Increase in gross primary production of boreal forests balanced out by increase in ecosystem respiration

Changes in the net carbon sink of boreal forests constitute a major source of uncertainty in the future global carbon budget and, hence, climate change projections. The annual net ecosystem exchange of carbon dioxide (CO 2 ) controlling the terrestrial carbon stock results from the small difference between respiratory CO 2 release and the photosynthetic CO 2 uptake by vegetation. The boreal forest, and the boreal biome in general, is regarded as a persistent and even increasing net carbon sink. However, decreases in photosynthetic CO 2 uptake and/or concurrent increases in respiratory CO 2 release under a changing climate may turn boreal forests from a net sink to a net source of CO 2 . Here, we assessed the interannual variability of the boreal forest net CO 2 sink-source strength and its two component fluxes from 1981 to 2018. Our remote sensing approach - trained by net CO 2 flux observations at eddy covariance sites across the circumpolar boreal forests - employs satellite-derived retrievals of snowmelt timing, landscape freeze-thaw status, and yearly maximum estimates of the normalized difference vegetation index as a proxy for peak vegetation productivity. Our results suggest that for the period 2000–2018, the mean annual evergreen boreal forest CO 2 photosynthetic uptake (gross primary productivity) was 2.8 ± 0.2 Pg C y −1 ( 1.6 ± 0.1 Pg C y −1 for Eurasia and 1.2 ± 0.1 Pg C y −1 for North America). In contrast to earlier studies results obtained here do not indicate a clear increasing trend in the circumpolar evergreen boreal forest CO 2 sink. The increase in photosynthetic CO 2 uptake is compensated by increasing respiratory releases with both component fluxes showing considerable interannual variabilities. • New approach to estimate the interannual dynamics carbon exchange. • Using CO 2 flux observations and satellite data on cryosphere. • Estimates on hemispheric net ecosystem CO 2 uptake and respiration for 1981–2018. • Producing estimates independent of terrestrial biosphere model predictions. • Showing that increases in CO 2 uptake compensated by increased respiratory releases.