A crucial component of the hypothesis that the one part in ten thousand in the atmosphere that represents human CO2 caused the atmospheric warming from the late 1970’s to the late 1990’s is the notion that the miniscule warming by this tiny fraction is amplified by the positive feedback of increased water vapor. The IPCC thinks this amplification is about 50%. The concept is plausible because the partial pressures of gasses between the ocean ant the atmosphere have opposite responses to temperature change. Simply put, a warmer ocean can hold less of any gas and a warmer atmosphere can hold more. Water vapor accounts for the majority of the greenhouse effect, somewhere between 30 and 90 percent. We don’t really know because the absorption bands of water vapor and CO2 overlap.
The fact that by all accounts water vapor is the prominent greenhouse gas requires a negative feedback beyond the Clausius-Clapeyron logarithmic diminution of effect with increasing concentration. Basically, anything CO2 can do to increase water vapor, water vapor can do for itself. If there were no negative feedback water would have boiled itself out of here eons ago.
Climate models have this peculiar notion that since water vapor cycles quickly with a typical residence time of 10 days it must be treated exclusively as a “feedback”. In a curious way this notion implies the negative feedback I speak of. Water vapor is treated as if it were already balanced and in need of a “forcing” like CO2. Yet photons are photons whether they come from a molecule cycling quickly or slowly and I suspect they are utterly unconcerned which term we bestow them in our algorithms.
Is rain the negative feedback? Negative for water vapor for a few days, but don’t forget the phase changes.Enthalpy of condensation warming the atmosphere;enthalpy of vaporization cooling the ocean, always a one way street with energy transfer from the ocean to the atmosphere. The warmed atmosphere absorbs more evaporation and “down radiates” IR to the ocean “skin”, a positive feedback. The cooled and more saline ocean “skin” away from synoptic upwelling zones will quickly overturn and be replaced by water warmed by visible and UV from the sun above the thermocline. The skin absorbs IR from the atmosphere but radiates it back efficiently, and it continues to evaporate. The rain either falls back in the ocean or on land and fairly quickly recycled. Snow might cycle seasonally or even be included in the ice caps, but at its formation enthalpy of fusion further warms the atmosphere.
I’m just not seeing the negative feedback, but as invisible planets are inferred from their perturbations, this negative feedback must certainly exist.Perhaps when this invisible hand is revealed we will better understand why the greenhouse effect of carbon dioxide appears not to be amplified by water vapor.
Update 12-16-12 : To credit Joel Shore for correction and striking of the sentence about runaway warming from water vapor (see comments), and to add another concept I had intended to include.
The atmosphere receives six times as much energy back from the surface as it receives from the sun. Amazingly, convection, evaporation and radiation back from the surface total twice the energy absorbed by the earth/atmosphere system directly from the sun.
Two thirds of this energy from the surface comes from the oceans. The atmosphere receives back from the oceans in the form of water vapor about half of the energy received from the sun by the entire surface of the earth.
Since 1997 the oceans have continued to warm but atmospheric temperature and water vapor have flatlined. If the oceans begin to cool soon this could be just a lagging effect, but it is hard to see how a warming ocean can allow the atmosphere to flatline in the absence of negative feedback to water vapor.
geosciencebigpicture 
