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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Basic rebuttal written by James Frank


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 5 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 201 to 225 out of 378:

  1. MA,  why is the link you provided for WV at 1000 and 600mb different from NOAAs dataset.  Here @1000mb and here @600mb

     

    Response:

    [TD] There is a post and thread devoted to this topic--the counter to the myth Water Vapor is Decreasing.

    [RH] Resized images. Please try to keep images below 500px otherwise they break the page formatting.

  2. My understanding is that the important increasing factor is the specific or absolute humidity and not the relative humidity.  Because warmer air holds more water vapor, if the temperature increases the specific humidity can increase while the relative humidity decreases.  This thread, referenced above, discusses the data for specific humidity.

  3. I believe that trends in relative humidity are far less important--and maybe completely unimportant--than trends in specific (absolute) humidity.  The latter reflects the absolute amount of water vapor, which is a greenhouse gas.  It is physically and logically completely possible for relative humidity to decrease at the same time specific humidity increases; that would mean only that specific humidity was not increasing as fast as the relative humidity allowed it to.  So increased greenhouse forcing from increased absolute amount of water vapor would not be defanged by a simulaneous decrease in relative humidity.

    The only caveat I can think of, is that relative humidity affects lapse rate, which certainly is key to the greenhouse effect.  But I've seen no mention of that counteracting increased specific humidity.

  4. Also, the global average of specific humidity is what matters by far the most.  Scales smaller than that will show variation, as described in Chapter 2 of the AR5, but such variation is no more important to the increasing global energy balance than are small scale variations in temperature.  The fake skeptics are using their standard tactics.

    Response:

    [Rob P] Indeed. Needless to say, the amount of moisture in the air is increasing - as physics would dictate in a warming atmosphere:

     

  5. POJO.
    Further to more recent comment & specifically addressing you comments:-
    @200, I think you should point out to the guys that the relevant line on page 164 of IPCC AR5 WG1 is "Observed change in the hydrological cycle, including precipitation and clouds, is assessed in Section 2.5."
    @201, the difference in the graphs is the difference between specific and relative humidity (as pointed out @199). You may note that the data is measured in g(H2O)/kg(air) in the one graph & % in the other.

  6. POJO, adequate answers have been provided. The warmer the air, the higher the water vapor content can be. Relative humidity is the precentage of total water vapor that could be contained in a volume of air at a given temperature. Absolute humidity is the actual water vapor content of the air in g/cubic meter. In aviation, relative humidity is given in weather reports and expressed, instead of a percent, as the temperature/dewpoint spread. The dewpoint is the temperature at which water vapor would condense if the air was cooled down to that temperature. The smaller the spread, the higher the relative humidity, hence the chance of visible moisture, which is of concern for aircraft operations. When considering climate, absolute humidity is more important because, as far as I can recall, it determines the warming feedback of water vapor.

    Furthermore, I would underline that the lower the relative humidity, the lower the chance of water condensing out of the air. A low relative humidity makes precipitation less likely; if your interlocutors are in California, perhaps they will realize that their argument does not foster a case of "it's nothing to worry about."

  7. Let's see now.  IR in intersepted by WV which immediatly thermalizes the energy. The increase in temperature evaporates more WV. Positive feedback in the lowest layers of the boundry layer, so far so good. Then this warmer air and WV convect to condensation altitudes where the heat is radiated away by the WV molecules.  We know that this is true since it is the only mechanism by which 3/4 of the earth heat is sent to space.  Additional warming by any forcing mechanism allways results in addition WV which compensate by radiating more heat to space.  Now you are going to tell me that any increment to the amount of WV is going to cause additional heating. Are you all out of your 'cotton picken' minds? know, it is mind boggeling complex but mother nature worked this out eons ago and it still works.  It is the only cooling mechanism that she has.  Has she changed it just for you? Or has Michael Moor's suggestion of reversing the laws of physics suddenly taken effec?

    Response:

    [RH] Please watch the tone of your comments if you want a productive engagement.

    [PS]

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive or off-topic posts. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site.
     
    Finally, please understand that moderation policies are not open for discussion.  If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  8. arationofreason, you did not ask an actual question, but I'm guessing that you were implying belief that the greenhouse gas effect of water vapor is saturated. If so, you should read the post about the myth of CO2's greenhouse gas effect being saturated, because although the details differ, the principles are the same.

    Response:

    [PS] Reading an accurate account of GHE eg as done here would help too.

