  Improvements in the representation of the land carbon cycle in Earth system
models participating in the Coupled Model Intercomparison Project Phase 6
(CMIP6) include interactive treatment of both the carbon and nitrogen cycles,
improved photosynthesis, and soil hydrology. To assess the impact of these
model developments on aspects of the global carbon cycle, the Earth System
Model Evaluation Tool is expanded to compare CO2 concentration and
emission-driven historical simulations from CMIP5 and CMIP6 to observational
data sets. Overestimations of photosynthesis (GPP) in CMIP5 were largely
resolved in CMIP6 for participating models with an interactive nitrogen cycle,
but remaining for models without one. This points to the importance of
including nutrient limitation. Simulating the leaf area index (LAI) remains
challenging with a large model spread in both CMIP5 and CMIP6. In ESMs, global
mean land carbon uptake (NBP) is well reproduced in the CMIP5 and CMIP6
multi-model means. However, this is the result of an underestimation of NBP in
the northern hemisphere, which is compensated by an overestimation in the
southern hemisphere and the tropics. Overall, a slight improvement in the
simulation of land carbon cycle parameters is found in CMIP6 compared to CMIP5,
but with many biases remaining, further improvements of models in particular
for LAI and NBP is required. Emission-driven simulations perform just as well
as concentration driven models despite the added process-realism. Due to this
we recommend ESMs in future CMIP phases to perform emission-driven simulations
as the standard so that climate-carbon cycle feedbacks are fully active. The
inclusion of nitrogen limitation led to a large improvement in photosynthesis
compared to models not including this process, suggesting the need to view the
nitrogen cycle as a necessary part of all future carbon cycle models.
