{"id":110,"date":"2005-01-19T23:52:19","date_gmt":"2005-01-20T03:52:19","guid":{"rendered":"\/?p=110"},"modified":"2009-06-01T07:36:40","modified_gmt":"2009-06-01T12:36:40","slug":"global-dimming-ii","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/01\/global-dimming-ii\/","title":{"rendered":"Global Dimming II Assombrissement Global II<\/lang_fr>"},"content":{"rendered":"
\n

Guest commentary on BBC documentary on “Global Dimming” aired on January 13th 2005 by Beate Liepert<\/strong>, LDEO, Columbia University<\/small><\/p>\n

I haven’t yet seen the documentary. I have only read the transcript and hence was spared the pictures of the potential apocalypse and the invocation of biblical-scale famines. However, as one of the lead scientists on the topic [and who was interviewed by the BBC for the Horizon documentary (transcript<\/a>, previous post<\/a>)], I feel I should explain a few things about it without using religious analogies and stoking unnecessary fear. <\/p>\n

First though, this is a nice example of the power of words: Gerry Stanhill coined the observed reduction in solar energy reaching the ground “global dimming”. He called it “global” dimming because the technical term for the radiative energy is called “global solar radiation” and it contrasts nicely with the more common “global warming”.<\/p>\n


\npar Beate Liepert<\/strong>, LDEO, Columbia University (traduit par Pierre Allemand)<\/small><\/p>\n

Je n\u2019ai pas encore vu le documentaire. J\u2019ai seulement lu la transcription, et je n\u2019ai donc pas \u00e9t\u00e9 touch\u00e9 par les images d\u2019une apocalypse potentielle et par l\u2019\u00e9vocation de famines \u00e0 l\u2019\u00e9chelle biblique. Cependant, en tant que l\u2019un des scientifiques leader du sujet, [et qui a \u00e9t\u00e9 interview\u00e9 par la BBC pour le documentaire de la s\u00e9rie Horizon, (transcription<\/a> et article pr\u00e9c\u00e9dent<\/a>)], je me sens dans l\u2019obligation d\u2019approfondir quelques d\u00e9tails sans utiliser d\u2019analogie religieuse ni d\u00e9clencher d\u2019inutiles inqui\u00e9tudes. <\/p>\n

Premi\u00e8re id\u00e9e : voici un bel exemple du pouvoir des mots : Gerry Stanhill qualifie la r\u00e9duction observ\u00e9e de l\u2019\u00e9nergie solaire atteignant le sol, d\u2019 “assombrissement global”. Il l\u2019a appel\u00e9 assombrissement “global” parce que le terme technique pour l\u2019\u00e9nergie de radiation est “rayonnement solaire global” et il s\u2019oppose ainsi \u00e9l\u00e9gamment au terme plus courant de “r\u00e9chauffement global”.
\n(suite…)<\/lang_fr>
\n<\/p>\n

Secondly, there are three published studies out on long term changes in solar radiation (or “global dimming” if preferred). All use the same data sources. Solar radiation has been measured at weather stations worldwide since about 1956-57. As with many other measurements most of the data are from the Northern Hemisphere and all are taken on land. A reduction in downward solar radiation of about 4% or about 7W\/m2<\/sup> from 1961 to 1990 was found at stations worldwide by Gilgen et al., (1998)<\/a>. Gilgen et al. did a quick analysis and used all the available data with increasingly shorter records for their trend statistics. Stanhill and Cohen (2001)<\/a> calculated a stronger reduction of about 8% per decade. The reason for the discrepancy might be that only 30 records were used in the latter study and it seems only the ones with the declining trend. My own analysis was based on 110 continuously recording stations worldwide from 1961 to 1990 (Liepert 2002)<\/a>. I confirmed Gilgen et al.’s estimate of a reduction of about 4% in three decades. Since the late 1980s a recovery seems to be occurring but the studies demonstrating this are not yet published.<\/p>\n

Why is solar radiation changing? From observations we can separate cloud-free skies and cloudy conditions. We can hence infer clouds or atmospheric transparency as possible causes for the dimming. In my study of the US data I identified clouds as the main reason for the dimming of sunlight. Only about a fifth of the dimming could be observed during cloud-free conditions.<\/p>\n

