{"id":391,"date":"2007-01-12T13:43:59","date_gmt":"2007-01-12T18:43:59","guid":{"rendered":"\/?p=391"},"modified":"2007-02-11T13:30:21","modified_gmt":"2007-02-11T18:30:21","slug":"arctic-sea-ice-decline-in-the-21st-century","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2007\/01\/arctic-sea-ice-decline-in-the-21st-century\/","title":{"rendered":"Arctic Sea Ice decline in the 21st CenturyD\u00e9clin de la banquise de l’Arctique au 21\u00e8me si\u00e8cle<\/lang_fr>"},"content":{"rendered":"
\n

Guest Commentary by Cecilia Bitz<\/a>, University of Washington<\/small><\/p>\n

Last month a paper I co-authored received considerable media attention. Headlines read “Experts warn North Pole will be ‘ice free’ by 2040”<\/a>, “The Big Melt: Loss of Sea Ice Snowballs<\/a>“, and “Arctic Clear for Summer Sailing by 2040: Models Predict Rapid Decline of Sea Ice<\/a>”. The story also reached NPR, BBC, CBC, the Discovery channel, and Fox News, among others. Dr. Marika Holland, the first author of the paper, was inundated with media attention. About a dozen journalists contacted me too. I was impressed by the questions they posed — questions that probably reflect what the public most wants to know. However, after giving lengthy interviews, I would read the resulting article and see my explanations boiled down to a few lines. In this essay, I’d like to explain the science in the paper and give my answers to the most often asked questions.
\nC\u00e9cilia Bitz<\/a>, Universit\u00e9 de Washington (traduit par Val\u00e9rie Masson-Delmotte)<\/small><\/p>\n

Ce mois-ci, un article dont j’\u00e9tais co-auteur a attir\u00e9 consid\u00e9rablement l’attention des m\u00e9dias. Les unes des journaux titraient : “Les experts tirent la sonnette d’alarme : le P\u00f4le Nord libre de glace d’ici \u00e0 2040”<\/a>; ““Fonte massive : perte de banquise en boule de neige”<\/a>; et “L’Arctique d\u00e9gag\u00e9e pour la navigation d’\u00e9t\u00e9 d’ici 2040 : les mod\u00e8les pr\u00e9voient un d\u00e9clin rapide de la banquise”<\/a>. Cette histoire a aussi gagn\u00e9 les cha\u00eenes de t\u00e9l\u00e9vision : NPR, BBC, CBC, Discovery Channel et Fox News, parmi d’autres. Le Dr Marika Holland, premier auteur de cet article, a \u00e9t\u00e9 submerg\u00e9e par les sollicitations m\u00e9diatiques. Parmi les douzaines de journalistes qui m’ont \u00e9galement contact\u00e9e, j’ai \u00e9t\u00e9 impressionn\u00e9e par les questions qui m’ont \u00e9t\u00e9 pos\u00e9es – des questions qui refl\u00e8tent probablement ce que le grand public veut savoir en priorit\u00e9. Cependant, apr\u00e8s avoir donn\u00e9 de longues interviews, je vois souvent mes explications r\u00e9duites \u00e0 quelques lignes dans les articles… Dans cet essai, je voudrais expliquer les r\u00e9sultats scientifiques de notre publication et mes r\u00e9ponses aux questions les plus fr\u00e9quentes. <\/p>\n

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

<\/p>\n

In our paper<\/a> (with co-author Bruno Tremblay), we examined the September Arctic sea ice cover in the 20th and 21st centuries in climate models, and found occasional decades of very rapid retreat. The most extreme case was a decrease from 6 to 2 million square kilometers in a decade (see Fig 1). This is about 4 times faster than the decline that has been observed in the past decade. <\/p>\n

<\/a>
\nFigure 1<\/em>: (a) Northern Hemisphere sea ice extent in September from one integration of the Community Climate System Model version 3 (CCSM3) with observations from the satellite era shown in red. The light blue line is a 5-yr running mean. The three lower panels show the September ice concentration (ice floes are separated by open water) in three select decades. <\/p>\n

It is common practice to run climate models multiple times with slight variations to the initial conditions. Because the system is chaotic, the natural variability in each run is random and uncorrelated from one run to the next. When an ensemble of runs is averaged, the natural variability is reduced in the ensemble mean, and it is easier to detect a significant trend.<\/p>\n

