Abstract
Abstract Humid air is lighter than dry air at a given temperature and pressure, as the molecular weight of water vapor is less than that of dry air. This effect is known as vapor buoyancy (VB). Although VB is a straightforward consequence of the ideal gas law, its influence on climate has been understudied. This study investigates VB’s influence on atmospheric temperature, radiation, and clouds. In mechanism-denial experiments, we remove VB from the dynamics and parameterizations of an idealized general circulation model (GCM). These experiments show that VB warms the tropical free troposphere by approximately 1 K in an Earth-like climate, and the magnitude of this effect increases with warming. This additional atmospheric warming causes greater outgoing longwave radiation (OLR) to be emitted as the climate warms. This constitutes a negative climate feedback which attains a magnitude of 0.1–0.2 W m−2 K−1 in the tropics. The simulations further show that, despite warming the atmosphere, VB does not introduce a countervailing positive water vapor feedback. Finally, we find that VB makes the net cloud feedback in the model more negative than it otherwise would be, reinforcing the clear-sky feedback.
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