Why should the atmospheric transparency change at all in cloud-free conditions? V. Ramanathan explained it in the BBC documentary. Sunlight is reflected by air pollution or absorbed in the atmosphere before it reaches the ground. Field campaigns like INDOEX<\/a> show this clearly (well, not “clearly” in the literal sense!). Advanced climate models include this “direct” aerosol effect and base their inputs on experiments like INDOEX.<\/p>\n

Why should clouds change? Global warming for example. Surprised? Most climate simulations predict some “global dimming” due to the water vapor and cloud feedback of greenhouse gas forced global warming. Global warming, however, affects the entire atmosphere whereas global dimming is only a surface and near-surface phenomena. Hence global warming and global dimming are not exclusive or contradictory. (Incidentally, the decline of solar energy at the surface inferred in my study is about 60% of the increasing longwave radiation in a typical global warming climate simulation (Feichter et al. 2004<\/a>)). With global warming, atmospheric moisture increases and this makes the atmosphere slightly less transparent to sunlight. Furthermore once clouds are formed, they tend to hold more water and therefore look a little darker.<\/p>\n

Anthropogenic aerosols were however singled out in the BBC documentary. They are correctly believed to change cloud reflectivity and cloud lifetime. Scientists are currently trying to assess the magnitude of these aerosol-cloud interactions and the impact on climate. Rotstyn’s<\/a> study on the Sahel drought is one example. But as Giannini et al.’s study<\/a> showed, you can look look at the Sahel drought with a number of different approaches. Clouds have always posed the greatest challenge for climatologists and I regard my own research on as a contribution to this ongoing debate. <\/p>\n

Currently, the best climate models include estimates of all these effects: anthropogenic greenhouse gas forcings, aerosols, natural solar cycles, and volcanic eruptions (see here<\/a> for example). The inclusion of aerosols in climate simulations has improved the model hindcasts when tested against past climate and dimming (Wild and Liepert, 1998<\/a>). The latest climate projections for the future therefore all include some estimates of aerosol changes. The scientifically interesting and new part is that introducing aerosols requires a closer scrutiny of surface energy and water budgets than was previously done. An example of this kind of analysis of a climate model in the context of \u201cglobal dimming\u201d can be seen in Liepert et al. (2004)<\/a>, but other model groups are currently performing similar analyses too.<\/p>\n

Finally a comment on language. It concerns me that articles from scientists and journalists alike have a tendency to use biblical and apocalyptic terms. This might be an appropriate way to describe a baroque church in Bavaria or a painting of P.P. Rubens (for instance in my favorite museum, the Alte Pinakothek<\/a> in Munich) but I would rather keep this emotive language out of scientific discussions.
\n
\nDeuxi\u00e8mement : trois \u00e9tudes ont \u00e9t\u00e9 publi\u00e9es sur les changements \u00e0 long terme du rayonnement solaire (ou “assombrissement global” si vous pr\u00e9f\u00e9rez). Toutes les trois utilisent les m\u00eames sources de donn\u00e9es. Le rayonnement solaire a \u00e9t\u00e9 mesur\u00e9 par des stations m\u00e9t\u00e9orologiques autour du monde depuis 1956-57. Comme beaucoup d\u2019autres mesures, la plupart des chiffres proviennent de l\u2019h\u00e9misph\u00e8re nord, et toutes ont \u00e9t\u00e9 mesur\u00e9es sur des zones terrestres. Une r\u00e9duction du rayonnement solaire d\u2019environ 4 % soit environ 7 W\/m_ de 1961 \u00e0 1990 a \u00e9t\u00e9 observ\u00e9e dans les stations m\u00e9t\u00e9os au niveau mondial par Gilgen et al., (1998)<\/a>. Gilgen et al. ont effectu\u00e9 une analyse rapide et ont utilis\u00e9 toutes les donn\u00e9es disponibles avec des dur\u00e9es de mesure de plus en plus courtes pour \u00e9tablir leurs statistiques de tendance. Stanhill and Cohen (2001)<\/a> ont abouti \u00e0 une r\u00e9duction plus importante de 8 % par d\u00e9cennie. La raison de cette divergence est peut-\u00eatre que pour cette derni\u00e8re \u00e9tude, seulement 30 s\u00e9ries de chiffres ont \u00e9t\u00e9 utilis\u00e9es, en prenant apparemment uniquement les s\u00e9ries qui montraient une tendance n\u00e9gative. Ma propre analyse \u00e9tait fond\u00e9e sur 110 s\u00e9ries continues de mesures par des stations m\u00e9t\u00e9os r\u00e9parties dans le monde, de 1961 \u00e0 1990 (Liepert 2002<\/a>).J\u2019ai confirm\u00e9 l\u2019estimation de Gilgen et al. A savoir, une r\u00e9duction d\u2019environ 4 % en trois d\u00e9cennies. Depuis les ann\u00e9es 1980, un r\u00e9tablissement semble se manifester, mais les \u00e9tudes qui le d\u00e9montrent n\u2019ont pas encore \u00e9t\u00e9 publi\u00e9es.<\/p>\n

