{"id":130,"date":"2005-03-10T13:56:01","date_gmt":"2005-03-10T17:56:01","guid":{"rendered":"\/?p=130"},"modified":"2011-03-15T12:30:22","modified_gmt":"2011-03-15T17:30:22","slug":"will-spring-2005-be-a-bad-one-for-arctic-ozone","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/03\/will-spring-2005-be-a-bad-one-for-arctic-ozone\/","title":{"rendered":"Will spring 2005 be a bad one for Arctic ozone? Le printemps 2005 comptera-t-il parmi les mauvais pour l\u2019ozone arctique ?<\/lang_fr>"},"content":{"rendered":"
\n

Guest Commentary by Drew Shindell (NASA GISS)<\/small><\/p>\n

The current winter and early spring have been extremely cold in the Arctic stratosphere, leading to the potential for substantial ozone depletion there. This has been alluded to recently in the press (Sitnews<\/a>, Seattle Post Intelligencer<\/a>), but what\u2019s the likely outcome, and why is it happening?
\n
\nPar Drew Shindell, NASA, GISS (traduit par Pierre Allemand)<\/small><\/p>\n

L\u2019hiver actuel et le d\u00e9but du printemps ont \u00e9t\u00e9 extr\u00eamement froids dans la stratosph\u00e8re arctique, ce qui est un facteur de r\u00e9duction substantielle de l\u2019ozone dans cette r\u00e9gion. La presse a r\u00e9cemment fait allusion \u00e0 ce ph\u00e9nom\u00e8ne, (Sitnews<\/a>, Seattle Post Intelligencer<\/a>),mais quels sont, en fait, les r\u00e9sultats, et qu\u2019est-ce qui fait que cela arrive ?<\/p>\n

(suite…)<\/a><\/p>\n

<\/lang_fr><\/p>\n


\n\"Min<\/a>(current conditions)<\/a><\/p>\n

First, let\u2019s go over some background. The recipe for massive springtime ozone loss in the polar regions, such as the annual ozone hole seen over Antarctica during the past two decades, is fairly simple. The two main ingredients are reactive halogen gases such as chlorine or bromine and sunlight. To prepare, keep the halogens at extremely cold temperatures, typically below \u201378 C (195 K). Use a strong polar vortex to mix the halogens to help achieve the required temperatures. When the mixture has been properly chilled, add sunlight and you\u2019ll get rapid ozone destruction.<\/p>\n

In the real world, both chlorine and bromine are readily available in the stratosphere worldwide, especially chlorine from chlorofluorocarbons which are well mixed in the lower atmosphere (where they are stable), before entering the stratosphere where they are photochemically decomposed. The halogens that are released generally end up in fairly unreactive forms where they have only a small effect on ozone. Under extremely cold conditions, however, ice and supercooled liquid droplets (so-called Polar Stratospheric Clouds – PSCs) can form even at the low densities present in the lower stratosphere (~15-25 km altitude). Chemical reactions on the surfaces of these particles can rapidly convert halogens into very reactive forms. Ozone depletion is therefore extremely sensitive to small changes in temperature when the stratosphere is near this freezing point. The temperatures themselves are greatly influenced by the strength of the polar vortex, a wind that swirls around the pole and when strong, can keep air confined throughout the winter in the polar night, allowing it to cool dramatically. The primary reason ozone depletion has been weaker over the Arctic than over Antarctica is than Arctic temperatures are typically about 10 degrees warmer as the Arctic vortex is generally weaker than its Antarctic counterpart. This is because of the differences in layout of the continents in the two hemispheres affects the dynamics of stratospheric circulation.<\/p>\n

The other key factor is that even if chemical conversion into reactive forms occurs during the cold, dark polar winter, the reactive chlorine must stick around until sunlight returns to the polar region for ozone destruction to take place. This is why ozone depletion over the poles is a springtime phenomenon. Even following a very cold winter, if temperatures warm quickly during spring very little ozone loss may take place. Alternatively, a milder winter, provided it was still cold enough to lead to chemical processing of halogens, could be followed by greater springtime ozone losses if temperatures stayed cold longer. Thus temperatures during the period when a lot of sunlight first returns to the polar areas following winter, March in the Arctic and September in the Antarctic, are crucial.<\/p>\n

This year has seen an exceptionally strong polar vortex over the Arctic (see the red line in the figure). Undoubtedly, chemical processing of halogens into reactive forms has taken place and the Arctic is primed for ozone depletion. Now that we\u2019re in March, sufficient sunlight is available to cause sizeable ozone losses. Should cold temperature persist for another couple weeks, ozone depletion could reach record levels for the Arctic. While the vortex was weakened and pushed to the side of the Arctic during the last week of February, temperatures below the critical freezing point are still present as of March 9 (see figure). The displacement off the pole also pushes the colder air into latitudes with more sunlight, enhancing ozone depletion in the short term. Ozone measurements from the first week of March already show a region over the North Atlantic with very low ozone levels (<250 Dobson units, versus minimum values of ~300 in the early 1980s). <\/p>\n

