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

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How do we know more CO2 is causing warming?

What the science says...

Select a level... Basic Intermediate Advanced

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence.

Climate Myth...

Increasing CO2 has little to no effect

"While major green house gas H2O substantially warms the Earth, minor green house gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940 has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of Increased Atmospheric Carbon Dioxide)


To make a statement like, "minor greenhouse gases such as CO2 have little effect", is to ignore 160 years of science history. So let's look at who figured out the heat-trapping properties of carbon dioxide and when.

Experiments involving various gas mixtures had demonstrated the heat-trapping properties of water vapour, CO2 and methane in the 1850s. But those effects were yet to be quantified - there were no meaningful numbers. It was to be another 40 years before that happened.

Swedish scientist Svante Arrhenius (1859-1927) was the person who crunched the numbers. The results were presented in a remarkable paper, "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground", in 1896.

The many calculations in the 1896 paper include estimates of the amounts of CO2 increase or decrease required to drive the climate into a different state. One example used was the Hothouse climate of the Cenozoic, around 50 million years ago. Another was the glaciations of the last few hundred millennia.

To get a temperature rise of 8-9°C in the Arctic, Arrhenius calculated that CO2 levels would have to increase by 2.5 to 3 times 1890s levels. To lower the temperature 4–5°C to return to glacial conditions, he calculated a drop in CO2 was needed of 0.62-0.55 times 1890s levels.

We know CO2 levels in the 1890s from ice-core data. They were around 295 ppm. Let's do the sums. A reduction factor of 0.55 to 0.62 on 295 ppm gives 162.2-183.9 ppm. Modern ice-core measurements representing the past 800,000 years show that in glacial periods, CO2 levels fell to 170-180 ppm.

What we now know due to additional research since 1896 when Arrhenius worked on this, is that CO2 was an essential 'amplifying feedback'. That means changes triggered by long term, cyclic variations in Earth's orbit cause warming or cooling and CO2 release or entrapment in turn. Those changes in CO2 levels affected the strength of Earth's greenhouse effect. Changes in the strength of the greenhouse effect then completed the job of pushing conditions from interglacial to glacial - or vice-versa.

Arrhenius also made an important point regarding water vapour: "From observations made during balloon voyages, we know also that the distribution of the aqueous vapour may be very irregular, and different from the ideal mean distribution." This statement holds true today: water vapour is a greenhouse gas but because water exists in gas, liquid and solid forms in the atmosphere, it is continually cycling in and out of the air. It is distributed in a highly uneven fashion and is uncommon in the upper atmosphere. That's where it differs from CO2.

Once CO2 is up there, it's up there for a long time. As a consequence it has a pretty even distribution: 'well-mixed' is the term. As Arrhenius quantified all that time ago, once it's up there it constantly absorbs and re-radiates heat in all directions. That's why dumping 44 billion tons of it into our atmosphere in just one year (2019 - IPCC Sixth Assessment Report 2022) is a really bad idea.

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

Good scientific theories are said to have ‘predictive power’. In other words, armed only with a theory, we should be able to make predictions about a subject. If the theory’s any good, the predictions will come true.

Here’s an example: when the Periodic Table of the chemical elements was proposed in 1869, many elements were yet to be discovered. Using the theory behind the Periodic Table, the Russian chemist Dmitri Mendeleev was able to predict the properties of germanium, gallium and scandium prior to their discovery in 1886, 1875 and 1879 respectively. His predictions were found to be correct.

The effect on Earth's greenhouse effect of adding man-made CO2 is predicted in the theory of greenhouse gases. This theory was first proposed by Swedish scientist Svante Arrhenius in 1896, based on earlier work by Fourier, Foote and Tyndall. Many scientists have refined the theory since Arrhenius published his work in 1896. Nearly all have reached the same conclusion: if we increase the amount of greenhouse gases in the atmosphere, the Earth will warm up.

