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How reliable are climate models?

What the science says...

Select a level... Basic Intermediate

Models successfully reproduce temperatures since 1900 globally, by land, in the air and the ocean.

Climate Myth...

Models are unreliable
"[Models] are full of fudge factors that are fitted to the existing climate, so the models more or less agree with the observed data. But there is no reason to believe that the same fudge factors would give the right behaviour in a world with different chemistry, for example in a world with increased CO2 in the atmosphere."  (Freeman Dyson)

Climate models are mathematical representations of the interactions between the atmosphere, oceans, land surface, ice – and the sun. This is clearly a very complex task, so models are built to estimate trends rather than events. For example, a climate model can tell you it will be cold in winter, but it can’t tell you what the temperature will be on a specific day – that’s weather forecasting. Climate trends are weather, averaged out over time - usually 30 years. Trends are important because they eliminate - or "smooth out" - single events that may be extreme, but quite rare.

Climate models have to be tested to find out if they work. We can’t wait for 30 years to see if a model is any good or not; models are tested against the past, against what we know happened. If a model can correctly predict trends from a starting point somewhere in the past, we could expect it to predict with reasonable certainty what might happen in the future.

So all models are first tested in a process called Hindcasting. The models used to predict future global warming can accurately map past climate changes. If they get the past right, there is no reason to think their predictions would be wrong. Testing models against the existing instrumental record suggested CO2 must cause global warming, because the models could not simulate what had already happened unless the extra CO2 was added to the model. All other known forcings are adequate in explaining temperature variations prior to the rise in temperature over the last thirty years, while none of them are capable of explaining the rise in the past thirty years.  CO2 does explain that rise, and explains it completely without any need for additional, as yet unknown forcings.

Where models have been running for sufficient time, they have also been proved to make accurate predictions. For example, the eruption of Mt. Pinatubo allowed modellers to test the accuracy of models by feeding in the data about the eruption. The models successfully predicted the climatic response after the eruption. Models also correctly predicted other effects subsequently confirmed by observation, including greater warming in the Arctic and over land, greater warming at night, and stratospheric cooling.

The climate models, far from being melodramatic, may be conservative in the predictions they produce. For example, here’s a graph of sea level rise:

Observed sea level rise since 1970 from tide gauge data (red) and satellite measurements (blue) compared to model projections for 1990-2010 from the IPCC Third Assessment Report (grey band).  (Source: The Copenhagen Diagnosis, 2009)

Here, the models have understated the problem. In reality, observed sea level is tracking at the upper range of the model projections. There are other examples of models being too conservative, rather than alarmist as some portray them. All models have limits - uncertainties - for they are modelling complex systems. However, all models improve over time, and with increasing sources of real-world information such as satellites, the output of climate models can be constantly refined to increase their power and usefulness.

Climate models have already predicted many of the phenomena for which we now have empirical evidence. Climate models form a reliable guide to potential climate change.

Basic rebuttal written by GPWayne

Last updated on 26 February 2014 by LarryM. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Further reading

Update

On 21 January 2012, 'the skeptic argument' was revised to correct for some small formatting errors.

Comments

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Comments 451 to 489 out of 489:

  1. If I were James Wilson, I would rant on about this paper being a religious paper and the authors not being able to refrain from political argument. I would then say how it got published because of a sympathetic reviewer and that it will certainly be torn apart in subsequent analyses etc etc...

    Yet I'm sure that, somehow, it will make Poptech's list. Isn't it nice to have flexible standards?
  2. Any ideas where the satellite data shown in the main picture comes from? It doesn't seem to match any data that I'm aware of.
    Response:

    [DB] "Any ideas where the satellite data shown in the main picture comes from?"

    Did you read the linked source?

  3. I noticed that the link to Tamino's graphic in the "further reading" box is broken.
    Response: [DB] Fixed link.
  4. mace @452, the satellite observations in the picture are for sea levels, not temperatures.
  5. mace Please avoid starting up a new discussion without first properly disengaging from another. You made an ill-informed comment here, which several contributors (including myself) took the time to address. It is rather rude to pose questions and the ignore the answers, so please go back to that thread and either acknowledge that you were mistaken or explain why the answers provided are not sufficient.

    Why should anybody bother respond to your posts if you appear to be ignoring those responses?
  6. The past 40ish years appears to have a fairly straight 0.016C/yr trend. There seems to be little indication of increased warming. Even when exogenous factors are removed, the signal doesn't seem to show any acceleration.

