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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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Further viewing

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Denial101x video(s)

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Expert interview with Steve Sherwood


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Comments 226 to 250 out of 392:

  1. Water vapour and CO2 at high altitude.

    The concentration of water vapour, a powerful greenhouse gas, at the earth's surface is usually tens of thousands of parts per million, which completely dwarfs the CO2's 400, let alone the changes in the concentration of the order of 100 parts per million. So say the skeptics.

    But this isn't the case at high altitudes. At 40,000 feet, at the top of the troposphere, the temperature is around -50C, and so the water vapour is frozen out, with a vapour pressure of about 4Pa, (1Pa at -60C). Atmospheric pressure at sea-level is about 100,000 Pa, and about 20,000Pa at 40000 feet. If I assume that the concentration of CO2 remains about the same, with a bit of turbulence stopping the heavy CO2 molecules settling out to lower altitudes, then the pressure of CO2 would be about 8Pa=20,000x(400/1,000,000)Pa, which is twice the pressure of the water vapour. So maybe an increase in the CO2 concentration does matter. We shall all get warmer, regardless of the height at which the heat flow into space is restricted.

  2. oldmanthames @ 226. Yes, you've nailed the underlying misunderstanding about that. The unlearned assume that it's about radiation from surface either getting to space or not, helped by the infotainment cartoons. But of course, it isn't and the energy shimmer is actually throughout the atmosphere since GHGs (inc. H2O) exist. I've heard it noted by a scientist that the *average*  transmission point to space must be at an average temperature of 255K and that is not the surface, it's at ~4.7 km altitude and most water vapour is below this point so it obviously cannot affect radiation from here heading off to space. I've been posting this table I made as a YouTube comment for 12months. I've been hoping to occasionally trigger some corrections because the source data was thin and fattened out by my estimates. But of course it's YouTube so I've never received a single worthwhile response.

    Water vapour *was* the most significant greenhouse gas when it was released into the atmosphere but now the effect is almost 100%

    saturated except for a 10% frequency band that gets from surface to space. It's been incapable of being the major driver of temperature

    change for billions of years because its freezing point is too high. The +CO2 "global warming" is happening at 6km-16km altitude and there's

    37.6x as much CO2 as H2O at 15km altitude. That's why climate scientists keep calling CH4, CO2, N2O, CFCs "well mixed", it means they go

    high without freezing out. Water doesn't. Like:----------
    alti- air air
    tude tempe- density --— atmospheric ppmv --
    km rature g/m3 CO2 water vapour
    0 16 1,290 400 14,000
    0.5 12 1,235 399 11,000 **84% already caught and shimmering around**
    1 9 1,180 398 8,500
    1.5 5 1,130 398 6,400
    2 2 1,075 397 4,900
    3 -6 965 395 2,900
    4 -13 860 394 1,700
    4.7 -18 783 392 1,200 This is the average point from where Earth's radiation is
    sent to space (temperature of 255K).
    More CO2 then water vapour above to intercept radiation.
    5 -20 750 392 1,000
    6 -27 680 390 600
    ** zone above approx. this height has more CO2 than H2O **
    ** zone above approx. this height is not "saturated" with GHGs **
    ** zone above approx. this height produces most +CO2 & +CH4 warming **
    7 -34 610 389 350
    8 -42 540 387 200 Contrails typically higher than this
    9 -49 470 386 120
    10 -56 420 384 70
    11 -56 370 384 40
    12 -56 320 381 25
    15 -56 200 376 10 ** 37.6x as much CO2 as H2O **
    ** not much +CO2 & +CH4 warming above approx. this height, air too thin **
    20 -56 90 368 8
    25 -52 40 360 8
    30 -47 20 352 8
    40 -25 5 344 8
    50 -3 1 336 7
    60 -18 0.39 328 7
    70 -50 0.125 320 5.5 to 6.5
    80 -83 0.027 312 2.5 avge (2 to 4.5)


  3. Water vapor has been capturing IR since the planet acquired water.  If as AR5 calculates there is an additional capture of IR by CO2 of 1.7 watts/m2 this represents ~1% additional forcing increase.  Through the mechanisms of IR capture, convection, latent heat release upon condensation, WV transfers heat to the upper troposphere where WV radiats heat to space.