  9. aror @207, consider the case where 100% of the IR radiation from the surface is absorbed by WV.  If your argument had any validity it would be valid in this case.  With an average surface temperature of 288 K, that means on average 390 W/m^2 of radiation  will be absorbed by the WV.  The energy is thermalized, and the WV rises through convection.  As it does do it cools due to adiabatic expansion, with the temperature falling by approximately 6.5 K per Km.  Suppose it rises by 2 Km, and then radiates its energy to space.  Therefore, it will radiate to space at a temperature of 275 K (on average).  At 275 K, the absolute maximum energy it can radiate to space is given by the Stefan-Boltzmann law, and is 324.3 W/m^2.  Therefore in this simplified scenario, less energy is radiated to space because of the existence of the WV.  As the temperature at altitude is governed by the surface temperature by the adiabatic lapse rate, and as Outgoing Longwave Radiation must equal incoming shortwave radiation to achieve equilibrium, that means the the Water Vapour warms the surface of the Earth.  You only manage to think otherwise because you ignore the fact that the WV cools as it rises, and the consequences that has on the energy balance.

    In real life, WV does not absorb all of the radiation.  Some makes it to space from the surface.  Equally, however, much of the radiation is from far higher altitudes than 2 Km.  A warming Earth not only increases the WV, thereby reducing the radiation to space due to increased absorption.  It also increases the altitude from which, on average, WV radiates to space thereby increasing the temperature differential between radiation from the surface, and radiation from WV, and hence increasing the strength of the greenhouse effect.

    Note to the moderator.  Last time I responded to aror I was reminded of the comments policy position on inflammatory tone.  aror's name is obviously chose to suggest that they have what others lack, ie, that ration of reason.  They are suggesting that they are adding a dose of reason to the discussion here that is otherwise lacking.  Ignoring the massive irony of that claim given the quality of their posts, that means their name itself violates the provisions agains inflammatory tone.  I request that you enforce the policy by requiring them to use a name without invidious implication.

    Response:

    [PS] Thank you for your positive contribution to the discussion. We do not have policy on pseudonyms to enforce. I thoroughly agree with the irony, but I think that it improves the chances of constructive discussion if we all resist provocation. We will see whether AROR is able to respond in conformance with comments policy.

  10. Moderator, My apology and thank you for your forebarance.

    I fully appreciate all of the technical analysis that I have read over the years which shows the incredible complexity of the problem even without the confusion which often arises for people who are not accustomed to working with feedback let alone positive feedback.  Yes all of these effects undoubtedly take place at different times and places throughout the atmosphere.  I merely wanted to make the point that many are local temporarly and spatially.  We may never have the measurements and computing power to untangle the physics with enough accuracy to settle the question and that we should look to nature for the solution. 

    The 10u window (negative FB ) aside.  The only tool left to balance the radiation by NET radiation to space is water vapor.  It has done a pretty good job (till now apparently).  The only simple point that I was trying to make is that in spite of (local) positive FB effects of increased water vapor, it is the only cooling mechanism there is.  As such its NET effect is cooling in response to increased forcing. How else can it keep up with variation in forcing and balance the earths radiation over the eons. It seems logical to me that increased water vapor will add cooling and is thus a net negative climate feedback in spite of all the complexities which are well noted and computed with our understanding of atmospheric physics.

    Response:

    [PS] This amounts to Argument from Personal Incredulity. Without supporting evidence, this is little better than sloganeering. "It seems logical to me that increased water vapor will add cooling" is an assertion that seems to me to reflect a poor understanding of the physics and should be supported by either data or references if you wish to persuade others of the value of your position.

  11. arationofreason, SkepticalScience's comment threads are for scientific discussion, not repetitions of what "seems logical" to you personally regardless of the science.  You started this discussion (rudely and vaguely), apparently without reading the original post of this thread, or at least without addressing any of its points. Several people then gave you more information, which you should have read and responded to, disagreeing if you desired.  Instead you ignored all that information and merely re-asserted your personal belief.  If you continue that behavior your comments will be deleted. Instead you need to respond specifically to the information you have been given.

  12. We have been led to believe that the warming effect of CO2 is linear but it's logarithmic, decreasing in proportion to saturation. This means that after about 200ppm adding more will do near nothing.
    Obviously if this were not true there would be no life on Earth.
    This can be seen with Venus and Mercury which have about the same daytime temps yet Venus has an atmosphere %96 CO2 and Mercury near none.
    This fact was brought up a few years ago and there was stunned silence then and stunned silence since, it killed the debate stone dead.
    Laughably the politicians soon started saying "The debate is over" but they didn't know why !

