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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 326 to 350 out of 378:

  1. There would be doubts about water feedback if measured total precipitable water differed from models. This comparison is discussed in opening observations chapter of the latest ipcc report. German translation is available I believe, so I strongly suggest that you check. 

  2. @Eclectic: True, all the water vapor has to be considered, not just the vapor at ground level. But I would not say that relative humidity is irrelevant. Because when you talk of a 'water vapor feedback', you imply that the increased capacity for water in air also will be used. But that does not follow logically.
    We know that relative humidity in higher altitudes is quite low. So when we have, let's say, 5 g/m^3 water capacity in air at a height of 10 km, but we only have a relative humidity of 20% (this would result in 1 g of water per m^3), then what is the effect of increasing the capacity to, let's say, 6 g/m^3? There will be no effect, because increasing the maximum limit (due to a higher temperature) will not automatically rise the water level in the atmosphere. The water has to come from somewhere. And if there would have been water available, it could have evaporated in the air before, because the air was not saturated with a relative humidity of 20%. Or in other words: In order for the water vapor feedback to be logically, you must assume a relative humidity of 100% at all times and all locations. Or you have to provide another law which links the value of relative humidity to temperature only, and thus makes relative humidity a function of temperature only and no other effects.
    To your second paragraph: I think you are mixing up some fundamental different terms. What you mean is the absorbance ( of a material. The absorbance defines, how much radiation will be absorbed by the material. The more molecules there are, the more radiation will be absorbed. How much radiation will be absorbed per molecule is defined by the 'molar attenuation coefficient' (, which is an intrinsic property, and thus will *not* change in higher altitudes. Thus water vapor at ground level and in high altitudes should be equally effectively, at least concerning this parameter.
    What indeed does change with higher altitudes (because with higher altitudes pressure changes as well), is the optical density as a synonym for 'refractive index'. But this index does not specify how much radiation will be absorbed by the material, but rather how fast electromagnetic waves travels through the material.

    @scaddenp: I looked up your reference. I found a graphic in the latest report (AR5): Figure 2.31 (b). It is not a good graphic, considering the fact that the axis label says something different than the description, and that the reverence value for the diagram is never stated. But one can see a trend that the water vapor is rising. This is not very surprising knowing that water vapor is a greenhouse gas and that the global temperature is also rising. But how can you be sure, that the rising water vapor is the effect, and not the cause of the rising global temperatures? When reading the text, though, there is a very interesting statement: "Satellite measurements also indicate that the globally averaged upper tropospheric relative humidity has changed little" (AR5, Chapter This would be at least an indication (but not a prove), that relative humidity is indeed a fixed value and does not fluctuate randomly. The only problem is: This statement is not proven sufficiently. Because when looking up the original source referenced in the AR5 (Shi and Bates, 2011), the authors never make such a statement. Anything I missed, or any other remarks?

  3. Silbersulfid @327 :

    I placed the term "optical density" in quotation marks because it is a term that really belongs in a different field (in certain liquid measurements).  Sorry, if that was leading your thoughts into an unhelpful direction.  I was hoping the term would suggest to you the idea of — how far the Infra-Red photons can penetrate into a volume containing a large number of water vapor molecules.

    At low altitudes, the IR photons can travel only a very short distance before being absorbed by another water molecule.  At sufficiently high altitude (where the inter-molecular distances become extremely large between water molecules) it becomes possible for significant numbers of IR photons to avoid re-absorption and escape to "outer space".

    It is the density of water molecules which controls the IR energy escape.  In other words, it is the absolute distance between molecules.  Therefore it is irrelevant whether the "relative humidity" is a bit higher or a bit lower (at this high altitude).

    Warmer air will contain more water vapor molecules — on average.  And on average, more water molecules will reach the high altitudes where their IR emissions will be lost to space.

    Therefore it is completely unnecessary to have 100% Relative Humidity.  Relative Humidity is irrelevant to the greenhouse mechanism.  It is all a matter of inter-molecular distances.  (And of course, the rate of IR energy loss will be related to the air temperature at this altitude.)

  4. @Eclectic: Ok I see your point, that makes sense. I see that we agree on the topic of water molecules and IR transmission.