An ensemble of runs offers an opportunity to evaluate rare events too, such as extreme sea ice decay. We were in search of evidence for “tipping points<\/a>“, which several authors have speculated might exist in sea ice. RealClimate places sea ice in the category of systems with “known unknowns” with regard to tipping points. This means we know there are thresholds involving sea ice (e.g., it can cease to exist), but we don’t know when, or if, the climate will arrive at one.<\/p>\n

Only one of seven ensemble members had an event as extreme as quoted above, and it resulted in near ice-free conditions for September by 2040 (see Fig 1d). (The sea ice grows back at least for some portion of winter for the duration of the 21st century.) However, every ensemble member had an event 5 years or longer at some time in the 21st century when the sea ice retreat was about 3 times faster than the observed retreat since 2001 (see Fig 2). These ensemble members took about 5–10 years longer to become nearly ice-free in September than the most extreme case.<\/p>\n

As illustrated in Fig 1, the sea ice retreat accelerates during the 21st century as the ice decays and more sunlight is absorbed by the ocean (the positive ice-albedo feedback). Increasing ocean heat transport under the sea ice adds to the melt back. The retreat appears abrupt when natural variability in the ocean heat transport into the Arctic Ocean is anomalously high. We did not find clear evidence of a threshold, which can be difficult to identify given the variability and complexity of the climate system. Therefore we can neither verify or rule-out the existence of a tipping point. Regardless, the rapid declines seen in our runs are a serious concern.<\/p>\n

<\/a>
\nFigure 2<\/em>: Northern Hemisphere sea ice extent in September for all seven integration of the CCSM3 with observations from satellite era shown in black. <\/p>\n

Most common questions asked by journalists<\/b><\/p>\n

1) How does our model compare with the trend in the observed record?<\/p>\n

The trends in the seven ensemble members for 1979-2006 span the trend in the observations: Some members retreat a little faster and some a little slower, as expected from the random natural variability in the runs (see Fig 2). The model also reproduces the mean and variance of the observations with good fidelity.<\/p>\n

2) Other scientists are predicting an ice-free Arctic in September by the year 2060-2080, why is this model predicting it 20-40 years sooner?<\/p>\n

First consider estimates based on extrapolation from the observational record. I’ve heard these numbers quoted in the media, but I have not seen a reference to a scientific paper that discusses the analysis in any detail. Figures 2 and 3 illustrate the danger of making an estimate of the future from the observational period. The future trend is not linear, the observational record is too short and the ice-free time is too far in the future to trust extrapolation. If one carries out such an exercise anyway, extrapolation from a linear fit to 1979–2006 gives a zero intersect (indicating the first ice-free year in the future) at about 2110 (see Fig 3). If instead one uses just the last decade, the extrapolation gives 2060. Both estimates are questionable, and so instead we turn to climate models.<\/p>\n

<\/a>
\nFigure 3<\/em>: Extrapolating into the future from the observational record.<\/p>\n

3) Is sea ice in our model retreating faster than in other models?<\/p>\n

Figure 4 shows September ice retreat in 16 models that were archived for the IPCC AR4. The most extreme predictions are from models that have too much or too little sea ice extent compared to observations, so it is important for a model to produce the correct sea ice coverage in the past. Some of the spread is expected from natural variability, but much depends on differing model sensitivity relating to the representation of sea ice, heat transport by the ocean, and cloud cover<\/a>. It is not possible to identify the most accurate model prediction, although I think it is safe to rule out some of the outliers owing to their poor match to the observations.<\/p>\n

About half of the models become ice-free in September during the 21st century. I included one ensemble member from our model, CCSM3, which is in the middle of the pack until about 2020. Our model run retreats faster than most after about 2020, but it isn’t radically different.<\/p>\n

There is considerable uncertainty in future model projections, and Figs 2 and 4 illustrate why it would be better not to focus too much on the year 2040, which to our dismay was highly publicized. The more important message from models is that all but a few outliers predict enourmous sea ice retreat this century. At least a few respectable models predict a nearly ice-free Arctic by midcentury, with a retreat that may be punctuated by rapid events.<\/p>\n

<\/a>
\nFigure 4<\/em>: Northern Hemisphere sea ice extent in September from model integrations submitted to the IPCC AR4 with observations from satellite era shown in black.<\/p>\n

4) Is it too late to save the sea ice? <\/p>\n

The future emissions scenario discussed here is one that assumes modest increases<\/a> in emissions. If humans can reduce the rise in emissions compared to this, then sea ice retreat would be slower and rapid events would be rarer, according to the IPCC AR4 models.<\/p>\n

