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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)

At a glance

If you hang a load of wet washing on the line on a warm, sunny day and come back later, you can expect it to be dryer. What has happened? The water has changed its form from a liquid to a gas. It has left your jeans and T-shirts for the air surrounding them. The term for this gas is water vapour.

Water vapour is a common if minor part of the atmosphere. Unlike CO2 though, the amount varies an awful lot from one part of the globe to another and through time. Let's introduce two related terms here: 'non-condensable' and 'condensable'. They set out a critical difference between the two greenhouse gases, CO2 and water vapour.

Carbon dioxide boils at -78.5o C, thankfully an uncommon temperature on Earth. That means it's always present in the air as a gas. Water is in comparison multitalented: it can exist as vapour, liquid and solid. Condensed liquid water forms the tiny droplets that make up clouds at low and mid-levels. At height, where it is colder, the place of liquid droplets is taken by tiny ice-crystals. If either droplets or crystals clump together enough, then rain, snow or hail fall back to the surface. This process is constantly going on all around the planet all of the time. That's because, unlike CO2, water vapour is condensable.

CO2 is non-condensable and that means its concentration is remarkably similar throughout the atmosphere. It has a regular seasonal wobble thanks to photosynthetic plants - and it has an upward slope caused by our emissions, but it doesn't take part in weather as such.

Although water vapour is a greenhouse gas, its influence on temperature varies all the time, because it's always coming and going. That's why deserts get very hot by day thanks to the Sun's heat with a bit of help from the greenhouse effect but can go sub-zero at night. Deserts are dry places, so the water vapour contribution to the greenhouse effect is minimal. Because clear nights are common in dry desert areas, the ground can radiate heat freely to the atmosphere and cool quickly after dark.

On the other hand, the warming oceans are a colossal source of water vapour. You may have heard the term, 'atmospheric river' on the news. Moist air blows in off the ocean like a high altitude conveyor-belt, meets the land and rises over the hills. It's colder at height so the air cools as it rises.

Now for the important bit: for every degree Celsius increase in air temperature, that air can carry another 7% of water vapour. This arrangement works both ways so if air is cooled it sheds moisture as rain. Atmospheric rivers make the news when such moisture-conveyors remain in place for long enough to dump flooding rainfalls. The floods spread down river systems, causing variable havoc on their way back into the sea.

Atmospheric rivers are a good if damaging illustration of how quickly water is cycled in and out of our atmosphere. Carbon dioxide on the other hand just stays up there, inhibiting the flow of heat energy from Earth's surface to space. The more CO2, the stronger that effect.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

When those who deny human-caused global warming use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as potent a greenhouse gas as water vapour, which there's already a lot of, adding a little more CO2 couldn't be that bad, they insist.

What this argument misses is the critical fact that water vapour in air creates what scientists call a 'positive feedback loop'. That means it amplifies temperature increases, making them significantly larger than they would be otherwise.

How does this work? The amount of water vapour in the atmosphere has a direct relation to the temperature in any given region and the availability of water for evaporation. Heard the weather-saying, "it's too cold to snow"? There's more than a grain of truth in that; very cold air has a low capacity for moisture.

But if you increase the temperature of the air, more water is able to evaporate, becoming vapour. There's a formula for this, the figure being 7% more moisture capacity for every degree Celsius of warming. All you then need is a source of water for evaporation and they are widespread - the oceans, for example.

So when something else causes a temperature increase, such as extra CO2 emissions from fossil fuel burning, more water can evaporate. Then, since water vapour is a greenhouse gas, this additional moisture causes the temperature to go up even further. That's the positive feedback loop.

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

The other factor to consider is that water evaporates from the land and sea and falls as rain, hail or snow all the time, with run-off or meltwater returning to the sea. Thus the amount of water vapour held in the atmosphere varies greatly in just hours and days. It's constantly cycling in and out through the prevailing weather in any given location. So even though water vapour is the dominant greenhouse gas in terms of quantity, it has what we call a short 'atmospheric residence time' due to that constant cycling in and out.