    Given that we're expected to hit +2C well before 2087 under BAU conditions:

    A) Why is it taking so long for the positive feedbacks to reveal themselves in the temperature record.

    B) How long will it be before the rate of change is 0.020 or 0.025C/yr?

    Ca) If emissions continue at ~BAU and short-term CS is >2.5C (which would result in a visible increase in warming), doesn't it follow that there is a very high chance that the next decade will contain one or more years that are dramatically hotter than '98/05/10?

    Cb) Isn't that really ****ing bad?
  7. Tristan As this thread is about the models being unreliable, do the models suggest that this acceleration should be visible over a 40 year timespan, above the noise we should expect to see in the observations due to sources of internal unforced variability such as ENSO?

    If so, then please give a reference to a paper or model output demonstrating this is the case. If not, then the reason we have not seen the clear accelleration is because of (i) the physics of climate suggest we shouldn't have seen it yet and/or (ii) there is so much noise in the observations it may be there but is obscured by the noise so we can't reliably/unequivocally detect it.
  8. Tristan -"There seems to be little indication of increased warming"

    Nonsense. Over 90% of global warming is going into the oceans. Did you miss Dana's recent blog post? Check out the last 40-ish years:



    I guess a blog post and graphic(s?) is necessary to clear up this confusion because a lot of the fake-skeptics don't seem to grasp this.
  9. DM

    F&R2011 removed a lot of that noise to reveal a fairly constant 0.16/decade trend. The 4AR predicts that 2011-30 will be +0.64-0.69C vs 1980-99. We won't get there at the current warming rate. Therefore either the warming rate must increase or the projections were too high. Which is it, do we know?

    RP

    I meant "There seems to be little indication of increased atmospheric warming."
  10. Tristan,
    I find it impossible to believe that the 4AR would make such a narrow projection. Can you provide a cite to the page where this projection was made? If you cannot provide a cite, please withdraw the question.

    The uncertainty in aerosol pollution alone is enough to account for the difference you note.
  11. I'd advise a little caution in making such strong statements, Michael.

    WG1 Ch10.ES Mean Temperature

    There is close agreement of globally averaged SAT multi-model mean warming for the early 21st century for concentrations derived from the three non-mitigated IPCC Special Report on Emission Scenarios (SRES: B1, A1B and A2) scenarios (including only anthropogenic forcing) run by the AOGCMs (warming averaged for 2011 to 2030 compared to 1980 to 1999 is between +0.64°C and +0.69°C, with a range of only 0.05°C). Thus, this warming rate is affected little by different scenario assumptions or different model sensitivities, and is consistent with that observed for the past few decades.
  12. Tristan,
    Thank you for the cite. Since it says "and is consistent with that observed for the past few decades." and the recent Foster and Rahmstorf (2011) paper claims the rate of warming is unchanging, what is left to explain?
  13. what is left to explain?

    The gap between 0.16 and 0.20.
  14. Tristan#461:

    Here is a map of GISS temperature anomaly for the year 2011, using 1980-1999 as a base period:


    --source

    You should note the average anomaly of 0.26C shown in the upper right corner. Using FR2011's linear trend of 0.18C per decade, by 2030 (two decades hence), we could easily see an anomaly (relative to 1980-99) in excess of 0.6C.
  15. Muon

    A) The average of the 5 records is .163C/decade, not 0.18C.

    B) 2030 may be +0.6C compared to 80-99. However, the 4AR states warming averaged for 2011 to 2030 compared to 1980 to 1999 is between +0.64°C and +0.69°C. If the anomaly is currently +0.26 and it reaches +0.6 by 2030, the average will be a lot less than +0.64.
  16. Tristan#465:

    You're forecasting using the average rate rather than the current rate? Based on this average of all five adjusted data sets, the warming trend has not slowed significantly in recent years (0.163°C per decade from 1979 through 2010, 0.155°C per decade from 1998 through 2010, and 0.187°C per decade from 2000 through 2010).

    Either way, what's your point? The thread here is model reliability, not model infallibility. Does pointing out a supposed flaw in an IPCC document somehow nullify AGW?
  17. (-Snip-)

    Do I need to state that I don't dispute any of the mechanics of climate change before people here actually read my posts properly?

    The AR4 is doing the forecasting. Not me.

    Based on the current rate of warming given by F&R2011 atmospheric temperatures will not reach the 2011-30 mean projected by the AR4.

