650,000 years of greenhouse gas concentrations 650 000 années de concentrations de gaz à effet de serre

The latest results from the EPICA core in Antarctica have just been published this week in Science (Siegenthaler et al. and Spahni et al.). This ice core extended the record of Antarctic climate back to maybe 800,000 years, and the first 650,000 years of ice have now been analysed for greenhouse gas concentrations saved in tiny bubbles. The records for CO2, CH4 and N2O both confirm the Vostok records that have been available for a few years now, and extend them over another 4 glacial-interglacial cycles. This is a landmark result and a strong testament to the almost heroic efforts in the field to bring back these samples from over 3km deep in the Antarctica ice. So what do these new data tell us, and where might they lead? Les derniers résultats du forage EPICA en Antarctique viennent juste d’être publiés dans le numéro de cette semaine de Science (Siegenthaler et al. et Spahni et al.). Cette carotte glaciaire a permis d’étendre l’enregistrement du climat Antarctique jusqu’à 800 000 ans, alors que les concentrations de gaz à effet de serre, piégées sous forme de minuscules bulles de gas, ont été analysées pour les 650 000 dernières années. Ces enregistrements de CO2, CH4 et N2O sont en accord avec ceux de Vostok (également en Antarctique) disponibles depuis quelques années, et permettent d’observer 4 cycles d’alternance glaciaire/interglaciaire supplémentaires. Ces travaux sont remarquables, et justifient des efforts gigantesques effectués sur le terrain pour ramener des échantillons enfouis jusqu’à 3km de profondeur dans la glace Antarctique. Que nous disent ces nouvelles données, et dans quelle direction peuvent elles nous mener ?

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Composite CO2: Click to enlargeFirst of all, the results demonstrate clearly that the relationship between climate and CO2 that had been deduced from the Vostok core appears remarkably robust. This is despite a significant change in the patterns of glacial-interglacial changes prior to 400,000 years ago. The ‘EPICA challenge’ was laid down a few months ago for people working on carbon cycle models to predict whether this would be the case, and mostly the predictions were right on the mark. (Who says climate predictions can’t be verified?). It should also go almost without saying that lingering doubts about the reproducibility of the ice core gas records should now be completely dispelled. That a number of different labs, looking at ice from different locations, extracted with different methods all give very similar answers, is a powerful indication that what they are measuring is real. Where there are problems (for instance in N2O in very dusty ice), those problems are clearly found and that data discarded.

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