To investigate the effect of CO₂ observations in Siberia on the surface CO₂ flux analyses, two experiments using observation data sets with and without Siberian measurements were performed. While the magnitude of the optimized surface CO₂ flux uptake in Siberia decreased, that in the other regions of the Northern Hemisphere increased for the experiment with Siberian observations. It is expected that the Siberian observations play an important role in estimating surface CO₂ flux in the future.