There is much debate over whether this has anything to do with climate change. Some climate models suggest that increasing greenhouse gases may be leading to a gradual strengthening of the Arctic vortex and hence increasing ozone losses, while others do not. Observations show that the vortex was typically more stable in the 1990s than during the 1980s, but the present decade has been mixed thus far. Temperature during the winter as a whole have generally decreased over the past two decades, likely as a result of climate change<\/a>, but the sensitivity of ozone loss to the exact timing of March warming events makes ozone depletion a much more variable quantity. With only one winter vortex per year, it will take many more years to determine if the exceptionally cold 2004-2005 winter is part of a trend or simply a single cold event. Prepare to see a lot of press coverage if this ends up being a big year though….<\/p>\n

Update:<\/strong> Indeed it was.<\/a>
\n<\/p>\n

\"Min<\/a>(conditions actuels)<\/a><\/p>\n

Tout d\u2019abord, passons en revue certains fondamentaux. La recette pour une perte massive d\u2019ozone au printemps dans les r\u00e9gions polaires comme le trou annuel d\u2019ozone au dessus de l\u2019Antarctique au cours des deux derni\u00e8res d\u00e9cennies est tr\u00e8s simple. Les deux ingr\u00e9dients principaux sont des gaz halog\u00e8nes r\u00e9actifs comme le chlore ou le brome, et de la lumi\u00e8re solaire. Pour la pr\u00e9paration, gardez les halog\u00e8nes \u00e0 des temp\u00e9ratures extr\u00eamement basses, typiquement au-dessous de \u201378\u00b0C (195 \u00b0K). Utilisez un vortex polaire puissant pour m\u00e9langer les halog\u00e8nes afin d\u2019atteindre plus facilement la temp\u00e9rature requise. Quand le m\u00e9lange a \u00e9t\u00e9 correctement refroidi, ajoutez la lumi\u00e8re solaire, et vous obtiendrez rapidement la destruction de l\u2019ozone.<\/p>\n

Dans le monde r\u00e9el, le chlore et le brome sont tous les deux facilement disponible partout dans la stratosph\u00e8re en particulier le chlore \u00e0 partir des chlorofluorocarbures qui sont bien r\u00e9partis dans la basse atmosph\u00e8re (o\u00f9 ils sont stables), avant d\u2019entrer dans la stratosph\u00e8re o\u00f9 ils sont d\u00e9compos\u00e9s photochimiquement.
\nLes compos\u00e9s halog\u00e9n\u00e9s qui r\u00e9sultent de la d\u00e9composition se retrouvent sous une forme relativement inerte ayant peu d\u2019effet sur l\u2019ozone. N\u00e9anmoins, dans des conditions de temp\u00e9ratures tr\u00e8s basses, des gouttelettes de glace et de liquide en surfusion (appel\u00e9es nuages stratosph\u00e9riques polaires [NdT : PSC, Polar Stratospheric Clouds]) peuvent se former, m\u00eame dans les conditions de faible densit\u00e9 pr\u00e9sentes dans la basse stratosph\u00e8re (de 15 \u00e0 25 km d\u2019altitude).
\nDes r\u00e9actions chimiques \u00e0 la surface de ces gouttelettes peuvent rapidement convertir les compos\u00e9s halog\u00e9n\u00e9s en une forme tr\u00e8s r\u00e9active. La destruction de l\u2019ozone est donc extr\u00eamement sensible aux faibles changements de temp\u00e9rature quand la stratosph\u00e8re est proche du point de cong\u00e9lation. Les temp\u00e9ratures elles-m\u00eames sont hautement influenc\u00e9es par l\u2019intensit\u00e9 du vortex polaire, un vent qui tourbillonne autour du p\u00f4le, et qui s\u2019il est suffisamment fort, peut garder l\u2019air confin\u00e9 pendant tout l\u2019hiver de la nuit polaire, ce qui provoque sont refroidissement intense.
\nLa raison premi\u00e8re pour laquelle la destruction de l\u2019ozone est plus faible en Arctique qu\u2019en Antarctique est que les temp\u00e9ratures arctiques sont typiquement plus hautes de 10 degr\u00e9s, car le vortex arctique est g\u00e9n\u00e9ralement plus faible que son pendant antarctique. Ceci parce que la diff\u00e9rence de disposition des continents entre les deux h\u00e9misph\u00e8res affecte la dynamique de la circulation stratosph\u00e9rique. <\/p>\n