    But you say, that "warmer air will contain more water vapor molecules - on average. And on average, more water molecules will reach the high altitudes". But where do you take this wisedom from? There must be a whole lot more factors, which influence the amount of water vapor molecules in air (also called humidity, and relative humidity being the humidity related to the maximum possible humidity for air of a special temperature). I could imagine that wind, cloud coverage and cloud condensation nuclei would influence the humidity of air as well, to just name a few. But I am not originally a meteorologist or a clima scientist, so for weather phenomena you should be way more firm than me. To me it seems just very over-simplified to assume that the humidity is solely a function of temperature and nothing else. It also makes absolutely no sense to me to refer to the "Clausius–Clapeyron Relation", since this relation only describes the maximum capacity of water in air, but not the actual humidity air will have for a certain temperature. So when you never refer to 100% relative humidity (which means that the air actually reached its maximum capacity of water), there is absolutely no use then to mention this relation, and to mention that air can hold more water now. When air never holds is maximum capacity, why mention that the maximum capacity just increased? This seems to be totally irrelevant.

  5. Silbersulfid: No one claims temperature is the only influence on humidity. Everyone agrees that other factors also affect humidity. Your assertion that anyone thinks temperature is the only influence, is a “straw man” argument. That means you constructed a straw man just so you could claim victory in knocking it down.

    Those other factors, plus variations in temperature locally in time and locally in space, plus the probabilistic nature even of the dependence of humidity on temperature, all together are responsible for relative humidity being less than 100% (and more than 100%) often and in many places—even over bodies of water. No one disputes that. Despite all that variation of humidity over time and space, it is perfectly legitimate, sensible, and useful to compute average humidity over time and space.

    Temperature can increase without any of those other factors changing. That increases the water-vapor carrying-capacity of the air everywhere that the temperature has increased—in other words, the relative humidity has decreased. But that reduction is fleeting, because quickly the newly extra carrying capacity is filled by extra water vapor molecules—in other words, the relative humidity quickly returns to whatever it was before the temperature increased. The new presence of more water vapor molecules is described as an increase in absolute humidity. All that happens without changing the variabilities of both relative and absolute humidities across time and space.

  6. I'm active in the regenerative agriculture community and keep coming across the following assertion: 

    "Do you know how much of the global heat dynamics on earth are regulated by carbon? You might be surprised to learn that it's just 4%. How much is regulated by water? 95%."

    As this tends to be an obfuscating claim put forward by cc deniers I was surprised to find that in this instance it comes not from a climate change denier but from a retired Australian CSIRO soil microbiologist called Walter Jehne who holds some interesting albeit controversial ideas about how to address climate change. This paper, for example, sets out his thinking and how he came up with those numbers. 

    "[water] and its unique capacity to absorb, retain, transfer
    and dissipate heat, via absorption, evaporation,
    clouds, condensation and precipitation, plus some
    60-80% of the natural greenhouse effect, that
    governs over 90% of the earth’s natural heat
    dynamics and heat balance (10). Can these water
    and heat processes help us in mitigating global

    By contrast CO2 influences less than 4% of the
    earth’s total heat balance as it provides some 20%
    of the natural greenhouse effect which contributes
    some 18% of the earth’s net heat balance (11). It
    follows that the 35% increase in CO2 levels from 280
    to 382 ppm over the past 250 years may have increased the
    global heat balance by perhaps 1%. This 1% change in the
    global heat balance has been assumed in conventional
    climate models to be the cause of global warming."

    Regardless of the merits of his proposals for mitigation, can such a percentage attribution even be made (95% / 4% of earth's heat dynamics governed by H20 and CO2 respectively)?  After doing a lot of reading here and going through the marvellous geophysics course on cc by Bob Trenwith (U of Chicago) <incorrectly attributed name removed> I'm having my doubts but it would be great if someone more 'steeped' in this subject matter could take a look. THX


    [DB] Fixed link

    [PS] Removed incorrect attribution as request by uploader.

  7. indagar @331,

    Jehne's paper (your link to this paper has picked up an extra SkS URL) isn't at all clear what it means by "net heat balance" and that this is the same as "total heat balance" suggests a poor piece of writing. The references Jehne cites are rather long and old (10) = Budyco (1958) 'The Heat Balance of the Earth;s Surface'. (11) = Schneider (1989) 'Global Warming: Are We Entering the Greenhouse Century?'. Some concept "heat balance" that would have the 18% of it comprising the GH-effect doesn't leap out at me from either.

