CO2 in, CO2 out

by Roger Andrews

Some time ago I posted a graph showing how the IPCC’s 21st century temperature projections for the “worst case” RCP85 emissions scenario could be replicated almost exactly using the IPCC’s CO2 radiative forcing estimates for the scenario, a climate sensitivity of 2.2C and nothing else. (Note that I don’t claim this as an original discovery. Others, I think Clive Best is one, have presented similar graphs in the past):

Figure 1

Author note, early am UK time August 28th : Dave Rutledge has pointed out that Figure 1 gives a climate sensitivity of around 3.0C when calculated using the present-day CO2 concentration of ~400 ppm and a 2100 concentration of 936 ppm, the IPCC’s official estimate for RCP85. This is higher than my best-fit estimate of 2.2C because I used 289 ppm CO2 in 1750 as the baseline and the IPCC’s official 2100 estimate of 1,233 ppm CO2 equivalent, which includes other greenhouse gases such as methane and nitrous oxide expressed as CO2.

Although there’s actually nothing very surprising about this result because CO2 is effectively the only input the IPCC’s climate models receive in the 21st century. Only about half of the ~4 degrees C of warming they project under the RCP85 scenario, however, is contributed by the direct radiative impacts of CO2. The other half comes from positive feedbacks generated by the interaction of CO2 with other model variables, so if the models are to predict the right amount of warming it’s important that they correctly quantify these feedbacks.

And do they?

Well, we really haven’t checked. We have enough model output and observational data to say whether they do or not, but nobody seems to have looked at them. So here we will rectify this omission.

There are three basic feedback mechanisms that operate, at least in theory, to enhance the warming impacts of CO2 (IPCC AR4 Section 8.6 discusses them in more detail):

Water vapor, itself a greenhouse gas, will cause a strong positive temperature feedback if temperatures increase and relative humidity stays the same and will cause an appreciable positive feedback even if it decreases slightly.

Clouds: Clouds block sunlight, causing negative feedbacks, and also absorb outgoing longwave radiation, causing positive feedbacks. Whether the net result is positive or negative depends on a number of poorly-understood factors.

Ice and snow: Less ice and snow causes positive feedbacks by lowering the Earth’s albedo and by allowing increased absorption of solar radiation.

Output data for three model variables that simulate humidity, clouds and ice/snow, either directly or indirectly, are available from the CMIP5 (Coupled Model Intercomparison Project Phase 5) model results posted on KNMI Climate Explorer. The three variables are:

Near-surface relative humidity

Precipitation (which since precipitation comes from clouds should be a proxy for clouds – there is no model variable that quantifies clouds per se)

Sea ice area (no snow cover model variable is available)

And the acid test of whether these model variables provide meaningful results is of course whether they adequately hindcast observations. Let’s see how well they do:

Figure 2 compares the CMIP5 near-surface relative humidity multi-model means with observed relative humidity at 1000mb (data from ESRL). Observations show short-term fluctuations of up to a percent and an overall decrease in relative humidity since 1948. The models show a flat line with a few minor wiggles:

Figure 2

Figure 3 compares the CMIP5 global sea ice area means with global (Arctic + Antarctic) sea ice extent data since 1978 obtained from NASA. The model hindcasts diverge significantly from observations in the mid-1990s and after 2007. (The model sea ice area fractions have been converted into ice extent in sq km so they can be compared directly with observations. The conversion will not be exact but should be close enough for comparison purposes.)

Figure 3

Figure 4 compares observed precipitation over land (data from the CRU TS3.21 data set at KNMI) with CMIP5 model precipitation hindcasts over land since 1900. The models show a decrease in rainfall between 1962 and 1992 that isn’t reflected in the observations and also fail to hindcast the observed increase in precipitation after 1940:

Figure 4

It’s possible that some of these mismatches may reflect observational inaccuracies, but as shown in Figure 5 we run into further difficulties when we compare model output for the three variables with CO2 radiative forcings between 1900 and 2100 (estimated from observed CO2 concentrations before 2010 and from the RCP85 CO2-equivalent concentrations after 2010 using w/sq m = 5.35 ln(CO2(2)/CO2(1)). Since there is no way of reconciling the units in these cases the scales are adjusted for an “eyeball” best fit:

Figure 5

Relative humidity shows an overall negative correlation with CO2 forcings in the 20th century but a strong positive correlation in the 21st, which seems implausible (note that the scale is inverted so that CO2 and relative humidity move in the same sense in the 21st century). Model precipitation tracks CO2 forcings after 2000 but not before, which also does not inspire confidence in the model precipitation predictions. Sea ice area shows a consistent inverse relationship with CO2 between 1900 and 2100 (note the inverted scale again) but the model hindcasts can’t be checked before 1978 because there are no global sea ice cover data to compare them against.

