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Climate Hustle

What does the full body of evidence tell us about humidity?

What the science says...

To claim that humidity is decreasing requires you ignore a multitude of independent reanalyses that all show increasing humidity. It requires you accept a flawed reanalysis that even its own authors express caution about. It fails to explain how we can have short-term positive feedback and long-term negative feedback. In short, to insist that humidity is decreasing is to neglect the full body of evidence.

Climate Myth...

Humidity is falling
"...the largest of all the positive or temperature-amplifying feedbacks in the UN’s arsenal is the water-vapor feedback. The UN gets this feedback wrong in numerous fundamental ways. For instance, its models tend to treat column absolute humidity as being uniform at all altitudes, when in fact – as Paltridge et al. (2009) have demonstrated recently – the upper troposphere (the only place where adding CO2 to the atmosphere could make any difference to temperature) is considerably drier than the models are tuned to expect." (Christopher Monckton)

Water vapor provides the most powerful feedback in the climate system. When surface temperature warms, this leads to an increase in atmospheric humidity. Because water vapor is a greenhouse gas, the increase in humidity causes additional warming. This positive feedback has the capacity to double the initial surface warming. So when temperatures rise, we expect humidity to also increase. However, one study using weather balloon measurements found decreasing humidity (Paltridge et al 2009). To get to the truth of the matter, the full body of evidence regarding humidity is perused in a new paper Trends in tropospheric humidity from reanalysis systems (Dessler & Davis 2010).

To give an overview of humidity trends, Dessler and David compare the results from Paltridge's 2009 paper to a number of other reanalyses of humidity. Figure 1 shows the trend in specific humidity from 1973 to 2007 over the tropics. The Paltridge reanalysis (thick black line) shows considerable divergence in the upper troposphere, with a strong negative trend while the other reanalyses all give consistent results, both with each other and theoretical expectations.


Figure 1: Various reanalyses showing the trend in specific humidity from 1973 to 2007 in the tropics (Dessler 2010 also looks at the Northern and Southern extra-tropics - only the tropic data is shown here for simplicity and as it shows the greatest contrast between Paltridge 2009 and the other reanalyses).

To gain more insight into the nature of the observed water vapor feedback, Dessler and Davis examine the relationship between humidity and surface temperature. They plot specific humidity directly against surface temperature - this gives a measure of the amount of water vapor feedback. They compare the short-term trend in water vapor feedback (under 10 years) to the long-term trend (greater than 10 years) for the 5 different reanalyses:

Figure 2:  Short-term (a) and Long-term (b) plots of the slopes of the regression between specific humidity and surface temperature, in the tropics. Trends are divided by the average specific humidity over the entire time period, so they are expressed in percent per degree K.

For the short-term trends, all five reanalyses produce consistent results, with surface warming associated with increasing humidity (eg - positive water vapor feedback). However, there is poorer agreement in the long-term trends. The Paltridge 2009 reanalysis is a distinct outlier, with long-term and short-term trends going in opposite directions, unlike the results from the other studies.

This leads to an interesting question: could water vapor feedback be opposite over short and long-term time scales? There is no theory that can explain how short-term feedback could be positive while long-term feedback is negative. The water vapor response to a climate fluctuation with a time scale of a few years (e.g., ENSO) should be about the same as for long-term warming.

Long-term positive feedback is confirmed by several independent sources. An analysis of long-term measurements of upper tropospheric water vapor shows a positive water vapor feedback in 22 years of satellite data (Soden et al 2005). In addition, analysis of long-term paleoclimate records is also inconsistent with a negative long-term water vapor feedback (Köhler et al 2010).

So why does Paltridge 2009 show decreasing humidity? The authors of Paltridge 2009 themselves point out the well-documented problems with radiosonde humidity observations in the upper troposphere. Comparisons of Paltridge 2009 with satellite measurements (NASA’s Atmospheric Infrared Sounder - AIRS) find the Paltridge 2009 reanalysis has large biases in specific humidity in the tropical upper troposphere. Additionally, Paltridge 2009 doesn't show any large increase in specific humidity during the 1998 El Niño. Direct measurements indicate the tropical atmosphere does indeed moisten during El Niño events and such moistening is seen in the other reanalyses.

