Four decades increase in gross photosynthesis of boreal forests balanced out by increase in ecosystem respiration

Abstract 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 projections1. The annual net ecosystem exchange of carbon dioxide (CO2) controlling the terrestrial carbon stock is the result of the small difference between respiratory CO2 release and the photosynthetic CO2 uptake by vegetation2. The boreal forest, and the boreal biome in general, is regarded as a persistent and even an increasing net carbon sink3,4. However, decreases in photosynthetic CO2 uptake and/or concurrent increases in respiratory CO2 release under a changing climate may turn boreal forests from a net sink to a net source of CO21. Here, we assessed the dynamics of boreal forest net CO2 sink-source strength and its components from 1981 to 2018. Our remote sensing approach - trained by net CO2 flux observations at eddy covariance sites across the circumpolar boreal forests - employs satellite-derived retrievals of snow melt timing, landscape freeze-thaw status, and yearly maximum estimates of the normalized difference vegetation index as a proxy for peak vegetation productivity. Our results for the period 1981-2018 suggest that the mean annual evergreen boreal forest CO2 sink was 0.4 Pg C y-1 (0.3 Pg C y-1 for Eurasia and 0.2 Pg C y-1 for North America). In contrast to earlier investigations3,4 our results do not indicate an increasing trend in the circumpolar evergreen boreal forest CO2 sink, as the increase in CO2 uptake is compensated by increasing respiratory releases with both components of CO2 sink-source strength showing considerable interannual variabilities.