According to this source, water vapor is not only a greenhouse gas, but totally dwarfs all other sources. If true, this pretty much ends the debate. Can anyone provide the Straight Dope?
Water vapor is indeed a very significant greenhouse gas, and may well be the dominant one. And, in fact, having some greenhouse gas is a good thing, because without it, our planet would be a frozen iceball. But it doesn’t take all that much change in the total amount of greenhouse gasses to go from “enough” to “too much”.
This is where carbon dioxide, methane, and the others come in. The amount of water vapor in the atmosphere can’t really be changed: If you try to add more, it’ll just rain that much more, and keep more or less the same equilibrium. However, the amount of carbon dioxide can be changed dramatically, and in fact the overwhelming evidence is that humans have in fact changed it dramatically.
You can think of greenhouse gases, other than water, as amplifiers of water’s effect.
If you add enough CO[sub]2[/sub] to a dry atmosphere to raise the temperature 1.0 degrees, that same amount of CO[sub]2[/sub] would raise the temp. of an atmosphere in dynamic equilibrium with liquid water by say 1.5 degrees. Why? Because the saturation vapor pressure of water depends upon air temperature. In fact, as seen in this chart, the saturated vapor pressure of water increases nonlinearly with temperature, so the more CO[sub]2[/sub] you add to a wet (equilibrium) atmosphere, the greater the amount of warming you see per unit of CO[sub]2[/sub] added.
One reason that you get a so-called ‘runaway’ greenhouse effect is the feedback of small changes from other gases on the water vapor content of the atmosphere. An increase of 0.5 C from CO2 etc., results in some ice cap melting (increasing available water in the system) and increases the water vapor capacity of the atmosphere. So, you get more greenhouse heating from that added water vapor, which raises the average global temperature some more, and before you know it, the planet looks like Venus.
Does that mean that if / when we switch to hydrogen fueled cars that emit water vapor that we are in for climate change in the form of increased rain?
You’d need to specify the precise form of the fuel cell, as well as some sort of ‘miles per gallon’ figure to make a direct comparison with the water emission of gasoline powered cars. However, burning a molecule of gasoline produces 8 molecules of CO[sub]2[/sub] and 9 molecules of H[sub]2[/sub]O. So fuel cells would have to have a pretty low mileage rating to massively exceed the water emissions of a gasoline engine.
All this proves the old saw is right!
IT AINT THE HEAT IT’S THE HUMIDITY!
LOL! 
We’ve certainly had extended periods in the distant past with much higher temperatures than today and we are not like Venus. Your argument seems unpersuasive (the issue of clouds comes to mind most readily).
I read somewhere a few years ago that there’s a threshold temperature for this runaway greenhouse iffect. IIRC, it’s something like 50°C.
I am a complete idiot, because it never occured to me why the humidity makes heat so much worse. “Sure it’s 112, but it’s a dry heat!”
A few weeks back I saw an episode on the Science Channel (or National Geographic Channel or whatever) about some guy braving the earth’s hottest climates. Early in the show, they headed to a jungle, where the heat was bad, but the humidity was approaching 100%.
The guy was in bad shape, explaining that it was so humid that your sweat couldn’t evaporate, meaning that not only did it not cool you off at all, but that it just sat on you and accumulated, so that you became drenched.
That was a lightbulb moment for me, when I realized that the higher the humidity, the less evaporation, so the less cooling effect you get from sweating. Duh!
Yeah, I know, I should have learned that in 3rd grade. I must have slept through that day in science class. In fairness, I never really gave the topic any thought before seeing that show.
The amount is totally trivial. A single outdoor swimming pool can evaporate tens of thousands of gallons of water in a season. You’ll never notice the increase from exhaust vapor.
That’s because you live in the wrong end of the country and (unless you’ve traveled) never experienced the so-called Dry Heat.
In the Spring here, the daytime temp easily reaches the mid-90s to a hundred but the humidity is 5 to 15%. If you’re in the sun and/or exercising you’re gonna get hot, but if you’re just settin’ in the shade you’re quite comfortable. Not exactly cool, but a lot more comfy than someone in, say Atlanta, would be at 95° You don’t get the sweat trickling down from your armpits or making the shirt stick to your back, but you are sweating. The sweat’s evaporating and keeping you cooler and everything’s jake . . . until you run out of water. Then you get heat prostration and literally keel over.
As a consequence, if you’ve been here more than a year or two, you’re very conscious of keeping hydrated. The first thing a hostess asks when you arrive at her home is if you need anything to drink, and we were carrying around those half-liter belt-bottles long before the rest of the country.
Later on, it gets to 110 - 116° and you’re hot no matter how much you’re sweating. Then the monsoon starts and the temp drops some, but the humidity rises to 35-45% and we get the sweaty clothes and are still miserable.
Oh, and in support of the water vapor holds the heat theory, when the humidity’s low, by dawn the temp has dropped to 70-75° When the humidity’s high, dawn temperatures of 90° are not rare, even with the somewhat lower daytime high. I remember one miserable morning when as I was driving to work at 4am, the weatherman reported the temp as 103 (!)
DD