why couldn't we change it?
I've been hearing about a water shortage for decades
if more ice melts, isn't there a change there will be more water vapor ?
There is no such thing as a "water shortage" at the planet level. The issue we have is droughts which cause a shortage of freshwater suitable for human consumption. But the Earth has plenty of water.
And creating more water vapor via warming is likely a bad thing as it's a GHG and would result in even more warming. However there is SOME suggestion that it would create more clouds which might reflect the sun's energy. But there are other suggestions that cloud levels are at lower altitude than the CO2 which is capturing IR in the upper atmosphere, so it's possible that clouds reflecting sunlight wouldn't actually prevent that warming. It's a very complex topic, and we don't know if increased water vapor in the atmosphere would be a positive or negative feedback to warming.
The answer to "why couldn't we change it" is that water vapor in the air is an effect, not a cause. We can't change the total amount of water on the planet. We could boil a bunch of it, turning it into vapor, but it would just rain right back down on us because we can't keep it in the atmosphere. It's a cycle.
There is also a carbon cycle. CO2 is emitted by natural sources (respiration, decomposing plants, volcanoes) and it is taken up by natural sources (growing plants, the ocean). But CO2 is more persistent. While water vapor levels can change on the daily level, CO2 changes occur over decades or centuries. The difference is that we have been emitting enough CO2 to overcome the Earth's ability to sink that carbon into new plants or the oceans, at least on a time scale of importance to human interests. Hence per Gigem it is actually a knob that we can turn. We've increased atmospheric CO2 (and methane) due to human activity, and it's something that we could reduce if we determined it was important enough.
Meaningfully, we have no knob to turn on water vapor.
