT program.

Additional assumptions are also important to the results of the scenario models. In the EDF model, assumptions about the dates of market expansion and maturity and the ultimate capacity levels chosen for the economic regions strongly influence the outcomes. The EDF, FESG, and DTI models all use statistics of traffic/air traffic from international organizations and OECD countries, as well as numerous other recently published sources, and adopt one or more of the IPCC IS92 scenarios to describe the long-term development of worldwide economic growth and population. The FESG, EDF, and DTI models also use information from the NASA and ANCAT/EC gridded inventories of traffic flows and related emissions. The FESG models used new, partly proprietary, information from industry as a base to project emissions in the year 2050.

None of the 3-D gridded inventories for 2050 assume any changes in design that would alter the cruise altitudes of subsonic aircraft. Furthermore, no consideration was given in any of the 2050 scenarios reported here to the possible stimulative (or otherwise) effect of HSCT introductions on traffic.

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