What do we conclude from these results? Well, they’re not as bad as they could be – at least the models give absolute values for relative humidity, sea ice cover and precipitation that are close to observed values. But do they hindcast observed trends well enough to allow us to place faith in the projections? Look at it this way. If these were temperature hindcasts, would we accept them as proof that the IPCC’s warming projections for the 21st century were realistic? I don’t think we would.

So here we have another problem with the IPCC’s climate models. Feedbacks roughly double predicted warming in the 21st century, but the model variables that most closely define the amplitude and sense of these feedbacks don’t adequately simulate what is happening in the atmosphere.

But if they don’t simulate what is happening in the atmosphere, what do they simulate? That isn’t hard to answer. They simulate CO2. Figure 6 compares 21st century CMIP5 output for ten RCP85 model output variables with 21st century CO2 radiative forcing inputs, and every one of them increases or decreases in lockstep or near-lockstep with CO2 – even soil moisture content. (Note that scales are again sometimes inverted and that the sea level pressure data are from the CMIP3 a1b emissions scenario used in the IPCC AR4 – the CMIP5 models don’t give usable sea level pressure values.)

Figure 6

Results like these make it difficult to escape the conclusion that the IPCC’s climate models are basically nothing more than CO2 recyclers. We give them CO2 in. They give us CO2 out.

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47 Responses to CO2 in, CO2 out

  1. Phil Chapman says:

    If the model variables supposedly representing the physical conditions leading to feedbacks actually just simulate CO2, then the whole IPCC feedback argument reduces to statements such as “We know that feedback X increases with CO2 because CO2 is increasing.” In other words, the models are no more than very complex examples of petitio principii, the fallacy of assuming the conclusion.

    We see similar circular reasoning in the use of GCMs (which assume a priori that changes in total solar irradiance are the only significant solar forcing) to “prove” that TSI is the only significant solar forcing.

    The IPCC has become The Gang That Can’t Think Straight.

    • It occurred to me some time ago that I could have duplicated the IPCC’s 21st century temperature projections for the four RCP scenarios to within acceptable limits with a hour’s work on a spreadsheet. I would have charged the IPCC only a million bucks for this valuable service, thereby saving them a lot of money. 😉

    • roberto says:

      “The IPCC has become The Gang That Can’t Think Straight.”

      Well said, I concur… in fact I’m reading the book “The Delinquent Teenager Who Was Mistaken for the World’s Top Climate Expert”

      … and the chapter on the “modelers” is quite amusing and entertaining!… it says it all on the accuracy of their results and predictions.



  2. Euan Mearns says:

    Roger, its worth by starting with Murry Salby who reached the same conclusions that you do and that is IPCC models reduce all other variables to constants and hence CO2 is the only variable left to explain ALL climate change. That is why climate scientists are now seeking to explain “record cold” in N America, “record storms” in England and “record snowfall” in Scotland all by global warming. The snowfall thing in Scotland is important since we were told about a decade ago that Scottish skiing would become a thing of the past. Permanent snowfields would become a thing of the past. In fact, ski resorts were closed this winter because of too much snow and the number of summer snow fields is growing fast.

    Murry Salby in Hamburg, 1 hour long video

    The chart below summarises the IPCC view of the world from AR5 earlier this year. It shows huge a growing amounts of anthropogenic climate forcing with virtually zero natural forcing (the tiny band for solar). This is tantamount to the IPCC declaring that natural climate change does not exist and that all climate change has to be explained by humans. This is so far removed from reality it is really difficult to understand how so many scientists can be duped into supporting what is a transparent lie.

