{"id":114,"date":"2005-01-27T10:20:46","date_gmt":"2005-01-27T14:20:46","guid":{"rendered":"\/?p=114"},"modified":"2020-02-03T08:45:05","modified_gmt":"2020-02-03T13:45:05","slug":"what-if-the-hockey-stick-were-wrong","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/01\/what-if-the-hockey-stick-were-wrong\/","title":{"rendered":"What If … the “Hockey Stick” Were Wrong? Et si \u2026. la “Crosse de Hockey” \u00e9tait fausse ? <\/lang_fr>"},"content":{"rendered":"
\n

The \u201chockey stick\u201d reconstruction of temperatures of the past millennium has attracted much attention \u2013 partly as it was high-lighted in the 2001 IPCC report as one of the important new results since the previous IPCC report of 1995, and partly as it has become the focus of a number of challenges. Discussion about the \u201chockey stick\u201d is conducted with considerable fervor in the public media, where this curve is often presented as if it were a proof, or even the most important proof, of anthropogenic influence on climate.<\/p>\n

As someone who has not worked on the past millennium, I do not want to discuss the merits of the often rather technical challenges (which have been dealt with elsewhere on this site<\/a>). Rather, I want to discuss the \u201cwhat if…\u201d question: what if really some serious flaw was discovered in the \u201chockey stick\u201d curve? What would that mean?
\n
\nPar Stefan Rahmstorf (traduit par Claire Rollion-Bard)<\/small><\/lang_fr><\/p>\n

La reconstruction “crosse de hockey” des temp\u00e9ratures du dernier mill\u00e9naire a beaucoup attir\u00e9 l’attention – en partie car elle a \u00e9t\u00e9 mise en avant dans le rapport IPCC 2001 comme l’un des nouveaux r\u00e9sultats importants depuis le pr\u00e9c\u00e9dent rapport IPCC de 1995, et en partie car elle est devenue le point de mire d’un certain nombre de d\u00e9fis. La discussion sur la “crosse de hockey” est transmise avec une ferveur consid\u00e9rable dans les m\u00e9dias, o\u00f9 cette courbe est souvent pr\u00e9sent\u00e9e comme une preuve ou m\u00eame la preuve la plus importante de l’influence anthropog\u00e9nique sur le climat.<\/p>\n

En tant que personne n’ayant pas travaill\u00e9 sur le dernier mill\u00e9naire, je ne veux pas discuter des m\u00e9rites des d\u00e9fis techniques (qui sont discut\u00e9s dans une autre section du site<\/a>). Je veux plut\u00f4t discuter de la question “et si\u2026” : et si de s\u00e9rieux d\u00e9fauts \u00e9taient trouv\u00e9s dans la courbe “crosse de hockey” ? Qu’est ce que cela signifierait ?<\/p>\n

(suite\u2026)<\/a><\/p>\n


\nSo let\u2019s assume for argument\u2019s sake that Mann, Bradley and Hughes made some terrible mistake in their statistical analysis, so we need to discard their results altogether. This wouldn\u2019t change our picture of the last millennium (or anything else) very much: independent groups, with different analysis methods, have arrived at similar results for the last millennium. The details differ (mostly within the uncertainty bounds given by Mann et al, so the difference is not significant), but all published reconstructions share the same basic features<\/a>: they show relatively warm medieval times, a cooling by a few tenths of a degree Celsius after that, and a rapid warming since the 19th Century. Even without Mann et al, we\u2019d still be stuck with a \u201chockey stick\u201d type of curve \u2013 quite boring.<\/p>\n

So let\u2019s try some more exciting \u201cwhat ifs\u201d. In mid-20th Century, medieval temperatures are exceeded in all the reconstructions, hence recent (last 10-15 years, say) temperatures appear to be unprecedented for at least a millennium (that even holds for the alternative histories presented by the \u201chockey stick\u201d critics). Now what if that <\/em>were wrong \u2013 if all proxy reconstructions as well as model simulations of the past millennium were fundamentally in error?<\/p>\n

