Fresh contrail affected by aircraft vortices, observed 12 May 2001, 15h55 UT over Bilthoven.

Present-day commercial jet aircraft have cruise altitudes of 8-13 km. The emissions from these aircraft change the atmospheric composition:

• Directly: by emitting carbon dioxide (CO₂), nitrogen oxides (NO_x = NO + NO₂), water vapour, unburnt hydrocarbons, soot, and sulfate particles.
• Indirectly: by a chemical reaction chain similar to smog-formation the greenhouse gas ozone (O₃) can be formed. In this reaction chain nitrogen oxides act as a catalyst under the influence of sunlight. The formed ozone has a lifetime of several weeks up to months in the troposphere. As a result of these chemical reations also the concentration of methane (CH₄), another greenhouse gas, decreases. This methane decrease is relatively longer lived (about 10 years) and leads to a (small) ozone decrease on the same time scale.

• The increase in the tropospheric ozone column due to subsonic aviation leads to a reduction in the amount of ultraviolet radiation (UV) reaching the surface.

The emitted sulphur and nitrogen compounds are removed from the atmosphere through scavenging by precipitation (acid rain) and by dry deposition at the earth's surface. This constitutes a small, but non-negligible, contribution to acidification and eutropication.

• Ozone, CO2, and water vapour are greenhouse gases and their increase has a warming effect.
• Methane is also a greenhouse gas and its decrease has a cooling effect.
• Aerosols (sulfate particles, soot) could have a cooling effect.
• Contrails formed due to the emission of water vapour increase the cloud cover in the upper troposphere. Contails lead to a reduction of the solar radiation reaching the surface (surface cooling) but they also enhance the greenhouse effect (surface warming) by absorbing longwave radiation welling up from the earth and atmosphere below. This might result in net cooling or heating depending on the size and optical depth of the ice crystals of which the contrails consist. Contrails thus lead to surface warming at night and generally to surface cooling at daytime. Presently it is estimated that contrails lead to a net warming effect over the full day.
• There may be changes in (non-contrail) upper level clouds: Most contrails decay after minutes to hours, but some continue to exist and are then not distinguishable anymore from natural cirrus clouds (thin upper level ice clouds) for the human eye. The climate effect of changes in cirrus cloud cover due to aviation are very uncertain.

The subject of metrics is much debated recently. Traditionally, the climate impact of emissions is expressed in terms of radiative forcing and Global Warming Potentials (GWPs), see e.g. the IPCC assessments. GWP corresponds to radiative forcing integrated over a certain time interval. The chosen time horizon for GWPs is somewhat arbitrary, but usually set to 100 years.
An alternative metric is for instance the Global Temperature Potential (GTP). Some scientists are now starting to realize that the metric to be chosen depends on the target that has to be achieved. This is where policymakers come into play. A target canot be chosen without consulting policymakers. GTP would be a metric suitable for the target that the global temperature increase should not exceed 2 degrees. However, depending on the chosen target many other metrics are possible. Some scientists have suggested to include the rate of temperature change in the metric; others, living in coastal areas, may be interested to include sea level rise in the metric. One might even want to formulate a metric in terms of the financial costs of climate change damages, mitigation and adaptation.
Another important discussion point is whether a discount rate should be applied.
Note that the 4th assessment report of IPCC (AR4) recommends that Radiative Forcing Index (RFI, the ratio between radiative forcings of different emissions) should not be used as a metric, since it does not account for the different lifetimes of the different emitted species (see P. Forster, K. Shine and N. Stuber (2006) "It is premature to include non-CO2 effects of aviation in emission trading schemes", Atmospheric Environment 40(6), p. 1117, doi:10.1016/j.atmosenv.2005.11.005). Hence, the use of the RFI factor of 2.7 as a multiplier to quantify the full (apart from changes in upper level clouds) climate impact of aviation emissions relative to the climate impact of only aircraft CO₂ emissions is not valid.

 

Contrails observed over De Bilt in a high pressure situation.

