{"id":192,"date":"2005-10-05T11:21:02","date_gmt":"2005-10-05T15:21:02","guid":{"rendered":"\/?p=192"},"modified":"2010-02-19T03:41:11","modified_gmt":"2010-02-19T08:41:11","slug":"global-warming-on-mars","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/10\/global-warming-on-mars\/","title":{"rendered":"Global warming on Mars? R\u00e9chauffement global sur Mars ? <\/lang_fr>"},"content":{"rendered":"
\n

Guest contribution by Steinn Sigurdsson<\/a>.<\/small><\/p>\n

Recently, there have been some suggestions that “global warming” has been observed on Mars (e.g. here<\/a>). These are based on observations of regional change around the South Polar Cap, but seem to have been extended into a “global” change, and used by some to infer an external common mechanism for global warming on Earth and Mars (e.g. here<\/a> and here<\/a>). But this is incorrect reasoning and based on faulty understanding of the data.
\nArticle invit\u00e9 par Steinn Sigurdsson<\/a>. (traduit par Claire Rollion-Bard)<\/small><\/p>\n

R\u00e9cemment, il y a eu des suggestions qu’un “r\u00e9chauffement global” a \u00e9t\u00e9 observ\u00e9 sur Mars (par exemple, ici<\/a>). Ceci est bas\u00e9 sur des observations d’un changement r\u00e9gional autour de la calotte polaire sud, mais semble avoir \u00e9t\u00e9 \u00e9tendu \u00e0 un changement “global” et utilis\u00e9 par certains pour en d\u00e9duire un m\u00e9canisme commun externe pour le r\u00e9chauffement global sur la Terre et sur Mars. (par exemple, ici<\/a> et ici<\/a>). Mais c’est un raisonnement incorrect et bas\u00e9 sur une mauvaise compr\u00e9hension des donn\u00e9es. <\/lang_fr>
\n<\/p>\n

<\/a>
\nA couple of basic issues first : the Martian year is about 2 Earth years (687 days). Currently it is late winter<\/a> in Mars’s northern hemisphere, so late summer in the southern hemisphere. Martian eccentricity<\/a> is about 0.1 – over 5 times larger than Earth’s, so the insolation (INcoming SOLar radiATION) variation over the orbit is substantial, and contributes significantly more to seasonality than on the Earth, although Mars’s obliquity (the angle of its spin axis to the orbital plane) still dominates the seasons. The alignment of obliquity and eccentricity due to precession is a much stronger effect than for the Earth, leading to “great” summers and winters on time scales of tens of thousands of years (the precessional period is 170,000 years). Since Mars has no oceans and a thin atmosphere, the thermal inertia is low, and Martian climate is easily perturbed by external influences, including solar variations. However, solar irradiance is now well measured by satellite and has been declining slightly<\/a> over the last few years as it moves towards a solar minimum. <\/p>\n

So what is causing Martian climate change now? Mars has a relatively well studied climate, going back to measurements made by Viking, and continued with the current series of orbiters, such as the Mars Global Surveyor<\/a>. Complementing the measurements, NASA has a Mars General Circulation Model<\/a> (GCM) based at NASA Ames. (NB. There is a good “general reader” review of modeling the Martian atmosphere by Stephen R Lewis<\/a> in Astronomy and Geophysics<\/i>, volume 44 issue 4. pages 6-14.<\/a>)<\/p>\n

Globally, the mean temperature of the Martian atmosphere is particularly sensitive to the strength and duration of hemispheric dust storms<\/a>, (see for example here<\/a> and here<\/a>). Large scale dust storms change the atmospheric opacity and convection; as always when comparing mean temperatures, the altitude at which the measurement is made matters, but to the extent it is sensible to speak of a mean temperature for Mars, the evidence is for significant cooling from the 1970’s, when Viking made measurements, compared to current temperatures. However, this is essentially due to large scale dust storms that were common back then, compared to a lower level of storminess now. The mean temperature on Mars, averaged over the Martian year can change by many degrees<\/a> from year to year, depending on how active large scale dust storms are.<\/p>\n