Let us assume that medieval temperatures after all had been warmer than the present. Even that would tell us nothing about anthropogenic climate change. The famous conclusion of the IPCC, \u201cThe balance of evidence suggests that there is a discernible human influence on global climate\u201d, does not depend on any reconstruction for the past millennium. It depends on a detailed analysis of 20th Century data. In fact, this conclusion is from the 1995 IPCC report, and thus predates the existence of quantitative proxy reconstructions like the \u201chockey stick\u201d.<\/p>\n

Climate changes can have several different reasons, and the cause of any particular climate change needs to be investigated on a case by case basis. It cannot be found by looking at one temperature curve. Had medieval climate been warmer than the present, this would probably have been due to some natural cause \u2013 perhaps a peak in solar output. That would only tell us that in principle, natural causes can cause warming larger than what we\u2019ve seen in the past decades. But we know that already \u2013 one need only go back far enough in time (e.g., fifty million years) to find examples of unquestionably warmer climates than today. However, it would be naive to conclude that the observed strong 20th Century warming therefore also must have a natural cause.<\/p>\n

Investigating the cause of 20th Century warming is done in so-called detection and attribution<\/em><\/a> studies, which analyze the various forcings (e.g., solar variations, greenhouse gases or volcanic activity) and the observed time and space patterns of climate change in detail. These studies, with a range of different techniques, have invariably concluded that the dominant cause of 20th Century warming is man-made greenhouse gases.<\/p>\n

In the spirit of this article, let\u2019s assume these studies were also wrong, in addition to all of the above. Let\u2019s assume these studies somehow greatly underestimated natural variability in the climate system, so that the \u201csignal\u201d of anthropogenic climate change has not yet emerged from the \u201cnoise\u201d of natural variations (i.e., the above-cited \u201cdiscernible human influence<\/em>\u201d had not been detected after all). Surely, then we wouldn\u2019t need to worry about global warming, and the world could hold off with the Kyoto protocol?<\/p>\n

Unfortunately, that also doesn\u2019t follow. The only thing that would follow in that case is that our data are not yet good enough to prove that anthropogenic climate change is already happening. That would not be so surprising \u2013 the expected amount of anthropogenic global warming to date (based on the radiative effects of the greenhouse gases and aerosols emitted by humans thus far) is only ~0.5 \u00baC. It is a small signal that is not easy to detect amongst the natural variability; most of the anthropogenic warming is still to come (the point of conducting science is to give an early warning, rather than just wait until the facts are obvious to everyone).<\/p>\n

The main reason for concern about anthropogenic climate change is not that we can already see it (although we can). The main reason is twofold.
\n(1)\tCarbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity. This is a measured fact not even disputed by staunch \u201cclimate skeptics\u201d.
\n(2)\tAny increase in carbon dioxide and other greenhouse gases will change the radiation balance of the Earth and increase surface temperatures. This is basic and undisputed physics that has been known for over a hundred years.<\/p>\n

But how strong<\/em> is this warming effect? That is the only fundamental doubt about anthropogenic climate change that can still be legitimately debated. We climatologists describe this in terms of the climate sensitivity, the warming that results in equilibrium from a doubling of CO2. The IPCC gives the uncertainty range as 1.5-4.5 \u00baC. Only if this is wrong, and the true value is lower, can we escape the fact that unabated emissions of greenhouse gases will lead to the warming projected by the IPCC.<\/p>\n

Chances for that are not good. A new large uncertainty analysis that appeared this week in Nature <\/em>shows that it is very difficult to get a climate sensitivity below 2 \u00baC in a climate model, no matter how one changes the parameters. And climate history, with its Ice Ages and other large changes, also speaks strongly against low climate sensitivity.<\/p>\n

Discussion about the temperature evolution of the past millennium will no doubt continue in the coming years. The most fundamental problem \u2013 the sparseness of data \u2013 will not be fixed quickly, but eventually better reconstructions with smaller uncertainties will become available. However, this discussion needs to be conducted in a sober and unexcited manner; it does not help to overburden the \u201chockey stick\u201d with symbolic meaning. In some media reports, the \u201chockey stick\u201d has even been hyped as \u201ca pillar of the Kyoto protocol\u201d (which was agreed in 1997 and thus predates it) or as \u201cproof that humans are warming the Earth\u201d. This is a serious misunderstanding of the scientific meaning of these data.<\/p>\n