    Response:

    [TD] See the post "Is the CO2 Effect Saturated?" Read the Basic tabbed pane there, then watch the video lower on that page. Then read the Intermediate tabbed pane, then the Advanced tabbed pane. Then read the article linked in the "Further Reading" box below the video. Then if you still are unconvinced, say so in a comment on that post's thread, not this one, and I will point you to two articles on RealClimate.org and some on ScienceOfDoom.com. Before commenting again, gain some knowledge and lose some attitude.

  13. AJ Virgo @212...  "We have been led to believe that the warming effect of CO2 is linear but it's logarithmic..."

    Nope. The entire premise of climate sensitivity is a function of a change in temperature per doubling of CO2. That is, in itself, is a logarithmic function.

    And that very premise dates back to Svante Arrhenius in the late 1800's through the early 1900's.

  14. re:AJ Virgo @212, I can well imagine a stunned silence when it was claimed the daytime temperatures of Mecury and Venus being "about the same" disproved the existence of a atmospheric greenhouse effect, but it is the stunned silence that shows complete disbelief that anybody could so flaunt their ignorance of basic science.

    Let's start with the basics.  Mercury is closer to the Sun than Venus.  Therefore, because of the inverse square law, Mercury recieves far more radiation than Venus.  Three and a half times as much per meter squared, as it happens, and six and two/thirds times as much as is recieved by the Earth per meter squared.  All else being equal, as a result of this we would expect the skin temperature of Mercury to be 120 K greater than that of Venus, and 169 K greater than that of the Earth.

    All else is not equal, of course.  In particular, the bond albedo of Mercury is just 0.068, compared to 0.9 for Venus and 0.306 for Earth.  That is, Mercury reflects away just 6.8% of radiation that falls on it from the Sun, compared to 90% for Venus and 30.6% for the Earth.  Once we factor that into the equation, we expect a skin temperature of Mercury of 439 K, compared to just 184 K for Venus, and 254 K for Earth.  Ergo, absent any greenhouse effect, we absolutely do not expect the daytime (and night time, winter or summer) surface temperature of Venus to excede the maximum surface temperature on Mercury, but at 737 K, it does indeed exceed the 700 K maximum daytime temperature of Mecury.

    Having ignored elephants like the effect of Sun-planet distance on radiation recieved, and the effect of albedo, it is no surpise that AJ Virgo also ignores subleties such as heat distribution, comparing daytime maximum temperatures with Venus mean annnual temperature (which coincidentally is also its daytime maximum temperature due to its thick atmosphere and strong greenhouse effect).  For what it is worth, the skin temperature, on a planet with no atmosphere, equals the surface temperature.  As it happens the observed mean surface temperature of Mercury is 440 K, near identical to the calculated vaue.

  15. Well, personally I think that talking about "daytime" temperature on a planet that takes longer to rotate on its axis than to orbit around the Sun speaks more about one's ignorance than any other enormity he could profer...

  16. PhilippeChantreau @215, perhaps, but half the planet remains in night at all times even so.  I suspect I have missed your point.

  17. Venus is better described as having a bright side and a dark side, because of it slow rotation; talking about "daytime" implies that a location on Venus is going to have diurnal temperature variations as experienced on Earth, while not only it is better described as a "yearly" variation but there is also no significant difference between bright and dark side temperatures on Venus. "Daytime" is not an appropriate qualifier of anything on this planet.

  18. The note that H2O causes a +GMST feedback of ~100% causes me to infer that H2O feedback is 50%. Example: suppress additional water vapor by technology and raise GMST by 1.0 degrees. Release the suppression of additional water vapor. This causes additional 0.5 degrees, which causes additional H2O feedback at 50% of 0.25 degrees, causing additional 0.125 degrees,...and so on. An asymptotic (not exponential runaway) +ve feedback loop of 0.5+0.25+0.125+0.0625..... adding to 1.0 degrees 100% feedback. Basic feedback>100% is runaway until the fuse blows. Below 100% feedbacks can be huge multiples of the initial cause or smaller in accordance with the basic feedback %age.

  19. arationofreason @210 You state water vapour "has done a pretty good job" by balancing the radiation by NET radiation to space. I disagree on 2 points. I disagree below on another assertion of yours. 

