{"id":246,"date":"2006-01-31T20:31:19","date_gmt":"2006-02-01T00:31:19","guid":{"rendered":"\/?p=246"},"modified":"2007-08-15T09:16:05","modified_gmt":"2007-08-15T14:16:05","slug":"can-2c-warming-be-avoided","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2006\/01\/can-2c-warming-be-avoided\/","title":{"rendered":"Can 2\u00b0C warming be avoided? Un r\u00e9chauffement de 2\u00b0C peut-il \u00eatre \u00e9vit\u00e9?<\/lang_fr>"},"content":{"rendered":"
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Guest comment by Malte Meinshausen, Reto Knutti and Dave Frame<\/small><\/p>\n

Yesterday\u2019s BBC article<\/a> on the “Avoiding Dangerous Climate Change”<\/a> report of the Exeter meeting<\/a> last year, carried two messages that have left some a little confused. On the one hand, it said that a stabilization of greenhouse gases at 400-450 ppm CO2<\/sub>-equivalent concentrations is required to keep global mean warming below 2\u00b0C, which in turn is assumed to be necessary to avoid ‘dangerous’ climate change. On the other hand, people are cited saying that “We’re going to be at 400 ppm in 10 years’ time”. <\/p>\n

So given that we will exceed 400 ppm CO2<\/sub> in the near future, is a target of 2\u00b0C feasible? To make a long story short: the answer is yes. Commentaire par Malte Meinshausen, Reto Knutti and Dave Frame (traduit par Alain Henry)<\/small>
\nHier, un article de la BBC<\/a> sur le rapport ” Eviter les changements climatiques dangereux”<\/a> de la conf\u00e9rence d’Exeter<\/a> de l’an dernier comportait deux messages qui pourraient entretenir une certaine confusion. D’une part, il mentionne qu’une stabilisation des gaz \u00e0 effet de serre (GES) \u00e0 400-450 ppm de CO2<\/sub> \u00e9quivalent est n\u00e9cessaire pour garder le r\u00e9chauffement global en dessous de 2\u00b0C, qui \u00e0 son tour est suppos\u00e9 n\u00e9cessaire pour \u00e9viter les changements climatiques ‘dangereux’. D’autre part, plusieurs personnes sont cit\u00e9es en disant \u00ab Nous atteindrons 400 ppm dans 10 ans \u00bb.<\/p>\n

Donc, \u00e9tant donn\u00e9 que nous allons d\u00e9passer les 400 ppm de CO2<\/sub> dans un futur proche, est-ce que la cible de 2\u00b0C est-elle faisable? En un mot, la r\u00e9ponse est oui.
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The following paragraphs attempt to shed a little light on the background on why 2\u00b0C and 400 ppm are mentioned together. First of all, ‘CO2<\/sub>-equivalent concentration’ expresses the radiative forcing effect of all human-induced greenhouse gases and aerosols as if we only changed CO2<\/sub> concentrations. We use that as shorthand for the net human perturbation – it’s not the same as real CO2<\/sub> being at 400 ppm because of the substantial cooling effect from aerosols. However, the other greenhouse gases such as methane and N2<\/sub>O increase the forcing and compensate somewhat for the aerosol effects. Thus the CO2<\/sub>-equivalent concentration is roughly equal to current levels of real CO2<\/sub>.<\/p>\n

The ecosystems on our planet are a little like a cat in an oven. We control the heating (greenhouse gas concentrations) and the cat responds to that temperature. So far, we have turned the controls to a medium level, and the oven is still warming-up. If we keep the oven control at today\u2019s medium level, the cat will warm beyond today\u2019s slight fever of 0.8\u00b0C<\/a>. And if we crank the control up a bit further over the next ten years and leave it there, the fever is going to get a little worse \u2013 and there is even a 4 in 1 chance that it could exceed 2\u00b0C. <\/p>\n

Where does that probability come from? If we knew the real climate sensitivity, then we would know the equilibrium warming of our oven\/planet if we left the CO2<\/sub>-equivalent concentrations at, say, 400 ppm for a long time. For instance, if the climate sensitivity were 3.8\u00b0C, such an oven with its control set to 400 ppm would warm 2\u00b0C in the long-term1<\/sup><\/a>. But what are the odds that the climate sensitivity is actually 3.8\u00b0C or higher? The chances are roughly 20%, if one assumes the conventional IPCC 1.5-4.5\u00b0C uncertainty range is the 80% confidence interval of a lognormal distribution2<\/sup><\/a>. Thus, if we want to avoid a 2\u00b0C warming with a 4:1 chance, we have to limit the greenhouse gas concentrations to something that is equivalent to 400 ppm CO2<\/sub> concentrations or below. (Note, though, that one might want to question, whether a chance of 4:1 is comforting enough for avoiding fever of 2\u00b0C or more…). <\/p>\n