Two of the newer reanalyses shown in the figures above, MERRA and ECMWF-Interim, correct for well documented biases introduced by changes in the observing system. These newer reanalyses are in better agreement with theory, other reanalyses and independent observations.

To claim that humidity is decreasing requires you ignore a multitude of independent reanalyses, including newer ones with improved algorithms, that all show increasing humidity. It requires you accept a flawed reanalysis that even its own authors express caution about. It fails to explain how we can have short-term positive feedback and long-term negative feedback (indeed there is no known mechanism that can explain it). In short, to insist that humidity is decreasing is to neglect the full body of evidence.

Intermediate rebuttal written by John Cook

Update July 2015:

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

Last updated on 26 October 2016 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 36:

  1. I was reading in Mark Bowen's book Censoring Science about the Vostok ice core. It says,

    "Methane, like carbon dioxide, rose and fell with temperature, whereas dust tended to go the other way. This made sense, as methane and carbon dioxide levels fell and the air cooled, it would have lost water vapor through feedback and become drier and more dusty."

    Yet, I also recall reading in a US Army Southwest Asia (Middle East) manual that as the temperature increases in summer the soil dries. Without water to hold it down dust goes into the air. So in that case warmer air is dustier.

    How can we be sure that increased levels of dust in layers of an ice core indicate lower humidity?
  2. There is a great article on water vapor trends at Science of Doom, today.

    Water Vapor Trends – Part Two
  3. "To claim that humidity is decreasing requires you ignore a multitude of independent reanalyses, including newer ones with improved algorithms, that all show increasing humidity"

    Doesn't changing algorithms that give you what you want to see raise a red flag for anyone? Perhaps they are better algorithms, but the number of times " (-intimations of fraud, impropriety and malfeasance snipped-)".

    Also, (-intimations of fraud, impropriety and malfeasance snipped-).
    Response: [DB] Please review this site's comments policy (link adjacent to the comments box) before making further comments.
  4. Jeff313 @3, you make two substantive claims. The first is that +CO2 - H2O equals no change in greenhouse gases. Apparently this is regardless of the magnitude of the changes in each, as you express no quantities. That is an unusual form of algebra you indulge in, but one I feel no need to follow.

    The second is that "our graphs are misleading", but our graphs are just reprints of the graphs from Desler & Davis 2010. Further, the purpose of Desler and Davis is to compare all of the reanalysis products. Consequently you are complaining that SkS is misleading because they accurately report the data from a study that looks at all reanalysis products.

    Frankly, at this point your comment is not making much sense to me. Bad algebra, and an objection to showing all the data seems to be all the counterargument you can muster.

    You do, of course, claim that all reanalysis products other than NCEP use similar algorithms. Would you care to document that? Or should we add "truth by decree" to your other modes of irrational argument?
  5. Judging from the draft, it seems likely the IPCC will report that there is no trend in atmospheric water vapor.
  6. Doesn't changing algorithms that give you what you want to see raise a red flag for anyone? Perhaps they are better algorithms, but the number of times "

    (-intimations of fraud, impropriety and malfeasance snipped-)".

    I understand the reasons for you assuming that fraud is primary thing implied when I mentioned a "red flag", as it is often brought up in this argument.  However, making clear I am now not talking about anything that could be considered a personal attack, we can not burry our heads in the sand as to human nature and human frailities that is true to all (including myself).  What I am now talking about is mistakes based on a biases that might spring from anything, even frustration arguing points with "contarians" such as myself.  As such I would think that scientists would have to keep a earnest watch for such things.  And when there is a pattern that changes to algorithms seems to always help your points, it should raise a red flag.  Again, not to accuse anyone of anything that I would not be given to myself and it is only made to help other understand secpticism and perhaps a need to double check and, yes question, the accuracy of some work.

