Are Terrestrial Biosphere Models Fit for Simulating the Land Carbon Sink?

The Global Carbon Project estimates that the terrestrial biosphere has absorbed about one-third of anthropogenic CO2 emissions during the 1959-2019 period. This value is derived from an ensemble of terrestrial biosphere models collectively referred to as the TRENDY ensemble. Given the pivotal role of the biosphere in the global climate system, the purpose of our study is to understand how well TRENDY models reproduce the processes that drive the terrestrial carbon sink. One challenge is to decide what level of agreement between model output and observation-based reference data is adequate, considering that observations are prone to uncertainties. To define such a level of agreement we compute benchmark scores that quantify the similarity between independent reference data sets. Models are then considered to perform well if their model scores reach benchmark scores. Results show that (i) poor model scores can result from observational uncertainties rather than model deficiencies, (ii) model performance is better than expected when taking observational uncertainties into account, and (iii) ample opportunities for improvements remain. The latter includes reducing biases in leaf area index (global), gross primary productivity (boreal North America), latent heat flux (tropics), and net ecosystem exchange (global). Our findings can inform future model development that focuses on reducing the uncertainties of the land carbon sink.