    The numbers for CO2's contribution to the GHG (and also the water vapour contribution) is roughly correct but in CO2 contributing some 20% to the ~33ºC GH-effect. By such a count, AGW from CO2 rising from 280ppm to 382ppm would provide 1.3% to the GH-effect through direct CO2 forcing and 4% with feedbacks. Jehne is back-of-fag-packet calculating the 280-382ppm rise as 35% of CO2's pre-industrial GHG contribution with 20% of all the pre-industrial GH-effect which would yield 7%, a value that is a ong way high.

    It would be a puzzle for somebody with some time to spare to sort what Jehne is actually on about.

  8. Sounds like another "gone emeritus" conservative. His "it follows" is patently false - it treats CO2 and water vapour as independent variables.

  9. The reason I choose to remain an AGW skeptic at this time is because a series of unscientific but still logical and true facts bring me there by deduction (which is all that readings and models happen to do also).

    1. Some warming is preferable to some cooling (humans are warm blooded and do not do well without insulation against the cold (0 deg.C). When I physically begin to feel uncomfortable from the relentless heat, I may then prefer a cooling trend.

    2. Positive and negative feedbacks associated with increased water vapor and cloud formation can cancel one another out and complicate matters. The actual balance between them is an active area of climate science research and therefore the science is actually not yet settled. Both oceans and atmosphere are fluid, dynamic and vast and average measurements can only indicate trends.

    3. As long as terrestrial and deep ocean volcanoes exist, and as long as I do not have difficulty breathing (O2 supply) then I am not going to worry about how much "heat" biological life (see above) contributes to global warming.

    4. I remain open to new findings and information and accept that a GW trend is currently occurring. 

  10. 1/ You choose your beliefs around your personal preferences? Hardly logical. A logical basis for believe is where the actual evidence points you.

    Again, it is not a question of what the temperature actually is, but how fast that it is changing that is the main problems. Rapid change threatens infrastructure and agriculture.

    2/"can cancel one another out". I am not aware of any evidence supporting that. Where did you get that from? Cloud feedback is very complicated (is it net positive or negative?) but able to cancel out water vapour feedback? Again, all the actual evidence whether from paleoclimate, models, TCS estimates etc. puts sensitivity in range 1.5 - 4.5 with likely value of 2.8-3.0. You appear to be accepting some hand-wavy arguments in favour of what you would like to believe rather then any actual evidence.

    "Both oceans and atmosphere are fluid, dynamic and vast and average measurements can only indicate trends." Not sure what your point is here? The error range associated with measurement of both ocean heat and atmosphere are well documented and I cannot see how they would support your argument.

    3. Well that is logical, because biological life contributes next to zero to global warming. Its burning fossil fuel that does the damage.

    4. Good, but actually understanding the existing findings and information would be good idea. You seem a little prone to ignoring observations you dont like.


    I am still keen to see a response to reply on other thread. Particularly your source for laws pushing people off land on basis projected sealevel rise.

  11. Just to further at your point 1. You seem to be stating it is reasonable to be AGW skeptic because prefer warming. In places where death toll in 1000s from heatwaves, then they would rather a cooling trend. Is it logical to for each person to determine the truth of what is actually happening on basis of their preference. Or is it, "I all right Jack" ergo AGW is alarmist conspiracy?

  12. bArt @334,

    While I agree with the comments @335&336, I would say that the meaning of much of what you write is not at all clear. So let me present what I interpret you as saying along with my own understanding of its context.

    You accept the world is warming and are open to "new findings and information" (4), but this is an exceedingly low base from which to establish the reality of AGW.

    You don't give a hoot about humanity (1) or other biological life (3) as long as you are not too hot and have oxygen to breath. Interestingly Arrhenius thought that a little more heat would be good for the world, he living in Sweden which is a tad cold come the winter. There was even discussion of setting fire to coal mines so AGW could be created without having to mine the stuff before you burn it. If Arrhenius had lived in the tropics (as do 40% of humanity) or a less Euro-centric world, he would surely have thought differently.

    Your need for volcanoes (3) remains a mystery.

    The failure of science to nail down ECS more exactly cannot really be seen as a reason to ignore the serious nature of AGW. Identifying the upper limits of ECS is always going to be difficult as a high ECS is only different from a medium ECS after 100 years or so. The work of folk trying their hardest to demonstrate tiny values for ECS (or TCR) and thus to diminish AGW, such work doesn't really hold water outside the narrow constructs they set it out within. So yes, in a narrow sense "the science is actually not yet settled" but the bit of science you rest your faith in (2) is narrower than narrow and those wholly engaged in that sliver of science are simply refusing to leave the last-chance-saloon at closing time.

