Abstract

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

1. Max Planck Institute für Biogeochemie, Jena, Germany
2. IGBP/GAIM, University of New Hampshire, Morse Hall, Durham, New Hampshire 03824, USA
3. LSCE Unité mixte CEA-CNRS, Bat. 709, CE L’Orme des Merisiers, 91191, Gif sur Yvette France
4. Laboratoire de Biogéochimie Isotopique, Unite mixte CNRS-UPMC-INRA, 4 Place Jussieu, 75252 Paris, France
5. Natural Resources Canada, Canadian Forest Service Northern Forestry Center, 5320 122 Street, Edmonton, Alberta, Canada
6. NCAR, 1850 Table Mesa Drive, Boulder 80303, USA
7. Potsdam Institute for Climate Impact Research, Telegrafenberg C4, 14473 Potsdam
8. GCTE International Project Office, CSIRO Sustainable Ecosystems, PO Box 284, Canberra, ACT 2601, Australia;
9. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523-1371, USA
10. Carnegie Institution of Washington, Department of Plant Biology, 260 Panama Street, Stanford, California 94305, USA
11. Woods Hole Research Center, PO Box 296, Woods Hole, Massachusetts 02543, USA
12. Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA
13. University of New Hampshire, Institute for the Study of Earth, Oceans and Space, Durham, New Hampshire 03824, USA
14. GCTE Impacts Center for Southeast Asia, Jalan Raya Tajur Km 6, POB 116, Bogor, Indonesia
15. Ecosystem Dynamics, RSBS, Australian National University, POB4 Acton, Canberra, ACT 0200, Australia
16. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Hampshire 08544-1003, USA
17. CSIRO Division of Land and Water, GPO Box 1666, Canberra ACT 2601, Australia
18. CSIRO-DAR, PMB #1, 3195 Aspendale, Australia
19. Environmentek, CSIR, PO Box 395, Pretoria 0001, South Africa
20. IGBP Secretariat, Box 50005, S104-05, Stockholm, Sweden
21. Present address: National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, Colorado 80305, USA.

Correspondence to: D. S. Schimel^1,21 Correspondence and requests for materials should be addressed to D.S.S. (email: Email: schimel@ucar.edu).