{"id":122,"date":"2005-02-15T06:26:27","date_gmt":"2005-02-15T10:26:27","guid":{"rendered":"\/?p=122"},"modified":"2010-03-14T12:01:29","modified_gmt":"2010-03-14T17:01:29","slug":"moberg-et-al-highly-variable-northern-hemisphere-temperatures","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/02\/moberg-et-al-highly-variable-northern-hemisphere-temperatures\/","title":{"rendered":"Moberg et al: Highly variable Northern Hemisphere temperatures? Moberg et coll. : une plus grande variabilit\u00e9 climatique pass\u00e9e dans l’H\u00e9misphere Nord ?<\/lang_fr>"},"content":{"rendered":"
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by William Connolley and Eric Steig<\/small><\/p>\n

The 10th Feb edition of Nature<\/i> has a nice paper “Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data<\/a><\/i>” by Anders Moberg, DM. Sonechkin, K Holmgren, NM Datsenko, & W Karlin (doi:10.1038\/nature03265). This paper takes a novel approach to the problem of reconstructing past temperatures from paleoclimate proxy data. A key result is a reconstruction showing more century-scale variability in mean Northern Hemisphere temperatures than is shown in previous reconstructions. This result will undoubtedly lead to much discussion and further debate over the validity of previous work. The result, though, does not fundamentally change one of the most discussed aspects of that previous work: temperatures since 1990 still appear to be the warmest in the last 2000 years.
\npar William Connolley et Eric Steig (traduit par Pierre Allemand)<\/small><\/p>\n

L’\u00e9dition du 10 f\u00e9vrier de Nature contient un int\u00e9ressant article “Les grandes variations de temp\u00e9ratures de l\u2019h\u00e9misph\u00e8re Nord reconstitu\u00e9es \u00e0 partir d\u2019observations \u00e0 basse et haute r\u00e9solution <\/a>” par Anders Moberg, DM Sonechkin, K Holmgren, NM Datsenko, et W Karlin (doi:10.1038\/nature03265). Cet article adopte une nouvelle approche du probl\u00e8me de la reconstitution des temp\u00e9ratures pass\u00e9es \u00e0 partir de marqueurs (“proxies”) des pal\u00e9oclimats. Un des principaux r\u00e9sultats est une reconstitution montrant une variabilit\u00e9 \u00e0 l\u2019\u00e9chelle du si\u00e8cle dans les moyennes des temp\u00e9ratures de l\u2019h\u00e9misph\u00e8re nord plus importante que celle montr\u00e9e dans les reconstitutions pr\u00e9c\u00e9dentes. Ce r\u00e9sultat am\u00e8nera sans doute beaucoup de discussions et de d\u00e9bats futurs concernant la validit\u00e9 des travaux pr\u00e9c\u00e9dents. N\u00e9anmoins, le r\u00e9sultat ne change pas fondamentalement un des aspects les plus discut\u00e9s de ces travaux, \u00e0 savoir que les temp\u00e9ratures depuis 1990 semblent \u00eatre les plus chaudes des 2000 derni\u00e8res ann\u00e9es.<\/p>\n

(suite…<\/a>)
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\"Click<\/a><\/p>\n

The novel thing about this paper is the use of wavelets (a statistical tool common in image-processing software) to separate the low and high-frequency components of the data. The temperature reconstruction is then formed by combining the high-frequency (< 80 y) signal from tree rings with the low-frequency (> 80 y) signal from lake sediments and other such non-annually resolved proxies. This does two things that may be important: it allows the non-annually resolved proxies to be used (recent previous reconstructions, e.g. MBH98, Esper et al., used only those with at least a value each year, to allow calibration against the instrumental record; Moberg’s approach allows the use of data that only provide 50 y means); and it throws away the long-term signal from the tree rings, which they consider to be untrustworthy. Other techniques have also been used or suggested for the merging of the low and high frequency signals (Guiot, 1985; Rutherford et al. (2005)<\/a>).<\/p>\n

