Water vapour act as a powerful greenhouse gas absorbing long-wave radiation. If the atmospheric water vapour concentration increases as a result of a global warming, then it is expected that it will enhance the greenhouse effect further. It is well known that the rate of evaporation is affected by the temperature and that higher temperatures increase the (saturated) vapour pressure (the Clausius-Clapeyron equation). This process is known as the water vapour feedback. One important difference between water vapour and other greenhouse gases such as CO2 is that the moisture spends only a short time in the atmosphere before being precipitated out, whereas the life time of CO2 in the atmosphere may be longer than 100 years.
Interesting post, I’d welcome more detail.
[Response: There is a link to a discussion of this topic in IPCC TAR in ref#1 and ref#2. rasmus-]
I’m not sure that the “short lifetime” of water vapour in the atmosphere is really relevant – after all it is being constantly replenished by evaporation.
It would also be interesting to know the relative overall contribution of water vapour compared with CO2.
[Response: You’re right about moisture being replenished, and that’s why it’s part of a feedback process. The important aspect, I think, is the rate at which misture is being added to the atmosphere versus the rate at which it is being removed. Water vapour is believed to account for the bulk of the greenhouse effect (according to Wikipedia, the free encyclopedia 60%, as opposed to 26% for CO2), but since the accumulation of vapour concentration in the atmosphere is affected by the rate of precipitation whereas the CO2 accumulates more readily, we can be more sure about the effect of CO2-emissions. rasmus-]
“Feedback” is a misnomer. The word you are looking for is “amplifier”. A true positive feedback would be exponentially unstable and, the Earth would have turned into a scalding ball of steam whizzing around the Sun eons ago.
[Response:Not so. Even a local positive feedback can be balanced by other negative feedbacks at finite amplitudes which is what happens in the climate system. – gavin]
If the system is continuous and negative feedback dominates, then the system is stable and, you have nothing to worry about.
It isn’t feedback because the extra water vapor does not create more CO2, just heat. Thus, there is no feedback loop, just an amplifier effect on the heat.
If the system is not continuous, or highly nonlinear, then the tradeoff between negative and positive feedback will result in a limit cycle.
I’m not trying to be a critic. Just trying to bring your terminology in line with my rather extensive experience with feedback loops.
But, I’m still not convinced there is a feedback loop here. Correct me if I am wrong but, the way I have interpreted what you have written is that CO2 creates heat which creates more water vapor which creates more heat. That is an amplifier effect. If the extra heat creates more CO2 that creates more water vapor and on and on, then you have a feedback loop.
Amplifier or feedback, it’s still a bad thing so, don’t feel you are giving up anything if what I am saying is correct. I know very little about your climate models, just a lot about feedback.
I’ll stop posting now. Sorry for the multiple posts…
Response: The thing to remember is that many different feedbacks (under anyone’s definition) operate in the system. Overall, the total feedback for the planet is negative (mostly because of the negative feedback of long-wave radiation increasing with temperature), but there are enough positive feedbacks in the system to allow for significant gain for small perturbations. -gavin]
I’m currently engaged in a discussion with a (person I consider to be a) contrarian. He claims that a paper produced by Minschwanner & Dressler 2004, shows that observations of the increase of Water Vapour are about 1/10 of those used in GCMs, He also claims that this is backed up by work done by Douglas Hoyt, solar physicist.
[Response:I believe that the results presented in that paper turned out to be wrong, as this is the impression I got from an IPCC workshop on climate model analysis in Honolulu March 1-4, 2005. But, I have not seen any papers on it, and I will not yet make a strong statement about this. I anticipate reading more about it in the next IPCC report. -rasmus]
Thus he is contesting that the results of the models show too much warming.
[Response:The GCMs do reproduce the past global mean temperature evolution. See Fig. 4 on p. 11 in IPCC 2001 (summary for policy makers) -rasmus]
Has anyone heard of this? From my (very) basic understanding it’s wrong. But my knowledge of evaporation isn’t great and I’m unable to find out if he has a point, i.e. I can’t refute it.
[Response:
See also the discussion on Water vapour: feedback or forcing? and further discussion of the IPCC workshop. -rasmus]
Thanks for the prompt reply Rasmus, I’ll follow up your leads. To be specific what’s being claimed is that Gravity is a factor limiting evaporation that’s been overlooked. I find this highly questionable, and it’s not covered on RealClimate, but I will pursue it myself.
I’ve already repeatedly pointed out that ‘the model doesn’t know whether it’s 1900 or 2000’ to little effect. That’s contrarism. But I don’t like leaving this stuff unchallenged.
Once again thanks to you, and to all at RealClimate, this is an invaluable resource.
[Response This is a serious overstatement of the M&D results (which were discussed a little in the other referenced post above). They were looking specifically at the uppermost troposhere, and not at the water vapour feedback in general – which is close to the feedback as simulated by GCMs (see Soden et al – links from previous post). Hoyt’s claim that gravity (or more correctly potential energy) limits any increase in water vapour feedback is completely bogus. The potential energy term is simply a energy storage mechanism and is released as soon as the water vapour falls to the ground as rain – there is more than enough kinetic energy to mix almost any amount of water to the stratosphere – the reason why it doesn’t get there is due to the sink processes (clouds and rain) not ‘gravity’. Argon is heavier than water and is well mixed almost all the way up. – gavin]
It isn’t feedback because the extra water vapor does not create more CO2, just heat. Thus, there is no feedback loop, just an amplifier effect on the heat.
The feedback refers to the warming effect. CO2 warming is enhanced by H2O warming feedbacks.
Amber is making an interesting point, which highlights the difficulty some scientists have with the concept of feedback and mathematical chaos. I have just read that, when James Clerk Maxwell first saw James Watt’s governor, he said “It is a fine invention, but try as I may, its analysis defies me” [Lovelock, 2006.] So anyone having this difficulty is in good company :-)
The greenhouse effect of carbon dioxide raises the temperature, and that causes more water vapour to evaporate. The greenhouse effect of H2O then acts as an amplifier of the CO2 effect, increasing the temperature further. However, that increase in temperature causes more water vapour, and so the water vapour is in a feedback loop with the surface/atmospheric temperature. The water vapour effect is a feedback, but the carbon dioxide effect is a forcing.
A point to note is that saturated water vapour density increases subexponetially with temperature. Hence the amount of amplification will depend on the initial temperature, mainly the sea surface temperature (SST). Since the SST varies with latitude etc., and does not respond immediately to an increase in the greenhouse effect of CO2 or H2O, it is pretty obvious (at least to me) that the idea of a fixed climate sensitivy is rather ridiculous, since the amplification of the CO2 effect by H2O will vary with the global and local initial temperatures. In other words, although the CO2 effect increases with the logarithm of its concentration, that is not true of the total effect which includes that from H2O.
Cheers, Alastair.
Lovelock, James, 2006 “The Revenge of Gaia, Why the Earth is Fighting Back – and How We Can Still Save Humanity” Allen Lane, London p. 37.
The greenhouse effect of carbon dioxide raises the temperature, and that causes more water vapour to evaporate. The greenhouse effect of H2O then acts as an amplifier of the CO2 effect, increasing the temperature further. However, that increase in temperature causes more water vapour, and so the water vapour is in a feedback loop with the surface/atmospheric temperature. The water vapour effect is a feedback, but the carbon dioxide effect is a forcing.