Stratospheric Water Vapor under Global Warming: Climate Feedback and Impacts on Stratospheric Temperature and Circulation

Stratospheric water vapor increases in response to global warming due to increasing CO₂. Increase in stratospheric water vapor enhances tropospheric and surface warming and has a positive climate feedback. Increase in stratospheric water vapor also enhances stratospheric cooling and could significantly affect stratospheric temperature and circulation. However, the importance of these effects is still under debate. The purpose of this study is to quantify the climate impact of stratospheric water vapor under global warming using the coupled atmosphere-ocean Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM).

Two sets of quadrupling CO₂ simulations were conducted to quantify the impacts of stratospheric water vapor change on surface and stratosphere climate. The results show that stratospheric water vapor increases by 2.5 ppmv under CO₂ quadrupling. The stratospheric water vapor climate feedback in GEOSCCM is 0.11 Wm^-2K^-1, which increases global surface warming by about 10%. Increase in stratospheric water vapor causes stratospheric cooling of about 1.5 K. The cooling pattern due to stratospheric water vapor is different from that due to CO₂ with stronger cooling in the Arctic and tropical lower stratosphere, and Antarctic middle stratosphere. This cooling pattern causes acceleration of the subtropical jet in the lower stratosphere, leading to a stronger Brewer-Dobson circulation. It is found that 30% of Brewer-Dobson circulation acceleration under CO₂ quadrupling is attributed to stratospheric water vapor change. However, increase in stratospheric water vapor does not significantly affect tropospheric circulation.