Global Dimming? Assombrissement global?

It just so happens that most of the posts on this site have tried to counteract arguments from those who would sow fake “uncertainty” in the climate debate. But lest our readers feel that we are unjustifiably certain about our knowledge, let us look at a recent example of the opposite tendency: too much certainty.

A recent BBC Horizon documentary (transcript) raised the issue of ‘global dimming’ and argued that this ‘killer’ phenomena’s newly-recognised existence would lead to huge re-assessments of future global warming. As part of the hyperbole, the process of global dimming was linked very clearly to the famines in Ethiopia in the 1980s and the implication was left that worse was to come. Media reports with headlines like “Fossil Fuel Curbs May Speed Global Warming” swiftly followed. So what’s the real story?

par Gavin Schmidt (traduit par Pierre Allemand)

Il est un fait que la plupart des interventions sur ce site ont essayé de contrer les arguments de ceux qui voudraient semer une “incertitude” trompeuse dans le débat sur le climat. Mais, de peur que nos lecteurs pensent que nous sommes abusivement certains de nos connaissances, examinons un exemple récent de la tendance inverse : trop de certitude.

Un récent documentaire de la série “Horizon” de la BBC (transcription) a soulevé la question d’un “assombrissement global” et a indiqué que l’existence récemment reconnue d’un tel phénomène « tueur » pourrait amener à reconsidérer très fortement l’hypothèse d’un futur réchauffement global. Cet audacieux raisonnement faisait un lien très clair entre l’assombrissement global et les famines en Ethiopie durant les années 1980, le plus grave restant encore à venir. Des dépêches de presse avec des titres du genre « Le coup de frein sur les énergies fossiles pourrait accélérer le réchauffement global » suivaient dans la foulée. Mais, qu’en est-il exactement ?


Global dimming is indicated by measurements over land areas in many regions in the world and may therefore be a real phenomena. Though there are serious issues with the quality of some of the data (birds drinking out of uncovered evaporation pans, drift and inhomogeneities in the solar radiation measuring instruments), in the most global assessment, Beate Liepert estimated that there was globally a reduction of about 4% in solar radiation reaching the ground between 1961 and 1990. While more recent indications are that the trend may have reversed in the last decade, it could still be significant. Assuming for the sake of argument that these data are valid, what could have caused this? A change of that magnitude in the incoming solar radiation itself is not possible since satellite observations would have seen it. Thus, it must be something that is happening in the atmosphere to intercept solar radiation. There are only a few possibilities: clouds, water vapour or aerosols.

First of all it is important to note that even pure greenhouse gas forcing will lead to a slight decrease in surface solar radiation (due to the concurrent increased humidity) and potential cloud feedbacks. Cloud cover and thickness are both like to vary as a function of climate change.

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