    Warmists, professionals and amateurs alike, like to declare that CO2 is a greenhouse gas and must therefore lead to warming. Anyone who does not sign up to this mantra is some kind of science denier. I keep trying to understand the greenhouse and enhanced greenhouse effect but struggle to understand the basic physics. I dare say that those who simply spout that CO2 is a GHG and must lead to warming do not understand the physics at all.

    CO2 absorbs outgoing IR radiation at very specific wavelengths. All (most of) of the IR radiation in these wavelengths is already absorbed by CO2 at pre-industrial levels. Hence adding more CO2 cannot result in more IR radiation being absorbed and the primary process cannot alter Earth’s radiation balance. What raising CO2 concentrations does do is to reduce the height of the column of CO2 in the atmosphere required to absorb all outgoing IR in these specific bands. As I (and Clive Best) understand things this is not the physical process modelled by the IPCC.

    These observations led to a rejection of CO2 greenhouse warming by physicists a century ago. This leaves a residual effect where IR re-emitted by the already saturated layer of CO2 close to surface gets re-absorbed by CO2 higher in the atmosphere. Evidently satellite measurements confirm this, but this is not surface warming and the physics of the whole process is incredibly complex.

    Clive Best: Doubling CO2 and basic physics

    • Murry Salby? Please, try to find someone “little bit” more serious and credible, want want to discuss science here, I hope!


      • Euan Mearns says:

        Roger, its worth by starting with Murry Salby who reached the same conclusions that you do and that is IPCC models reduce all other variables to constants and hence CO2 is the only variable left to explain ALL climate change.

        You need to be careful about engaging in ad hominem arguments where you use someones views on one topic to discredit their views on another. If you want to counter the argument that IPCC climate models ignore all variables other than CO2 ± some other human factors please present the evidence.

      • A C Osborn says:

        Eaun, remember or email discussion about a certain Doctor?
        Well now you have the SS on your case as well, or at least someone quoting SS.
        You are definitely making an impact.

      • Dave Rutledge says:

        Hi Alex,

        “would want to discuss science”

        I would characterize the post you linked to a drive-by hit piece, and not a science discussion.


    • Euan:

      Salby’s main argument is that the increase in atmospheric CO2 was natural rather than anthropogenic – an argument that I happen to disagree with – but whether it was or not doesn’t affect the conclusion that the IPCC models reduce all variables other than CO2 (and other anthropogenic forcings) to constants.

      And as far as the IPCC is concerned there is only one natural variable – total solar irradiance, which is held constant through the 21st century (what they actually do is project the most recent complete solar cycle – cycle 23 – out ad infinitum; for all the impact this has they might as well not bother). Natural ocean cycles that are known to have a major impact on global temperatures, such as the AMO, PDO and ENSO, aren’t even identified as variables. Why not? Because CO2 feedbacks are supposed to induce them, which of course they don’t.

      • Euan Mearns says:

        Roger, I am heading out this evening with a friend for beer and curry. I don’t think we disagree too much over the provenance of some of the elevated CO2 levels today, but there are a number of points to discuss, amongst them the residence time of CO2 in the atmosphere, the origins of approximate co-variance of temperature and CO2 in ices cores, especially Vostok etc. Perhaps tomorrow, or better still in a separate post.

        I started to write a post on the C cycle a while back but hit a brick wall when it came to finding reliable data on deforestation.

        • Euan: The question of where the CO2 came from is something I’ve done a good bit of work on, and yes, it’s a separate issue. Could put up a post if you like, but it would be long and complicated and the subject has already been discussed on other blogs without so far as I can see changing any entrenched positions.

          • Euan Mearns says:

            Roger, I was planning a post called “The Carbon Cycle for Beginners”. Did quite a lot of work on this until hitting a brick wall. Maybe I could lay out the ground work leaving some unanswered questions and you could follow on with the more sophisticated analysis and some answers.

            the subject has already been discussed on other blogs without so far as I can see changing any entrenched positions.

            I don’t think that matters. One primary objective here is to educate, and hopefully a group of readers that don’t read other blogs.

            A question I’d have right now for readers who lean towards warming and accepting IPCC results (or anyone else for that matter): “what is the residence time of anthropogenic CO2 in the atmosphere, and what is the evidence to support this?”