    I hope this general topic is not off limits for this board, but to be safe, even though I think it is a valid point that both sides should admit to for honest conversation, I will not mention this topic again.


    [DB] Quite frankly, lacking specific examples in your concerns about "red flags", any discussion by you of uncertainties in your understandings clearly highlights that very point: you lack command and comprehension of what you are trying to discuss.

    Given that, the appropriate next step would be to isolate those areas where you lack understanding and first study more, and to then ask questions when you get stuck. Starting with the assumption, as you do, that since you do not understand something that therefore a likely possibility is that mistakes were made upon the part of the scientists and researchers in this area. This is a fallacy known as the Dunning-Kruger Effect and may also reflect cognitive dissonance and cognitive bias on your part.

  7. Tom curtis

    My concept that "+CO2 - H2O equals no change in greenhouse gases" is a general concept.  The quantities is the debate.  It is in now way bad algebra   If CO2 reduces the amount of another green house gas, the effects are minimized.  This is the same equation (with a negative value) that would be used to say that warming is amplified by increased H2O.  No one would argue that the algebra is bad in that case just because someone did not give the exact quanities of the H20 increase.  My only point was to make clear the importance of a full understaning to the processes in play.

    I would add to this argument that even NASA has estimated that some of the warming over the last century is due to increased solar activity.  I my option, direct warmth added to the sun would of course create an increase in humidity.  So it is no suprise or proof (in and of itself) that increased humidity prove theories that CO2 released into the atmosphere increases H2O.  I only question this because in Chemistry 101 in phase diagrams there are 2 factors that would effect the evaporation of water, heat and pressure.  I have only heard the heat being considered in this process.  Increased pressure would decrease the phase shift.  Quantities are again the debate.

  8. The debate? What debate? Your post is full of confusion and it is hard to figure exactly what you're trying to say. Solar activity is monitored by satellites. CO2 release does not increase the water vapor content of the atmosphere, increased temperature does that, regardless of the forcing. Temperatures have continued to increase as solar activity has decreased.

    Another fact: scentists do their work and normally do not pay attention to the noise coming from contrarians who have clearly no clue. Unfortunately, in some case, that noise becomes such a distraction that they do have to spend time addressing nonsense.

  9. Jeff313 wrote "I would add to this argument that even NASA has estimated that some of the warming over the last century is due to increased solar activity."

    You do know that the IPCC AR4 WG1 scientific basis report explicitly states the much the same thing?  However, that is for the warming of the first half of the 20th century, thus it doesn't support your hypothesis as since then humidity has been rising, whilst solar forcing has been steady or declining slightly.

  10. Jeff313 @7, thankyou for admiting that your formula was merely sloganeering, not analysis.  Once, however, you admit that the effect depends on the values, unless you analyse the actual values and the effect they have, which you have not done, you are unable to say anything regarding the net effect.  This is particularly the case given that increases water vapour provides both a positive (the WV feedback) and negative (the lapse rate feedback) feedbacks.  If WV is increased at medium altitudes, but decreased at high altitudes, that can have the effect of neutralizing the lapse rate feedback, while strengthening the water vapour feedback.  Ergo, your slogan is not valid even as a "general statement".

  11. (-multiple off-topic and inflammatory snipped-)


    [DB] Discussions of temperatures, temperature acquisition methods, siting issues, models, etc need to be placed individually on the most appropriate discussion threads...AFTER first reading the OP of that thread AND each and every comment on those threads to best determine if your issue/question has already been addressed and resolved. THIS thread is about the discussion of the effects of humidity in the climate system and what the science has to say about it. Period.

    Please use the Search function located in the upper left section of every page to search for the most appropriate thread for your comments. The Taxonomic listing is also very valuable, providing insight into the nature of the structure of skeptic arguments. Also, please re-read this site's Comments Policy and then ensure your comments fully comply with it.