The result is a more long-term variable signal than, e.g., Mann, Bradley and Hughes (1998; henceforth MBH98). Moberg et al. end up with two “warm peaks” in the smoothed record around 1000 and 1100 A.D., at 0 \u00baC on their anomaly scale. A few individual years within these intervals are almost +0.4 \u00baC warmer than the average. In comparison, the most recent data from the instrumental record post 1990 peak at +0.6 or a bit more, on the same scale. The coldest years of the so called “Little Ice age” occur around 1600 and are about -0.7 colder than average, with individual years down to -1.2 \u00baC.<\/p>\n

These results are bound to stir up interest beyond the scientific community, since the “hockey stick” shape of previous reconstructions has become so totemic (although just about everyone agrees that there is no need for this “totemising”<\/a>). We hope that press reports about this paper that mention the increased variability will also emphasize the other key result: that there is “no evidence for any earlier periods in the last millennium with warmer conditions than the post-1990 period – in agreement with previous similar studies (1-4,7)” where (1)is MBH98, (2) is MBH99, (7) is Mann and Jones ’03. The “News” article in Nature<\/i> explicitly rejects the idea that this means we’re not causing the current warming. And it quotes statistician Hans von Storch (who has been quite critical of the earlier work): “it does not weaken in any way the hypothesis that recent observed warming is a result mainly of human activity”.<\/p>\n

There are a couple of technical concerns with the paper that are worth discussing, and which the compressed space of the Nature<\/i> publication didn’t leave the authors space to address fully.<\/p>\n

First, its not too clear that the use of wavelets has added much to the mix. Moberg et al. use the wavelets to merge the high-frequency data from the tree rings with low-frequency data from the other sources, which have lower temporal resolution. But that means the low-res proxies are doing all the work, and the tree rings are just adding a fringe of noise that your eye reads as sort-of error bars.<\/p>\n

Second, because they have used the wavelets, they end up with a non-dimensional signal which has to be normalised against the instrumental record from 1859 to 1979. So if (for example) their reconstruction was too flat in that period, the renormalization would pump it up in the pre-industrial period. Or if too noisy, it would get toned down. The adjustment is done to match “mean value and variance” but (being in Nature<\/i>) this is a bit brief: do they mean the variance of the smoothed or full series? If the full series (as we suppose), then most of the variance is probably coming from the interannual variations of the tree rings, *but* the bit that’s really important is the long-term signal. If its the long term signal that is being matched, then you only have 1 or 2 degrees of freedom in the calibration period (1859-1979). To some extent this uncertainty is eased by their figure 2a, which shows the original records used, with the scaling in \u00baC. Properly nailing down all these issues is going to take a while.<\/p>\n

These points will undoubtedly be answered, and the authors of other temperature reconstructions (including other authors at RealClimate) will have comments on the method. It is worth noting that, in any case, the results of Moberg et al., if they prove correct, would not require any change in the the IPCC TAR summary for policymakers<\/a>, which says “the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year (Figure 1b). Because less data are available, less is known about annual averages prior to 1,000 years before present…”. The last statement is based largely on MBH99. All this remains true with the Moberg reconstruction, and is actually somewhat strengthened, since their results go back 1000 years before MBH99.<\/p>\n

References<\/p>\n

http:\/\/www.grida.no\/climate\/ipcc_tar\/wg1\/005.htm<\/a> – IPCC TAR SPM
\nHockey Stick Myths<\/a><\/p>\n

This post is released under the GFDL<\/a>; the picture in this article is from Wikipedia<\/a><\/small>
\n<\/p>\n

