Climatic effect of water vapor release in the upper troposphere

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Abstract
Water vapor is released into the Goddard Institute for Space Studies (GISS) global climate middle atmosphere model at the locations and cruise altitude of subsonic aircraft. A range of water vapor values is used to simulate not only current and 2015 projected emissions but also to provide larger signal‐noise ratios. The results show that aircraft water 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 few tenths degrees Celsius globally and locally, while with emissions 300 times the 2015 values, a global warming of 1°C results. However, with releases this large, only about 5% actually stays in the atmosphere. The larger emissions increase the specific humidity most in the tropical lower troposphere, partly as a result of increased evaporation due to the global warming; at flight altitudes, relative humidity and cloud cover increase at latitudes of emission, and temperature decreases. Surface warming is relatively independent of latitude, and only a slight longitudinal aircraft footprint is found in the warming for the most extreme experiment. Comparison to increased CO2experiments of similar magnitude warming shows that the upper tropospheric response is greater in the water vapor release experiments, but the high‐latitude surface temperature response is larger with increased CO2due to more effective cryospheric feedbacks.