{"id":58,"date":"2004-12-07T11:21:03","date_gmt":"2004-12-07T15:21:03","guid":{"rendered":"\/?p=58"},"modified":"2007-09-30T12:45:30","modified_gmt":"2007-09-30T17:45:30","slug":"why-does-the-stratosphere-cool-when-the-troposphere-warms","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2004\/12\/why-does-the-stratosphere-cool-when-the-troposphere-warms\/","title":{"rendered":"Why does the stratosphere cool when the troposphere warms? Pourquoi la stratosph\u00e8re refroidit alors que la troposph\u00e8re se r\u00e9chauffe ?<\/lang_fr>"},"content":{"rendered":"
\n

This post is obsolete and wrong in many respects. Please see this more recent post<\/a> for links to the answer.<\/strong><\/p>\n

14\/Jan\/05: This post was updated in the light of my further education in radiation physics.<\/strong>
\n25\/Feb\/05: Groan…and again.<\/strong><\/p>\n

Recent discussions of climate change (MSU Temperature Record<\/a>, ACIA<\/a>) have highlighted the fact that the stratosphere is cooling while the lower atmosphere (troposphere) and surface appear to be warming. The stratosphere lies roughly 12 to 50 km above the surface and is marked by a temperature profile that increases with height. This is due to the absorbtion by ozone of the sun’s UV radiation and is in sharp contrast to the lower atmosphere. There it generally gets colder as you go higher due to the expansion of gases as the pressure decreases. Technically, the stratosphere has a negative ‘lapse rate’ (temperature increases with height), while the lower atmosphere’s lapse rate is positive.
\nPar Gavin Schmidt (traduit de l’anglais par Vincent No\u00ebl)<\/small>
\nDes \u00e9tudes r\u00e9centes du changement climatique (MSU temp\u00e9rature Record<\/a>, ACIA<\/a>) ont mis en \u00e9vidence un refroidissement de la stratosph\u00e8re, en parall\u00e8le a un apparent r\u00e9chauffement de la surface et la basse atmosph\u00e8re (troposph\u00e8re). La stratosph\u00e8re se situe entre 12 et 50 km d’altitude environ. Elle se caract\u00e9rise par un profil de temp\u00e9rature qui augmente avec l’altitude, en raison de l’absorption des radiations solaires ultraviolettes par l’ozone stratosph\u00e9rique. Les choses sont tr\u00e8s diff\u00e9rentes dans la troposph\u00e8re (de 0 a 12 km d’altitude environ), ou, en g\u00e9n\u00e9ral, la temp\u00e9rature baisse lorsque l’altitude augmente, en raison de l’expansion des gaz alors que la pression atmosph\u00e9rique diminue. En d’autres termes, la stratosph\u00e8re a un gradient de temp\u00e9rature n\u00e9gatif, alors que la troposph\u00e8re a un gradient positif.
\n(suite…)<\/a>
\n<\/lang_fr>
\n<\/p>\n

(New.<\/strong> A brief discussion of the greenhouse effect is useful here. You can read the technical reasons below, but the key point for this discussion is that increasing greenhouse gases increases the temperature gradient from the surface. <\/p>\n

[Technical digression:<\/strong> Imagine an atmosphere with multiple isothermal layers that only interact radiatively. At equilibrium each layer can only emit what it absorbs. If the amount of greenhouse gas (GHG) is low enough, each layer will basically only see the emission from the ground and so by Stefan-Boltzmann you get for the air temperature (Ta) and the ground temperature (Tg) that 2 Ta4<\/sup> = Tg4<\/sup> , i.e. Ta=0.84 Tg for all layers (i.e. an isothermal atmosphere). On the other hand, if the amount of GHGs was very high then each layer would only see the adjacent layers and you can show that the temperature in the top layer would approximate 0.84n<\/sup> Tg<\/strike> (n+1)-1\/4<\/sup> Tg, (see note<\/strong>) where n is the number of layers – much colder than the low GHG case. Hence the increased GHG steepens the surface-to-top temperature gradient.]<\/p>\n

In the case of the Earth, the solar input (and therefore long wave output) are roughly constant. This implies that there is a level in the atmosphere (called the effective radiating level) that must be at the effective radiating temperature (around 252K). This is around the mid-troposphere ~ 6km. Since increasing GHGs implies an increasing temperature gradient, the temperatures must therefore ‘pivot’ around this (fixed) level. i.e. everything below that level will warm, and everything above that level will cool. <\/p>\n