          • Euan Mearns says:

            Here’s an example of some of the stuff I was looking at.

          • roberto says:


            “A question I’d have right now for readers who lean towards warming and accepting IPCC results (or anyone else for that matter): “what is the residence time of anthropogenic CO2 in the atmosphere, and what is the evidence to support this?””

            As far as I know this paper is often cited…


            … “The mean lifetime of anthropogenic CO2 is dominated by the long tail, resulting in a range of 30-35 kyr. ”

            Most of the CO2 is gone in a few 100s years… but there’s a long tail exceeding plutonium’s half-life… 🙂


          • roberto says:

            … forgot the link to another of Archer’s papers, more recent…different journal…


            “We argue that a better shorthand for the lifetime of anthropogenic CO2 would be “hundreds of years plus a significant fraction that changes climate forever”.”


          • Roger Andrews says:

            Hi Euan:

            Your very excellent forest area vs. CO2 graph could be a game-changer. Could you send me the forest numbers please? (I already have CO2). I’d like to play around with them for a bit. Thanks

  3. Willem Post says:

    Another good article. I wonder if the IPCC folks will read it.

    The present smoggy atmosphere is more loaded with small particles than before industrialization. On these particles is condensed water vapor.

    Thus the atmosphere contains much more water than indicated by humidity measurements. This may have a cooling, or warming, or cloud cover effect, but certainly will increase sudden, large rainfalls, as the atmosphere already is “primed for action”.

    • Euan Mearns says:

      Willem, I think this observation may apply to the Far East, but certainly not to Europe or N America where particle emissions are significantly reduced compared to 50 years ago.

      • roberto says:


        … with a nice conclusion:

        “We conclude that while historical changes in the total SSI are likely due to aerosols via direct and indirect effects, future changes in the cloud‐free SSI will be dominated by increases in water vapor concentrations resulting from the water vapor feedback.

        —> While it is widely appreciated that water vapor is a strong and effective greenhouse gas, the impacts upon the solar spectrum are frequently overlooked.” <—–

  4. A C Osborn says:

    The hindcast of CO2 sensitivity almost matches the “Quality Controlled” temp record. But does not hindcast at all the real raw data, which also happens to match the observed climate of those historical days.
    The keepers of the temp record are desperately trying to get the record to match the “Hockey Stick” and the CO2 narrative.

  5. Glen Mcmillian says:

    There are some things that are very likely to add to positive warming feed backs not mentioned here in this article.

    We are stripping the natural vegetative cover off of more land on a continuing basis and replacing it with buildings and asphalt and planting crops on this land.

    A lot of crop land is bare a lot of the year in temperate zones and thus absorbs heat more readily than forested land.

    There is a rise in particulates that needs to be taken into account too. There seems to be good evidence that soot is darkening the snow cover in some places and increasing melt rates are thus to be expected.

    The models most definitely should be updated to take into account the factors mentioned here today insofar as possible.

    I am not professionally qualified to comment on the details of the models being neither a physicist nor a statistician.

    BUT I am a student of politics and history and the general scientific consensus has a very good record of being validated in recent times over a broad range of questions.

    Until such time as NUMEROUS well trained professionals start telling me there is nothing to worry about I will continue to worry. I have spent many hundred of hours cruising the news and articles on the internet and I haven’t found very many people who are properly qualified who think co2 levels are nothing to worry about- after weeding out the ones whose salaries and speaking fees depend on thinking it is nothing to worry about.

    In the meantime it seems wise to keep the precautionary principle in mind.

    This is not to say that I want to shut down the coal fired electricity industry on short notice or even over the medium term of the next fifty years but rather that we need to be working on conservation and efficiency pedal to the metal on a continuous basis.This planet is not capable of supporting the current level of consumption very much longer; we are burning thru our ecological capital rather than living off the interest.

    The people who assured us two decades ago that oil would be cheap for the foreseeable future were wrong by a factor of five about the present day price of oil.Maybe they will be as far wrong about the price of natural gas too. I suspect given growing population and hopefully rising living standards that gas is going to go the same way as oil price wise within the next decade.Except for taxes the wind and the sun are always going to be free and it will not be too much longer before renewables really are cheaper than fossil fuels if I am right.Pushing renewables now will bring the tipping point on costs in favor of renewables by a decade or more.

