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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)

The mistaken idea that the Greenhouse Effect is 'saturated', that adding more CO2 will have virtually no effect, is based on a simple misunderstanding of how the Greenhouse Effect works.

The myth goes something like this:

  • CO2 absorbs nearly all the Infrared (heat) radiation leaving the Earth's surface that it can absorb. True!
  • Therefore adding more CO2 won't absorb much more IR radiation at the surface. True!
  • Therefore adding more CO2 can't cause more warming. FALSE!!!

Here's why; it ignores the very simplest arithmetic.

If the air is only absorbing heat from the surface then the air should just keep getting hotter and hotter. By now the Earth should be a cinder from all that absorbed heat. But not too surprisingly, it isn't! What are we missing?

The air doesn't just absorb heat, it also loses it as well! The atmosphere isn't just absorbing IR Radiation (heat) from the surface. It is also radiating IR Radiation (heat) to Space. If these two heat flows are in balance, the atmosphere doesn't warm or cool - it stays the same.

Lets think about a simple analogy:

We have a water tank. A pump is adding water to the tank at, perhaps, 100 litres per minute. And an outlet pipe is letting water drain out of the tank at 100 litres per minute. What is happening to the water level in the tank? It is remaining steady because the flows into and out of the tank are the same. In our analogy the pump adding water is the absorption of heat by the atmosphere; the water flowing from the outlet pipe is the heat being radiated out to space. And the volume of water inside the tank is the amount of heat in the atmosphere.

What might we do to increase the water level in the tank?

We might increase the speed of the pump that is adding water to the tank. That would raise the water level. But if the pump is already running at nearly its top speed, I can't add water any faster. That would fit the 'It's Saturated' claim: the pump can't run much faster just as the atmosphere can't absorb the Sun's heat any faster

But what if we restricted the outlet, so that it was harder for water to get out of the tank? The same amount of water is flowing in but less is flowing out. So the water level in the tank will rise. We can change the water level in our tank without changing how much water is flowing in, by changing how much water is flowing out.

water tank

Similarly we can change how much heat there is in the atmosphere by restricting how much heat leaves the atmosphere rather than by increasing how much is being absorbed by the atmosphere.

This is how the Greenhouse Effect works. The Greenhouse gases such as carbon dioxide and water vapour absorb most of the heat radiation leaving the Earth's surface. Then their concentration determines how much heat escapes from the top of the atmosphere to space. It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface.

So how does changing the concentration of a Greenhouse gas change how much heat escapes from the upper atmosphere? As we climb higher in the atmosphere the air gets thinner. There is less of all gases, including the greenhouse gases. Eventually the air becomes thin enough that any heat radiated by the air can escape all the way to Space. How much heat escapes to space from this altitude then depends on how cold the air is at that height. The colder the air, the less heat it radiates.

(OK, I'm Australian so this image appeals to me)

So if we add more greenhouse gases the air needs to be thinner before heat radiation is able to escape to space. So this can only happen higher in the atmosphere. Where it is colder. So the amount of heat escaping is reduced.

By adding greenhouse gases, we force the radiation to space to come from higher, colder air, reducing the flow of radiation to space. And there is still a lot of scope for more greenhouse gases to push 'the action' higher and higher, into colder and colder air, restricting the rate of radiation to space even further.

The Greenhouse Effect isn't even remotely Saturated. Myth Busted!

Basic rebuttal written by dana1981

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 7 July 2015 by pattimer. View Archives

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Further reading

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


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Comments 251 to 300 out of 627:

  1. Stealth, I have replied to your comments about the quality of models on a more appropriate thread:  the counterargument to "Models are Unreliable."

  2. Scaddenp @242: I don’t think I’m confusing models. I think understand the differences between them and their various purposes, at least in a high-level way.

    As for the 5.35 value, I read a paper (which I cannot find now) that was trying to find the CO2 finger print in satellite measurements, and they only looked at clear skies over the Pacific Ocean. I thought it was Harries et al 2001, but it must have been something related. Nevertheless, the forcing value for CO2 has been stated as:

    ΔF = 5.35*ln(C/C0) W/m2

    Is the 5.35 value true for clear skies only, or entire average of the whole atmosphere? Others in this thread have implied that this is the forcing for CO2, which implies all conditions. But the empirical measurements seemed to be done only for clear skies. If it has been measured in clear skied only, then what does the forcing equation look under cloud conditions? If clouds completely absorb all IR in the bands that CO2 can affect, then this would imply that CO2 cannot absorb any more and the forcing function would be zero when there are clouds. If 65% of the earth is covered with clouds then wouldn’t the 5.35 value be multiplied by .35 since only 35% of the sky is clear? This would imply that the value is 1.87. Or, is the value for clear skies really 15.29 and then when it is averaged over the globe it becomes 5.35? I am not making any claims about the value, just asking if this forcing equation for CO2 is valid through clouds.

  3. Stealth - "Is the 5.35 value true for clear skies only, or entire average of the whole atmosphere?"

    Myhre 1998 refers to and builds upon Myhre 1997, which states that "Spatial and temporal variation in the radiative forcing due to variations in temperature, humidity, and cloudiness has been taken into account on the basis of observed data." This supported the rational for (and accuracy of) using multiple atmospheric columns in computing the Myhre 1998 results. So, yes, the CO2 forcing function is for the atmosphere as a whole, not just clear skies. 

  4. As for getting source, I’ve chased down through many of the links provided and yet have been able to get to any source code. I recall doing this a while ago, and getting code is not as easy as you all imply. The NASA GISS ModelE is restricted to those with a IP address. When I try to download one of the ModelE snapshots, I just get an invalid ZIP archive. CESM from NCAR/UCAR only talks about the various versions, but I don’t see any accessible source code. EdGCM 4D appears to be built on top of ModelE and doesn’t provide source code. It looks like a pre built package that one can install and play with, but no source code. The subversion source code repository for CCSM4.0 requires a username and password.

    Well, this is enough wasted time looking for model source code with no luck. I’ll look more at a later time. Have any of you that provided links (most of whom implied that I'm an idiot that can't do a 2 second Google search) actually download any source code for any model?

    But I was stuck by this statement from the Azimuth project (

    “General circulation models used to simulate weather and climate do not operate at fine enough grid resolutions to resolve many observed regional weather and climate features.”

    This comment from the Met Office on the same page is also interesting:

    “Clouds could create positive or negative climate feedbacks and are an ongoing area of research. One example is low-level clouds, especially stratocumulus, which help reflect sunlight and keep the Earth cool. The more stratocumulus we get over the planet, the more cooling effect. If our warming climate creates more low cloud, this would be a negative feedback — helping to offset the heating by reflecting more sunlight away from Earth. If our current climate change means there will be less low cloud overall, then this would be a positive feedback — contributing further to the warming by allowing more sunlight in.”

    The climate models seem to be coarse in resolution, and there seems to be uncertainty as to whether or not clouds are a positive or negative feedback. This seems to be an admission that the models a) do not model clouds well, and b) that they could either warm or cool the planet, and c) induce a lot of doubt about the accuracy of the models. How do you develop so much confidence in the accuracy of the climate models when the model developers make statements like this? They seem to be agreeing with me and saying there is sizable uncertainty.


    [TD] Cloud feedback claimed to be negative is covered here.  But the bottom line of course is how well the models have actually performed, both in hindcasting and in forecasting; that topic is covered here.  If the models perform well enough to tell us what action we should take, then it really doesn't matter whether anyone personally believes that they should perform poorly, nor does it matter how much they might be improved.

  5. Stealth, how about Clear Climate Code?

  6. Stealth @252, from Myhre et al 1998, the mean global clear sky forcing of CO2 in 1994 was 1.78 W/m^2 (model range 1.759-1.8 W/m^2).  The mean global cloudy sky forcing of CO2 was approximately 1.32 W/m^2 (model range 1.313 to 1.37 W/m^2).  The global value for 1994 CO2 concentrations (358 ppmv) using their simple formula is 1.315 W/m^2.  From that I deduce that by "cloudy sky" they meant what would now be called "all sky" conditions.