5) Have we crossed a tipping point?<\/p>\n

I don’t think we have yet. If we fix the greenhouse gas and aerosol levels at year 2000 values and run the model into the 21st century, the sea ice retreats for only another decade or two and then levels off (some of the ensemble members even recover a little bit). So according to our model, the sea ice does not appear to have passed a threshold yet. We have not done an exhaustive study of any years beyond today, so unfortunately we cannot say with certainty that no tipping points exist. The bottom-line: The retreat can be surprisingly rapid even without clear evidence of a tipping point.<\/p>\n

I thank Dr. Holland for valuable suggestions to improve this post and providing Fig 1. I thank Ian Eisenman for computing ice extent from the IPCC AR4 models shown in Fig 4. I look forward to reading your comments and questions.<\/small>
\n
\nDans notre publication<\/a> (avec notre co-auteur Bruno Tremblay), nous avons examin\u00e9 la couverture de glace de mer de l’Arctique au mois de septembre dans des simulations climatiques des 20\u00e8me et 21\u00e8me si\u00e8cles, et observ\u00e9 certaines d\u00e9cennies correspondant \u00e0 des retraits tr\u00e8s rapides. Le cas le plus extr\u00eame correspondait \u00e0 une diminution de 6 \u00e0 2 millions de kilom\u00e8tres carr\u00e9s en dix ans (voir la Figure 1). Ceci est pr\u00e8s de 4 fois plus rapide que le d\u00e9clin observ\u00e9 au cours de la derni\u00e8re d\u00e9cennie.<\/p>\n

<\/a>
\nFigure 1<\/em> : Extension de la glace de mer dans l’h\u00e9misph\u00e8re nord produite dans une simulation du CCSM3 (Mod\u00e8le de Syst\u00e8me Climatique Communautaire, version 3) (noir) et observ\u00e9e par satellite (rouge). La courbe bleu clair correspond \u00e0 une moyenne glissante sur 5 ans. Les 3 cartes du bas montrent la concentration de glace de mer en Septembre au cours de trois d\u00e9cennies sp\u00e9cifiques.<\/p>\n

Il est d’usage courant de faire tourner les mod\u00e8les de climat \u00e0 de multiples reprises, en modifiant l\u00e9g\u00e8rement les conditions initiales. Comme le syst\u00e8me est chaotiques, la variabilit\u00e9 naturelle est al\u00e9atoire dans chaque simulation et n’est pas corr\u00e9l\u00e9e d’une simulation \u00e0 l’autre. Quand un ensemble de simulations est moyenn\u00e9, la variabilit\u00e9 naturelle est r\u00e9duite dans la moyenne d’ensemble et il est plus facile de d\u00e9tecter une tendance significative.<\/p>\n

Un ensemble de simulations permet \u00e9galement d’\u00e9valuer l’occurrence d’\u00e9v\u00e8nements rares, comme un d\u00e9clin extr\u00eame de la glace de mer. Nous sommes \u00e0 la recherche de preuves d’existence de ““points de basculement”<\/a>; plusieurs auteurs ont fait l’hypoth\u00e8se que de tels points de basculement pourraient exister dans le fonctionnement de la glace de mer. RealClimate situe la glace de mer parmi la cat\u00e9gorie de syst\u00e8mes associ\u00e9s \u00e0 des “incertitudes connues” concernant les points de basculement. Cela signifie que nous savons qu’il y a des seuils impliquants la glace de mer (par exemple, celle-ci peut cesser d’\u00eatre pr\u00e9sente), mais nous ne savons pas quand ni si le climat va parvenir \u00e0 ces seuils.<\/p>\n

Parmi les 7 simulations de notre ensemble, seule l’une de ces simulations a produit un \u00e9v\u00e8nement aussi ext\u00eame que celui d\u00e9crit ci-dessus, entra\u00eenant une disparition quasi compl\u00e8te de la couverture de banquise en Septembre d’ici \u00e0 2040 (Figure 1d) (la couverture de glace de mer se reforme partiellement au cours de l’hiver pendant toute la dur\u00e9e du 21\u00e8me si\u00e8cle). Cependant, chaque simulation de notre ensemble avait un \u00e9v\u00e8nement de 5 ans ou plus, au cours du 21\u00e8me si\u00e8cle, pendant lequel la vitesse de disparition de la banquise \u00e9tait environ 3 fois plus rapide que la diminution observ\u00e9e depuis 2001 (voir Figure 2). Ces simulations prenaient 5 \u00e0 10 ans de plus pour devenir libres de glace en septembre que dans le cas le plus extr\u00eame.<\/p>\n

