In the water vapor greenhouse effect, how does an H2O molecule know if it is part of the forcing or part of the feedback?

Hmm, why would you think that a H2O molecule knows anything?

Water vapor would not exist without heat, so it is always a feedback. It is a always a positive feedback if it doesn't form clouds.

Kano is partially right. When it joins together with other H2O molecules it forms clouds, the clouds reflect light and that would be mostly a negative feedback. However those same clouds at night are a positive feedback. (You might have noticed that overcast days tend to be cooler, while at night, it is the lack of clouds that make it colder.)

Kano's link is to an activist denier site. (A great site if you want to reinforce your misconceptions, useless if you want to learn about global warming)

You shouldn't anthropomorphize physical things, it leads to scientific confusion. A water molecule is just a water molecule--it doesn't "know" anything. In the Earth's climate system it can exist as vapor, liquid or solid. Words like "forcing" or "feedback" are human ways of interpreting the physics of the situation. There is a very large sink for water vapor in the climate system--the Earth's oceans, so if we were to follow a water molecule around it would not stay long as vapor in the atmosphere, perhaps a week or two. Water is continually evaporating from the ocean surface and falling back down as rain or snow (or condensing into clouds).

The overall water content of the atmosphere closely follow the equilibrium value given by the Clausius-Clapeyron relation, with an average relative humidity in the tropical atmosphere of about 80% (that is, 80% of the equilibrium value). If the surface temperature is increased, the relative humidity stays approximately constant while the absolute amount of water vapor goes up about 7% for each kelvin of increase. You could change the relative humidity by changing the continental distribution or the average wind speed, but we don't expect either of those to change significantly over the next few hundred years. It would also change if the land use changes significantly, and that may be a factor as tropical rainforests are removed and the boreal forests expand.

Because the relative humidity remains fairly constant with temperature, the only way to significantly change the water vapor content of the atmosphere is to change the temperature--humans aren't pumping enough water vapor into the atmosphere through our industrial processes to significantly change the water vapor concentration of the atmosphere, which is about 0.4% by volume.

The carbon dioxide content, on the other hand, IS being significantly changed by human processes. So far humans have increased the amount of CO2 in the atmosphere by about 40%, so it's now about one-tenth of the abundance of water vapor, or 0.04% by volume. There are also sinks for CO2, both on land (biomass) and in the ocean (in solution), but they are not fast enough to keep up with the rate of release into the atmosphere, so the amount in the atmosphere is increasing. As it increases, it changes the longwave infrared optical properties of the atmosphere and warms the surface. As the surface warms, more water evaporates and the water vapor content of the atmosphere goes up in response to the temperature increase. This further changes the longwave optical properties of the atmosphere and warms the surface.

Of course, this all happens according to the laws of physics in a self-consistent manner--trying to break it into separate pieces that occur in a particular sequence is a construction of the human mind to better understand the process--it is not part of the physics. We call the release of CO2 into the atmosphere a "forcing" because we have control over that and it persists over human time scales. We call the increase of atmospheric water vapor a "feedback" because humans don't directly control that, it's simply a response to temperature. If we had some way of dumping huge amounts of water vapor into the atmosphere very quickly (quickly with respect to the 9 day average lifetime of a water molecule in the atmosphere) then we might call our water vapor emissions a "forcing" also, but we can't do that, so we don't.

I think I can clear this up, though, as others have said, H2O does only operate as a feedback; not causing warming, but amplifying any warming caused by anything else. So H2O molecules have no need for an iPad.

It is CO2 molecules that require these devices, as they need to know whether they occur naturally or are part of the AGW Conpiracy to take all of our money and freedom. If they get the word "natural" they continue to do what they have always done; intercepting radiation and directing it towards the Sun, which then warms a little, thus warming the Earth by a tiny amount. (also Mars and Pluto, but not the Moon as it has no atmosphere)

If they get the word "conspiracy" they simply redirect the radiation towards the nearest thermometer! (that's why you get more warming over the US; more iPad sales, but less over the oceans as there's often no signal)

Hope that helps. :)

It does both together, the warming effect of H20 is expotential (huge) but thankfully its evaporation and precipitation cooling is expotentially huge too.

Read this

https://wattsupwiththat.com/2016/07/28/precipitabl...