  12. Jeff313 This article is concerned with humidity, it appears that you no longer want to discuss that topic.  If this is the case, please find a more appropriate thread for your discussion, and please avoid "gish gallops" where a number of different topics are introduced all at once.  All that achieves is to disrupt the discussion, so that none of the topics can be dealt with in detail.

  13. You're citing Steven Goddard? Seriously?The Steve Goddard of Antarctic carbonic snow fame, who covered himself and WUWT with so much ridicule that Watts eventually had to distance himself from him? The guy who couldn't grasp the message contained in the phase diagram of CO2?  The Steve Goddard who averages percent of snow cover without area weghing so he can come up with ridiculous numbers? The same guy who is now arguing that the Arctic is seeing an unprecedented ice gain (one of his funniest yet)? That's you source? You trust it?

    The precision that can be derived from thousands of measurements gathered from thousands of sources is much greater than the precision of one individual source. Research that before jumping to conclusion. Why would you believe that the scientist studying this don't do their homework?

    Instead of gish galloping across areas of which you seem to have limited understanding, why not stick to the subject. Earlier you made an argument that appeared to suggest that CO2 "displaces" H2O as a greenhouse gas. That is quite new and exotic, you should elaborate on that with scientific references (Steve Goddard does not qualify as such).

    Response: [DB] Jeff313 has been counseled to find more appropriate threads to place his concerns, as made more fully compliant with the Comments Policy, on more appropriate threads than this.
  14. dwm:  The NASA press release you quoted was written in 2004--ten years ago, and one of the authors was Dessler, whose more recent work is cited in the orginal post at the top of this thread you are reading now.

  15. The information I quoted is on the NASA website.  Are you suggesting that the research has been "done" now on humidity levels and the issue is completely understood and settled?

    As you know, humidity is technically hard to measure accurately at all alititudes for the whole world, and there is not general certainty about what those levels are, or what they have been historically.  It is a very weak spot of most current climate models.

  16. dwm @15.

    The information you quoted on the NASA website (linked @14) is titled:-

    Humidity Relative to Earth's Temperature                         07.20.04

    It would perhaps have been better if it said 20th July 2004 but that probably would not make its message less of a honeypot for skeptical argument as its presence on the NASA website could be argued to show continued relevance.

    The UN IPCC Assessment Reports stand as a pretty definitive account of the present science. If you are having difficulty with the content of this SkS post, I woud thus recommend you read AR5 Chapter 2 Section 2.5.5 pp206-8.

  17. dwm:  I regret to inform you that your tesseract is malfunctioning.  In this timeline that the rest of us inhabit, and that you have dropped into, the year 2004 is earlier, not later, than the year 2010.  Research on humidity levels has indeed continued, but in the time direction of 2004 forward to 2014, not the reverse. Therefore the newer results of Dr. Dessler and others were reported in their papers published in 2010 as summarized in the orginal post at the top of this thread.

    By the way, which manifestation of The Doctor are you?  My favorite is the one with the curly red hair.

  18. Personal insults aside, I'll repeat since you tried dodging it:

    Are you suggesting that the research has been "done" now on humidity levels and the issue is completely understood and settled?  As you know, humidity is technically hard to measure accurately at all alititudes for the whole world, and there is not general certainty about what those levels are, or what they have been historically. It is a very weak spot of most current climate models.


    [JH] Please document the source of your assertion that humidity is "a very weak spot of most current climate models."

  19. Research is never "done" but what the latest published papers referenced here show that with the best of observation and reanalysis so far, humidity levels are not falling and furthermore the reasons for the problem with Paltridge 2009 is understood. The evidence available shows humidity is rising.

  20. From the IPCC AR5, Ch. 2, section Reanalyses, p. 208:

    In summary, radiosonde, GPS and satellite observations of tropospheric water vapour indicate very likely increases at near global scales since the 1970s occurring at a rate that is generally consistent with the Clausius-Clapeyron relation (about 7% per degree Celsius) and the observed increase in atmospheric temperature. Significant trends in tropospheric relative humidity at large spatial scales have not been observed, with the exception of near-surface air over land where relative humidity has decreased in recent years (Section 2.5.5).

