Water is the only gas of the three that condenses in atmospheric conditions. The total amount of water vapor that air (or the atmosphere) can hold depends on temperature more than anything else, excess water remains in the atmosphere for days only. This seems like good overview.

https://www.newscientist.com/article/dn11652-climate-myths-c...

Water is the biggest contributor to the greenhouse effect (~60% of the total), but that's because it's the most abundant by far (wikipedia indicates about 0.25% globally [0], by mass, which translates to something like half a percent by volume; meanwhile CO2 is 0.04% and CH4 is 0.0002% by volume). And as others mentioned, it's mostly not human-generated, and it stays in the air for a much shorter period of time (on the order of days instead of years or centuries because of a localized phenomenon called, um, rain).

Methane, on the other hand, is much more than 2x as potent as CO2 (estimates range from 21x to 40x the warming effect over the span of 100 years, but most are centered around 25x, when taking into consideration that that methane's atmospheric lifetime is only ~12 years).

You could argue that there's a 3x multiplier in the comparable difference in weight, but you've still got an 8x multiplier. Even if we assume that CO2 and H2O are comparable in terms of warming potential by volume (it's hard to measure H2O for various reasons), there's still a 3x multiplier over that 100-year period, compared to 1x CO2 and 2x H2O.

Why would some molecules be more potent than others? It's a matter of the infrared wavelengths they absorb, but in particular how they cover the spectrum relative to other atmospheric gases. CO2 absorbs strongly in parts of the spectrum that H2O absorbs more weakly, and CH4 absorbs strongly in parts that aren't covered by either CO2 or H2O [2].

[0] https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Compositio...

[1] https://www.epa.gov/ghgemissions/overview-greenhouse-gases#C... - "Pound for pound, the comparative impact of CH4 is 25 times greater than CO2 over a 100-year period."

[2] https://www.acs.org/content/acs/en/climatescience/greenhouse... which depicts the absorption spectra for various gases on the right: https://www.acs.org/content/acs/en/climatescience/greenhouse...

The amount of water in the atmosphere is probably independent of how much water vapour we emit.

Water is the most powerful greenhouse gas because it’s so abundant. Because it is so abundant though, the absorption lines are pretty much maxed. Besides the available quantum states (1 and 2 atom molecules cannot be GHGs) which make CFCs more powerful, the main impact on GHG strength is relative abundance. Methane is rare, so every atom is important, CO2 is uncommon, and H2O is common. I guess that makes CFCs a foil, lol.

H2O turns into a sort of gravity-propelled fluid form known as "rain", and then its greenhouse effect in the atmosphere ends quickly.

I thought that clouds reflected sunlight back to space and had a cooling effect. Interesting that I may be wrong about that.