What is not taken into account (even the computer models don't know how to do it) is cloud formation, more cloud formation means increased albedo and cooling (especially low cloud) no-one can predict what clouds will form, how much cloud, and what the overall effect will be.
Until we have computer models that can forecast accurately and reliably and are proven by weather and climate changes, they are a waste of time, no model or theory is worth anything if results disagree with it.
Look at the physics.
Water vapor does not have a direct impact on temperature. Neither does CO2 or any other greenhouse gas, and the scientists studying global warming don't claim so. Greenhouse gasses act by intercepting photons and randomizing their directions. The energy in those photons shows up eventually as heat, which eventually affects temperature. Ice melting is a good example of an exchange of heat with no temperature change, so is the condensation of water vapor into clouds.
Your premise is based on a mistaken reinterpretation of "impacts".
The premise of your question is incorrect. If direct carbon dioxide effects were ignored (as you claim they are), then there would also be no warming from water vapor.
The saturation vapor pressure of water is given by the Clausius-Clapeyron relation. If the temperature goes up (due to anything: more CO2, higher solar output, whatever) then there will be more water vapor in the air and a stronger greenhouse effect due to water vapor also. It's a pretty strong dependence: each 1 kelvin increase in temperature increases the saturation vapor pressure by about 6%.
To ignore water vapor effects would be like examining the action of a gun and only looking at the mechanical effects of pulling the trigger, while ignoring the explosion that takes place when the hammer strikes the bullet.
EDIT: The warming naturally comes out of the physical equations that go into the models. This is outside Clark's area of expertise (geology), so perhaps he does not understand this. I'll make this more explicit for you: to deny a connection between temperature rise and the increased effects from water vapor, you either have to pretend that the Clausius-Clapeyron relation does not exist, or you have to hypothesize that there are other effects that will counter it.
In other words, the effects due to water vapor arise naturally from the physics; to remove them would require fudging the science--is that what you think Clark is advocating?
EDIT for James: I'm not sure if you're referring to my answer, but it boils down to this: if the temperature goes up, more water evaporates. Are you saying that is not true?
Another EDIT: I find Kano's argument interesting--he is sort of following the Richard Lindzen idea that yes, the climate will change, but it will change the cloud distribution on Earth and possibly not the temperature. Frankly, why are we supposed to believe that changing the cloud cover is any better than changing the temperature? As far as I'm concerned, saying that the climate will change, but in an unknown way, is even worse than the current situation--it should be an argument FOR controlling CO2, not against it.
By the way, while cloud parameterizations are far from perfect in climate models, they are getting better and better, just as they are in weather models. The successful predictions of the East Coast blizzard and Superstorm Sandy are ample evidence that models can be very useful even without the ability to resolve clouds.
All things atmospheric are related to all other things. If you recall your high school algebra, what you do on one side of the equation you have to balance on the other side or X will never equal Y. Idealy isolation...the added energy from the sun on one side of the Earth is equaled by the outgoing radiation of infrared energy on the dark side of the planet. The heat energy of a car engine is cooled to the same degree by the car's radiator. If not for that the engine would burn up. If not for the radiation of heat energy into space at night the Earth would burn up. Still with me?
The atmosphere moderates this heat to cold cycle. CO2 holds some of the daytime heat..if not for that every time the sun set the dark side temperature would plunge to close to absolute zero. The situation today is an ever increasing amount of of CO2....more heat is retained than the amount required to equalize the Earth's temperature. Fortunately in the short run the Earth has a lot of ice. Instead of radiating out X amount of heat energy at night because of the CO2 load the excess heat energy goes to melt ice and warm seawater. Enter water vapor.
Water vapor is also a 'greenhouse gas'. Warm seawater will evaporate more of this vapor into the atmosphere than cold seawater. Seawater that's even a tiny fraction of a degree warmer will evaporate a massive amount of water vapor mainly because there's a shed load of ocean.
Conclusion: What we have is a feedback loop involving solar heat, CO2, diminshed heat radiation, warming seawater and an increase in water vapor leading to an ever increasing 'greenhouse effect'. The point here is that this is a 'system' where a change in part of the equation forces other parts to respond. The rules of heat and atmospheric physics demand that increasing the CO2 and methane content of the atmosphere even slightly will trap more heat. The heat has to go somewhere...cold ice and cool seawater in this case. Warm water evaporates more water vapor and the beat goes on. Unless all the laws of heat and atmosphere are wrong....AGW is the culprit that needs to be addressed.
All but 3 of the models in the ipcc ar4 assume there is a positive feedback, co2 to water vapour.
The assumption is that more water vapour=further atmospheric warming. But the observations to date don't confirm the model projection in this regard.
It may well be that more water vapour=more cloud, which is a net negative forcing.
Because CO2 by itself isn't threatening enough. When you are trying to use CO2 emissions to fight capitalism, you need to have something that will credibly cause a threat. So they theorized that CO2 increased temperature which then increased water vapor which then will cause a catastrophic warming. The problem is that it doesn't appear to be true. It is only true in the models. All their alarmism is based on models. The real world is not their ally.
Because water vapor is the number 1 global warming gas and rabid global warming alarmists must associate CO2 with it in order to illustrate their positive feedback fantasy which fuels their global warming hysteria.
One of Pat's answers to a previous question had a link to Professor Ian Clark. The link did not work when I tried it, but Pat's good answer motivated me to find the You Tube link below which is a brief testimony by Professor Ian Clark to the Canadian Senate about rising temperature.
http://www.youtube.com/watch?v=hDKSkBrI-TM
The evidence he puts forward clearly outlines CO2 as NOT being a major forcing for rising temperature. Water vapour is presented as the major greenhouse gas contributor for rising temperature. Further, modelled CO2 impacts are impacts to water vapour and not directly to temperature. So water vapour is used in the models to drive global warming rather than CO2 directly. These findings are also well documented and viewed by many eminent skeptics as a major concern with AGW claims.
I am sure the AGW advocates will have thier opposition to these findings. Lets hear from both the skeptics and AGW advocates about what they think of Professor Ian Clark's testimony.
