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Most global climate models project a considerable stratospheric moistening during global warming. It is important to quantify how significantly the increase of stratospheric humidity affects the radiation budget and contributes to the surface warming. Here, we conduct a mechanism denial experiment to investigate the warming effect of the stratospheric water vapor (SWV) using a global climate model. By locking the SWV in a quadrupling CO2 experiment, we find that the surface warming effect of SWV is not as significant as previously thought, increasing the global mean surface warming by about 2%. This is due to compensating changes in other feedback, especially those of tropospheric temperature and cloud, affected by SWV.

Plain Language Summary How significantly does the increase of stratospheric humidity as projected by many global climate models contribute to the surface warming? Here, we conduct a mechanism denial experiment to address this question. By locking the SWV in a quadrupling CO2 experiment, we find that the warming effect of SWV is not as significant as previously thought, due to compensating changes in other feedback.