Radiative forcing by persistent contrails and its dependence on cruise altitudes

The expected increase of global air traffic raises concern over the impact of aviation on climate, for instance via the creation of persistent contrails that influence the radiative budget of the Earth. Here the radiative forcing due to persistent contrails due to potential new aircraft, which are added to the present fleet, is studied as a function of cruise altitudes. A sophisticated radiative transfer model is used to calculate the forcing, assuming a contrail cover derived from ECMWF analyses combined with information on air traffic from the AERO2k inventory for 2002. The global net forcing of the 2002 fleet is found to be 6 mW/m(2) with a large uncertainty of about 50%, mainly due to the sparseness of contrail cover observations, assumptions about the optical depth of the contrails, and the small size of the net forcing, which results from a large cancellation of longwave and shortwave forcings of opposite signs. By increasing air traffic by, e. g., 10% in 2000 ft thick horizontal global layers in turn, it is found that contrails produced around 10 km have the largest impact on the radiative forcing per flown kilometer. A clear linear behavior was observed: in the most sensitive layer each percent of increase in air traffic leads to a 0.25% increased radiative forcing. A comparison between using ECMWF model outputs with different cloud schemes, allowing for or neglecting ice supersaturation, is presented.