{"id":5915,"date":"2019-12-13T16:37:28","date_gmt":"2019-12-13T15:37:28","guid":{"rendered":"https:\/\/blog.mr-int.ch\/?p=5915"},"modified":"2019-12-13T16:46:25","modified_gmt":"2019-12-13T15:46:25","slug":"performance-of-climate-model-projections","status":"publish","type":"post","link":"https:\/\/blog.mr-int.ch\/?p=5915","title":{"rendered":"Performance pr\u00e9visionnelle des mod\u00e8les climatiques"},"content":{"rendered":"\n

Article \u00ab\u00a0Evaluating the performance of past climate model projections*,
Zeke Hausfather et al., AGU 2019, doi: 10.1029\/2019GL085378<\/a> <\/p>\n\n\n\n

My comments\nwhile reading this review article.<\/p>\n\n\n\n

From the plain language summary:<\/p>\n\n\n\n

\u201cModel projections rely on two things to accurately match observations: accurate modeling of climate physics, and accurate assumptions around future emissions of CO2<\/sub> and other factors affecting the climate.\u201d<\/em><\/p>\n\n\n\n

Not only is\naccurate modeling necessary, but it must also be comprehensive, covering all the\nphenomena involved. Also, the unavoidable simplification making necessary parameter\ntuning is a practical way to get to some results; however, this has little\nscience in it.  A posteriori model\nevaluation has to distinguish between what the model calculates and how fit\nwere the assumptions made about the future. To do the first, a model hindcast\nneeds to be performed by running it against the actual changes of independent\nvariables that took place during a long enough historical period. No model is\nrequired to qualify the scenario accuracy of the then projected emissions and\nother artificial or natural forcing; it suffices to compare actual data series with\nthe scenario that was designed at the time. The model quality should not depend\non the validity of the scenarios.<\/p>\n\n\n\n

\u201cThis research should help resolve public confusion around the performance of past climate modeling efforts, and increases our confidence that models are accurately projecting global warming.\u201d<\/em><\/p>\n\n\n\n

Setting\nsuch an objective to a research project is admitting at the onset the\nwillingness to perform what is called \u201cadvocacy research\u201d, i.e. proving an\nargument rather than looking hard for falsification.<\/p>\n\n\n\n

Fourteen\nmodel projections were assessed over the period of time between the date of the\nmodel publication and the end of 2017, with the IPCC\u2019s Fourth Assessment Report\n(2007) having a too short projection period to draw significant conclusions.\nComparison is made against five published global mean surface temperature time\nseries (GMST). However, the validation of models does not require any future\nprojection; hindcast should be made by re-running today the old models with the\nsame initial conditions and tuning parameters, and applying a single scenario,\nthat of the actual history of emissions and other causes to anthropogenic forcing.\nTherefore, CMIP 4 and 5 models could have been included.  <\/p>\n\n\n\n

An \u201cImplied Transient Climate Response\u201d (TCR, a measure of short to mid-term response of GMST to the forcing associated with a doubling of the atmospheric CO2<\/sub> concentration) is defined as  <\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

In its\nturn, this implication implies two assumptions:<\/p>\n\n\n\n

  1. that the anthropogenic forcing component is known (3.7 W m-2<\/sup> for 2xCO2<\/sub> ?)
    I suppose this is the primary radiative forcing due to the absorption by the greenhouse gas CO2<\/sub> of the Earth\u2019s surface infrared emission.<\/li>
  2. that the climate response is linearly depending on it.
    The authors argue that it is the case because it is expected to be so with a slowly varying forcing.    <\/li><\/ol>\n\n\n\n

    The\nsubsequent feedbacks responding to the primary forcing are the object of the\nmodel computations from which a calculated TCR should be derived. No linearity should\nbe assumed. <\/p>\n\n\n\n

    Also, to\ndraw an implied TCR from temperature observations is not only difficult but\nimpossible to do\u2026 without applying the same model in reverse to observed\ninstrumental data. This methodology has quite a tautological character. <\/p>\n\n\n\n

    The part of\nGMST to be attributed to anthropogenic forcing can be explicit in the models,\nbut not in the observed temperature time series that depend on much more\nvariables. Thus, to speak of an \u201cobserved TCR\u201d is not just a semantic error, it\nis wrong. Notwithstanding this objection, this is precisely what is done in\nthis article.<\/p>\n\n\n\n

    Climate\nsensitivity remains one of the major stumbling blocks (if not the major one) of\nclimate science.
    <\/p>\n\n\n\n

    Results<\/h3>\n\n\n\n

    In its Fig\n2, actual vs calculated temperature rate of warming and \u201cimplied TCR\u201d are shown:\n<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    Surprisingly,\nthe range of the observed temperatures (top chart) is getting wider with more\nrecent models (on the right of the graph); would this indicate that, during the\nmore recent observation periods, temperature had actually a larger variability?\nOr is it due to some statistical artefact?<\/p>\n\n\n\n