Pourquoi le rayonnement solaire change-t-il ? Les r\u00e9sultats d\u2019observations permettent d\u2019\u00e9tablir une distinction entre les cas de ciels sans nuage et les cas de conditions nuageuses. Nous pouvons donc supposer que les nuages ou la transparence atmosph\u00e9rique sont des causes possibles de cet assombrissement. Dans mon \u00e9tude sur les donn\u00e9es provenant des USA, j\u2019ai identifi\u00e9 les nuages comme la raison principale de la diminution de la lumi\u00e8re solaire. En effet, en l\u2019absence de nuage, l\u2019assombrissement n\u2019est plus que le cinqui\u00e8me de sa valeur.<\/p>\n

Pourquoi la transparence atmosph\u00e9rique changerait-elle en l\u2019absence de nuage ?
\nV. Ramanathan l\u2019a expliqu\u00e9 dans le documentaire de la BBC. La lumi\u00e8re du soleil est r\u00e9fl\u00e9chie par la pollution de l\u2019air ou absorb\u00e9e dans l\u2019atmosph\u00e8re avant d\u2019atteindre le sol. Les mesures sur le terrain comme INDOEX<\/a> l\u2019indiquent clairement (Oui, “clairement” dans son sens litt\u00e9ral !). Des mod\u00e8les climatiques avanc\u00e9s int\u00e8grent cet effet “direct” des a\u00e9rosols, et s\u2019appuient sur les chiffres donn\u00e9s par des exp\u00e9riences comme INDOEX. <\/p>\n

Pourquoi les nuages changeraient-ils ? A cause du r\u00e9chauffement global, par exemple. Vous \u00eates surpris ? La plupart des simulations climatiques pr\u00e9disent un “assombrissement global” d\u00fb \u00e0 la vapeur d\u2019eau et aux nuages r\u00e9sultant du r\u00e9chauffement global par le for\u00e7age des gaz \u00e0 effet de serre. Le r\u00e9chauffement global, n\u00e9anmoins, affecte l\u2019ensemble de l\u2019atmosph\u00e8re, alors que l\u2019assombrissement global est un ph\u00e9nom\u00e8ne limit\u00e9 \u00e0 la surface ou \u00e0 sa proximit\u00e9 imm\u00e9diate. Ainsi, le r\u00e9chauffement global et l\u2019assombrissement global ne sont ni exclusifs l\u2019un de l\u2019autre, ni contradictoires. (Accessoirement, la diminution de l\u2019\u00e9nergie solaire \u00e0 la surface du sol signal\u00e9e dans mon \u00e9tude repr\u00e9sente environ 60 % de l\u2019augmentation du rayonnement \u00e0 longueur d\u2019onde \u00e9lev\u00e9e dans une simulation de r\u00e9chauffement global classique (Feichter et al. 2004<\/a>)). Sous l\u2019influence du r\u00e9chauffement global, l\u2019humidit\u00e9 atmosph\u00e9rique s\u2019accro\u00eet, ce qui rend l\u2019atmosph\u00e8re l\u00e9g\u00e8rement moins transparente \u00e0 la lumi\u00e8re solaire. De plus, une fois que les nuages se sont form\u00e9s, ils tendent \u00e0 retenir une plus grande quantit\u00e9 d\u2019eau et, de ce fait, apparaissent un peu plus sombres.<\/p>\n

