The ability for CO2 to warm the surface of a planet through the absorption of infrared radiation is well known. (skepticalscience.com)
He knew that the absorption of infrared radiation was due to CO2 and water vapour, and he also knew that CO2 is a forcing while water vapour is a feedback. (planet3.org)
Where water vapor is important is as a feedback effect... whereby the warming of the atmosphere due to increased CO2 causes the «equilibrium» concentration of water vapor to increase and this then enhances the warming because of water vapor's absorption of infrared radiation. (realclimate.org)