On the other hand, CO2 doesn't take an active part in the weather. It does hitch a lift on it by being slowly removed from the air as weak solutions of carbonic acid in rainwater. These solutions are key weathering agents, affecting rocks on geological time-scales. Weathering is a key part of the slow carbon cycle, with the emphasis on slow: CO2 thus stays in our atmosphere for years and even centuries. It has a long atmospheric residence time. Even a small additional amount of CO2 thus has a greater long-term effect - and in our case that additional amount is far from small.

To summarize: what deniers are ignoring when they say that water vapour is the dominant greenhouse gas, is that the water vapour feedback loop actually amplifies temperature changes caused by CO2.

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.

Last updated on 23 July 2023 by John Mason. View Archives

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Comments 276 to 300 out of 378:

  1. JeffDylan @273 , I hope you found TD's and Scaddenp's posts helpful.

    The hypothetical fully frozen world is locked in stable equilibrium (since it lacks CO2 in its atmosphere) but of course it is only an illustrative example of how CO2 or H2O greenhouse effect works.

    The real physical world is the one we are interested in.  And in the real world, it is largely CO2 which works as the "control knob".  Real-world perturbations (e.g. your "forest fires, volcanoes, lightning, or even a solar flare") are brief and relatively small, compared with the decades/centuries-long cumulative effects of CO2 planetwide.

    You might also be interested to look at the (intermediate) "CO2 Lags Temperature" [Climate Myth Number 12, via the Home Page] where the Milankovitch cycles are mentioned.   The biggest take-home message there, is that subtle alterations in albedo (Northern Hemisphere ice cover & vegetation changes) connected with summer/winter axial tilt, being enormously amplified in effect by consequent outgassing (or the opposite) of CO2 from the oceans.  Hence the lag effect, mentioned.

    Climate science is like a 1000-piece jigsaw puzzle where all of the pieces fit together nicely to produce a portrait.  Even with a piece or two missing, the portrait still presents a clear and unmistakable picture (and does not produce "alternative" pictures).     JeffDylan, I sense you feel that [so to speak] one of the pieces is upside down or in the wrong place.   If that is so, then please clarify your thoughts on the matter (but I strongly suspect the overall picture will remain the same — and that your unease stems from a semantics problem, not a physical-reality problem).

  2. Eclectic — I'm not sure I understand how we arrive at the frozen world scenerio you described in 268.  If we take the earth and atmosphere as is, and made every atmospheric CO2 molecule vanish, we would certainly lose the greenhouse effect contribution from CO2, but remember that H2O is the stronger and more abundant greenhouse gas.  Therefore, we would expect a small reduction in greenhouse heating possibly causing temperatures to drop by a few degrees C, but not at all like the "frozen world" you describe. 

    Remember that the earth, atmosphere, and solar radiation as they are today must share an energy balance relation.  That is, whatever energy the earth and atmosphere absorb from the solar radiation must be re-radiated from the earth and atmosphere, at least in the long term.  Since CO2 has only a minor impact on the greenhouse effect, I would not expect the temperatures after CO2 removal to be much different from what they are now in order to maintain energy balance.

  3. @JeffDylan

    "If we take the earth and atmosphere as is, and made every atmospheric CO2 molecule vanish, we would certainly lose the greenhouse effect contribution from CO2, but remember that H2O is the stronger and more abundant greenhouse gas. Therefore, we would expect a small reduction in greenhouse heating possibly causing temperatures to drop by a few degrees C, but not at all like the "frozen world" you describe."

    Real scientists have examined exactly that.  FYI, Water vapor is a condensible GHG. As such, and by definition, it cannot be a driver of temperature changes but can only serve as a feedback to them.

    Carbon dioxide, on the other hand, is the most important temperature control knob on the planetary thermostat.