    Note that the earth is radiating IR to space to balance the solar energy absorbed from the sun to stabilize the earth temperature.  WV does the major share of this job.  We know this since there is no other significant physical material and physics available to do this heat transfer to space.  As such WV responds to any forcing by increasing this heat transfer to space.  Responding to heating by cooling is the definition of Negative feedback. Thus the 'science is settled'. Without significant Positive feedback of some sort, there is not enough CO2 greenhouse heating to be of consequence.

    Either the '97%' of the science community is too stupid to understand this simple logic or else they are willing to sell their reputations for their share of the $2.6 billion research funding from the govetrnment.  Since I refuse to participate in adhominin attacks, I must assume that answer lies with the former assertion.



    The hubris involved in assuming physicists are wrong when in fact you have misunderstood the physics is frankly astonishing.

    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, off-topic posts or intentionally misleading comments and graphics 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.


  4. shorter arationofreason @228, "I have had this random thought bubble.  This random thought bubble disagrees with the conclusions of 1000s of scientists who have devoted tens of thousands of person hours to the problem.  Therefore the 1000s of scientists must be wrong, and so wrong that only utter stupidity or corruption could explain the error."

    The breath taking arrogance displayed demonstrates that 'arationofreason' is on a very short ration of reason indeed.

    Among the facts that arationofreasonneglects in his thought bubble are that:

    1)  The CO2 contribution to the total greenhouse effect is about 20%, not 1%;

    2)  The 1.7 W/m^2 is the change in the CO2 contribution since the preindustrial, not the total contribution;

    3)  Because CO2 does not precipitate out with reduced temperature, while water does, the upper troposphere has very much less H2O than does the surface, but very close to the same amount of CO2, resulting in CO2 dominating IR absorption and reemission in the upper troposphere;

    4)  Because H2O precipitates out with reduced temperature, eliminating all CO2 (and hence 20% of the total greenhouse effect) would result in the precipitation of a great deal of the H2O in the atmosphere, reducing the IR impact of the remaining H2O.  That in turn would precipitate out yet more H2O, and so on.  The net effect is that in the absence of all long lived greenhouse gases, and assuming there had been no increase in the extent of ice sheets and/or deserts (ie, that albedo only changed due to reduced cloud cover, and increase in snowfields), only 10% of the current H2O would remain in the atmosphere, and temperatures would plumet by around 35 degrees celcius.

    You don't need to be stupid or corrupt to not realize the above.  All you need to do is to mistake abysmal ignorance for wisdom.


    [PS] You know better but thanks for tackling the substance.

  5. richpender elsewhere suggests that there is 80 times by volume more H2O and CO2 in the atmosphere.  

    According to Trenberth and Smith (2005) the total dry mass of the atmosphere is 5.1352 +/-0.0003 x 10^18 kg, while the mean mass of H2O in the atmosphere is 1.27 x 10^16 kg.  According to the CDIAC FAQ on Climate Change, each ppmv of CO2 in the atmosphere has a mass of 2.13 Gigatonnes.  The most recent IPCC report (AR5) has corrected that to a value of 2.12 Gigatonnes, or 2.12 x 10^12 Kgs.  That means with 400 ppmv of CO2, there are approximately 848 x 10^12 Kgs CO2 in the atmosphere.  In other words, by mass, there is approximately 1.5 times as much H2O in the atmosphere as there is water.  CO2 is heavier than H2O, however.

    To run the comparison of the ratio by volume, however, we need to convert from Mass to molar units.  Apparently water vapour has a molar mass of 18.02 Kg per kilomole, while CO2 has a molar mass of 44.01 Kg per kilomole.  It follows that there are 705.8 x 10^15 moles of H2O in the atmosphere; but only 19.3 x 10^15 moles of CO2 at 400 ppmv.  The ratio of H2O to CO2 by volume is therefore 36.6, just under half of richpender's estimate.

    I am not sure why richpender got the estimate wrong.  Taking a median value on surface humidity, the ratio is approximately 53 moles of H2O per mole of CO2 - a value which does not include clouds (unlike Trenberth's figure).  Perhaps richpender has used an area weighted mean of humidity, which would give a larger ration at the surface.  Crucially to the strength of the greenhouse effect, what such comparisons nearly always get wrong is using only surface values.  Because the volume of H2O in the atmosphere is closely tied to temperature, the H2O concentration falls rapidly with altitude, as seen in this graph by Science of Doom:

    (Note that the graph only shows about 320 ppmv for CO2, not the current 400 ppmv).

