On the structure of water vapour feedbacks in climate models

Abstract

This paper quantifies the structure of water vapour feedback in a model by evaluating its meridional and vertical distribution and decomposing it into conceptually simplified components. First is a division into changes from fixed and varying relative humidity (RH). The first term is further divided into responses associated with the global mean surface temperature change, changes in horizontal gradients, and changes to the lapse rate. The uniform global temperature change component explains the majority of water vapour feedback. The horizontal gradient and varying RH terms are small. The component of water vapour feedback associated with lapse rate changes makes a significant contribution to the global water vapour feedback. However, there is a roughly equal and opposite temperature feedback from the lapse rate changes themselves. These results suggest both a very simple conceptual model for the bulk of the global water vapour feedback, as well as a relative insensitivity of combined temperature and water vapour feedbacks to the details of lapse rate changes. The effect of clouds on all feedback contributions is also quantified.