Comme le montre la Figure 1, le retrait de la banquise s’acc\u00e9l\u00e8re au cours du 21\u00e8me si\u00e8cle, lorsque la glace dispara\u00eet et que l’oc\u00e9an absorbe davantage d’\u00e9nergie solaire (c’est la r\u00e9troaction positive due \u00e0 l’alb\u00e9do de la glace). L’augmentation du transport de chaleur par l’oc\u00e9an sous la banquise augmente la fonte. Le retrait devient abrupt lorsque la variabilit\u00e9 naturelle du transport de chaleur par l’oc\u00e9an vers l’Oc\u00e9an Arctique est anormalement \u00e9lev\u00e9e. Nous n’avons pas trouv\u00e9 d’indice de seuil, ce qui peut \u00eatre difficile \u00e0 d\u00e9terminer \u00e0 cause de la variabilit\u00e9 et de la complexit\u00e9 du syst\u00e8me climatique. Par cons\u00e9quent, nous ne pouvons pas ni v\u00e9rifier ni \u00e9liminer l’existence d’un point de basculement. Cela \u00e9tant, les d\u00e9clins rapides identifi\u00e9s dans nos simulations sont inqui\u00e9tants.<\/p>\n

<\/a>
\nFigure 2 <\/em>: Extension de la glace de mer dans l’h\u00e9misph\u00e8re Nord pour chacune des 7 simulations du mod\u00e8le CCSM3; les observations par satellite figurent en rouge.<\/p>\n

Questions les plus fr\u00e9quemment pos\u00e9es par les journalistes<\/b><\/p>\n

1) Comment votre mod\u00e8le se compare-t-il aux tendances observ\u00e9es?<\/p>\n

Les tendances des 7 simulations de notre ensemble couvrent la tendance des observations entre 1979 et 2006 : certaines simulations montrent un retrait un peu plus rapide, d’autres un retrait un peu plus lent, comme il est attendu de la variabilit\u00e9 naturelle al\u00e9atoire des simulations (Figure 2). Le mod\u00e8le reproduit correctement la moyenne et la variance des observations.<\/p>\n

2) D’autres scientifiques pr\u00e9disent un Arctique libre de glace en septembre d’ici aux ann\u00e9es 2060-2080, pourquoi ce mod\u00e8le le montre-t-il 20 \u00e0 40 ans plus t\u00f4t?<\/p>\n

Les premiers ordres de grandeur sont bas\u00e9s sur une extrapolation des observations. J’ai entendu ces nombres cit\u00e9s dans les m\u00e9dia, mais je n’ai jamais vu de r\u00e9f\u00e9rence \u00e0 un article scientifique qui discute en d\u00e9tail cette analyse. Les Figures 2 et 3 montrent le danger de faire des pr\u00e9visions d’\u00e9volutions futures \u00e0 partir de la p\u00e9riode d’observation par satellite. La tendance future n’est pas lin\u00e9aire, la p\u00e9riode d’observation est trop courte et le moment de la disparition de la glace de mer en \u00e9t\u00e9 est trop lointain pour que les extrapolations soient fiables. Si l’on poursuit cependant cet exercice, l’extrapolation d’une tendance lin\u00e9aire \u00e0 partir de 1979-2006 donne une intersection nulle (indicant la premi\u00e8re ann\u00e9e libre de glace \u00e0 venir) vers 2110 (Figure 3). A l’inverse, si l’on utilise uniquement la derni\u00e8re d\u00e9cennie, l’extrapolation donne ce r\u00e9sultat en 2060. Les deux estimations se discutent, et donc nous nous tournons vers les mod\u00e8les de climat.<\/p>\n

<\/a>
\nFigure 3 <\/em>: extrapoler les observations vers le futur…<\/p>\n

3) Est ce que la glace de mer se retire plus vite dans notre mod\u00e8le que dans d’autres mod\u00e8les?<\/p>\n

La Figure 4 montre le retrait de la glace de mer en Septembre dans les 16 mod\u00e8les archiv\u00e9s pour l’IPCC AR4. Les pr\u00e9dictions les plus extr\u00eames proviennent de mod\u00e8les ayant soit trop soit pas assez de couverture de glace de mer par rapport aux observations, et il est donc important pour un mod\u00e8le de produire l’extension correcte de glace de mer au cours du temps. Une partie de la dispersion provient de la variabilit\u00e9 naturelle, mais la plus grande partie r\u00e9sulte de sensibilit\u00e9s diff\u00e9rentes des mod\u00e8les vis \u00e0 vis de la repr\u00e9sentation de la glace de mer, du transport de chaleur par l’oc\u00e9an, et de la couverture nuageuse<\/a>. Il n’est pas possible d’identifier les mod\u00e8les fournissant la pr\u00e9vision la plus pr\u00e9cise, mais je pense qu’il est prudent d’\u00e9liminer quelques exceptions (mod\u00e8les aberrants) en utilisant le crit\u00e8re du r\u00e9alisme par rapport aux observations.<\/p>\n

