Water vapor is also a powerful greenhouse gas - but it’s (a bit more) complicated
Water vapor is indeed a greenhouse gas and contributes to global warming. However, the story is (a bit) more complicated than the case of CO2 and methane. Here is why.
The water vapor positive feedback loop
A positive feedback loop is a self-reenforcing cycle. Here are a couple of examples:
if you work out you build strength and as a consequence have more energy and work out more.
the baby feeding stimulates the mother’s milk production which causes further feeding
Here s how this is relevant: warm air is capable of holding more water vapor than cooler air, so increasing temperatures (because of CO2 and methane) lead to more H2O in the air. This water vapor itself is a greenhouse gas which helps increase temperatures even more leading to even more water in the air.
If we reduce warming due to CO2, methane and others greenhouse gases, the temperatures will go down and less water will be present in the air reducing the temperatures even further. The same feedback loop applies on the way down as applied on the way up.
Unless we want to cover all open sources of water (e.g. oceans) there is no way to control the amount of water vapor in the air. If we were to remove some of it, new one would simply enter the atmosphere from oceans, lakes, rivers, etc. until the equilibrium for the given temperature is reestablished.
The important lesson here is: water vapor does not control the Earth’s temperature. Instead, the amount of water vapor is controlled by the temperature. Source