The discussions about the past millennium are not <\/em>discussions about whether humans are changing climate; neither do they affect our projections for the future. In fact, if humanity takes no action and this century will bring a temperature rise of 2 \u00baC, 3 \u00baC or even more, the current discussions over whether the 14th Century was a few tenths of a degree warmer or the 17th a few tenths cooler than previously thought will look rather academic.<\/p>\n

Supposons donc que Mann, Bradley et Hughes ont fait une terrible erreur dans leur analyse statistique et que nous devons renoncer \u00e0 tous leurs r\u00e9sultats. Cela ne changerait pas beaucoup notre vision du dernier mill\u00e9naire : des groupes ind\u00e9pendants, avec diff\u00e9rentes m\u00e9thodes d’analyses, sont arriv\u00e9s \u00e0 des r\u00e9sultats similaires pour le dernier mill\u00e9naire. Les d\u00e9tails diff\u00e8rent (la plupart dans les limites d’incertitude donn\u00e9es par Mann et al, donc la diff\u00e9rence n’est pas significative), mais toutes les reconstructions publi\u00e9es partagent les m\u00eames caract\u00e9ristiques basiques : elles montrent une \u00e9poque m\u00e9di\u00e9vale relativement chaude, ensuite un refroidissement de quelques dixi\u00e8mes de degr\u00e9, puis un r\u00e9chauffement rapide depuis le 19\u00e8me si\u00e8cle. M\u00eame sans Mann et al, on resterait toujours coinc\u00e9 avec une courbe du type “crosse de hockey” – ce qui est assez ennuyeux.<\/lang_fr><\/p>\n

Essayons donc d’autres “et si” plus excitants. Au milieu du 20\u00e8me si\u00e8cle, dans toutes les reconstructions, les temp\u00e9ratures m\u00e9di\u00e9vales sont au-dessus, d’o\u00f9 des temp\u00e9ratures r\u00e9centes (c’est-\u00e0-dire les derniers 10-15 ans) apparaissant \u00eatre sans pr\u00e9c\u00e9dent depuis au moins un mill\u00e9naire (m\u00eame si on tient compte les reconstructions alternatives des critiques de la “crosse de hockey”). Maintenant et si cela est faux – si toutes les reconstructions \u00e0 partir de marqueurs aussi bien que les simulations des mod\u00e8les du dernier mill\u00e9naire \u00e9taient fondamentalement dans l’erreur ?<\/p>\n

Supposons, apr\u00e8s tout, que les temp\u00e9ratures m\u00e9di\u00e9vales ont \u00e9t\u00e9 plus chaudes que l’actuel. M\u00eame si cela ne nous dit rien sur le changement anthropog\u00e9nique du climat. La conclusion fameuse de l’IPCC, “L’\u00e9quilibre des preuves sugg\u00e8re qu’il y a une influence humaine discernable sur le climat global”, ne d\u00e9pend d’aucune reconstruction du dernier mill\u00e9naire. Elle d\u00e9pend d’une analyse d\u00e9taill\u00e9e des donn\u00e9es du 20\u00e8me si\u00e8cle. En fait, cette conclusion est tir\u00e9e du rapport IPCC 1995, et ainsi pr\u00e9c\u00e8de l’existence de reconstructions quantitatives \u00e0 partir de marqueurs comme la “crosse de hockey”.<\/p>\n

