Minimizing the climate impacts of aviation is among the most difficult challenges facing aviation industry and air traffic management, requiring a reliable assessment of aviation-induced climate change. Substantial scientific uncertainties are currently hampering a targeted reduction of climatic effects. A sound metric is not available that could be used to compare the climate forcings caused by aviation with those from other natural and anthropogenic sources. An acknowledged scientific basis is lacking that would enable appropriate consideration of aviation in the emissions-trading schemes discussed in the European Union.
Global climate change requires measures for reduction of all anthropogenic sources, including those from aviation. Aircraft emit carbon dioxide (CO2) and nitrogen oxides (NOx) and therefore contribute to greenhouse gases (e.g., ozone). Contrary to CO2, the climate impact of NOx depends on the emission altitude and atmospheric conditions. Moreover, aviation contributes to climate change by the formation of condensation trails and changes in cloudiness. Long-range traffic has a larger share than short-range traffic. Because air traffic growths 5% per year and thus twice as much as the global economy, reductions of CO2 and NOx according to the goal of the Advisory Council for Aeronautics Research in Europe (ACARE) are of high priority. In contrast, condensation trails cannot be avoided by altering the air frame or engine technology.