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Byline: L. Rivier (1), Ph. Peylin (2), Ph. Ciais (1), M. Gloor (3), C. Rodenbeck (4), C. Geels (5), U. Karstens (4), Ph. Bousquet (1), J. Brandt (5), M. Heimann (4) Abstract: Approximately half of human-induced carbon dioxide (CO.sub.2) emissions are taken up by the land and ocean, and the rest stays in the atmosphere, increasing the global concentration and acting as a major greenhouse-gas (GHG) climate-forcing element. Although GHG mitigation is now in the political arena, the exact spatial distribution of the land sink is not well known. In this paper, an estimation of mean European net ecosystem exchange (NEE) carbon fluxes for the period 1998--2001 is performed with three mesoscale and two global transport models, based on the integration of atmospheric CO.sub.2 measurements into the same Bayesian synthesis inverse approach. A special focus is given to sub-continental regions of Europe making use of newly available CO.sub.2 concentration measurements in this region. Inverse flux estimates from the five transport models are compared with independent flux estimates from four ecosystem models. All inversions detect a strong annual carbon sink in the southwestern part of Europe and a source in the northeastern part. Such a dipole, although robust with respect to the network of stations used, remains uncertain and still to be confirmed with independent estimates. Comparison of the seasonal variations of the inversion-based net land biosphere fluxes (NEP) with the NEP predicted by the ecosystem models indicates a shift of the maximum uptake period, from June in the ecosystem models to July in the inversions. This study thus improves on the understanding of the carbon cycle at sub-continental scales over Europe, demonstrating that the methodology for understanding regional carbon cycle is advancing, which increases its relevance in terms of issues related to regional mitigation policies. Author Affiliation: (1) Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, IPSL, Paris, France (2) Laboratoire de Biogeochimie des Milieux Continentaux, CEA-CNRS-UVSQ, IPSL, Grignon, France (3) School of Geography, University of Leeds, Leeds, LS2 9JT, UK (4) Max-Planck-Institut fur Biogeochemie, Jena, Germany (5) National Environmental Research Institute, University of Aarhus, Roskilde, Denmark Article History: Registration Date: 22/06/2010 Received Date: 05/01/2009 Accepted Date: 15/06/2010 Online Date: 15/07/2010