    Having said this much I fervently hope there really is enough readily available and affordable fossil fuel to support the business as usual economy long enough for more and substantial progress to be made in harvesting and storing renewable energy.Global warming in my estimation is not as big a threat as an energy and resource driven WWIII.

    We are stuck here at the bottom of the gravity well on this planet and even if we had a starship Enterprise we would have nowhere to go with it except sightseeing.

    The regulars here , the authors at least, seem to be engineering types. Would any of you build a big dam to hold back a cubic mile of water if the geologists you consulted warned you that the ground JUST MIGHT NOT be suitable for such a job?

    You are to be commended for bringing some questionable assumptions to light. There is nothing as useful as sunshine and and plenty of ventilation for cleaning up a mess or a mistake or a misunderstanding .

    Gadflies are among the most useful individuals of all humanity.

    • roberto says:

      “In the meantime it seems wise to keep the precautionary principle in mind.”

      No, absolutely NOT! The precautionary principle is good for amusement parks, like “you ride at your own risk”… had it been applied earlier in the evolution of mankind we would all be living in caves.


    • A C Osborn says:

      Glen, I have spent 1000s of hours over the last 10 years looking at the evidence.
      Any time you want to have an off forum discussion, or some pointers on where to look just let me know as I would really like to help you stop worrying.
      But just ask yourself this, if the IPCC is correct, why does none of the current data match up with a single prediction of how Catastrophic Anthropogenic Global Warming has supposed to have affected us all over the last 20 years or so let alone in the future?

      • Glen Mcmillian says:

        Twenty years is merely an eye blink in terms of geological and climatic time. There are many possible natural cycles not yet known or well understood that could account for your argument as well as plain old random variations.

        There seems to be a clear warming trend in ocean waters which could account for a substantial amount of heat energy accumulation without air temperatures rising very much over the last few years.IF this proves to be the case we can expect to see results as this heat is recycled into the air over coming years.

        This is a statistical game and it is well known that there are ” lies, damned lies, and statistics”.You can believe who you please. I will continue to believe who I please while I contemplate the fact that cargo ships are transiting the Arctic at this very minute.Having examined a good bit of evidence that would not pass muster with you but that impresses me nevertheless I believe we are in a long term warming trend and that there is plenty of reason to expect it to continue and to accelerate.

        The basic black box problem is simple enough and I have talked to a number of professionals who have no skin in the game – no grant money, no professional status at stake, working in different areas etc and ALL of them are convinced that warming is a very real problem. All these people are well trained in related or relevant fields.

        We all are subject to unconsciously cherry picking data so as to reinforce our existing beliefs.Now one way I judge whether a person is a serious thinker is to consider his opinions concerning topics which I happen to be very familiar with such as the historical price trends of non renewable mineral resources. Anybody who does not take depletion and the cost of extracting ever poorer quality ores seriously is in my estimation living in la la land.

        Most of the people here in my estimation seem to have their heads buried pretty deep in the sand when it comes to fossil fuel supplies and the risks of depending on them in a world where they in so many cases must be imported and paid for even as they are purchased from historical enemies.The price of oil has risen about five times in the last decade and a half. Now I am not saying that the s

        This does not indicate to me that I should depend on their interpretation of other data sets.

        The price of oil has risen about five times in the last decade and a half. Now I am not saying that the same thing will happen to the price of natural gas…. but it might.

        Burning another billion tons of coal may not in fact result in any warming. BUT it might. And the results could be delayed for a long time.I have friends that have been assured by their physicians that they would be dead in five years or less who are still alive after twenty years.But in the end they will die of the fatal diseases that they are afflicted with unless something else kills them first.

        A quantitative prediction can be off in time but still be right in the end.

        What will be will be because only an idiot could possibly believe that the human species is going to give up coal and oil and natural gas until forced to do so by the fact that these fuels are no longer affordable.

        That time is coming unless I am complete and utter fool. My seat of the pants guess is that oil prices will double again within another decade. Natural gas prices may go up even faster.

        Nobody really knows how much gas can be extracted or at what cost until the wells are drilled but we do know about growing populations and rising demand as the result of rising living standards.

