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Is the CO2 effect saturated?

What the science says...

Select a level... Basic Intermediate Advanced

The notion that the CO2 effect is 'saturated' is based on a misunderstanding of how the greenhouse effect works.

Climate Myth...

CO2 effect is saturated

"Each unit of CO2 you put into the atmosphere has less and less of a warming impact. Once the atmosphere reaches a saturation point, additional input of CO2 will not really have any major impact. It's like putting insulation in your attic. They give a recommended amount and after that you can stack the insulation up to the roof and it's going to have no impact." (Marc Morano, as quoted by Steve Eliot)


This myth relies on the use (or in fact misuse) of a particular word – 'saturated'. When someone comes in from a prolonged downpour, they may well exclaim that they are saturated. They cannot imagine being any wetter. That's casual usage, though.

In science, 'saturated' is a strictly-defined term. For example, in a saturated salt solution, no more salt will dissolve, period. But what's that got to do with heat transfer in Earth's atmosphere? Let's take a look.

Heat-trapping by CO2 in the atmosphere happens because it has the ability to absorb and pass on infra-red radiation – it is a 'greenhouse gas'. Infra-red is just one part of the electromagnetic spectrum, divided by physicists into a series of bands. From the low-frequency end of the spectrum upwards, the bands are as follows: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Gamma rays thus have a very high-frequency. They are the highest-energy form of radiation.

As our understanding of the electromagnetic spectrum developed, it was realised that the radiation consists of particles called 'photons', travelling in waves. The term was coined in 1926 by the celebrated physicist Gilbert Lewis (1875-1946). A photon's energy is related to its wavelength. The shorter the wavelength, the higher the energy, so that the very high-energy gamma-rays have the shortest wavelength of the lot.

Sunshine consists mostly of ultraviolet, visible light and infra-red photons. Objects warmed by the sun then re-emit energy photons at infra-red wavelengths. Like other greenhouse gases, CO2 has the ability to absorb infra-red photons. But CO2 is unlike a mop, which has to be wrung out regularly in order for it to continue working. CO2 molecules do not get filled up with infra-red photons. Not only do they emit their own infra-red photons, but also they are constantly colliding with neighbouring molecules in the air. The constant collisions are important. Every time they happen, energy is shared out between the colliding molecules.

Through those emissions and collisions, CO2 molecules constantly warm their surroundings. This goes on all the time and at all levels in the atmosphere. You cannot say, “CO2 is saturated because the surface-emitted IR is rapidly absorbed”, because you need to take into account the whole atmosphere and its constant, ongoing energy-exchange processes. That means taking into account all absorption, all re-emission, all collisions, all heating and cooling and all eventual loss to space, at all levels.

If the amount of radiation lost to space is equal to the amount coming in from the Sun, Earth is said to be in energy balance. But if the strength of the greenhouse effect is increased, the amount of energy escaping falls behind the amount that is incoming. Earth is then said to be in an energy imbalance and the climate heats up. Double the CO2 concentration and you get a few degrees of warming: double it again and you get a few more and on and on it goes. There is no room for complacency here. By the time just one doubling has occurred, the planet would already be unrecognisable. The insulation analogy in the myth is misleading because it over-simplifies what happens in the atmosphere.

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

This myth relies on the use of a word – saturated. When we think of saturated in everyday use, the term 'soggy' comes to mind. This is a good example of a word that has one meaning in common parlance but another very specific one when thinking about atmospheric physics. Other such words come to mind too. Absorb and emit are two good examples relevant to this topic and we’ll discuss how they relate to atmospheric processes below.

First things first. The effect of CO2 in the atmosphere is due to its influence on the transport of 'electromagnetic radiation' (EMR). EMR is energy that is moving as x-rays, ultraviolet (UV) light, visible light, infrared (IR) radiation and so on (fig. 1). Radiation is unusual in the sense that it contains energy but it is also always moving, at the speed of light, so it is also a form of transport. Radiation is also unusual in that it has properties of particles but also travels with the properties of waves, so we talk about its wavelength.

The particles making up radiation are known as photons. Each photon contains a specific amount of energy, and that is related to its wavelength. High energy photons have short wavelengths, and low energy photons have longer wavelengths. In climate, we are interested in two main radiation categories - firstly the visible light plus UV and minor IR that together make up sunshine, and secondly the IR from the earth-atmosphere system.

The Electromagnetic Spectrum

Fig. 1: diagram showing the full electromagnetic spectrum and its properties of the different bands. Image: CC BY-SA 3.0 from Wikimedia.

CO2 has the ability to absorb IR photons – it is a 'greenhouse gas'.So what does “absorb” mean, when talking about radiation? We are all familiar with using a sponge to mop up a water spill. The sponge will only absorb so much and will not absorb any more unless it's wrung out. In everyday language it may be described, without measurements, as 'saturated'. In this household example, 'absorb' basically means 'soak up' and 'saturated' simply means 'full to capacity'. Scientific terms are, in contrast, strictly defined.