    Because of the rapid decline of H2O concentration with altitude, in the upper troposphere where the greenhouse effect has its major impact, CO2 has a far greater impact than would be expected from surface calculations alone.  Indeed, CO2 contributes 20% of the total greenhouse effect, compared to 50% by water vapour and 25% by clouds.  The ratio of the instantanous impact of water vapour (plus clouds) to CO2 for the total greenhouse effect is not 80 to 1, not 36.6 to 1, but 3.75 to 1.

    That is the instantanious impact only.  Because the concentration of H2O in the atmosphere is highly temperature dependent, if the CO2 were eliminated from the atmosphere, the amount of H2O in the atmosphere would fall drastically due to the reduced temperature.  That in turn would lead to a further reduction in atmospheric H2O, and a further fall in temperature.  The end of the process would leave very little H2O in the atmosphere, and a total greenhouse effect a very small fraction of current values.

    Conversely, increasing CO2 increases the amount of H2O in the atmosphere, amplifying the impact of that increase in CO2.  This is called the water vapour feedback.

  6. Water Vapour melts ice... it's time to talk hysteresis as some say it takes 40 years for carbon emissions to reveal themelves as the true problem they are hypothesised to potentially be!

  7. In my view, water vapor is effectively anti-greenhouse. Here is how. In 10k-40k feet height, water vapor in the atmosphere gets converted into tiny ice crystals. These water crystals floating in upper atmosphere reflect the sun rays away from the earth (back to the empty space) in the same way that cirrus clouds do. If the skies were clear, the sun rays in pristine form, falls on the ground and ocean, creating the appropriate differential of pressure and temperature to create conducive condition for precipitation (rain). Absence of a strong sunlight is what preventing timely rains. And this is what probably damaging the world weather systems.

    The biggest question is how can water vapor go to upto 40k feet altitude? The culprit is — aviation exhaust (i.e. exhaust made by jet airplanes). The exhaust mainly contain CO2 + water vapor. We must cut down on water vapor emitted by air traffic if we want to save the planet's weather system.

    Further reading:



    [PS] Fixed links: This does not really belong here. Replies to this thread please where the Burkhardt and Karcher paper is already discussed in comments. 

  8. I haven't posted here for years but I re-iterate that dry gases in the atmosphere do not radiate significant energy - where can it come from?  Only when aggregated in their billions do molecules gather sufficient energy to radiate. Even then, the process takes place at the discontinuity between a body - solid or liquid - in a depth of a handful of molecules. No radiation penetrates far into the solid earth's surface and likewise any that emerges comes from a thin surface skin. I'm sorry, the accepted science of climate regulatio in my days has been forgotten by a generation lost in inappropriate minutiae. It is the dynamic operation of cloud cover which regulates energy getting into earth's envelope and it is urbanisation and drainage which is restricting water vapour - and hence cloud cover - so giving rise to climate change.  It's  not difficult.

  9. Imagine a body radiating steadily in equilibrium with its energy source. Now throw an enclosure round it which reflects every other photon. For simplicity assume photons leave regularly of equal energy. Immediately after the first photon has been returned, the surface generator detects a rate of arrival of energy twice what it expected. The next photon out will therefore leave in half the time and so on. the balance will be maintained as before and no temperature difference arises. If you connect an identical body without  the enclosure, no energy transfer would occur. This is why GHG's do not raise earth energy, they are inert, undergo no changes and have to defy the Law of Thermodynamics to achieve it.


    [PS] Creating a caricature of GHG theory and then criticizing it is hardly a valid form of argument.

  10. old sage:

    If you want to argue "the greenhouse effect violates the second law of thermodyamics", then the proper place to do that is over on this thread. The last comment there was made in late 2014 (#1481), but you can read through the Basic and Intermediate tabs, then the comments, and then see if you have anything to say that hasn't already been debunked.

