ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-12-9587-2012 Reconstruction of the carbon isotopic composition of methane over the last 50 yr based on firn air measurements at 11 polar sites Sapart C. J. 1 Martinerie P. 2 Chappellaz J. 2 van de Wal R. S. W. 1 Sperlich P. 3 van der Veen C. 1 Bernard S. 2 Sturges W. T. 4 Blunier T. 3 Witrant E. 5 Schwander J. 6 Etheridge D. 7 Röckmann T. 1 Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, Utrecht, The Netherlands UJF – Grenoble1/CNRS, Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), UMR5183, Grenoble, 38041, France Center for Ice and Climate (CIC), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark School of Environmental Sciences, Universtity of East Anglia (UEA), Norwich, NR15 1RL, UK UJF – Grenoble1/CNRS, Grenoble Image Parole Signal Automatique (GIPSA-lab), UMR5216, B.P. 46, 38402 St Martin d'Hères, France Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland Centre for Australian Weather and Climate Research/CSIRO Marine and Atmospheric Research, Private Bag 1, Aspendale, VIC 3195, Australia 13 04 2012 12 4 9587 9619 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/12/9587/2012/acpd-12-9587-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/12/9587/2012/acpd-12-9587-2012.pdf

Methane is a strong greenhouse gas and large uncertainties exist concerning the future evolution of its atmospheric abundance. Analyzing methane atmospheric mixing and stable isotope ratios in air trapped in polar ice sheets helps reconstructing the evolution of its sources and sinks in the past. This is important to improve predictions of atmospheric CH<sub>4</sub> mixing ratios in the future under the influence of a changing climate. We present an attempt to reconcile methane carbon isotope records from 11 firn sites from both Greenland and Antarctica to reconstruct a consistent &delta;<sup>13</sup>C(CH<sub>4</sub>) history over the last 50 yr. In the firn, the atmospheric signal is altered mainly by diffusion and gravitation. These processes are taken into account by firn transport models. We show that isotope reconstructions from individual sites are not always mutually consistent among the different sites. Therefore we apply for the first time a multisite isotope inversion to reconstruct an atmospheric isotope history that is constrained by all individual sites, generating a multisite "best-estimate" scenario. This scenario is compared to ice core data, atmospheric air archive results and direct atmospheric monitoring data.

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