There is a common claim from climate change deniers that ninety eight percent of the greenhouse effect is caused by water vapour so that changes in Carbon Dioxide and other greenhouse gasses are not significant or important.
This is plainly and obviously a falsehood. The facts are very clear that, CO2 accounts for about fifty percent of the effect. We then have clouds that contribute twenty five percent and the remaining amount is accounted for by the other greenhouse gasses. The concentration of water vapour in the air increases as temperature rises and there is evidence that this is acting as a feedback mechanism further increasing dangerous warming.