    (1) "a pretty good job" is relative and you haven't related it to anything. In point of fact water vapour has done a lousy job of radiating energy to space compared to the job Earth's surface could do if we could get rid of all the dratted water vapour and well-mixed GHGs in the atmosphere with a giant Kleenex. It's 35 degrees worse. It's a slacker employee. That's because it radiates in random directions but Earth's surface radiates upwards only. This random directionality means WV & GHGs in the atmosphere that send 83% of the needed radiation to space to match Sun's non-reflected input must send a matching 83% downwards into WV & GHGs in the atmosphere layers below. Furthermore, there must be more energy radiating in the atmosphere layers below because, in a coarse 2-layer simple example, atmosphere below must send 166% downwards to match the 166% it sent upwards, and so on down to the surface. I've overstated it by using a coarse 2-layer example with the lower layer getting 0% radiation to space because it's actually 333 w/m**2 getting sent back down to the surface in order to get a large enough shimmer of radiation into WV & GHGs in the atmosphere to get 199 w/m**2 sent to space. So, WV & GHGs efficiency radiating to space is 199/532=37%. Considering that the surface would radiate to space at 100% efficiency if all the WV and GHGs in the atmosphere were gone, that 37% is not a "pretty good job" at all. It's lousy efficiency is why there's the current mix of life on Earth. Thus, if there were no WV and GHGs in the atmosphere at all then efficiency radiating to space is 100% (100% of radiation from Earth's surface goes straight to space). With the current WV & GHGs in the atmosphere efficiency radiating to space is 37% (I've explained why above, due to radiation in random directions from WV and GHGs in the atmosphere, the actual quantification 199 & 333 w/m**2 was developed by climate scientists). Obviously, it didn't jump from 100% to 37% efficiency in a unit step at some magical WV and GHG quantity, it steadily reduced efficiency in step with increasing WV and GHG quantity. Obviously, there's nothing magical about the present quantities and the present 37% so efficiency will continue lowering from 37% with increasing WV and GHG in the atmosphere. Since the 199 w/m**2 needs to get sent to space to limit the imbalance at the present 0.7 w/m**2 then if increasing WV and GHG in the atmosphere reduces efficiency to, say, 36% as it eventually must if WV and GHG keep increasing as per prior simple obviously-correct logic, then it follows that 199/36%-199 = 354 w/m**2 must get sent back down to the surface in order to get a large enough shimmer of radiation into WV & GHGs in the atmosphere to get 199 w/m**2 sent to space. That example is an increase of 354 - 333 = 21 w/m**2 at Earth surface. That extra downward LWR warms land surface and traps some extra Sun's SWR in the oceans' top few centimetres, from where it gets mixed down and is variously available both for bigger El Ninos and for warmer deep ocean for millenia.

    (2) It isn't only WV radiating to space as you assert, the GHGs do it also.

    (3) The "local temporarly and spatially" complex, unpredictable variations you mention are not relevant to the strategic topics of "global warming" and ecosphere heat increase. They are relevant to the inability of current science to predict timing, location and severity of specific severe weather events, to predict which periods of several years will have less warming and which will have more warming and to predict how climate of specific regions will change over time scales of only a few decades with the obvious exception of the well-known "Arctic amplification". We all know this. Climate scientists call it "natural variation". When temperature data are analyzed for suffiently long periods over large enough geographic areas, they are clear enough.

  20. What you are saying is that if the global temperature increases by 1 degree, water vapour will then increase global temperature by another degree. But won't that degree cause water to increase the global temperature by another degree. And won't that degree cause water to increase by another degree ... and so forth? Either that means that your assumptions are wrong, or global warming is not caused by humans but by a run away water effect.

    Response:

    [PS] Please see the myth "Positive feedback leads to runaway warming".

  21. TheNumberOne @220, no!

    What the OP is saying is that an increase in temperature due to CO2 will cause an increase in temperature due to WV of 0.5 C, which will cause a further increase in temperature due to WV of 0.25 C, which will cause a further increase in temperature due to WV of 0.125 C, and so on, with the entire series of increases adding up to a total increase due to WV of 1 C, with feedbacks on feedbacks on feedbacks already included.

    Mathematicaly,  G = 1/(1-f) where G is the total response, and f is the initial response of the feedback.  Provided f is less than 1, G is finite.

  22. There seems to be an obsession with positive feedback here. Earth's stability arises from the fact that as it warms, more water vapour is produced. That then rises and condenses out into water droplets. These collect translational energy from all the molecules which dissolve in them - h2o, co2, n2,o2, converting it to vibrational together with latent heat of condensation.  The raindrops - being the only generator of radiation of any significance in the atmosphere - then get on with the job of pushing more energy out, the higher they are and the more complete the cloud cover the greater the portion leaving earth.  It is radiation from clouds which keeps the temperature up under a cloudy sky. It's radiartion from clouds which keeps earth warmer under cloudy skies, not radiation from water vapour. 

    Response:

    [PS] Attempted to fix html.

    Can I please suggest that before you criticize the science, that you take some time to first understand it. There are good resources on this site to do so. Repeatedly making nonsense assertions will be regarded as sloganeering.

