Water vapour makes up between zero and 3% of the Earth's atmosphere, making it the third, fourth or fifth most abundant gas therein (argon makes up about 0.9% and carbon dioxide 0.4%). Water vapour contributes to warming by absorbing longwave radiation and radiating it back to the surface. According to the Climate Change Connection website, "the water vapour feedback process is most likely responsible for a doubling of the greenhouse effect when compared to the addition of carbon dioxide on its own."
© Haydn Thompson 2022