Where there is less agreement is with respect to the exact amount of warming. This issue is called 'climate sensitivity', the amount the temperatures will increase if CO2 is doubled from pre-industrial levels. Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F). A key reason for this range of outcomes is because of the large number of potential climate feedbacks and their variable interactions with one another. Put simply, some are much better understood than others.

What Goes Down…

The greenhouse effect works like this: Energy arrives from the sun in the form of visible light and ultraviolet radiation. The Earth then emits some of this energy as infrared radiation. Greenhouse gases in the atmosphere 'capture' some of this heat, then re-emit it in all directions - including back to the Earth's surface.

Through this process, CO2 and other greenhouse gases keep the Earth’s surface 33°Celsius (59.4°F) warmer than it would be without them. We have added 42% more CO2, and temperatures have gone up. There should be some evidence that links CO2 to the temperature rise.

So far, the average global temperature has gone up by more than 1 degrees C (1.9°F):

"According to an ongoing temperature analysis led by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by at least 1.1° Celsius (1.9° Fahrenheit) since 1880. The majority of the warming has occurred since 1975, at a rate of roughly 0.15 to 0.20°C per decade."

The temperatures are going up, just like the theory predicted. But where’s the connection with CO2, or other greenhouse gases like methane, ozone or nitrous oxide?

The connection can be found in the spectrum of greenhouse radiation. Using high-resolution FTIR spectroscopy, we can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.

Greenhouse spectrum

Figure 1: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapour is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

Sure enough, we can see that CO2 is adding considerable warming, along with ozone (O3) and methane (CH4). This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.

...Must Go Up

How long has CO2 been contributing to increased warming? According to NASA, “Two-thirds of the warming has occurred since 1975”. Is there a reliable way to identify CO2’s influence on temperatures over that period?

There is: we can measure the wavelengths of long-wave radiation leaving the Earth (upward radiation). Satellites have recorded the Earth's outgoing radiation. We can examine the spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.

Change in outgoing radiation

Figure 2: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries et al. 2001).

This time, we see that during the period when temperatures increased the most, emissions of upward radiation have decreased through radiative trapping at exactly the same wavenumbers as they increased for downward radiation. The same greenhouse gases are identified: CO2, methane, ozone and so on.

The Empirical Evidence

As temperatures started to rise, scientists became more and more interested in the cause. Many theories were proposed. All save one have fallen by the wayside, discarded for lack of evidence. One theory alone has stood the test of time, strengthened by experiments.

We have known CO2 absorbs and re-emits longwave radiation, since the days of Foote, Tyndall and Arrhenius in the 19th Century. The theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur.

Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation. Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths.

Last updated on 16 July 2023 by John Mason. View Archives

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Further reading

A good summation of the physics of radiative forcing can be found in V. Ramanathan's Trace-Gas Greenhouse Effect and Global Warming.

Denial101x video


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Comments 426 to 440 out of 440:

  1. devcarr @424... I believe that's referred to as emission altitude or effective emission height.

  2. Thanks. I see it is also called the Effective Radiating Level (ERL). But I can't find information on how much is has increased in altitude.

    I am surprised by the graph showing that temperature rise stops almost immediately when emissions stop. I would think temperature would rise until the Effective Radiating Temperature, at the ERL, caused enough outgoing radiation to balance the incoming radiation from the sun. Does that happen so quickly that it can be ignored?

    Is the ERL very close to the top of the troposphere, below it, or above it?

  3. Devcarr:

    The emission altitude varies with the wavelength of IR light that you examine.  Some IR is emitted from the surface while other wavelengths are emitted from 15 kilometers up.  The average emission altitude is about 10,000 meters which is near the average height of the troposphere.  The top of the troposphere also varies in height.

    The temperature in the troposphere varies according to the lapse rate.  The lapse rate is about 6C per kilometer (this also varies).  Temperatures have increased about 1.2C.   !.2 devided by 6 is .2 km or about 200 meters.  That is a very rough estimate.  I was very surprised when I first learned that the increase in the emission altitude is so small.  (the change is slightly different for different wavelengths of IR light).