        There does at least appear to be plenty of coal well enough distributed over the planet that we may not fight WWIII over access to it at least not within the next few decades.

        Basically your argument is one that depends on interpretation of the data. If I were a statistician myself and had plenty of time to look into it I could come to my own conclusions without relying to the expertise of others.

        I will continue to worry not that it will do me any good.

        • A C Osborn says:

          “I will continue to believe who I please while I contemplate the fact that cargo ships are transiting the Arctic at this very minute”

          Would you care to name those ships, are you suggesting that they can do so without the Aid of an Ice Breaker?
          As the North West Passage is decidedly not open at the moment, even though it was in the early 1900s.

        • A C Osborn says:

          Twenty years is merely an eye blink in terms of geological and climatic time. There are many possible natural cycles not yet known or well understood that could account for your argument as well as plain old random variations.

          I agree completely.

          So why do you also believe that the only possible cause of the 20 years of warming in the 80s & 90s was caused by CO2?
          Especially as Climate Scientists are now admitting that CO2 is not dominant and hence the “pause”.

    • Phil Chapman says:

      The rapid decline in solar activity in the last couple of cycles suggests that we are already in a “grand solar minimum.” The last such events were the Maunder and Dalton Minima, which were strongly associated wight he coldest periods in the Little Ice Age (LIA). We may or may not be facing a new LIA — but if you believe in the Precautionary Principle, you don’t need to wait until we find out. Since global cooling is much worse than global warming, you MUST do everything you can to promote warming,

      In reality, the Precautionary Principle is nonsensical propaganda designed to persuade the ignorant. You won’t hear much about it any more, now that it demands pumping out as much GHG as possible.

      • Euan Mearns says:

        you MUST do everything you can to promote warming

        Or rather we must do what we can to stay warm. What will perform best during occasional long, very cold winters? Solar PV and wind or nuclear power? I seem to recall last winter in N America that it was coal, gas and nuclear power that kept folks alive.

        For a long time I wanted to write a post called “Energy decline in a freezing climate”, maybe now is the time.

        What will the Sun do next? I think the next solar cycle will be interesting together with the period in between. For readers this is good source of solar data:

        and sunspot / solar activity charts

        this headline caught my attention:

        Small sunspot, big flare

        Could it be that a quiet Sun is more prone to explosive eruptions?

        • roberto says:

          ” I seem to recall last winter in N America that it was coal, gas and nuclear power that kept folks alive.”

          Actually, I’ve read that wind conditions during the long cold spell have been exceptional in US and Canada during the period that you’ve mentioned… but I concur with you that “n” is better as a baseload source.


  6. Sam Taylor says:


    I must admit, I’m somewhat sceptical of the accuracy of the IPCC models for various reasons, but as far as I’m aware, it’s our current best model. In lieu of better data, are we to ignore the warnings of it’s predictions, in the hope that it’s wrong? I’m sure there are plenty of unknown unknowns when it comes to climate modelling, and I doubt it’s an easy business, but we have to make do with what we have at the moment, surely?

    • Sam: I’m glad you asked that question because I don’t think the IPCC models are our current “best models”. There’s another class, called “phenomenological” or “empirical” (a lot easier to say) models that allow for both CO2 forcings and for the ~60 year temperature cyclicity, closely related to ocean cycles such as the PDO and AMO, that’s clearly visible in the global temperature record. The models are constructed by adjusting the natural-anthropogenic weightings until a best fit to observations is achieved and then by projecting the cycles and the CO2 forcings. This procedure is of course not totally robust, but it gives much better model-observed fits than do the IPCC models, and having run a lot of empirical models myself I can confirm that you can’t get a good fit by throwing CO2 out altogether.

      Here’s a comparison between the results of Nicola Scafetta’s latest empirical model and the IPCC projections FYI.

      The peer-reviewed paper it comes from (rather long and complicated I’m afraid) is at:

    • Euan Mearns says:

      Sam, I think as an intelligent species we can do better than to assume that CO2 is the sole driver of climate change and to then spend billions trying to prove our assumption. From what I read, and the work I did with Clive Best on clouds, climate sensitivity to CO2 is less that 1.5˚C. I.e. doubling CO2 from 280 ppm to 560 ppm may result in less than 1.5˚C warming.

