Stratospheric Water Vapor Feedback and Its Modification by Interactive Ozone Chemistry

Stratospheric water vapor has a positive climate feedback effect: a warming climate increases stratospheric water vapor, and the increased stratospheric water vapor enhances surface warming. There is strong debate on the importance of this feedback. Previous studies reported a wide range of stratospheric water vapor feedback strength from 0.02 to 0.3 Wm^-2K^-1, with the high-end magnitude comparable to that of the surface albedo or cloud feedback. Furthermore, the stratospheric water vapor feedback is modified by interactive ozone chemistry through changes in the Brewer-Dobson circulation and tropical tropopause temperature. But the magnitude of this chemical modification is highly uncertain. The purpose of this study is to quantify the stratospheric water vapor feedback, its impact on surface warming, and its modification by interactive ozone chemistry in the Goddard Earth Observing System Model, Version 5 (GEOS-5).

The stratospheric water vapor feedback is quantified by comparing two doubling CO₂ experiments: one that includes the feedback, and the other that eliminates the feedback by fixing the stratospheric water vapor concentrations. We find a large stratospheric water vapor feedback of 0.12 Wm^-2K^-1 in GEOS-5, which increases global surface warming by nearly 20%. Additional doubling CO₂ experiments with interactive stratospheric chemistry are conducted in order to quantify the chemical modification of stratospheric water vapor feedback. The results show that ozone chemistry causes tropical tropopause cooling and reduces stratospheric water vapor feedback by more than 50%. Our results suggest that stratospheric water vapor and ozone feedbacks significantly contribute to the climate sensitivity