What do the CMIP5 models tell us about the water vapor feedback?

The water vapor feedback refers to the process whereby an initial warming of the planet, caused for example by an increase in atmospheric greenhouse gas abundance, causes an increase in the specific humidity of the atmosphere. Because water vapor is itself a greenhouse gas, the increase in specific humidity causes additional warming. In this talk, I will show calculations of the magnitude of the feedback in the CMIP5 models in response to long-term global warming and short-term interannual variations. The differences in the feedbacks is related to differences in the pattern of surface warming for these different climate variations — in particular, the amount of tropical warming vs. the amount of extratropical warming. I'll also show that calculations based on alternative decompositions that combine temperature and water vapor feedbacks show better agreement vs. observations.