    The relevance of your final sentence in (2) is not evident.

  13. bArt - your point 4/ raises interesting question. How do you really go about about evaluating an issue. There is a lot about critical thinking versus motivated reasoning here.

    One good starting point is to decide what evidence would make you change your mind (and please dont insist on nature doing something that climate science says cant happen like monotonic temperature rise).

    You might ask, what would it take for me to decide AGW is wrong and I think this post outlines at the bottom what discoveries would certainly cause me to change my mind.

  14. @bArt 334.  "The reason I choose to remain an AGW skeptic at this time is because a series of unscientific but still logical and true facts bring me there by deduction (which is all that readings and models happen to do also)."

    I disagree, unscientific is not logical.  There is a reason more than 97% of mainstream science says AGW is not only real but a serious problem.  And there is absolutely no doubt that the increased CO2 and warming is anthropogenic.  Its plain straight forward chemistry shown through carbon isotopes.  No ifs, ands, or buts.

  15. I can't add scientific value to this debate though I am a voracious reader on the subject. Just want to say how refreshing it is to see Chris & Mizimi having a scientific debate without the usual vitriolic climate denier criticism.

    Have to say I feel Mizimi has a slight edge & remain convinced that the sun is the primary driver of climate change.


    [DB] The incoming energy from the sun has been well-documented and studied, for literally generations of scientists.  That the sun plays almost no role in the warming of the surface of the Earth over the past 50+ years is also well-researched, well-documented and uncontentious in any scientific sense.

    Per NASA, it's not the sun



    From the 4th National Climate Assessment, Volume 2, released by the Trump Administration:

    "Scientists have understood the fundamental physics of climate change for almost 200 years. In the 1850s, researchers demonstrated that carbon dioxide and other naturally occurring greenhouse gases in the atmosphere prevent some of the heat radiating from Earth’s surface from escaping to space: this is known as the greenhouse effect.

    This natural greenhouse effect warms the planet’s surface about 60°F above what it would be otherwise, creating a habitat suitable for life. Since the late 19th century, however, humans have released an increasing amount of greenhouse gases into the atmosphere through burning fossil fuels and, to a lesser extent, deforestation and land-use change. As a result, the atmospheric concentration of carbon dioxide, the largest contributor to human-caused warming, has increased by about 40% over the industrial era.

    This change has intensified the natural greenhouse effect, driving an increase in global surface temperatures and other widespread changes in Earth’s climate that are unprecedented in the history of modern civilization.

    Global climate is also influenced by natural factors that determine how much of the sun’s energy enters and leaves Earth’s atmosphere and by natural climate cycles that affect temperatures and weather patterns in the short term, especially regionally.

    However, the unambiguous long-term warming trend in global average temperature over the last century cannot be explained by natural factors alone.

    Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence.

    Without human activities, the influence of natural factors alone [which includes the sun] would actually have had a slight cooling effect on global climate over the last 50 years."


  16. I'll also add that even if water vapour wasn't a positive feedback loop, it doesn't matter if water vapour keeps in a huge amount of heat compared to CO2, That doesn't somehow make the CO2 induced warming insignificant. Such a claim is trying to trick you by warping your perspective. 

  17. Which also addresses Mizimi's comment about how animals make a bigger difference in terms of heat.  I havent confirmed anything on that study, but methane doesn't last in the air for the same amount of time CO2 does, it's significantly shorter.  Thus it is already in a stable equilibrium.  As more methane is put In, the old methane is coming out of the system at the same time.  Its not the same for CO2 as we haven't been putting it in the system at a stable rate for long enough that the oldest will start coming out of the system. 

  18. And to whoever asked, a positive feedback loop isn't necessarily runaway.  It can give diminishing returns, due to many other factors at play.  Which would be why we don't see runaway heating from water vapour

  19. I am working on a paper that includes depictions of water vapor's role in our current dire situation, I am experiencing research fatigue and would appreciate constructive comments. In the following excerpt I am attempting to provide clear support to well established facts, but I am encountering source variability. Here goes:

    Water vapor is not considered to be a primary forcer in that it does not initiate global thermal loading, even though its presence in the atmosphere has the largest impact.
    This may not seem to fully hold when some details are considered. When any hydrocarbon is burnt, water vapor is always a component of the exhaust. This is particularly significant in regard to commercial air travel. High altitude jet exhaust includes both water vapor and aerosols which are perfect in the formation of cirrus clouds in the cold upper troposphere along contrails. This can be augmented by additional water vapor present in a supersaturated state, which seems to be now more and more the case. Such clouds impart a major greenhouse effect. Also, methane emitted there has an easy way into the stratosphere wherein it is oxidized into twice as many water molecules as that of CO2 by the abundant hydroxyl radicals there present. Water vapor in stratosphere has a greatly amplified greenhouse effect. Otherwise, not much water vapor makes it there.