  4. devcarr @427,
    I assume your surprise at the graphic @425Response showing the AGW temperature rise halting as soon as emissions stop because your questioning didn't expect the residual warming-yet-to-come to be pretty-much balanced by the reduction in climate forcing. Following the end of emissions, the GHGs are no longer being boosted by those emissions but instead falls as GHGs are naturally drawn out of the atmosphere.

    Perhaps your expected a response to your question @425 to be for the time for warming to end and equilibrium to be reached for a constant level of GHGs. This is of course a different question.

    The radiative imbalance is running at something like 1 Wm^-2 and if that were allowed to play out by keeping GHG levels constant, it would take a century or more before equilibrium is effectively achieved, with ECS=+3.0ºC suggesting an additional +0.8ºC. The time for this is uncertain as it is the longer-term processes that are poorly quantified when the ECS is assessed, and the value of ECS is famously poorly bounded.

    Hansen et al 2011 fig4

    The left-hand graphic here is Fig 4a from Hansen et al (2011) 'Earth’s energy imbalance and implications'. It shows a large part of the warming-to-come appearing in the first decade and that followed by a further large part in the following century. The big uncertainty is in the longer-term warming and this is the major cause of ECS being so poorly defined. And that long-term warming is not entirely a thermodynamic thing. Major longer term warming due in albedo can be caused by a minor and quite insignificant temperature imbalance acting over a long period. So any such long-term warming will result from the warming-already-achieved (+1.0ºC) as well as warming-to-come (+0.8ºC). And so if climate forcing were maintained, this longer term could then become quite significant.

  5. devcarr @427,
    You ask about the altitude of the planet's IR emissions out into space, this ERL as you term it. The physical altitude is dependent on wavelength so there is no single altitude.

    The graph below shows contours of absolute temperature and when these emission altitudes are within the troposphere, the altitude can be inferred from these temperatures. The big 'bite' out of the emissions spectrum at 666 Wavenumber is due to CO2, the smaller one at 1050 Wavenumber is ozone. Note the small central spike within each of these 'bites'. These are caused by the emissions altitude of the central region of these wave bands being so high that they are up in the stratosphere where the temperature starts to rise again. Thus the spike yields a higher emissions temperature than the rest of the 'bite'.
    Being up in the stratosphere means that as the central spike expands with additional CO2, this specific region of the affected wave band will act to cool the planet while other parts of the 'bite' expand to warm the planet. The net effect remains a warming one.

    emissions spectrum from Clive Best

    Significant change in CO2 concentration in the atmosphere will impact the shape of the 'bite' at 666 Wavenumber markedly, as well as kicking off other 'bites' (or perhaps 'nibbles') close to the ozone 'bite'. This is described in Zhong & Haig (2013) 'The greenhouse efect and carbon dioxide'.

  6. Can someone please comment on this article?  Why is overall radiation still constant over last couple decades?

  7. rkcannon,

    If Zoe Phin really thinks she has a better grasp than the Kramer et al (2021) team, she should go hack it in the litterature, I'm sure that GRL would be interested. In the meantime, the Kramer et al study is even more recent than Zoe's blog post, peer-reviewed and published, so it was picked up by the SkS team. It uses the data from CERES, whic includes the ERB instruments. Kramer et al find 0.5 W/sq meter increase just between 2003 and 2018.

    Kramer et al (2021)

    NASA's press release on the paper.


  8. Rkcannon, permit me to add a comment, as a non-expert in these matters.   Measuring the alterations in outgoing IR radiation from Earth, is a matter of measuring a very small quantity against the background of a very large quantity.   Rather like measuring your bodyweight on ordinary bathroom scales ~  with and without wearing your wristwatch.   It is hard to get an accurate assessment of the weight of your wristwatch, even though you repeat the measurements daily over many years!

