The potential climate impact of contrails and alterations in the lifetime of background cirrus due to subsonic aircraft water and aerosol emissions has been investigated in a set of experiments using the GISS GCM connected to a q-flux ocean. Cirrus clouds at a height of 12--15 km, with an optical thickness of 0.33, were input to the model x percentage of clear-sky occasions along subsonic aircraft flight paths. The percentage x is varied from 0.05 to 6%. Two types of experiments were performed: one with the percentage of cirrus cloud increase independent of flight density along the flight paths, the other with the percentage related to the density of fuel expenditure. The overall climate impact was similar with the two approaches, due to the feedbacks of the climate system. Fifty years were run for each of the eight experiments, with the following conclusions based on the stable results from years 31--50. The equilibrium global mean response shows that altering high-level clouds by 1% changes the global mean temperature by 0.43 ¿C. The global temperature response is highly linear (linear correlation coefficient of 0.996) for high cloud cover changes between 0.1 and 5%. The warming is amplified in the Northern Hemisphere, more so with greater cloud cover change. The temperature effect maximizes around 10 km, more so as the overall warming increases (warming greater than 4 ¿C occurs there with a 4.8% increase in upper level clouds). The surface temperature response is dominated by the feedbacks and shows little geographic relationship to the high cloud input outside of the hemispheric difference. Considering whether these effects would be observable, changing upper level cloud cover by as little as 0.4% produces warming greater than 2 standard deviations in the microwave sounding unit (MSU) channels 4, 2, and 2r (although the effect would be most noticeable in the upper troposphere channel 3 were standard deviations available). Given estimates of current aircraft impacts, this would require some increase relative to present-day effects, but a signal should be clear given the projections for 2050 aircraft. In comparison to increased CO2 experiments, in these runs the Northern Hemisphere clearly warms more relative to the Southern Hemisphere, and warming due to cloud height changes exceeds that due to the water vapor feedback. Despite the simplified nature of these experiments, the results emphasize the sensitivity of the modeled climate to high-level cloud cover changes and thus the potential ability of aircraft to influence climate by altering clouds in the upper troposphere. ¿ 2000 American Geophysical Union