    Although clouds will reduce the CO2 forcing, only thick clouds with very high tops will eliminate it entirely.  Other than cumulo-nimbus clouds, most low clouds will have very little effect on the CO2 forcing because their tops are well below the altitude at which CO2 radiates to space.  Those clouds still have a greenhouse effect because their absorb and radiate across the entire IR spectrum, including in regions in which well mixed gases do not absorb and radiate.

    In the more recent Lacis et al (2010) (Preprint version) shows the clear sky CO2 greenhouse effect is 26% stronger than the all sky effect.  The single factor addition effect (ie, no overlaps with water vapour) is 29% stronger than the all sky effect.  That compares with about 33% stronger for Myhre et al.

  7. Stealth @254, of course the models have large margins of error.  Have you not seen the error bars on the temperature predictions:

    That does mean we might luck out and bring in a temperature increase of 2 C (the upper limit of what is considered safe) by 2100 even doing nothing; but we equally might get unlucky and see a temperature increase over 5 C.  Further, the two scenarios are not equal, with the increase in damage for each extra C of increase in temperature being highly non-linear.  Therefore rational policy making should weight the unfortunate outcomes more. 

    One thing is very clear, however, even given the wide margins of error.  Not curtailing emissions will see temperature increases to values that have never been seen by Homo sapiens, and which are likely to radically reshape our world.  

  8. Stealth,

    As for getting source, I’ve chased down through many of the links provided and yet have been able to get to any source code. I recall doing this a while ago, and getting code is not as easy as you all imply.

    Step 1: Click on the link I provided to RealClimate.

    Step 2: Click on the very first link in the list, "GISS ModelE".

    Step 3: Scroll down to the section "CMIP3 Model Configuration and Description". Where it says "The frozen version of the ModelE code used for CMIP3 simulations and the controls for model description papers is denoted as ModelE1 (internal version number 3.0, dated Feb. 1, 2004). This code can be freely downloaded (as a 1.2 MB gzip-ed tar file) from modelE1.tar.gz.", click on "modelE1.tar.gz". This is the source code for the version used in AR4.

    Step 4: Un-zip and un-tar the source code from modelE1.tar.gz. I use 7-zip under windows.

    Step 5: Open the "model" folder. Inside you'll find "all the model source code used in all possible configurations".

    There are also instructions for downloading the historical forcings to be used as input. doc/modelE.html gives a brief overview. The NASA GISS page links to papers, etc.

    I hope that wasn't too difficult.

    The climate models seem to be coarse in resolution, and there seems to be uncertainty as to whether or not clouds are a positive or negative feedback.

    Of course they're coarse. They're not trying to forecast weather, they're trying to forecast climate, and they're trying to do so in a reasonable time frame. You can predict that summer will be hotter than winter on average, and Miami will be hotter than Boston on average, without knowing what the exact weather will be on a micro-scale.

    And as Tom points out, those uncertainties are already baked in to the forecasts. But note that the uncertainty about what we're going to do about AGW is larger than the models' uncertainties about what the consequences of each course of action will be.

    Regarding clouds, the reason why we can't be certain whether they will be a positive or negative feedback is precisely because so far it seems to be a wash; the latest results indicate they may be slightly positive. If they were a strong feedback either way then it would be easy to identify. To argue that the models significantly over-state future warming on that basis is to argue that clouds will become signficantly negative in the future despite no evidence to support that — in other words, wishful thinking.

  9. KR @ 248: I get the impression that you do not understand physics; fudge factors are not an accusation of fraud in any way. The comment of “fudge factors” is from Dr. Freeman Dyson, a world renowned physicist -- I am certain he is qualified to speak to both physics and fudge factors. Fudge factors are values that are derived from observation or educated guesses. Often they are fit to curves in order to simplify an equation. As an example, in the CO2 forcing equation, ΔF = 5.35*ln(C/C0) W/m2, the 5.35 value is a “fudge factor”, and so is the natural logarithm function. Over a much larger sample of data, for example, a logarithm with a base of pi may fit better than one with a base of e.

    The “laws of physics” are not something given to us from on high. They are simply a mathematical representation of what we think is happening, or a way to describe how something behaves. The laws of physics (i.e. equations with coefficients) are used to make predictions, not some ordained fact. Your statement of “They are full of physics” as a way to assert truth or correctness of the models, is both meaningless and ignorant.

    Tom Dayton @255: I downloaded CCC, but it appears to be a model to reconstruct the global temperature anomaly from land and sea records in GISTEMP. While I did ask for “any model”, I was really looking for something like a GCM, and (drum roll) I have finally downloaded CMIP3 source code.

    Tom Curtis @ 256: I’m glad you chimed in – I like your responses the best. You provide nice data, graphs, and explanations. Some of the other folks here are a little combative or condescending. That can get me whipped up and want to lash back, and I really try to avoid that because it is not productive.

    Based on your explanation, it seems that the forcing function for CO2 is more complicated than stated, and I fully expected that to be true. The coefficient of 5.35 isn’t static, but a function of the amount of clouds (and probably other things). It may be 5.35 for conditions seen today, but if the amount of clouds changes over time (which they will), then that equation begins to break down. This is what I mean when I say “all models are wrong.” This doesn’t mean they are not useful, but it means they are limited in their effectiveness because of the underlying assumptions. How GCMs handle this effect is critical across every equation they use. My primary concern is that almost every equation in GCMs have various limitations or assumptions because they are calibrated to measurements made recently. How those equations, coefficients, and assumptions hold up over time is critical for accuracy.

    Tom Curtis @257: What model, or models, produced the chart in your example? I see that it is an IPCC chart, so it might be an aggregation of many models -- if so -- do you know which ones?

    If I am reading the chart correctly, even if CO2 emissions were stopped in 2000, then the models predict that temperature will continue to increase at a low rate for far into the future. If this is correct, then I think we’re about to have one heck of a big test relative to solar activity. As everyone knows, the sun is entering an exceptional quite spell, and something that may approach the Maunder Minimum and the Little Ice Age. Many people, and myself included, believe that the sun plays a fairly large role in the climate, whereas many people here and in the peer reviewed literature believe it doesn’t. If global temperature falls in response to the quite sun, and CO2 continues to rise, I think this will be proof that the models and peer reviewed literature are not accurate enough in order to take action about curbing CO2 emissions.

    JasonB @258: I followed your instructions and still failed. Trying to extract the ModelE1 tarball using Windows and WinZIP fails. However, I did go to my Linux machine and tried there using the ‘tar’ command, and succeeded. So now I have actual Fortran code for a 10 year old model. That’s a start, I guess. I fully expected this to be hard, if only because model developers and their organizations tend to be protective of their code due to competitiveness.


    [TD] Facts trump belief, so for the strength of the Sun's influence versus greenhouse gases see the counterargument to "It's the Sun," and note that it has Basic, Intermediate, and Advanced tabbed panes.  For how much effect a Maunder minimum would have, see the Intermediate tabbed pane in the counter to "We're Heading Into an Ice Age."

    Please help keep Skeptical Science comprehensible and therefore useful by putting your comments on the appropriate threads.  Your comments are valuable to other people only if those people who are interested in your topics can find your comments.  Someone interested in the accuracy of GCMs is not going to find your comments on that topic in this "CO2 is Saturated" thread, because they will be looking in the "Models are Unreliable" thread.  Your off-topic comments in this CO2 is Saturated thread make it more difficult for people interested in the saturation topic to find comments that are related to that topic.  Everybody else, please set a good example by replying in the appropriate threads to off-topic comments here.  Stealth, you can see all recent comments regardless of the thread they are in by clicking the "Comments" link in the blue horizontal bar that is at the top of every page.

  10. Stealth @259, thank you for the complement, though I am not sure it is deserved.  On occasion I have been quite acerbic myself.

