1. LSCE, UMR CEA-CNRS, Bâtiment 709, CE, L'Orme des Merisiers, F-91191 Gif-sur-Yvette, France
2. Laboratoire de Biogéochimie Isotopique, LBI, Bâtiment EGER, F-78026 Thiverval-Grignon, France
3. Max Planck Institute for Biogeochemistry, PO Box 100164, 07701 Jena, Germany
4. Department of Biology, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium
5. Faculté de foresterie et de géomatique, Université Laval, Sainte-Foy, Quebec G1K 7P4, Canada
6. Department of Ecology, Peking University, Beijing 100871, China
7. Climate Research Division, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada
8. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA
9. Department of Ecology, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
10. USDA Forest Service Northern Research Station, 271 Mast Road, Durham, New Hampshire 03824, USA
11. Finnish Meteorological Institute, FIN-00101 Helsinki, Finland
12. Department of Physical Geography and Ecosystems Analysis, Lund University, SE-22362 Lund, Sweden
13. Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA
14. Department of Physical Sciences, University of Helsinki, PO Box 64, FIN-00014 Helsinki, Finland

Correspondence to: Shilong Piao¹Philippe Ciais¹ Correspondence and requests for materials should be addressed to S.L.P. (Email: slpiao@lsce.ipsl.fr) or P.C. (Email: philippe.ciais@lsce.ipsl.fr).

Abstract

The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring^{1, 2, 3, 4}, with spring and autumn temperatures over northern latitudes having risen by about 1.1 °C and 0.8 °C, respectively, over the past two decades⁵. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity^{6, 7}. These observations have led to speculation that spring and autumn warming could enhance carbon sequestration and extend the period of net carbon uptake in the future⁸. Here we analyse interannual variations in atmospheric carbon dioxide concentration data and ecosystem carbon dioxide fluxes. We find that atmospheric records from the past 20 years show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend cannot be explained by changes in atmospheric transport alone and, together with the ecosystem flux data, suggest increasing carbon losses in autumn. We use a process-based terrestrial biosphere model and satellite vegetation greenness index observations to investigate further the observed seasonal response of northern ecosystems to autumnal warming. We find that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. Our simulations and observations indicate that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC °C^-1, offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously suggested^{9, 10}.

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