Les a\u00e9rosols anthropog\u00e9niques \u00e9taient n\u00e9anmoins distingu\u00e9s des autres dans le documentaire de la BBC. On pense avec raison qu\u2019ils changent la r\u00e9flectivit\u00e9 et la dur\u00e9e de vie des nuages. Les scientifiques essayent actuellement de d\u00e9terminer la magnitude de ces interactions nuages \/ a\u00e9rosols et leur impact sur le climat. L\u2019\u00e9tude de Rotstyn<\/a> concernant la s\u00e9cheresse au Sahel en est un exemple. Mais, comme l\u2019\u00e9tude de Giannini et al.<\/a> l\u2019a montr\u00e9, on peut \u00e9tudier la s\u00e9cheresse au Sahel selon diff\u00e9rentes approches. Les nuages ont toujours \u00e9t\u00e9 l\u2019occasion des plus grands “challenges” pour les climatologues, et je consid\u00e8re mon propre travail comme une contribution personnelle au d\u00e9bat qui s\u2019annonce. <\/p>\n

Actuellement, les meilleurs mod\u00e8les climatiques comportent des estimations des effets suivants :
\nfor\u00e7age par les gaz \u00e0 effet de serre anthropog\u00e9niques, a\u00e9rosols, cycles solaires naturels et \u00e9ruptions volcaniques (voir par exemple ici<\/a>).
\nLe fait d\u2019inclure les a\u00e9rosols dans les simulations climatiques a am\u00e9lior\u00e9 la r\u00e9ponse des mod\u00e8les climatiques sur les tests concernant le climat pass\u00e9 et l\u2019assombrissement (Wild and Liepert, 1998<\/a>). Les derni\u00e8res projections climatiques pour le futur prennent donc en compte certaines estimations des changements des a\u00e9rosols. L\u2019int\u00e9r\u00eat scientifique et la nouveaut\u00e9 r\u00e9side dans le fait que la prise en compte des a\u00e9rosols requiert l\u2019\u00e9tude plus pr\u00e9cise qu\u2019auparavant de l\u2019\u00e9nergie de surface et des bilans aqueux. Un exemple de ce genre d\u2019analyse d\u2019un mod\u00e8le climatique dans le contexte de “l\u2019assombrissement global” peut \u00eatre trouv\u00e9 chez Liepert et al. (2004)<\/a>, mais d\u2019autres groupes effectuent aussi le m\u00eame type d\u2019analyse.<\/p>\n

Finissons sur un commentaire de langage. Je suis frapp\u00e9 par la tendance pour des articles tant de scientifiques que de journalistes \u00e0 utiliser des termes bibliques et apocalyptiques. Ce pourrait \u00eatre une mani\u00e8re appropri\u00e9e de d\u00e9crire une \u00e9glise baroque en Bavi\u00e8re ou une tableau de P. P. Rubens ((par exemple dans mon mus\u00e9e pr\u00e9f\u00e9r\u00e9, la Alte Pinakothek \u00e0 Munich<\/a>) mais je pr\u00e9f\u00e8rerais tenir ce langage \u00e9motif en dehors des discussions scientifiques. <\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Guest commentary on BBC documentary on “Global Dimming” aired on January 13th 2005 by Beate Liepert, LDEO, Columbia University I haven’t yet seen the documentary. I have only read the transcript and hence was spared the pictures of the potential apocalypse and the invocation of biblical-scale famines. However, as one of the lead scientists on […]<\/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":[17,5,1],"tags":[],"class_list":{"0":"post-110","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-aerosols","7":"category-climate-modelling","8":"category-climate-science","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/110","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=110"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/110\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=110"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=110"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=110"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}