Now let's look at the atmosphere. The greenhouse effect works like this: energy arrives from the sun in the form of visible light and ultraviolet radiation. A proportion reaches and warms Earth's surface. Earth then emits the energy in the form of photons of IR radiation.

Greenhouse gases in the atmosphere, such as CO2 molecules, absorb some of this IR radiation, then re-emit it in all directions - including back to Earth's surface. The CO2 molecule does not fill up with IR photons, running out of space for any more. Instead, the CO2 molecule absorbs the energy from the IR photon and the photon ceases to be. The CO2 molecule now contains more energy, but that is transient since the molecule emits its own IR photons. Not only that: it's constantly colliding with other molecules such as N2 and O2 in the surrounding air. In those collisions, that excess energy is shared with them. This energy-sharing causes the nearby air to heat up (fig. 2).

CO2 heat transfer

Fig. 2: The greenhouse effect in action, showing the interactions between molecules. The interactions happen at all levels of the atmosphere and are constantly ongoing. Graphic: jg.

The capacity for CO2 to absorb photons is almost limitless. The CO2 molecule can also receive energy from collisions with other molecules, and it can lose energy by emitting IR radiation. When a photon is emitted, we’re not bringing a photon out of storage - we are bringing energy out of storage and turning it into a photon, travelling away at the speed of light. So CO2 is constantly absorbing IR radiation, constantly emitting IR radiation and constantly sharing energy with the surrounding air molecules. To understand the role of CO2, we need to consider all these forms of energy storage and transport.

So, where does 'saturation' get used in climate change contrarianism? The most common way they try to frame things is to claim that IR emitted from the surface, in the wavelengths where CO2 absorbs, is all absorbed fairly close to the surface. Therefore, the story continues, adding more CO2 can’t make any more difference. This is inaccurate through omission, because either innocently or deliberately, it ignores the rest of the picture, where energy is constantly being exchanged with other molecules by collisions and CO2 is constantly emitting IR radiation. This means that there is always IR radiation being emitted upwards by CO2 at all levels in the atmosphere. It might not have originated from the surface, but IR radiation is still present in the wavelengths that CO2 absorbs and emits. When emitted in the upper atmosphere, it can and will be lost to space.

When you include all the energy transfers related to the CO2 absorption of IR radiation – the transfer to other molecules, the emission, and both the upward and downward energy fluxes at all altitudes - then we find that adding CO2 to our current atmosphere acts to inhibit the transfer of radiative energy throughout that atmosphere and, ultimately, into space. This will lead to additional warming until the amount of energy being lost to space matches what is being received. This is precisely what is happening.

The myth reproduced at the top – incorrectly stating an analogy with roof insulation in that each unit has less of an effect - is misleading. Doubling CO2 from 280 ppm to 560 ppm will cause a few degrees of warming. Doubling again (560 to 1130 ppm) will cause a similar amount of additional warming, and so on. Many doublings later there may be a point where adding more CO2 has little effect, but recent work has cast serious doubt on that (He et al. 2023). But we are a long, long way from reaching that point and in any case we do not want to go anywhere near it! One doubling will be serious enough.

Finally, directly observing the specific, global radiative forcing caused by well-mixed greenhouse gases has - to date - proven elusive. This is because of irregular, uncalibrated or limited areal measurements. But very recently, results have been published regarding the deep reinterrogation of years of data (2003-2021) from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua Satellite (Raghuraman et al. 2023). The work may well have finally cracked the long-standing issue of how to make finely detailed, consistent wavelength-specific measurements of outgoing long-wave radiation from Earth into space. As such, it has opened the way to direct monitoring of the radiative impact (i.e. forcing + feedback) of greenhouse gas concentration changes, thereby complimenting the Keeling Curve - the longstanding dataset of measured CO2 concentrations, down at the planet's surface.

Note: Several people in addition to John Mason were involved with updating this basic level rebuttal, namely Bob LoblawKen Rice and John Garrett (jg).

Last updated on 31 December 2023 by John Mason. View Archives

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Further reading

V. Ramanthan has written a comprehensive article Trace-Gas Greenhouse Effect and Global Warming.

Further viewing

Video by Rosh Salgado on his "All about Climate" YouTube channel in which he debunks Will Happer's claim that the CO2 effect is saturated in the atmosphere:


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Comments 476 to 500 out of 715:

  1. LTO @472 ,

    I must say I am puzzled by your assessment of the Science of Doom website.   The Greenhouse Effect is understood through observational studies combined with well-established basic physics.   Any usage of models came much later, and is certainly not foundational to the science of it all.

    Please do not be discouraged.  The CO2/Greenhouse Effect is actually quite simple & straightforward ~ once you have gotten your head around it.   But it is not immediately intuitive.

    Just like the Galileo/Tower-of-Pisa/falling-weights tale . . . and like the concept of Gravity . . . and Newton's Laws of Motion.   All these things can be "unsimple" to explain in a few paragraphs ~ but are quite simple and obvious, after you have grasped the concepts.   But for previous thousands of years, they were not intuitive at all !   And still are not ~ until you take a scientific approach and think things through.

