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Stern Science La science de SternA ciência de Stern

Filed under: — group @ January 28th, 2007

Halldór Björnsson, William Connolley and Gavin Schmidt

Late last year, the UK Treasury’s Stern Review of the Economics of Climate Change was released to rapturous reception from all sides of the UK political spectrum (i.e. left and right). Since then it has been subject to significant criticism and debate (for a good listing see Rabbett Run). Much of that discussion has revolved around the economic (and ethical) issues associated with ‘discounting’ (how you weight welfare in the future against welfare today) – particularly Nordhaus’s review. We are not qualified to address those issues, and so have not previously commented.

However, as exemplified by interviews on a recent Radio 4 program (including with our own William Connolley), some questions have involved the science that underlies the economics. We will try and address those.
Halldór Björnsson, William Connolley and Gavin Schmidt (traduit par Etienne Pesnelle)

A la fin de l’an dernier, le Trésor britannique a publié le rapport Stern sur les conséquences économiques du changement climatique, qui a été reçu avec enthousiasme par l’ensemble de la classe politique du Royaume-Uni, c’est à dire la gauche et la droite Depuis, il a fait l’objet de nombreux débats et critiques (voir la liste qu’a établie Rabbett Run). L’essentiel de la discussion a tourné autour des problèmes économiques (et éthiques) associés à “l’actualisation” (c’est-à-dire comment mesurer le bien-être futur à l’aune du bien-être actuel), ce dont traite notamment Nordhaus . Nous ne sommes pas qualifiés pour discuter de ces points, aussi ne les avons-nous pas commentés précédemment.

Toutefois, comme l’illustrent les interviews données lors d’une émission récente de Radio 4 (dont une avec notre William Connolley), certaines questions ont concerné la science qui sous-tend les calculs économiques. Nous allons essayer de les aborder.
Halldór Björnsson, William Connolley e Gavin Schmidt (traduzido por F. M. Ramos e I. B. T. Lima)

No fim do ano passado, o Tesouro britânico publicou o Relatório Stern sobre as conseqüências econômicas das mudanças climáticas, que foi recebido com entusiasmo pelo conjunto da classe política do Reino Unido, isto é a esquerda e a direita. Depois, ele foi objeto de inúmeros debates e críticas (ver a lista que preparou Rabbett Run). O essencial da discussão realizou-se em torno dos problemas econômicos (e éticos) associados à “atualização” (isto é, como medir o bem-estar futuro em comparação ao bem-estar atual) – particularmente o Relatório Nordhaus. Nós não estamos qualificados para comentar estes assuntos, assim como não comentamos no passado.

No entanto, como ilustram as entrevistas concedidas durante um recente programa de Radio
4
(das quais uma com William Connolley), certas questões diziam respeito à ciência que sustenta os cálculos econômicos. Vamos tentar abordá-las a seguir. Ao contrário de um relatório mais antigo da Câmara dos Lordes, Stern não perde tempo tentando trapacear, e essencialmente busca a ciência no relatório do IPCC, com algumas atualizações de trabalhos mais recentes. A maior parte da ciência está resumida no capítulo um, e um leitor casual familiarizado com o relatório IPCC encontrará poucas surpresas em seções que incluem afirmações como “Uma massa esmagadora de evidências científicas indica que o clima da Terra está mudando rapidamente, predominantemente pelo efeito do crescimento dos gases de efeito estufa causado pelas atividades humanas” etc. Entretanto, as possibilidades científicas em Stern são ponderadas de maneira levemente diferente que nos relatórios do IPCC uma vez que, como ele afirma, “os tomadores de decisão devem levar em conta os riscos extremos, além das previsões médias, por que seria muito grave se estes riscos viessem a se materializar” (Stern reply to Byatt et al).

Há três componentes científicas no relatório Stern: a sensibilidade climática, as emissões futuras dos gases de efeito estufa, e os impactos de uma dada mudança, expressas na forma de anomalia de temperatura global por razões de comodidade.

