The amount of water (humidity) varies from place to place, time to time. Water remains in the atmosphere for a relatively short time, unlike excess carbon dioxide which can remain for centuries.
But human activity is putting water in the atmosphere indirectly, by the increased heat caused by increased CO2.
This is a feedback loop. As we put carbon dioxide in the air, temperatures rise which cause more evaporation of water, further increasing temperatures. The only way to reduce or stabilize this GHG, water, is to reduce or stabilize temperature. That takes us back to CO2 emissions.