Quote:
Originally Posted by Nimwe
Water is a greenhouse gas, it's true, and on a per molecule basis has a similar magnitude of infrared absorbance - and therefore radiative forcing potential - as CO2. However, the effect of water vapor on global atmospheric heat trapping is strongly dependant on its average atmospheric concentration. That, in turn, tends to be strongly heterogeneous regionally, varying on Earth from about 0 to 4% depending on latitude, average surface temperature, wind patterns, and availability of evaporating liquid water at the surface.
On a planet as described in the narrative, absolute global H2O vapor concentration would be likely limited by freeze-out on the night side and low availability of surface liquid on the day side. So the relative humidity, and therefore the absolute concentration of water vapor, would be globally low, with a relative maximum near the terminator (over a liquid ocean) and a minimum at both poles. So, the average global effect of H2O on absolute radiative forcing would likely be minimal.
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I think you'd need some pretty special topography to make that work. What's stopping a massive water flow from the cold to the hot side? Unless you assume all the highland is on the day side and all the ocean basins are on the cold side. If there is an ocean basin that spans the terminator then you're going to have massive evaporation/boiling there. You only have to evaporate a small percentage of an ocean to add massively to the atmospheric greenhouse, and evaporates that condensed out as snow and rain are likely to fall where they are most available for flow back to the hot side, close to the terminator, not at the pole of cold.
Exactly what happened along the way to replace the primordial hydrogen, helium, and methane atmosphere with carbon dioxide isn't explained.