\"Cliquez<\/a><\/p>\n

La nouveaut\u00e9, dans cet article est l\u2019utilisation d\u2019ondelettes (un outil statistique d\u2019usage courant dans le traitement des images par ordinateur) pour s\u00e9parer les composantes \u00e0 haute et basse fr\u00e9quence. La reconstitution des temp\u00e9ratures est alors obtenue en combinant l\u2019information \u00e0 haute fr\u00e9quence (< 80 ans) provenant des anneaux de croissance (cernes) des arbres avec l\u2019information \u00e0 plus basse fr\u00e9quence (>80 ans) provenant des s\u00e9diments lacustres ou d\u2019autres marqueurs \u00e0 r\u00e9solution pluri-annuelle. Cela a deux r\u00e9sultats qui peuvent \u00eatre importants : permettre l\u2019utilisation d\u2019observations \u00e0 fr\u00e9quence non annuelle (les reconstitutions r\u00e9centes pr\u00e9c\u00e9dentes, MBH98, Esper et al., utilisaient des observations comportant au moins une valeur par an pour permettre un calibrage avec les mesures instrumentales ; l\u2019approche de Moberg autorise seulement l\u2019utilisation de moyennes sur 50 ans et rejette les observations provenant des anneaux de croissance sur une longue p\u00e9riode, qu\u2019elle consid\u00e8re comme non fiable. D’autres techniques ont \u00e9t\u00e9 \u00e9galement employ\u00e9es ou sugg\u00e9r\u00e9es pour la combinaison des signaux \u00e0 basse et haute fr\u00e9quence (Guiot, 1985; Rutherford et al. (2005)<\/a> ).
\nLe r\u00e9sultat est un allongement de la p\u00e9riode du signal, par rapport \u00e0, par exemple, Mann, Bradley et Hughes, 1998 (MBH98). Moberg et al.<\/i> montrent 2 \u00ab p\u00e9riodes de chaleur anormales\u00bb dans la courbe liss\u00e9e, autour des ann\u00e9es 1000 et 1100, \u00e0 0\u00b0C sur leur \u00e9chelle d\u2019anomalie. Quelques ann\u00e9es \u00e0 l\u2019int\u00e9rieur de ces p\u00e9riodes atteignent presque +0,4\u00b0C par rapport \u00e0 la moyenne. A titre de comparaison, les mesures instrumentales les plus r\u00e9centes atteignent +0,6\u00b0C et m\u00eame un peu plus dans la p\u00e9riode qui suit 1990, sur la m\u00eame \u00e9chelle. Les ann\u00e9es les plus froides de ce qu\u2019il est convenu d\u2019appeler le \u00ab Petit \u00e2ge glaciaire \u00bb se trouvent autour de 1600 et se situent \u00e0 environ – 0,7\u00b0C par rapport \u00e0 la moyenne, certaines ann\u00e9es atteignant \u20131,2 \u00b0C.
\nCes r\u00e9sultats vont raviver l’int\u00e9r\u00eat au del\u00e0 de la communaut\u00e9 scientifique, en raison de la “tot\u00e9misation” de la forme \u00ab en crosse de hockey \u00bb des reconstitutions pr\u00e9c\u00e9dentes (encore que chacun convient qu’il n’y avait aucun besoin de cette “tot\u00e9misation” ). Nous esp\u00e9rons que les commentaires de la presse sur cet article qui mentionnent l\u2019accroissement de la variabilit\u00e9 souligneront \u00e9galement l’autre r\u00e9sultat principal : \u00e0 savoir qu’il n’y a ” pas d\u2019autre \u00e9vidence au cours du dernier mill\u00e9naire de conditions plus chaudes que la p\u00e9riode suivant 1990 \u2013 et cela en accord avec les \u00e9tudes semblables pr\u00e9c\u00e9dentes (1-4.7)” o\u00f9 (1) = MBH98, (2) = MBH99, (7) = Mann et Jones(03). L’article'”News” de nature rejette explicitement l’id\u00e9e que ceci signifie que nous ne sommes pas la cause du r\u00e9chauffement actuel. Et il cite le statisticien Hans von Storch (qui a \u00e9t\u00e9 assez fortement critique sur les premiers travaux): “il n’affaiblit en aucun cas l’hypoth\u00e8se selon laquelle le r\u00e9chauffement observ\u00e9 r\u00e9cemment est d\u00fb principalement \u00e0 l\u2019activit\u00e9 humaine”.
\nIl y a deux questions techniques concernant cet article qui valaient la peine d\u2019\u00eatre discut\u00e9es, et que les auteurs n\u2019ont pas pu traiter, faute de place dans Nature.
\nD’abord, ce que l\u2019utilisation des ondelettes a apport\u00e9 n\u2019appara\u00eet pas tr\u00e8s clairement. Moberg et al. ont utilis\u00e9 les ondelettes pour combiner les donn\u00e9es \u00e0 fr\u00e9quence haute des anneaux de croissance avec des donn\u00e9es \u00e0 fr\u00e9quence plus basse provenant d\u2019autres sources, qui avait une r\u00e9solution temporelle plus basse. Mais cela signifie que les observations \u00e0 basse r\u00e9solution deviennent en fait le facteur pr\u00e9pond\u00e9rant, les anneaux de croissance ajoutant simplement un bruit de fond qu\u2019on interpr\u00e8te visuellement seulement comme une sorte de barre d\u2019erreur.