    Per Lacis et al 2010:

    CO2 is the most powerful GHG

    CO2 is the most important GHG

    CO2 is the most important GHG


    "Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth's atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can, and does.

    Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect.

    Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state."

    Per Lacis et al 2013:

    "The climate system of the Earth is endowed with a moderately strong greenhouse effect that is characterized by non-condensing greenhouse gases (GHGs) that provide the core radiative forcing. Of these, the most important is atmospheric CO2. There is a strong feedback contribution to the greenhouse effect by water vapor and clouds that is unique in the solar system, exceeding the core radiative forcing due to the non-condensing GHGs by a factor of three. The significance of the non-condensing GHGs is that once they have been injected into the atmosphere, they remain there virtually indefinitely because they do not condense and precipitate from the atmosphere, their chemical removal time ranging from decades to millennia. Water vapor and clouds have only a short lifespan, with their distribution determined by the locally prevailing meteorological conditions, subject to Clausius-Clapeyron constraint.

    Although solar irradiance is the ultimate energy source that powers the terrestrial greenhouse effect, there has been no discernible long-term trend in solar irradiance since precise monitoring began in the late 1970s. This leaves atmospheric CO2 as the effective control knob driving the current global warming trend.

    Over geologic time scales, volcanoes are the principal source of atmospheric CO2, and the weathering of rocks is the principal sink, with the biosphere participating as both a source and a sink. The problem at hand is that human industrial activity is causing atmospheric CO2 to increase by 2 ppm/yr, whereas the interglacial rate has been 0.005 ppm/yr. This is a geologically unprecedented rate to turn the CO2 climate control knob. This is causing the global warming that threatens the global environment."


    "If there had been no increase in the amounts of non-condensable greenhouse gases, the amount of water vapor in the atmosphere would not have changed with all other variables remaining the same.

    The addition of the non-condensable gases causes the temperature to increase and this leads to an increase in water vapor that further increases the temperature.

    This is an example of a positive feedback effect. The warming due to increasing non-condensable gases causes more water vapor to enter the atmosphere, which adds to the effect of the non-condensables."



  4. Thanks DB.  I particularly like your first chart, which presents a "balanced" argument  ;-)    ..... though at the most elementary level [suited to my mind!!] it doesn't make the important distinction between "feedback" and "forcing".

    JeffDylan @277 , we arrive at the frozen world scenario (discussed earlier) because there is a huge difference between CO2 and H2O as GreenHouse gasses.

    At first glance , you might think there is not much difference — because under present-day Earth conditions, CO2 contributes [IIRC] roughly 30 watts/squ.meter of GreenHouse warming and H2O (vapor) roughly 70 watts/squ.meter of GH warming.    But there is a huge difference in negative feedback once you "perturb" things toward lower levels of GH gasses.   The essential cause of the difference is that at habitable temperatures, H2O transforms between ice / water / vapor over a very narrow temperature range (while CO2 remains as a gas, and exerting its usual GH effect).

    How this difference works out "so differently" — is through the albedo effect.   Currently the Earth has an albedo of about 0.3 , so roughly 30% of the sun's heat (predominantly as light) is reflected away, and 70% is absorbed by the planetary "surface".    Yet snow/ice has an albedo of about 0.9 — so as a greater percentage of the planet gets covered in ice, there's a vast reduction in absorption of the sun's heat.   It is a very strong feedback, since cooler air loses H2O vapor which is precipitated as snow and frost (and more extensive sea-ice) and more ice leads to even colder conditions.  And so it goes.  And hence the (hypothetical) fully frozen world, which stays frozen [if without CO2].

    You can see a more graduated (and fortunately limited!!) effect occurring in the real planet Earth.  So the scientists are well-justified in calling CO2 the "control knob" and describing H2O vapor as the tail on the dog [metaphorically speaking].

    Yes, the dog's H2O tail is bigger than the CO2 dog [at present temperatures] -— but it is the dog which controls where it goes!!