In 2001, Malin et al published a short article in Science<\/a> (subscription required) discussing MGS data showing a rapid shrinkage of the South Polar Cap. Recently, the MGS team had a press release discussing more recent data showing the trend had continued. MGS 2001 press release<\/a> MGS 2005 press release<\/a>. The shrinkage of the Martian South Polar Cap is almost certainly a regional climate change, and is not any indication of global warming trends in the Martian atmosphere. Colaprete et al in Nature 2005<\/a> (subscription required) showed, using the Mars GCM, that the south polar climate is unstable due to the peculiar topography near the pole, and the current configuration is on the instability border; we therefore expect to see rapid changes in ice cover as the regional climate transits between the unstable states.<\/p>\n

Thus inferring global warming from a 3 Martian year regional trend is unwarranted. The observed regional changes in south polar ice cover are almost certainly due to a regional climate transition, not a global phenomenon, and are demonstrably unrelated to external forcing. There is a slight irony in people rushing to claim that the glacier changes on Mars are a sure sign of global warming, while not being swayed by the much more persuasive analogous phenomena<\/a> here on Earth…
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\n<\/a><\/p>\n

Tout d’abord, quelques bases : l’ann\u00e9e martienne correspond \u00e0 environ 2 ann\u00e9es terrestres (687 jours). Couramment, il y a un hiver tardif<\/a> dans l’h\u00e9misph\u00e8re Nord de Mars et donc un \u00e9t\u00e9 tardif dans l’h\u00e9misph\u00e8re Sud. L’excentricit\u00e9<\/a> martienne est environ 0,1 – soit plus de 5 fois plus grande que celle de la Terre, donc la variation de l’insolation (INcoming SOlar radiATION) sur l’orbite est substantielle et contribue significativement plus \u00e0 la saisonnalit\u00e9 que sur la Terre, bien que l’obliquit\u00e9 de Mars (l’angle entre son axe et le plan de l’orbite) domine toujours les saisons. L’alignement de l’obliquit\u00e9 et de l’excentricit\u00e9 d\u00fb \u00e0 la pr\u00e9cession est un effet beaucoup plus fort que pour la Terre, entra\u00eenant de grands \u00e9t\u00e9s et hivers sur des \u00e9chelles de temps de dizaines de milliers d’ann\u00e9es (la p\u00e9riode de pr\u00e9cession est 170 000 ans). Puisque Mars n’a pas d’oc\u00e9an et une atmosph\u00e8re fine, l’inertie thermique est faible, et le climat de Mars est facilement perturb\u00e9 par des influences externes, incluant les variations solaires. Cependant, la radiation du Soleil est maintenant bien mesur\u00e9e par les satellites et a baiss\u00e9<\/a> l\u00e9g\u00e8rement ces derni\u00e8res ann\u00e9es et tend vers un minimum solaire.<\/p>\n

Quelle est, alors, la cause du changement actuel du climat martien ? Mars a un climat relativement bien \u00e9tudi\u00e9, allant des mesures faites par Viking et continuant avec les s\u00e9ries d’orbiteurs comme le Mars Global Surveyor<\/a>. En compl\u00e9ment des mesures, la NASA a un mod\u00e8le de circulation g\u00e9n\u00e9rale de Mars<\/a> (GCM) bas\u00e9 au NASA Ames (NB : il y a une bonne revue g\u00e9n\u00e9rale du mod\u00e8le de l’atmosph\u00e8re de Mars par Stephen R. Lewis<\/a> dans Astronomy and Geophysics<\/i>, volume 44 issue 4, pages 6-14)<\/a><\/p>\n

