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Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models

Authors

  • JAMES T. RANDERSON,

    1. Department of Earth System Science, Croul Hall, University of California, Irvine, CA 92697, USA,
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  • FORREST M. HOFFMAN,

    1. Oak Ridge National Laboratory, Computational Earth Sciences Group, PO Box 2008, Oak Ridge, TN 37831, USA,
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  • PETER E. THORNTON,

    1. Climate and Global Dynamics, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA,
    2. Oak Ridge National Laboratory, Environmental Sciences Division, PO Box 2008, Oak Ridge, TN 37831, USA,
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  • NATALIE M. MAHOWALD,

    1. Department of Earth and Atmospheric Sciences, 2140 Snee Hall, Cornell University, Ithaca, NY 14850, USA,
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  • KEITH LINDSAY,

    1. Climate and Global Dynamics, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA,
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  • YEN-HUEI LEE,

    1. Climate and Global Dynamics, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA,
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  • CYNTHIA D. NEVISON,

    1. Department of Earth System Science, Croul Hall, University of California, Irvine, CA 92697, USA,
    2. Institute for Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA,
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  • SCOTT C. DONEY,

    1. Department of Marine Chemistry and Geochemistry, MS 25, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA,
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  • GORDON BONAN,

    1. Climate and Global Dynamics, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA,
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  • RETO STÖCKLI,

    1. Department of Atmospheric Sciences, Colorado State University, Ft Collins, CO 80523, USA,
    2. MeteoSwiss, Climate Service, Federal Office of Meteorology and Climatology, CH-8044 Zurich, Switzerland,
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  • CURTIS COVEY,

    1. Program for Climate Model Diagnosis and Intercomparison, 7000 East Avenue, Bldg. 170, L-103, Livermore, CA 94550-9234, USA,
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  • STEVEN W. RUNNING,

    1. Numerical Terradynamic Simulation Group, College of Forestry & Conservation, University of Montana, Missoula, MT 59812, USA,
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  • INEZ Y. FUNG

    1. Department of Earth and Planetary Science and Department of Environmental Science, Policy, and Management, 307 McCone, Mail Code 4767, University of California, Berkeley, CA 94720, USA
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Jim Randerson, tel. +949 824 9030, fax +949 824 3874, e-mail: jranders@uci.edu

Abstract

With representation of the global carbon cycle becoming increasingly complex in climate models, it is important to develop ways to quantitatively evaluate model performance against in situ and remote sensing observations. Here we present a systematic framework, the Carbon-LAnd Model Intercomparison Project (C-LAMP), for assessing terrestrial biogeochemistry models coupled to climate models using observations that span a wide range of temporal and spatial scales. As an example of the value of such comparisons, we used this framework to evaluate two biogeochemistry models that are integrated within the Community Climate System Model (CCSM) – Carnegie-Ames-Stanford Approach′ (CASA′) and carbon–nitrogen (CN). Both models underestimated the magnitude of net carbon uptake during the growing season in temperate and boreal forest ecosystems, based on comparison with atmospheric CO2 measurements and eddy covariance measurements of net ecosystem exchange. Comparison with MODerate Resolution Imaging Spectroradiometer (MODIS) measurements show that this low bias in model fluxes was caused, at least in part, by 1–3 month delays in the timing of maximum leaf area. In the tropics, the models overestimated carbon storage in woody biomass based on comparison with datasets from the Amazon. Reducing this model bias will probably weaken the sensitivity of terrestrial carbon fluxes to both atmospheric CO2 and climate. Global carbon sinks during the 1990s differed by a factor of two (2.4 Pg C yr−1 for CASA′ vs. 1.2 Pg C yr−1 for CN), with fluxes from both models compatible with the atmospheric budget given uncertainties in other terms. The models captured some of the timing of interannual global terrestrial carbon exchange during 1988–2004 based on comparison with atmospheric inversion results from TRANSCOM (r=0.66 for CASA′ and r=0.73 for CN). Adding (CASA′) or improving (CN) the representation of deforestation fires may further increase agreement with the atmospheric record. Information from C-LAMP has enhanced model performance within CCSM and serves as a benchmark for future development. We propose that an open source, community-wide platform for model-data intercomparison is needed to speed model development and to strengthen ties between modeling and measurement communities. Important next steps include the design and analysis of land use change simulations (in both uncoupled and coupled modes), and the entrainment of additional ecological and earth system observations. Model results from C-LAMP are publicly available on the Earth System Grid.

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