    Regarding the models, they are just the GCM used for the IPCC AR4, which are listed on table 8.1 of the WG1 report.

    Regarding Clear Climate Code, I believe they are working cooperatively with NASA GISS to produce a python version (from memory) of the GISS Model E.  I am unsure how far that has advanced, and if completed I am unsure who is hosting it.

    Regarding "fudge factors", there is in fact a quite reasonable use of the term in physics as you point out.  If this was just physics, however, nobody would be disputing Michael Mann's Hockey Stick, and nobody would be disputing the existence of global warming.  There are, unfortunately, people whose approach to this debate is entirely governed by politics.  Those people would, and have in the past, take words like "fudge factor" and interpret them out of context to use as just one more weapon in their propaganda war.  Therefore we have to recognize words that could be so used, and be very carefull to use exact words that are not prone to misinterpretation.

    Regarding the Sun, currently many solar physicists are predicting an extended and significant minimum.  Those predictions, from what little I know, have been aound for ten years at most, and are not based on models of solar physics, but on short term correlations.  They may be correct, or we may simply be getting a repeat of the very low solar activity around 1910 (which we have not yet dropped below).  However, it has been shown that a genuine repeat of the Maunder Minimum would only very slightly delay global warming.  That is, with such a minimum and ongoing business as usual, the temperatures we would otherwise have reached in the early 2090s will be reached in 2100.

    Finally, forcing is always defined by reference to a particular year.  Therefore, any change in cloud cover with respect to that year would constitute a feedback rather than a reduction or enhancement of the CO2 forcing.  As it happens, the reference year used by convention is 1750, so any difference in the extent or type of cloud cover between 1994 and 1750 would result in an error in calculating the greenhouse forcing.  Of course, whatever the size of that error, it will be matched by an equal and opposite error in calculating greenhouse component of the cloud feedback.  So, while your point is technically correct if properly stated, it is largely an academic issue about the use of terms.  It is also an unresolvable issue in that we cannot hope to have sufficiently accurate historical records or proxies of cloud cover in 1750 to resolve the point. 


    [TD] Will everyone please help get the conversations onto the right threads, since Stealth is not cooperating but his/her comments and everyone's responses are too useful to delete?

  11. Moderator [TD] @260: My apologies; I will try harder to stay on topic.

    Back to the CO2 forcing equation: ΔF = 5.35*ln(C/C0) W/m2

    I have read the Myhre 1998 paper, and it appears that the 5.35 coefficient was derived from a combination of using LBL, BBM, NBM, and HITRAN models.

    I have read the Schmidt 2010 paper ( and it uses the NASA GISS ModelE as an attempt to arrive at CO2 affect at absorbing IR radiation. The paper also references peer reviewed sources for CO2 effects that are all over the map: Houghton 1990 states water vapor is 60% to 70% of the greenhouse effect and CO2 is 25%. Lindzen 1991 states water vapor is 98% and CO is 2%. The effects of clear sky, clouds, water vapor distribution vary enormously. Schmidt also states that many “cloud treatments tended to be quite simple.”

    It appears that coefficients for the CO2 forcing function equation have been derived from models -- is this true, or have I misread these papers? Are there any empirical measurements made with any land or space based instruments, and if so, how did they separate out CO2 absorption from IR absorption of other gases?

  12. SASM, is this what you're looking for:

  13. Stealth @261:

    1)  You appear to not understand what models are.  They are simply the worked out predictions of scientific hypotheses.  In the case of LBL models, they are the worked out predictions of hypotheses that have been repeatedly tested to very greate detail in laboratories along with more or less accurate estimates of atmospheric temperature, humidity and gas concentration profiles.  The working out is not mathematically complex, although it is tedious.  Nor is it chaotic, so errors in initial conditions will be proportional to errors in the final results.  In that context, saying that an LBL model predicts something is no different from saying that the only well confirmed radiation theory predicts that thing.  It would be like disputing the planned trajectories of space craft as entirely hypothetical because they were worked out on a computer using only Newton's laws of graviation and the known masses and trajectories.  Indeed, more absurd in that Newton's laws of gravitation and motion for more than two bodies and a sufficient time are chaotic.

    2)  Despite that, satellite observations have been used to directly confirm the accuracy of LBL models.  One of the earliest attempts to do so was published in 1970:


    In this case, the data about temperature and humidity profiles was gathered by a radiosonde near simultaneiously with the satellite observation.  The accuracy, even back in 1970, was extraordinary.

    More recently (2008), a comparison between two LBL models and satellite observations was made.  The following is a scatterplot of measured OLR for 134,862 observations between the satellite and the more accurate of the two LBL models:


    And here is a fuller comparison of both models with the satelite:

    Note that the least accurate of the two models is never less accurate than 1.33% error, while the more accurate is never less accurate than 0.33% error.

    Similar comparisons can be found for even broad band models such as Modtran:

    Broadband models use the same well understood physics as LBL models, but use simplifying approximations to reduce the number of calculations needed.

    It should be very clear from this that uncertainty from radiative models is almost entirely from uncertainty about the specific composition of the atmosphere; and that Myhre et al's model based estimate of 3.7 W/m^2 +/-10% is sufficiently justified by the data.

    3)  Your suggestion that estimates of the CO2 contribution appears to be based solely on Lindzen's 1991 throw away comment.  Certainly the 1990 IPCC (Houghton) estimate of 60-70% water vapour and cloud contribution is reasonably close to the 1997 Khiel and Trenberth estimate of 72-79%, which is yet closer to the Schmidt et al 2010 estimate 75%.  That seems like a fairly smooth refinement of a value as methods (and computers) improve, with the initial estimate being by no means unreasonable.

    In contrast Lindzen's "estimate" has no basis.  He does not even provide sufficient information as to how the estimate was done to know whether he was estimating the current increase in radiative forcing of CO2 as a proportion of the total greenhouse effect (as assumed by Schmidt et al), or whether he estimated the relative contributions to back radiation (as I have previously assumed).  In the first case he is, completely without warrant, assuming that H2O and clouds accounted for 100% of the natural greenhouse effect.  In the latter case he is showing a fundamental misunderstanding about the nature of the greenhouse effect (which is about the TOA energy balance).  In neither case is his estimate a scientific estimate because he has not provided the necessary detail for reproducibility.

    What is more, you are showing a throrough inconsistency.  You are calling into doubt detailed and accurate estimates because they are "model based", but are prepared to give Lindzen's "estimate" credence when it appears to be based on no more than a back of the envelope calculation.  Frankly, all Lindzen's estimate shows is that he stopped doing science and became a simple denier far earlier in his career than I had previously estimated.

  14. Stealth, in addition to Tom Curtis's excellent reply to you, see Chris Colose's post "Adding Up the Greenhouse Effect: Attributing the Contributions," being sure to click on his links to coverage of that topic by RealClimate among others.

  15. As Tom Curtis correctly points out, the LBL codes are quite accurate and are direct computations from the spetroscopy, not complex models, confirmed by satellite meaasures. And the primary uncertainty in direct effects is from varying composition of the atmosphere, not the LBL models. 

    However - the uncertainties (+/- 5-10%?) in direct forcing are once again smaller than, and subsumed by, uncertainties in climate sensitivity, which is estimated at 2°C to 4.5°C per CO2 doubling. A high estimate in direct CO2 forcing will lower the CS estimate, and vice versa, as they are related by the total effects on the climate. 

    So: ΔF = 5.35*ln(C/C0) W/m2 is a demonstrably solid estimate for CO2 direct forcing, quite small uncertainties, and incidentally not affected by the computation methods of GCMs. To bring this discussion back to the opening post, CO2 direct forcing is by no means saturated; it has a logarithmic relationship to CO2 concentration. And the total effects on the climate of GHG increases are bounded by climate sensitivity, not the far far smaller uncertainties with line-by-line radiative code. 

    Can we leave this particular dead horse in peace now?