    And hence the emphasis on the vital importance of reducing atmospheric CO2.

  5. Daniel Bailey @278

    I'm not sure what you mean by a "real scientist", but I do have a doctorate degree in physics which includes some course background in atmospheric physics including the greenhouse effect. Also, I have tried for over ten years to sort out what the "experts" are saying about climate change and anthropogenic global warming (AGW). Therefore, I feel I should be at least qualified to comment on the claims made by these experts.

    Do you actually believe the statement you quoted from Lacis et. al. 2010

    Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.

    So if we take the earth and atmosphere as is, and remove the atmospheric CO2, the earth would lose its entire greenhouse effect despite the fact that the H2O vapor caused about 95 percent of the greenhouse heating — I don't think so. The laws of physics say that no matter how we label H2O vapor (forcing, feedback, both, or neither), those molecules are going to continue capturing and re-emitting IR photons for as long as they stay up there. Furthermore, all H2O molecules that we lose from the atmosphere through condensation or precipitation we gain through evaporation. Therefore, we will still have most of the greenhouse heating that we had before we removed the CO2. Temperatures would likely drop a few degrees C, but we won't see that "frozen world" resulting from a total shutdown of the greenhouse effect. That's absurd!

  6. JeffDylan @280, CO2 is responsible for approximately 20% of the Total Greenhouse Effect (TGE), ie, Total upward IR radiation at the surface minus total upward IR radiation at the top of the atmosphere.  Based on IPCC AR5, the TGE is 159 W/m^2.  Therefore removing the CO2 completely would increase upward IR radiation at the TOA by 31.8 W/m^2.  With no feedbacks, that would result in a temperature decline of 8.9oC to Global Mean Surface Temperature (GMST).  That decline in temperature would result in a fall in total column water vapour, which in turn would increase the lapse rate (a negative feedback) and reduce the contribution of water vapour to the greenhouse effect (a positive feedback).  Combined, the two feedbacks have a feedback factor of +1.1 (+0.9 to +1.3) )W/m^2)/oC.  That results in a further drop in GMST of 4oC, for a total of 12.9oC. 

    That is, eliminating all CO2 from the atmosphere, and with the combined WV/Lapse Rate feedback, the GMST would drop from 14oC to 1oC.  That is from two to three times the reduction in GMST of the Last Glacial Maximum relative to the Holocene.  Naturally, it will result in increased sea ice exent.  Indeed, it would push the sea ice extent significantly south of the southern tip of the United Kingdom (its extent in the LGM):

    That increased sea ice extent would result in a significant increase in the Earth's albedo.  Based on Lacis et al (2010), it would increase it from 0.3 to 0.4 (second diagram in DB's post).  Using that figure, that would result in a fall of 34 W/m^2 in the net incoming Short Wave radiation.  From that we can expect a further fall of >13oC in GMST from the increase in albedo plus the effect of the WV&Lapse Rate combined feedback.

    All up, we are looking at a >26oC drop in GMST just from those two feedbacks.  That is less than that shown by Lacis et al, but their model was not restricted to just two feedbacks (although it did exclude very slow feedbacks, such as the formation and growth of ice sheets).  >26oC is not "a few degrees C".  In fact, the suggestion of a drop of just a few degrees C from the removal of all CO2 is absurd even for just the 9oC Planck responce, let alone once any feedbacks are taken into account.  Given this, I suggest you reconsider your position carefully.  If you want to insist on your absurdly low estimate, you at least owe us an explanation of how it was calculated (I know how deniers typically mispresent the science in calculating these values, but you may have simply assumed a low value without calculation, or be making a different mistake, so I will ask you to describe the calculation in your own words.)

  7. "That's absurd!"

    Your utter lack of actual analysis is duly noted.  FYI:  arguments from your personal incredulity carry no weight in a science-based venue, such as this.