    Either the warming must increase or the prediction must fall. My completely unscientific guess is that both of these will be the case. I presume that someone with a lot more knowledge than me can give me more information about this disparity.

    We are not on track for a mean 2011-30 anomaly of +0.64C versus 1980-99 without a very visible acceleration in warming.
    Response:

    [DB] Improper ideological categorizations snipped.

  18. Tristan, it looks like you used the multi-model mean forecast for global temperatures as if it were an exact prediction. Model ensembles don't work that way. If the multi-model mean was projected to be 0.64-0.69, what was the spread of the individual models around that? Some will be lower, some will be higher. How does the current rising trend in global temperature (on the assumption it remains at its recent trend) interact with the ensemble spread? Before you claim incorrectness, you need to know this.
  19. Tristan I understand the discrepancy you highlight. Are you able to sift through the relevant segments of the IPCC and report back? I find it takes too damn long reading through IPCC reports to find the nugget one is after. So I'm not volunteering.
  20. Tristan wrote "F&R2011 removed a lot of that noise to reveal a fairly constant 0.16/decade trend."

    You are missing the point. A model that had a slight upward (or indeed downward) curvature would fit the observations almost as well (e.g. in terms of the log likelihood) as the "best estimate" given in F&R2011. That is because there is enough signal in the noise to get a reasonable estimate of the basic trend, but not enough to get a reliable indication of any curvature.

    "The 4AR predicts that 2011-30 will be +0.64-0.69C vs 1980-99."

    page reference please.

    "We won't get there at the current warming rate. Therefore either the warming rate must increase or the projections were too high. Which is it, do we know?"

    That is impossible to answer without knowing *exactly* what was claimed in AR4.
  21. Tristan Sorry, I see you did give the quote. However, the quote gives the range of model ensemble means under different scenarios. However this doesn't mean that we would expect the observations to lie within that range, but instead would be within the spread of the multi-model ensemble, which would be very much broader. By saying that the multi-model mean is consistent with current observations, they would mean that they lie within the spread of the ensemble (which they undoubteldy do).
  22. Tristan#459: "The AR4 is doing the forecasting. Not me. "

    Wasn't this you in #465?: "We won't get there at the current warming rate."

    Sounds like a forecast to me. Again, so what? Especially now that we see you're talking about the average of models, not any specific model.
  23. The section in AR4 that Tristan seems to feel is a problem is here.
  24. Thanks for the responses.

    If it turns out that the 2011-2030 temperatures are markedly lower than the ensemble mean predicted, this suggests a systemic error in the way temperature was being forecasted in '07.

    I will be interested to see how the AR5's predictions compare to AR4's. Undoubtedly another 5 years of science and temperatures would have shed even more light on the nature of the climate's response to emissions.
  25. Tristan#474: 'Ensemble mean' is not a prediction; a difference between actual and mean does not imply systemic error.
  26. If the temperatures end up outside the range of the models, then that would be interesting. Remember every model run represents a possible future climate given those forcings.

    I would expect Ar5 to be better absolutely. It will probably reflect ongoing research into the aerosol effect and size of current aerosol forcing.
  27. The models are unreliable because there is an implicit assumption that backradiation from a cooler atmosphere is capable of either (a) warming or (b) slowing the rate of cooling of a (significantly) warmer surface.

    In a new extension of the work of Einstein and Planck, computations on blackbody radiation have show conclusively that this is a physical impossibility.

    For any given temperature, a surface emits at a peak frequency (proportional to the absolute temperature) this cut-off frequency being determined by Wien's Displacement Law - see Wikipedia. (There is a maximum a bit above the peak as the distribution is strongly attenuated.)

    Coherent radiation which the Earth's surface receives and which has frequencies above the maximum (ie SW solar insolation) will all be converted to thermal energy which can be stored and subsequently emitted with the appropriate (lower) IR frequencies, or diffused into the atmosphere or transferred by evaporation. Such transfers by these thermodynamic means reduce the remaining energy available for radiation. This is why the Earth's surface does not act anything like a perfect blackbody.

    However, radiation which has frequencies significantly below the peak in the emitting spectrum cannot be converted to thermal energy and thus has no effect. It is immediately emitted with the same spectrum and intensity, thus leaving no energy behind. It might as well have been reflected because the end result is similar.