Environ la moiti\u00e9 des mod\u00e8les deviennent libres de glace en septembre au cours du 21\u00e8me si\u00e8cle. J’ai inclus l’une des simulations de notre mod\u00e8le, CCSM3, qui est au milieu de la m\u00eal\u00e9e jusqu’en 2020 environ. Notre simulation montre un retrait plus rapide que la plupart des mod\u00e8les apr\u00e8s ~2020, mais elle n’est pas radicalement diff\u00e9rent des autres.<\/p>\n

Il y a des incertitudes consid\u00e9rables dans les projections d’\u00e9volution future du climat obtenues \u00e0 l’aide des mod\u00e8les, et les Figures 2 et 4 illustrent pourquoi il vaut mieux ne pas se focaliser sur l’horizon 2040, l’ann\u00e9 qui a \u00e9t\u00e9 mise en avant par les m\u00e9dias – \u00e0 notre consternation. Le message le plus important est que tous les mod\u00e8les, \u00e0 quelques exceptions (aberrantes) pr\u00e8s, pr\u00e9disent un retrait massif de la banquise au cours de ce si\u00e8cle. Quelques mod\u00e8les respectables pr\u00e9disent un Oc\u00e9an Arctqiue quasiment libre de glace d’ici au milieu de ce si\u00e8cle, avec un retrait qui peut \u00eatre ponctu\u00e9 par des \u00e9v\u00e8nements rapides.<\/p>\n

<\/a>
\nFigure 4<\/em> : extension de glace de mer dans l’h\u00e9misph\u00e8re Nord en septembre, dans toutes les simulations conduites pour l’IPCC AR4. Les observations par satellite figurent en noir.<\/p>\n

4) Est-ce trop tard pour sauver la banquise?<\/p>\n

Le sc\u00e9nario de rejets de gaz \u00e0 effet de serre discut\u00e9 ici est celui qui fait l’hypoth\u00e8se d’une augmentation mod\u00e9r\u00e9e<\/a> des \u00e9missions. Si les humains peuvent diminuer l’augmentation des \u00e9missions par rapport \u00e0 ceci, alors le retrait de la banquise serait plus lent, et les \u00e9v\u00e8nements rapides seraient plus rares, selon les mod\u00e8les de l’IPCC AR4.<\/p>\n

5) Avons-nous franchi un point de basculement?<\/p>\n

Je ne pense pas que ce soit d\u00e9j\u00e0 le cas. Si nous fixons les concentrations de gaz \u00e0 effet de serre et d’a\u00e9rosols aux niveaux de l’ann\u00e9e 2000 et que nous faisons tourner le mod\u00e8le pour le 21\u00e8me si\u00e8cle, alors la glace de mer diminue pendant une \u00e0 deux d\u00e9cennies seulement, puis se stabilise (certaines simulations de notre ensemble montrent m\u00eame une r\u00e9cup\u00e9ration partielle). Donc, selon notre mod\u00e8le, la banquise ne semble pas encore avoir pass\u00e9 de seuil critique. Nous n’avons pas fait d’\u00e9tude d\u00e9taill\u00e9e de ce type pour les ann\u00e9es \u00e0 venir, et donc nous ne pouvons pas affirmer avec certitude qu’il n’existe pas de point de basculement. Corollaire : le retrait peut \u00eatre \u00e9tonnamment rapide, sans indice net de point de basculement.
\n<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Guest Commentary by Cecilia Bitz, University of Washington Last month a paper I co-authored received considerable media attention. Headlines read “Experts warn North Pole will be ‘ice free’ by 2040”, “The Big Melt: Loss of Sea Ice Snowballs“, and “Arctic Clear for Summer Sailing by 2040: Models Predict Rapid Decline of Sea Ice”. The story […]<\/p>\n","protected":false},"author":12,"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":[12,1],"tags":[],"class_list":{"0":"post-391","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-arctic-and-antarctic","7":"category-climate-science","8":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/391","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=391"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/391\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=391"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=391"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=391"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}