{"id":124,"date":"2005-02-23T14:38:33","date_gmt":"2005-02-23T18:38:33","guid":{"rendered":"\/?p=124"},"modified":"2005-03-22T13:26:55","modified_gmt":"2005-03-22T17:26:55","slug":"why-looking-for-global-warming-in-the-oceans-is-a-good-idea","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/02\/why-looking-for-global-warming-in-the-oceans-is-a-good-idea\/","title":{"rendered":"Why looking for global warming in the oceans is a good idea Voici pourquoi rechercher le r\u00e9chauffement global dans les oc\u00e9ans est pertinent<\/lang_fr>"},"content":{"rendered":"
\n

A lot of press and commentary came out this week concerning a presentation and press release<\/a> from Tim Barnett and Scripps colleagues presenting at the AAAS meeting (The Independent<\/a>, John Fleck<\/a> ,(and again<\/a>) David Appell<\/a>…etc). Why did this get so much attention given that there is no actual paper yet?
\npar Gavin Schmidt (traduit par Pierre Allemand)<\/small><\/p>\n

Un grand nombre d\u2019articles de presse et de commentaires sont sortis cette semaine concernant une pr\u00e9sentation et un communiqu\u00e9 de presse<\/a> de Tim Barnett, Scripps et coll\u00e8gues au congr\u00e8s de l\u2019AAAS (American Society for the Advancement of Science), (The Independent<\/a>, John Fleck<\/a> ,(et de nouveau ici<\/a>) David Appell<\/a>…etc. (NdT : sites en anglais). Pourquoi cela a-t-il donn\u00e9 lieu \u00e0 autant d\u2019attention, alors qu\u2019aucune publication n\u2019est encore disponible ?<\/p>\n

(suite…)<\/a><\/p>\n

<\/lang_fr>
\n
\nBasically, it is because it is a really good idea. The background is this: whenever forcings change, there is a delay in the climate response because of the large thermal inertia of the oceans – it’s takes a long time to warm them up or cool them down. While they are changing, there will be a ‘radiation imbalance’ at the top of the atmosphere. In the case of an increase in greenhouse gases (which cause a warming), that implies that the planet will be absorbing more solar radiation than it emits as longwave radiation. This imbalance will persist until the planet regains it’s ‘normal’ quasi-equilibrium. (The planet is never in perfect equilibrium of course, there are small fluctuations in the annual mean that occur in the absence of any forcings, but these fluctuations are small compared to the current anthropogenic forcings and the implied net imbalance). <\/p>\n

This imbalance is really an important quantity – estimates of how much warming is in the ‘pipeline’, the size of the aerosol cooling effect etc. all depend on knowing what this number is. However, it is very difficult to measure from space. Getting an accurate global average of all the long wave energy coming out, along with a correctly calibrated estimate of all the solar radiation coming in and estimating the difference between them to the necessary accuracy (fractions of a W\/m2<\/sup>) is currently beyond our technical capabilities. <\/p>\n

So then, how do we estimate it? All of that energy has to be going somewhere, and it is easy to show that neither the land surface nor the glaciers can be storing this energy. Therefore it is going into warming the oceans and indeed historical analysis of ocean temperatures by Levitus and colleagues<\/a> in 2000, and updated recently in GRL<\/a> does show warming that is consistent with the radiative imbalance suggested by climate models (around 0.5 W\/m2<\/sup> from 1955 to 1995 and possibly as high as 0.7 W\/m2<\/sup> over the last decade, Hansen et al (2002)<\/a>). <\/p>\n

The advantage of the ocean heat content changes for detecting climate changes is that there is less noise than in the surface temperature record due to the weather that affects the atmospheric measurements, but that has much less impact below the ocean mixed layer. The disadvantage is that comprehensive ocean measurements do not go back very far. <\/p>\n

Previous work<\/a> by Barnett’s group showed that coupled models when forced with greenhouse gases did give ocean heat content changes similar to that shown in the data. But questions remained concerning the degree of decadal variability, the length of the record and the balance in the models between aerosol forcing and climate sensitivity (which can’t really be disentangled using this measure). (A recent report from the National Academies discusses this in more detail<\/a>). With the latest round of modelling results now having been performed and archived<\/a> for the IPCC 4th Assessment Report, the time is appropriate to revisit the question with more up-to-date models and observations.<\/p>\n