    As Tom mentioned in #463 : in air, the molecular collisions occur at a rate many orders of magnitude above the "relaxation times" of a CO2 molecule (where a CO2 molecule accepts the energy of a 15um InfraRed photon, and later "relaxes" to emit an equivalent IR photon in a random direction).    Even where you reduce that collision rate by a hundred-fold (by reducing density & temperature, e.g. in the upper troposphere), you still get the situation where the collision rate is still vastly greater than the IR relaxation rate.   When you think it through, you see that the end result gives a negligible difference in the actual effect [ e.g. comparing the bulk difference between 99.99% and 99.9999% ].


    ** And LTO ~ a word in your ear.  While I myself am sweetly naive and unsuspicious that you might be uttering some phrasing of words which is alas too often heard coming from the mouths/keyboards of trollish science-deniers . . . nevertheless you have managed to cause Tom's ears to vibrate, by your using terms of the type: "incredulity / hoax-like / too-complex-to-be-an-honest-description / etcetera [obviously I am harshly paraphrasing your comments]."

    Those sorts of phrasings are common among science-deniers [= faux-skeptics] who subconsciously wish to reject reality ~ and who summon all their powers of distraction & rhetoric, in order to deceive themselves.

    ( I do read the WattsUpWithThat website, for entertainment.  Half the posters commenting there, are angry-crazies & political-extremists who are still in complete denial that CO2 & other Greenhouse gases have any global warming effect at all . . . and many of the other half are intelligent but so deeply affected by their Motivated Reasoning, that they distract themselves by using rhetorical smoke & mirrors ~ basically for deceiving themselves into a viewpoint that "there's nothing really unusual going on, and there's little or no global warming happening . . . and even if it is happening, then it's gonna be good for us, and with no major downsides". )

  2. Michael: fantastic thank you

    1. I find this a little difficult to believe in its face (which isn't to say it isn't true!) for two reasons.  First, while the number of collisions is high, the relaxation rate of an excited co2 molecule is presumably quite fast, and so the net effect could be considerably more than it appears from the raw collision rate itself. Second, and more importantly, if temperature and pressure don't make much of a difference then why is the increase in escape altitude so important? I'm still not clear on why the relatively small decrease in temperature as the escapeealtitude increases affects the emission of a discrete photon so significantly.

    On the ppm point, difficulty isn't a great excuse in my view! The constancy of co2 in ppm is a bit of a red herring, if i'm understanding things correctly, because really it's the molar concentration combined with pressure that gives effect to the phenomena. For example, both venus and mars have comparable (very high) co2 ppm levels, but clearly completely different effects, at least in psrt because of the difference in atmospheric pressure (as I understand it).

    2. That's really useful re the troposphere thanks. The complexity in escape altitude mustmmake things very difficult to model.

    That link is really interesting for  two reasons. First, it implies that actually the hole in the ozone layer should uave been responsible for a significant portion of troposphere warming, as opposed to CO2. Presumably the UV light that would otherwise have warmed the stratosphere warmed the troposphere instead.  Do we know how much troposphere warming is attributable to this?

    Second, the point on CO2 cooling the upper atmosphere really didn't make sense to me. The argument appeared based on an assertion that the earth is always radiating the same amount of heat so an increase in troposphere temp must lead to a decrease in upper atmosphere temp, but I thought the whole point was that the earth was retaining more heat. Why isn't the increasing CO2 conc in the stratosphere leading to increased heat retention of photons released by co2 in the troposphere and concomitant warming? Could be related to the pressure point above, but in that case why isn't it a relevant consideration for increasing escape altitude?

    3. I will do! May take a while - hopefully will get answered here first.

  3. LTO @477 ,

    we seem to have cross-posted at 21:54 PM.

    Your first paragraph shows that you are still a long, long way from understanding the Greenhouse Effect.

    The "complexity in escape altitude" does not require a "model" of mathematical ingenuity & tour-de-force.  You can do a good approximation on the back of the proverbial envelope, using a blunt pencil.   Basically, use the temperature lapse rate of around 6.5 degrees per 1000m altitude.   (Of course, the escape altitude is not a razor-thin layer, but a fuzzy zone . . .but your can treat it as one particular altitude (as described in some of the comments upthread) . . . while always remembering that other Greenhouse gases have different escape altitudes.

    The stratospheric cooling is interesting, in that it demonstrates that modern global warming is not of solar initiation.   But in practical terms, the stratosphere is so low density, as to have minimal effect (and similarly with the thermosphere).

    Better for now, to focus your thoughts on understanding the GHE.   Think about the transient condition, where (as right now) there is a nett inflow of heat into our planet . . . compared with future condition, where the Greenhouse influence has stabilized at a higher surface temperature.