  11. old sage's post @233 is essentially a rehash with slightly different (and very confused terminology) of his post @222 and is more than sufficiently rebutted @225.  In particular, his claim that "dry gases in the atmosphere do not radiate significant energy" is abundantly refuted by the graph @225 showing (at one location) the downward radiation from those dry gases which he claims to be insignificant.  His rebuttal of the abundant observational evidence refuting his claim consists entirely of the rhetorical question, "where can it [the energy] come from?"

    That would be from the energy transported to the atmosphere by shortwave radiation (79 W/m^2), evaporation (84 W/m^2), sensible heat (20 W/m^2) and, most importantly, IR radiation (approximately 378 W/m^2).  (378 rather than 398 because of the approximately 20 W/m^2 atmoshperic window)

    Note: 378 W/m^2 rather than 398 W/m^2 because of the approximately 20 W/m^2 atmoshperic window.  Adding the sums will show the total to balance the 219 W/m^2 upward radiation from the atmosphere (ie, 239 minus the atmospheric window) plus the 342 W/m^2 downward IR radiation.  Because the energy radiated from the atmosphere is continuously resupplied, the thermal capacity of the atmosphere is largely irrelevant except as it effects diurnal and seasonal temperature ranges.

    old sage's rhetorical question only has bite of you maintain a scrupulous ignorance about what observations show about energy flows into and out of the atmosphere.

  12. old sage @234 presents an incoherent, simplified model that he claims refutes the greenhouse effect.  While it could be dealt with as a 2nd law of thermodynamics argument, it is simpler to simply note that:

    1)  Contrary to old sages claim, the immediate response to placing his half mirrored shell is to increasing incoming photons by 50% (not twice as he claims).  The number of photons increases to double only once equilibrium is achieved and nothing in his model indicates that equilibrium will be achieved instantaneiously.  He merely assumes it.

    2)  Unless and untill he refutes the very well established (for over a century) Stefan Boltzmann Law that equates the radiation from a body with its temperature, "the balance will be maintained as before and no temperature difference arises" is simply false.  Indeed, given the Stefan Boltzmann law and related results it cannot be the case.

    3)  Between the time of the placement of the half mirrored shell and temperature rising sufficiently to accomplish equilibrium, there is a positive energy imbalance at the surface in his model.  That positive energy balance provides the additional energy needed to raise the surface temperature.  Once equilibrium is achieved, there is no further change in temperature as there is no energy imbalance to drive the change.

    So, all that is necessary for old sage's model to justify his conclusions from it is that a physical law that is both confirmed by literally hundreds of thousands of laboratory and field observations, and derivable from quantum mechanics be false (and hence that quantum mechanics also be false), and that equilibrium be achieved instanteiously given any perturbation.  Given that, old sage's conclusions from his model are pushing well into the anti-science level achieved by geocentrists and flat earthers.

  13. So much gobbledegook here its difficult to know where to start. Atmospheric gases are governed by kinetic theory, they engage in inelastic collisions and conserve energy and momentum, no surplus energy is generated to form photons. Stefan Boltzmann BB radiation applies only to a special situation  in which a continuum of energy levels are provided by the body in question. Any other body, earth included, will discharge its energy with a modified spectrum and gases, which obey quantum mechanics and are below the threshold for excitation do not radiate except with a very low probability.

    I should not need to explain how the condensate of h2o, o2 ,co2, n2 etc into raindrops scavenges energy from  the atmosphere and plays the critical role it does in getting energy away but if you critics care to think about it, you can work it out for yourselves.

  14. old sage.

    Lots of gobbledegook in your comment. So lets look at some basics.

    Molecules in a gas can contain energy in multiple forms. They have kinetic energy of linear translation - they are zipping along. They have kinetic energy of rotation, they are rotating and if their moment of inertia is non-negligible then there is energy in that rotation. The individual atoms within the molecule are in motion relative to the whole. The intra-molecular bonds are non-rigid so the individual atoms are jiggling around relative to the centre of the molecule. As in any spring-like system their is kinetic energy and potential energy. Then, within individual atoms, their electrons can be in higher or lower energy states.

    Then there is potential energy due to the separation of the molecules. Inter-molecular forces such as Van Der Waals forces tend to attract molecules together so that when they are separated there is potential energy due to that separation just like in a gravitational field. And as they draw closer to each other potential energy is converted to kinetic energy of translation.

