The impact of cruise altitude on contrails and related radiative forcing

Within the framework of the European Fifth Framework Project TRADEOFF, the impact of changing cruise altitudes on contrail coverage and corresponding radiative forcing was investigated. On the basis of the reference year 1992, a series of aircraft emissions inventories with changed flight altitudes was prepared. These emission scenarios provide flown distances, fuel consumption and NOx emissions on a three-dimensional grid. The vertical resolution of these inventories was significantly increased over that used in former inventories. With a downshift of cruise altitude by 2000 ft (Throughout this paper we denote flight levels in ft. 2000 ft convert to approximately 610 m.), 4000 ft, and 6000 ft global annual mean contrail coverage is reduced in an approximately linear manner, reaching a maximum decrease of almost 45% for a 6000 ft lower cruise altitude. Contrary to this, a slight increase by 6% of global annual mean contrail coverage resulted for a 2000 ft higher maximum flight altitude. Relative changes of corresponding radiative forcing were shown to be very similar to those of contrail coverage. For changes in contrail coverage and radiative forcing associated with changes in flight altitudes, a strong seasonal and regional variability was found. This study only considers contrail radiative forcing. Trade-offs from other aviation related radiative impacts, e.g., from CO2 or O-3 , have not been studied.