Les changements du climat peuvent avoir diff\u00e9rentes raisons, et la cause d’un changement particulier du climat a besoin d’\u00eatre examin\u00e9e sur la base du cas par cas. Elle ne peut pas \u00eatre trouv\u00e9e en regardant une courbe de temp\u00e9rature. Le climat m\u00e9di\u00e9val a \u00e9t\u00e9 plus chaud qu’actuellement, cela serait probablement d\u00fb \u00e0 des causes naturelles – peut-\u00eatre un pic dans l’activit\u00e9 solaire. Cela nous dirait seulement que, en principe, les causes naturelles peuvent provoquer un r\u00e9chauffement plus important que celui que nous avons observ\u00e9 dans les derni\u00e8res d\u00e9cennies. Mais nous le savons d\u00e9j\u00e0 – il suffit de remonter assez loin dans le temps (e.g. 50 millions d’ann\u00e9es) pour trouver des exemples de climats indiscutablement plus chauds qu’aujourd’hui. Cependant, il serait na\u00eff de conclure que le fort r\u00e9chauffement observ\u00e9 au 20\u00e8me si\u00e8cle devrait donc avoir une cause naturelle.<\/p>\n

L’examen de la cause du r\u00e9chauffement du 20\u00e8me si\u00e8cle est fait dans les \u00e9tudes nomm\u00e9es d\u00e9tection et attribution, qui analysent en d\u00e9tail les diff\u00e9rents for\u00e7ages (e.g. variations de l’activit\u00e9 solaire ; des gaz \u00e0 effet de serre ou de l’activit\u00e9 volcanique) et le temps et l’espace observ\u00e9s pour le changement climatique. Ces \u00e9tudes, r\u00e9alis\u00e9es avec diff\u00e9rentes techniques, ont invariablement conclu que la cause dominante du r\u00e9chauffement du 20\u00e8me si\u00e8cle est les gaz \u00e0 effet de serre cr\u00e9\u00e9s par l’homme.<\/p>\n

Dans l’esprit de cet article, supposons que ces \u00e9tudes ont \u00e9galement tort, en plus de tout ce qu’il y a au-dessus. Supposons que ces \u00e9tudes, d’une certaine mani\u00e8re, sous-estiment grandement la variabilit\u00e9 naturelle dans le syst\u00e8me climatique, de telle mani\u00e8re que le “signal” du changement anthropique du climat n’a pas encore \u00e9merg\u00e9 du “bruit” des variations naturelles (i.e. “l’influence discernable de l’homme” sus-cit\u00e9e n’a pas encore \u00e9t\u00e9 d\u00e9tect\u00e9e). Nous n’aurions donc certainement pas besoin de nous inqui\u00e9ter du r\u00e9chauffement global, et le monde pourrait \u00e9carter le protocole de Kyoto ?<\/p>\n

Malheureusement cela ne marche pas non plus. La seule chose qui marcherait dans ce cas est que nos donn\u00e9es ne sont pas encore assez bonnes pour prouver que le changement anthropog\u00e9nique du climat a d\u00e9j\u00e0 lieu. Cela ne serait pas si surprenant – la quantit\u00e9 attendue du r\u00e9chauffement anthropog\u00e9nique global est \u00e0 ce jour seulement de 0,5 \u00b0C (bas\u00e9 sur les effets radiatifs des gaz \u00e0 effet de serre et des a\u00e9rosols \u00e9mis par les humains). C’est un faible signal qui n’est pas facile \u00e0 d\u00e9tecter parmi la variabilit\u00e9 naturelle ; la majeure partie du r\u00e9chauffement anthropog\u00e9nique reste encore \u00e0 venir (le but de la science est de donner une alerte pr\u00e9coce, plut\u00f4t que juste attendre que les faits soient \u00e9vidents pour tout le monde).<\/p>\n

La principale raison d’\u00eatre concern\u00e9 par le changement anthropog\u00e9nique du climat n’est pas que nous pouvons d\u00e9j\u00e0 le percevoir (bien que nous puissions). La principale raison est double :
\n(1) Le dioxyde de carbone et les autres gaz \u00e0 effet de serre augmentent rapidement dans l’atmosph\u00e8re \u00e0 cause de l’activit\u00e9 humaine. C’est un fait mesur\u00e9 m\u00eame pas remis en cause par les sceptiques affirm\u00e9s.
\n(2) Toute augmentation du dioxyde du carbone et des autres gaz \u00e0 effet de serre changera l’\u00e9quilibre de rayonnement de la Terre et augmentera les temp\u00e9ratures de surface. C’est de la physique basique et non discut\u00e9e qui est connue depuis une centaine d’ann\u00e9es.<\/p>\n