  16. Tom Curtis @263:

    It has been a little while for me. Sorry for responding slowly and appearing to drag out a topic, but I read what you guys send, I do some research, and I think about things a bit. I’m trying not to jump to conclusions and I am really trying to dig into things to get a better understanding.

    1) I think I have a pretty good idea of what models are, why they are built, how they are used, and what many of their weakness are: I have a physics degree and computer science degree and have been software building models for 30 years for aircraft, threats, radars, missiles, various aircraft sensor and weapons, and earth components such as terrain and weather (not climate, but weather as it impacts these systems). I test my models for aircraft with other models, measurement data in labs and anechoic chambers (, tests from poll models ( and actual flight tests. I know from first hand experience that testing is critical and that models and lab tests do not always match very well to what happens in the real world.

    My question @261 was more related to verification and validation of the various LBL models as they relate to the atmosphere. My concern was for the accuracy of the 5.35 coefficient in the CO2 forcing equation. If this value was derived from models without any real world measurements to back it up, then I would be highly suspicious of it.

    I would not dispute Newtonian mechanics for orbital prediction since that is easy physics. The climate, however, is very complex and making accurate predictions is hard. Think more along the lines of trying to predict where a gold ball will land after being hit with a golf club. Let’s say we need 1% or 2% accuracy too. That is wicked hard even if you know the exact initial conditions of the ball when it leaves the club face. You will have to run a complex and iterative numerical-methods simulation that considers many things: winds, ball drag (which is a function of many things), spin on the ball, lift induced by rotating body, and so on -- hard stuff to be accurate. Think about why we have GPS and laser guided bombs (which I know a lot about) – we do not use dumb unguided bombs because they are very inaccurate -- we can model the physics with extreme precision but still cannot hit anything with a dumb bomb. There simply is too much uncertainty in the environment, even on a very small scale. Therefore, we install guidance and targeting systems to compensate for errors and drift as it develops during the bomb delivery. (I also think the iterative growth in errors within GCM is a huge problem, but I’ll get into that over on the models topic).

    2) I dug into Dessler et al 2008 in JOGR, and that was an impressive study and test. That helps me gain confidence that the models are at least in the ball park in terms of accuracy with respect to the real world. Excellent charts, data, and a great study on Dessler’s part.

    3) I was wondering if the Lindzen reference was the same guy at MIT. I wasn’t sure and I wasn’t claiming or supporting it at all. It was just a data point in a list of points about the effects of CO2 and how they varied a lot. Granted, Lindzen was an extreme outlier and I withdraw his data point from my previous point @261.

    I spent a bit of time playing with MODTRAN running numbers on the reduced IR flux from CO2 and cloud effects. Since the chart you reference above shows that MODTRAN matches fairly close to IRIS Satellite data, I think my analysis below should be accurate.

    I found that for no clouds and CO2 at 294.3 ppm the IR flux is 288.97 W/m2. At 800 ppm CO the IR flux is 284.39 W/m2. The ratio of 800 to 294.3 is approximately e, so the natural log of this is very close to1. Using this I compute that coefficient for the CO2 forcing function is 4.58. This is 15% less than Myhre 1998, and his value of 5.35 is 15% less than the previous IPCC value of 6.3. I am sure that Myhre is a good scientist, but it seems that MODTRAN has been shown to be accurate, yet its output disagrees with Myhre. I find the wide range of values from MODTRAN, Myhre, and IPCC over the last decade not very reassuring in that scientists have a solid grip on these values. I know and fully accept that it is a very noisy world, so I fully expect that this value is hard to determine.

    Furthermore, when I run similar numbers using clouds in MODTRAN, I get an IR flux of 261.78 W/m2 for 294.3 ppm CO2, and 258.11 W/m2 at 800 ppm CO2. This yields a CO2 forcing function coefficient of 3.61, which is 21% less than MODTRAN with no clouds, and a full 33% less than Myhre. So while CO2 is not technically fully saturated, its effect is small relative to clouds. The reduced IR flux just from clouds alone is 27 W/m2 when compared to clear sly. Since 60% to 70% of the earth is covered with clouds, it seems logically correct to me that clouds are a major player in IR flux. If doubling CO2 reduces IR flux by 3.7 W/m2, but clouds reduce it by 27 W/m2, then clouds are over 7 times stronger than CO2. A small change in cloud coverage could easily overwhelm the effect of CO2, making the issue of whether or not CO2 is fully saturated really a moot point. Is this not correct?

    Given the magnitude of the effect of clouds, how has climate science determined that most of the global warming since 1950 is most likely due to CO2? That seems like an impossible thing to determine. In 1950 CO2 was about 310 ppm and today it is 400 ppm. Even using Myhre’s CO2 forcing coefficient of 5.35, CO2 has only blocked 1.36 W/m2 in the clear sky portion of the atmosphere. Where there are clouds the effect is even less. Isolating CO2 when so many other moving parts of the earth’s energy balance are also changing seems impossible, especially given we do not have very good data on clouds since before satellites (about 1970). How can the IPCC claim that it is very confident that CO2 has caused most of the recent warming?

    KR @265. The devil is always in the details. The horse is never dead. There is nothing that is *true*. Science can only falsify things and cannot prove something to be true. Some scientific theories are accepted as true only because they have withstood the test of time and have not been shown to be false. I seriously doubt that climate science is any where near achieving this status.


    [DB] You are all over the map here, with much of your comment off-topic for this thread.  Please direct comments towards models to one of the model thread.  Your question about attributions is discussed on a number of threads, summarized here.

    Assuming scientists do not know what they are doing because it disagrees with your preconceptions is simply arguing from your personal ignorance.  Much of what you claim is unknown is actually fairly well-understood by science.

    It would be helpful to adopt a more streamlined approach to both your comment construction, the threads on which you place them and operate with less of a presumption that the science must be wrong.

  17. Please can we make sure that you are not confusing models in the sense of GCM (which would have a complexity considerably larger than say  CFD models for airplane simulation) and LBL models which would be at least an order of magnitude simpler, mathematically and computationally. I cant see why you think the accuracy of 5.35 is so important - how much difference does it make whether it is 5.1 or 5.8? To my mind, this part of the equation pales to insignificance compared to uncertainities in feedback. 

    It also puzzling how you can read and Dessler (read the more recent papers) and yet make a statements about effect of cloud uncertainties. The best we have would put the net effect of clouds about approximately zero. If you have a substantial criticism then perhaps follow up here or here. Note that clouds are both positive and negative. 

    It would also be interesting to know what your alternative hypothesis is. If the obviously change in net forcings is not cause of observed climate, what do you propose instead and what is your case for this making physical sense.

  18. Sorry looking further up, I see you have answered the question about the distinction in models.

  19. Moderator [DB] post to my comment @266: I cover several things in my post, but they covered the three items in Tom Curtis’ post at @263. He was not called out for “being all over the map.”

    He claims in 263:1 that I do not seem to understand what models are, and I show that I build software models for a living. I may not know a lot about LBL models, but I know a lot about software, physics based models, testing models, and so on.

    Then Tom Curtis covers lots of excellent data in 263:2 and the Dessler 2008 paper. This shows that LBL models are accurate in computing reduced IR flux for CO2. This is directly related to whether not CO2 is fully saturated, is it not? I was questioning this because I was concerned about the accuracy of models because Myhre 1998 use models to arrive at the 5.35 coeffcient in the generally accepted CO2 forcing function (ΔF = 5.35*ln(C/C0) W/m2). TC showed support that models (including MODTRAN) are pretty accurate. I believe that now, for the most part.

    In 263:3 he jumped on Lindzen with valid comments, so I with drew my usage of his comment about the effects of CO2.

