A comprehensive general circulation model (GCM) is used to estimate the climate impact of aviation emissions of black carbon (BC) and sulfate (SO4) aerosols. Aviation BC is found not to exert significant radiative forcing impacts, when BC nucleating efficiencies in line with observations are used. Sulfate emissions from aircraft are found to alter liquid clouds at altitudes below emission (approximate to 200 hPa); contributing to shortwave cloud brightening through enhanced liquid water path and drop number concentration in major flight corridors, particularly in the N. Atlantic. Global averaged sulfate direct and indirect effects on liquid clouds of 46 mWm(-2)are larger than the warming effect of aviation induced cloudiness of 16 mWm(-2). The net result of including contrail cirrus and aerosol effects is a global averaged cooling of -21 +/- 11 mWm(-2). These aerosol forcings should be considered with contrails in evaluating the total global impact of aviation on climate.