  21. For all those wanting to engage dwm in his continued trolling, please note that when he says "the issue is completely understood and settled" (my bolding) he is engaging in step 4 of The 5 characteristics of scientific denialism (Impossible expectations of what research can deliver).

    It's the old "if you don't know everything, you know nothing" gambit.

  22. "It's the old "if you don't know everything, you know nothing" gambit."  Bizarrely, such a 'gambit' usually stands as the corollary of "I understand everything that is known and totally disagree with your position. Therefore..."

  23. JH and others:  as I said, we are years away from having reliable humidity data for the various levels of the atmosphere, and moreover, as humidity is not uniform, we understand very little of the complex interactions of having different humidities in different layers of the atmosphere and in different regions of the earth.  The facts that we don't have the data, and that we don't understand the complexities of the interactions, don't allow you to indulge in the weak defense of "It's the old "if you don't know everything, you know nothing" gambit." 

    If that is not the case, please tell me where I can find the data showing humidity levels of all layers of the earth's atmosphere for the different regions, tropic, sub-tropic..., for the past century, or heck, how about just the past 20 years.

    Without a very good idea of what humidity leves have been, climate models are left in the "best guess" scenario, and since water vapor is the most important greenhouse gas, their reliability is very much in question.


    [JH] Your personal opinions matter little on this website. If you cannot document your assertions, merely repeating them constitutes sloganeering- which is prohibited by the SkS Comment Policy.

  24. dwm - are you seriously suggesting that Clausius-Clapeyron relation doesnt hold in the atmosphere? Furthermore water vapour are constrained by OLR measurement. What is your estimate (I would love to know your source...) that the error range in the humidity estimates invalidate model estimates.

    Next question, suppose for whatever reason you decide that you dont trust models are the best predictors of future climate, and yet the physics of AGW is not in doubt. What predictor of future climate do you suggest policy makers use which you think has better skill than climate models?

    Uncertainity cuts both ways. Suppose the errors are such that warming ends up much faster than predicted? The precautionary principle would seem to apply.

  25. scaddenp, I pointed out that we are all still waiting for a good source (of a data record), and I asked for a good source if you have one, so why are you asking me for my source?

    This is getting off topic, but since you asked.. I don't disagree with the precautionary principle. The problem lies with how you define precautionary.  For instance, I am alarmed by those who advocate for pro-active measures such as geoengineering based on climate science which is, in my opinion, still in its infant stage. 

  26. dwm, for data you've got the references in the original post (in particular, the AIRS instrument on AQUA), plus two people pointing you to AR5, plus an early comment pointing to Science of Doom (which now has a Part 7).  It is necessary for you to click and read, and (horrors!) sometimes then click and read those sources' cited sources.

  27. dwm:

    Please note that posting comments here at SkS is a privilege, not a right. This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive or off-topic posts. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site. Finally, please understand that moderation policies are not open for discussion. If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing.

    Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.

  28. In terms of quantifying feedback, there are well-acknowledged uncertainities in what the value of climate sensitivity is. However, you have asserted " we understand very little of the complex interactions of having different humidities in different layers of the atmosphere and in different regions of the earth"  and I cant find backing for this in science that I am aware of. 

    Geoengineering is discussed as only as method of last resort if humanity doesnt do the obvious step - reduce emissions. Reducing emissions is safe, since it takes us to takes us back to known state. Since you accept the precautionary principle, I assume you are good with that.

  29. Hi Tom,(-snip-)

    I am familiar with the water vapor articles on the Doom site, but I hadn't read the latest, thank you for that.

