evidence: The CO2 Human Emission (CHE) project has produced a global CO2, CH4 and CO nature run simulation at 9km resolution for 2015 based on the Copernicus Atmosphere Monitoring Service (CAMS) forecasting system which is part of the Integrated Forecasting System (IFS) at ECMWF. The aim of the CHE nature run is to produce realistic atmospheric tracer variability and to provide a reference for Observing System Simulation Experiments. The simulations include tagged tracers that estimate the contribution of the biogenic and anthropogenic fluxes in the simulated atmospheric variability. We find that the largest source of errors in the atmospheric CO2 variability comes from the online biogenic fluxes in the land surface model (CTESSEL). However, with the evaluation of atmospheric CO2 it is difficult to separate errors coming from transport and surface fluxes. The ICOS ecosystem data will be used to evaluate the modelled biogenic flux uncertainty at different temporal scales from sub-diurnal to synoptic to seasonal. The modelled fluxes will also be compared to other flux products, such as FLUXCOM and the CAMS CO2 inversion product, in order to assess the errors in the flux gradients and the large-scale budgets of the biogenic fluxes.