  4. LTO @472 &475,
    I think I should add to the message from Tom Dayton @473.
    You talk of "unknowns in the system" and "taking someone else's word for it."
    I appreciate that getting a grasp of AGW can be frustrating. I remember when I first encountered the Greenhouse Effect and the idea that there could be some equivalent to a sheet of glass allowing light in but preventing IR escaping seemed a bit much to accept. What you tend not to find, even now, is convincing explanations as to why the Earth's atmosphere is more hermitically sealed than any greenhouse or blanket. So I'll say it here. The atmosphere is incredibly well balanced vertically. Outside hurricanes, volcanic eruptions and other relatively rare events, the day-to-day reality is the motion of the Hadley Cells, They are responsible for most of the up-down movement in the troposphere and they take about two weeks to rise from surface to tropopause. Yet this is not something you will readily learn if you start asking folk. I share with you here my own frustrations from a few decades ago.
    Yet there are some (probably very many) apparent incongruities that can be expressed about AGW that are not readily answerable in a simple way using non-scientific argument. This SkS site addresses many but there are always different flavours of incongruity to consider in such a complex system.

    Yet such incongruities do in no way support the notion of potential "unknowns in the system" where we have to "tak(e) someone else's word for it." And in particular here we are discussing an aspect of AGW that is in no way in dispute as, despite the complexity, there are absolutely no "unknowns in the system."

  5. LTO,

    Unfortunately I do not have much time.

    1) The number of collisions at the surface is about 1 million times faster than the relaxation time.  The change in temeprature to 10 kmn is about 40C.  That is about a 20% change in speed and collision rate.  The pressure change is about 40 kPa.  About a 40% change which changes the collision rate a little more than 40%.  Combined they change the collision rate less than a factor of 5.  At the escape altitude there are 200,000 times as many collisions as emissions.

    The atmosphere is a black body.  At a lower temperature it emits less energy.  Science of Doom will have a graph of energy emitted compared to temperature.  Black body radiation changes relative to T to the fourth power so small changes are a much larger effect.

    Complex models can exactly calculate the emission spectrum of the entire atmposphere at any level or all combined but are not needed to explain the greenhouse effect.  They demonstrate that scientists know what they are talking about. (I do not have time to find a reference, sorry)

    2) Most of the UV light is still absorbed.  There is not that much energy in the UV remaining (it can be calculated and is considered by climate scientists).

    The Stratosphere is a completely different situation than the Troposphere.  The Troposphere is heated by energy coming up from the surface.  CO2 blocks this energy from escaping (until it reaches the escape altitude) so the Troposphere warms.

    The Stratosphere is warmed by UV light from the Sun.  Increased CO2 causes increased emission of IR energy.  Since the Stratosphere is above the escape altitude (as discussed above) the increased IR emission results in increased loss of energy and cools the Stratosphere.  The key understanding is the escape altitude (which is very complicated but we simplify to 10 km for these discussions)

    Scientists predicted this effect in advance.  It is a key signature of the greenhouse effect.  Post hoc explainations about unknown "global oscellations" do not hold the same weight as predictions made in advance.  I know of no alternate explainations for how the Troposphere could warm as the Stratosphere cools.

  6. Michael: thanks for taking the time. Apologies on the collision point, i see it's a bit of a red herring. I'm still not getting the importance of temperature on the escape altitude. I understand black body radiation (kind of) but not how that relates to the discrete emission of a photon from an excited molecule. In other words, the collision between bulk property thermodynamics and a discrete quantum event. This may just be my own deficiencies.

    What i don't follow is your point on UV and stratosphere. The article you linked to was clesr that tbe decrease in ozone layer was the main cause in a large decrease in stratosphere temperature through decreased UV absorption. It *must* therefoelre be the case that this energy was transferred to the troposphere instead. Given ozone depletion very simply predicts a decrease in stratosphere temps, the greenhouse effect prediction would seem to carry less weight: the two effects would need to be disambiguated, increasing uncertainty, particularly as according to that article the ozone effect is dominant. 


    I'd suggest its unfair to call explanations based on temperature cycles post hoc - this idea is obviously very well supported by the pre-industrial historical record. The weakness in that argument is of course that it predicts anything, amd therefore predicts nothing .NNevertheless, the challenge is to show something unusual is happening, which is difficult to do persuasively when we've only been measuring certain metrics for a short period. That's a very different topic to this one though!

  7. To clarify my query: I understand well enough the principles behind the greenhouse effect. However, what I recently only became aware of was that there is already a vast overabundance of co2 to absorb all the IR emitted at 15 um, so the effect of adding more co2 at current levels must have a vastly smaller effect than adding thse same amount of co2 at much lower levels. This seems to be common ground amongst those in the know, but I'd say unknown to 99% of people.

    The explanation for why it matters nevertheless seems to be twofold, and here is where I'm struggling to understand quantitatively how significsnt the effects are.

    1. CO2 also has other minor bands of absorption, which may depend on concentration amongst other factors, that arent saturated. My question here is just how much additional energy this actuslly captures and re-radiates back to the ground. Is it really significant in the grand scheme.