    So collections of molecules have energy in multiple forms, held in different parts of the system. A molecule in a gas can move in three axes - it has 3 degrees of freedom. Similarly it can rotate around 3 axes, and if the Moment of Inertia around each axis is non-trivial, there are up to 3 more degrees of freedom. There are from zero (for a single atom) to 1 (for a diatomic molecule that can only experience stretching) to many multiple degrees of freedom associated with poly-atomic molecules that can undergo mutiple different forms of stretching and bending of the intra-molecular bonds. And the different electron energy levels for individual atoms within the molecule constitute additional degrees of freedom.

    A basic principle in Physics is the Equi-Partition Theorem. In a collection of molecules the total available energy will tend to be equally distributed between each degree of freedom. And the Thermodynamic (or kinetic) definition of temperature says that temperature is proportional to the energy in the translational only degrees of freedom. So a gas at temperature X has a certain amount of its energy in translational movement, and the rest in other modes of action. This is the basis for the calculation (and observation) of the specific heats of different substances. Not all added energy goes into something that impacts temperature - linear translation.

    An important wrinkle in all of this is quantum mechanics. Translational movements aren't quantised whereas rotational, vibrational and electron energy levels are. This constrains the possible energy changes that can occur, and as an aside, modifies the specific heat of substances. And changes to the number of degrees of freedom available to a substance in its solid, liquid and gas forms, along with potential energy changes as molecules move apart during vaporisation is a big factor in why the enthaplies of Fusion and Vaporisation for a substance can be large compared to their specific heat.

    So when molecules collide, it is an extremely complex interaction. They accelerate towards each other as the collision approaches, drawn together by Van der Waals forces, converting potential energy to kinetic energy as they approach. After the collision, the reverse happens and they experience a deceleration as they move apart, losing kinetic energy to potential energy in the process.

    During the collision, the forces between molecules and atoms come into play creating a collision with a defined, non-zero duration, and complex interplay between all the degrees of freedom of the molecules. The conservation laws of Energy, Momentum and Angular Momentum apply but many complex interactions are possible. For example, as the atoms are vibrating, jiggling, they are momentarily changing the Moments of Inertia of the molecule. To conserve Angular Momentum, the angular velocity needs to change.

    All these complex interactions allow energy transfer between the many degres of freedom, within the constraints imposed by quantum mechanics. So all this complexity is driving the Equi-partition theorem, tending to ensure that all the different modes of energy storage, across all available molecules, tend to have the same share of the total energy, within the constraints imposed by quantum mechanics.

    So how does a gas absorb and emit photons? One potential way is electron energy level transitions within individual atoms. This is the basis of the classic Lyman and Balmer series for hydrogen for example. But the energy levels needed for such transitions are typically beyond the energy content of IR photons. Most electron energy level transitions need quanta of energy associated with visible light and above. Not Infrared.

    How else can photons be absorbed or emitted? Basic theory, from Maxwells Eqns, says that an electric charge that is accelerating can potentially create a photon. Similarly, a passing photon can excite movement in an electric charge. This is the basis of transmitting and receiving antennae.

    But a molecule is electrically neutral, it doesn't expose its internal electric charges, from electrons and positrons, to the outside world does it? Yes.

    This depends on the 'electro-negativity' of the individual atoms within a molecule. Electro-negativity for an atom essentially describes how strongly that atoms tends to hold electrons to it. Oxygen is strongly electo-negative for example, while Hydrogen is weak. So in a molecule with atoms having differing electro-negativities, where the atoms are arranged assymetrically, this can produce a 'charge-separation'; electrons tending to cluster on one side of the molecule. Water is the classic example of this with the Oxygen side of the molecule tending to be negatively charged, and the Hydrogen side positive.

    So now the molecule is 'displaying' an electric charge to the outside world. Potentially able to interact with a passing photon or to emit one. This is the property that allows molecules to absorb or emit IR photons. It requires an exposed electric charge and acceleration. Linear motion of molecules can't supply this - apart from the briefest moments during a collision, they aren't accelerating. But vibration and rotation can. Both involve accelerations, and if a molecule has a charge separation then absorption and emission events are possible.

    So some gas molecules can absorb and emit infrared photons because they can generate a charge separation and they are assymetrical. And when they tend to gain or lose energy from their rotational and vibrational degrees of freedom due to absorption or emission events, the intricate interplay of collisions, billions of times per second re-establishes the balance.

