Could Airplanes Change the Atmosphere's Temperatures?
Two major aircraft companies, Boeing and McDonnall Douglass (which recently merged), foresee growth in passenger demand. Air traffic in North America-Asia routes, for example, is expected to increase by 10.1% between 1990-2000. To meet future passenger demand, the aircraft industry may build a larger subsonic fleet by 2015 which could generate exhaust 2.3 times current levels. Concern exists that the increased exhaust, i.e., water vapor, nitrous oxides, carbon dioxide and aerosols, could change atmospheric radiative properties and photochemistry and, in turn, affect our health and living conditions at the surface.
Water vapor is the most interactive effluent and could have a strong impact on atmospheric temperatures. Exhaust is produced at subsonic cruise altitudes in the upper troposphere (10-12 km) and mostly in the northern hemisphere — especially over the northeastern US, western Europe and Japan.
As an experiment, water vapor was added to cruise corridors in a NASA/GISS computer climate model. Microwave maps of the resultant model atmosphere were calculated to get snapshots of the water vapor's impact on atmospheric temperatures. These maps were then compared with real data. Figure 1 shows such real data — maps of annual average temperatures across several heights from 10 years of data gathered by the Microwave Sounding Unit (MSU) instruments on NOAA satellites. MSU also measured the microwave variability or "noise" from year to year for the period of 1982-1991 (see Fig. 2). If the added water vapor caused changes greater than twice the observed microwave noise, then that water vapor addition was labeled significant and detectable.
Detectable signals only appeared when extreme, unrealistic amounts of water vapor were added to the climate model. An addition of 15 times the expected 2015 aircraft water vapor or 35 times 1990 aircraft water vapor was needed to create significant changes (see Fig. 3). The upper troposphere near cruise altitudes shows the largest warming (< 0.5 K), with almost no lower stratospheric signal at 18 km and with decreasing tropospheric warming at 5 km and 2 km. In the climate model, 95% of the increased atmospheric water vapor was rained out. The 5% that remained in the atmosphere caused greenhouse warming and heightened evaporation of surface water. So, realistic amounts of water vapor expected from a larger 2015 subsonic fleet did not appear to change climate model temperatures to any notable degree.
Shah, K., et al. 1997. Aircraft and increased CO2 impacts on MSU stratospheric and tropospheric temperatures. J. Climate, submitted.
Rind, D., P. Lonergan, and K. Shah 1996. The climatic effect of water vapor release in the upper troposphere. J. Geophys. Res. 101, 29395-29405.