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 
Here is where your argument goes wrong:
Actually, no. This is the old confusion about the fact that some infinite series have finite sums.
So, for example, let’s imagine that some extra water vapor gets into the air and this water vapor causes the temperature to rise by 1 C. What happens next? Well, that temperature rise will cause more water vapor to evaporate, which will then cause a further temperature rise…which will then cause more water vapor to evaporate and more temperature rise…and, pretty soon, you have boiled away all the water, right? Well, no, not necessarily. It depends how large the positive feedback is. Let’s say it is such that for each degree rise in temperature, the additional water vapor that evaporates then causes another 0.5 C rise.
So, the amount temperature rise of 1 C causes the amount of water vapor to rise so that temperature rises by 0.5 C. Then that additional temperature rise of 0.5 C cause the amount of water vapor to rise so that temperature rises by 0.25 C…and so on. In the end, what you have is the infinite geometric series 1 + (1/2) + (1/4) + (1/8) + … and that series does not diverge. Instead, it converges to 2. So, the effect of the water vapor feedback in this case is to double any perturbation (including a perturbation originally produced by water vapor itself).
There does not need to be a negative feedback to prevent the water vapor feedback from “blowing up” and boiling all of the water on the planet.
“So, for example, let’s imagine that some extra water vapor gets into the air and this water vapor causes the temperature to rise by 1 C.”
This would be very strange, since evaporation of water cools both the water surface and the air when %RH rises. We even build air coolers based on this principle, evaporative coolers.
I find that climate science based on the greenhouse theory gets many things wrong. Both water vapor and co2 are well known to lower temperature, and they are used for this purpose in industrial applications.
Oh…and by the way…I don’t think climate models make any assumptions that water vapor acts only as feedback and needs to be forced by something like CO2. Water vapor is free to fluctuate but the fluctuations don’t cause the temperature to blow up because, again, the water vapor feedback is not strong enough to do this…It just magnifies the fluctuations by a finite factor.
Joel,
My source for feedback only was the NASA GISS website and I believe the author was Dr. Lacis. I would hope that modern models have eliminated this because it makes no sense.
I am not an expert on finite sums. Are you saying that any amplification less than one will resolve to a finite number and not diverge?
gymnosperm,
I’d be curious to see what the website you are talking about says. Do you have the link?
And, yes, if by “amplification” you means the “first order response relative to the perturbation”. I.e., if the first-order effect of the increase in water vapor in response to a temperature increase of 1 C is a temperature increases of less than 1 C, then it will not diverge. (By first order response, I mean just the response to the original perturbation and not a response to the additional temperature increase caused by the water vapor feedback.)
In particular, for an amplification factor A, the total response would be an amplification of the original perturbation by a factor of 1 + A + A^2 + A^3 + A^4 + … This is what is called a geometric series and the series converges to the value 1/(1-A). So, the example I gave before where A = 1/2 gives a total amplification by a factor of 2 whereas A = 9/10 would give a total amplification by a factor of 10. As A approaches 1, the amplification gets bigger and bigger, but it is not infinite as long as A is less than 1.
Joel,
Here is the link:
http://www.giss.nasa.gov/research/briefs/lacis_01/
“The numerical climate experiment described in Fig. 2. demonstrates the fundamental radiative forcing role of the non-condensing GHGs, and the feedback (only) role of water vapor and clouds”
I actually recall a more specific discussion of the fast cycle time regarding feedback only somewhere on that site but I can’t find it.
I think you are right and I am going to strike out the runaway sentence but I think the prior sentence that water vapor doesn’t need CO2 remains true. The argument might be made that water vapor has “converged” already but I don’t see how we could know that given the clear evidence of extremely rapid regional warming episodes in the Pleistocene. I wonder how the convergence relates to a “tight corner” in the Clausius Curve where water vapor becomes essentially unresponsive to any forcing.
H2O content of the lower atmosphere is often modeled as a positive feedback to increasing CO2 because added CO2 is suppose to mean warmer air which will allow it to hold more H2O. Greenhouse gases act as a radient insulator showing the rate of flow or IR radiation flowing out of the atmosphere. As greenhouse gases operate to warm the lower atmosphere the opposite is true in the upper atmosphere were the earth radiates to space. With input radient energy constant, to gain more energy the earth has to radiate less to space At equlibrum the earth looks like a roughly -18C black body. If there were no green houe gasses this temperature would appear at the surface of the earth but green house gasses rase this level to about 9 kilometers in altitude so that the surface is roughly 33C warmer then what the earth looks llike as a radiator to space. If added CO2 allows the earth to gain energy by adding more insulation then it will lower the -18C temperature at high altitude.. The lowering of the temperature at high altitude will mean that the capacity to hold H2O at high altitude will decrease. H2O is also a greenhouse gas. Less H2O in the upper atmosphere will decrease the insulating green house gas effect and act to stableize IR radiation to space. So in the upper atmosphere H2O acts as a negative feedback to increases in CO2 resulting in climate stability.
willhaas,
What you say is true but it is important to remember that unlike the “well mixed” greenhouse gasses including Carbon dioxide water vapor is highly concentrated in the lower troposphere. While we might expect the lower troposphere to warm and the upper to cool as a result of additional greenhouse gas, the lower troposphere has spent the last sixteen years defying that expectation in spite of the reality that nearly 1/3 of all historic human CO2 emissions have occurred in the last decade and a half. Cooling of the upper troposphere for any reason will reduce its water content, but the absolute volume reduction will be small because the initial concentration was small. The cooling and desiccation should also reduce the effective radiative altitude and increase the earth temperature from space as you suggested.
So we are faced with the problem that the upper troposphere has been cooling a bit even though the lower troposphere has not been warming. Furthermore, the lower troposphere has declined to accept either heat or very much water vapor from a warming ocean.
Thank you for reading my reply and commenting. In the lower troposphere the humidity levels are usually close to saturation anyway so trying to boost them just creates more rain. Subtracting H2O from the upper atmosphere should to act to lower the effective altitude of where radiation to space takes place. Adding CO2 should act to raise the level but then lowering H2O as a consequence will lower that level again. Raising the level results in higher temperatures at ground and level and vis versa. The altitude os a matter of the temperature lapse rate and is a function of parameters such as the pressure profile and heat capacity of the atmosphere. The amount of CO2 that we are talking about has not appreciable effect on the lapse rate. This feedback mecanism in the upper atmosphere deffinitely middigates the effect of adding CO2 to the atmosphere. In my mind how much it midigates is in question. Of course climate has been relatively stable over the past billion years so changes in green house gasses have not caused runaway climate instability. There must be some mechanism that stablizes the climate. There is one researcher, Ferenc M. Miskokzi that calculates that this feedback mechanism that I speak of cancles out all effects of adding CO2 to the atmosphere. If correct, Miskokzi’s theory blows away the theory of CO2 induced global warming.
willhaas,
It is wonderful to see people actually thinking instead of spouting jargon. Since we obviously have no idea what is going on everything is on the table. Even the lapse rate is not simple. The moist differs from the dry. It is conceivable that a necessarily small reduction in upper tropospheric water vapor could cancel CO2 because after all CO2 concentration is very small without amplification. Amplification is obviously very small because the lower atmosphere has accepted only a very small increase in vapor in spite of dramatically increased human CO2.I will check out Miskokzi.
Please keep thinking and let me know what you find.