    2. Increasing CO2 increases the altitude of emission (perfectly happy here), and because 15 um photons are being released at this higher altitude therefore global warming (this is where I'm getting lost).

  8. LTO @482,

    An unsaturated GHG (like methane) does provide for stronger warming effects with rising concentrations, these being roughly linear increases in warming with rise concentrations, rather than the logarithmic relarionship found with CO2.

    (1) What you mean by "bands of absorption" is not clear. The bendy wobble absorption band at 15 microns is made up of a set of wavelengths which are weaker the further away from the central part of the band. Thus the CO2 absorption appears as a wide dip at 15 microns as per this graph below.

    EarthIR spectrum

    Note the small spike in the centre of the dip. This is the strongest part of the CO2 bendy wobble absorption. Here at this precise wavelkength you would be up into the stratosphere before a photon has a clear shot at space. (It is an upward spike because the stratosphere is warmer at that altitude than the upper troposphere.

    One of the effects of adding CO2 to the atmosphere is to widen the broad CO2 dip as there are weaker wavelengths at the edges that are not saturated and in dry air would allow a photon to be emitted by CO2 and have a clear shot at space from ground level.

    If you mean by "band" an energy of photon that imparts a different wobble into CO2, there are none of consequence operating in the IR range, the closest being 4.3 microns.

    (2) The impact of altitude-increase is that (and here your question up-thread was hidden by your additional comments on 'rate of collision') through the troposphere temperature drops with altitude and so the Stephan-Boltzmann relationship applies. A colder gas is unable to emit as much IR. See the contours on the graph above. Less photons emitted to space, more energy accumulating on the planet, a warming planet until the energy fluxes are balanced

  9. MA Rodgers:  Thanks for posting that graph.


    Black body radiation is the net of all the photon emitting events.  When the temperature is higher more photons are emitted.  According to the Boltzman equation, the number of photons emitted is proportional to T raised to the fourth power.  A small change in T means a large change in photons emitted.

    UV radiation not absorbed in the stratosphere passes through the Troposphere and is absorbed at the surface.  There has to be an absorbing molecule, like ozone in the stratosphere, for the energy to be absorbed.

    It appears to me that you are applying a double standard.  Unknown "Global cycles" do not need evidence while scientific explainations require every T crossed and I dotted.  Fortuantely, the T's and I's have all been done.  Keep  reading scientific sites and you will find out what you seek.  Be careful of reading "Skeptic" sites as they traffic in nonsense which has to be unlearned.

    1) Exact numbers are beyond my pay grade.  Look at MARodgers graph.

    2) The temperature at the escape altitude is essentially fixed because it must be high enough to allow all the energy incoming from the sun to be emitted.  The lapse rate of the atmosphere (the decrease in temperature with increasing altitude) is a physical constant and is also fixed.  When the escape altitude increases, the temperature at the new escape altitude also increases to ensure conservation of energy.  When the temperature increases at the new escape altitude the increase propagates down to the surface to comply with the lapse rate.

    While at the surface the 15 micron absorbtion is saturated at the escape altitude it is not.  Therefore increasing CO2 increases temperature.  People who do not understand the greenhouse effect think because absorbtion at the surface is saturated temperature cannot increase.  The escape altitude is where the action is .

    Someone else suggested that the temperature at the escape altitude did not change as much as I think it does.  I think there is an issue of different simplifications of a complex subject.

  10. LTO @482 ,

    forgive my bluntness, but your recent questions show that you are still floundering rather than "understanding the principles [behind the GHE]".

    The GHEffect is multi-faceted, but straightforward.   Take your time, think things through and put the pieces together in your mind.   There is no trickery, no hidden or undiscovered "unknown unknown" factors . . . it is all simply very basic physics [high school level physics will be quite adequate].

    Picture the Earth of 300 years ago, when things were very close to equilibrium [though to be more accurate, the Earth has been cooling very gradually for around 5 thousand years].   The air CO2 level was about 280ppm, and the "escape altitude" was at the appropriate level.   Now look at today : CO2 level 410ppm, and the escape altitude has risen 100 or 200m higher and colder ~ fewer and slower molecular collisions.   And therefore marginally less excitation and emission of 15um photons to space.   And yet we still have about the same incoming heat energy from solar radiation.   Result : imbalance.

    Now jump 100 years into the future.   Wise political leaders have (of course!) long ago brought "zero nett carbon emission" into being, and have fostered projects which incorporate carbon (dioxide) into the soil . . . bringing air CO2 levels down to the low 400's.   Planetary surface temperature is 1 degreeC above 2019 levels [i.e. 2 degrees above pre-industrial levels] and is steady.   The escape altitude is at (say) +300m, but the air at that point has [stabilized] become slightly warmer . . . enough for the 15um IR loss-to-space to have increased back to the pre-industrial amount ~ so the Earth is in thermal equilibrium again (solar radiational input and terrestrial radiational output are matched).   But we on the surface here are 2 degrees warmer than pre-industrial.

