Aviation and the Global Atmosphere

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9.1. Introduction

The nature and composition of aircraft emissions has been described in Chapter 1, and their effects on the composition of the atmosphere are described in Chapters 2 and 3. Chapter 4 uses aircraft emissions data in modeling studies to provide chemical perturbations that feed into the ultraviolet (UV) irradiance and radiative forcing calculations presented in Chapters 5 and 6, respectively. In this chapter, the aircraft emissions data that were used in calculations described in Chapters 4 and 6 are presented and discussed.

Compilation of global inventories of aircraft NOx emissions has been driven by requirements for global modeling studies of the effects of these emissions on stratospheric and tropospheric ozone (O3). Aircraft carbon dioxide CO2) emissions are easily calculated from total fuel burned. Early studies used one- (1-D) and two-dimensional (2-D) models of the atmosphere (see Section 2.2.1). Most of these early studies considered effects on the stratosphere (e.g., COMESA, 1975), but some also included assessments of the (then) current subsonic fleet on the upper troposphere and lower stratosphere (e.g., Hidalgo and Crutzen, 1977; Derwent, 1982). An early height- and latitude-dependent emissions inventory of aircraft NOx was given by Bauer (1979), based on earlier work by A.D. Little (1975). This work was used by Derwent (1982) in a 2-D modeling study of aircraft NOx emissions in the troposphere.

Later estimations of global aircraft emissions of NOx were still made by relatively simple methods, using fuel usage and assumed EI(NOx) (e.g., N��er and Schmitt, 1990; Beck et al., 1992). Concerted efforts were subsequently made by a number of groups to construct high-quality global 3-D inventories of aircraft emissions. Such work was undertaken for a variety of programs and purposes: United Kingdom input to ICAO Technical Working Groups (McInnes and Walker, 1992); the U.S. Atmospheric Effects of Stratospheric Aircraft (AESA) Program (Wuebbles et al., 1993); the German "Schadstoffe in der Luftfahrt" Program (Schmitt and Brunner, 1997); and the ANCAT/EC Emissions Database Group (ANCAT/EC, 1995), which combined European efforts to produce an aircraft NOx inventory for the AERONOx Program (Gardner et al., 1997). Subsequently, methodologies for the production of global 3-D inventories of present-day aircraft NOx emissions (based on 1991-92) have been refined and have produced results that have largely superseded earlier work. These inventories cover the 1976-92 time period and have been extended to the 2015 forecast period. These gridded inventories-which calculate aviation emissions as distributed around the Earth in terms of latitude, longitude, and altitude-have been produced by NASA, DLR, and ANCAT/EC for national and international work programs (Baughcum et al., 1996a,b; Schmitt and Brunner, 1997; Gardner, 1998).

This chapter is not the first attempt to synthesize information on aircraft emissions inventories; earlier assessments were made by the World Meteorological Organization (WMO)/ United National Environment Programme (UNEP) (1995) Scientific Assessment of Ozone Depletion, ICAO's Committee on Aviation Environmental Protection (CAEP) Working Group 3 (CAEP/WG3, 1995), the NASA Advanced Subsonic Technology Program (Friedl, 1997), and the European Scientific Assessment of the Atmospheric Effects of Aircraft Emissions (Brasseur et al., 1998).

Any assessment of present and potential future effects of subsonic and supersonic air transport emissions relies heavily on input emissions data. Thus, considerable effort has been expended on understanding the accuracy of present-day inventories and the construction of forecasts and scenarios. Forecasts are quite distinct from scenarios, as noted in Chapter 1. Forecasts of aviation emissions for a 20-25 year time frame are generally considered possible, whereas such confidence is not the case for longer time frames. Thus, scenarios generally rely on many more assumptions and are less specific than forecasts.

In planning this Special Report, it was clear that there were no gridded emission scenarios of NOx emissions from subsonic aircraft for the year 2050 that could be used as input to 3-D chemical transport models (see Chapters 2 and 4). The IPCC made a request to ICAO to prepare 3-D NOx scenarios, which was carried out under the auspices of ICAO's FESG (CAEP/4-FESG, 1998). The UK DTI also responded to this requirement, producing an independent 3-D NOx scenario for 2050 (Newton and Falk, 1997). The EDF had also published scenarios of aircraft emissions of NOx and CO2 extending to 2100 (Vedantham and Oppenheimer, 1994, 1998), but these scenarios were not gridded; thus, although the aviation CO2 scenarios could be used in radiative forcing calculations (see Chapter 6), the NOx scenarios could not be used to calculate O3 perturbations and subsequent radiative forcing. Other scenario data exist for aircraft emissions, including those from WWF (Barrett, 1994) and MIT (Schafer and Victor, 1997). As with the EDF data, these scenarios were not gridded for NOx emissions, therefore could not be used in O3 perturbation calculations in Chapter 4. Furthermore, the MIT data do not explicitly represent aircraft emissions; instead, they cover high-speed transport modes, including some surface transportation modes.

HSCT scenarios prepared for NASA's AESA Program are considered distinct from subsonic scenarios; these HSCT scenarios represent a technology that does not yet exist but might be developed. Therefore, the HSCT scenarios represent a quite different set of assumptions from other long-term scenarios, which only consider continued development of a subsonic fleet. The HSCT scenarios were used in modeling studies (Chapters 4 and 6) as sensitivity analyses for studying the effects of their emissions on stratospheric O3.

In this chapter, methodologies of inventory and forecast construction are compared, and a review and assessment of long-term scenarios and their implicit assumptions provided. This is the first detailed consideration of long-term scenarios and their implications.

By way of background, Section 9.2 provides an overview of factors that affect aircraft emissions, such as market demand for air travel and developments in the technology. The aircraft emissions data discussed in this chapter are of four distinct types: Historical inventories (e.g., for 1976 and 1984); inventories that represent the "present day" (i.e., 1991-92); forecasts for 2015; and long-term scenarios for 2050 and beyond. The methodologies and a comparison of historical, present-day, and forecast inventories are presented in Section 9.3. Section 9.4 describes and comments on available long-term scenarios for 2050 and beyond. Scenarios of high-speed civil transport (HSCT) that incorporate certain assumptions about the development of a supersonic fleet and its impact on the subsonic fleet are presented separately in Section 9.5. Finally, Section 9.6 discusses underlying assumptions and drivers of long-term subsonic scenarios. The plausibility of the assumptions are also considered in terms of implications for fleet size, infrastructure requirements, and global fossil-fuel availability.

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