Aircraft engines produce emissions that are similar to other emissions resulting from fossil fuel combustion. However, aircraft emissions are unusual in that a significant proportion is emitted at altitude. These emissions give rise to important environmental concerns regarding their global impact and their effect on local air quality at ground level.

A comprehensive assessment concerning aviation's contribution to global atmospheric problems is contained in the Special Report on Aviation and the Global Atmosphere, which was prepared at ICAO's request by the Intergovernmental Panel on Climate Change (IPCC) in collaboration with the Scientific Assessment Panel to the Montreal Protocol on Substances that Deplete the Ozone Layer and was published in 1999. This told us inter alia:

• that aircraft emit gases and particles which alter the atmospheric concentration of greenhouse gases, trigger the formation of condensation trails and may increase cirrus cloudiness, all of which contribute to climate change; and
   
• that aircraft are estimated to contribute about 3.5 per cent of the total radiative forcing (a measure of change in climate) by all human activities and that this percentage, which excludes the effects of possible changes in cirrus clouds, was projected to grow.

The Report recognized that the effects of some types of aircraft emissions are well understood, revealed that the effects of others are not, and identified a number of key areas of scientific uncertainty that limit the ability to project aviation impacts on climate and ozone.

Against this background, the ICAO Assembly in 2001 urged States to promote scientific research aimed at addressing the uncertainties identified in this Report and requested the Council to continue to cooperate closely with the IPCC and other organizations involved in the definition of aviation's contribution to environmental problems in the atmosphere and the need to take initiatives for a scientific understanding of the problems (Assembly Resolution A35-5 (pdf), Appendix H). This was reiterated by the Assembly in 2007 (Assembly Resolution A36-22 (pdf), Appendix I). ICAO has requested the IPCC to include an update of the main findings of the 1999 report in its Fourth Assessment Report published in 2007.

The new findings related to aviation emissions are inter alia:

• Due to developing scientific knowledge and more recent data estimates of the climate effects of contrails have been lowered and aircraft in 2005 are now estimated to contribute about 3.0 % of the total of the anthropogenic radiative forcing by all human activities;
• Total CO₂ aviation emissions is approximately 2 % of the Global Greenhouse Emissions;
• The amount of CO₂ emissions from aviation is expected to grow around 3-4 percent per year; and
• Medium-term mitigation for CO₂ emissions from the aviation sector can potentially come from improved fuel efficiency. However, such improvements are expected to only partially offset the growth of CO₂ aviation emissions.

The Expanded Scope of Policy-making

In the past, ICAO's policy-making to address the environmental impact of aircraft engine emissions focussed primarily on the ground level effects. In recent years, the scope has been expanded to include the global impact of aircraft engine emissions.

In this regard, the Kyoto Protocol (pdf) (1997) to the United Nations Framework Convention on Climate Change (UNFCCC) is of particular importance. The Protocol, which entered into force on 16 February 2005, requires countries listed in Annex I to the Convention (industrialized countries) to reduce their collective emissions of six greenhouse gases, the one most relevant to aviation being carbon dioxide (CO₂). International aviation emissions are currently excluded from the targets. Instead, Article 2, paragraph 2 of the Kyoto Protocol states that the responsibility for limiting or reducing greenhouse gas emissions from aviation bunker fuels shall fall to the Annex I Parties, working through ICAO.

In 2007, the ICAO Assembly requested the Council (Assembly Resolution A36-22 (pdf), Appendix K) to continue to study policy options to limit or reduce the environmental impact of aircraft engine emissions and to develop concrete proposals and provide advice as soon as possible to the Conference of the Parties to the UNFCCC. It called for special emphasis to be placed on the use of technical solutions while continuing consideration of market-based measures, and taking into account potential implications for developing as well as developed countries.

Engine Certification Standards

Aircraft are required to meet the engine certification standards adopted by the Council of ICAO. These are contained in Annex 16 — Environmental Protection, Volume II — Aircraft Engine Emissions to the Convention on International Civil Aviation. These were originally designed to respond to concerns regarding air quality in the vicinity of airports. As a consequence, they establish limits for emissions of oxides of nitrogen (NO_x), carbon monoxide, unburned hydrocarbons, for a reference landing and take-off (LTO) cycle below 915 metres of altitude (3 000 ft). There are also provisions regarding smoke and vented fuel.

While these standards are based on an aircraft's LTO cycle, they also help to limit emissions at altitude. Of particular relevance is the standard for NO_x, a precursor for ozone, which at altitude is a greenhouse gas. The standard for NO_x was first adopted in 1981, then made more stringent in 1993, when ICAO reduced the permitted levels by 20 per cent for newly certificated engines, with a production cut-off on 31 December 1999. In 1999, the Council further tightened the standard by about 16 per cent on average for engines newly certificated from 31 December 2003. The latest review of medium- and long-term technology goals for NO_x [Independent Experts NO_x Review and the Establishment of Medium and Long Term Technology Goals for NO_x (Doc 9887)] will be published in 2008. While during CAEP/7 long term technology goals for NO_x were discussed, (Long Term Technology Goals for NO_x Presentation (PDF)).

The latest ICAO NO_x Emissions Standards became applicable in November 2005 and were applied to engines manufactured after December 2007.

