Da Yang, Seth Seidel

The molar mass of water vapor is significantly less than that of dry air. This makes a moist parcel lighter than a dry parcel of the same temperature and pressure. This effect is referred to as the vapor buoyancy effect and has often been overlooked in climate studies. We propose that this effect increases Earths outgoing longwave radiation (OLR) and stabilizes Earths climate. We illustrate this mechanism in an idealized tropical atmosphere, where there is no horizontal buoyancy gradient in the free troposphere. To maintain the uniform buoyancy distribution, temperature increases toward dry atmosphere columns to compensate reduction of vapor buoyancy. The temperature difference between moist and dry columns would increase with climate warming due to increasing atmospheric water vapor, leading to enhanced OLR and thereby stabilizing Earths climate. We estimate that this feedback strength is about O(0.2 W/m$^2$/K), which compares with cloud feedbacks and surface albedo feedbacks in current climate.