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Biogeosciences Discuss., 10, 20113-20177, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper is under review for the journal Biogeosciences (BG).
Trends and drivers of regional sources and sinks of carbon dioxide over the past two decades
S. Sitch1, P. Friedlingstein1, N. Gruber2, S. D. Jones3, G. Murray-Tortarolo1, A. Ahlström4, S. C. Doney5, H. Graven2, C. Heinze6,7,8, C. Huntingford9, S. Levis10, P. E. Levy11, M. Lomas12, B. Poulter13, N. Viovy13, S. Zaehle14, N. Zeng15, A. Arneth16, G. Bonan10, L. Bopp13, J. G. Canadell17, F. Chevallier13, P. Ciais13, R. Ellis9, M. Gloor18, P. Peylin13, S. Piao19, C. Le Quéré3, B. Smith4, Z. Zhu20,21, and R. Myneni22
1University of Exeter, Exeter EX4 4QF, UK
2Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zürich, Switzerland
3Tyndall Centre for Climate Change Research, University of East Anglia, Norwich NR4 7TJ, UK
4Lund University, Sölvegatan 12, 223 62 Lund, Sweden
5Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA
6Geophysical Institute, University of Bergen, Bergen, Norway
7Bjerknes Centre for Climate Research, Bergen, Norway
8Uni Climate, Uni Research AS, Allégaten 70, 5007 Bergen, Norway
9Centre for Ecology and Hydrology, Benson Lane, Wallingford OX10 8BB, UK
10National Center for Atmospheric Research, Boulder, Colorado, USA
11Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
12Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
13Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, 91191 Gif-sur-Yvette, France
14Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, P.O. Box 10 01 64, 07701 Jena, Germany
15Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20740, USA
16Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany
17Global Carbon Project, CSIRO Marine and Atmospheric Research, Canberra, Australia
18University of Leeds, School of Geography, Woodhouse Lane, LS9 2JT, Leeds, UK
19College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
20State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, 100101, China
21Center for Applications of Spatial Information Technologies in Public Health, Beijing, 100101, China
22Department of Geography and Environment, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA

Abstract. The land and ocean absorb on average over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 "sinks" are modulated by climate change and variability. Here we use a suite of nine Dynamic Global Vegetation Models (DGVMs) and four Ocean Biogeochemical General Circulation Models (OBGCMs) to quantify the global and regional climate and atmospheric CO2 – driven trends in land and oceanic CO2 exchanges with the atmosphere over the period 1990–2009, attribute these trends to underlying processes, and quantify the uncertainty and level of model agreement. The models were forced with reconstructed climate fields and observed global atmospheric CO2; Land Use and Land Cover Changes are not included for the DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon sink of −2.4 ± 0.7 Pg C yr−1 with a small significant trend of −0.06 ± 0.03 Pg C yr−2 (increasing sink). Over the more limited period 1990–2004, the ocean models simulate a mean ocean sink of –2.2 ± 0.2 Pg C yr–1 with a trend in the net C uptake that is indistinguishable from zero (−0.01 ± 0.02 Pg C yr−2). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small trend of −0.02 ± 0.01 Pg C yr−2. Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink are driven by increasing net primary production (NPP) whose statistically significant trend of 0.22 ± 0.08 Pg C yr−2 exceeds a significant trend in heterotrophic respiration of 0.16 ± 0.05 Pg C yr−2 – primarily as a consequence of wide-spread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (−0.04 ± 0.01 Pg C yr−2), with almost no trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counteract the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, and on the influence of land use and land cover changes on regional trends.

Citation: Sitch, S., Friedlingstein, P., Gruber, N., Jones, S. D., Murray-Tortarolo, G., Ahlström, A., Doney, S. C., Graven, H., Heinze, C., Huntingford, C., Levis, S., Levy, P. E., Lomas, M., Poulter, B., Viovy, N., Zaehle, S., Zeng, N., Arneth, A., Bonan, G., Bopp, L., Canadell, J. G., Chevallier, F., Ciais, P., Ellis, R., Gloor, M., Peylin, P., Piao, S., Le Quéré, C., Smith, B., Zhu, Z., and Myneni, R.: Trends and drivers of regional sources and sinks of carbon dioxide over the past two decades, Biogeosciences Discuss., 10, 20113-20177, doi:10.5194/bgd-10-20113-2013, 2013.
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