    The alternatives are rather worse, if we allow the GHE to push things up 3 or 4 or 5 degrees.

  11. MA, Michael - thank you for this; very helpful, and it's tht first time Ive heard about the lapse rate as an explanation for how global warming works. There are a few things I'm still not following:

    1. How much of a difference does the effective widening lf the bands actually makw at the concentrations werew talking about? Eg if the effective drop in radiance (ie area under thr curve) at 400 ppm co2 was 100, what would it be at 800 ppm? 

    2. I'm not following the black body radiation argument, because co2 excitation and subsequent emission of 15 um isn't a black body phenomena (correct me if I'm wrong here). A single co2 molecule in an excited state will release a 15 um photon, and this is separate to black body radiation.

    3. I'm not sure I'm following why the temperature of the escape altitude increases. As indicated by MA earlier, there isn't necessarily energy transfer to the surrounding gas as excited co2  molecules increasing the temp  of surrounding air through collision then in principle increases the number of co2 molecules excited through collisions with surrounding air, hence minimising net energy transfer. I'd thought thr main mechanism behind the greenhouse effect was re-radiation of 15 um photons back to the surface (or water vapour),  not through heating of surrounding air. Is that wrong? Or are these minor secondary effects that are only really relevant once you get past the primary saturation point?

    4. Even if the temperature of the escape altitude is increased, it's not clear to me why that is necessarily transmitted to the ground through the lapse rate. What is the mechanism? Presumably not convection. Given the existence of eg temperature inversions in the troposphere and the day-night temperature cycling, it isn't clear to me why that is necessarily the case.


  12. LTO,

    SkS is always happy to help those who want to learn the science.

    1) My understanding is that line broadening is a very small effect on Earth.  It is important on Venus.  It is not necessary to understand line broadening to get the basic greenhouse effect.

    2)  CO2 molecules emit a variety of radiation lines with15 micron being the most important.  The number of photons emitted by a section of the atmosphere (with a great many CO2 molecules in it) is determined by the black body equation.  Most of the CO2 molecules that absorb a photon coming up from below transfer the energy of the photon to other molecules in the air through collisions.  They do not re-emit the photon they absorbed.

    There is always a population of excited CO2 molecules that can emit a photon.  These molecules are excited by collisions with other molecules.  The size of this population is determined by the black body equation.  When it is hotter there are more molecules that are excited and more photons emitted.  When cooler less excited molecules, less photons.  The number of photons increases with Temperature to the fourth power.  The population of excited molecules is the important idea, not individual molecules.

    3) Let us imagine the escape altitude is 10.00 km and the Earth is at equilibrium.  Exactly the same amount of energy is emitted from the molecules at the escape altitude as is absorbed by the Earth (the energy comes from the Sun and is primarily absorbed on the surface).  The Earth receives 240 W/m2 and emits 240 W/m2.  The Earth is at a stable temperature.   The temperature at the escape altitude is 255.0K.  

    Someone adds 1,000 gigatons of CO2 to the atmosphere.  This causes the CO2 concentration to double.  This causes the escape altitude to increase to 10.50 km (500 meters). 

    The temperature of the atmosphere decreases with height according to the lapse rate (6C per km).  The temperature at the new altitude is only 252.0K (255 - [0.5km x 6C/km]).  Because it is colder less energy is emitted from the Earth (the amount can be calculated using the Boltzman equation.  It takes me a long time to calculate with this equation.).  For the purpose of discussion let us say at the new altitude only 239 W/m2 is emitted.

    The Earth is no longer at equilibrium.  It is absorbing 1 W/m2.  It starts to heat up.  The temperature at the escape altitude must increase to 255.0K in order for the Earth to emit 240W/m2 again. (There are some complications like a small increase in surface area that do not matter). 

    The atmosphere always has a lapse rate of 6C/km.  Since the temperature at 10.30 km has increased 3.0C the rest of the atmosphere also increases.  The lapse rate is a measured physical property so it must be applied.

    I do not understand your question about energy.  Most of the absorbed energy is transferred to the surrounding atmosphere.  That is how energy reaches the escape altitude and is emitted to space.

    The main effect is sometimes in the eye of the beholder.  I think the main effect is to  increase the temperature of the atmosphere.  That occurs because CO2 (and other greenhouse gases) absorb upwelling IR radiation and slow the emission of energy into space.  Both re-radiating energy back to the surface and heating the surrounding air are important.  The most important effect is increasing the escape altitude.

    4) The maintenance of the lapse rate in the atmosphere is complex (scientists who study the lapse rate understand how it works).  See this article for background information (found using Google).  Convection is involved but there are other factors.

    When we say the escape altitude is 10 km that is an average over the entire Earth: Tropics to Arctic, night and day (a few wavelengths escape from the surface).  The escape altitude is not the same everywhere on Earth.  In the Tropics it is higher than in the Arctic.  The lapse rate is an average property of the entire atmosphere, individual storms or other phenomena can violate the lapse rate (and the escape altitude) for periods of time.