    As Tom Curtis notes, bring actual credible evidence for support of your claims.  And remember to construct your comments in compliance with this venue's Comments Policy...

  8. Tom Curtis @281

    Your analysis presumes the concept of H2O as only a feedback to the CO2 forcing, which no one has proven (even though climate "experts" use it all of the time), and I believe is false. Also, nothing coming from the IPCC or the United Nations in general carries any credibility with me. I'm afraid that unless you can come up with no arm-waving proof that CO2 is the "control knob" for the greenhouse effect (even though the greenhouse contribution from H2O vapor is mathematically much larger), there is really nothing further to discuss. You are only building castles on quicksand in your postings.


    [JH] Antagonisitic and baseless sloganeering snipped. 

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

    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.

  9. JeffDylan - I'm afraid that your personal incredulity is not a argument; CO2 as the control knob of global temperature is quite frankly as well established as the existence of gravity. Non-condensible gases set the thermal equilibrium, condensible gases (water vapor) can only act as feedback because they respond so quickly to a temperature change, even if their overall effect is quite large. 

    Again, your claim of disbelief in the science carries no weight by itself - try the peer-reviewed science, such as Lacis et al 2010, for actual discussions of the evidence.

  10. JeffDylan, which university gave you your doctorate in physics, along the way teaching you atmospheric physics while claiming the very concept of relative humidity is a fraud invented by the United Nations and the IPCC in the 1700s through early 1800s, and promulgated ever since by everyone including your local meteorologist?

  11. Tom Dayton @285 ,

    I hope you are not intending a disparagement of Trump University. 

  12. JeffDylan @283, first, even absent any water vapour feedback (but including the albedo feedback), my analysis shows a drop on GMST from the removal of all CO2 in the order of 17oC, ie, a drop to a GMST of -3oC.  That in itself shows your claim of a drop of "a few degrees C" to be nonsense.  Indeed, even the no feedback drop of 13oC demonstrates that.

    Second, my analysis does not assume water vapour is a feedback on CO2, but on temperature, as formalized by the Clausius-Claperyon relation, which is basic laboratory physics.  It is also demonstrated in the atmosphere by the relation of the total column water vapour to latitude:

    Or the change in relative humidity over land over time in recent decades, plotted as a scatterplot against global land surface temperature:

    (Black is observations (HadISDh), pink a reanalysis product (ERA-40), with the other three being model ensembles for historical greenhouse gas forcings (yellow), historical natural forcings (green), and all historical forcings (purple).)

    You may want to assume total column water vapour is constant regardless of GMST, but when you do you render your views no more interesting, or realistic, than those of flat earthers.

  13. Well, I was hoping that with the climate expertise and reverence to science claimed on this site, someone here would be able to give me a clear, valid explanation as to why condensible greenhouse gases can't be a radiative forcing while they are in the gas state.  Obviously, I was mistaken.


    [DB]  You've been given explanations and citations to credible sources.  You've simply ignored them and not read them.

    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 intentionally misleading comments or simply make things up. 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, as no further warnings shall be given.

  14. JeffDylan @288, if that was what you were after, perhaps you should have asked for it?

    A Radiative Forcing is "...the change in the net, downward minus upward, radiative flux (expressed in W m–2) at the tropopause or top of atmosphere due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun". (My emphasis.)  That is the technical definition of the term.  In contrast, a climate feedback is:

    "An interaction in which a perturbation in one climate quantity causes a change in a second, and the change in the second quantity ultimately leads to an additional change in the first. A negative feedback is one in which the initial perturbation is weakened by the changes it causes; a positive feedback is one in which the initial perturbation is enhanced. In this Assessment Report, a somewhat narrower definition is often used in which the climate quantity that is perturbed is the global mean surface temperature, which in turn causes changes in the global radiation budget. In either case, the initial perturbation can either be externally forced or arise as part of internal variability."

    Both definitions from here.

