Surely a ten-year old can see that this is complete and utter nonsense?

From one of my favourite Warmenist Porn sites:

Climate Myth:

Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour.

The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost.

If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius.

That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas. (Tim Ball)

Basic rebuttal written by James Frank:

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right?

What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa.

So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

Has he not heard of the concept of an equilibrium?

There are two ways of interpreting the rebuttal:

1. There is no equilibrium. So a CO2 induced increase of 1C leads to an overall 3C increase. Which in turn would lead to another 9C increase. Which would lead to a further 27C increase. And then we'd all be boiled alive. Clearly not true.

2. There is an equilibrium, and 3C warmer is the 'new normal'. OK. But the Warmenists insist, with calculations, that the entire 1C increase since the Little Ice Age is down to increases in CO2, with water playing little or no role.

In which case, why isn't it already 3C warmer? That's clearly not true.

If they insisted that of the 1C increase, 0.3C was due directly to CO2 (this is in fact just about plausible) and the rest was due to water vapour "and other feedback loops" (as yet unspecified), well fine, but none of them has ever said that.
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Finally, he does not give a starting point for the CO2 level at which such positive water/CO2 feedback kicks in.

Let's change his wording slightly: "So when something else causes a temperature increase (such as extra CO2 from fossil fuels bright sunlight or atmospheric pressure), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback."

That's clearly nonsense.

Should we assume that up to 'pre-industrial levels' of 280 ppm of CO2, there is no positive feedback, and it starts at 281 ppm? Or current levels of 420 ppm (and rising)? Or does the positive feedback start at 1 ppm?