• Aircraft effects upon the atmosphere are studied by KNMI in narrow cooperation with other international, mainly European, research groups, and funded by the European Union (EU):
  • In the framework EU Environment and Climate Programme project Aeronox first estimates of the effects of emissions of nitrogen oxides by aircraft upon the global atmospheric composition have been made in the first half of the 1990s.
  • Within the EU projects POLINAT 1 and 2 (1994-1997) model calculations were confronted with measurements made by the research aircraft Falcon in the North Atlantic Flight Corridor.
  • Further studies, evaluations and intercomparisons of global atmospheric chemistry models, such as our TM-model, were performed in the EU projects AEROCHEM-2 (Modelling of the impact on ozone and other chemical compounds in the atmosphere from airplane emissions, 1998-2000).
  • In the EU TRADEOFF (Aircraft emissions: Contributions of Various Climate Compounds to Changes in Composition and Radiative Forcing- Tradeoff to Reduce Atmospheric Impact, 2000-2003) project an update on the radiative forcing estimates of the IPCC special report was calculated.
  • In the most recent EU project QUANTIFY (2005-2010) the impacts of aviation, shipping and road traffic on climate have been quantified and compared.
• At the national level we have contributed to the bill Air Pollution and Aviation (Nota Luchtverontreiniging en Luchtvaart - Lulu, 1995). This bill is now somewhat outdated and nnot available on the internet anymore. The most recent document describing the national policy of the Netherlands is the "Luchtvaartnota" of april 2009.
• We have contributed to the environmental and climate modules of the integrated assessment model AERO (Aviation Emissions and evaluation of Reduction Options), see e.g. S. Vlek and M. Vogels in Air & Space Europe, Volume 2, Issue 3, p. 41-44, May-June 2000 (doi:10.1016/S1290-0958(00)80062-3), KNMI TR-185, "Interim report on the KNMI contributions to the second phase of the AERO-project", by Wauben et al., 1995, and KNMI TR-196, "Environmental and climatic consequences of aviation : final report of the KNMI contributions to the AERO-project" by Wauben et al. (1997).
• Ernst Meijer has performed a Ph.D. study on the impact of aviation on the atmosphere. Amongst others, he incorporated a parametrization of subgrid aircraft plume processes in our global atmospheric chemistry model TM. This resultied in a reduction by about 20-30 percent of the ozone perturbation due to aircraft NOx-emissions relative to the case where the NOx-emissions are directly emitted into grid cells of the global TM model. His Ph.D. thesis (defended 21 november 2001 at Eindhoven University of Technology) can be downloaded from KNMI publications. Zie ook het persbericht.
• KNMI co-organized the first international presentation of the "IPCC special report on Aviation and the Global Atmosphere" in Amsterdam in 1999 (together with NLR) and the second International Conference on Transport, Atmosphere and Climate TAC-2 in 2009 (together with DLR).

• Some first statements about the effects of aircraft emissions on the atmosphere can be found in the excecutive summary of "The scientific assessment of ozone depletion : 1994" by WMO/UNEP.
• Brasseur et al. (1998) published a "European scientific assessment of the atmospheric effects of aircraft emissions" in Atmospheric Environment, Vol. 32, No. 13, p. 2329-2418, 1998. This was intended to serve as input to:
• The IPCC special report on Aviation and the Global Atmosphere (1999)
  See also:
  • SPM (Summary for PolicyMakers).
  • The IPCC 1999 input Technical reports on supersonic scenarios should be available from NASA ...
  • Press release (in Dutch) about a symposium held in Amsterdam on 23/24 June 1999 to present the IPCC special report to the aviation sector and policy makers.
• Based on the results from the EU Tradeoff project, Sausen et al., provided an update for 2000 of the radiative forcing estimates from IPCC 1999, published in Meteorologische Zeitschrift (Vol. 14, No. 4, 555-561, August 2005) .
• Most recently, the 4th assessment report of IPCC summarized the present state of knowledge about Contrails and Aircraft-Induced Cloudiness in section 2.6 and about radiative forcing in section 2.9.
• The EU project ATTICA has produced a review paper published in Atmospheric Environment (Lee et al., 2009).

