At cruising altitudes of 30,000-43,000 feet, aircraft produce gases and particles that can affect the global climate by contributing to the "greenhouse effect". These emissions are not thought to contribute to the thinning of the ozone layer in the stratosphere. In environmental terms, the most important emissions from aircraft are carbon dioxide (CO2), water vapour (H20), in the form of condensation trails and nitrogen oxides (NOx).

Local level

In the direct vicinity of airports, engine emissions of NOx, HC, CO and visible smoke contribute to local air quality concerns. Emission of these substances is subject to international standards set by ICAO to improve more sever limits to the certification of new engines. The proportion of aircraft-related emissions is generally small compared to the total amount generated by ground airport activities and car traffic around airports.

Global level

At the global level, both carbon dioxide (CO2) and water vapour (H2O) emissions directly contribute to global warming, whereas nitrogen oxides (NOx) contribute to it indirectly through the creation of ozone (O3). Aviation's overall contribution to human climate change is currently thought to be about 3.5%. This share can be expected to grow to 5% in 2050 due to the high level of demand for aviation services.

Further reducing emissions

Over time, the introduction of new technologies will bring about further reductions in fuel consumption, directly helping to decrease emissions of CO2 and H2O. The accelerated introduction of communications, navigation, surveillance and air traffic management systems (CNS/ATM) and additional infrastructure will also enable airlines to fly shorter distances and reduce fuel consumption combined with operational improvements within airlines, these measures have the potential to improve fuel consumption up to 18%. Airframe and engine manufacturers and airlines are participating in research and technology programmes in all fields that improve fuel consumption, and, therefore, emissions. ICAO is evaluating the potential role of market-based options such as emissions trading to control CO2 emissions - these may provide opportunities to limit emissions from aviation at a lower cost and in a more flexible manner than traditional regulatory measures.