Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration^{1, 2}. But although severe regional heatwaves may become more frequent in a changing climate^{3, 4}, their impact on terrestrial carbon cycling is unclear. Here we report measurements of ecosystem carbon dioxide fluxes, remotely sensed radiation absorbed by plants, and country-level crop yields taken during the European heatwave in 2003. We use a terrestrial biosphere simulation model⁵ to assess continental-scale changes in primary productivity during 2003, and their consequences for the net carbon balance. We estimate a 30 per cent reduction in gross primary productivity over Europe, which resulted in a strong anomalous net source of carbon dioxide (0.5 Pg C yr^-1) to the atmosphere and reversed the effect of four years of net ecosystem carbon sequestration⁶. Our results suggest that productivity reduction in eastern and western Europe can be explained by rainfall deficit and extreme summer heat, respectively. We also find that ecosystem respiration decreased together with gross primary productivity, rather than accelerating with the temperature rise. Model results, corroborated by historical records of crop yields, suggest that such a reduction in Europe's primary productivity is unprecedented during the last century. An increase in future drought events could turn temperate ecosystems into carbon sources, contributing to positive carbon-climate feedbacks already anticipated in the tropics and at high latitudes^{1, 2}.

1. Laboratoire des Sciences du Climat et de l'Environnement, LSCE, F-91191, Gif sur Yvette, France
2. Department of Forest Environment and Resources, DISAFRI, University of Tuscia, I-01100 Vitterbo, Italy
3. Potsdam Institute for Climate Impact Research, Telegrafenberg C4, D-14473 Potsdam, Germany
4. Ecologie et Ecophysiologie Forestières, Centre de Nancy, F-54280 Champenoux, France
5. Functional Ecology and Environmental Physics, Ephyse, INRA, F-33612 Villenave d'Ornon, France
6. Grassland Ecosystem Research, INRA, 234 Avenue du Brézet, Clermont-Ferrand, F-63039, France
7. Institute of Plant Sciences, Universitätsstrasse 2, ETH Zentrum LFW C56, CH-8092 Zuerich, Switzerland
8. Faculté des Sciences Agronomiques, Avenue de la Faculté d'Agronomie 8, B-5030 Gembloux, Belgium
9. Department of Meteorology, Institute of Hydrology and-Meteorology, Technische Universität Dresden, D-01062 Dresden, Germany
10. Fundacion CEAM C/ Charles Darwin, Parque Tecnologico de Paterna, E-46980 Valencia, Spain
11. Department of Physical Sciences, University of Helsinki PO-Box 64, Finland
12. Max-Planck Institute for Biogeochemistry, Postfach 10 01 64, D-07701 Jena, Germany
13. Department of Environmental Science Policy and Management, Ecosystem Science Division, University of California, Berkeley, California 94720, USA
14. Laboratoire de Glaciologie et Géophysique de l'Environnement, 54 rue Molière, F-38402, Saint Martin d'Hères, France
15. Institute for Environment and Sustainability, Joint Research Center European Commission, TP 280, I-21020 Ispra, Italy
16. IBIMET-CNR, Ple delle Cascine, 18, I-50144 Firenze, Italy
17. Dream CEFE-CNRS, 1919 route de Mende, F-34293 Montpellier, France
18. Biosystems Department, Risø National Laboratory, 4000 Roskilde, Denmark
19. †Present addresses: Centro di Ecologia Alpina, Viote del Monte Bondone, I-38040 Trento, Italy (G. Manca); ISAFOM-CNR, Via Cavour 4−6, I-87036, Rende, Italy (G. Matteucci)

Correspondence to: Ph. Ciais¹ Correspondence and requests for materials should be addressed to Ph.C. (Email: philippe.ciais@cea.fr).

Received 30 March 2005; Accepted 23 June 2005

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