At the heart of the second statement (\u201cWe’re going to be at 400 ppm in 10 years’ time\u201d) is the fair judgment that we seem committed to crank up concentrations not only to 400 ppm CO2<\/sub> but beyond: anything less implies we would have to switch off our power plants tomorrow. Current CO2<\/sub> concentrations are already about 380ppm and they rose about 20ppm over the last ten years<\/a>. <\/p>\n

\"CO2<\/a> Figure: A schematic representation of (a) fossil CO2<\/sub> emissions, (b) CO2<\/sub>-equivalent concentrations, and (c) global mean temperature for two scenarios: Firstly, an \u201cimmediate stabilization\u201d which implies rising CO2<\/sub>-equivalent concentrations up to around 415 ppm in 2015 and stable levels after that (red dashed line). This scenario is clearly hypothetical as the implied emission reductions in 2015 and beyond would hardly be feasible. Secondly, a peaking scenario (green solid line), which temporarily exceeds and then returns to a 415 ppm stabilization level. Both scenarios manage to stay below a 2\u00b0C target – for a climate sensitivity of 3.8\u00b0C or lower. This is roughly equivalent to a 4:1 chance of staying below 2\u00b0C3<\/sup><\/a>. <\/em><\/p>\n

Indeed, avoiding concentrations of \u2013 say – 475ppm CO2<\/sub> equivalent (see Figure a) will require quite significant reductions in emissions. However, as long as we reduce emissions decisively enough, concentrations in the atmosphere could lower again towards the latter half of the 21st century and beyond. The reasons are the relatively short lifetimes of methane, nitrous oxide, other greenhouse gases and some CO2<\/sub> uptake by the oceans (as discussed here<\/a>). <\/p>\n

Now, what is going to happen to our cat, if we turn up the heat control of our oven to about 475 ppm and then reduce it again? If we react quickly enough, we might be able to save the cat from some irreversible consequences. In other words, if concentrations are lowered fast enough after peaking at 475 ppm, the temperature might not exceed 2\u00b0C. Basically, the thermal inertia of the climate system will shave off the temperature peak that the cat would otherwise have felt if the oven temperature reacted immediately to our control button. <\/p>\n

Thus, to sum up: Even under the very likely scenario that we exceed 400 ppm CO2<\/sub> concentrations in the very near future, it seems likely that temperatures could be limited to below 2\u00b0C with a 4:1 chance, if emissions are reduced fast enough to peak at 475 ppm CO2<\/sub> equivalent, before sliding back to 400 ppm CO2<\/sub>-equivalent. <\/p>\n

Peaking at 475 ppm CO2<\/sub>-equivalent concentrations and returning to 400 ppm certainly comes at a cost, though: Our oven will approach 2\u00b0C more quickly compared to a (hypothetical) scenario where we halt the build-up of CO2<\/sub> concentrations at 400 ppm (see Figure c). Thus decisions would need to be different if we care more about the rate of warming than the equilibrium. In fact, some emission pathways and some models also suggest that peaking at 475 ppm CO2<\/sub> equivalent and returning to 400 ppm might even slightly decrease our chances to stay below 2\u00b0C (see Chapter 28 in the DEFRA report<\/a>). Depending on the actual thermal inertia of the climate system, the peak temperature corresponding 475 ppm might be very close to the 400 ppm equilibrium temperature. This points to a more fundamental issue: Rather than discussing ultimate stabilization levels, it might be more worthwhile for policy and science to focus on the peak level of greenhouse gas concentrations. By the time that we managed to peak concentrations we could still decide whether we want to stabilize at 400 ppm or closer to pre-industrial levels. We will most likely be able to make wiser decisions in the future given that we certainly have learnt something about the cat\u2019s behavior in its current fever.
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\n1.<\/a> The equilibrium warming dT can be easily estimated from the CO2 equivalent stabilization level C, if one would know the climate sensitivity S, with the following little formula: dT=S*ln(C\/278 ppm)\/ln(2)<\/p>\n

2.<\/a> This is of course a probability that merely reflects the uncertainty in our knowledge. The climate sensitivity is not random, it is just unknown. It expresses our degree of belief in that outcome, and is of course subject to future modification in the light of new evidence. <\/p>\n

