1. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523, USA
2. CSIRO Atmospheric Research, PMB 1, Aspendale, Victoria 3195, Australia
3. National Center for Atmospheric Research (NCAR), Boulder, Colorado 80303, USA
4. Laboratoire des Sciences du Climat et de l'Environnement (LSCE), F-91198 Gif-sur-Yvette Cedex, France
5. Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration (NOAA), 326 Broadway R/CG1, Boulder, Colorado 80303, USA
6. Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02141, USA
7. AOS Program, Princeton University, Sayre Hall, Forrestal Campus, PO Box CN710, Princeton, New Jersey 08544-0710, USA
8. Center for Atmospheric Sciences, McCone Hall, University of California, Berkeley, California 94720-4767, USA
9. Max-Planck-Institut fur Biogeochemie, D-07701 Jena, Germany
10. Meteorological Service of Canada, Environment Canada, Toronto, Ontario M3H 5T4, Canada
11. Quality Assurance Section, Atmospheric Environment Division, Observations Department, Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan
12. Institute for Global Change Research, Frontier Research System for Global Change, Yokohama 236-0001, Japan
13. Earth System Science, University of California, Irvine, California 92697-3100, USA
14. Divisions of Engineering and Applied Science and Geological and Planetary Sciences, California Institute of Technology, Mail Stop 100-23, Pasadena, California 91125, USA
15. National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa Tsukuba, Ibaraki 305-8569, Japan
16. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA

Correspondence to: A. Scott Denning¹ Correspondence and requests for materials should be addressed to A.S.D. (e-mail: Email: denning@atmos.colostate.edu).

Abstract

Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO₂ concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models^{1, 2, 3, 4, 5, 6, 7, 8, 9}. Here we report estimates of surface–atmosphere CO₂ fluxes from an intercomparison of atmospheric CO₂ inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO₂ in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO₂. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO₂ observation network within the tropics.