    Then my longest part is a lot of data about MODTRAN and the reduction of IR flux. MODTRAN has been shown to be accurate by Tom Curtis in 263:2 and Dessler 2008, so I am using its data. My ultimate point is that both Myhre and MODTRAN cannot be correct. The entire topic is about whether no CO2 is fully saturated. This is all about the CO2 forcing function (ΔF = 5.35*ln(C/C0) W/m2) from Myrhe 1998 and the coefficient of 5.35. I run MODTRAN and I get 4.58 in clear skies. In cloudy skies I get a value of 3.61. Most of the planet is cloudy, so the 3.61 value, if MODTRAN is correct, is probably more accurate. These values from MODTRAN disagree with Myrhe 1998. I am not claiming incorrectness on anyone’s part – only showing that they are inconsistent. You guys cannot claim that CO2 is not fully saturated and it has a large effect because of the 5.35 coefficient, then claim that MODTRAN is accurate.

    Does this help clear up my point? And does it show that I am actually on topic?

  20. I've returned to see what discussion followed the questions I asked a while back and have a few comments.

    First, I would like to say that I have enjoyed reading the questions posed by Stealth.  I am six years into a graduate program in physics and appreciate that even some of the most "basic" phenomena can be hard to model.

    That said, I would like to point out that Stealth comes across to me as a true skeptical scientist.  A quick Google of define:skeptical yields:

    1. Not easily convinced; having doubts or reservations.
    2. Relating to the theory that certain knowledge is impossible.

    As I was reading the responses to his questions, I also felt they were needlessly "combative".  Some of them, I think, would certainly qualify as "ad hominem"...

    Whatever.  Just sort of disappoints me when I see the response to a well-thought out question on a website that intends to teach is criticism of the asker.  That said, the parts of the responses that were science I liked.

    Back to the original topic (and my prior question), it is true that the measured outgoing spectra and the atmospheric temperature profile are coupled.  It is also true that, in Harries et al's more recent publication, no significant difference in absorption for CO2 is observed between 1997 and 2003.  The original post here uses the observed difference as definitive proof that CO2 absorption is not saturated.  Should the conclusion be changed to something like "Assuming the validity of the temperature profiles resulting from simulations to match the outgoing spectra, CO2 absorption is not saturated."?

  21. basnappl - " Harries et al's more recent publication, no significant difference in absorption for CO2 is observed between 1997 and 2003"; a reference please? I am not aware of such a paper. 

    Chen and Harries et al 2007, "Spectral signatures of climate change in the Earth’s infrared
    spectrum between 1970 and 2006" clearly indicates the same top of atmosphere (TOA) spectral changes as predicted by LBL models and described in Harries 2001. 

    Speaking for myself, some of the strong responses were due to Stealth dismissing (repeatedly) the accuracy of well supported LBL spectral calculations by inappropriately considering them to have the uncertainties associated with far more complex global circulation models (GCMs). Those are not equivalent, and his comments thereof were unsupportable. 

  22. Hello KR,

    In post 234, I quote Harries 2007 paper (Griggs, J. A., J. E. Harries, 2007: Comparison of Spectrally Resolved Outgoing Longwave Radiation over the Tropical Pacific between 1970 and 2003 Using IRIS, IMG, and AIRS. J. Climate, 20, 3982–4001.)

    Regarding Stealth's "unsupportable" comments:  It seems to me that at least part of his question is to what extent are lab models of radiative transfer representative of what is actually occurring in the atmosphere.  I think this is a fair question.  He did add a perhaps unneccessary bit of opinion but I took that as supplying context for why he might feel compelled to post here at all.  Essentially, I think, he's saying that he has a lot of experience with applying non-climate physics based models to the real world and that this experience, along with Dyson etc's statements and what appears to be variability in a pretty foundational number (CO2 forcing), has led him to be skeptical about climate models.

    It bothers me that his skeptisism wasn't greeted more warmly.  I understand that this is an enormously complex system and it must be tedious for those that know a lot about it to try to bring others up to speed.  I imagine it doesn't help when those asking questions start with the hypothesis that this is all a bunch of hooey (which I don't think Stealth was).  But, that's the way it is with climate science.  There's no denying it's been politicized and lots of people who have no business voicing an opinion do.  That's not (as far as I can tell) what the case was with Stealth.  Bear in mind, the entire purpose of this website is at least partially political.  It wouldn't make much sense for me to make a skepticalscience page regarding high-temperature superconductors even though it's an open, important problem.  This site is, in part, meant to be a political counterbalance to non-science.  I think that's a good thing, but I also think that attacking people is an ineffective way to change their minds.

  23. Reading that paper I see: "In Fig. 8, both the observations (Obs) and simulation
    (NCEP) show a rather flat difference spectrum, close to zero except in the 2
    band of H2 O. This would be expected, since in the short time interval between 1997
    and 2003 little growth of greenhouse gases occurred" Note Obs and simulation showing little difference.

    On the other hand, the paper is yet another (using new instruments) showing that the real atmosphere very much behaves in accordance with the models. Skepticism is good, but it seemed to me in the end, Stealth was determined to misunderstand, more like someone looking desparately for a reason to dismiss climate science rather than understand it.

    The fact remains -the observational evidence from multiple sources supports the contention that LBL models are very accurate. The errors in estimating CO2 forcing from change in GHG are very small compared to errors in estimating feedbacks so I struggle to understand the hang-up.

  24. basnappl - scaddenp has an excellent point; 7 years is a very short time in terms of CO2 increase, and given little GHG change we can but expect little spectral change over such a short period. But _all_ of the Harries papers provide solid support for the accuracy of LBL models. 

    As I noted above, I believe some of the response to Stealth involves the fact that he repeated unsupported assertions of uncertainty in LBL models after and through multiple pointers to the data. IMO - It's one thing to discuss a point in question, another entirely to ignore responses. 

  25. I’m new to this thread, and I confess that I’ve not yet read all the preceding comments. I’m still trying to construct a good mental image of the role of CO2 in global warming.

    According to the article that starts this thread, “Consider the CO2 absorption band around 15 μm (about 650 cm-1), it is strong enough to not let any light go through after a few tens of meters at surface temperature and pressure.”

    Now I believe this statement to be even more generally true because the earth’s blackbody radiation in the entire 750 to 600 cm-1 band is around 3x1022 photons per square meter per second. Furthermore, I estimate that near the earth’s surface there are about 1x1022 CO2 molecules per cubic meter. Thus a few tens of meters of the near-earth atmosphere should be plenty to absorb all photons in the band, not just the central emission at 650 cm-1. The high rate of collision between bending-mode CO2 (v2 CO2*) and the other atmospheric molecules will transfer that vibrational energy to translational energy quickly, thus converting the photon energy to thermal energy. The v2 CO2* population should stay very close to the distribution predicted by Boltzmann statistics for the observed temperature and pressure ranges. Any increase in CO2 concentration within these “few tens of meters” will not lead to additional warming.

    Of course the observed increase in CO2 is not confined to very near the surface of the earth.

    This is where I’m at so far in my mental picture. I want to make sure I’ve got this right before I continue. Any corrections or suggestions would be most appreciated.

  26. MThompson @275, that mental picture is largely correct.  What is missing at this point is that the probability of a CO2 molecule absorbing a photon falls of towards the wings of the absorption band so that near those wings, the distance before nearly all light is absorbed is 100s of meters, or at the very fringes, thousands.

    More important, however, is the next step, which is described here (which, if you have not already covered that material, I recomend you read before going further).

  27. Excuse me, I am but a poor and ignorant engineer.  Also, as MThompson , above, I have not read all the pages.

    so, with that, I venture the following question:

    with ring down IR spectrophotometers with path lenghts of hundreds of meters, would not the premise that "saturation" in the heating of the atmosphere from increasing CO2 occurs..

    also mean that the spectrophotometers might become useless at a % CO2 level which intercepts all their source laser IR energy?

    Which doesn't happen, up to 100% CO2.

    Probably I'm missing something here, your elucidation is solicited.

  28. davidwell, regarding comment 277"

    If a ring-down optical path lengh was say 900 meters, then the pulse duration would be 3 microseconds. If the lifetime of the v2 CO2* state is much longer than that would some probe beam intensity survive even at 100% CO2?  I ask this because I'm guessing that the excited state (v2 CO2*) does not absorb the probe beam. Anyway, I'm just a guy that likes science, and your question is interesting to me. 

