How to think about the shortwave water vapor feedback

Earth's climate feedback quantifies how Earth's energy budget responds to temperature changes and thus directly determines climate sensitivity. To a large extent, the climate feedback is controlled by water vapor, the most powerful greenhouse gas in Earth's atmosphere. Water vapor strongly absorbs longwave radiation emitted by Earth, but also absorbs shortwave radiation from the sun. Recent studies have introduced analytical models for the longwave component, however, to our knowledge, no corresponding study exists for the shortwave. Here we introduce such an analytical model for the shortwave water vapor feedback, building on the longwave model of Jeevanjee [1]. In our model, the shortwave water vapor feedback is proportional to the change with temperature in the square of atmospheric transmissivity, which depends on the abundance of water vapor in Earth's atmosphere and its absorption of solar radiation. We show that to accurately represent the shortwave water vapor feedback, one needs to account for the strong variation in water vapor absorption at different frequencies. Our results demonstrate that a simple analytical model of the shortwave water vapor feedback captures most of its magnitude and its variation with surface temperature. This way, we expand the conceptual understanding of an important but understudied feedback component.