    Long story short. Gases can and do absorb and emit IR photons due to the presence of charge separations across the molecules.

  15. Mr Tamblyn, please spare me a graduate level explanation of the various form of energy which can exist and put some effort into quantifying them.  Spontaneous generation of energy from gases at atmospheric temperature and pressures - is negligeable, look up the numbers yourself in any decent text book. If you want CO2 to radiate, get it to low pressure in a vacuum tube and put a high voltage across it and it will be accelerated sufficiently to generate, it will produce hardly anything left to itself.

    According to wikipedia, the average solar energy 24 hrs/day per unit area is 141 w/m2, the energy needed to get the world average rainfall -99cms- to vapour, at the surface, is 70w/m2.  Half of all energy is occupied in the vapourisation  of water - and that is assuming all the energy gets past the cloud cover shutter and makes it to the surface. Just get away from your models, look around, and take note of the physical manifestation of the automatic negative feedback provided by the shutter which operates courtesy of water vapour whether in the form of droplets or aerosols. Water vapour is the critical component.

    Before it became an imperative to diversify from fossil fuel energy sources, what I am saying was settled science.  GHG's are the result of a conspiracy of silence by physicists who know their subject. If you can prove that heating from CO2 stops at a certain concentration and why, you will have proven it doesn't happen in the first place.


    [PS] Old sage. Time to reread the comments policy if you wish to comment further on this site. In particular:

    Sloganeering: If you make an assertion, back it with your sources. Use the link button in the editor.

    Tone: If you want to troll without the slightest interest in a reasonable discussion, this this is not the forum for you.

    In short, you are busy demonstrating a misunderstanding of the science and complete unwillingness to learn different. Wilful ignorance cannot be helped, but people here will try to correct misunderstandings. Disparaging their efforts and repeating nonsense will not advance a conversation.

    If you actually want a sensible discussion, then I suggest you look at where you think measurements of real world observations contradict theory. You could begin by providing an alternative explanation of what is measured by instruments measuring back-radiation, the extreme accuracy by which the Radiative Transfer Equations predict observations of radiative spectra, etc.

    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.


  16. old sage @240:

    "Before it became an imperative to diversify from fossil fuel energy sources, what I am saying was settled science."

    The "settled science" from before"... it became an imperative to diversify from fossil fuel energy sources" can be seen easilly in the following clip from a 1950s film:

    Clearly old sage's assertion to the contrary is a pure fiction, and one for which he will have zero citations.  old sage has a repeated history of such inventions asserted without evidence with the intention to deceive and I wonder that the moderators tolerate it.

    I will note that the imperative to diversify from fossil fuels is a consequence of the science, not the other way round as old sage, contrary to his tin foil hat assertions in his final paragraph.

  17. Old Sage @240,

    You say that "according to wikipedia, the average solar energy 24 hrs/day per unit area is 141 w/m2." It would be worthwhile if we other commenters had sight of this Wiki reference. I have attempted to find it for myself but it is remaining elusive. Do provide the link.

  18. old sage @238:

    "Any other body, earth included, will discharge its energy with a modified spectrum and gases, which obey quantum mechanics and are below the threshold for excitation do not radiate except with a very low probability."

    Indeed, "Comparing the radiative rate kR (the inverse of the lifetime) to the collisional deactivation rate kM[M], provides a quick estimate that only one out of 100,000 CO2 molecules excited into the (0,10,0) by collision or absorbing a photon, will emit." (source)  The collisional deactivation rate is "~ 105 s-1" (same source).  Further, "At 300 K about 6% of the CO2 molecules in the atmosphere are vibrationally excited and can radiate." (Source)  Multiplying out, that means approximately 6% of CO2 molecules will radiate in any given second at 300 K at 15 microns wavelength in any second.  But with 9.52 x 10^21 molecules of CO2 per meter cubed at standard atmospheric temperatures and pressures, that is 0.572 x 10^21 photons emitted per second from gaseous CO2 per meter cubed per second across the 15 micron band.

    At 15 microns wavelength, each photon has 13.25 x 10^-21 Joules of energy, so that each meter cubed of air at standard pressure and 300 K radiates approximately  7.58 Joules per second, or 7.58 Watts, with the energy divided equally among all directions.  Added up over the thickness of the troposphere and the the resultant radiation is very significant.

