We assessed air-sea CO2 fluxes in the ocean circulation ecosystem model FESOM-REcoM. FESOM is a finite element sea ice-ocean model, with a variable resolution ocean mesh. The mesh used here has a nominal resolution of 150 km in the open ocean and reaches 25 km in the tropics and in the Arctic region. While FESOM-REcoM has previously been used to study biogeochemical cycles and physics-ecosystem interactions, we have now evaluated the air-sea CO2 exchange in a preindustrial control simulation and in a historical simulation with varying climate and increasing atmospheric CO2 concentrations. We evaluate the total annual CO2 uptake and its regional distribution of the historical run and compare modelled pCO2 to observed pCO2 from the SOCAT data-base. The relative interannual variability mismatch and RMSE are similar to that calculated with the same biogeochemical model coupled to the MITgcm ocean circulation model. These numbers and further metrics for model evaluation e.g. natural CO2 fluxes, mismatch time-series, seasonal cycle are set into context by providing the same evaluation for the Global Carbon Budget (GCB) Models. This closes a gap, as these estimates of the ocean carbon sink are used in the community, but their performance has not been documented in detail. We’ll further present methodological updates to the ocean carbon sink estimate in the latest GCB release.