3.<\/a> The depicted peaking scenario is the EQW-S475-P400 scenarios as presented in Chapter 28 of the DEFRA report<\/a>. The \u201ccombined constraint\u201d (see as well Chapter 28) has been chosen to find aerosol forcing and ocean diffusivity values for a 3.8\u00b0C climate sensitivity, which allow an approximate match to historic temperature and ocean heat uptake records. The historic fossil CO2<\/sub> emission data is taken from Marland et al.<\/a>, the CO2<\/sub> observations from Etheridge et al. and others are as given here<\/a> and here<\/a>, and the temperature observations and their uncertainties are from Jones, Folland et al.<\/a>. The simple climate model that was used is MAGICC 4.1 as in Wigley and Raper (2001 \u2013 see here<\/a>).
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\nLes paragraphes qui suivent essaient d\u2019expliquer les raisons pour lesquelles 2\u00b0C et 400 ppm sont cit\u00e9s ensemble. Tout d’abord, la concentration \u00e9quivalente en CO2<\/sub> exprime l’effet du for\u00e7age radiatif de tous les GES et des a\u00e9rosols \u00e9mis par les activit\u00e9s humaines (anthropiques) comme si la seule concentration du CO2<\/sub> \u00e9tait modifi\u00e9e. Nous utilisons ce raccourci pour d\u00e9crire la perturbation humaine totale nette \u2013 ce n’est pas comme si le CO2<\/sub> \u00e9tait vraiment \u00e0 une concentration de 400ppm, \u00e0 cause de l’effet de refroidissement important des a\u00e9rosols. Cependant, les autres GES tels que le m\u00e9thane et le N2<\/sub>O augmente le for\u00e7age et compense en partie l’effet des a\u00e9rosols. La concentration en CO2<\/sub> \u00e9quivalent est donc approximativement \u00e9gale au niveau actuel du vrai CO2<\/sub>. <\/p>\n

Les \u00e9cosyst\u00e8mes de notre plan\u00e8te sont un peu comme un chat dans un four. Nous contr\u00f4lons la temp\u00e9rature (les concentrations de GES) et le chat r\u00e9agit \u00e0 celle-ci. Jusqu’\u00e0 pr\u00e9sent, nous avons r\u00e9gl\u00e9 le four sur une temp\u00e9rature moyenne, et il est en train de pr\u00e9chauffer. Si nous laissons le r\u00e9glage au niveau moyen actuel, le chat se r\u00e9chauffera au-del\u00e0 de la l\u00e9g\u00e8re fi\u00e8vre de 0,8\u00b0C<\/a> qu’il a aujourd’hui. Et si nous montons un peu le r\u00e9glage dans les 10 ans qui viennent et le laissons ainsi, la fi\u00e8vre s’aggravera un peu \u2013 et il y a m\u00eame une chance sur 4 qu’elle d\u00e9passe 2\u00b0C. <\/p>\n

D’o\u00f9 vient cette probabilit\u00e9? Si nous connaissions la vraie sensibilit\u00e9 climatique, alors nous conna\u00eetrions le r\u00e9chauffement que notre four\/plan\u00e8te atteindrait \u00e0 l’\u00e9quilibre avec une concentration de CO2<\/sub> qui resterait \u00e0 \u2013 disons \u2013 400 ppm sur une longue p\u00e9riode. Par exemple, avec une sensibilit\u00e9 climatique de 3,8\u00b0C, ce four, avec un r\u00e9glage \u00e0 400 ppm, se r\u00e9chaufferait de 2\u00b0C \u00e0 long terme1<\/sup><\/a>. Mais quelles sont les chances que la sensibilit\u00e9 climatique soit de 3,8\u00b0C ou plus? Les chances sont approximativement de 20%, si on suppose que l’intervalle d’incertitude conventionnel du GIEC de 1,5 \u00e0 4,5\u00b0C est l’intervalle de confiance \u00e0 80% d’une distribution lognormale2<\/sup><\/a>. Donc, si nous voulons \u00e9viter un r\u00e9chauffement de 2\u00b0C (avec une probabilit\u00e9 de 75%), nous devons limiter les concentrations de GES \u00e0 un maximum d’environ 400ppm de CO2<\/sub> \u00e9quivalent. (Notez, pourtant, qu’on pourrait s’interroger pour savoir si cette chance de 75% d’\u00e9viter une fi\u00e8vre de 2\u00b0C ou plus est suffisamment r\u00e9confortante.)<\/p>\n