    (Nevertheless, basic physics and common sense do combine to tell you that the wristwatch has a real positive weight, not a negative or zero weight.)

    Taking a step back and looking at the climate situation :-  over many decades, the observed surface temperature is rising, and the observed Ocean Heat Content is rising, and the observed planetary ice-sheets are melting, and the observed sea-evel is rising.   And these changes are in accord with our understanding of radiational basic physics, too.

    So only a fool (or scoundrel) would assert that Global Warming is not occurring.   (Despite the difficulties inherent in a situation of continual variations and distributions of planet-wide cloud types.)

    Speaking of which :- the NoTricksZone  website has an appalling track record of presenting distorted and/or misleading information.   It is clear that "NTZ"  has a strong agenda of presenting disinformation via misquotes and misinterpeting of scientific papers.   Yes, I am making an ad hominem comment ~  and it is a very well deserved ad hom in the case of NTZ  and its chief editor.   Whenever you see something "scientific" reported on NTZ  website, your own proper skepticism should immediately go to Triple Red Alert overdrive status.

    There are several versions of reporting circulating about an initial study (Kramer et al., 2021).    NTZ's  effort mentions a Zoe Phin, who is IIRC one of these "GreenHouse Effect does not exist" people ~  so again, your skepticism should result in a close examination of what's being put forward.  (Unless you wish to dismiss it all as a huge waste of time for you to investigate.  Just as you do when faced with a complicated "proof" of Flat Earth . . . or a new Perpetual Motion Machine . . . or a complicated screed of mathematics supplied by AGW-deniers like Christopher Monckton.)

  9. I would note that Zoe Phin's credentials are "B.S. Economics, M.S. Quantitative Finance, 10-year Wall Street veteran"

    These are most certainly excellent degrees and experience to have, but they are far from what you would want to rely on for topics like, changes in planetary energy balance and the earth's greenhouse effect.

    When you need an expert in economics and finance, you certainly don't go looking for a phycisist or a climate scientist. I'm not sure why people think the reverse is rational.

  10. Rob Honeyutt @434,

    Even though Zoe Phin's credentials are not that of a climatologist, I would suggest her work providing the basis for the 'article' presented @431 (her work is set out here) is so poor, she should approach her college and ask for her money back. Evidently her education has entirely failed.

    What Zoe Phin manages to show is no more than "Look!! Lots of numbers with decimal points. So I'm right and AGW is fake!!!" Or in her own  baseless words "The greenhouse effect hypothesis is simply incorrect and should be abandoned for the sake of empirical science."

    The first table of numbers presented by Zoe Phin are presumably taken from CERES and shows that high, uppermid & lowermid clouds have increased through the period 2003-19 while low cloud shows no statistical trend. However I don't see any use of this data within the analysis provided.

    The second and third tables (again presumably from CERES but quite where from CERES is a different matter as this is modelled data not measured data) shows annual global average values for surface upward & downward IR and TAO upward for clear-sky/all-sky and with/without areosols. These numbers are not the sort of thing that can be measured globally and further are a ridiculously good fit with GISS annual global average SAT which shows to anybody with half a brain who thought to examine them that the values are modelled numbers not measured numbers.

    The extent of the analysis provided by Zoe Phin is simply to compare the 2003-19 averages of these annual global average IR numbers and declare the upward surface IR do not vary enough between clear-sky/all-sky for clouds to play any role in any greenhouse effect. Thus Zoe declares AGW must be fake.

    The averaged clear-sky/all-sky TOA upward IR numbers vary by 25Wm^-2, enough to provide 20% of a theoretical +33ºC greenhouse effect, not a million miles from what would be anticipated. But the analysis dismisses the relevance of TOA IR. We are told "Less top-of-atmosphere outgoing radiation doesn’t cause surface heating."  So what does less energy leaving the planet do? Where does the energy that is now failing to exit planet Earth go? These are the questions Zoe Phin needs to answer before she continues to make a total fool of herself.