    It is very obvious that because total column water vapour (and specific humidity), and therefore the effect of water vapour on radiative forcing, vary with temperature, it is an internal driver of the net radiative flux.  Consequently it is a climate feedback.  The relationship between specific humidity and temperature shown in the second image in my preceding post shows conclusively that the effect of water vapour on radiative flux is a feedback, in that it depends on an internal climate state - namely, the global temperature field.

    Of course, I have not been the first person to make this point.  Indeed, it has been repeatedly made, and repeatedly ignored by you.  Your argument, therefore, gives every appearance of a Humpty Dumpty argument, ie, you insist that water vapour is a radiative forcing, not because its concentration is not temperature dependent, but because you insist that words be used your way rather than according to the definition and usage of the community who defined and use those words. 

  15. JeffDylan @288 , you say you have studied climate matters for over ten years — and yet you fail to grasp the concept of "forcings"??

    As an exercise, I have just googled "condensable greenhouse gases", and the very first entry gave an explanation in that regard.  (See the website of the ACS : the American Chemistry Society.)  Not to mention subsequent entries — including the famous R. Pierrehumbert, physicist, who publishes on such planetary matters.

    Jeff, were I more cynical, I would suspect you of being disingenuous.

  16. Eclectic @290

    I have so grasped the concept of "forcings" and "feedbacks". It doesn't mean, however, that I always agree with how they are used. It is important to realize that they are not scientific terms, but only inventions to help keep cause and effect consistent in our thinking and models. Also, there is no rule saying that the presumed cause and effect (or forcing and feedback) will always be correct.

    I looked up the ACS website and found nothing new about condensible greenhouse gases. Apparently, these molecules shut down their absorption/re-emission systems as they enter the gas phase.


    [PS] You are straying into straight sloganeering. People are responding with evidence and references. You are so far providing nothing in support. All words are inventions, and are useful to communication when everyone uses them the same way. And you need extraordinary evidence, not speculation if you want to contradict laws of physics. So far evidence is missing from your comments, and you dont appear to be bothering to study resources people have provided - especially given your egregious final statement which no one and no resource has asserted. 

  17. JeffDylan

    From your earlier comment.

    "It is the greenhouse heating from water vapor driving the water vapor feedback that actually dominates the greenhouse effect."

    The starting point in considering this is the observed fact that water vapour content in the atmosphere is governed by temperature. This is simply an everyday meteorological observation.

    So imagine the atmosphere in a particular state, at some sort of equilibrium. So certain amounts of CO2 and H2O contributing to the GHE in this equilibrium state.

    Now perturb this by adding some CO2. Temperature rises due to the CO2.

    So water content now rises. The rapid turn-over time for water in the atmosphere is what ensures that a strong, quick response occurs.

    Then a further temperature rise occurs. Lets call this the initial feedback response of water to the initial CO2 perturbation.

    But this water response has raised temperature by some further amount, resulting in a further increase in water content. Now this additional  water is acting as a feedback to the initial water feedback raising temperatures further. Feedbacks on feedbacks. And this will continue, like a number series, feedbacks on feedbacks on feedbacks...

    So what would happen, eventually? If the gain of the water feedback were greater than 1, then each increment of warming would produce greater warming and this would be a runaway positive feedback leading to runaway warming. So too any initial cooling would produce runaway cooling.

    In short the weather system (because the short residence time of water in the atmosphere ensures rapid responses so we are talking about weather timescales) would be totally unstable. Even a gain = 1 would still diverge rapidly.

    Which doesn't happen - we would notice that I think.

    So our daily experience shows that the gain on the water feedback is <1. And any such feedback converges on a final value, like a number series that converges to a value.

    So although some part of the water feedback is in response to prior water feedbacks, ultimately the total of the water vapor response to a perturbation is finite. And this water feedback cannot occur without the initial perturbation.

