{"id":187,"date":"2006-01-19T23:00:00","date_gmt":"2006-01-20T03:00:00","guid":{"rendered":"\/?p=187"},"modified":"2006-02-21T09:49:13","modified_gmt":"2006-02-21T13:49:13","slug":"atlantic-circulation-changes","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2006\/01\/atlantic-circulation-changes\/","title":{"rendered":"Atlantic circulation change summary Changement de la circulation Atlantique \u2013 R\u00e9sum\u00e9<\/lang_fr>"},"content":{"rendered":"
\n

Nature<\/em> this week has an excellent summary<\/a> of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it’s still worth reading if you want to understand how results like the Bryden et al paper<\/a> – that suggested that the Atlantic overturning had reduced by 30% in recent decades – are assimilated into the scientific picture.Nature a publi\u00e9 cette semaine un excellent<\/a> r\u00e9sum\u00e9 de l’\u00e9tat de la science en ce qui concerne les changements possibles de la circulation thermohaline (ou m\u00e9ridionale) oc\u00e9anique dans l’Atlantique et son impact sur le climat. M\u00eame s’il cite certains d’entre nous, cela vaut n\u00e9anmoins la peine de le lire pour comprendre comment les r\u00e9sultats de l’article de Bryden et al. <\/a>– qui sugg\u00e9raient que le retournement atlantique s’\u00e9tait r\u00e9duit de 30% dans les derni\u00e8res d\u00e9cennies – sont assimil\u00e9s dans la communaut\u00e9 scientifique.
\n(suite…)<\/a>
\n<\/lang_fr><\/p>\n

\nThe [Bryden et al] result came as a surprise to those in the field. Few scientists had thought that such dramatic slowing of the thermohaline circulation could happen so soon. Models suggest that the increase in fresh water needed for a conveyor shutdown would not be expected without a global warming of 4\u20135 C; warming in the twentieth century is currently put at 0.6 C (ref. 3). The most complex computer models of the climate and oceans, the sort used to make climate predictions for the Intergovernmental Panel on Climate Change (IPCC), suggest that the flow might be expected to slow by an average of 25% by the end of the twenty-first century, but not to shut down completely.\n<\/p><\/blockquote>\n

The reason for this surprise is that other data of relevance, such as the changes in salinity or deep convection or lack of observed cooling in the North Atlantic, aren’t obviously consistent with such a large change:<\/p>\n

\n[Ruth Curry’s] recent analysis of 1950 to 2005 salinity data suggests that 4,000 cubic kilometres \u2014 eight times the annual outflow of the Mississippi river \u2014 of fresh water have accumulated in the upper ocean layers since the 1960s. \u201cThe extra freshwater input is beginning to affect density,\u201d she says. But the amount of fresh water needed to shut down the thermohaline circulation in Rahmstorf \u2019s comparisons is an order of magnitude greater than the flux reported by Curry, and she agrees that the circulation will not be unduly affected this century. Peter Wadhams, an oceanographer at the University of Cambridge, UK, last year reported a substantial weakening of convection \u2018chimneys\u2019 down which surface water flows in the Greenland sea, but it is unknown how much of the observed effect is due to natural variability.\n<\/p><\/blockquote>\n

It is possible that the Bryden result was an unlucky statistical artefact: \u201cThe results are based, after all, on just five snapshots of an extremely noisy and under-sampled system,\u201d says Carl Wunsch. His own analysis<\/a> doesn’t suggest any significant change over the last decade or so (though it’s probably worth pointing out that Wunsch’s opinions<\/a> of the relative (un)importance of the Atlantic circulation for climate change are probably not (yet?) mainstream in the community).<\/p>\n

Everyone quoted is however agreed on one thing: “the notion that [a future change in the themohaline circulation] may trigger a mini ice age is a myth\u201d. The evidence of previous changes for instance at the Younger Dryas or during the 8.2 kyr event <\/a> is quite strong, and significant coolings were observed particular around the North Atlantic, but even such localised coolings are not predicted to occur if the circulation slows as an effect of global warming.<\/p>\n

It is however a complicated business, and the stability of this circulation depends on many aspects of climate that are poorly observed and uncertainly modelled. So it may yet be some time before new observations (such as the permanant monitoring array recently installed along a section of the ocean), better modelling, and a better appreciation of the paleo-climatic data add up to a coherent understanding of this interestingly counter-intuitive aspect of climate change.
\n<\/p>\n

\nLe r\u00e9sultat de Bryden et al est surprenant pour les sp\u00e9cialistes de ce domaine. Peu de scientifiques avaient pens\u00e9 qu’un tel ralentissement de la circulation thermohaline pouvait arriver si t\u00f4t. Les mod\u00e8les sugg\u00e8rent que l’augmentation en eau douce n\u00e9cessaire pour un arr\u00eat de la circulation ne sont pas attendu sans un r\u00e9chauffement global de 4 \u00e0 5\u00b0C; le r\u00e9chauffement du vingti\u00e8me si\u00e8cle est actuellement estim\u00e9 \u00e0 0,6\u00b0C (r\u00e9f 3). Les mod\u00e8les les plus complexes du climat et des oc\u00e9ans, tels que ceux utilis\u00e9s pour r\u00e9aliser les pr\u00e9visions climatique pour le Groupe intergouvernemental d’experts sur le climat (GIEC), sugg\u00e8rent qu’on peut s’attendre \u00e0 ce que le flux ralentisse d’une moyenne de 25% \u00e0 la fin du vingt et uni\u00e8me si\u00e8cle, mais pas qu’il s’arr\u00eate compl\u00e8tement.\n<\/p><\/blockquote>\n

