Pease cite this work:
Zhang, X. Z., Shushi Peng, Philippe Ciais, Ying-Ping Wang, Jeremy D. Silver, Shilong Piao, and Peter J. Rayner. 2019: Greenhouse gas concentration and volcanic eruptions controlled the variability of terrestrial carbon uptake over the last millennium. Journal of Advances in Modeling Earth Systems, in press, https://doi.org/10.1029/2018MS001566.