{"id":116,"date":"2004-11-28T11:11:43","date_gmt":"2004-11-28T15:11:43","guid":{"rendered":"\/?p=116"},"modified":"2005-03-28T07:13:50","modified_gmt":"2005-03-28T11:13:50","slug":"climate-sensitivity","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2004\/11\/climate-sensitivity\/","title":{"rendered":"Climate sensitivity Sensibilit\u00e9 climatique<\/lang_fr>"},"content":{"rendered":"
\n

Climate sensitivity is a measure of the equilibrium global surface air temperature change for a particular forcing<\/a>. It is usually given as a \u00b0C change per W\/m2<\/sup> forcing. A standard experiment to determine this value in a climate model is to look at the doubled CO2<\/sub> climate, and so equivalently, the climate sensitivity is sometimes given as the warming for doubled CO2<\/sub> (i.e. from 280 ppm to 560 ppm). The forcing from doubled CO2<\/sub> is around 4 W\/m2<\/sup> and so a sensitivity of 3\u00b0C for a doubling, is equivalent to a sensitivity of 0.75 \u00b0C\/W\/m2<\/sup>. The principal idea is that if you know the sum of the forcings, you can estimate what the eventual temperature change will be. <\/p>\n

We should underscore that the concepts of radiative forcing and climate sensitivity are simply an empirical shorthand that climatologists find useful for estimating how different changes to the planet’s radiative balance will lead to eventual temperature changes. There are however some subtleties which rarely get mentioned. Firstly, there are a number of ways to define the forcings. The easiest is the ‘instantaneous forcing’ – the change is made and the difference in the net radiation at the tropopause is estimated. But it turns out that other definitions such as the ‘adjusted forcing’ actually give a better estimate of the eventual temperature change. These other forcings progressively allow more ‘fast’ feedbacks to operate (stratospheric temperatures are allowed to adjust for instance), but the calculations get progressively more involved. <\/p>\n

Secondly, not all forcings are equal. Because of differences in vertical or horizontal distribution of forcings, some changes can have a more than proportional effect on temperatures. This can be described using a relative ‘efficacy’ factor that depends on the individual forcing. For instance, the effect of soot making snow and sea ice darker has a higher efficacy than an equivalent change in CO2<\/sub> with the same forcing, mainly because there is a more important ice-albedo feedback in the soot case. The ideal metric of course would be a forcing that can be calculated easily and where every perturbation to the radiative balance had an relative efficacy of 1. Unfortunately, that metric has not yet been found!
\nLa sensibilit\u00e9 climatique est une mesure de la variation de temp\u00e9rature d’\u00e9quilibre globale de surface atmosph\u00e9rique pour un forcage<\/a> donn\u00e9. Son unit\u00e9 est g\u00e9n\u00e9ralement des \u00b0C change par W\/m2<\/sup> de for\u00e7age. Pour d\u00e9terminer sa valeur, une exp\u00e9rience classique est de regarder le climat pour une concentration en CO2<\/sub> atmosph\u00e9rique doubl\u00e9e ; ce qui fait que la sensibilit\u00e9 est donn\u00e9e quelquefois donn\u00e9e en terme de r\u00e9chauffement pour un doublement du du CO2<\/sub>(c.a.d. une augmentation de 280 ppm a 560 ppm). Le for\u00e7age pour un doublement du CO2<\/sub> est d’environ 4 W\/m2<\/sup> ce qui implique qu’une sensibilit\u00e9 de 3\u00b0C pour un doublement est \u00e9quivalente a une sensibilit\u00e9 de 0.75 \u00b0C\/W\/m2<\/sup>. L’id\u00e9e fondamentale est que si on conna\u00eet la somme de tous les for\u00e7ages, il est alors possible d’estimer l’amplitude du changement de temp\u00e9rature induit.<\/p>\n

Il est important de noter que les concepts de for\u00e7age radiatif et de sensibilit\u00e9 climatique sont des raccourcis empiriques que les climatologues trouvent utiles pour estimer l’impact de changements dans le bilan radiatif terrestre en termes de changements de temp\u00e9ratures. Quelques nuances doivent \u00eatre mentionn\u00e9es. Premi\u00e8rement, il existe diff\u00e9rentes mani\u00e8res de d\u00e9finir un for\u00e7age. La plus simple est le ‘for\u00e7age instantan\u00e9’ – le changement est appliqu\u00e9 et la diff\u00e9rence nette de radiation est estim\u00e9e a la tropopause. Mais, en r\u00e9alit\u00e9, d’autres d\u00e9finitions, comme le ‘for\u00e7age ajust\u00e9’ donnent de meilleurs estimations du changement de temp\u00e9rature final. Ces autres for\u00e7ages autorisent progressivement la mise en place de plus de r\u00e9troactions ‘rapides’ (les temp\u00e9ratures stratosph\u00e9riques peuvent s’ajuster par exemple), mais le niveau de calcul augmente en retour.<\/p>\n

Deuxi\u00e8mement, tous les for\u00e7ages ne sont pas \u00e9gaux. En raison de diff\u00e9rences dans les distributions verticales ou horizontales des for\u00e7ages, certains changements peuvent avoir un effet sur les temp\u00e9ratures sup\u00e9rieur a celui directement proportionnel. Ceci peut \u00eatre d\u00e9crit comme un facteur relatif d’efficacit\u00e9’, sp\u00e9cifique a chaque for\u00e7age. Par exemple, l’effet des suies a assombrir la neige et la glace de mer a une efficacit\u00e9 plus \u00e9lev\u00e9e qu’un changement \u00e9quivalent en CO2<\/sub> avec le m\u00eame for\u00e7age, principalement en raison d’une r\u00e9tro-action glace-alb\u00e9do dans le cas des suies. Id\u00e9alement, un for\u00e7age pourrait \u00eatre quantifi\u00e9e par une m\u00e9thode facile et dans laquelle chaque perturbation du bilan radiatif aurait une efficacit\u00e9 relative de 1. Malheureusement, une telle m\u00e9thode n’a pas encore \u00e9t\u00e9 trouv\u00e9e !<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Climate sensitivity is a measure of the equilibrium global surface air temperature change for a particular forcing. It is usually given as a \u00b0C change per W\/m2 forcing. A standard experiment to determine this value in a climate model is to look at the doubled CO2 climate, and so equivalently, the climate sensitivity is sometimes […]<\/p>\n","protected":false},"author":2,"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":[6],"tags":[],"class_list":{"0":"post-116","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-glossary","7":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/116","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=116"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/116\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=116"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=116"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=116"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}