  11. MA Rodger,

    It may be too late for that. Down the comment thread at NTZ she seems to argue against space being a heat sink.

    Other commenters refer to G&T as if it held any value.

    It seems NTZ is every bit as much a waste of time as WUWT.

  12. Is there more continuous data available than the individual year comparisons of change in brightness temperature shown?

  13. Long Knoll:

    Google Scholar lists 233 citations of the source for figure 2 (Harries 2001). You could try looking through those to see if they provide additional data that meets your concerns.

  14. Long Knoll @437,

    There are papers that repeat the spectal comparison for a period of a decade or so as per Harries et al (2001), such as Chen et al (2007)Bantges et al (2016), Rentsch (2020). But measuring outgoing IR is difficult enough without the spectral aspect to the analysis so yearly data would be likely lost in the error bars. Feldman et al (2015), a paper I haven't read, do say they have managed to show a statistically significant result using downwelling surface IR.

  15. Thanks all! Will have a look as soon as I have the time!

  16. The best data on atmospheric transmissivity of Infra-red earth radiation is the data obtained by NASA in 1991.  Unfortunately, this data was buried in an Appendix to an obscure paper and not available to the Scientific community.  I found it, and published it on Amazon as "Infra-red Transmissivity of the Atmosphere--NASA Satallite report".  It is in the public domain, and now can be found by search engines.

    Inter alia, it shows that zero energy escapes the earth in the 16-14 micron wavelength range, the range where CO2 is effective.  Showing that more CO2 can not cause global warming.

    Thus, we must divert the money being wasted on the now disproven assumption to discovering what is actually causing global warming.

    Further analysis of this is in my little book "Carbon Dioxide-Not Guilty".  Confession, I make 6c a copy on kindle because they won't let me price below 99 cents!  For free, email me at and I will send you a pdf copy which you can use as you will, because I put it in the public domain.   

  17. Bobhisey @441 , you really need to explain your statement:

    "zero energy escapes the earth in the 16-14 micron wavelength range"

    ~ is this a massive typographical error by you?  

    Please look at the graph in post #430  [above]  which shows the satellite-measured radiation leaving the Earth i.e. "escaping the earth".   Clearly, the energy escaping the planet at 16-14 micron range is (roughly) half the intensity at 12 or 18 microns for example.  Enormously higher than "zero".

    Or were you meaning to convey something entirely different?  But what?

  18. Bobhisey @441:

    In addition to explaining the graphic that Eclectic has pointed you to in comment 442, you may wish to read the "CO2 is saturated" thread, where much more information is available. Your argument about transmissivity falls into that category.

    And - like so many climate science "critics" that have made this argument - it looks like your "transmissivity" position (as presented here) completely fails to account for the simple fact that CO2 that absorbs radiation in the 14-16um range will also emit radiation in the 14-16um range. What is seen from space (in any wavelength) is the total of what is emitted from the surface and transmitted through the atmosphere (however small that may be) plus what is emitted at every other height in the atmosphere (including the stratosphere) and transmitted.

    All radiation of any specific wavelength is the same. You can't tell where a 14um photon was emitted, or at what temperature the emitting object was. It could have been emitted near the surface at near 288K, or it might have been emitted in the stratosphere at a much colder temperature - and with a much shorter path length (and smaller probability of being absorbed) to space.

    Satellite measurements see it all. To assess where ti comes from, you need to do some detailed modelling. Modelling that shows that adding CO2 does have an effect.

  19. thank you all for your post, I fully support respectfull debate as a way to create your own opinion. I relly concern about global warning and the future of our planet and as technologist i belive that make the right question leads you to the right asnwer. I read a lot about global warning and its consequences nevertherless i am not an expert. Thus, an after reading here posts from 2001. I reached (learnt) a basic conclusion: the key concept is balance between the ratios( IR(14-16um) absortiom / IR (14/16) emission) by a molecule of CO2  from IR comming from the sun(day) and from the earth surface(night) also considering CO2 saturation . right?