Au coeur de la deuxi\u00e8me affirmation (\u00ab Nous atteindrons 400 ppm dans 10 ans \u00bb) se retrouve l’\u00e9valuation correcte du fait que nous allons probablement faire monter les concentrations jusque 400 ppm et au-del\u00e0: pour faire moins, il faudrait d\u00e8s demain arr\u00eater nos centrales \u00e9lectriques. La concentration actuelle de CO2<\/sub> est d\u00e9j\u00e0 de 380 ppm et elle a augment\u00e9 de 20 ppm sur les 10 derni\u00e8res ann\u00e9es<\/a>. <\/p>\n

\"CO2<\/a> Figure : Une repr\u00e9sentation sch\u00e9matique (a) des \u00e9missions de CO2<\/sub> d’origine fossile, (b) des concentrations en CO2<\/sub> \u00e9quivalent et (c) de la temp\u00e9rature moyenne globale pour deux sc\u00e9narios : premi\u00e8rement, une \u00ab stabilisation imm\u00e9diate \u00bb qui implique que les concentrations de CO2<\/sub> augmentent jusque 415 ppm en 2015 et se stabilisent par apr\u00e8s (ligne rouge en trait interrompu). Ce sc\u00e9nario est purement hypoth\u00e9tique, car les r\u00e9ductions d’\u00e9missions en 2015 et par apr\u00e8s seraient difficilement faisables ; deuxi\u00e8mement, un sc\u00e9nario de pic (la ligne verte continue), qui d\u00e9passe temporairement les 415 ppm puis y retourne en se stabilisant. Ces deux sc\u00e9narios restent en dessous de la cible de 2\u00b0C \u2013 pour une sensibilit\u00e9 climatique de 3,8\u00b0C ou moins. C’est plus ou moins \u00e9quivalent \u00e0 75% de chance de rester en dessous de 2\u00b0C3<\/sup><\/a>. <\/em><\/p>\n

En effet, \u00e9viter des concentrations de, disons, 475 ppm de CO2<\/sub> \u00e9quivalent (voir figure a) implique des r\u00e9ductions tr\u00e8s significatives d’\u00e9missions. Toutefois, pour autant que nous r\u00e9duisions nos \u00e9missions de fa\u00e7on suffisamment d\u00e9cisive, les concentrations dans l’atmosph\u00e8re pourraient baisser \u00e0 nouveau dans la seconde moiti\u00e9 du 21\u00e8me si\u00e8cle et au-del\u00e0. Les raisons en sont les relativement courtes dur\u00e9es de vie du m\u00e9thane, du protoxyde d’azote, d’autres GES et d’une partie de l’absorption du CO2<\/sub> par les oc\u00e9ans (comme discut\u00e9 ici<\/a>). <\/p>\n

Maintenant, que va-t-il arriver \u00e0 notre chat si nous augmentons le r\u00e9glage de notre four \u00e0 environ 475 ppm pour le r\u00e9duire ensuite ? Si nous r\u00e9agissons assez vite, nous pourrions \u00eatre \u00e0 m\u00eame de sauver le chat de cons\u00e9quences irr\u00e9versibles. Autrement dit, si les concentrations baissent assez vite apr\u00e8s le pic \u00e0 475 ppm, l’augmentation de temp\u00e9rature pourrait ne pas d\u00e9passer 2\u00b0C. Concr\u00e8tement, l’inertie thermique du syst\u00e8me climatique va \u00e9cr\u00eater le pic de temp\u00e9rature que le chat aurait senti si la temp\u00e9rature du four r\u00e9agissait imm\u00e9diatement au bouton de r\u00e9glage. <\/p>\n

Donc, en r\u00e9sum\u00e9: m\u00eame dans le sc\u00e9nario tr\u00e8s probable o\u00f9 nous d\u00e9passons les 400 ppm de CO2<\/sub> \u00e9quivalent dans un avenir tr\u00e8s proche, il semble probable que l’augmentation des temp\u00e9ratures pourraient \u00eatre limit\u00e9e \u00e0 2\u00b0C avec une chance de 75% si les \u00e9missions sont r\u00e9duites suffisamment vite pour atteindre un maximum de 475 ppm de CO2<\/sub> \u00e9quivalent avant de redescendre \u00e0 400 ppm de CO2<\/sub> \u00e9quivalent. <\/p>\n

