Darwinist asked in EnvironmentGlobal Warming · 8 years ago

How strong is the water vapour feedback?

My understanding of water vapour is that the maximum amount in the atmosphere is dependent on temperature (lets assume pressure is constant, for simplicity). If this amount is exceeded, the excess precipitates out and falls as rain. The only way it can be increased is if there is a rise in temperature.

So, the simple version of the question; for a 1 dec C in global average temperature, for whatever reason, how much additional warming would be caused by the extra water vapour in the atmosphere?

... and the compound version; given that water vapour is a greenhouse gas, and that warming oceans would give off CO2; also a greenhouse gas, how much warming in total, assuming enough time to reach equilibrium?

What other feedbacks (positive or negative) would modify this figure? ... and by how much?


Sorry; in case anyone didn't get it, that should read 'for a 1 dec C RISE in global average temperature ...'

Update 2:

jim; of course you are correct when you say it isn't 100 percent humidity everywhere, but what possible mechanism could could cause an increase simply by adding water vapor only where it isn't at 100% of the starting value? By chance, that has to be extremely unlikely at best ... doesn't it?

Update 3:

Spider boy; <<More water vapor means more clouds (maybe).>> ... I would have thought yes!

<<This can have a cooling effect (maybe).>> ... again, yes, as a result of increased albedo. But don't clouds also have a warming effect? There can't be many who haven't noticed the contrast between mild, cloudy nights and cold, starry nights. Overall, from what I've learned here, clouds are most likely slightly positive (warming) in their effect, though there is uncertainty on this; they could possibly be negative.

<<We do know that it has not warmed in 16 years.>> We do not 'know' this; even if temps were dropping, there is not yet enough data to support that conclusion. Statistically, the warming continues!

Update 4:

Spider boy; your additional comments:

<<You would have thought "yes"? But the atmosphere is (supposedly) warmer. Therefore water vapor has less of a chance of condensing.>>

Initially yes, assuming the temperature rise was rapid, but if temperatures are warmer, then there would be more evaporation. The warmer atmosphere would soon take up its increased capacity and then precipitation would balance evaporation. So I would have thought 'yes' there would be more clouds.

<<Regarding your reply regarding clouds: Read YOUR reply carefully. Isn't "maybe" what you are saying? First you say "yes" they cool. Then you say "no" they warm.>>

Not quite; they both cool and warm by different mechanisms.

As for 'it has not warmed in 16 years' that's clearly not true; just go to any of the main global data sets! You don't need to 'carefully pick' anything!

But please; the question is about the water vapour feedback, can we not get too bogged down with side issues?

10 Answers

  • 8 years ago
    Best Answer

    The studies reviewed by the IPCC concluded that water vapor doubles the CO2 effect, but that when interactions with yet other feedbacks are considered the water vapor effect could be 3.5 times the CO2 effect.


    A more recent study confirmed that previous conclusions the water vapor could as much as double the CO2 effect.


    A study several years prior used Mt Pinatubo to try to measure the water vapor effect and concluded that water vapor added about +60% to CO2 (not including the amplification of other feedbacks.)


    So, the best estimate is about doubling initially, but when that doubling increases the albedo and methane feedbacks it becomes much more than that.

    Source(s): For an easy to follow video about water ... http://www.youtube.com/watch?v=LAtD9aZYXAs
  • Al P
    Lv 7
    8 years ago

    Effectively, the earth is a divergent sink of IR flux

    resulting in a mean positive temperature gradient.

    This is due to in phase feedback between the flux and

    increasing CO2 and H20 particle density to name

    just two. Although out of phase feedbacks are

    present, they are not on average greater than positive



    The data, if true, indicates that there are slowly increasing

    levels of CO2 and H2O over a time scale measured in decades.

    In the gas phase (and liquid) this means more kinetic and quantum potential

    excited states of energy on the surface and in the atmosphere. Again, assuming

    the data I've studied is correct, a simple application of quantum theory,

    kinetic theory, and deductive reasoning proves the case to me beyond a reasonable

    doubt. Nature has way of cycling up and down. Currently, she is on the upswing.

    To answer your question directly without proof if I understand your terminology and

    meaning correctly:

    Water vapor has a average positive feedback that is stronger now than in the past.

  • 8 years ago

    Temperature is only one factor governing the maximum amount of water vapor in the atmosphere.

    Temperature does not control precipitation - energy and water saturation are the two factors. Energy is dependent on the phase of the water. Precipitation requires airborne particulates to initiate condensation (which releases energy) and turbulent mixing of air (which transfers energy).