La raison de cette surprise est que d’autres donn\u00e9es pertinentes, comme les changements de salinit\u00e9, la convection profonde ou le refroidissement de l’Atlantique Nord qui n’est pas observ\u00e9, ne sont pas coh\u00e9rents de fa\u00e7on \u00e9vidente avec un si grand changement. <\/p>\n

L’analyse r\u00e9cente des donn\u00e9es de salinit\u00e9 de 1950 \u00e0 2005 de [Ruth Curry] sugg\u00e8re que 4000 kilom\u00e8tres cube \u2013 8 fois le d\u00e9bit annuel du Mississippi \u2013 d’eau douce se sont accumul\u00e9s dans les couches sup\u00e9rieures de l’oc\u00e9an depuis 1960. \u00ab L’eau douce suppl\u00e9mentaire commence \u00e0 influencer la densit\u00e9 \u00bb dit-elle. Mais la quantit\u00e9 d’eau douce n\u00e9cessaire pour arr\u00eater la circulation thermohaline dans les comparaisons de Rahmstorf est d’un ordre de grandeur sup\u00e9rieur aux quantit\u00e9s rapport\u00e9es par Curry et elle est d’accord avec le fait que la circulation ne sera pas excessivement affect\u00e9e pendant ce si\u00e8cle. Peter Wadhams, un oc\u00e9anographe de l’universit\u00e9 de Cambridge, UK, a rapport\u00e9 l’ann\u00e9e derni\u00e8re un affaiblissement substantiel des chemin\u00e9es convectives qui entra\u00eenent les eaux de surface vers le bas dans le mer du Groenland, mais on ne sait pas quelle part de l’effet observ\u00e9e est du \u00e0 une variabilit\u00e9 naturelle. <\/p><\/blockquote>\n

Il est possible que le r\u00e9sultat de Bryden soit le r\u00e9sultat d’un artefact statistique malchanceux. \u00ab Les r\u00e9sultats sont bas\u00e9s, apr\u00e8s tout, sur cinq \u00e9chantillons d’un syst\u00e8me comportant beaucoup de bruit et sous \u00e9chantillonn\u00e9, \u00bb dit Carl Wunsch. Sa propre analyse<\/a> ne sugg\u00e8re aucun changement significatif au cours de grosso modo la derni\u00e8re d\u00e9cennie (mais cela vaut probablement la peine de signaler que l’opinion<\/a> de Wunsch sur l’importance (ou la non-importance) relative de la circulation atlantique pour le climat n’est probablement pas courante dans la communaut\u00e9). <\/p>\n

Tout ceux qui sont cit\u00e9s sont cependant d’accord sur une chose: \u00ab la notion qu’ [un futur changement de la circulation thermohaline] pourrait d\u00e9clencher un mini \u00e2ge glaciaire est un mythe. \u00bb La preuve de changements pr\u00e9c\u00e9dents, par exemple au Dryas R\u00e9cent ou durant l’\u00e9v\u00e9nement d’il y a 8200 ans<\/a> est relativement forte, et des refroidissements significatifs furent observ\u00e9s en particulier autour de l’Atlantique Nord, mais m\u00eame de tels refroidissements localis\u00e9s ne sont pas pr\u00e9vus si la circulation ralentit \u00e0 la suite du r\u00e9chauffement global.<\/p>\n

C’est toutefois une histoire compliqu\u00e9e, et la stabilit\u00e9 de la circulation d\u00e9pend de nombreux aspects du climat qui sont mal observ\u00e9s, et mod\u00e9lis\u00e9s avec incertitude. Il pourrait donc s’\u00e9couler du temps avant que de nouvelles observations (notamment gr\u00e2ce au r\u00e9seau permanent d’observation r\u00e9cemment install\u00e9 le long d’un section de l’oc\u00e9an), une meilleure mod\u00e9lisation et une meilleure \u00e9valuation des donn\u00e9es pal\u00e9o-climatique permettent une compr\u00e9hension coh\u00e9rente de ces int\u00e9ressants aspects contre-intuitifs des changements climatiques.<\/p>\n

(Traduit de l’anglais par Alain Henry)<\/em> <\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Nature this week has an excellent summary of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it’s still worth reading if you want to understand how results like the Bryden […]<\/p>\n","protected":false},"author":2,"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":[1,19,2],"tags":[],"class_list":{"0":"post-187","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-oceans","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\/187","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\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=187"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/187\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=187"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=187"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=187"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}