Contrails - Research, comments and links

Contrails and aviation-induced cirrus-clouds

IPCC (01)

Releasing a scientific report on the atmospheric impacts of aviation


Geneva, Friday, 4 June 1999 - The Intergovernmental Panel on Climate Change (IPCC) has today released a special report on the effect of aviation on the global atmosphere entitled "Aviation and the Global Atmosphere". The report has been developed over the last two years by a group of more than 100 scientists from around the world and was endorsed during a three-day IPCC meeting in San Jose, Costa Rica in mid-April 1999. It represents the results of unprecedented collaboration between the IPCC and the Scientific Assessment Panel of the Montreal Protocol on Substances that Deplete the Ozone Layer, and provides a detailed assessment of the impact of aircraft engine emissions on climate and atmospheric ozone.

Air travel is projected to grow by about five per cent annually until 2015, burning three per cent more fuel per year in that period. This report finds that aircraft emissions, and their impacts, will be far greater in 2050 unless new technologies and operational modes are developed. Fuel consumption by civil aviation is expected to reach 300 million tonnes in 2015 and 450 million tonnes in 2050, compared to 130 million tonnes in 1992, with corresponding high emissions of greenhouse gases such as carbon dioxide and water vapour as well as nitrogen oxides and sulphur oxides.

The aviation industry has undergone rapid growth and projections suggest that the trend is likely to continue. It is, therefore, highly relevant to consider current and possible future effects of aircraft on the atmosphere. The report considers all the gases and particles emitted by aircraft into the atmosphere and the role which they play in climate change, and modification of the ozone layer. The report also considers how potential changes in aircraft technology, air transport operations and the institutional, regulatory and economic framework might affect emissions in the future. It describes the state of scientific knowledge together with associated uncertainties.

A unique aspect of this report is the integral involvement of technical experts from the aviation industry, including airlines and airframe engine manufacturers, alongside atmospheric scientists. This involvement has been critical in producing what IPCC believes is the most comprehensive assessment available of the effects of aviation on the global atmosphere

The report is forwarded to the Parties to the United Nations Framework Convention on Climate Change (UNFCCC) and to the International Civil Aviation Organization (ICAO) to inform them about the possible influence of the aviation sector on climate change. The report was prepared at ICAO request.

The IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), to make periodic assessments of the science, impacts and socio-economic aspects of climate change and of adaptation and mitigation options to address it; and to provide, on request, scientific-technical advice to the Conference of the Parties to the UNFCCC and its bodies. After publishing two major influential reports in 1990 and 1995, IPCC is now working on its Third Assessment Report to which the Special Report on Aviation and the Global Atmosphere will contribute.

Note to Correspondents:

The "Summary for Policy Makers" of the Report is available in Arabic, Chinese, English, French, Russian and Spanish at the IPCC Secretariat.

For more information please contact:

Taysir M Al-Ghanem
Public Information Office, IPCC Secretariat
7 bis, Avenue de la Paix, CH-1211 Gen�ve 2, Switzerland
Tel: (41 22) 730 8315 
Fax: (41 22) 733 2829 or 7308027


Aviation and the Global Atmosphere (IPCC, 1999)

Summary for Policymakers
4.5. Contrails

In 1992, aircraft line-shaped contrails are estimated to cover about 0.1% of the Earth�s surface on an annually averaged basis with larger regional values. Contrails tend to warm the Earth�s surface, similar to thin high clouds. The contrail cover is projected to grow to 0.5% by 2050 in the reference scenario (Fa1), at a rate which is faster than the rate of growth in aviation fuel consumption.

This faster growth in contrail cover is expected because air traffic will increase mainly in the upper troposphere where contrails form preferentially, and may also occur as a result of improvements in aircraft fuel efficiency. Contrails are triggered from the water vapor emitted by aircraft and their optical properties depend on the particles emitted or formed in the aircraft plume and on the ambient atmospheric conditions. The radiative effect of contrails depends on their optical properties and global cover, both of which are uncertain. Contrails have been observed as line-shaped clouds by satellites over heavy air traffic areas and covered on average about 0.5% of the area over Central Europe in 1996 and 1997.


6.8.6 Aerosol Indirect Effect on Ice Clouds Contrails and contrail-induced cloudiness

Using meteorological and air traffic data scaled to regional observations of contrail cover, Sausen et al. (1998) estimated the present day global mean cover by line-shaped contrails to be about 0.1%. This results in a global and annual mean radiative forcing by line-shaped contrails of 0.02 Wm-2 (Minnis et al., 1999), subject to uncertainties in the contrail cover, optical depth, ice particle size and shape (Meerk�tter et al., 1999). We follow Fahey et al. (1999) and retain a range of 0.005 to 0.06 Wm-2 for the present day forcing, around the best estimate of 0.02 Wm-2.

Contrails can evolve into extended cirrus clouds. Boucher (1999) and Fahey et al. (1999) have shown evidences that cirrus occurrence and coverage may have increased in regions of high air traffic compared with the rest of the globe. Smith et al. (1998) reported the existence of nearly invisible layers of small ice crystals, which cause absorption of infrared radiation, and could be due to remnant contrail particles. From consideration of the spatial distribution of cirrus trends during the last 25 years, Fahey et al. (1999) gave a range of possible best estimates of 0 to 0.04 Wm-2 for the radiative forcing due to aviation-induced cirrus. The available information on cirrus clouds was deemed insufficient to determine a single best estimate or an uncertainty range. Impact of anthropogenic aerosols on cirrus cloud microphysics

Measurements by Str�m and Ohlsson (1998) in a region of high air traffic revealed higher crystal number concentrations in areas of the cloud affected by soot emissions from aircraft. If the observed enhancement in crystal number density (which is about a factor of 2) is associated with a reduction in the mean crystal size, as confirmed by the measurements of Kristensson et al. (2000), a change in cloud radiative forcing may result. Wyser and Str�m (1998) estimated the forcing, although very uncertain, to be in the order of 0.3 Wm-2 in regions of dense air traffic under the assumption of a 20% decrease of the mean crystal size. No globally averaged radiative forcing is available.

The sedimentation of ice particles from contrails may remove water vapour from the upper troposphere. This effect is expected to be more important in strongly supersaturated air when ice particles can fall without evaporating (Fahey et al., 1999). The impacts of such an effect on cirrus formation, vertical profile of humidity and the subsequent radiative forcing have not been assessed.

Aerosols also serve as ice nuclei although it is well recognised that there are fewer ice nuclei than cloud condensation nuclei. It is conceivable that anthropogenic aerosols emitted at the surface and transported to the upper troposphere affect the formation and properties of ice clouds. Jensen and Toon (1997) suggested that insoluble particles from the surface or soot particles emitted by aircraft, if they serve as effective ice nuclei, can result in an increase in the cirrus cloud coverage. Laaksonen et al. (1997) argued that nitric acid pollution is able to cause an increase in supercooled cirrus cloud droplet concentrations, and thereby influence climate (see Chapter 5, Section 5.3.6). Such effects, if significant at all, are not quantified at present.

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Contrails are Bad News