  29. I am technically a laymen in the world of climate change, but I have a decent understanding of climate change principles. I am by no means near the level of understanding as all the people posting on here. But I have a quick question that I don't think would require much thought for most of you. 

    I was in a back and forth with someone on a comment board and he brought up the saturation argument. I then sent him the link to this article. He then said that this article contradicts the IPCC:

    "CO2 has reached its reflective saturation limit, we accept that. However, rising CO2 levels cause a rise in relative atmospheric humidity and water vapor is amount the most powerful global warming forces. So CO2 causes higher humidity and that causes global warming. This is the OFFICIAL IPCC explanation that there is a supposed consensus on.

    If you can’t see how your source contradicts the actual explanation the climate scientists give at IPCC and you can’t admit that you had no clue as to the actual explanation, we are done, I have no more time to waste on you."

    I don't see what the contradiction is. Am I missing something? Thanks for any thoughts.


    [JH] You are correct. There are no contradictions between the OP and the IPCC report. The role of water vapor is explained in the SkS rebutal article, Explaining how the water vapor greenhouse effect works.

  30. Dan Smith @279, your respondent is thoroughly confused, having mistaken a response to temperature increases (the water vapour feedback) for the initial cause of temperature increase (the radiative forcing).  The theory that CO2 is saturated, ie, that increases in CO2 in the atmosphere will cause not increase in radiative forcing does, however, contradict the IPCC:

    In fact the IPCC gives the formula for radiative forcing due to a change in concentration of CO2 as being:

    ΔF = 5.35 x ln(C/C0),

    where ΔF is the change in radiative forcing, C is the atmospheric CO2 concentration in part per million by volume (ppmv, or ppm), and C0 is the CO2 concentration in the period to which you are comaring, and ln is the natural logarithm.  The specify this in the supplementary material to chapter 8 (section 8.SM.3), as they have done in the two preceding reports.

    Anybody who thinks CO2 is saturated does not understand the basics of the greenhouse effect.  The change in CO2 concentration reduces IR radiation to space.  Ignoring the stratosphere, any increase in CO2 concentration means the IR radiation to space must come from a higher level, and hence cooler level.   Because the amount that a gas will radiate depends on its temperature, this means it will radiate less to space, and hence the total IR from the Earth to space will decrease.  To restore radiative balance, some change in the atmosphere will need to occur.  As changes in the atmosphere are driven by changes in temperature, and as increasing temperature increase radiation (and hence radiation to space), that will certainly require an increase in temperature.  I have explained this in more detail here.    

  31. Dan Smith @279. Further to Tom Curtis's response I would add that the following statement is incorrect.

    However, rising CO2 levels cause a rise in relative atmospheric humidity .... So CO2 causes higher humidity and that causes global warming.

    There is no physical mechanism that allows CO2 to "suck" more water vapour into the atmosphere except its ability to warm the atmosphere itself. In other words, if the CO2 warming potential is saturated (which it is not) then water vapour concentrations in the atmosphere would not be able to rise.

    This is the OFFICIAL IPCC explanation that there is a supposed consensus on.

    This statement is simply incorrect. I would challenge your respondent to find this explanation in the IPCC reports, available here.

  32. That clarifies things. Thank you all very much for your thorough explanations.

  33. CO2 and similar assymetric species are necessary in order to radiate IR into space, for the same reason that they absorb IR.

    IR frequencies are those of the bond-stretching modes and couple to the EM field via molecular dipoles (and molecular rotation to conserve angular momentum).

    Suppose the upper atmosphere contained only N2 and O2. These molecules' bond motions cannot couple to the EM field [to 1st order] so cannot radiate IR. The heat would be trapped until the temperature rose enough to allow electron modes to radiate.

    But there is CO2. And if the partial pressure of the CO2 is increased, that should provide more opportunities for the upper atmosphere to radiate, and so cool the upper atmosphere.

    We already know that vertical heat transport in the lower atmosphere is dominated by convection [the IR "greenhouse effect is saturated there].

    So doesn't that mean that extra CO2 in the upper atmosphere is an advantage to shedding IR into space?

    I suspect that human changes to land use will turn out to be the dominant anthopogenic contribution to climate change.

  34. rational being - I'm not entirely certain what your question is. CO2 at the top of the atmosphere radiates energy into space in the IR bands, with an effective emission altitude generally defined as where 50% or more of the emission escapes without absorption. That altitude is determined by the total amount of IR absorbing gases above that point in the atmosphere, and is rather directly related to the partial pressure of GHGs.

    As CO2 levels increase, that effective emission altitude increases. Given the lapse rate, an increase in altitude means a decrease in temperature, hence a reduction in the IR energy radiated, an imbalance at the top of the atmosphere. That imbalance will persist as energy accumulates on the surface, warming the entire atmospheric column including the CO2 at the effective radiating altitude, until the energy radiated equals the energy incoming from the sun.

    Summary: Increased GHGs -> increased radiating altitude thus colder radiating gases -> less energy leaves -> climate warms -> upper atmosphere warms as a result -> amount radiated equals amount received. 

  35. rational being @284, the Earth's surface emits IR radiation upward at approximately 390 W/m^2.  Absent IR absorbing molecules in the atmosphere, that IR radiation would radiate to space, making the total IR radiation to space from the Earth 390 W/m^2.

    As it happens, some of that IR radiation is trapped by IR absorptive molecules, which then radiate based on their temperature.  On average, IR radiation from water vapour radiates from an altitude of (very approximately) 4 km.  At that altitude, temperatures are on average 26 K cooler than at the surface due to the lapse rate, so the IR radiation to space from water vapour is at (very approximately) 267 W/m^2.  On average IR radiates to space from 10 km altitude, and hence from a temperature of 213 K.  Consequently its IR radiation is at (very approximately) 116 W/m^2.  

    Combined across all factors, including the IR radiation from cloud tops, the IR radiation from the surface through the atmospheric window, and the differences in altitudes in radiation at different latitudes (along with the differences in surface temperatures), the total IR radiation to space averages at 240 W/m^2.  That is, it very closely matches the incoming solar radiation averaged across the Earth's surface.  Absent the IR active gases, however, it would radiate at the much higher level of the Earth's surface.  That, of course, would create an energy imbalance leading to the rapid cooling of the Earth's surface until outgoing IR radiation matched incoming solar radiation again, with the Earth's average surface temperature near 255 K.

  36. Thank you Tom Curtis and KR for taking the time to reply.

    Are we saying that increasing upper atmosphere CO2 raises the altitude of the tropopause? And is the warming argument, then, that the temperature of the tropopause is fixed by the needs of radiative balance, so that a higher tropopause implies a warmer surface?

    The altitude of the tropopause varies over the globe from around 9km at the poles to almost double in the tropics. I suppose it is where convective heat transport gives way to radiation as the dominant mode. The details are complex enough that I am not sure simple averaging arguments work well.

  37. rational being - That's correct, increased GHGs (not just CO2) raise the tropopause, the slope of the lapse rate remains constant, and the entire atmosphere and surface are warmer as a result of increased effective radiating altitude. There's a fair bit of literature on that (see Google Scholar here), for example Santer et al 2003 states:

    Observations indicate that the height of the tropopause—the boundary between the stratosphere and troposphere—has increased by several hundred meters since 1979. Comparable increases are evident in climate model experiments. The latter show that human-induced changes in ozone and well-mixed greenhouse gases account for 80% of the simulated rise in tropopause height over 1979 –1999. (emphasis added)

  38. rational being @286, the increased warmth of the atmosphere from the increased greenhouse effect does in fact raise the tropopause, but it does so by increased convection due to the surface warming.  The change in radiative forcing would occur whether or not that happened.  I have explained the actual method of warming in greater detail and clarity than I can in a comment here.  I recommend you read it and comment further on that thread if you want to explore the issue in detail.