    Looking it over, I found several qualifying statements in the article that basically agree with the opinions I have been posting.  You seem to have missed them so here they are:

    This quote from Doom corroborates exactly what I said (in bold):
    "A major problem with analyzing UTWV is that most historic measurements are poor for this region. The upper troposphere is very cold and very dry – two issues that cause significant problems for radiosondes."
    This quote from Doom also agrees with what I wrote, water vapour is a critical issue (hence potential weak spot), and it is massively complex (hence hard to calculate and easy to get wrong, exhaustive study is necessary before having confidence):
    "The question of how water vapor responds to increasing surface temperature is a critical one in climate research.
    vapor concentration in the free troposphere is dependent on the global circulation, making it dependent on the massive complexity of atmospheric dynamics."
    This quote from Doom concedes it is "very possible" that climate models get it "wrong" when he writes that
    some people may “acknowledge that climate models attempt to calculate humidity from some kind of physics but believe that these climate models get it wrong. That is of course very possible."

    Gettelman & Fu concede that their short (and scattered) data sample by itself is “not sufficient”, or in other words, only one step in a long road yet ahead before we can conclude with confidence that the models are accurate:
    "The hypothesis we seek to test is whether water vapor in the model responds to changes in surface temperatures in a manner similar to the observations. This can be viewed as a necessary but not sufficient condition for the model to reproduce the upper-tropospheric water vapor feedback caused by external forcings such as anthropogenic greenhouse gas emissions."


    [Dikran Marsupial] Moderation complaints snipped.  Moderation complaints are by definition off-topic and will be deleted (after reading).  I have snipped the moderation complaint this time, but in future the whole comment will be deleted as moderators do not have the time to edit posts.  If you want to make a substantive point, leave the moderation complaints out of it.  Please read the comments policy and abide by it, compliance is non-negotiable.

  30. I meant to mention, Tom, that I’m glad you brought up AQUA. Here are a couple of statements by Dr. Spencer’s who is the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer flying on NASA’s Aqua satellite.

    "we do not have enough accurate global data for a long enough period of time to see whether there are natural warming mechanisms at work”

    "the climate system is quite insensitive to humanity’s greenhouse gas emissions and aerosol pollution."

    He has a website if you'd be open to checking out an alternative viewpoint.

  31. dwm - firstly I note that you are reading links, which is good, but to defend your position why do you need to do so? I would assume that you would adopt a position on the basis of papers you have read or had least been reported on but so far you havent shown us what these were.

    Second, it is a bit of a stunning leap to jump from the bolded quote to your conclusions. In particular, why are historical values of any importance to climate models? Also, you seem to have missed the actual conclusion of the Gentleman and Fu paper. The important point is that if climate models had it badly wrong, then they would not be modelling OLR which is sum of those processes.


    For Spencer, I have responded to you here.

  32. dwm:  I strongly suspect that Bob Loblaw is correct about your intention.  But I'll try one more time:  You are incorrect in expecting scientists to say that they are absolutely certain about anything.  One of the criteria for writing a good scientific paper for peer-review and then publication is to always describe what further research should be done; usually that means describing ways in which your own research that you have described in this paper fails to answer all the questions anyone might have.  Such admissions of shortcomings are not appropriately interpreted as implying that the currently reported research is inadequate for drawing any conclusions.  But that, as Bob pointed out, is what you are doing.  The question always is what conclusions are adequately supportable by this research.  As scaddenp pointed out, Gentleman and Fu concluded their research was sufficient for the purpose of modeling OLR.  Similarly, the Science of Doom blog author concluded:

    Still, that’s a different story from acknowledging that climate models attempt to calculate humidity from some kind of physics but believing that these climate models get it wrong. That is of course very possible.

    At least from this paper we can see that over this short time period, not subject to strong ENSO fluctuations or significant climate change, the satellite date shows upper tropospheric humidity increasing with surface temperature. And the CAM model produces similar results.

    A broader survey of the literature was done by the IPCC.  Their conclusion that I quoted to you earlier is that the empirical evidence is more than sufficient.  Your opinion to the contrary is meaningless unless you can cite specific, concrete reasons for the empirical evidence being insufficient for the purpose to which the IPCC is using it.