A sensibilidade climática (já discutida aqui anteriormente) foi considerada como provavelmente estando no intervalo de 1.5 a 4.5 C do IPCC TAR, e no intervalo de 2 a 5 C nos modelos utilizados no relatório Stern. No entanto, a probabilidade de valores maiores tem um papel importante no relatório. Especificamente, Meinshausen* (2006) [estabelece] que há “entre 2% a 20% de probabilidade que o aquecimento possa ser maior que 5 C”. Isto é verdade, mas o relatório esquece de mencionar que outros novos estudos (Annan and Hargreaves; Hegerl et al) sugerem que é insignificante a probabilidade que a sensibilidade climática seja superior a 5 C.

A incerteza sobre o aquecimento futuro não se reduz à incerteza sobre a sensibilidade, mas depende também daquela relacionada aos níveis futuros dos gases de efeito estufa (GEE). Existe uma ampla gama de cenários e de estimativas sobre níveis futuros de GEE que são utilizados nos relatórios do IPCC. O cenário utilizado pelo Relatório Stern é o A2, mas neste cenário, os níveis de GEE na segunda metade do século XXI são superiores àqueles do cenário A1b, por exemplo. A questão não é se o cenário A2 é menos sólido que o A1, mas simplesmente que o Relatório Stern escolheu trabalhar com um dos cenários de “fortes emissões”. Além disso, o relatório reconhece também a grande incerteza (mas não claramente quantificável) de feedbacks positivos nas emissões de CO2 e CH4 de origem natural.

Com relação aos impactos das mudanças climáticas, a estória é semelhante: a maior parte dos impactos são declarados mas sua probabilidade de ocorrência é sujeita à debate. Por exemplo: o enfraquecimento da corrente termohalina sob 1 grau de aquecimento, risco de colapso em 3 graus, risco de derretimento irreversível da calota de gelo da Groenlândia para um aquecimento de 2 graus, a elevação dos mares de 5 a 12 metros durante muitos séculos, – estas eventualidades são questionáveis, e deveriam ser consideradas como “o cenário adverso” dentre os possíveis impactos.

Em conclusão: Stern de um modo geral utiliza bem a ciência do clima, mas desvia-se para o lado das estimativas mais impactantes e as utiliza em seu sumário. Este viés altista faz com que o relatório seja vulnerável a acusações de “alarmismo”. O relatório é justo em apontar que os danos e seus custos crescem de maneira desproporcional com o aumento da mudança de temperatura e portanto, dada esta assimetria, os tomadores de decisão têm razão de levá-los em conta. Entretanto, parece que a maior crítica deste relatório será atribuída (em outros foros) à parte econômica.

NB: De modo previsível, alguns dos “céticos” habituais atacaram igualmente a ciência do relatório Stern. No entanto, uma indicação de sua falta fundamental de seriedade é que, quando há realmente importantes incertezas (por exemplo, a probabilidade de que a sensibilidade seja superior àquela geralmente estimada), eles as ignoram para fazer as mesmas repetitivas, desinteressantes e incorretas afirmações que sempre fazem.

*Meinshausen, M. (2006): ‘What does a 2C target mean for greenhouse gas concentrations? A brief analysis based on multi-gas emission pathways and several climate sensitivity uncertainty estimates’ (“O que significa um alvo de +2°C em termos de concentração de gás de efeito estufa? Uma rápida análise fundamentada em caminhos de emissão multi-gás e várias estimativas de incerteza da sensibilidade climática”), Avoiding dangerous climate change (Evitando uma perigosa mudança climática), in H.J. Schellnhuber et al. (eds.), Cambridge: Cambridge University Press, pp. 265 a 280.