    The bottom\nof the chart represents \u201cmodel calculated TCR\u201d vs \u201cmodel calculated TCR from\nobserved temperature series\u201d. They overlap quite well, which is not surprising\ndue to the tautological methodology implied.<\/p>\n\n\n\n

    One model\nthat got much public attention was the 1988 assessment made by Hansen et al. It\nshows larger discrepancies between calculated and observed values. <\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    The authors\nattribute this discrepancy to the chosen scenarios. Therefore, nothing can be\nsaid about the intrinsic model quality. Indeed, the evolution of forcing\nassumed in these three scenarios are given in the supplemented material. The\n2017 projected values are a CO2<\/sub> concentration of 408 ppm for A, 402\nppm for B, and 368 for C, while the actual value was 405 ppm. Thus, the \u201cbetter\u201d\nprojection C was achieved with a wrongly assumed CO2<\/sub> concentration.\nThis is no quality certificate for neither the model nor the scenario.<\/p>\n\n\n\n

    A more\ncritical mind would rather suggest that, if the model sensitivity would have\nbeen reduced to approx. 40% of the basic model assumption, then a correct\nhindcast would have been delivered. <\/p>\n\n\n\n

    Conclusions<\/h3>\n\n\n\n

    The\nconclusion of the authors is one of satisfaction for the \u201cskillfulness\u201d of\nmodels, also earlier ones, in predicting subsequent warming in the years after\npublication. They also underline the mismatch of so-called high-profile models where\nobserved forcing (CO2<\/sub> emissions) were actually weaker than projected.<\/p>\n\n\n\n

    A\nnon-involved reader may not reach to such conclusion. <\/p>\n\n\n\n

    The trick\nof extracting a TCR value out of observed temperature series cannot be\ntolerated. <\/p>\n\n\n\n

    The wide\nrange of results as compared with the actual variability of the global climate\nsystem does not allow for significance; thus, what may stay within ballpark for\na few decades may have no validity for extrapolation to longer periods, up to\nthe end of the century and even beyond. <\/p>\n\n\n\n

    If it may be satisfactory that most ranges are overlapping, the example of the Hansen 1988 case indicates that the climate model community, or at least the authors of this article, have a hard time to make a clear distinction between true observations, model hindcast simulations that should validate them, and the plausibility of forecast scenarios to calculate possible climate projections. <\/p>\n\n\n\n

    There are practical reasons to not be in condition to re-run old models. But it should be reminded that, in its time, each model was tuned to match the temperature evolution then in progress; up to that point, it was designed to be accurate. The same applies for today\u2019s models. This does not lead to their validation. If, at a later date, they would show a good match with reality, it may be a lucky strike \u2013 as in case C of Hansen 1998 \u2013 or it may stem from a true skilfulness. A mere comparison as made in this article does not prove anything and a deeper diagnosis would be required. <\/p>\n\n\n\n

    Furthermore, knowing that the most extreme and unlikely scenarios (e.g. RCP 8.5 in the CMIP 5 comparisons) are those that are retained to frighten everyone and to demand immediate and drastic mitigation measures, the scientific division of the Global Climate Business should avoid claiming the high quality of such approximate matches; on the contrary, they should stress their lack of confidence and the limits of their knowledge.<\/p>\n","protected":false},"excerpt":{"rendered":"

    Un article r\u00e9cemment publi\u00e9 par Zeke Hausfather et al. conclut que les mod\u00e8les ont \u00e9t\u00e9 jusqu’\u00e0 pr\u00e9sent tr\u00e8s habiles \u00e0 pr\u00e9voir le r\u00e9chauffement climatique futur. Cependant, ce certificat de satisfaction doit \u00eatre relativis\u00e9 parce qu’il soutient que la sensibilit\u00e9 climatique peut \u00eatre observ\u00e9e \u00e0 partir de mesures instrumentales des temp\u00e9ratures et aussi parce qu’il ne distingue pas clairement le r\u00f4le des mod\u00e8les de celui des sc\u00e9narios utilis\u00e9s pour simuler l’avenir. Les climatologues devraient plut\u00f4t essayer de r\u00e9futer (falsify) les th\u00e8ses actuelles plut\u00f4t que de se charger de les prouver.
    \nVoici mes commentaires sur cet article de synth\u00e8se (en anglais).<\/p>\n","protected":false},"author":2,"featured_media":2373,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"iawp_total_views":7,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[38],"tags":[],"class_list":["post-5915","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-climat"],"_links":{"self":[{"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/posts\/5915","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5915"}],"version-history":[{"count":0,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/posts\/5915\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=\/wp\/v2\/media\/2373"}],"wp:attachment":[{"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5915"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5915"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.mr-int.ch\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5915"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}