    Precipitation is a phenomena of the troposphere and the lower stratosphere. CO2 affects heat retention in the mesosphere and thermosphere.

    Increased water vapor is a symptom, not a cause of global warming. By definition the feedback effect is mathematically insignificant.

    You are using a phenomena dependent on local conditions and trying to extrapolate globally. The rules don't apply. Microscopic and macroscopic phenomena are different.

  • 8 years ago

    Too many interdependent variables that are poorly quantified to draw a reliable conclusion.

    More water vapor means more clouds (maybe). This can have a cooling effect (maybe). Will water vapor increase the snowfall in permanent Ice fields in Antarctica locking up this water? Will more rain fall? Less rain? Will the long term consequence be more forestation? Less? Will it affect the albedo? Which way?

    No one can answer those questions with a definite answer.

    We do know that it has not warmed in 16 years. The ice is growing in Antarctica and the sea level is dropping. Who predicted that?

    Simple computer models diverge quickly enough. Complicated ones are a joke. The only thing that can be predicted is the speed of divergence--in that respect, the models have behaved as you would expect.


    You would have thought "yes"? But the atmosphere is (supposedly) warmer. Therefore water vapor has less of a chance of condensing.

    Regarding your reply regarding clouds: Read YOUR reply carefully. Isn't "maybe" what you are saying? First you say "yes" they cool. Then you say "no" they warm.

    Maybe "maybe" is correct, eh?

    <<We do know that it has not warmed in 16 years.>> We do not 'know' this; even if temps were dropping, ------------------------Prima facia.

    >>But please; the question is about the water vapour feedback>> OK, my bad.

    >>Not quite; they both cool and warm by different mechanisms. >>

    So,....we're back to 'maybe'? How can you possibly conclude (or even conjecture) that the water vapor feedbacks are positive? It seems ALL feedbacks are positive with you guys. How has the planet's climate stayed so stable when we have had all these positive feedbacks lurking? I guess that one molecule in ten thousand was the tipping point, eh?

    Source(s): "Statistically, the warming continues! " If you VERY CAREFULLY pick your truncation points.
    • If the total water vapour in the atmosphere increases, the optical thickness of the atmosphere will increase (the basis of any claim for positive feedback). The optical thickness has remained constant at 1.82 for 60 years (the measurement period for NASA data).

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  • Rio
    Lv 6
    8 years ago

    Using NASA's average seasonal atmospheric pressure adjustment, an ~ 1.8 F increase in a warming. Will increase water vapor and trap an additional ~2watts per sq meter... Or a factor of two for warming.

    Your use of (maximum) has left me befuddled: snow can bypass the liquid transition phase, which is not restricted to temperature increases. But altitude and wind, or are you literally stating a unformed constant pressure for everything or trying to apply one or more STP formulas?

    There are modeling attempts concerning sea spray, aerosol salts, and BVOCs, those are usually specific studies. I couldn't with any degree of accuracy state an overall net effect. But then I'm not alone.

  • 4 years ago

    Ottawa Mike usually avoids mentioning the denier-blog sources he endlessly trawls, but this time he provides one link for his denier-fantasies about water vapor: non-scientist Marcel Crok. Anti-science non-scientists, like JimZ, often pretend to be scientists. Having taken some classes in geology, however, does not make someone a scientist except in the sense that everyone, even the most non-skeptical fake-skeptics, has some degree of interest in scientific questions.

  • 8 years ago

    Assuming the parties above who say the thing is too complicated to work out are correct you might work it out yourself using a very simply model. This sort of exercise tends to be interesting and informative.

    Perhaps: A disk, the earth, has a 'law of atmospheres' atmosphere hanging above it and down through this shines parallel rays of heat. It shines back with light appropriate to its temperature.

    Warm the disk and water vaporizes into the atmosphere. This increases the volume of the atmosphere - would this 'useless work' squeeze some of the water vapour in the atmosphere into liquid? - and increases its pressure, due to the partial pressure of the additional water vapour.

    That sort of thing. It'll probably turn out to be vastly more complicated than it looks.

    In the event you get this working, could you mail me with it?

  • 8 years ago

    I would go with Baccheus. His is the only answer addresses the question, makes sense, and provides credible links. Unless you think he is really missing something, I would suggest that you could stop there with no need to continue to ask further variations of this same question.

  • JimZ
    Lv 7
    8 years ago

    Since it isn't 100 percent humidity everywhere, obviously it could be increased simply by adding water vapor where it isn't at 100%, IMO.

  • 8 years ago


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