    The upshot is that while the probability of any given molecule radiating between excitation and collision is very small, the probability of each molecule radiating in a given second is only moderately small (6%) and there are a very lot of molecules.  Of course, we don't need this maths because this is just another attempt by old sage to prove that the observed IR radiation from CO2 in the atmosphere does not exist.  No matter how many times he hides his eyes from the observational data, it does not go away.

  19. M.A. Roger- do a google search and you will find the average surface energy per m2 is of the order 150w. This is to compare like with like for average surface rain fall of 99cms taking up 70w/m2.

    Tom Curtis - 7w /m3 for radiation by CO2, 40 times as much H2O vapour of similar characteristics in the air means 280 w/m3. My living room is 90m3 so why do I bother heating it. Have you even considered how the tiny weight of CO2 in a m3 could generate 7 watts?

    This column needs to be renamed so as to include surreal.

  20. Just a couple of points of clarification:

    M.A. Rogers - the gross incident energy is of course 1360/4 kw/m2. But I agree, the Google results are somewhat suspect and need further inspection.

    Tom Curtis: The 6% of excited molecules you allege must come about from externally derived illumination. Vibrational excitation most certainly cannot arise from mutual collisions. It's a question of the difference between absorption and emission, your number, if true, varies with the strength of source rather than temperature of the gas.

  21. M.A. Rogers - oops 1360/4 w/m2, not kw.  

  22. old sage @246,

    I'm disappointed by your responses. What you're telling me is that you were wrong to write @340 that "according to wikipedia, the average solar energy 24 hrs/day per unit area is 141 w/m2." In itself this is fine. Who does not make unfortunate mistakes from time to time? But you sadly dodge any explanation/apology. That is poor do's.

    My reason for asking for a link was that it would provide us with the same hymn-sheet to sing from. As it is you provide no link and instead @244 provide the vaguest of description - "The average surface energy per m2..." and attach the vaguest of values to it - " of the order 150w."

    Can I cut through your reticence and suggest that the graphic above @236 provides a source of this data. The 1360/4 = 340W/sq m of TSI you sign up to @ 245/46 is given there. We also have the average solar energy absorbed at the surface "of the order of 150w" = 161W/sq m. And your 70W/sq m is also within the given range of 70-85W/sq m. (I should point out that 99cm of precipitation would require more than 70W/m sq to evaporate from the surface unless the evaporating water was at 100ºC, which is why the graphic gives a range running higher than your 70W/sq m.)

    So are we happy with the graphic @236?

  23. old sage @245 @246:

    1)  I quote again:

    "Comparing the radiative rate kR (the inverse of the lifetime) to the collisional deactivation rate kM[M], provides a quick estimate that only one out of 100,000 CO2 molecules excited into the (0,10,0) by collision or absorbing a photon, will emit."

    It follows that on average, where CO2 absorption equals CO2 emissions, out of every 100,000 molecules dropping from a (0,10,0) state by emission, 99,999 will have been excited to that state by collision.  Your statement that "Vibrational excitation most certainly cannot arise from mutual collisions" is nonsense.  Classically, there is no better way to induce vibrations in an object than to subject it to a collision.  Within quantum mechanics, the vibrational mode must have a particular energy level, so that the proportion of collisions conveying that level comes into question.  However, as temperature rises, the mean energy of motion of the gas molecules rises and hence the proportion of collisions having the correct energy also increases.  I quote more fully:

    "At 300K (27 C) ground state CO2 molecules are continually colliding with oxygen, nitrogen and other molecules. The average collision has an energy equivalent to kT where k is Boltzmann’s constant. In units usual to the field this is ~200 cm-1 (multiply by the speed of light in cm/sec and Planck’s constant to get Joules) but some of them have much more energy. A few of the energetic collisions can vibrationally excite the CO2 to the same 650 cm-1 excited vibrational level. Because collisional processes are fast wrt radiation, the number of vibrationally excited CO2 molecules can be characterized by a Boltzman distribution. At 300 K about 6% of the CO2 molecules in the atmosphere are vibrationally excited and can radiate."

