ENERGY USE, EMISSIONS, AND ENVIRONMENTAL IMPACT
The growth in air transportation volume has important global environmental impacts associated with the potential for climate change. On local to regional scales, noise, decreased air quality related primarily to ozone production and particulate levels, and other issues, such as roadway congestion related to airport services and local water quality, are all recognized as important impacts. In this section, the focus is on emissions-related impacts; because of its relative importance, some additional detail on the aviation role in climate change is provided.
The total mass of emissions from an aircraft is directly related to the amount of fuel consumed. Of the exhaust emitted from the engine core, 7-8% is composed of carbon dioxide (CO2) and water vapor (H2O); another 0.5% composed of nitrogen oxides (NOx), unburned hydrocarbons (HC), carbon monoxide (CO), and sulfur oxides (SOx); there are other trace chemical species that include the hydroxy family (HOx) and the extended family of nitrogen compounds (NOy), and soot particulates. Elemental species such as O, H, and N are also formed to an extent governed by the combustion temperature. The balance (91.5-92.5%) is composed of O2 and N2.
Emissions of CO2 and H2O are products of hydrocarbon fuel combustion and are thus directly related to the aircraft fuel consumption, which in turn is a function of aircraft weight, aerodynamic design, engine design, and the manner in which the aircraft is operated. Emissions of NOx, soot, CO, HC, and SOx are further related to details of the combustor design and, to some extent, to postcombustion chemical reactions occurring within the engine. These emissions are thus primarily controlled by the engine design, but total emissions can be reduced through improvements in fuel efficiency. Such emissions are therefore typically quoted relative to the total amount of fuel burned as an emission index (e. g., grams of NOx/kilogram of fuel). A host of minor constituents exist in very small, trace amounts.
The climate effects of aviation are perhaps the most important of the environmental impacts, both in terms of economic cost and the extent to which all aspects of the aviation system, operations, and technology determine the impact. Because a majority of aircraft emissions are injected into the upper troposphere and lower stratosphere (typically 913 km in altitude), resulting impacts on the global environment are unique among all industrial activities. The fraction of aircraft emissions that is relevant to atmospheric processes extends beyond the radiative forcing effects of CO2. The mixture of exhaust species discharged from aircraft perturbs radiative forcing two to three times more than if the exhaust was CO2 alone. In contrast, the overall radiative forcing from the sum of all anthropogenic activities is estimated to be a factor of 1.5 times CO2 alone. Thus the impact of burning fossil fuels at altitude is approximately double that due to burning the same fuels at ground level. The enhanced forcing from aircraft compared with ground-based sources is due to different physical (e. g., contrails) and chemical (e. g., ozone formation/destruction) effects resulting from altered concentrations of participating chemical species and changed atmospheric conditions. However, many of the chemical and physical processes associated with climate impacts are the same as those that determine air quality in the lower troposphere.
Estimates of the radiative forcing by various aircraft emissions for 1992 offered by the Intergovernmental Panel on Climate Change (IPCC) and the 1999 projections from Penner et al. for the year 2050 are shown in Fig. 1. The estimates translate to 3.5% of the total anthropogenic forcing that occurred in 1992 and to an estimated 5% by 2050 for an allsubsonic fleet. Associated increases in ozone levels are expected to decrease the amount of ultraviolet radiation at the surface of the earth. Future fleet composition also impacts the radiative forcing estimate. A supersonic aircraft flying at 17-20 km would have a radiative forcing five times greater than a subsonic equivalent in the 9- to 13-km range. It is important to note that these estimates are of an uncertain nature. Although broadly consistent with
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FIGURE 1 Radiative forcing estimated for 1992 (0.05 W/m2 total) and projected to 2050 (0.19 W/m2 total). Note differences in scale. Note also that the dashed bars for aviation-induced cirrus cloudiness describe the range of estimates, not the uncertainty. The level of scientific understanding of this potential impact is very poor and no estimate of uncertainty has been made. Cirrus clouds are not included in the total radiative forcing estimate. Reproduced from Penner et al. (1999), with permission.
these IPCC projections, subsequent research reviewed by the Royal Commission on Environmental Protection (RCEP) in the United Kingdom has suggested that the IPCC reference value for the climate impact of aviation is likely to be an underestimate. In particular, although the impact of contrails is probably overestimated in Fig. 1, aviation-induced cirrus clouds could be a significant contributor to positive radiative forcing; NOx- related methane reduction is less than shown in Fig. 1, reducing the associated cooling effect, and growth of aviation in the period 1992-2000 has continued at a rate larger than that used in the IPCC reference scenario.