Unlike an earlier report by the House of Lords, Stern spends no time quibbling, and essentially takes the science from the IPCC report, though somewhat updated by more recent work. Most of the science is flipped through fairly quickly within chapter one, and casual readers familiar with the IPCC report will find little to surprise them with sections including statements such as “An overwhelming body of scientific evidence indicates that the Earth’s climate is rapidly changing, predominantly as a result of increases in greenhouse gases caused by human activities” etc. However, the scientific possibilities in Stern are weighted slightly differently than in the IPCC reports since, as he states, “policymakers need to take into account the risks of greater dangers, as well as central expectations, because the consequences if these risks were to materialise would be very serious” (Stern reply to Byatt et al).

There are three strands to the science in Stern: the climate sensitivity, future emissions of greenhouse gases and the impacts of any particular level of change (scaled to the global mean temperature anomaly for convenience).

The climate sensitivity (as discussed here previously) was given a likely range of 1.5 – 4.5 C in IPCC TAR, and with a range of 2 – 5 C in the models used in that report. However, the probability of higher values plays a significant role in the report. Specifically, Meinshausen (2006) that there is “between a 2% and 20% chance that climate sensitivity is greater than 5C” but in the key message section of chapter 1 this is distilled as: “Several new studies suggest up to a 20% chance that warming could be greater than 5C”. This is true, but the report neglects to mention other new studies (Annan and Hargreaves; Hegerl et al) that suggest a negligible probability of CS greater than 5 C.

Uncertainty about future warming is not just the uncertainty about sensitivity, but also about the future greenhouse gas levels (GHG). There is a wide range of scenarios and estimates of future GHG levels that are used in the IPCC reports. The scenario used by the Review is the A2 one, but in this scenario GHG in the latter part of the 21st century is higher than in say, the A1b scenario. The point here is not that A2 is less sound than the A1b scenario, but simply that the Review chooses to work with one of the “high emission” scenarios. Additionally, the report also acknowledges the highly uncertain (but not clearly quantifiable) the possibilities of positive feedbacks in natural CO2 and CH4 emissions.

For impacts of climate change the story is similar: many of the impacts mentioned possible but their likelihood is debatable. For example, the weakening of the THC under 1 degree of warming, a risk of collapse for 3 degrees, risk of irreversible melting of the Greenland Ice sheet at 2 degrees warming, sea level changes of 5 – 12 meters over several centuries, – these eventualities are debatable, and should certainly be viewed as the “adverse tail” of possible impacts.

In conclusion: Stern gets the climate science largely right, though he strays on the high side of various estimates and picks the high side to talk about in the summary. This high-end bias lends the Review open to charges of “alarmism”. The report does make the fair point that the damages and their cost grows disproportionally with increasing temperature change and so, given that asymmetry, policymakers are correct in taking note of them. However, it looks like the major criticism of his work will be directed (in other fora) at the economics.

NB. Rather predictably, some of the usual contrarian suspects have also attacked the science in Stern. It is, however, a measure of their fundamental lack of seriousness that when there really are important uncertainties (i.e. the likelihood that climate sensitivity is higher than generally thought), they ignore them in favour of making the same repetitive uninteresting and incorrect claims they always make.

*Meinshausen, M. (2006): ‘What does a 2C target mean for greenhouse gas concentrations? A brief analysis based on multi-gas emission pathways and several climate sensitivity uncertainty estimates’, Avoiding dangerous climate change, in H.J. Schellnhuber et al. (eds.), Cambridge: Cambridge University Press, pp.265 280.

Contrairement à un rapport plus ancien de la Chambre des Lords , Stern ne perd pas de temps à chicaner, et il va essentiellement chercher les éléments scientifiques dans le rapport du GIEC, avec parfois des mises à jour issues de récent travaux. La plupart de ces éléments semble avoir été rapidement récoltée en feuilletant le premier chapitre, et le lecteur un tant soit peu familier du rapport du GIEC trouvera peu de choses qui le surprendront dans des chapitres où on trouve des assertions comme “une masse écrasante de preuves scientifiques indique que le climat terrestre est en train de changer rapidement, essentiellement du fait de l’augmentation des gaz à effet de serre générés par les activités humaines”, etc. Cependant, les perspectives scientifiques de Stern sont pondérées de façon légèrement différentes de celles des rapports du GIEC puisque, déclare-t-il, “les décideurs doivent prendre en compte les risques de dangers extrêmes, en plus des prévisions moyennes, car les conséquences de tels risques, s’ils se matérialisaient, seraient très sérieuses” ( réponse de Stern à Byatt et al ).

