FLUXNET and modelling the global carbon cycle ANDREW D. FRIEND *, ALMUT ARNETH w , NANCY Y. KIANG z, MARK LOMAS§, JE ´ RO ˆ ME OGE ´ E } , CHRISTIAN RO ¨ DENBECK k, STEVEN W. RUNNING **, JEAN-DIEGO SANTAREN *, STEPHEN SITCH ww 1 , NICOLAS VIOVY * , F. IAN WOODWARD§ and SO ¨ NKE ZAEHLE ww 2 *Laboratoire des Sciences du Climat et de l’Environnement, CEA – Centre de Saclay, Orme des Merisiers LSCE ba ˆt. 712, Point courrier 132, F-91191 Gif sur Yvette Cedex, France, wDepartment of Physical Geography and Ecosystems Analysis (INES), Centre for GeoBiosphere Science, Lund University, So ¨lvegatan 12, 223, 62 Lund, Sweden, zNASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA, §Department of Animal & Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK, }EPHYSE (Functional Ecology and Environmental Physics), INRA, BP81, F-33883 Villenave d’Ornon Cedex, France, kMax-Planck-Institut fu ¨r Biogeochemie, PO Box 100164, D-07701 Jena, Germany, **School of Forestry, University of Montana, Missoula, MT 59812, USA, wwPotsdam Institut fu ¨r Klimafolgenforschung (PIK) e.V., Telegrafenberg, PO Box 60 12 03, D-144 12 Potsdam, Germany, Abstract Measurements of the net CO 2 flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO 2 source–sink patterns, CO 2 flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/ validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO 2 (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, informa- tion not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model-data comparison. Flux measurements made over four vegetation types were used to calibrate the land- surface scheme of the Goddard Institute for Space Studies global climate model, sig- nificantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land-surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO 2 uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite-measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO 2 measurements. Combining the PEM, DGVM, and inversion results suggests that CO 2 fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO 2 uptake (i.e. 33% of global NEE), as well as its interannual variability. The Correspondence: Andrew D. Friend, fax 1 33 (0)1 69 08 30 73, e-mail: Andrew.Friend@cea.fr 1 Present address: UK Met Office (JCHMR), Maclean Building, Crowmarsh-Gifford, Wallingford OX10 8BB, UK. 2 Present address: Laboratoire des Sciences du Climat et de l’Environnement, CEA – Centre de Saclay, Orme des Merisiers LSCE ba ˆt. 712, Point courrier 132, F-91191 Gif sur Yvette Cedex, France. Global Change Biology (2007) 13, 610–633, doi: 10.1111/j.1365-2486.2006.01223.x r 2007 The Authors 610 Journal compilation r 2007 Blackwell Publishing Ltd