    Note that the Maxwell-Boltzmann distribution that governs this is a function of temperture, not of ambient radiation.  The upshot is that your claim is entirely wrong.

    2)  Again, you are merely arguing against well established emperical data.  In that, you are like the flat earthers who, in an era of international flights and GPS insist that the Earth is flat based on misunderstood scientific principles.  This is made graphic by the use of IR cameras filtered to 4.2 to 4.4 microns at which wavelength CO2 absorbs and emits radiation, but H2O and other major atmospheric components do not.  Using a better quality, IR filtered camera, we can see CO2 production in a variety of circumstances:

    Note the CO2 coming from exhaust pipes have come from conditions of near complete darkness.  Their high rate of emissions shown by their bright temperatures, therefore are almost entirely the product of thermal collisions.  As a side note, the lack of visible smoke or water vapour from the stacks (1:30 on the video) confirm that they are not emitting H2O and are emitting limited quantities of soot and hence that it is primarilly CO2 that is emitting the radiation.  The CO2 from peoples breath is relatively cool, and typically cooler than the background and shows up as slightly dark.  Again, level of radiation is temperature dependent.

    As a technical note, this sort of footage relies on wavelengths near the edge of the absorption band so that CO2 has a low emissivity other wise distant shots would be impossible.  As a consequence, the low levels of radiation associated with room temperatures do not show emissions in these cameras.  Using a 15 micron filter, they would but so also would H2O.

    3)  Each meter cubed of dry air at standard pressure and 300 K results in the emission of about 7 Joules of energy per second, but a stack of such cubes will result in absorption of some of the emissions which limits the total amount transmited.  At the the 15 micron emission peak for CO2, such absorption may well occur within the first meter, so that the total amount leaving each cubic meter may be a little less than the 7 joules.

    More importantly, the CO2 emits the energy, it does not generate it.  It is only able to emit the energy because it is continuously absorbing an equivalent amount of energy from thermal radiation and collisions.  Therefore it cannot heat a room anymore than the high emission from room temperature walls will heat a room.  In short, the laws of thermal emission do not violate the laws of thermodynamics (and if you think they do, move your discussion to the Second Law of thermodynamics thread). 

  24. M A Rodgers please stop quibbling, the absorption of 70 w/m2 by vapourising water is a very significant proportion of the energy arriving at the surface. If you cover 1 hectare of surface with concrete and drain it, the resulting surface area presented by 1cms of rainfall shrinks from 10,000 sq m to 100 sq m as it is collected in drains.  Therefore, the amount of vapour is significantly reduced.

    I repeat what every schoolboy should know, that reduces cloud cover and lets more solar energy into earth's environment. The form factor and altitude of cloud formations radiate externally far more than vertically downwards.

    Finally, I ask Tom Curtis to explain why when a given amount of CO2 has raised earth's temperature by t degrees - as we are asked to believe by theIPCC - it then stops retaining a single calorie of energy despite everything being in place to keep the 'blockage' of outgoing energy effective?

    And please don't try to drown me in a deluge of 'off the point' rambling over radiation from gases at STP, they scatter, they absorb, but most definitely they do not spontaneously radiate from their translational kinetic energy to any significant extent. All they  do is convey energy from one place to another in an energy neutral fashion.


    [JH] Your argumentative, word-salad responses to MA Rogers and Tom Curtis constituite sloganeering which is prohibited by the SkS Comments Policy.   

  25. As a research scientist, the most stimulating discussion was got from others playing devil's advocate - sloganeering, is that how this particular column shuts down unwelcome thoughts.  No wonder I haven't  bothered to tune in for years. I repeat, what physical process shuts down the alleged entrapment of heat by CO2 after it has reached its target temperature per concentration. If you can show that, you will see the rise is snuffed out from the outset and T does not rise whatever the concentration. 


    [PS] Moderation complaint snipped. If you are research scientist, then you could try arguing like one, with accurate data and logic and especially a demonstration that you have understood the theory you are arguing against. At the moment, it is not clear that you even understand response being made to you. If you are have not read Ramanathan and Coakley 1978 then I suggest it is high time you did. Continuing to assert statements about physics of gases without backing evidence (and in defiance of well-established experimental evidence) is simply sloganeering. Put up evidence to back your assertions- otherwise your comment will be deleted.

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