    I kindly rise these questions:

    1) IS the IR range the same for absortion/emission (14-16)? what happen with the termal radiation between 8um-14um also emtitted by the earth surface?

    2) Saturated CO2 is not able to absorve but it emits during 12 hours aprox? how much time does a C02 molecula to be satured? I guess is close to 0 seconds, right?

    3) even most of the IR from the sun are under 3um, it seems that there is low percentage in the termal range (8-16un), but considering the global IR emmitted by the sun, it seems termal IR emitted by the sun overpass IR emmitted by the Earth. is this right?

    4) A satured CO2 emits energy a very low pace, right? if we consider it is exposed during the day to sunlight and during the night to the thermal IR. When CO2 emits energy?

    5) someone has made calculations on how much themal IR (earth surace) and how much IR from sun is absorved by CO2?

    6) as CO2 accounts for 0.035% how we can explain their impact in global warning?

    thank you in advance for all your asnwers.

  20. Manuel2001nyc  @444 ,

    The IR thermal energy absorbed by a CO2 molecule is very quickly passed (by impact) to nearby N2 and O2 molecules.  N2 and O2 molecules can give vibrational energy to a CO2 molecule ~ which may then emit a 15-micron IR photon . . . or more probably instead pass vibrational energy back to N2 or O2 molecules.   All this happens over and over again ~ many times per second.   Similar processes happen with H2O and other Greenhouse gasses.

    Therefore "saturation" is unimportant near the Earth's surface ~ but becomes important at very high altitudes in the atmosphere.

    Your 6 questions are far too few for an education in the physics.  You first need to do much reading to understand the complex mechanisms of radiation entering and leaving the planet.

  21. manuel2001nyc...  You've got quite a mishmash of errors embedded into all of your questions here. It seems quite a mess to untangle, but here are a few of examples:

    "...absortiom / IR (14/16) emission) by a molecule of CO2 from IR comming from the sun(day)..."

    Incoming energy from the sun is higher frequency and does not interact with CO2.

    Your questions 1 and 2 are completely indecipherable to me. Maybe others understand what you're trying to get at.

    " seems termal IR emitted by the sun overpass IR emmitted by the Earth. is this right?"

    Here again, you're not understanding that incoming energy is not primarily in the IR spectrum. There is some short wave IR but it doesn't have a lot of effect. CO2 interacts with LWIR (long wave IR).

    Your question 4 is also hard to decipher but also seems to contradict your own statement in question 2.

    "someone has made calculations on how much themal IR (earth surace) and how much IR from sun is absorved by CO2?"

    Same problem here. Incoming energy is in the short wave frequencies, primarily in the visible light spectrum.

    Here's a diagram that may help you:

    "6) as CO2 accounts for 0.035% how we can explain their impact in global warning?"

    Simple answer, yes. This question has been repeatedly asked and answered essentially the same way for well over 150 years.

  22. Yes, manuel2001nyc, there are some major things missing in the questions  you pose in comment #444.

    I'll start by trying to address questions 1 and 2 that Rob Honeycutt skips over, but I'll need to explain a bit of background, first.

    To begin, your description of "saturation" suggests that you think of CO2 as a reservoir of energy that gradually fills up to the point that it can't hold any more. This is not the case.

    • Individual molecules can only store energy at certain levels, dictated by their molecular structure. Electrons "jump" from one energy level to another, absorbing or releasing energy in specific quanta as they "jump". Encyclopedia Britannica has a nice explanation.
    • When the absorption or release of energy occurs via radiation, the photons absorbed or released must have a wavelength that matches the energy quanta - photon energy is uniquely linked to wavelength.
    • This link between molecular structure, energy levels, and photon energy explain why molecules such as CO2 only absorb and emit certain wavelengths or radiation.