Il y a trois composantes scientifiques dans le rapport Stern : la sensibilité climatique, les émissions futures de gaz à effet de serre et les impacts d’un changement donné, exprimé en anomalie de température globale pour raison de commodité.

La sensibilité climatique (déjà discutée ici précédemment) était donnée comme probablement dans la fourchette 1,5-4,5 °C par le Troisième Rapport d’Evaluation (TRE) du GIEC, et comprise dans la fourchette 2-5°C par les modèles utilisés dans le rapport Stern. Cependant, la probabilité d’atteindre des valeurs supérieures joue un rôle significatif dans le rapport. En particulier, Meinshausen* (2006) [établit] qu’il y a “entre 2% à 20% de probabilité que la sensibilité climatique est supérieure à 5°C” , mais dans le résumé du chapitre 1 ceci est condensé en “de nombreuses nouvelles études semblent indiquer qu’il y a jusqu’à 20% de probabilité que le réchauffement pourrait être supérieur à 5°C”. Cela est vrai, mais le rapport néglige de mentionner d’autres nouvelles études (Annan et Hargreaves ; Hegerl et al) qui semblent indiquer le caractère négligeable de la probabilité que la sensibilité climatique soit supérieure à 5°C.

L’incertitude sur le réchauffement futur ne se réduit pas à l’incertitude sur la sensibilité, mais dépend aussi de celle sur les niveaux futurs des gaz à effet de serre (GES). Il y a une grande batterie de scénarios et d’estimations des niveaux futurs des GES qui sont utilisés dans les rapports du GIEC. Le scénario utilisé par le Rapport Stern est le A2, mais, dans ce scénario, les niveaux de GES dans la seconde partie du 21ème siècle sont supérieurs à ceux du scénario A1, par exemple. La question ici n’est pas que le scénario A2 est moins solide que le A1, mais simplement que le Rapport Stern a choisi de travailler avec un des scénarios “à fortes émissions”. En plus, le rapport reconnaît aussi la grande incertitude (mais pas clairement quantifiable) des possibilités de rétroactions positives des émissions de CO2 d’origine naturelle et de méthane.

De même pour les impacts du changement climatique : la plupart des impacts sont déclarés possibles mais leur probabilité d’occurrence est matière à débat. Par exemple : l’affaiblissement de la circulation thermohaline en-dessous d’un degré de réchauffement, le risque d’effondrement pour 3 degrés, le risque de fonte irréversible de la calotte du Groenland pour un réchauffement de 2 degrés, l’élévation de 5 à 12 mètres du niveau des mers sur plusieurs siècles – ces éventualités sont matière à débat, et ne devraient certainement pas être considérées comme le “plus petit commun multiple” des impacts possibles.

En conclusion : Stern a largement bien utilisé la climatologie, bien qu’il se soit égaré parmi les fourchettes hautes des nombreuses estimations et qu’il les ait prises comme référence dans son résumé. Ce parti pris haut de gamme fait qu’on peut accuser son rapport d’ “alarmisme” : il ne souligne pas, alors que c’est juste, que les dommages et leurs coûts augmentent de façon non proportionnelle avec la température et que, compte tenu de cette asymétrie, les décideurs ont justement raison d’en prendre note. Toutefois, il semble que la critique majeure de cette oeuvre concernera (sur d’autres forums) sa partie économique.