When this is done, will it really provide the ‘final proof’ of man-made global warming? As I indicated above, the preliminary work by Levitus, Barnett, Hansen and others has already demonstrated that this is a good approach. Thus it is more a question of refining the details, rather than suddenly ‘proving’ global warming. If the latest round of models compare better to the data than they did before (as claimed at the AAAS), and if the result is robust to some of the remaining uncertainties (in aerosol forcing, ocean model components etc.), then it will certainly add to the ‘balance of evidence’ that man-made global warming is already here. <\/p>\n


\nFondamentalement, c\u2019est parce qu\u2019il s\u2019agit v\u00e9ritablement d\u2019une bonne id\u00e9e. L\u2019id\u00e9e sous-jacente est celle-ci : quels que soient les changements dans les for\u00e7ages, il y a un certain d\u00e9lai dans la r\u00e9ponse climatique en raison de la grande inertie thermique des oc\u00e9ans – Les r\u00e9chauffer ou les refroidir prend beaucoup de temps. Pendant ce changement, il y aura un \u201cd\u00e9s\u00e9quilibre radiatif\u201d dans la couche sup\u00e9rieure de l\u2019atmosph\u00e8re. Dans le cas d\u2019un accroissement des gaz \u00e0 effet de serre (qui causent un r\u00e9chauffement), cela implique que la plan\u00e8te absorbera plus de radiations solaires qu\u2019elle n\u2019en \u00e9met sous la forme de radiations \u00e0 de grandes longueurs d\u2019onde. Ce d\u00e9s\u00e9quilibre persistera jusqu\u2019\u00e0 ce que la plan\u00e8te retrouve son quasi-\u00e9quilibre \u00ab normal \u00bb. (La plan\u00e8te n\u2019est, \u00e9videmment, jamais en \u00e9quilibre parfait, il y a de petites fluctuations dans les moyennes annuelles qui se produisent en l\u2019absence de tout for\u00e7age, mais ces fluctuations sont petites compar\u00e9es aux for\u00e7ages anthropog\u00e9niques actuels, et au d\u00e9s\u00e9quilibre net correspondant).
\nCe d\u00e9s\u00e9quilibre est vraiment une grandeur importante – les estimations du potentiel de r\u00e9chauffement restant dans le ‘tuyau’, l\u2019importance de l\u2019effet de r\u00e9chauffement par les a\u00e9rosols, etc. d\u00e9pendent tous de la connaissance de cette valeur. N\u00e9anmoins, il est tr\u00e8s difficile d\u2019effectuer des mesures depuis l\u2019espace. Obtenir la valeur globale exacte de toute l\u2019\u00e9nergie rayonn\u00e9e \u00e0 des longueurs d\u2019onde \u00e9lev\u00e9es en m\u00eame temps qu\u2019une estimation correctement calibr\u00e9e de l\u2019\u00e9nergie solaire re\u00e7ue sous forme de rayonnement et estimer la diff\u00e9rence entre les deux avec une pr\u00e9cision suffisante (fractions de W\/m2) se situe au-del\u00e0 de nos possibilit\u00e9s actuelles.
\nBien, dans ces conditions, comment l\u2019estime-t-on ? Toute cette \u00e9nergie doit aller quelque part, et il est facile de montrer que ni la surface des terres \u00e9merg\u00e9es, ni les glaciers ne peuvent stocker enti\u00e8rement cette \u00e9nergie. En cons\u00e9quence, celle-ci va r\u00e9chauffer les oc\u00e9ans, et en fait, l\u2019analyse historique des temp\u00e9ratures des oc\u00e9ans, par Levitus et collaborateurs<\/a> en 2000, r\u00e9cemment mise \u00e0 jour ( dans les Geophysical Research Letters<\/a>) montre un r\u00e9chauffement qui est en ad\u00e9quation avec le d\u00e9s\u00e9quilibre radiatif sugg\u00e9r\u00e9 par les mod\u00e8les climatiques (environ 0,5 W\/m2 de 1955 \u00e0 1995 et probablement jusqu\u2019\u00e0 0,7 W\/m2 au cours de la derni\u00e8re d\u00e9cennie, Hansen et al (2002)<\/a>).