    So, for question 1, yes CO2 absorbs and emits at the same wavelengths. And it does not matter whether the IR radiation came from other CO2 molecules or the earth's surface, etc. A 14um photon is just a 14um photon, with no memory of where it came from.

    As for question 2: the term "saturation" makes no sense in this context. Individual molecules do not get "saturated". When they absorb a photon, their energy level increases. They very rapidly pass this energy on to any other gas molecules (CO2 or anything else) they collide with. This will increase the energy levels of those molecules. This will raise the overall average temperature of the air.

    • Note that "temperature" is not a property of individual molecules. It is the average kinetic energy of a very large number of molecules. Temperature will "average out" because those molecules are always colliding with each other and sharing energy.
    • Sometimes, when a molecule such as CO2 obtains energy from a collision with another molecule, it will shed that energy by emitting a photon instead of colliding with a different molecule.
    • The warmer the air, the more often a CO2 molecule will obtain extra energy via collision, and the more often it will emit a photon. This is how temperature affects total radiation energy emission - by emitting more photons, not by emitting more energy in each photon.

    For additional information on the process of absorption and emission, you might benefit from reading this post here at Skeptical Science. In particular, some of the comments talk more about emission. (The original post is more about absorption.)

  23. manuel2001nyc @444,
    You are very dedicated. It is a super-human task attempting to learn from the comments in a long comments-thread such as this.

    I would guess the term "saturation" you used concerns more the saturation as in laser intensity** rather than any climatological/atmospheric consideration. (**That is, an excited CO2 molecule cannot absorb IR until it has returned to its unexcited state. Such consideration doesn't really apply here.)
    As other commenters have explained, 15μm IR can be absorbed by CO2 and the resulting excited CO2 molecules would almost always then be in collision with other atmospheric molecules and lose its excitation in that manner. Thus the absorbed IR energy is converted into thermal energy in the atmosphere. This process is because these collisions with molecules in the atmosphere will occur within microseconds while the relaxation period allowing the CO2 to emit IR is on average in the tenths of seconds.
    But that does not mean there is little 15μm IR emitted by CO2 or that such emissions are rare. As well as taking-away the excitation from a CO2 molecule, these numerous collisions can also impart excitation into CO2 molecules. Thus the vast majority of the excited CO2 is because of these impacts. And if the Earth's surface is at the same temperature as the local atmosphere, the CO2 will be shooting off the same amount of 15μm IR back down at the surface as the surface is shooting upwards. (Note the surface only shoots upwards while the CO2 will shoot both up and down)

    So, to answer yor first question.
    It is a rough rule in physics that the absorption and emission of photons balance, usually. If they don't there will be a net flow of energy, with more/less emissions cooling/heating the substance and thus reducing/increasing emissions. Thus the absorption-emission balance would be achieved.

    The 8μm-14μm waveband is often called the Infrared Window because in a dry atmosphere the IR from Earth's surface has a clear path out into space. Clouds would block that clear path and water vapour acts as a weak absorber.

    There is other IR absorbtion in the 8μm-14μm waveband. There is a strong ozone absorption band at 9.6μm. Also, if we do let CO2 concentrations rise far enough, CO2 absorption bands will appear close to 10μm, these starting to become significant for CO2 concentrations above 1,000ppm.

  24. Please note: the basic version of this rebuttal has been updated on July 16, 2023 and now includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @

    Thanks - the Skeptical Science Team.

  25. Most of the major climate indices (ENSO, AMO, PDO, QBO, AO, SAM, MJO, NAO, SOI) show no signs of AGW, as the characteristic secular trend is missing, The only one that does is the IOD, as the West IOD shows a much larger trend than East. In any case, all the indices can be explained by a tidal mechanism, which should be good news to those that worry that natural climate change has no constraints — tides always revert to a mean of zero =>

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