    I recommend you accept the lapse rate and escape altitude on faith while you learn how the greenhouse effect works.  After you understand the basics you can add other effects that you are interested in.  Line broadening, convection, heat transfer by phase changes, clouds and other effects all occur in the atmosphere and alter the greenhouse effect.  Climate models have to deal with all these effects but they do not alter the basics.

  13. LTO:

    I found a Boltzmann Equation calculator on line (Google)

    It finds 239.8 W/m2 at 255 K and only 228.7W/m2 at 252K.  That means the Earth heats up faster than if the difference was only 1W/m2 but in the end the temperature at the escape altitude must increase to 255K so all the energy is emitted.

  14. LTO @486,

    I'm conscious that directly answering some of your queries would lead toward some rather incongruous implications with complex explanations required to sort them out. So I'm torn between simply answering #486, going back to first principles as an explanation or introducing a mathematical model into the mix. Haven't decided which yet.

    But I will pitch in with (1).

    michael sweet @487 mixes up the broadening of the CO2 dip in the IR spectrum (most important) and pressure broadening (not important). These are two different phenomena.

    The 15 micron wave band absorbed by CO2 is flanked by weaker bands which result from spinning CO2 molecules. Spin being a quantum process, there are only certain speeds of spin that can happen, resulting in the graph below (I assume it is for 1 atm).

    CO2 absorption probabilities

    It is the strenghtening in these flanking bands that broadens the CO2 IR dip.

    But you will also note there is a small probability of absorption at wavelengths between the seperate bands. This is the pressure broadening which is a big effect on Venus with its 90bar atmosphere.

    As for your actual question, the effect of this broadening of the CO2 dip with an increase 400-to-800ppm relative to a 280-to-400ppm increase (=100). I think, as a component of a logorithmic ratio of 194/100, it would possibly be something like 400/100. By 800ppm, the emissions height for the central part of the band is increasingly in the stratosphere and so acts as a cooling mechanism counteracting much of the warming through the strengthened absorption at the edges of the CO2 dip. You may find Zhong & Haigh (2013) 'The greenhouse effect and carbon dioxide' Figure 5b a useful reference.

  15. I recall Gavin Schmidt going at length over this at RC and saying basically the same thing: the absorbtion in the "wings" is where the additional watts/sq.m happen as concentration goes up. It adds up significantly. This may even figure still in the "Saturated gassy argument" posts linked below the thread.

  16. Chris Colose is a scientist who studies Climate Change who used to write sometimes for SkS.  He wrote a description of the greenhouse effect here.  His sumary states:

    "So…review: Because of energy balance, the planet must get rid to space as much energy as it receives from the sun. Averaged over the Earth, taking into account the albedo and geometry, this is about 240 W m-2. In the absence of an atmosphere, this flux of radiation is lost by the surface by \sigma T^{4}_{s}. With an atmosphere, this flux of radiation is allowed to emanate from upper, colder layers of the atmosphere, say on average at some altitude H. Increasing greenhouse gases increases the altitude of H, a height in the atmosphere which depends on wavelength, and characterizes a level of mean emission to space. Because the atmosphere is now emitting from colder levels of the atmosphere, the OLR has decreased, and the result is that the planet must warm to re-establish radiative equilibrium."

    I think my description is similar to his.  His summary is more technical and those who want to increase their knowledge of the greenhouse might want to read it.

    Apparently I mistook line broadening and pressure broadening.  Line broadening is important for the greenhouse on Earth.  Both these effects, and many others, contribute to the magnitude of the greenhouse effect.  If we double the concentration of CO2, the CO2 will directly cause heating of about 1C.  Some of that will be due to line broadening.  Feedbacks from other causes like increased water vapor and changes in clouds will contribute additional heating.  The feedbacks are difficult to calculate exactly but if the climate sensitivity is 3.0C (about midrange in the estimates) they will contribute 2C.

    Philippe Chantreau: can you link Gavin Schmidt's comments, I could not find them at Realclimate.

    As I said above, there are many phenomena that contribute to the greenhouse effect.  Different scientists sometimes emphasize different phenomena as important.  All these effects together make the greenhouse effect.

  17. michael sweet @391,

    I cannot fault what you say. Particularly, that "there are many phenomena that contribute to the greenhouse effect" is one of the difficulties in setting out a succinct statement of how it operates. Yet the simple energy balance is an overriding principle - if energy-out is different from energy-in, there has to be consequences, in the present case a period of global warming. The point with this aspect of AGW is that it is more than "settled", it is cast in concrete! All that people are lacking is an explanation appropriate for their needs.

    I have been putting a bit of thought on a journey to introduce these GHG mechanisms in a way I've not seen before. I'm hoping it may be useful to folk like LTO. I've not quite routed out that journey yet, but it is looking useful.

  18. The links are in the notes below and take you to the RC threads, where Ray Pierrhumbert did most of the initial comments. In fact, I don't know that anything recently discussed above is not adequately addressed in the notes right here below the comments, where there are many good links.

