Because of the international nature of aviation and the need to promote international standards of safety, regulation of air transport is harmonized on a worldwide basis as much as possible through ICAO, a UN specialized agency established in 1944 under the Convention on International Civil Aviation (the "Chicago Convention") (ICAO, 1997a). With the technical support of states and the aviation community, ICAO has developed international standards and recommended practices for aviation. These standards and practices are attached as annexes to the Chicago Convention and are considered binding. However, if any state finds it impossible to comply with them, the state is required to inform ICAO of any differences. ICAO also provides guidance material for states on various aspects of international civil aviation.
In addition to operating under the harmonization achieved through ICAO, international flights are also subject to bilateral air service agreements between states. Although ICAO's mandate relates to international civil aviation, most states also take ICAO's international standards and recommended practices and guidance material into account in regulating their domestic aviation.
The ICAO strategic action plan acknowledged "a perception that the aviation sector may be contributing unduly to both existing and future environmental problems." The ICAO program of implementation action on the strategic action plan for the 1996-98 triennium lists several activities that could be relevant to environmental issues (ICAO, 1997c).
In the environmental field, ICAO has established standards for aircraft noise and aircraft engine emissions (Annex 16), as well as standards and procedures for operational measures such as landing and departure (ICAO, 1993a,b). It has also developed broader policy guidance on fuel taxation and charging principles that have relevance in an emissions context (ICAO, 1994, 1997b).
Much of ICAO's work in the environmental field is undertaken by its Committee on Aviation Environmental Protection (CAEP). CAEP examines the effectiveness and reliability of proposed aircraft certification schemes from the viewpoint of technical feasibility, economic reasonableness, and environmental need. CAEP also examines other issues related to emissions from aircraft engines-including international and national research into the impact of emissions and possible means of controlling emissions, such as operational measures and emissions-related levies. CAEP works closely with regional bodies and national airworthiness authorities to discuss and propose changes in recommended environmental standards.
Although the United Nations Framework Convention on Climate Change (UNFCCC) does not specifically refer to emissions from aviation, its coverage includes emissions from all sources. One of the commitments in the Convention is that parties to the Convention compile national inventories of their emissions sources (UNFCCC, 1992).
IPCC guidelines provide advice on the quantification of aviation emissions (IPCC, 1996e). Even after aviation emissions have been quantified, an important distinction is whether such emissions are domestic or international. For domestic flights, emissions are considered to be part of the national inventory of the country within which the flights occur. The IPCC guidelines require international aviation emissions to be estimated by the country where the fuel is sold, although such emissions are not to be included in that country's total emissions. For international flights, the problem is how to allocate the emissions (referred to as "emissions from international aviation bunker fuels" in UNFCCC terminology, although "international" is not always specified) to national inventories. A similar problem exists for shipping.
In an attempt to resolve this problem, the UNFCCC Subsidiary Body for Scientific and Technological Advice (SBSTA) has been presented with the following allocation options for emissions from international aviation and marine bunker fuels (UNFCCC, 1996):
. Option 1-No allocation
. Option 2-Allocation of global bunker sales and associated emissions to parties in proportion to their national emissions
. Option 3-Allocation according to the country where the bunker fuel is sold
. Option 4-Allocation according to the nationality of the transporting company, or to the country where an aircraft or ship is registered, or to the country of the operator
. Option 5-Allocation according to the country of departure or destination of an aircraft or vessel; alternatively, emissions related to the journey of an aircraft or vessel shared by the country of departure and the country of arrival
. Option 6-Allocation according to the country of departure or destination of passengers or cargo; alternatively, emissions related to the journey of passengers or cargo shared by the country of departure and the country of arrival
. Option 7-Allocation according to the country of origin of passengers or owner of cargo
. Option 8-Allocation to a party of all emissions generated in its national space.
SBSTA subsequently noted that there were three separate issues: adequate and consistent inventories, allocation of emissions, and control options. Appropriate allocation of responsibility for emissions from international bunker fuels would be connected to inventory and control issues. Reviewing the foregoing eight options, SBSTA decided that Options 1, 3, 4, 5, and 6 should be the basis for further work on this issue (UNFCCC, 1997).
To date, however, there has been no agreement among parties to the Convention on which option to choose. At Kyoto in December 1997, the Conference of Parties to the UNFCCC urged SBSTA to elaborate further on the inclusion of emissions from fuel sold to aircraft and ships engaged in international transport in overall greenhouse gas inventories (UNFCCC, 1998a).
The Kyoto Protocol to the UNFCCC, which has not yet entered into force, requires Annex I (industrialized) countries to reduce their collective emissions of greenhouse gases by approximately 5% by 2008-2012 compared with 1990 levels, with the reduction varying from country to country. The agreed-on targets apply to national totals of greenhouse gases. Consequently, each Annex I country can determine how the various emission-producing sectors in its economy should be called upon to assist in achieving the country's national target.
Because international aviation and marine emissions are not included in national inventories, they are currently excluded from the agreed-on targets. However, the treatment of aviation and marine emissions was considered in Kyoto in the context of discussions on policies and measures to be pursued by Annex I countries, and a provision was included in the Kyoto Protocol. The relevant text (Article 2, paragraph 2) reads as follows:
"The Parties included in Annex I shall pursue limitation or reduction of emissions of greenhouse gases not controlled by the Montreal Protocol from aviation and marine bunker fuels, working through the International Civil Aviation Organization and the International Maritime Organization, respectively." (UNFCCC, 1998b)
A question has since arisen in the aviation community regarding whether this provision covers emissions from international aviation only or emissions from both international and domestic aviation. Although this issue has not been resolved, ICAO commented in a statement to the Conference of Parties to the UNFCCC in November 1998 that ICAO's mandate (from the 1944 Convention on International Civil Aviation) does not extend to domestic aviation but that ICAO's standards, recommended practices, and procedures in many circumstances have a de facto application domestically (ICAO, 1998c).
At the ICAO assembly in September/October 1998, the ICAO Council was asked to study policy options to limit or reduce greenhouse gas emissions from civil aviation-taking into account the findings of this IPCC Special Report and the requirements of the Kyoto Protocol-and to report to the Assembly at its next ordinary session in 2001. ICAO was also asked to work with SBSTA to consider the various options for allocation of emissions from international aviation (ICAO, 1998b).
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