Even though the stratosphere has an opposite lapse rate to the troposphere because of the ozone absorption, the effect of increasing GHGs is the same, i.e. since it is above the effective radiating level, it will cool. The cooling will be greatest as you go higher. In the troposphere, there are important other effects that change the temperature, cheifly moist convection, and that smears out the temperature changes you expect from a pure radiative atmosphere. So while the troposphere does warm as a function of increasing GHGs, the maximum change is not at the surface, but actually in the mid-troposhere. End new)<\/strong><\/p>\n

To be sure, this is a very rough picture and where other feedbacks are important (due to clouds, convection, dynamics etc.) the picture locally can be significantly different from this one-dimensional cartoon. Nonetheless, at the global mean level, this is the dominant effect. <\/p>\n

Another important climate forcing, volcanic aerosols, can also give a similar opposing trend between the stratosphere and troposphere. In this case, large amounts of sulphate aerosols (small particles) are injected into the stratosphere by large explosive eruptions (the most recent one being Mt. Pinatubo in 1991). These aerosols are reflective and increase the albedo of the planet. This reduces the amount of solar radiation reaching the surface and therefore cause a cooling in the troposphere. However, they also absorb some radiation, and so in the lower stratosphere, they actually cause a warming. <\/p>\n

So do all climate changes cause opposing trends in stratospheric and tropospheric temperatures? No, it depends on the physics of each case. A good counter-example is that of solar forcing. An increase in the sun’s irradiance such as occurs over the 11-year sunspot cycle (or potentially longer time scales) warms the stratosphere (due to increased absorbtion by ozone) but it also warms the troposphere. <\/p>\n

25\/Feb\/2005 Note:<\/strong> thanks to JBS for the correction to the n-shell calculation (see comment 11) <\/p>\n

NB. The following text was originally in the post (paragraph 2), and has subsequently turned out to be wrong. It is left here so that the comments on it can remain comprehensible.<\/em>
\n
\nThe effect on local temperatures of increasing greenhouse gases depends on this lapse rate. Greenhouse gases (like CO2<\/sub>, CH4<\/sub> or water) absorb and re-radiate infra-red (IR) radiation that is emitted from the planet’s surface at rates that depend on the temperature (the Stefan-Boltzmann law). If the temperature below is warmer than the local temperature, IR radiation that is re-radiated is less than is absorbed, the net effect of the greenhouse gases is to warm that layer. Conversely, if the temperatures below are cooler, the local emissions will be larger than the IR radiation absorbed, and thus the net impact of the GHG will be to cool. In steady state, these effects are balanced principally by convection in the troposphere, and by ozone UV absorbtion in the stratosphere. As GHG levels change though, especially in the case of the well mixed gases like CO2<\/sub>, the tendencies described above will be enhanced, and thus in the troposphere, where GHGs warm, they will warm further, and conversely, in the stratosphere, where they cool, they will cool further. Thus the impact of GHGs locally is dependent on the local lapse rate. <\/strike>
\nVous pouvez sauter le paragraphe suivant, c’est tr\u00e8s technique. En r\u00e9sum\u00e9, une augmentation de la concentration des gaz a effet de serre entra\u00eene une augmentation du gradient de temp\u00e9rature a la surface.<\/p>\n

[Explication technique. Imaginez une atmosph\u00e8re constitu\u00e9e de couches isothermes, qui n’interagissent que de fa\u00e7on radiative. A l’\u00e9quilibre, chaque couche ne peut \u00e9mettre que ce qu’elle a absorb\u00e9. Si la quantit\u00e9 de gaz a effet de serre (GES) est faible, chaque couche ne voit que les \u00e9missions de la surface, et donc par Stefan-Boltzmann on peut d\u00e9duire que 2Ta4 = Tg4, avec Ta la temp\u00e9rature de l’air et Tg celle du sol. Ainsi Ta=0.84 Tg pour toutes les couches (dans une atmosph\u00e8re isotherme). D’un autre cot\u00e9, si la concentration en GES est tr\u00e8s \u00e9lev\u00e9e, chaque couche ne voit que les \u00e9missions de ses voisines, et on peut montrer que la temp\u00e9rature de la couche sup\u00e9rieure serait donn\u00e9e par (n+1)-1\/4 Tg, avec n le nombre de couches. Cette derni\u00e8re temp\u00e9rature est bien plus froide que dans le cas des faibles GES. Donc l’augmentation des GES accro\u00eet le gradient de temp\u00e9rature atmosph\u00e9rique.]<\/p>\n

