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Earth system models (ESMs) explicitly simulate the interactions between thephysical climate system components and biogeochemical cycles. Physical andbiogeochemical aspects of ESMs are routinely compared against theirobservation-based counterparts to assess model performance and to evaluatehow this performance is affected by ongoing model development. Here, weassess the performance of version 4.2 of the Canadian Earth system modelagainst four land carbon-cycle-focused, observation-based determinants ofthe global carbon cycle and the historical global carbon budget over the1850–2005 period. Our objective is to constrain the strength of theterrestrial CO2 fertilization effect, which is known to be the mostuncertain of all carbon-cycle feedbacks. The observation-based determinantsinclude (1) globally averaged atmospheric CO2 concentration,(2) cumulative atmosphere–land CO2 flux, (3) atmosphere–land CO2flux for the decades of 1960s, 1970s, 1980s, 1990s, and 2000s, and (4) theamplitude of the globally averaged annual CO2 cycle and its increaseover the 1980 to 2005 period. The optimal simulation that satisfiesconstraints imposed by the first three determinants yields a net primary productivity (NPP) increase from  ∼  58 Pg C year−1 in 1850 toabout  ∼  74 Pg C year−1 in 2005; an increase of  ∼  27 %over the 1850–2005 period. The simulated loss in the global soil carbonamount due to anthropogenic land use change (LUC) over the historical period isalso broadly consistent with empirical estimates. Yet, it remains possiblethat these determinants of the global carbon cycle are insufficient toadequately constrain the historical carbon budget, and consequently thestrength of terrestrial CO2 fertilization effect as it is represented inthe model, given the large uncertainty associated with LUC emissions over thehistorical period

geoscientific model development

Constraining the strength of the terrestrial CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fertilization effect in the Canadian Earth system model version 4.2 (CanESM4.2)