      We need to balance the risks, on the other side of the equation, to 7 billion people who are deprived access to cheap and reliable energy upon which Human prosperity is built.

  7. Dave Rutledge says:

    Hi Roger,

    Thanks for a terrific post.

    “at least the models give absolute values for relative humidity, sea ice cover and precipitation that are close to observed values.”

    With the extensive parametrization in the models, I would argue that the absolute values for these quantities and temperature are effectively fits and do not indicate any model skill.


    • Hi Dave:

      I don’t know how much of the good agreement between absolute model values and observations is a result of “tweaking”, but I do know that you only get it when you average lots of models together. Here’s a plot showing precipitation output from each of the 38 individual CMIP5 RCP85 models (annual values) that were averaged together to obtain the mean precipitation values I used. The range in the 20th century was from ~1.8 to ~2.5 mm/day, a difference of 40%.

      • Dave Rutledge says:

        Hi Roger,

        That is an extremely interesting observation. Thank you for an amazing plot.


      • Euan Mearns says:

        Eyeballing this, the real data from 1850 to 2000 is completely flat but folks can’t agree what the observed numbers are? And the forecast data has precipitation rising in the future – in response to rising CO2?

        • Roger Andrews says:

          Euan & Dave: I think what may be happening here is that it’s impossible to stabilize all the model variables at initial state conditions. Temperatures, for example, may be a couple of degrees off, but when the model is tweaked to match them to real-world conditions feedback mechanisms cause other variables – relative humidity, SLP, whatever, to depart from real-world conditions. What we’re seeing is the modelers’ best efforts to match everything at once. I think.

  8. euanmearns says:

    @ Roberto, thanks for the links. The reason for asking the question is that a few weeks ago I came across this article by Phil Chapman who has been commenting here. Phil’s CV is worth checking out.

    I was not aware of the bomb 14C data but have since seen reference to it on WUWT. This seems to be pretty solid evidence that the anthropogenic bomb CO2 had been sequestered in about 20 years and presumably all emissions based CO2 from the bomb test period with it.

    Where’s the catch? How can the IPCC, and all of climate science simply ignore this evidence? Or is there another explanation.

    I know Roger doesn’t agree with all of Phil’s conclusions. But I think there’s and interesting discussion to be had.

    • Sam Taylor says:


      Isn’t the point here not the residence time of any individual CO2 molecule in the atmosphere, which the graph ably demonstrates is of the order of a few decades before something presumably absorbs it, but the residence time of any additional increment of CO2 added to the atmosphere before it returns to some equilibrium value, since it’s this that the IPCC would be more concerned with.

      I fail to see how figuring out how long any particular molecule takes to be absorbed by something, on average, really impacts the debate one way or another?

      • Euanmearns says:

        Sam, the equivalent of 50% of all CO2 emissions have been naturally sequestered, and yet we are told that CO2 emissions will be with us for centuries. Even if we stop burning FF today, the legacy is in the atmosphere, there is nothing we can do about it, the planet is going to melt. But in fact the data suggest that if we do stop burning FF, that the problem (that may or may not exist) will resolve itself in 20 odd years.

        • sam Taylor says:


          I’m not trying to be obtuse but I don’t see how that follows. It just looks like a diffusion. Since the carbon cycle wouldn’t send the same carbon atoms out as went in, presumably.

          Secondly I have to imagine that since all the carbon burned in the intervening years would be millions of years old and devoid of c14 that would also go done way to decreasing the concentration itself.

          I really don’t think this particular line of attack has a great deal of merit.

    • Euan Mearns says:

      Phil Chapman says in his post:

      This result proves beyond any reasonable doubt that the residence time of CO­2 in the atmosphere is much shorter than assumed by the IPCC. This has profound implications for the theory of anthropogenic global warming (AGW), as it indicates that most of the CO2 emitted due to human activities has been absorbed long ago. Calculation shows that the quantity remaining in the atmosphere is less than 300 GT, or 40% of the measured increase in the atmospheric load.

      Since 1965, using BP emissions data and a 15% annual decline (sequestration) rate I get 200 Gt. 15% annual sequestration is what is required to reduce annual emissions to near zero in 20 years as per bomb data.

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