CLIMATIC EFFECT OF WATER-VAPOR RELEASE IN THE UPPER TROPOSPHERE

Water vapor is released into the Goddard Institute for Space Studies ( GISS) global climate middle atmosphere model at the locations and crui se altitude of subsonic aircraft. A range of water vapor values is use d to simulate not only current and 2015 projected emissions but also t o provide larger signal-noise ratios. The results show that aircraft w ater vapor emissions do not significantly affect the model's climate, either at the surface or in situ. With emissions some 15 times higher than the 2015 projection, a small impact is observed, amounting to a f ew tenths degrees celsius globally and locally, while with emissions 3 00 times the 2015 values, a global warming of 1 degrees C results. How ever, with releases this large, only about 5% actually stays in the at mosphere. The larger emissions increase the specific humidity most in the tropical lower troposphere, partly as a result of increased evapor ation due to the global warming; at flight altitudes, relative humidit y and cloud cover increase at latitudes of emission, and temperature d ecreases. Surface warming is relatively independent of latitude, and o nly a slight longitudinal aircraft footprint is found in the warming f or the most extreme experiment. Comparison to increased CO2 experiment s of similar magnitude warming shows that the upper tropospheric respo nse is greater in the water vapor release experiments, but the high-la titude surface temperature response is larger with increased CO2 due t o more effective cryospheric feedbacks.