    I agree that averageing across the Earth's surface creates a multitude of problems.  They are not as large as often imagined, however, because at the effective altitude of radiation to space, temperatures are far more similar over a range of latitudes than they are at the surface (in part because of the higher tropopause in the tropics).  However, the alternative to using globally averaged values is (more or less) to develop a full scale AOGCM, which is a bit much for blog comments.  As it is, observations and AOGCM's show that globally averaged values give good back of the envelope estimates, though not accurate enough for detailed prediction (obviously).

  39. I was attracted to this particular article because I think that the causality case can be made most convincingly from the properties of the Carbon Dioxide molecule itself... especially its absorption spectrum.

    I have got into the practice of screen-recording things as I learn it, so that three to six months from now when I have the opportunity to pick up where I left off, I might remember some of what I was thinking.  


    Here is a short list of where I think I'm still confused.

    •What is the meaning of "Brightness Temperature"  Isn't brightness usually measured in Watts/meter^2?
    •I didn't fully grasp how the "Pressure and Doppler Broadening" but that may have been for lack of time and effort... 
    • What I see, though, is that the absorption coefficient doesn't drop off instantaneously... I would think that any absorption coefficient below 1/(10 km) is going to be NOT saturated.  I think maybe the problem involves a lot more detailed calculus though because in those 10 km to to the top of the troposphere, there are pretty massive changes in the density and pressure, wouldn't there?  
    •According to the graph at LINK (which I used in the video above) it appears that Carbon Dioxide absorbs about 2% or more of the light in a continuous spectrum from 1.5 micrometers to 30 micrometers. 
    Is that graph accurate?  And if so, wouldn't you say that Carbon Dioxide does *not* saturate the spectrum in the wavelengths where it is absorbing 2% of the light?

    Finally, have you thought about trying to put together some kind of quantifiable problem...  Could you give a functional representation of the absorption coefficient of CO2, as a function of wavelength and concentration?

    Then a representation of the power-distribution emitted by the surface of the earth, as a function of wavelength (Planck distribution, yes, I know) 

    Then a calculation of the heat capacity of the atmosphere at large, with 70% nitrogen, 29% oxygen, etc.  

    And maybe a description of whether heat flows at the boundaries of air and water, and between troposphere and stratosphere... What kind of models are used in predicting heat conduction between the layers.  I guess convection between air and water is completely halted, since clearly the water doesn't flow into the air, and the air doesn't flow into the water...  But what about conduction?  Is the phase change just as dramatic between the troposphere and the stratosphere?


    [RH] Hot linked urls.

  40. Jonathan Doolin...  Just want to get something clear first. You're going to throw your lot in with two computer science guys, who have no special training in any of the science that they're commenting on, over that of 30,000+ actively publishing climate researchers, and all the National Academies, and pretty much every scientific organization who has a statement on AGW.

    Have I got that right?

  41. Jonathan, you'll get a response here, but if you're actually interested in going through the maths, save some time by going to SoD.

  42. Johnathan Doolin @289, to begin with, the graph you rely on from Jo Nova incorrectly shows the distribution of IR radiation from the Earth.  To get a better idea of the distribution, here are three satellite observed spectra of outgoing IR radation:

    Units of wavenumber may be unfamiliar to you.  They are a measurement of frequency in terms of number of waves per cm.  For ease of conversion, here is another satellite observed spectrum showing both wave numbers and wavelengths, this time from over Barrow in Alaska, and also showing a simultaneous downward spectrum at the Earth's surface:

    These graphs are drawn such that an equal area under the grap corresponds to an equal total power (in W/m^2) emitted to space at the top of the atmosphere (or in one instance at the bottom of the atmosphere to the Earth's surface).  The large feature at about 666 cm-1 wave number, or 15 micrometers wavelength is the CO2 absorption/emission band.  As you can see, it is displaced in the Jo Nova graph to suggest CO2 absorbs very little outgoing radiation - but from the actual observations above, it is evident that that displacement is (to be far kinder than she deserves) an error.

    As an aside, all five graphs also show the blackbody curves at different temperatues.  The "brightness temperature" is just the absorption spectrum rescaled at different wavelengths such that the black body curves form parralel lines with the x-axis.  It is convenient for some purposes but not for others.

    The most important fact shown in the graphs above is that at atmospheric temperatures, CO2 both absorbs and emits IR radiation at the same wavelengths.  This can be seen in the top three images in the tiny spike of increased radiation from the point of greatest absorptivity by CO2.  Because CO2 absorbs so efficiently at that wavelength, it also emits efficiently.  More importantly, at that precise wavelength, most IR radiation as seen from space looking down comes from the stratosphere, which is warmer than the nearby troposphere, resulting in a peak in net emissions.

    The emissions can also be seen (very obviously) in the downward spectrum at Barrow, where the near surface air is much warmer than the near tropospheric air.  As a result, the emissions seen from space (which can see no further down than the upper troposphere) are very low and much lower than the nearby wavelengths without CO2 absorption where we can see down to the lower 4 kms of the troposphere (H2O band = 400-800 cm-1) or the surface ("atmospheric window" = 800-1000 cm-1), which being warmer emit more intensively.  Seen from the surface, however, all emissions in the CO2 and H2O bands come from the lowest kilometer of the atmosphere and are much warmer than that from the neighbouring atmospheric window (where they effectively come from space).

    Turning to your fog model, it contains three essential errors.  First, at IR wavelengths, CO2 both absorbs and emits radiation.  That is an important disanalogy to your headlights in the fog, for fog will absorb visible light, but not emit it.  Second, early in the you define "saturation" in terms of whether or not headlights can be seen in the fog.  You say, "If you can't see the headlights in front of you at all, that means the light is completely blocked."  But, if you cannot see the headlights at all, then headlights at a shorter distance may well not be blocked.  Absorption is a function of distance.  Third, the theory of the greenhouse effect is a theory of radiative balance relative to space.  You apply your assumptions from the perspective of light leaving the ground, but the proper perspective for the greenhouse effect is that of light escaping to space.

    So, consider a hypothetical case in which a gas that absorbs equally in all frequencies.  That gas will also emit as a black body, and hence emit according to its temperature.  Supose also that the gas is thick enough in the atmosphere as to block all light from the surface.  It cannot, however, at Earth's temperatures block all IR radiation, for it emits some.  The higher in the atmosphere it is, the higher in the atmosphere from which it will emit so that while it may block all sight of the surface, it can never block all IR emission.  So the question becomes, what is the lowest from which you can see an IR beacon on a satellite when looking up?  Because from that same altitude, IR radiation emitted by that gas can escape to space.  Looking from space, you will see a (very tiny) amount of IR radiation from that altitude, and more from higher levels.  If that level is above the surface, the atmosphere is saturated, but that in no way prevents IR radiation from reaching space.  It only prevents it from reaching it from the surface.

    Suppose, that we take an atmosphere containing that gas, which is just saturated.  You can just see the IR beacon from a km above the Earth's surface and from no lower.  Now we double the concentration of the gas.  It follows that the lowest altitude from which we can see the beacon will rise.  Ergo, the IR escaping to space will come from a higher altitude.  But, because temperatures fall with greater altitude, it will have a correspondingly less powerfull emission based on the Stefan Boltzmann law.  As less energy is escaping to space, the result will be a build up in energy stored in the system until radiative balance is restored, ie, the temperature of the levels of the atmosphere from which IR radiation emitted to space rise to match those of the lower levels from which they previously were emitted.

  43. Further, and minor points:

    1)  Absorption is best specified by molar units.  The reason is that the atmosphere becomes less dense as you rise, so that the number of moles in a vertical column that is a meter squared at the base decreases, (ie, absorption per meter decreases with altitude).

    2)  IR can only absorb or emit CO2 at very specific frequencies, based on the natural resonant frequency of the molecular bonds.  That frequency of absorption is blurred by the motion of the particles.  A CO2 molecule moving in the same direction of the light will see the light as being redshifted (longer wavelength) and will consequently be able to absorb light of a slightly shorter wavelength than would normally be the case.  Likewise in reverse.  The doppler effect broadens the effective bandwidth of IR light that can be absorbed.