    From the above one could argue that it is a prime forcer. However, apart from the effects of air travel, tropospheric effects are mostly short lived. Since its mean residency period is not much more than a week being largely controlled by condensation at tropospheric dew point encounters, it cannot become well-mixed or be independently sustainable. If forcers suddenly decline, it cannot persist or continue to promote other feedbacks. Furthermore, if other forcers dip below baseline values subsequent declines in water vapor will produce a proportional negative feedback. It is powerful, but passive, sort of like when control levers on earth moving equipment are moved by the operator and the hydraulic system performs monumental tasks.
    Absolute and relative humidity is highly variable from about 0.01 to 3.0 % typically and to about 4.0 % more exceptionally. However, most of the Earth’s surface is wet and able to produce a pronounced feedback. Also, with elevating condensation threshold zones that are now being seen to develop, the residency time will increase as well the total volume. This could increase its temperature response sensitivity. Certainly, in its reliable and large feedback response to all other longer term forcing factors one could consider it to be a co-forcer.

    At current climate sensitivity estimates, a doubling of CO2 will add one degree Celcius to the global mean temperature in itself and water vapors total feedback effect, accounting for all iterations of self-looping, will add another 1.7 degrees. Fortunately, it is apparent that the initial feedback is well below unity and self-limiting at about 0.6. If this sensitivity value reaches 0.7, which is at the threshold of becoming exponential, conceivably, by itself, it could go runaway. We don’t need this as CO2 is already sprinting. Apart from possible PTE or early Venusian extremities, it seems that this has not previously happened. Furthermore, Earth’s persistent resiliency while maintaining abundant free water, logically, precludes it.


    [JH] Put draft paragraphs in block quote for ease of understanding. Please learn how to do this yourself using the edit tools.

  20. 3-d Construct @344 , 

    I apologize for my inexpert comments : but I gather that you are in that state of mind (analogous to writer's block) where useful thoughts may be triggered by re-encountering stuff you've already been acquainted with . . . or even by comments which are vapid & klutzy.

    So :-

    *  Quantification.  Exactly how important is H2O in the stratosphere?  In absolute terms or relative terms.  Lower or upper stratosphere.  Re-visit lapse rates and TOA concepts?

    *  Quantification of "supersaturated" water vapor at high altitude.  Important, or too transient?

    *  There are 350 million square Km of ocean surface interacting with a thin rind [a few dozen Km] of planetary atmosphere.  Is the high-altitude human contribution of water vapor genuinely significant?

    *  In comparison, there were subtle but measurable alterations in regional heat flux for the [ocean-free] expanse of the continental USA during the 3-day "shutdown" of jet flights immediately following the 911 terrorist event.   IIRC, the lack of contrails did have the expected effect : cooler nights, warmer days.   But of course the other 98% of the planet surface has more ocean and/or less air traffic.

    *  I gather Hansen has rather "walked back" his earlier comments about the dangers of runaway warming . . . and, as you say, the paleo evidence points to "moderate stability/resiliency" of global surface temperature.

  21. Hi,

    I'm dealing with a very disengenious climate denier who tries to fool everyone that he's a self made scientist. *rolled eyes!

    What denier misinformation are they playing when they post things liek this?  I think they don't fully understand the graph they posted.

    CO2 absorbs at 2.7 microns, 4.3 microns and 15 microns.

    Since Earth does not emit Black Body Radiation at 2.7 microns, we only have to look at 4.3 microns and 15 microns, and we'll apply Wien's Law to both.

    Wien's Law T (Temperature) = b / wavelength in micrometers, where "b" is a constant equal to 2,900 um-K.

    T = 2,900 um-K / 15 um = 193°K = -112°F

    T = 2,900 um-K / 4.3 um = 673.9°K = 753°F

    What we can infer from science is that 4.3 microns has far greater energy than 15 microns, however the amount of Black Body Radiation Earth emits at 4.3 microns is minuscule, as this link proves:

    Water vapor is far more significant.

    Water vapor is the primary absorber of incoming radiation and the largest and most significant reflector of out-going radiation.