Un pic \u00e0 475 ppm de CO2<\/sub> \u00e9quivalent et un retour \u00e0 400 ppm par la suite a pourtant un co\u00fbt. Notre four va atteindre les 2\u00b0C plus rapidement que compar\u00e9 \u00e0 un sc\u00e9nario (hypoth\u00e9tique) o\u00f9 les concentrations de CO2<\/sub> se stabiliseraient \u00e0 400 ppm (voir figure c). Donc les d\u00e9cisions seraient diff\u00e9rentes si nous nous pr\u00e9occupions plus du taux de r\u00e9chauffement que de l’\u00e9quilibre. En fait, certains types d’\u00e9volution des \u00e9missions et certains mod\u00e8les sugg\u00e8rent \u00e9galement qu’un pic \u00e0 475 ppm de CO2<\/sub> \u00e9quivalent avec retour par la suite \u00e0 400 ppm pourraient faire diminuer nos chances de rester en dessous de 2\u00b0C (voir chapitre 28 du rapport du DEFRA<\/a>). En fonction de l’inertie thermique r\u00e9elle du syst\u00e8me climatique, la temp\u00e9rature maximum correspondant au pic \u00e0 475 ppm pourrait \u00eatre tr\u00e8s proche de la temp\u00e9rature d’\u00e9quilibre de 400 ppm. Ceci nous am\u00e8ne \u00e0 un point fondamental: plut\u00f4t que de discuter le niveau ultime de stabilisation, il pourrait \u00eatre plus utile pour la politique et la science de se concentrer sur le niveau de pic des concentrations de GES. Quand nous aurions atteint ce pic, nous pourrions encore d\u00e9cider si nous voulons stabiliser \u00e0 400 ppm ou plus pr\u00e8s des niveaux pr\u00e9industriels. Nous serons tr\u00e8s probablement plus \u00e0 m\u00eame de prendre des d\u00e9cisions plus avis\u00e9es \u00e0 l’avenir \u00e9tant donn\u00e9 que nous avons certainement appris quelques chose sur le comportement du chat dans sa fi\u00e8vre actuelle.
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\n1.<\/a> Le r\u00e9chauffement d’\u00e9quilibre dT peut facilement \u00eatre estim\u00e9 \u00e0 partir du niveau de stabilisation de CO2<\/sub> \u00e9quivalent C, si on conna\u00eet la sensibilit\u00e9 S, avec la petite formule suivante: dT=S*ln(C\/278 ppm)\/ln(2)<\/p>\n

2.<\/a> C’est bien entendu une probabilit\u00e9 qui ne refl\u00e8te que l’incertitude de nos connaissances. La sensibilit\u00e9 climatique n’est pas al\u00e9atoire, elle est simplement inconnue. Ceci exprime notre degr\u00e9 de croyance dans l’un ou l’autre r\u00e9sultat, et est bien entendu sujet \u00e0 r\u00e9vision future \u00e0 la lumi\u00e8re de nouvelles informations.<\/p>\n

3.<\/a>Le sc\u00e9nario de pic d\u00e9crit est le sc\u00e9nario EQW-S475-P400 tel que pr\u00e9sent\u00e9 au chapitre 28 du rapport du DEFRA<\/a>. La \u00ab contrainte combin\u00e9e \u00bb (voir aussi chapitre 28) a \u00e9t\u00e9 choisie pour identifier les valeurs du for\u00e7age des a\u00e9rosols et la diffusivit\u00e9 des oc\u00e9ans pour une sensibilit\u00e9 climatique de 3,8\u00b0C, qui permettent de faire correspondre approximativement les temp\u00e9ratures historiques et les enregistrements d’absorption de chaleur par l’oc\u00e9an. Les donn\u00e9es historiques d’\u00e9mission fossile de CO2<\/sub> sont prises de Marland et al.<\/a>, les observations de CO2<\/sub> de Etheridge et al. et les autres sont donn\u00e9es ici<\/a> et a href=”http:\/\/cdiac.ornl.gov\/ftp\/trends\/co2\/lawdome.combined.dat”>ici, et les observations de temp\u00e9rature et leurs incertitudes sont de Jones, Folland et al.<\/a>. Le mod\u00e8le climatique simple qui a \u00e9t\u00e9 utilis\u00e9 est MAGICC 4.1 , d\u00e9crit par Wigley and Raper (2001 \u2013 voir ici<\/a>).
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Guest comment by Malte Meinshausen, Reto Knutti and Dave Frame Yesterday\u2019s BBC article on the “Avoiding Dangerous Climate Change” report of the Exeter meeting last year, carried two messages that have left some a little confused. On the one hand, it said that a stabilization of greenhouse gases at 400-450 ppm CO2-equivalent concentrations is required […]<\/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":[1,3],"tags":[],"class_list":{"0":"post-246","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-greenhouse-gases","8":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/246","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=246"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/246\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=246"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=246"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=246"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}