    Although the total response to a perturbation is more than a simple single feedback, ultimately the total water feedback to a perturbation , including feedbacks on feedbacks etc, is finite. To me this says that water acts only as a feedback to exernal perturbations, even though the magnitude of the feedback is more than just the simpleinitial response.

    If we consider the frozen world scenario. Progressively remove CO2. This lowers temperatures and water content in the atmosphere.

    The approximate change, from the Clausius-Clapyeron eqn is that a 1 C change changes water content by 7%. Taking Tom Curtis's earlier figure of -8.9C temperature drop just from removal of CO2,that would result in a decline in water in the atmosphere of 0.93^8.9 or down to 52% of current water values. Add in removal of the other minor GH gases, the decline in cloud contribution to the GH effect as well and you will have an even larger decline.

    Will there be some residual amount of water in the atmosphere after all the other GH gases are removed? Yes. But the temperatures will be sufficiently lower that ice is vastly more extensive, temperatures are much lower still and water much loweragain. So in an ice world water vapor levels would be very, very low.

    In essence, 2 factors force water to only have a feedback role.

    1. The gain on the water feedback must be <1 otherwise we would observe that the system is totally unstable. So water cannot indefinitely drive the response to any perturbation, even from changes in water.
    2. Water content in the atmosphere is totally temperature dependent. It cannot rise or fall without a prior temperature change. Therefore the GH effect of water depends on temperature.

      The conclusion is that water cannot initiate a change by itself, be a forcing. Because for that to occur there needs to be an initial temperature change to trigger a change of water vapor content. Nothing can change water content without an initial temperature change. And any temperature change must force a water content change. Water can't itself perturb the system. And it must respond if there is a perturbation. Even if random variations, local weather events, El Ninos etc introduce small perturbations, triggering small water responses, when those perturbations reverse, the water response reverses.

      A pure feedback-only mechanism. Water can only be a forcing, an initiator of a perturbation, if its atmospheric concentration can be changed by something other than temperature change. If it could accumulate like CO2 does for example. But it can't.
  18. Glenn Tamblyn, for JeffDylan:

    The starting point in considering this is the observed fact that water vapour content in the atmosphere is governed by temperature. This is simply an everyday meteorological observation.

    And via the aforementioned Clausius-Clapeyron relation that formalized this everyday meteorological observation, established by 1850, Arrhenius was able to derive the first CO2 'greenhouse' climate model, which he laboriously worked through on paper and published in 1896.  His estimate of ECS after CO2 forcing and H20 feedback (among other feedbacks) is no more than a factor of 2 higher than current estimates.

    With all due respect to Swedish chemist and Nobelist Svante Arrhenius: do you see how easy this is?

  19. By extension: see how absurd it is to suspect a hoax launched at least 190 years ago by wholly self-interested yet superhumanly far-sighted illuminati, who then enlisted thousands of scientists illustrious and obscure from around the world, all with the superhuman discipline to keep the secret to the present day?

  20. Mal Adapted @294

    Please just can the sarcasm!  Scientific theories come and go all of the time.  If some "illuminati" is using Svante Arrhenius' work now, it is probably because they dug it up recently and is not part of some conspiracy spanning two centuries.  BTW, I thought there were rules against conspiracy theories on these postings, or at least that's what I get held to.


    [DB] This user, yet another sock puppet of spammer cosmoswarrior, has outlived its usefulness and will no longer participate in this forum, where genuine people genuinely seek to promote actual and active dialogue and learning.

  21. John Cook

    Are you certain it is the Clausius-Clapeyron (CC)relation that governs the amount of water vapor brought into the atmosphere by evaporation? Remember that this relation predicts equilibrium (or saturation) values of the water vapor concentrations, which may or may not be the actual concentrations. If the CC equation is used for obtaining H2O vapor concentration as you explain, we would be assuming 100 percent relative humidity everywhere, which I believe leads to a great over-estimation of the water vapor positive feedback you describe.