Le deuxi\u00e8me facteur-cl\u00e9 est que m\u00eame si la conversion chimique en une forme r\u00e9active intervient durant la nuit froide de l\u2019hiver polaire, le chlore r\u00e9actif doit rester en place jusqu\u2019\u00e0 ce que la lumi\u00e8re du soleil revienne dans la r\u00e9gion polaire pour que la destruction de l\u2019ozone s\u2019effectue. Voil\u00e0 pourquoi la destruction de l\u2019ozone au dessus des p\u00f4les est un ph\u00e9nom\u00e8ne printanier. M\u00eame apr\u00e8s un hiver tr\u00e8s froid, si les temp\u00e9ratures remontent rapidement au printemps, la perte d\u2019ozone sera tr\u00e8s faible.
\nInversement, un hiver plus doux, pourvu qu\u2019il ait \u00e9t\u00e9 assez froid pour permettre la transformation chimique des compos\u00e9s halog\u00e9n\u00e9s, peut \u00eatre suivi par une perte d\u2019ozone plus importante au printemps si les temp\u00e9ratures restent basses plus longtemps.
\nAinsi, les temp\u00e9ratures durant les p\u00e9riodes o\u00f9 la lumi\u00e8re solaire revient dans les r\u00e9gions polaires apr\u00e8s l\u2019hiver, \u00e0 savoir : mars dans l\u2019Arctique et septembre dans l\u2019Antarctique, sont cruciales.<\/p>\n

Cette ann\u00e9e a vu un vortex exceptionnellement intense sur l\u2019Arctique (voir la courbe rouge sur la figure). Sans aucun doute, la transformation des compos\u00e9s halog\u00e9n\u00e9s en une forme r\u00e9active a eu lieu, et l\u2019Arctique est pr\u00eat pour la destruction de l\u2019ozone.
\nMaintenant que nous sommes en mars, il y a suffisamment de lumi\u00e8re solaire pour provoquer des pertes d\u2019ozone substantielles. Si des basses temp\u00e9ratures persistent pendant encore 2 semaines, la destruction de l\u2019ozone pourra atteindre un niveau record pour l\u2019Arctique. Alors que le vortex s\u2019affaiblissait et se trouvait repouss\u00e9 vers le bord de l\u2019Arctique durant la derni\u00e8re semaine de f\u00e9vrier, des temp\u00e9ratures au dessous du point critique de cong\u00e9lation r\u00e8gnent encore le 9 mars (voir la figure).
\nLe d\u00e9placement du vortex en dehors du p\u00f4le pousse l\u2019air plus froid vers des latitudes plus \u00e9clair\u00e9es par le soleil, ce qui accro\u00eet dans l\u2019imm\u00e9diat la destruction de l\u2019ozone. Les mesures du taux d\u2019ozone depuis la premi\u00e8re semaine de mars montrent d\u00e9j\u00e0 un niveau d\u2019ozone tr\u00e8s bas dans une r\u00e9gion au dessus de l\u2019Atlantique Nord (<250 unit\u00e9s Dobson, \u00e0 comparer \u00e0 des minima d\u2019environ 300 au d\u00e9but des ann\u00e9es 80).
\nLe fait de savoir si ce ph\u00e9nom\u00e8ne a un rapport avec le changement climatique est tr\u00e8s d\u00e9battu. Certains mod\u00e8les climatiques pr\u00e9disent qu\u2019un accroissement des gaz \u00e0 effet de serre pourrait conduire \u00e0 un renforcement graduel du vortex arctique, et donc \u00e0 l\u2019accroissement des pertes d\u2019ozone, d\u2019autres non. Les observations ont montr\u00e9 que le vortex a \u00e9t\u00e9 typiquement plus stable dans les ann\u00e9es 90 que durant les ann\u00e9es 80, cependant que la d\u00e9cennie actuelle se pr\u00e9sente plus mitig\u00e9e jusqu\u2019\u00e0 aujourd\u2019hui. Les temp\u00e9ratures hivernales ont globalement chut\u00e9 au cours des deux derni\u00e8res d\u00e9cennies, probablement \u00e0 cause du changement climatique<\/a>, mais la sensibilit\u00e9 du ph\u00e9nom\u00e8ne de perte d\u2019ozone \u00e0 l\u2019arriv\u00e9e exacte du r\u00e9chauffement de mars rend la destruction d\u2019ozone beaucoup plus variable quantitativement. Comme il n\u2019y a qu\u2019un vortex par an, cela prendra beaucoup plus d\u2019ann\u00e9es pour d\u00e9terminer si l\u2019hiver exceptionnellement froid de 2004-2005 participe \u00e0 une tendance ou reste un \u00e9v\u00e9nement froid isol\u00e9.
\nPr\u00e9parons nous cependant \u00e0 une couverture dans la presse sur le fait de savoir si cette ann\u00e9e est sp\u00e9ciale ou non.
\n<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Guest Commentary by Drew Shindell (NASA GISS) The current winter and early spring have been extremely cold in the Arctic stratosphere, leading to the potential for substantial ozone depletion there. This has been alluded to recently in the press (Sitnews, Seattle Post Intelligencer), but what\u2019s the likely outcome, and why is it happening? Par Drew […]<\/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":[12,1],"tags":[],"class_list":{"0":"post-130","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\/130","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=130"}],"version-history":[{"count":3,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/130\/revisions"}],"predecessor-version":[{"id":7169,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/130\/revisions\/7169"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=130"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=130"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=130"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}