  23. old sage - Um, no. The only phrase in your comment that is correct is that "..as it warms, more water vapor is produced". Not even a complete sentence. 

    Water vapor acts as a feedback to CO2 driven temperature changes, and while it radiates a significant portion of climate energy to space, it's strictly through thermal emission from water vapor - not collecting energy from molecules 'dissolved in water droplets', which is complete, utter, nonsense. Nor by any means is it the only pathway to Earth emissions - CO2, CH4, CFCs, clouds, the atmospheric window, etc, all share in IR emissions. 

    The limiting factor in climate feedback is the Stephen-Boltzmann law, wherein energy radiated to space scales with total Earth emissivity and T4 temperatures. 

    I strongly suggest, 'old sage', that you do some reading before posting additional misunderstandings - The Discovery of Global Warming by Spencer Weart is an excellent and approachable place to start. 

  24. In a press release issued yesterday (Nov 9, 2015), the World Meterological Organization (WMO) states:

    Water vapour and CO2 are the two major greenhouse gases. But it is CO2 which is the main driver of climate change. Water vapour changes are the so-called feedback mechanisms and happen as a response to the change in CO2. For a scenario considering doubling of is CO2 concentration from pre-industrial conditions, i.e. from about 280 to 560 ppm, water vapour and clouds globally would lead to an increase in atmopsheric warming that is about three times that of long-lived greenhouse gases, according to the Bulletin*. 

    Greenhouse Gas Concentrations Hit Yet Another Record, WMO Press Release, Nov 9, 2015

    *WMO Greenhouse Gas Bulletin issued Nov 9, 2015.  

  25. The misnamed "old sage" (@ 222) asserts that raindrops are "... the only generator of radiation of any significance in the atmosphere".  That is, he claims the vast majority of radiation in the atmosphere comes, not from water vapour, but from water in a liquid (or, presumably, frozen) state.  He provides no source or data for the claim.  It is, however, a claim shown easily to be false by clear sky surface, downward IR spectrums such as this one from Wisconsin:

    Note that the H2O lines shown are for water vapour, not liquid water which has a different spectral signature, being effectively a black body in the IR spectrum.

    The relative contribution of liquid water to atmospheric radiation can be checked by comparing the cloudy and clear sky radiation using a radiation model.  Using modtran, with tropical atmosphere, 0 km altitude looking up, but otherwise default settings, the back radiation is 347.91 W/m^2 in the clear sky state, and 418.25 W/m^2 with a low cumulus cloud cover.  That is, the low cloud cover increases the backradiation by 70.34 W/m^2 or by 20.2%; but that liquid water contributes >50% of the total radiation where it is present.

    It contributes greater than 50% because the other radiation is still emitted, but then absorbed by liquid water drops.  Indeed, where present, if we consider all the radiation emitted by liquid water drops and then absorbed by other drops before escaping the cloud layer, the total emission by liquid water, where present is >> 50%, but how much greater would be very difficult to determine (and beyond the capabilities of public access radiation models).  It may be this fact that "old sage" is relying on in his claim.  If so, it still does not justify the claim as it only applies where the liquid water (ie, clouds) is present.  

    A full examination of the issue using global cloud cover and allowing for all emissions may justify the claim that liquid water in the atmosphere is responsible for the vast majority of all emissions in the atmosphere - or not.  That however, is irrelevant to "old sages" specific claim, however, which was that it was "the only generator of radiation of any significance".  As seen above, under some circumstances adding the cloud layer only increases backradiation by 20%, so that emission from liquid water in those circumstances is of significantly less (approx a quarter of) significance than emission by gases (including WV), because it only makes about a 25% difference to the total backradiation.

    Even that, however, is overly generous to "old sage".  An increase in backradiation makes no net difference to the surface energy balance because it will be balanced by increased evaporation, or increased convection (as we have all seen in the formation of cumulo-nimbus clouds).  In contrast, a TOA energy balance is, in the end, only eliminated by a change in surface temperature.  Therefore it is the TOA energy balance which determines the effect on GMST of changes in radiative components of the atmosphere.  And there we are fairly confident about the relative effects.  Specifically, CO2 contributes about 20% of the total greenhouse effect, clouds (liquid and frozen water in the atmosphere) about 25% and WV about 50%.

    So, in the end, what "old sage" says may be true if "significance" is interpreted purely in terms of quantity of emission (although he has not shown it).  If that is how he intended it, however, it is doubly misleading.  Misleading because, by focussing on emission only and ignoring absorption, he grossly overstates the relative impact on daily weather (backradiation); and misleading because by focusing on backradiation he completely ignores the primary factor effecting global climate change.

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