    Water vapor typically averages 13 TRILLION tons and by weight is far greater than CO2: 0.33% H2O vs 0.04% CO2.

    In terms of relative humidity, Earth is about 75% at ground level, decreasing to 45% at about 5,000 meters. That means water vapor is concentrated near the Earth, unlike CO2.

    Water vapor absorbs at 5.9, 6.5, 6.9, 7.2, 7.6, 8.2 and 9.6 microns.

    Wien's Law:

    T = 2,900 um-K / 5.9 um = 491°K = 424°F
    T = 2,900 um-K / 6.5 um = 446°K = 343°F
    T = 2,900 um-K / 6.9 um = 420°K = 296°F
    T = 2,900 um-K / 7.2 um = 402°K = 263°F
    T = 2,900 um-K / 7.6 um = 381°K = 226°F
    T = 2,900 um-K / 8.2 um = 353°K = 175°F
    T = 2,900 um-K / 9.6 um = 302°K = 83°F

    As you can see from the graph and from Wien's Law, water vapor is far more powerful than CO2 could ever hope to be and generates far more energy than CO2 ever will.

    Water vapor is the driver of climate, not CO2.


    What the heck does Wien's Law have to do with it?

  22. Ops sorry for my typos above.

    This is the website the denier took the graph from.

    Solar Radiation and the Earth's Energy Balance.

  23. TVC15 @347,

    The (Fig 10) graph is non-controversial & quite well travelled.

    TVC15 @346,

    Indeed!! Wien's displacement Law is a new one on me. And it is totally irrelevant. Your "very disengenious climate denier" is speaking through a wrong orifice and note he totally fails to quantify the "far more powerful" nature of H2O. He goes not further than effectively say 'See!! Lots of numbers!!!!'

    The three primary absorption bands of CO2 do lie at 2.7, 4.3 & 15 microns. The 2.7 micron band features in the tail end of the solar radiation part of the spectrum while the 4.3 micron one sits between the incoming and outgoing part of the spectrum. Our friend ignores the compound absorption bands at 10 microns which is today quite insignificant but would begin to significantly add AGW above 3,000ppm.

    Different bands can have a more powerful absorption than others. So the 4.3 micron CO2 band is stronger than the 15 micron one, but of course it requires radiation to operate and there is effectively no radiation at 4.3 microns.

    The height of the GHG in the atmosphere is very relevant. If H2O were not "concentrated near the Earth, unlike CO2", its GHG effect would be far srtonger - the "very disengenious climate denier" gets this arse-about-face.

    The "very disengenious climate denier" is however correct in saying that H2O provides a far greater amount of GHG-effect that CO2. Without CO2, if H2O levels were maintained somehow, the GHG-effect would be 80% or so of its present strength. But without the CO2, that 80% cannot be maintained as in a cooler atmosphere the H2O levels are lower, and lower and lower as it cools until effectively the GHG-effect disappears.

    Thus it is plain. With no CO2, there is no GHG-effect on planet Earth. So saying "water vapor is the driver of climate, not CO2" is another arse-about-face assertion from your "very disengenious climate denier."

  24. If water vapor is the driver of climate, perhaps we should stop combining fossil hydrogen with atmospheric oxygen and inject more of it in the atmosphere then. Unless one accepts that it is not a forcing and subscribes to the standard model of Earth climate. Oh well, it's not like this has not been extensively studied by people who actually know what they're doing.

  25. Philippe Chantreau "349,

    That is an interesting thought. A rough back-of-fag-packet calculation (using Global Carbon project 2018 figures & H2O/CO2 = 2 for natural gas and 0.8 for oil) puts the count of fossil emissions of these "far more powerful than CO2 could ever hope to be" H2O molecules as 75% of the allegedly 'ever-hopless' CO2 molecules. In weight works out as 11 Gt(H2O) per year.

    Of course, there are about ten times more H2O molecules in the atmosphere than there are CO2 molecules so molecule-for-molecule H2O is not so "powerful". And they get rained out in days so the extra fossil-sourced H2O is irrelevant, except all that extra water has to go somewhere. So the back-of-fag-packet answer is that burning fossil hydrogen is adding water onto the surface at a rate that's enough to raise sea levels by 3mm in a century. But even in this, H2O is weaker than CO2-powered AGW as the extra absolute humidity in the atmosphere  due to today's AGW is enough to reduce sea levels by about 5mm in a century.

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