Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration1,2. But although severe regional heatwaves may become more frequent in a changing climate3,4, 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 latitudes1,2.
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This work is part of the CARBOEUROPE-IP research program funded by the European Union. Eddy-covariance flux towers are also funded by national programmes. M.R. is supported by a European Union Marie-Curie Fellowship. Supercomputing facilities are provided by the French Commissariat à l'Energie Atomique. We thank H. Dolman, M. Heimann and J. Grace for discussions while preparing this manuscript. Author Contributions The first three authors contributed equally to this work: Ph.C. did the analysis, M.R. did the eddy covariance data harmonization and interpretation, and N.V. did the modelling.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
a, Eddy covariance forest site measurements of GPP, TER and NEP from July to September in 2002 and in 2003. b. Same quantities simulated by the biosphere model used to upscale productivity changes over the European continent. c, Measurements of July to September air temperature and annual precipitation changes at each site between 2003 and 2002. (PDF 36 kb)
a, Average crop harvest converted to crop NPP for wheat and maize for selected countries in year 2002 and 2003. b, Comparison between observed and modelled changes in crop NPP during 2003 vs. 1998-2002. c, Country averaged July to September air temperature and annual precipitation changes between 2003 and 1998-2002 over cropland areas. (PDF 20 kb)
a, Observed versus modelled difference of mean daily gross primary production for each site and month. b, Histogram of observed versus modelled difference of mean daily gross primary production, GPP, for all sites and months. (PDF 39 kb)
From top to bottom. Regression of observed and modelled NEE, GPP and TER differences in 2003 vs. 2002 at the eddy-covariance sites as a function of observed air July to September temperature differences (left) and of April to October rainfall differences (right). On the same scale is shown the regression of simulated NEE, GPP and TER changes in 2003 vs. 2002 for all the model grid points over Europe, defined here as the area bounded by 10°W and 37°E in longitude and by 36°N and 69°N in latitude. The eddy covariance observations and the model simulations over Europe illustrate the fact that there is a larger correlation of flux changes with rainfall than with temperature changes. The largest correlation is found for GPP. (PDF 44244 kb)
Tree circumference measurements performed at Hesse for the period 1999-2003 on 11 beech trees from the codominant and dominant crown classes. (PDF 40 kb)
Full text explanations to accompany the above Supplementary Figures. (DOC 42 kb)
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