[pjc50]

Yes, water vapor is a contributor - but at an equilibrium rate determined by the temperature! In the narrow band of temperature we live in, increases in CO2 drive an increase in vapor which magnifies their effect.

There's even a nice explainer in IPCC AR5 about this ("FAQ 8.1"): https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapt...

> Currently, water vapour has the largest greenhouse effect in the Earth’s atmosphere. However, other greenhouse gases, primarily CO2, are necessary to sustain the presence of water vapour in the atmosphere. Indeed, if these other gases were removed from the atmosphere, its temperature would drop sufficiently to induce a decrease of water vapour, leading to a runaway drop of the greenhouse effect that would plunge the Earth into a frozen state. So greenhouse gases other than water vapour provide the temperature structure that sustains current levels of atmo-spheric water vapour. Therefore, although CO2 is the main anthropogenic control knob on climate, water vapour is a strong and fast feedback that amplifies any initial forcing by a typical factor between two and three. Water vapour is not a significant initial forcing, but is nevertheless a fundamental agent of climate change.