    Part 2 is more interesting from the technical point of view, especially the extra absorbtion in the wings of the spectrum.

    This is old news. The RC posts are almost 12 years old. It has all been worked out with the highest level of precision in HITRAN. The appropriate physics are in the models. It is not an area of very active research or debate. HITRAN was pretty much as far it was worth going with it, from any practical point of view.

  19. Thanks all, very useful. MA: that sounds like it would be extremely helpful for me and others.

    The point on many contributing phenomena is the key one - not just the existence of the phenomena, but how quantitatively significant it is. This no doubt varies with co2 concentration, and easily leads to confusion. The current understanding I have is that the most significant co2-induced warming mechanism (which is the one most people know about - re-radiation of IR photons to the surface) is indeed saturated at much lower co2 concs, and it is other effects that are purported to combine to trap significant amounts of energy in the troposphere. Once comfortable on this point, the point on energy balance is a given.

    One really interesting point that has come up is what happens when the altitude of emission is in the stratosphere. I understand that the tropopause varies between about 9 and 17 km, while co2 concs are relatively homogeneous across the globe, therefore the altitude of emission at ~10 km is already in the stratosphere around thr poles. it would seem that increasing co2 concs will, by raising the altitude of emission, progressively increase the proportion of the atmosphere in which this is happening. Will this offset troposphere warming effects to some extent?

  20. LTO @494 ,

    there are worrisome ambiguities in your comments.

    The "tropopause" is a very different concept from the "escape altitude i.e.  emission altitude".  For CO2 that altitude is dependent on the absolute density of CO2, while the tropopause is a temperature-related concept.  If I have correctly understood the figures quoted earlier, the [CO2] altitude you are interested in does always remain in the troposphere (not the stratosphere).

  21. Hi Eclectic, it doesn't seem to me like you've correctly understood the figures quoted earlier (or my post@494), but others may have a different view. Why do you think the altitude of emission must always remain in the troposphere?

  22. LTO,

    I am not an expert on the tropopause but I doubt that the escape altitude will move into the stratosphere to a significant amount (I do not have a reference for that opinion).

    In the poles where the stratosphere is lowest the troposphere is very cold.  That lowers the escape altitude so that the escape altitude is still below the stratosphere (we discussed that the escape altitude varies across the planet from the tropics to the poles).  Perhaps Science of Doom can answer this question.

    These detailed questions are secondary to how the basic greenhouse effect works. 

     Keep in mind that the description of the greenhouse we have discussed is a description of some of the most important basic features of a very complex phenomenon. Many additional complications exist. If it were simple to evaluate the error bars on climate sensitivity would not be so large.

  23. Philippe,

    Thank you for the references.

    I have noticed that it is becoming harder to find direct answers to some basic questions because they were answered so long ago.

  24. LTO @496 ,

    I commented (@495) because I was surprised that you (@494) were giving the impression that you thought the escape/emission altitude (for 15um IR) was in the stratosphere at some latitudes.

    The tropopause is a temperature- & weather-related concept.   OTOH, the 15um photon "escape" (for CO2 emission) is dependent on absolute CO2 density ~ and of course also dependent on temperature of "local" air which energizes the CO2 molecules to emit a sufficient energy flux to achieve the appropriate contribution to cooling the planet.   (The "altitude" you are interested in is not an ultra-thin single altitude [for any particular latitude] but is a weighted average).   The emission is from a fuzzy band (of altitude), so we mustn't oversimplify too far.

  25. Golly!! More complexity being considered.

    The tropopause is a temperature thing. The tropopause height drops at night, it drops through winter and it drops greatly with latitude towards the poles. The emissions height is a pressure thing (as well as a wavelength thing). Averaged across the globe, there are parts of the CO2 emissions band that have an emissions height up in the stratosphere, and an increase in CO2 concentrations (& thus the emissions height) will thus see wider wavelengths with emission heights up in the stratosphere. A rising stratosphere emissions height sees an increase in emissions temperature which would counteract part of the the otherwise full CO2 AGW effect. But with rising CO2, there will also be more wavelengths becoming significant to CO2 absorption, for instance the two compound bands at roughly 10 microns.

    As for the poles, these are small in area relative to the tropics and emit much less radiation from the surface (which can be absorbed by CO2) being colder. The tropopause at the poles drops to ~250mbar from ~100mbar in the tropics, so 2.5x the atmosphere above it with CO2 content. So at the poles the balance between above/below the tropopause emissions height will be greatly slanted towards stratosphere, at the poles relative to the tropics. But averaged over the globe, the poles are a small part of the equasion. It is all latitudes that are averaged out to give the logarithmic relationship between CO2 and CO2 forcing, a relationship which stands (from memory) up to 1,200ppm.

    Beyond that, the reference linked @489 is reporting that the CO2 forcing would be greater than logorithmic above 1,200ppm. Thus the rise of CO2 emissions heights for more wavelengths into the stratosphere would not see an end to CO2-powered AGW.

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