NB. De façon plutôt prévisible, quelques-uns des « sceptiques » habituels ont également mis en cause la climatologie utilisée dans le rapport Stern. Cependant, une indication de leur manque fondamental de sérieux est que, lorsqu’il y a vraiment d’importantes incertitudes (ex : la probabilité que la sensibilité climatique est supérieure à celle généralement estimée), ils les ignorent pour continuer à faire les mêmes contestations répétitives et inintéressantes qu’ils ont toujours faites.

*Meinshausen, M. (2006): ‘What does a 2C target mean for greenhouse gas concentrations? A brief analysis based on multi-gas emission pathways and several climate sensitivity uncertainty estimates’ (“Que signifie une cible de +2°C en terme de concentration de gaz à effet de serre ? Une rapide analyse fondée sur les schémas d’émissions multi-gaz et plusieurs estimations de l’incertitude de la sensibilité climatique”), Avoiding dangerous climate change (Eviter un dangereux changement climatique), in H.J. Schellnhuber et al. (eds.), Cambridge: Cambridge University Press, pages.265 à 280.


55 Responses to “Stern Science La science de SternA ciência de Stern

  1. 51
    Hank Roberts says:

    Bogus. That’s from the 1950s, an atomic bomb “duck and cover” school education film.
    I remember seeing it the first time around.

  2. 52
    David Price says:

    The Stern report was flawed in that he is accused of overstating the damage of agw and understaing the costs of dealing with it. It is therfore rightly accused of bieng propagandist. Remember he works for a government which has a history of producing dodgy dosiers. To command public confidence he will have to clean up his act a bit.
    We need a balanced view of the cost/benefit ratio of carbon reduction. Only this will carry the public with it.

  3. 53
    Dan says:

    re: 52 and “the costs of dealing with (AGW)”. The fact is that business has an absolutely miserable record and very little credibility if any re: estimating the costs of reducing pollution. And by “miserable”, I mean by spreading fear by far over-estimating costs both to business and to the economy. For example, in the US, each time the Clean Air Act has been amended since 1972, fossil-fuel power companies and their associated think-tanks screamed bloody murder that sulfur dioxide and nitrogen oxide emission reductions would essentially destroy the economy (all but ignoring the obvious health and environmental benefits that did come to fruition). Far from it. In fact, after the Clean Air Act Amendments of 1990, the economy grew quite well across the board. Additional mandated or voluntarily implemented controls of other pollutants since 1972 (the year of the “first” US so-called “Clean Air Act”) have also not brought the economy down. Recessions that occurred during the period were due to other unrelated issues/factors. In hindsight, after the 1990 Amendments, it was the US government’s cost estimates that proved to be much more accurate than business estimates.

  4. 54
    SolarNTrains says:

    #49, matt, I am glad to see there are a few pragmatists here. I always find it unfortunate when the Davids and Al denigrate the industrial technologists and scientists, b/c that is who we need to help us pull the wood out of the fire, so to CO2 speak. And a lot them are conservative as well (or NeoCon mouthbreathers, as they are usually referred to by the realclimatorati). If you actually look at some of the studies on alternative energy, we have a long way to go just to keep up with growth, must less replace some of the current sources. I work with solar, and even with the spectrum conversion improvements, we will be lucky if it provides 9% of our energy needs in the next decades. So nukes are probably a necessity, unless you intend to give up economically unnecessary activities (like blogging) or ecologically bankrupt geographical areas (like Southern California).

  5. 55
    SolarNTrains says:

    re: #53; Dan, can’t comment on your opinion here, as I haven’t seen the data. However, I would expect there will always be a diversity of opinion between government and business. In fact, I would be concerned if they agree, and would really want to hold onto my wallet tightly!
    Businesses will be concerned first and foremost with profits, cash flow, expenses, employment, et cetera on a forward looking (and continually reviewed) basis. Governments, except during elections years, tend to be more collection focused (both information and taxes!). Usually, the government only responds when a majority of opinion means it is a popular decision. Thus, I would expect we will finally get a productive response from Congress on Kyoto this year.


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