A permafrost-enhanced biogeochemical process model using observed climate and CO2 data, and satellite-observed maps of forest composition and density predicts a moderate biomass increment and carbon sink of 74 and 131 Tg carbon per year (TgC/a) in the Russian forests during 1981--1999. The enhanced process model realistically represents ecosystem state in terms of river runoff, area burned by fires, vegetation productivity and biomass, in comparison to monitoring and inventory data. Rising atmospheric CO2 content is found to have been the main cause of the carbon sink. Amounting to 7% of carbon emissions from fossil fuel emissions in Eurasia, our results demonstrate a limited capability of the Russian boreal forest in its current state to compensate anthropogenic carbon emissions.