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Leading European researchers have concluded that aircraft emissions are small in comparison to all other man-made emissions, but could be significantly affecting atmospheric ozone and cloud cover with possible implications for climate change. These are the conclusions of a comprehensive review of the current state of knowledge carried out at the initiative of the European Commission and published today in the scientific journal "Atmospheric Environment".
The authors of the review, entitled "European Scientific Assessment of the Atmospheric Effects of Aircraft Emissions", which relies heavily on the results of successful European  and national  research programmes, conclude that:
- The 20-50% increase in the NOx abundance caused by aircraft traffic in the vicinity of their cruising altitude (10-12 km) has produced a 4-8% increase in the ozone concentration of the upper troposphere (maximum value during summertime) where ozone is a strong greenhouse gas. The warming effect associated with this ozone increase is comparable to the warming effect of CO2 emitted by aircraft (about 2-3% of all anthropogenic CO2 emissions).
- Climate perturbations could also have resulted from the formation of persistent contrails and high-level cirrus clouds produced in the busiest flight corridors. Additional effects on the radiative balance of the atmosphere could have been generated by the soot and sulphur particles released by aircraft engines. The warming effect of the changes in cloudiness is more difficult to assess but appears to be also of the same magnitude as the warming effect of CO2 emitted by aircraft.
- The total climate forcing caused by the present fleet of commercial aircraft (about 0.1 Wm-2) is a small contribution to the total forcing (2.4 Wm-2) associated with past industrial development. However, with air traffic in the next 20 years expected to grow faster than the global economy, the relative contribution of aviation to environmental changes (pollution, stratospheric ozone, climate) could become more significant, unless new, less-polluting engines and more fuel-efficient aircraft technologies are introduced.
- The development of a fleet of supersonic aircraft flying at high altitudes (17-20 km) could perturb the ozone layer in the stratosphere. Current models indicate, however that changes in the ozone column produced by a hypothetical fleet of 500 aircraft would be just a few percent and should lead to changes in the UV-B level at the Earth's surface of less than 2%. However, more research on the complex and poorly understood processes that affect the chemistry of ozone in the lower stratosphere is required before a conclusive assessment can be produced.
The assessment also identified a number of areas where improved knowledge could advance our understanding of how aircraft perturb the atmosphere. It stresses that the region where aircraft fly, straddling the tropospheric and stratospheric boundary at around 12km, is not sufficiently understood. A better understanding of the background ('natural') state of this region is required if the perturbations arising from aircraft emissions are to be known with confidence. For instance, the natural production of NOx from lightning needs to be better quantified before the impact of aircraft-induced NOx can be determined with confidence. In addition, the effect of aircraft emissions on the abundance of particles that provide the surface for complex heterogeneous reactions, needs to be carefully studied. The large uncertainty and the large potential for climatic impact due to possible changes in cloudiness induced by aircraft emissions, requires more emphasis on this topic than before. Finally, the relative importance of aircraft emissions may evolve in the course of future changes e.g. in tropospheric and stratospheric temperature, in water vapour concentration and in the residence time of other greenhouse gases like methane.
In light of the current scientific issues, the European Commission is continuing to support research on this subject through its Environment and Climate programme and the CORSAIRE initiative (Coordination of Research for the Study of Aircraft impact on the Environment). A CORSAIRE meeting was held last November in Cambridge, UK, at which these key scientific issues were discussed in the context of the on-going European research addressing the impact of aircraft on the atmospheric environment. The meeting brought together 40 experts from the three groups dealing with the problem: atmospheric scientists, engineers from aviation industry and government officials. Its purpose was to improve links between the three communities and to share ideas in order to sharpen the focus of future research into aircraft impacts.
The European Community will continue supporting research in the coming years within the Fifth Framework Programme which is under preparation. Research on the atmospheric effects of aircraft emissions and their relative importance to all other anthropogenic emissions will be carried out in the "Global Change, Climate and Biodiversity" Key Action, while all technological aspects of aviation will be addressed in the "New perspectives in Aeronautics" Key Action.
In making their decisions, industry is aware that their aircraft design choices of today will have an atmospheric impact lasting for several decades. Growing public concern is also motivating the airlines to seek guidance from scientists on how best to address environmental issues. Jet engines release several compounds into the atmosphere, including nitrogen compounds (NOx) which produce ozone in the troposphere, CO2, and particle emissions impacting cloud formation. All three, CO2, tropospheric ozone, and enhanced cloudiness contribute to global warming. For economic reasons, airlines are anxious to reduce CO2 emissions by improving fuel efficiency, but more must be done to understand the effects of NOx and cloud formation in both troposphere and the stratosphere so that the aviation industry and regulators can consider the total environmental impact when designing and ordering new aircraft, and developing emission standards.
European scientists are playing leading roles in research on this issue, and many are participating in the international assessment of the scientific and technical issues "Aviation and the Global Atmosphere" currently being carried out by the Intergovernmental Panel on Climate Change (IPCC). The IPCC assessment relies heavily on the European Assessment and on recent assessments in the U.S.A.
For further information, please contact:
Georgios Amanatidis and Giovanni Angeletti
Environment and Climate Programme, DG XII-D
Fax: + 32-2-296.30.24
Press and information officer
Communication Unit, DG XII
Fax: + 32-2-295.82.20