title: Net carbon dioxide losses of northern ecosystems in response to autumn warming.
abstract: The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring, with spring and autumn temperatures over northern latitudes having risen by about 1.1 degrees C and 0.8 degrees C, respectively, over the past two decades. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity. 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 future. 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 degrees 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.
authors.affiliation: LSCE, UMR CEA-CNRS, Bâtiment 709, CE, L'Orme des Merisiers, F-91191 Gif-sur-Yvette, France. slpiao@lsce.ipsl.fr
authors.forename: Shilong
authors.initials: S
authors.lastname: Piao
authors.forename: Philippe
authors.initials: P
authors.lastname: Ciais
authors.forename: Pierre
authors.initials: P
authors.lastname: Friedlingstein
authors.forename: Philippe
authors.initials: P
authors.lastname: Peylin
authors.forename: Markus
authors.initials: M
authors.lastname: Reichstein
authors.forename: Sebastiaan
authors.initials: S
authors.lastname: Luyssaert
authors.forename: Hank
authors.initials: H
authors.lastname: Margolis
authors.forename: Jingyun
authors.initials: J
authors.lastname: Fang
authors.forename: Alan
authors.initials: A
authors.lastname: Barr
authors.forename: Anping
authors.initials: A
authors.lastname: Chen
authors.forename: Achim
authors.initials: A
authors.lastname: Grelle
authors.forename: David Y
authors.initials: DY
authors.lastname: Hollinger
authors.forename: Tuomas
authors.initials: T
authors.lastname: Laurila
authors.forename: Anders
authors.initials: A
authors.lastname: Lindroth
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Richardson
authors.forename: Timo
authors.initials: T
authors.lastname: Vesala
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D018533:Biomass; D002245:Carbon Dioxide / Q000032:analysis / Q000378:metabolism*; D019069:Cell Respiration; D017753:Ecosystem; D005579:Fossil Fuels; D005843:Geography; D017752:Greenhouse Effect; D049673:History, 20th Century; D049674:History, 21st Century; D009792:Oceans and Seas; D010788:Photosynthesis; D018526:Plant Transpiration; D010944:Plants / Q000378:metabolism; D011891:Rain; D012621:Seasons; D012987:Soil / Q000032:analysis; D013696:Temperature; D014867:Water / Q000378:metabolism
chemical_list: D005579:Fossil Fuels; D012987:Soil; D014867:Water; D002245:Carbon Dioxide