Environmental issues regarding emissions from aircraft were originally related to their contribution to local air quality in the vicinity of airports. These considerations led to the introduction of legislation in the United States resulting in domestic regulatory standards. Subsequently, ICAO developed international standards and recommended practices for the control of fuel venting and emissions of carbon moNOxide, hydrocarbons, nitrogen oxides, and smoke from aircraft engines over a prescribed landing/take-off (LTO) cycle below 915 m (3,000 feet) (ICAO, 1981). Although the global environmental effects of aircraft emissions have been a matter of much debate at a scientific and technical level, there are no specific standards for the control of emissions from aircraft during cruise. However, the LTO standards in place do indirectly limit emissions from an engine during climb and cruise.
The UNFCCC seeks to stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Its coverage includes emissions from all sources and all sectors, although it does not specifically refer to aviation. However, the Kyoto Protocol to the Convention, which was agreed in December 1997, includes two elements that are particularly relevant to aviation. First, the Kyoto Protocol requires developed countries to reduce their total national emissions from all sources by an average of about 5% for the years 2008-2012 compared with 1990 (with differences for individual countries). It contains a provision for so called "flexible mechanisms"-including emissions trading, "joint implementation," and a "clean development mechanism." Secondly, the Kyoto Protocol contains a provision (Article 2) that calls on developed countries to pursue policies and measures for the limitation or reduction of greenhouse gases from "aviation bunker fuels," working through ICAO. These issues are discussed in Chapter 10.
Future mitigation options and strategies will need to consider the motivation for increasing operational efficiencies and reducing fuel use in light of other environmental effects of aviation, such as noise. The availability and cost of fuel in the overall budget of aircraft operators will continue to exert strong pressure for fuel efficiency. A number of technological improvements (e.g., to airframe aerodynamics, aircraft weight, and engine cycle performance) over the years have improved the fuel efficiency of subsonic aircraft and engines. These innovations have had a direct impact on the amount of CO2 and H2O emitted by aircraft (the less fuel consumed, the less CO2 and H2O emitted) and have reduced CO and hydrocarbon emissions. The effect on emissions of NOx and particles is not as simple, however. The drive to improve fuel efficiency and reduce aircraft noise has resulted in a general trend to higher operating pressures and temperatures in engines and increased production of NOx for a given type of combustor technology. Combustor design changes can offset this problem to some extent but may result in increased complexity and weight. Different considerations may apply to the potential second generation of civil supersonic aircraft. The current status and potential changes in the technology of engines and aircraft themselves, with consequences for emissions, are discussed in Chapter 7.
Alternative fuels to aviation kerosene are being investigated, and some of these fuels have some attractive environmental characteristics. For instance, hydrogen offers the potential for eliminating direct CO2 emissions, though at the expense of increased H2O production. None of the alternatives appear capable of eliminating both CO2 and H2O emissions. The use of such fuels also would require the development and implementation of new technology and infrastructure, and many factors would need to be considered, including overall energy use, energy density, availability, cost, indirect impacts through production, and environmental benefits. These issues are also discussed in Chapter 7.
Future emissions from aviation will also be influenced by the manner in which aircraft are operated. At present, there is non-optimum use of airspace and ground infrastructure. However, advances in digital communications technology and satellite systems should allow new flight management procedures involving greater use of computerized air traffic control systems. In principle, such systems could lead to reductions in the lengths of routes between certain cities and higher traffic volumes in heavily flown corridors. More efficient routing would directly reduce fuel use and emissions. Economic and environmental benefits also might be enhanced through greater use of meteorological information. Changes in flight altitudes and speeds could occur as a result of new aircraft designs and operating procedures; these changes would result in aircraft emissions occurring at different altitudes. These factors are described in Chapter 8.
The framework within which technical and operational changes occur is influenced by government and industry, with aircraft safety the most important objective. Operator fleet decisions are influenced primarily by aircraft mission, performance, and operating cost, though aircraft technology and regulatory acceptance are significant parameters. Economic instruments such as fuel taxes and emissions charges affect an aircraft's operating costs. Aircraft operating limitations such as emission caps could directly affect capital investment as well as operating costs. On the other hand, new and emerging market mechanisms such as aircraft emissions trading are policy instruments that could introduce flexibility into regulatory compliance schemes. Such issues are discussed in Chapter 10.
When evaluating possible options for limiting certain emissions in the future, it is important to keep a proper perspective. This report is the first detailed assessment of the global environmental effects of a single industrial sector. Air transport is only one of a number of transport modes that use fossil fuel, either directly or indirectly. Each of these modes may have specific advantages, globally or nationally. Other sources of greenhouse gases and other emissions also contribute to the composition of the global atmosphere and are likely to change with time. The environmental consequences of all emissions (transport and non-transport) and the economic impacts associated with different policy options will need to be balanced.
In this report, the relative importance of various aircraft emissions are assessed according to the best available knowledge of atmospheric effects and in the light of current knowledge of future technological options. Economic analyses will be required to investigate the consequences of possible mitigation strategies. Ideally, these analyses would take into account the wide range of activities in the aeronautics and aviation industries and assign monetary value to emissions and their effects. The current state of such economic analysis is discussed in Chapter 10.
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