Water vapor is the most significant greenhouse gas?

This talking point, like many others, starts with a "true fact," but by omitting other facts, it misleads the hearers. 

It's true that water vapor traps more infrared heat/light than carbon dioxide does.  And it's true that, as the upper graph shows, the total amount of water vapor in the atmosphere is increasing.  That forces us to ask the crucial question, did that rise in water vapor initiate modern warming, or did warming initiate a rise in water vapor?  It's the classic question:  which came first, the chicken or the egg?

The lower graph answers that question by displaying relative humidity.  Merriam-Webster defines relative humidity as "the ratio of the amount of water vapor actually present in the air to the greatest amount possible at the same temperature."  Therefore, if water vapor is causing modern warming, water vapor must increase before temperature does, so relative humidity would have to increase.  Yet we see relative humidity is decreasing.  This shows us that the warming is occurring first, and the additional warmth causes increased evaporation.  Then, that increased evaporation increases the overall amount of water vapor in the air, which traps still more heat.

Scientists make a clear distinction between water vapor and what they refer to as noncondensing greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and others.  From Lacis et al 2010:  "Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state."