One point on water vapor. Water vapor is not seen as a driving force since it is short term. It acts as a magnifying source. A change in CO2 levels will increase the heat slightly which will increase water vapor and that will in turn raise temperature even more. Within limits there is feedback, but that is controlled by the base temperature set by CO2. A cold snap can remove quite a bit of the water vapor from the atmosphere and then it will have to build up again form its base. Carbon dioxide is long term. It gets removed from the atmosphere very slowly. That is why it is more of a concern for long time warming than either water or methane.
Ah, this is getting more interesting. Let's start at the end where you mention Methane. I will assume that you know that most methane rapidly (8-9 years) breaks down into CO2 & H2O. Apart from a dozen other methane sources, global warming releases methane from permafrost, wetlands, oceans etc. Methane starts off as a 'super' greenhouse gas and subsequently become 2 lesser greenhouse gases. It is itself a major source of global CO2.
Back to water vapour. Water vapour in polar regions barely exists, it nearly all condenses into clouds. Water vapour mainly exists between the tropics - where there is of course an almost constant strong sun to warm the planet up. Warmer air can hold more water vapour so as the planet warms, the atmosphere as a whole can sustain more water vapour. That in turn acts as a greenhouse gas which starts the cycle again. There is nothing obvious to stop it until the day that the air can sustain no more water vapour and we end up with complete cloud cover, maybe triggering global cooling - who knows?
Local weather and temperature variation in the temperate zones can have very little effect on the total global water vapour (very difficult to measure by the way). Outside the tropics we do indeed get seasonal variations in temperatures which condense the vapour into clouds and subsequent precipitation but as the Northern hemisphere cools, the Southern hemisphere warms up (and visa-versa) which balances out the condensation.
Don't forget the methane and CO2 released by global warming and countless billions of living creatures. It certainly helps to drive temperatures up to sustain even more water vapour - the biggest greenhouse gas by far. We all know that CO2 is a moderately potent greenhouse gas but the theory that anthropogenic CO2 is THE major cause of global warming is just a theory. The IPCC carefully couch their reports with terms like 'It is likely that' or 'It is thought that' rather than stating a cause as a fact. They do that for very good reasons.
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