I’ve been trying to track down the issues on water in the climate models. Water is funny stuff, and its funny thermodynamic properties are responsible for the kind of world we live in.
Washington has been suffering from solid, particulate water that will eventually melt into liquid. It’s a mixed blessing for us that water’s melting point is within the normal range of temperatures where most of us live. Its boiling point is considerably above, but if you have a heat source, water is a great heat transfer medium for cooking food and can become part of that food, as in soup.
Even when it’s not boiling, though, water vaporizes. In our dry Santa Fe climate, snow and ice disappear without ever melting. And Washingtonians know well how much water the air can hold on a hot day.
Water is a greenhouse gas, and there is more of it in the atmosphere than any other, including carbon dioxide. But, from what I read, it is not included in the climate models in the same way as carbon dioxide; it is a feedback rather than an input. I include the qualification because I have not yet found an explanation of how water is treated that makes sense to me.
I have concluded that water concentrations in air are not included as an input to the models because water is everywhere on earth and it is so volatile. By volatile, I mean that it rapidly establishes an equilibrium among its liquid, solid, and gaseous forms. So if you include temperature as an input along with water’s thermodynamic properties, the model will calculate the correct concentration of water in the atmosphere. When added carbon dioxide increases the temperature, the additional water in the atmosphere is automatically calculated.
That’s my bottom line. Let’s see what a few of the climate blogs and websites have to say.
Skeptical Science seems to be the closest to my interpretation:
Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the temperature of the ocean and air, being governed by the Clausius-Clapeyron relation. If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapour levels to 'normal levels' in short time.A bit more here, but not substantially different.
Scholars and Rogues has a similar explanation.
Basically, water vapor is a more important greenhouse gas than CO2, but because CO2 will cause heating independently of water vapor, as man-made CO2 increases global heating, water vapor will increase too, boosting the amount of warming with a positive feedback loop. How much exactly is up for debate, and there’s not a long enough data series on water vapor in the atmosphere to know everything. But just because humans can’t increase or decrease water vapor in the air directly doesn’t mean that CO2 heating of the air won’t do so indirectly.But they precede it with an explanation I find confusing.
RealClimate’s argument is impossible to summarize in a short quote. This seems to be the center of it:
While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days). To demonstrate how quickly water reacts, I did a GCM experiment where I removed all the water in the atmosphere and waited to see how quickly it would fill up again (through evaporation from the ocean) . The result is shown in the figure. It’s not a very exciting graph because the atmosphere fills up very quickly. At Day 0 there is zero water, but after only 14 days, the water is back to 90% of its normal value, and after 50 days it’s back to within 1%.This comes after a calculation of the percentage that the various greenhouse gases contribute by removing them from model calculations. If someone is skeptical about models, I don’t think that the way to prove anything to them is through model calculations.
I also don’t like the use of the word forcing. It doesn’t have an immediately obvious meaning to the general public; it means any factor that raises or lowers the earth’s temperature. I’m not sure where the word came from. I haven’t seen it in chemistry or physics. I’m usually pretty good at figuring out meanings from context, but I didn’t feel confident about this one until I found an explicit definition. It's not going to go away, though; too implanted in the modeling community.
I’m not convinced that my interpretation of why water vapor is treated as a feedback rather than an input is correct, although writing this post has made me more confident of my logic. And it doesn’t conflict with any of the explanations I’ve quoted.