The ICAO Engine Exhaust Emissions Data Bank (Doc 9646), issued in 1995, contains a comprehensive database of aircraft jet engine emissions certification data. Subsequent updates of the data bank are available through the ICAO website.

ICAO is currently considering alternative parameters on which to base future high altitude emissions controls, taking into account trends in emissions reduction technology, as well as the performance of the whole aircraft and its productivity. Particular attention is being given to NO_x. In the case of CO₂, it has been decided not to develop an ICAO standard, since CO₂ production is directly related to fuel consumption and there is already intense economic pressure to keep fuel consumption to a minimum and, in addition, there would be significant difficulties in designing a certification condition.

The Use of Operational Practices

In 2001, the ICAO Assembly requested the Council to promote the use of operational measures as a means of limiting or reducing the impact of aircraft engine emissions.

ICAO has published guidance material, in the form of the ICAO Circular 303-AN/176 published in February 2004, on operational opportunities to minimize fuel use and reduce emissions. The Circular includes information on aircraft ground level and in-flight operations, as well as ground service equipment and auxiliary power units.

ICAO has emphasized to States that early implementation of new communications, navigation, surveillance and air traffic management (CNS/ATM) systems would be an effective means of reducing fuel burn and avoiding unnecessary emissions. The results of a preliminary study of the environmental benefits associated with CNS/ATM and the methodology for their assessment have been incorporated into the Global Air Navigation Plan for CNS/ATM Systems (Doc 9750).

The Use of Market-based Measures

The use of market-based measures to limit or reduce emissions is a subject where policy is under development.

In 2001, the ICAO Assembly requested the Council to continue to develop guidance for States on the application of market-based measures aimed at reducing or limiting the environmental impact of aircraft engine emissions, particularly with respect to mitigating the impact of aviation on climate change. The Assembly encouraged States and the Council, taking into account the interests of all parties concerned, to evaluate the costs and benefits of the various measures with the goal of addressing aircraft engine emissions in the most cost-effective manner, emphasising the need for States to take action in a consistent manner regarding both domestic and international aviation emissions (Resolution A36-22 (pdf), Appendix L).

The analyses that have been undertaken so far within ICAO regarding the possible use of market-based measures have focussed on CO₂.

One of the principal findings is that an emissions-trading system that is, is a system whereby the total amount of emissions is capped and allowances, in the form of permits to emit CO₂, can be bought and sold to meet emission reduction objectives. Such a system could serve as a cost-effective measure to limit or reduce CO₂ emitted by civil aviation in the long term, provided that it is open to all economic sectors.

The Assembly consequently endorsed the development of an open emissions trading system for international aviation. It requested the Council to develop as a matter of priority the guidelines for open emissions trading, focussing on establishing the structural and legal basis for aviation's participation in an open trading system, and including key elements such as reporting, monitoring, and compliance, while providing flexibility to the maximum extent possible consistent with the UNFCCC process. A draft guidance on the use of emissions trading has been published by ICAO. [Draft Guidance on the use of Emissions Trading for Aviation (Doc 9885)].

The analyses have also indicated that short-term voluntary measures – that is, a mechanism under which industry and governments agree to a target and/or to a set of actions to reduce emissions – could serve as a first step towards future actions to further reduce emissions. The Assembly consequently encouraged short term action by States and other parties involved to limit or reduce international aviation emissions, in particular through voluntary measures. It also urged the Council to facilitate actions by developing guidelines (such as for quantifying, monitoring and verifying emission reductions or actions) for such measures, including a template voluntary agreement as appropriate. The Assembly also recognized the need to ensure that those taking early action would benefit from such actions and would not subsequently be penalized for so doing.

ICAO has published guidance on emission-related levies [Guidance on Aircraft Emission Charges Related to Local Air Quality (Doc 9884)] – that is, charges or taxes. It also has long-standing policies covering charges in general (ICAO's Policies on Charges for Airports and Air Navigation Services, Doc 9082/6). ICAO has also developed separate policy guidance to States on taxation (ICAO's Policies on Taxation in the Field of International Air Transport, Doc 8632), which recommends the reciprocal exemption from all taxes levied on fuel purchased for international flights, a policy implemented in practice in the vast majority of bilateral air services agreements, and also calls on States to reduce or eliminate taxes related to the sale or use of international air transport.

In December 1996, the ICAO Council adopted a policy statement in the form of a resolution, concerning the use of emission-related levies. While the Council considered that the development of an internationally agreed environmental charge or tax that all States would be expected to impose would appear not to be practicable at that time, given the differing views of States and the significant organizational and practical implementation problems that would be likely to arise, the Council strongly recommended that environmental levies that States may introduce should be in the form of charges rather than taxes and that the funds collected should be applied in the first instance to mitigating the environmental impact of aircraft engine emissions. Such charges should be based on the costs of mitigating this impact, to the extent that such costs can be properly identified and directly attributed to air transport.

The Assembly recognized the continuing validity of the 1996 policy regarding emission-related levies, urged States to refrain from unilateral action to introduce emission-related levies inconsistent with the current guidance, and called for the Council to carry out further studies and develop further guidance on the subject.