Abstract

Stratospheric water vapor (SWV) is recognized as a potentially important positive feedback in global warming. The SWV change induces significant downward radiative flux perturbation at the tropopause and therefore is hypothesized to substantially amplify the surface warming. To test this hypothesis, we use a global climate model to quantify the surface warming contributed by the SWV change in the context of the quadrupled CO₂. By prescribing the SWV increase as an external forcing, we find that SWV only accounts for 0.42 K surface warming, making up merely 5.4 % of the total CO₂‐caused surface warming (7.7 K). The efficacy of the stratosphere‐adjusted SWV forcing is small (38%), where the efficacy is defined as the ratio of the global temperature response per unit radiative forcing relative to that of the CO₂ forcing. With the aid of a series of auxiliary experiments, we find that although the stratosphere‐adjusted SWV forcing at the top of atmosphere (TOA) is significant (1.13 W m^‐2), more than half of the forcing is offset by a high‐cloud decrease and an upper tropospheric warming in the tropospheric adjustment. The direct radiative impact of the SWV increase on surface temperature is negligible, and the SWV‐induced surface temperature change is a result of interactions between the radiative and non‐radiative processes.

Plain Language Summary

What role does the stratospheric water vapor play in global warming? Previous works demonstrated that stratospheric water vapor increase exerts significant downward radiative flux perturbation at the tropopause. This paper investigates whether this SWV radiative perturbation can substantially amplify the surface warming. Our simulation shows a negative answer to this question. The surface warming attributed to the stratospheric water vapor change is small, due to the effect of the tropospheric adjustment. The stratospheric water vapor increase causes a decrease in the high clouds and an increase in the upper tropospheric temperature, which offsets more than half of the stratospheric water vapor‐induced radiative perturbation.

Key Points

• The surface warming efficacy of the stratosphere‐adjusted SWV forcing is low (38%)
• SWV increase accounts for about 5% of total warming in the context of CO₂‐driven global warming
• More than half of the stratosphere‐adjusted SWV forcing is offset by the high cloud decrease and the upper tropospheric warming in the tropospheric forcing adjustment

This article appeared on the Journal of Geophysical Research Atmospheres website at https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JD032752