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"Why models run hot"

  1. Jan 22, 2015 #1

    phyzguy

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    I think this is now a legitimate topic. If not, I apologize. I would like to hear comments on the paper "Why models run hot", discussed in this article. Do people on this forum agree that the IPCC models overestimate the amount of positive feedback in the Earth's climate system.? Or is this new article flawed? I've attached a pdf of the article.
     
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  3. Jan 22, 2015 #2

    Quantum Defect

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    Lord Monckton?
     
  4. Jan 22, 2015 #3

    PeterDonis

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    He's one of four authors of the paper.
     
  5. Jan 22, 2015 #4

    PeterDonis

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    I've thought that for quite some time. It's important to keep the paper's claim in perspective, however. The paper, as I read it, is not saying that its model should be used to predict what the climate will be like at the end of the century; it is not saying that its model is based on a detailed understanding of how the climate works. It is only saying that its very simple model can make better predictions, as compared with the data over the last couple of decades, than the much more complicated IPCC models, and that should make us skeptical that the IPCC models are modeling things correctly.
     
  6. Jan 22, 2015 #5

    Bandersnatch

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    I'd be wary of papers whose primary author is a politician with no relevant scientific background and a clear agenda.
     
  7. Jan 22, 2015 #6

    PeterDonis

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    By PF rules, we should focus on the scientific content of papers and not on who authored them, provided they meet the basic guidelines for acceptable references. This paper was published in a peer-reviewed journal, and it looks like at least two of the four authors have scientific credentials. So it appears to be an acceptable reference.
     
  8. Jan 22, 2015 #7

    Bystander

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    May I paraphrase: "a very simple model provides a better fit of data collected over the past two decades than do complex models that have been generally accepted." Implication: the complex models are "overfitting" the available data.
     
  9. Jan 22, 2015 #8

    PeterDonis

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    Yes, this is better because it avoids the word "prediction", which doesn't really fit here. (The paper also includes predictions based on the simple model, but those aren't what the claim that the complex models are overestimating feedback is based on.)

    More precisely: the complex models overfit the data from prior to the past couple of decades, i.e., the data that were used to build the models. Hence, the model results for the past couple of decades don't match as well.
     
  10. Jan 22, 2015 #9

    DaveC426913

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  11. Jan 22, 2015 #10

    Bystander

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    There's a note at the head of the forum regarding this. Stick to refereed sources and omit the politics.

    Beg pardon --- there was --- do not ask me where it went.
     
    Last edited: Jan 22, 2015
  12. Jan 22, 2015 #11

    phyzguy

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    I have thought for a while that the IPCC climate models seem to have an unreasonably large amount of positive feedback. It appears, based on what PeterDonis said in Post #4, that he agrees. My logic was that if the climate system had a large amount of positive feedback, we would have seen more climate variability in the past due to various forcings. The authors of this paper say exactly the same thing at the beginning of Section 8.3.2, where they say,

    "A plausible upper bound to f may be found by recalling that absolute surface temperature has varied by only 1 % or 3 K either side of the 810,000-year mean [40 , 41 ]. This robust thermostasis [42 , 43 ], notwithstanding Milankovich and other forcings, suggests the absence of strongly net-positive temperature feedbacks acting on the climate."

    It seems possible that the developers of the IPCC models incorporated enough positive feedback to fit the temperature data through the early part of the 21st century, and are now finding that the positive feedback needs to be dialed back in order to fit the more recent data. Does anyone know whether there are other checks on the various components of feedback to verify if they are reasonable?
     
  13. Jan 22, 2015 #12

    Bystander

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    How many "feedbacks" have been proposed? Haven't bothered to keep up with them lately. Have they been run independently of one another? Or, do the models allow a "synergy" between/among them?
     
  14. Jan 22, 2015 #13

    PeterDonis

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    I think part of the issue is that we don't have a good understanding of the mechanisms behind the various feedbacks, so it's hard to check them independently. For example, I don't think there's agreement even on the sign (positive or negative) of the net feedback from clouds, because we don't understand the various effects clouds have well enough to know which ones dominate.

    There are also people working on estimating the feedbacks by looking at satellite data; this is basically doing it "from the outside", not testing the various mechanisms separately but just looking at the overall radiation in and out to estimate global effects. AFAIK there is still a lot of spread in the possible feedback values estimated this way as well--largely because we simply haven't been taking satellite data that long.
     
  15. Jan 22, 2015 #14

    berkeman

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    Thanks for the heads up! The stickie is back visible again. :-)
     
  16. Jan 22, 2015 #15

    Bystander

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    Thank you --- I'm not having a "golden age moment."
     
  17. Jan 22, 2015 #16

    phinds

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    Ah you probably are and berkeman is covering for you :smile:
     
  18. Jan 22, 2015 #17

    Bystander

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    You'll spend the rest of your days proofreading every line you post --- one typo and you're done for.
     
  19. Jan 22, 2015 #18

    Bystander

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    I got the impression while hunting for information for Loudzoo's thread on IR & ocean evaporation that the satellite data wasn't broad spectrum. Got any details whether they're collecting UV, vis., and IR?
     
  20. Jan 22, 2015 #19

    PeterDonis

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    I'm pretty certain satellites can get radiance measurements in the microwave and IR bands. I'm fairly sure they can get them in visible as well. Don't know about UV. As far as using satellite data to infer temperatures, since all of the temperatures in question correspond to spectrum peaks in the IR range, I'm not sure UV data would add much anyway; if you've got microwave, IR, and visible, you've covered the key portion of the spectrum.
     
  21. Jan 23, 2015 #20

    Bystander

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    Thee or me either one. I haven't actually sat down and designed a "dream data set" for satellite measurements, let alone whether instrument sensitivity is adequate for my "intuitive" sense that "global" measurements are probably going to be most meaningful if made from at least lunar distance. Sorting direct reflection from scattering? Reflected IR from emitted IR? Which details are even meaningful. Hopefully NASA, NOAA, and JPL have coordinated programs for such purposes, and one of these days it'll show up in "J. Gee. Whiz" for me to play with.
     
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