\nEn second lieu, en raison de l\u2019utilisation des ondelettes, ils aboutissent \u00e0 un signal sans dimension qui doit \u00eatre normalis\u00e9 par rapport aux mesures instrumentales entre 1859 et 1979. Ainsi, si (par exemple) leur reconstitution appara\u00eet trop plate dans cette p\u00e9riode, cette normalisation accentue les variations pour la p\u00e9riode pr\u00e9industrielle. Inversement, si elle appara\u00eet trop contrast\u00e9e, elle sera att\u00e9nu\u00e9e. L\u2019ajustement est fait pour adapter \u00ab valeur moyenne et variance \u00bb mais (d\u2019apr\u00e8s Nature<\/i>) c\u2019est un peu bref. Parle-t-on de la variance des s\u00e9ries apr\u00e8s lissage ou avant ? S\u2019il s\u2019agit (ce que nous supposons) des s\u00e9ries brutes, la plus grande partie de la variance vient probablement des variations inter annuelles des anneaux de croissance, *mais* la partie r\u00e9ellement importante est le signal \u00e0 long terme. Si c\u2019est le signal \u00e0 long terme qui doit \u00eatre pris en compte, alors il existe seulement 1 ou 2 degr\u00e9s de libert\u00e9 dans la p\u00e9riode de calibrage (1859 \u2013 1979). Cette incertitude appara\u00eet en un sens dans la figure 2a, qui montre les chiffres originaux utilis\u00e9s, avec une \u00e9chelle en \u00b0C. R\u00e9soudre point par point tous ces probl\u00e8mes risque de prendre beaucoup de temps.
\nUne r\u00e9ponse sera sans doute apport\u00e9e \u00e0 ces questions, et les auteurs d\u2019autres reconstitutions de temp\u00e9rature (y compris des auteurs de RealClimate) feront des commentaires sur cette m\u00e9thode. Il est \u00e0 noter que, dans tous les cas, les r\u00e9sultats de Moberg et al., s\u2019ils sont d\u00e9montr\u00e9s, n\u2019exigeront aucun changement dans le r\u00e9sum\u00e9 pour les preneurs de d\u00e9cision du GIGCC<\/a> qui dit : \u00ab l\u2019accroissement de temp\u00e9rature au cours du 20\u00e8me si\u00e8cle est sans doute le plus important de ces 1000 derni\u00e8res ann\u00e9es. Il appara\u00eet aussi que, dans l\u2019h\u00e9misph\u00e8re nord, les ann\u00e9es 1990 apparaissent comme la d\u00e9cade la plus chaude, et 1998 l\u2019ann\u00e9e la plus chaude (Figure 1b). Moins de choses sont connues, par suite du manque de donn\u00e9es, sur les moyennes annuelles ant\u00e9rieures aux 1000 derni\u00e8res ann\u00e9es… \u00bb Cette derni\u00e8re affirmation est largement fond\u00e9e sur MBH99. Tout cela reste vrai avec la reconstitution de Moberg, et se trouve r\u00e9ellement quelque part renforc\u00e9 puisque leurs r\u00e9sultats remontent jusqu\u2019\u00e0 1000 ans avant MBH99.
\n<\/><\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

by William Connolley and Eric Steig The 10th Feb edition of Nature has a nice paper “Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data” by Anders Moberg, DM. Sonechkin, K Holmgren, NM Datsenko, & W Karlin (doi:10.1038\/nature03265). This paper takes a novel approach to the problem of reconstructing past temperatures from […]<\/p>\n","protected":false},"author":12,"featured_media":0,"comment_status":"closed","ping_status":"open","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":[1,9,2],"tags":[],"class_list":{"0":"post-122","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-instrumental-record","8":"category-paleoclimate","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/122","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\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=122"}],"version-history":[{"count":2,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/122\/revisions"}],"predecessor-version":[{"id":3300,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/122\/revisions\/3300"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=122"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=122"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=122"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}