• Grootschalige verspreiding van vliegtuigemissies in de troposfeer en stratosfeer - relevante fysische processen, Van Velthoven e.a., Tijdschrift Lucht, 12e jaargang, nr. 4, December 1995, p. 129-133 (pdf).
• Klimaat- en milieueffecten van mondiaal vliegverkeer, Van Velthoven e.a., Nederlands Tijdschrift voor Natuurkunde 62/5, p. 71-75, 19 maart 1996 (pdf, samenvatting).
• Zelf rekenen aan strepen in de lucht, D. Olivie, Natuur en Techniek, Juli/Augustus 2000, 58-62.
• Ministerie van Verkeer en Waterstaat over Luchtvaart, zie Milieu en geluid luchtvaart. Hier staat ook de laatste luchtvaartnota d.d. 17 april 2009.
• Milieucentraal adviseert over klimaatvriendelijk vakantieverkeer.

• See the assessments listed above.
• The impact of NOx emissions from aircraft upon the atmosphere at flight altitudes 8-15 km, AERONOX, ed. U. Schumann. Office for Publications of the European Commission, Brussels (1995), pp. 471. (ISBN-92-826-8281-1)
• Pollution from Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT), ed. U. Schumann. Air Pollution Research Report 58, Report EUR 16978 EN, Luxembourg: Office for Official Publication of the European Communities (1997) pp. 303.
• Pollution from Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2), edited by U. Schumann, Air Pollution Research Report 68, Report EUR 18877 EN, Luxembourg: Office for Official Publication of the European Communities, ISBN 92-828-6197-X (1999) pp. 312.
• Also of interest are the proceedings of the international scientific conferences on Aviation, Atmosphere and Climate (AAC) and on Transport, Atmosphere and Climate (TAC-1 and TAC-2) organized by DLR and partners:
  • AAC (30 June-3 July 2003, Friedrichshafen at Lake Constance, Germany): R. Sausen, C. Fichter and G. Amanatidis (Eds.), 2004: European Conference on: Aviation, Atmosphere and Climate (AAC): Proceedings of an International Conference. Air pollution research report 83, European Commission, 369 pp. See also Meteorologische Zeitschrift, Volume 14, Number 4, August 2005 (contents).
  • TAC-1 (26-29 June 2006, Oxford): Sausen, R., A. Blum, D.S. Lee and C. Bruning, (Eds.) 2007: Proceedings of an International Conference on Transport, Atmosphere and Climate (TAC). Luxembourg, Office for Official Publications of the European Communities, ISBN 92-79-04583-0, 320pp.
  • TAC-2 (22-25 June 2009; Aachen, Germany, and Maastricht, the Netherlands): proceedings are in preparation
• An overview of measurements and modelling of the impact of aviation emissions on the atmosphere was given by Van Weele et al. in the KNMI Climate Research Department's Biennal Review (1999).

• EU:
  • Dg Environment on Aviation and climate
• NASA:
  • The Atmospheric Effects of Aviation Project (AEAP)
  • The SONEX project (SASS Ozone and Nitrogen Oxide Experiment). This project has cooperated with POLINAT.
  • The NASA SUbsonic Contrails & Clouds Effects Special Study : SUCCESS (at Nasa-Ames and at ESPO.
• The International Civil Aviation Organization (ICAO) on
  • Environment
  • Aircraft Engine Emissions
• The Air Transport Action Group (ATAG) on Environment
• Centre for Sustainable Tourism and Transport (NHTV, Breda)
• Airneth:
  • Documents on Climate Change and Aviation
• Platform Duurzame Luchtvaart over
  • Klimaat en Luchtverontreiniging

• Contrail formation and observations
• Contrails and Meteologica by R.C. van Waning show many examples of contrails observed near Amsterdam Airport.
• Wikipedia (Contrail)