    This is why frost lying in a shady spot does not get melted all day long by backradiation, even if the ground and air are slightly above freezing point. This is why a gas will not absorb when an emitter is radiating (spontaneously) until that emitter is warmed above the temperature of the receiving surface. (Spectroscopy confirms this.)

    So it is a fact of physics now proven (and never disproven) that the assumed warming effect of backradiation cannot happen. Thus an atmospheric greenhouse effect assumed as a result of radiative transfer is a physical impossibility.

    Those who are well read will know of the "Computational Blackbody Radiation" note to which I am referring which was written by a widely published Professor of Applied Mathematics whose name has (predictably) been somewhat slurred due to misinterpretation and lack of understanding. Having had over 45 years experience in Physics I can vouch for the accuracy of his results in such computations which I realise may well be above the heads of many readers here.
  28. Doug Cotton?
  29. That's the first name that came to mind - style is much the same.
  30. What an incoherent blabbermouth..
  31. Doug Cotton @477, the theory you have just proposed is complete nonsense, and in contradiction to inumerable experiments conducted primarily by the USAF to understand the behaviour of IR radiation in order to design effective heat seeking missiles, and FLIR cameras.

    If you expect us to believe us, you need to refer us to the actual scientific papers explaining the experiments you purport prove your theory, and giving the experimental results. Failing that, we will recognize it for the con it is, and continue to believe that IR lamps will warm a surface, contrary to the theory you have proposed.
  32. However, radiation which has frequencies significantly below the peak in the emitting spectrum cannot be converted to thermal energy and thus has no effect. It is immediately emitted with the same spectrum and intensity, thus leaving no energy behind. It might as well have been reflected because the end result is similar.


    Anyone can go into their kitchen and disprove this canard. Please, Mr Cotton, tell me then how the humble microwave oven is able to heat my dinner. The microwave radiation, according to your obscure physicist's incorrect theory, is incapable of adding energy to my lunch, which naturally emits (dimly) in the longwave infrared band. After exposure to the microwave radiation, my lunch is warmer than it was before, but you say this is not possible...

    Perhaps microwave manufacturers are all in the worldwide conspiracy?
  33. Tom Curtis & Skywatcher @481 & 482

    Regarding gases absorbing, see second paragraph here

    Regarding frost not melting, anecdotal only here

    For mathematical proof (which I have studied and agree with and which is not disproved) read Computational Blackbody Radiation.

    I am only interested in seeing any experiment (eg metal plates receiving backradiation at night) which demonstrates warming. Two identical radiators in open air warming up together will not help each other to warm faster because neither is hotter than the other. If they did you'd have energy creation. When the Earth surface and the first 1mm of the atmosphere are very close in temperature S-B law says there would be very little radiation.

    Microwaves (and lasers) are red herrings - they are not emitting spontaneous blackbody radiation - which is the subject above. Microwaves are a very special form of waves anyway which mostly only warm things like fat and water molecules up to boiling point only. They are irrelevant regarding backradiation.
  34. Doug Cotton @483, scientists are interested in theories which are wrong in interesting ways, ie, wrong in such a way that you learn something new in trying to refute it. Claes Johnson is wrong in that boring old way of just being absurd.

    He purports to derive a new theory of black body radiation which differs significantly from Planck's Law and the Stefan-Boltzmann Law, both of which are fundamental to the theory of radiation and have been multiply confirmed by observation. In place of these he offers a theory with no experimental confirmation, which we no will be disconfirmed experimentally because its predictions differ from those of a theory which is well confirmed experimentally; and whose only intellectual virtue is that it would refute a common misunderstanding of the greenhouse effect. That is not an interesting theory. Of course nobody has tried to disprove it, anymore than actual geologists don't waste their time trying to disprove hollow earth theories.