  33. My understanding is that specific (or absolute? I always get those confused) humidity is increasing.  Because this would neccessarily capture heat, the atmosphere should warm — thereby increasing its capacity to absorb water.  This increase in capacity would result in a lower measurement of relative humidity.  You can see then how an H2O positive feedback loop would operate, even in the absence of CO2.  I don't personally have any instruments to measure it!  The prediction would be rising temperatures, rising absolute (or specific?  U guys figure it out) humidity and decreasing relative humity as the planet goes to hell.

  34. Specific humidity is increasing, relative humidity is expected to be approximately constant. The processes are somewhat complex however and are discussed in considerable detail with measurements in sections 2.5.4 and 2.5.5 of the AR5 WG1.

  35. Technical Question about Water Vapor Data:

     SpectralCalc has in their Atmospheric Browser section, data on the concentration of water vapor in the atmosphere versus altitude in terms of volume molecular ratio. SpectralCalc uses the 1976  U.S. Standard Atmosphere for this APP. These data can also be plotted graphically and the resulting graph is quite like the corresponding graph in Modtran Infrared Light in the Atmosphere (MILA). Since SpectralCalc states their input is from the 1976 U.S. Standard Atmosphere I suspect the same is true for MILA. 

     For CO2 the present day concentration is 400 ppm whereas in 1976 it was 330 PPM and therefore, to do a SpectralCalc calculation involving present day CO2 one uses a CO2 scale factor of 1.212 instead of one.

     What scale factor relative to the 1976 value should one use, then, for water vapor?

      But water vapor is not a well mixed gas as is CO2 and the measurement would be more complex. Nevertheless: 

      Why don't they - whoever "they" are - "just" go ahead repeat all the 1976 measurements in the present day...(Did they use mostly weather ballons in 1976 ?) compare, by the same methods in the same locations, the1976 average water vapor volume molecular ratios to present day volume molecular ratios?

     That would seem like a good way to quantify the changes between water vapor concentration in 1976 versus now.

  36. curiousd,  for Modtran using the default tropical setting, at 0 Km altitude in the second section on "atmospheric profile" it shows RH, which I take to be relative humidity.  In the third section under H2O it gives a value of 1.90E+01, unit not specified.  The value for 0 Km under H2O changes to 5.89E+00 for the US Standard Atmosphere, and to 6.24E+00 in the US Standard Atmosphere with a temperature offset of +1 C provided you have the model set to Hold Fixed  "relative humidity" rather than "water vapor pressure".  I have not gone through all of the standard settings with and without constant relative humidity, but it would not take a great effort to do so.

    Clearly with this function, if you offset the surface temperature by the difference between 1976 and today, holding fixed relative humidity in the UChicago version of Modtran, you would automatically adjust for the change in water vapour pressure as well.

    This does create a slight problem if you are trying to calculate radiative forcings, which are the difference in upwelling IR radiation at the tropopause after the stratosphere has reached radiative equilibrium, but before the troposphere has had any feedbacks.  The latter clause means without andy adjustment in H2O vapour presssure.  Technically that means if you are calculating the radiative forcing between 280 ppmv and 400 ppmv the model would need to be set for the relative humidity at an equilibrium temperature for 280 ppmv, and retain a constant water vapour pressure when calculating the the radiative forcing outgoing IR radiation at 400 ppmv.  That in turn would require knowing the offset in temperature from 1976 to the temperature equilibrium.  In practise, and in the absense of historical data (which we probably lack on a global scale for when the when the CO2 level was 280 ppmv), it means assuming a climate sensitivity factor (ie, a temperature change at equilibrium for a given change in radiative forcing) and making successive approximations on the temperature offset.  It also means that the radiative forcing for an increase in CO2 from 280 ppmv to 400 ppmv would be slightly different to that from a decrease from 400 ppmv to 280 ppmv due to the different base H2O vapour pressure.  For small changes in CO2 the difference should be small enough in practise that it can be ignored.

    I should note that there exists a technique for adjusting for stratospheric equilibrium in calculating the strict radiative forcing, which I have seen explained by David Archer.  Unfortunately, I remember neither the explanation, nor the page on which it was located, so I cannot help you with that.  I mention it, however, incase you want to follow it up. 

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