1 ICAO is the United Nations specialized agency that has global responsibility for the establishment of standards, recommended practices, and guidance on various aspects of international civil aviation, including environmental protection.

2 The revenue passenger-km is a measure of the traffic carried by commercial aviation: one revenue-paying passenger carried 1 km.

3 Specific emissions are emissions per unit of traffic carried, for instance, per revenue passenger-km.

4 Radiative forcing is a measure of the importance of a potential climate change mechanism. It expresses the perturbation or change to the energy balance of the Earth-atmosphere system in watts per square meter (Wm^-2). Positive values of radiative forcing imply a net warming, while negative values imply cooling.

5 Airborne sulfate particles and soot particles are both examples of aerosols. Aerosols are microscopic particles suspended in air.

6 The historical breakdown of aviation fuel burn for civil (passenger plus cargo) and military aviation was 64 and 36%, respectively, in 1976, and 82 and 18%, respectively, in 1992. These are projected to change to 93 and 7%, respectively, in 2015, and to 97 and 3%, respectively, in 2050.

7 IPCC, 1992: Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment [Houghton, J.T., B.A. Callander, and S.K.Varney (eds.)]. Cambridge University Press, Cambridge, UK, 200 pp.

8 The two-thirds uncertainty range means there is a 67% probability that the true value falls within this range.

9 This value is based on satellite observations and model calculations. See WMO, 1999: Scientific Assessment of Ozone Depletion: 1998. Report No. 44, Global Ozone Research and Monitoring Project, World Meteorological Organization, Geneva, Switzerland, 732 pp.