    [DB] A general note to readership:  this user is yet another fake-account sock puppet created by serial spammer cosmoswarrior (who's real identity I shall refrain from publishing).  Posting rights rescinded as per the Comments Policy; this interchange shall remain for transparency and edification purposes.

  22. MartianSky: The CC relation by itself applies if all else is equal. But Earth's atmosphere is very diverse, so the CC relation applies in very local conditions. For example, when rain forms in the atmosphere, relative humidity is 100% at that precise spot at that precise time. But even as those raindrops fall, often they pass through air of such different pressure or temperature that the rain evaporates before hitting the ground. The atmosphere varies dramatically and dynamically in pressure and temperature at scales from tiny to huge, so the CC relation can be perfectly valid in each of all those locales, but the availability of water in a given piece of air at a given moment can be inadequate for the water to fully fulfill that role. The availability of particles that act as condensation nuclei also affect the ease of condensation; ultra clean and unmoving air can have relative humidity of more than 100%, similar to a supersaturated solution. In short, the CC relation is not the only thing relevant to relative humidity, and it applies only in approximation at scales larger than micro in the real atmosphere, due to the inhomogeneity of the real atmosphere.

    But none of that matters for the purpose of figuring out whether water vapor is a feedback or a forcing of temperature in Earth's conditions. What matters is that, all else being equal, water vapor injected into air at a given temperature and pressure will condense out--in 10 days as a global average.


    [DB] This is yet another iterative sock puppet of serial spammer cosmoswarrior.  Leaving the interchange up for edification purposes and transparency.

    [PS] Wont do anything to edify our sockpuppet master based on past ironclad ability to miscomprehend anything that contradicts his/her denial.

  23. "The starting point in considering this is the observed fact that water vapour content in the atmosphere is governed by temperature. This is simply an everyday meteorological observation." 

    My question is what happens when water is insulated from atmospheric temperature and then that insulation is removed? The temperature hasn't changed, but now there is more water in the atmosphere. Specifically, I'm referring to topsoil and the loss thereof. The loss of carbon in the topsoil since the dawn of agriculture is the largest single contributor to atmospheric CO2. Since topsoil contains more water by mass than carbon, it would seem to be a major source of water vapor as well and independent of atmospheric temperature.

  24. KanKapper: The limit on water vapor is not a lack of liquid water available to evaporate to become vapor. The planet is covered with vast pools of liquid water that constantly are evaporating, adding water vapor to the atmosphere. The limit is how much water vapor can remain in the atmosphere at the atmosphere's temperature. Water vapor constantly is condensing out of the atmosphere to become liquid (or solid) water. Any water vapor added to the atmosphere above the atmosphere's temperature-dictated capacity simply condenses out--about 10 days as a global average.

  25. Questions for John Cook concerning rebuttal

    I have a few questions about the paragraph in your intermediate level rebuttal that mentions the Clausius-Clapeyron relation. Specifically, you state

    Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature.

    You realize, of course, that more water vapor can be added to the air without changing its temperature if the air is below saturation (ie. 100 percent relative humidity). Therefore, the water vapor in the atmosphere can depend on parameters other than temperature. I would appreciate it if you could give some clarification to your statement.  Next, you state

    ... - the rate [of evaporation] depends on the temperature of the ocean and air, being governed by the Clausius-Clapeyron relation.

    The Clausius-Clapeyron relation says nothing about evaporation rates. It is a relation that allows the determination of the partial pressure (or equivalently, concentration) of water vapor that is in thermal equilibrium with nearby liquid water at a known temperature. This would be the saturation concentration (ie. concentration at 100 percent relative humidity) at the given temperature. Therefore, it seems to me that the Clausius-Clapeyron relation is not applicable here since much of the atmosphere is not at 100 percent relative humidity. Furthermore, assuming 100 percent relative humidity throughout the atmosphere would result in a gross over-estimation of the water vapor greenhouse effect. It would be great if you could clarify what you are doing here.

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