Mais quelle intensit\u00e9 a cet effet de r\u00e9chauffement ? C’est le seul doute fondamental sur le changement anthropog\u00e9nique du climat qui peut \u00eatre encore l\u00e9gitimement d\u00e9battu. Nous, climatologues, d\u00e9crivons le r\u00e9chauffement qui r\u00e9sulte, \u00e0 l’\u00e9quilibre, d’un doublement du CO2 en terme de sensibilit\u00e9 du climat. L’IPCC donne la gamme d’incertitude \u00e0 1,5 – 4,5 \u00b0C. Seulement si c’est faux et que la valeur vraie est plus faible, nous pouvons en tirer le fait que la non-diminution d’\u00e9missions de gaz \u00e0 effet de serre m\u00e8nera au r\u00e9chauffement projet\u00e9 par l’IPCC.<\/p>\n

Les chances d’avoir cela ne sont pas bonnes. Une nouvelle grande analyse d’incertitude qui est parue cette semaine dans Nature, montre qu’il est tr\u00e8s difficile d’obtenir une sensibilit\u00e9 du climat en-dessous de 2\u00b0C pour les mod\u00e8les climatiques, quelle que soit la mani\u00e8re dont on change les param\u00e8tres. Et l’histoire du climat, avec ses \u00e2ges glaciaires et ses autres grands changements, plaide fortement contre une faible sensibilit\u00e9 de climat.<\/p>\n

La discussion sur l’\u00e9volution des temp\u00e9ratures du dernier mill\u00e9naire continuera, sans aucun doute, dans les ann\u00e9es \u00e0 venir. Le probl\u00e8me le plus fondamental – l’\u00e9parpillement des donn\u00e9es – ne sera pas arrang\u00e9 rapidement, mais \u00e9ventuellement, de meilleures reconstructions avec des incertitudes plus faibles seront disponibles. Cependant, la discussion a besoin d’\u00eatre conduite de mani\u00e8re sobre et sans excitation ; cela ne sera d’aucune aide pour surcharger la “crosse de hockey” d’une signification symbolique. Dans certains rapports des m\u00e9dias, la “crosse de hockey” a m\u00eame \u00e9t\u00e9 exag\u00e9r\u00e9e et d\u00e9crite comme “un pilier du protocole de Kyoto” (qui a \u00e9t\u00e9 reconnue en 1997 et ainsi le pr\u00e9c\u00e8de) ou comme une “preuve que les hommes r\u00e9chauffent la Terre”. C’est une mauvaise interpr\u00e9tation s\u00e9rieuse de la signification scientifique de ces donn\u00e9es.<\/p>\n

Les discussions sur le dernier mill\u00e9naire ne sont pas des discussions sur le fait de savoir si les hommes changent le climat ; ni s’ils affectent nos projections dans le futur. En fait, si l’humanit\u00e9 ne prend aucune mesure et que ce si\u00e8cle apporte une augmentation de temp\u00e9rature de 2\u00b0C, 3\u00b0C ou m\u00eame plus, les discussions actuelles sur le fait que le 14\u00e8me si\u00e8cle \u00e9tait quelques dixi\u00e8mes de degr\u00e9 plus chaud ou que le 17\u00e8me quelques dixi\u00e8mes de degr\u00e9 plus froid qu’avant, sembleront plut\u00f4t acad\u00e9miques.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

The \u201chockey stick\u201d reconstruction of temperatures of the past millennium has attracted much attention \u2013 partly as it was high-lighted in the 2001 IPCC report as one of the important new results since the previous IPCC report of 1995, and partly as it has become the focus of a number of challenges. Discussion about the […]<\/p>\n","protected":false},"author":10,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[1,3,9,2],"tags":[],"class_list":{"0":"post-114","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-greenhouse-gases","8":"category-instrumental-record","9":"category-paleoclimate","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/114","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/10"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=114"}],"version-history":[{"count":1,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/114\/revisions"}],"predecessor-version":[{"id":22980,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/114\/revisions\/22980"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=114"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=114"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=114"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}