    Pressure broadening (and collisional) broadening also broadane the effective bandwidth, but the physics involved is above my pay scale.

    3)  You would probably find it instructive to play around with the Modtran model.  (Instructions and source code)  Modtran is a moderate resolution model of atmospheric transmission.  The version in the public domain dates from the late 1980s to early 1990s, and is slightly inaccurate for exact calculation.  It is, however, very informative about basic effects.

    Science of Doom developed his own model along similar lines, describing the process and maths involved in his blog as he did so.  Also very informative.  There are two relevant series of blog posts.

  44. Hello.  I found the graphs from Barrow Alaska very helpful.  

    The graphs from Barrow make it seem that Carbon Dioxide operates like a blackbody in wavelengths near 15 micrometers (667/cm) and is transparent in (most) other wavelengths.

    Looking up, in the 600/cm - 760/cm range, there is roughly 100 milliWatt's per (square meter • steradian • cm^-1).  Looking down, there's only 50.  There is a much higher photon count in that range looking up than there is looking down.  

    I did a little calculation using these numbers; based on the units of the vertical and horizontal parts of the Barrow Alaska graph...  I could draw a little rectangle 100 high and 150/cm wide.  

    This rectangle would have an area of  15,000 milliWatt per (square meter • steradian).  I would multiply by the area of the entire sky in steradians, which is about 6.25.  which comes out to about 93.75 Watts per square meter.


    The graph that I referenced was not directly from joannova, but was from comment #58 at

    which in turn comes from

    Except for the color, this seems identical to the graph here.

    What has been changed is that the infrared spectrum of Earth has been added.  

    Wein's Law says that lambda_peak * Temperature = .0029 meter • Kelvin

    But what temperature should you use?  290 Kelvin yields a peak wavelength around 10 micrometers.  When I did this earlier today, I thought the resilientEarth graph was too far to the left... (Using a temperature near 325 Kelvin, perhaps--like the Sahara.)  However, the Sahara graph has a peak elsewhere, I think... Is Wein's Law an approximation that doesn't work at these temperatures?


    I worked a good portion of the morning making another video, but unfortunately the screen-capture program crashed.  These weren't the only things I addressed but seemed worth mentioning.

  45. Jonathan Doolin @294:

    1) The formula you used is for wavelength.  That is, for a graph with a constant scale per unit wavelength, it shows the point with the highest value by wavelength.  For 320 K, I work out that wavelength to be 9 micrometers, which is equivalent to a wavenumber of 1100 cm-1.  For a graph with a constant scale for units of frequency, however, you should use νmax = 5.879 x 1010 x T, or 18.8*10^12 Hertz.  Converted into wavenumbers, that is 627 cm-1.  

    The reason for the difference is that one unit wavenumber corresponds to more units of wavelength at 627 cm-1 than at 1100 cm-1.  Therefore the area shown under the graph at 627 cm-1 must be divided among more units wavelength.  To retain the same area, it must show a correspondingly lower intensity per unit.

    The graph you originally linked to does actually show a long tail over the 15 micrometer peak absorption band for CO2, so that the upper curve may not be a mistake per se.  The red band, however, is deliberately drawn to exclude that peak even though it lies in the emission band and is fundamentally important.  Further, by using a wavelength scale, the CO2 band is placed on the wings where it is hard to judge its impact.  That impact will in fact be the same no matter whether you use a frequency or wavelength scale, and as can be seen on the frequency scale (wave number) is very important.

    In any event, I do not believe Wein's law to be an approximation, but of necessity it takes different forms for frequency and wavelength.

    2)  I have not repeated your calculation for the Barrow figure, but it sounds like it is in the correct ball park.

    Looking at the downward from space figure, you can see that in the absence of CO2 (and ignoring water vapour and clouds), the radiation to space around 666 cm-1 would follow the black body curve for 268 K, that is over the absorption band for CO2 it would have the same intensity as the downward radiation at the surface (or actually very slightly more).  Therefore, the presence of CO2 at that location has a warming effect of, using your calculation, around 45 W/m^2.  In fact, water vapour would create some of that warming because it does overlap, but at a lower and warme altitude.  Consequently its effect in the absence of CO2 would be less than that of CO2.    Therefore the warming effect of CO2 at that location at that time was probably closer to 20 W/m^2, and is impossible to calculate without a full fledged radiation model.

    It is often noted that water vapour has a greater greenhouse effect than CO2.  That, however, is because it has a lesser effect across a far wider band of frequencies.  In the frequency in which CO2 is active, CO2 has the stronger effect.  (Of course, water vapour only has any effect because the atmosphere is warm enough to evaporate, and without the warming contribution of CO2 that would not be the case, or almost entirely not the case.  Therefore CO2 drives temperatures more than water vapour, even though it has the weaker greenhouse effect.)

  46. Tom,

    Does it make a difference when the spectrum was measured over Barrow?  I cannot do the calculation, but it strikes me that in the winter there would be a different amout of radiation lost to space than during the summer.  In the tropics the radiation would be more constant, but might not give as clear a spectrum.

  47. Evans & Puckrin 2006 might be helpful, Jonathan.

  48. Jonathan Doolin - I think I see some of the issues here; the image you are referring to is, well, a bit misleading in presentation. 

    A more detailed graph shows not just the results of atmospheric absorption on incoming and outgoing radiation, in a more clear fashion shows what would be expected for an Earth without an atmosphere (at the same temperatures):

    Upward and Downward radiation

    Here you can more clearly see the absorption ranges for water vapor and CO2 that result in the more detailed spectra Tom Curtis linked. And see clearly the range of IR reduced by the greenhouse effect. 

    Quite frankly, the rather cartoon representation in the graph you linked gives the (incorrect) impression that the colored band is the sum radiation, wholly unaffected by GHG absorption, when in reality that band region is a complex spectra of what's left after passing through an atmosphere containing those GHGs. I consider your graph less than useful as a result. 

    As noted here and elsewhere, absorption of IR is effectively saturated near the ground at sea-level pressures, with the average absorption path length being quite short (in the order of meters). The more telling altitude is, however, that of effective radiation (roughly where half the radiated IR escapes to space without reabsorption). And that altitude rises as GHG concentrations increase, to cooler altitudes (by the lapse rate) that will radiate less energy. IR escapes across the thermal blackbody spectral range - but at levels determined by the temperature of where it radiates. 

    If you want to play with the math, I would suggest both looking into the freely available copies of MODTRAN as well as reading Myhre et al 1998, where they used line-by-line radiation codes for multiple locations (you cannot get correct global results by looking at a single locations, such as the Barrow atmospheric column) to compute the effective direct forcing change for 2x CO2, which results in the simplified formula:

    ΔF = 5.35*ln(C/C0)

  49. I don't know if Tom or KR has watched Jonathan's video, but I think there's a bit of an issue in how he's approaching the questions he's bringing up. But I don't know enough of the detailed science to address it.

    At one point, while reading through the SkS material above, he comes to a point where the author starts applying a metaphor for how radiative absorption operates. But then Jonathan skips that metaphor and applies his own metaphor, one where he likens visible light to IR and CO2 to fog. In other words, he's trying to think of radiative absorption as similar to car headlights in the fog.

  50. Rob Honeycutt - As I noted in a long ago comment here, you have to be very careful about reasoning from an analogy back towards a complex system that the analogy is trying to explain. In general the mapping from the complex system is only part of that system to one of the analogy relationships (forward mapping), and taking the analogy as 100% identity capable of outlining issues with the complex system (backward mapping) is an error. That's an incorrect use of analogy

    Far better to examine issues in the actual problem domain, as with the Myhre et al 1998 reference above - line by line radiative codes backed by detailed spectral data, later confirmed by satellite observations. Those are not amenable to nor contradicted by simple analogical reasoning and back of the envelope calculations. 

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