Quantifying and reducing uncertainty in global carbon cycle predictions: Lessons and perspectives from 15 years of data assimilation studies with the ORCHIDEE terrestrial biosphere model

Predicting terrestrial carbon, C, budgets and carbon-climate feedbacks strongly relies on our ability to accurately model interactions between vegetation, C and water cycles, and the atmosphere. However, C fluxes simulated by global, process-based terrestrial biosphere models (TBMs) remain subject to large uncertainties, partly due to unknown or poorly calibrated parameters. In this review, we present 15 years’ development of a C cycle DA system for optimizing C cycle parameters of the ORCHIDEE TBM. We analyze the impact of assimilating multiple different C cycle related datasets on regional to global-scale gross and net CO2 fluxes. We find that assimilating atmospheric CO2 data is crucial for improving (increasing) ORCHIDEE predictions of the terrestrial land C sink, predominantly due to the global-scale constraint these data provide for optimizing initial soil C stocks. When comparing the data-constrained ORCHIDEE land C sink estimates to the CAMS atmospheric inversion, we show that while the two approaches agree on the global C sink magnitude, they continue to differ in partitioning the global C sink between the northern hemisphere and tropics. We also discuss technical challenges faced in our C cycle DA studies, in particular the difficulty in characterizing the observation error covariance matrix. We offer our perspectives on how to tackle these challenges that we hope can serve as a roadmap for other TBM groups wishing to develop C cycle DA systems.

MACBEAN Natasha;  BACOUR Cedric;  RAOULT Nina;  BASTRIKOV Vladislav;  KOFFI N'Dri;  KUPPEL Sylvain;  MAIGNAN Fabienne;  OTTLÉ Catherine;  PEAUCELLE Marc;  SANTAREN Diego;  PEYLIN Philippe; 

2022-07-20

AMER GEOPHYSICAL UNION

JRC126347

0886-6236 (online),   

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2021GB007177,    https://publications.jrc.ec.europa.eu/repository/handle/JRC126347,   

10.1029/2021GB007177 (online),