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1.5. Mitigation
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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