    I ask you again to link to papers showing experimental confirmation of the theory. Your failure to either do so, or to reject the theory as unempirical nonsense would show you once again to be trolling, and I would ask the moderators to enforce against you the ban for repeating, unmitigated trolling that has led to your prior banning which you are currently violating.
  35. Cotton seems to think objects would be aware, when receiving radiation, the circumstances under which that radiation was emitted, whether blackbody or not. This is desperate, handwaving nonsense to defend a theory which, as Tom says, is not experimentally verified.
  36. skywatcher... the Hermeneutics of Doug's "cut-off frequency being determined by Wien's Displacement theory" is clear. Claes Johnson looked at Wien's displacement law saw the Peak wavelength / frequency described as maximum and interpreted this as a cut-off - although hedged with "heavily attenuated" if you look at his writing.
    1/ I can understand how a Swedish native speaker could confuse Maximum in the sense of Peak and in the sense of "the highest value possible". To a mathematical - rather than a physics - the error would be opaque.
    2/ the "heavily attenuated" hedge is a bit odd as, as we all know, the distribution of BB radiation falls both above and below the maximum...
    3/ none of this actually follows form Prof. Johnsonns conscious-quanta ... which stands alone in it's bizarrness.
    4/ Doug does not have the where-withall to either read Prof. Johnsonns material in enough detail to see this nor to understand empirical physics which demonstrate it one way or the other.
  37. Consider for a moment how backradiation is measured (a Pyrgeometer). The thermopile has to be heated by the incoming radiation to generate a voltage at all. According the imaginary 2nd Law postulated by Doug and others, this couldn't happen. And yet a pyrgeometer makes measurements that are completely consistent with what boring textbook versions of thermodynamics postulate. If backradiation cannot warm the surface, then what physics accounts for what a pyrgeometer measures?
  38. Climate-Change-Theory/Doug Cotton - Let's be clear here. Your arguments violate observations and physical laws, and go against even freshman physics. It's just not a viable objection.

    Please - go read a book or two... such objections are why many 'skeptics' are not taken seriously.
  39. elsa - Redirecting from an off-topic comment on another thread: "They then go on to say "computer models have recently shown that during periods when there is a smaller increase of surface temperatures, warming is occurring elsewhere in the climate system, typically in the deep ocean."

    To be clear on this, global circulation models incorporating the most accurate physics we have on atmospheric and oceanic circulation under various forcing conditions exhibit behaviors including decade long very low or very steep atmospheric warming, with the inverse generally showing in deep ocean regions. This is entirely consistent with observations of climate behavior under, for example, ENSO extremes (El Nino, La Nina cycles).

    "That statement, as I pointed out yesterday cannot be true. It is not within the power of a model to do such a thing."

    And here you would be incorrect.
  40. Having been requested to post my view on models here rather than Facebook am happy to add my four pence worth (-Snip-).

    The 2011 temperature was below the IPCC projection for no increase in CO2 (after quite a large one) while your own sea level example doesn't seem to be consistent with others either in the past (2003, reported in 2008) http://www.worldclimatereport.com/index.php/2008/05/06/slower-sea-level-rise/

    or the present (note that you said the projection may have been conservative, but in a truly chaotic system chaotic things happen, in fact they have to as that is its nature)

    http://www.real-science.com/sea-level-continues-historic-decline

    Your articles are only as good as the latest data and can so quickly become out of date, (-Snip-). (-Snip-).
    Response:

    [DB] Time to acquaint yourself with this site's Comments Policy.  You should be familiar with it:  that of the SkS FB page was modeled on it.

    Multiple violations of the Comments Policy snipped.

  41. I'll add a nice little discussion here following the revelation that on far easier to measure population data the true figure of climate refugees (based on what the models said would happen) in 2010 was zero.
    http://newsbusters.org/blogs/noel-sheppard/2011/04/12/flashback-2005-un-predicts-50-million-global-warming-refugees-2010
    You do not use computers to do anything more than play games with role playing software, you do not pretend you can guess the future. That tends to come back and bite you in the tushy.
  42. Climate models have no skill at decadal prediction where internal variability dominates, nor do they pretend to. For study of that internal variability, F&R demonstrate how ENSO, solar and aerosols account for most of variability while the underlying trend follows IPCC predictions. ENSO is hard to predict even months out let alone be part of climate models. You can find a better comparison of model versus data here

    Sealevel drop is not due to the ice suddenly stopping to melt because of cooler temperatures. Far from it - ice continues to decline. Instead it is due to La nina precipitation change dumping water on land. See here for more discussion and here for GRACE data showing where the water went.
  43. VoR- Did you read the original BBC? First, this is not based on climate models (which are matching predictions just fine), nor is about "climate" refugees. Second estimating whether a person is an environmental refugee is difficult and the number is absolutely not zero. A comparable number would have to come from the same source.