Dans le cas de la Terre, les radiations solaires sont a-peu-pr\u00e8s constantes. Ceci implique qu’il existe un niveau dans l’atmosph\u00e8re (appel\u00e9 le niveau de radiation effectif) a la temp\u00e9rature de radiation effective (environ 252K). Ce point est situ\u00e9 dans la troposph\u00e8re moyenne (environ 6 km d’altitude). \u00c9tant donne qu’une augmentation des GES implique une augmentation du gradient de temp\u00e9rature, les temp\u00e9ratures vont donc “pivoter” autour de ce point fixe : l’atmosph\u00e8re en-dessous de ce point va se r\u00e9chauffer, et l’atmosph\u00e8re au-dessus se refroidir.<\/p>\n

M\u00eame si la stratosph\u00e8re a un gradient de temp\u00e9rature oppos\u00e9 a celui de la troposph\u00e8re en raison de l’absorption par l’ozone, l’impact d’une augmentation des GES sera le m\u00eame : comme la stratosph\u00e8re est au-dessus du niveau de radiation effectif, celle-ci va se refroidir. Le refroidissement sera plus important aux hautes altitudes. Dans la troposph\u00e8re, beaucoup d’autres param\u00e8tres influencent la temp\u00e9rature, principalement la concentration en vapeur d’eau, et donc le changement est limit\u00e9 par rapport a une atmosph\u00e8re purement radiative. En conclusion, m\u00eame si la troposph\u00e8re se r\u00e9chauffe lors d’une augmentation de GES, le plus fort changement n’est pas observe a la surface, mais dans la troposph\u00e8re moyenne.<\/p>\n

Bien entendu, cette explication est une approximation simplifi\u00e9e, et d’autres m\u00e9canismes sont \u00e9galement importants (nuages, convection, dynamique, etc). Localement, le comportement atmosph\u00e9rique peut \u00eatre tr\u00e8s diff\u00e9rent. N\u00e9anmoins, a grande \u00e9chelle, ce m\u00e9canisme est l’effet dominant.<\/p>\n

D’autres acteurs importants du for\u00e7age radiatif, les a\u00e9rosols volcaniques, peuvent \u00e9galement mener a des changements oppos\u00e9s dans la stratosph\u00e8re et la troposph\u00e8re. Lors d’\u00e9ruptions explosives (la plus r\u00e9cente \u00e9tant celle du Mont Pinatubo en 1991), de fortes quantit\u00e9s d’a\u00e9rosols (tr\u00e8s petites particules) de type sulfate sont inject\u00e9es dans la stratosph\u00e8re. Ces a\u00e9rosols r\u00e9fl\u00e9chissent la lumi\u00e8re du soleil et donc augmentent l’alb\u00e9do de la plan\u00e8te. Ceci diminue la quantit\u00e9 de radiation solaire atteignant la surface, et donc refroidit la troposph\u00e8re. Par contre, ils absorbent simultan\u00e9ment une certaine quantit\u00e9 de ces m\u00eames radiations, ce qui m\u00e8ne a un r\u00e9chauffement de la stratosph\u00e8re basse.<\/p>\n

Est-ce-que tous les changements climatiques influencent de fa\u00e7on oppos\u00e9e la stratosph\u00e8re et la troposph\u00e8re ? Non, \u00e7a d\u00e9pend de la physique de chaque cas. Un bon contre-exemple est donn\u00e9 par le for\u00e7age radiatif solaire. Une augmentation globale de l’irradiation solaire (pendant un cycle solaire de 11 ans) r\u00e9chauffe la stratosph\u00e8re (plus d’absorption par l’ozone) mais aussi la troposph\u00e8re.<\/p>\n

[Ndt : Cette traduction est bas\u00e9e sur la version du 25 F\u00e9vrier 2005]<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

This post is obsolete and wrong in many respects. Please see this more recent post for links to the answer. 14\/Jan\/05: This post was updated in the light of my further education in radiation physics. 25\/Feb\/05: Groan…and again. Recent discussions of climate change (MSU Temperature Record, ACIA) have highlighted the fact that the stratosphere is […]<\/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":[15],"tags":[],"class_list":{"0":"post-58","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-attic","7":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/58","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=58"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/58\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=58"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=58"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=58"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}