Definition of the Problem

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 requested the IPCC to include an update of the main findings of the 1999 report in its Fourth Assessment Report (IPCC AR4) published in 2007.

The new findings related to aviation emissions in IPCC AR4 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 per cent 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 IPCC initiated the preparation of the Fifth Assessment Report (AR5), which is scheduled to be completed in 2014. ICAO participated in the IPCC process to ensure that issues related to scientific understanding of aviation’s impacts on global climate are covered in the AR5.

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. See “Climate Change” for further details.