\nPrendre en compte la chaleur de l\u2019oc\u00e9an pour d\u00e9tecter les changements climatiques permet de limiter le bruit de fond pr\u00e9sent dans les mesures de temp\u00e9ratures de surface, en raison des \u00e9v\u00e9nements m\u00e9t\u00e9orologiques qui affectent beaucoup plus les mesures atmosph\u00e9riques que les couches oc\u00e9aniques, constamment m\u00e9lang\u00e9es. L\u2019inconv\u00e9nient est que les mesures oc\u00e9aniques ne permettent pas de remonter tr\u00e8s loin dans le temps.
\nUn travail ant\u00e9rieur de l’\u00e9quipe de Barnett<\/a> a d\u00e9montr\u00e9 que les mod\u00e8les coupl\u00e9s forc\u00e9s avec des gaz \u00e0 effet de serre montrent un r\u00e9chauffement oc\u00e9anique similaire \u00e0 celui donn\u00e9 par les mesures instrumentales. Mais certaines questions restaient sans r\u00e9ponse, en particulier le degr\u00e9 de variation d\u00e9cennale, la dur\u00e9e des observations, et l\u2019importance relative, dans les mod\u00e8les du for\u00e7age des a\u00e9rosols sur la sensibilit\u00e9 climatique (qui ne peuvent pas \u00eatre r\u00e9ellement dissoci\u00e9s l\u2019un de l\u2019autre en utilisant ces mesures). (un rapport r\u00e9cent des National Academies pr\u00e9sente cela plus en d\u00e9tai<\/a>l). La mod\u00e9lisation de la s\u00e9rie la plus r\u00e9cente de r\u00e9sultats ayant \u00e9t\u00e9 effectu\u00e9e, et les chiffres mis en ligne<\/a> pour le 4\u00e8me Rapport d\u2019Evaluation du GIEC, il est maintenant temps de revoir la question avec des mod\u00e8les et des observations remis \u00e0 jour.
\nQuand cela sera fait, est-ce que cela fournira vraiment la ‘preuve finale’ du r\u00e9chauffement climatique d\u2019origine humaine ? Comme je l\u2019ai indiqu\u00e9 plus haut, les travaux pr\u00e9liminaires de Levitus, Barnett, Hansen et al. ont d\u00e9j\u00e0 d\u00e9montr\u00e9 que c\u2019\u00e9tait une bonne approche. Par cons\u00e9quent, il s\u2019agit plus de peaufiner les d\u00e9tails, que de \u2018prouver\u2019 ex-abrupto le r\u00e9chauffement global. Si la nouvelle s\u00e9rie de mod\u00e8les concorde mieux avec les donn\u00e9es que pr\u00e9c\u00e9demment, (comme communiqu\u00e9 \u00e0 l\u2019AAAS), et si le r\u00e9sultat appara\u00eet suffisamment probant vis-\u00e0-vis de certaines incertitudes encore pr\u00e9sentes (for\u00e7age par les a\u00e9rosols, composantes du mod\u00e8le oc\u00e9anique, etc\u2026), alors, celui-ci contribuera \u00e0 renforcer le ‘faisceau d\u2019\u00e9vidences’ montrant que le r\u00e9chauffement global d\u2019origine humaine est d\u00e9j\u00e0 effectif.
\n<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

A lot of press and commentary came out this week concerning a presentation and press release from Tim Barnett and Scripps colleagues presenting at the AAAS meeting (The Independent, John Fleck ,(and again) David Appell…etc). Why did this get so much attention given that there is no actual paper yet? par Gavin Schmidt (traduit par […]<\/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":[5,1,19],"tags":[],"class_list":{"0":"post-124","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-modelling","7":"category-climate-science","8":"category-oceans","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/124","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=124"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/124\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=124"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=124"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=124"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}