Uncertainty in land carbon budget simulated by terrestrial biosphere models

Lucas Hardouin (1) , Christine Delire (1) , Bertrand Decharme (1, 2) , David M Lawrence (3) , Julia E M S Nabel (4) , Victor Brovkin (4) , Nathan Collier , Rosie Fisher (3) , Forrest M Hoffman (5) , Charles D Koven (6) , Roland Séférian (1) , Tobias Stacke (7)

1 CNRM - Centre national de recherches météorologiques
2 UT - Université de Toulouse
3 NCAR - National Center for Atmospheric Research [Boulder]
4 MPI-M - Max Planck Institute for Meteorology
5 CCSI - Climate Change Science Institute [Oak Ridge]
6 LBNL - Lawrence Berkeley National Laboratory [Berkeley]
7 GKSS - Helmholtz-Zentrum Geesthacht

Abstract Global estimates of the land carbon sink are often based on simulations by terrestrial biosphere models. The use of a large number of models that differ in their underlying hypotheses, structure and parameters is one way to assess the uncertainty in the historical land carbon sink. Here we show that the atmospheric forcing datasets used to drive these terrestrial biosphere models represent a significant source of uncertainty that is currently not systematically accounted for in land carbon cycle evaluations. We present results from three terrestrial biosphere models each forced with three different historical atmospheric forcing reconstructions over the period 1850-2015. We perform an analysis of variance to quantify the relative uncertainty in carbon fluxes arising from the models themselves, atmospheric forcing, and model-forcing interactions. We find that atmospheric forcing in this set of simulations plays a dominant role on uncertainties in global gross primary productivity (75% of variability) and autotrophic respiration (90%), and a significant but reduced role on net primary productivity and heterotrophic respiration (30%). Atmospheric forcing is the dominant driver (52%) of variability for the net ecosystem flux, defined as the difference between gross primary productivity and respiration (both autotrophic and heterotrophic respiration). In contrast, for wildfire-driven carbon emissions model uncertainties dominate and, as a result, model uncertainties dominate for net ecosystem exchange. At regional scales, the contribution of atmospheric forcing to uncertainty shows a very heterogeneous pattern and is smaller on average than at the global scale. We find that this difference in the relative importance of forcing uncertainty between global and regional scales is related to large differences in regional model flux estimates, which partially offset each other when integrated globally, while the flux differences driven by forcing are mainly consistent across world and therefore which add up to a larger fractional contribution to global uncertainty.

Mots clés: uncertainty; land carbon sink; terrestrial biosphere model; atmospheric forcing dataset

Domaines: Sciences de l'environnement

Lucas Hardouin, Christine Delire, Bertrand Decharme, David M Lawrence, Julia E M S Nabel, et al.. Uncertainty in land carbon budget simulated by terrestrial biosphere models: the role of atmospheric forcing. Environmental Research Letters, 2022, ⟨10.1088/1748-9326/ac888d⟩. ⟨hal-03749645v2⟩