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A model-based constraint of CO2 fertilisation

Author(s): P. B. Holden | N. R. Edwards | D. Gerten | S. Schaphoff

Journal: Biogeosciences Discussions
ISSN 1810-6277

Volume: 9;
Issue: 7;
Start page: 9425;
Date: 2012;
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We derive a constraint on the strength of CO2 fertilisation of the terrestrial biosphere through a "top-down" approach, calibrating Earth System Model parameters constrained only by the post-industrial increase of atmospheric CO2 concentration. We derive a probabilistic prediction for the globally averaged strength of CO2 fertilisation in nature, implicitly net of other limiting factors such as nutrient availability. The approach yields an estimate that is independent of CO2 enrichment experiments and so provides a new constraint that can in principal be combined with data-driven priors. To achieve this, an essential requirement was the incorporation of a Land Use Change (LUC) scheme into the GENIE earth system model, which we describe in full. Using output from a 671-member ensemble of transient GENIE simulations we build an emulator of the change in atmospheric CO2 concentration change over the preindustrial period (1850 to 2000). We use this emulator to sample the 28-dimensional input parameter space. A Bayesian calibration of the emulator output suggests that the increase in Gross Primary Productivity in response of a doubling of CO2 from preindustrial values is likely to lie in the range 11 to 53%, with a most likely value of 28%. The present-day land-atmosphere flux (1990–2000) is estimated at −0.6 GTC yr−1 (likely in the range 0.9 to −2.0 GTC yr−1). The present-day land-ocean flux (1990–2000) is estimated at −2.2 GTC yr−1 (likely in the range −1.6 to −2.8 GTC yr−1). We estimate cumulative net land emissions over the post-industrial period (land use change emissions net of the CO2 fertilisation sink) to be 37 GTC, likely to lie in the range 130 to −20 GTC.
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