Globalement, la temp\u00e9rature moyenne de l’atmosph\u00e8re martienne est particuli\u00e8rement sensible \u00e0 la force et \u00e0 la dur\u00e9e<\/a> des temp\u00eates de poussi\u00e8res h\u00e9misph\u00e9riques (voir par exemple ici<\/a> et la<\/a>). Les temp\u00eates de poussi\u00e8res de large \u00e9chelle changent l’opacit\u00e9 atmosph\u00e9rique et la convection ; comme toujours quand on compare des temp\u00e9ratures moyennes, l’altitude \u00e0 laquelle la mesure est faite compte, mais dans la mesure o\u00f9 il est raisonnable de parler de temp\u00e9rature moyenne pour Mars, l’\u00e9vidence est pour un refroidissement significatif depuis les ann\u00e9es 70, quand Viking a fait les mesures, compar\u00e9es aux temp\u00e9ratures actuelles. Cependant, c’est essentiellement d\u00fb aux temp\u00eates de poussi\u00e8res qui sont revenues alors, compar\u00e9 \u00e0 un niveau plus faible de temp\u00eates maintenant. La temp\u00e9rature moyenne de Mars, moyenn\u00e9e sur une ann\u00e9e martienne, peut changer de plusieurs degr\u00e9s<\/a> d’ann\u00e9e en ann\u00e9e, d\u00e9pendant de l’activit\u00e9 des temp\u00eates de poussi\u00e8res.<\/p>\n

En 2001, Malin et al. ont publi\u00e9 un court article dans Science (abonnement requis)<\/a> discutant des donn\u00e9es de MGS montrant une rapide diminution de la calotte polaire Sud. R\u00e9cemment, l’\u00e9quipe du MGS a fait un communiqu\u00e9 de presse discutant des donn\u00e9es plus r\u00e9centes, qui montrent que la tendance a continu\u00e9. Communiqu\u00e9 de presse MGS 2001<\/a> Communiqu\u00e9 de presse MGS 2005<\/a>. La diminution de la calotte polaire sud martienne est presque certainement un changement climatique r\u00e9gional, et n’est pas une indication de tendances vers un r\u00e9chauffement global dans l’atmosph\u00e8re de Mars. Colaprete et al.<\/em> dans Nature 2005 (abonnement requis)<\/a> ont montr\u00e9, en utilisant le GCM de Mars, que le climat polaire sud est instable \u00e0 cause d’une topographie particuli\u00e8re pr\u00e8s du p\u00f4le, et la configuration actuelle en transit entre ces \u00e9tats instables.<\/p>\n

Ainsi d\u00e9duire un r\u00e9chauffement global \u00e0 partir d’une tendance r\u00e9gionale sur 3 ann\u00e9es martiennes est infond\u00e9. Les changements r\u00e9gionaux observ\u00e9s dans la couverture de glace du p\u00f4le sud sont presque certainement dus \u00e0 une transition du climat r\u00e9gional, non \u00e0 un ph\u00e9nom\u00e8ne global, et ne sont pas reli\u00e9s de mani\u00e8re prouv\u00e9e \u00e0 un for\u00e7age externe. Il y a quelque ironie dans le fait que les gens s’empressent de d’affirmer que les changements de glacier sur Mars sont un signe certain d’un r\u00e9chauffement global, alors qu’ils ne sont pas secou\u00e9s par les ph\u00e9nom\u00e8nes analogues <\/a>beaucoup plus convaincants ici sur Terre\u2026
\n<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Guest contribution by Steinn Sigurdsson. Recently, there have been some suggestions that “global warming” has been observed on Mars (e.g. here). These are based on observations of regional change around the South Polar Cap, but seem to have been extended into a “global” change, and used by some to infer an external common mechanism for […]<\/p>\n","protected":false},"author":12,"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":[5,1,13,4],"tags":[],"class_list":{"0":"post-192","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-modelling","7":"category-climate-science","8":"category-faq","9":"category-sun-earth-connections","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/192","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\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=192"}],"version-history":[{"count":2,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/192\/revisions"}],"predecessor-version":[{"id":3018,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/192\/revisions\/3018"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=192"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=192"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=192"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}