Les paramètres connus du changement climatique

POTSDAM – Le philosophe Daniel Dennett a déjà comparé la science à la structure d’une gigantesque pyramide. Sa base comprend la masse des connaissances bien établies, celles qui ne sont plus controversées et qui font rarement l’objet de discussions en dehors des cercles scientifiques. Des études plus récentes s’empilent au sommet de la pyramide, où la plupart de débats publics se déroulent. Cette métaphore sied comme un gant à la recherche dans le domaine des changements climatiques, et mérite d’être gardée à l’esprit pour la lecture du dernier rapport du Groupe d’experts intergouvernementaux sur l’évolution du climat (GIEC) publié par les Nations Unies.

Le cinquième rapport GIEC, le produit de plusieurs années de travail par des centaines de scientifiques internationaux spécialistes du climat, passe en revue l’état de nos connaissances en matière de changement climatique et explique des analyses plus récentes. Évidemment, les médias ont tendance à focaliser sur ce dernier volet ; comme les prédictions beaucoup plus élevées de la hausse du niveau de la mer que dans le rapport précédent du GIEC en 2007. Faisons abstraction pour l’instant de la tourmente médiatique et examinons de plus près la base solide de la pyramide de nos connaissances.

L’étude des phénomènes climatiques remonte à au moins deux siècles, à la découverte par Joseph Fourier des effets des gaz à effets de serre sur les conditions climatiques planétaires ; en 1859, John Tyndall démontrait aussi dans son laboratoire que les gaz étaient à l’origine de cet effet. Des mesures précises du rayonnement solaire au sol et au niveau des satellites ont prouvé depuis l’existence de l’effet de serre.

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