    "You do not use computers to do anything more than play games with role playing software, you do not pretend you can guess the future"
    That's sailing very close to the wind on the comments policy here. Please stick to supported facts.
  44. The article also makes claims based a map linked to here

    An interesting message can be found there now. Sounds like some journalist axe-grinding not science.
  45. I've been busy going through my own archives, someone here has taken the trouble to compare as many predictions from models with the following reality as possible, (-Snip-). But as people have incredibly short memories and attention spans unless recorded nearly everyone's off on a new trip (-Snip-), the more so the greater ahead they are predicted. 2100 would make a perfect one, as when picked by the IPCC no one on earth could fulfil the experiment as they'd have needed to be 110 at the very youngest. (-Snip-)
    http://www.c3headlines.com/predictionsforecasts/
    Response:

    [DB] Time to acquaint yourself with this site's Comments Policy.  You should be familiar with it:  that of the SkS FB page was modeled on it.

    Multiple violations of the Comments Policy snipped.

  46. vor. Well here's the very latest data-model comparison for the completed 2011 year at RealClimate.

    I'm not sure how much effort should be expended on detailed projections for 100+ years. You can just see how much difference arises from using various scenarios. Climate might be difficult, but predicting what people might do and when they might do it is even more so.

    One question. Were you looking at the 'basic' or the 'intermediate' version of this post.

    And a quiet word in your shell-like "That's a fine parameter for a scientific experiment, setting the end point beyond observable time, possible a clue as to the general integrity of the whole project?" is as close to an accusation of fraud as dammit is to swearing. Don't be surprised if the moderators give you a hard time.
  47. Adelady#496

    vor's objection to use of 2100 as an endpoint is also utterly without meaning. It's arbitrary. And vor provides a good example of the fact that everybody and their dog are checking those predictions at least annually.

    This is the real issue that vor doesn't want to face: What data do the checkers (and their dogs) use to check the predictions? Is it a carefully cherrypicked few years ('it hasn't warmed since last week!')? Is it just one parameter of particular interest ('it's snowing in Europe!')? Is it just one prediction ('my TV news said there aren't any climate refugees!')?

    Or is it the entire record (as F&R2011 analyzed, as BEST verified) and the entire spectrum of the evidence (as we see here)?
  48. VoR - let me acquaint you with another way this site works. Your "model predictions are wrong" is list of cherry-picked, long-debunked guff from likes of Tisdale, Watts, CO2"science". This is not how it is done. If you want to contest model/observations, first you reference the published science source that makes the prediction. That takes case of the strawman arguments. Next you reference the published data that refutes it. That way we can see if it fair prediction or just cherry-picking. Also, models are tied to scenarios. Ie "IF the forcings are this, THEN the climate will this". That beats false prediction.

    Now the models stuff out miles of predictions and some are more robust than others. eg trends in global climate parameters are pretty robust. Regional prediction is less robust especially when it depends on how ENSO will change (if it does) with rising temperature. That is an open question.
  49. vor: "You do not use computers to do anything more than play games with role playing software, you do not pretend you can guess the future. That tends to come back and bite you in the tushy."

    What the heck is that? Are you saying that any attempt to predict the future is useless? Or are you saying that computer models aren't the best way to predict (if so, you got something better?)? Or are you saying that a specific model has problems, problems which you are able to detail/explain? Or are you saying that one person's prediction about climate refugees casts doubt on the scientific work of thousands? A little clarity, if you don't mind.
  50. Continuing from a comment posted by Manny:

    Sorry, what is extraordinary about the claims of climate theory? They are fully consistent with everything else we know about physics.

    The time period for model validation is not some arbitrary no. (eg why stop at 100,000?) but more determined by internal variability which is calculable. However, the basic theory of surface temperature, if correct, must work throughout all times and on all planets.

    There are two problems going back long periods in time:
    1/ computer power - full resolution, 100 year runs take a lot of muscle and time. Its more normal to look at specific parts of past time (eg LGM, ememian peak, YD, PETM etc). You dont gain a lot with very long runs.
    2/ Uncertainty as to the inputs increase. ie what was past albedo, atmosphere composition, TSI etc. There are uncertainties in proxy records for those as well as uncertainties in the proxy temperature.

    You can say that paleoclimate temperature proxies are consistent with climate theory and proxies for climate forcings. Ditto for temperature regimes on other planets.

    However, paleoclimate isnt the only way to validate model - they after all push out predictions on huge no. of variables on various time scales. See Chpter 8 of that IPCC report.

    However, the modellers would also I am sure caution you that it is better to consider model skill rather than simple model validation. In considering the implication of the modelling for future planning, this is what matters.

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