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We determine the annual timing of spring recovery from spaceborne microwave radiometer observations across northern hemisphere boreal evergreen forests for 1979-2014. We find a trend of advanced spring recovery of carbon uptake for this period, with a total average shift of 8.1 d (2.3 d/decade). We use this trend to estimate the corresponding changes in gross primary production (GPP) by applying in situ carbon flux observations. Micrometeorological CO 2 measurements at four sites in northern Europe and North America indicate that such an advance in spring recovery would have increased the January-June GPP sum by 29 g·C·m carbon uptake | earth observation | snowmelt H igh-latitude warming and an associated reduction in spring snow cover are expected to have complex impacts on regional climate patterns and ecological responses (1, 2). The timing of spring recovery of photosynthesis in boreal evergreen forest following snowmelt is one of the major factors affecting the carbon balance across high latitudes (3-5). The integrated effect of anthropogenic impacts on climate causes the total radiative forcing to be positive, which increases the heat balance of the atmosphere. The largest individual cause of warming is the anthropogenic increase in CO 2 concentration (6), which is controlled by the response of the global carbon cycle to anthropogenic CO 2 emissions. Recent studies have documented, with very high confidence, that the world's forests constitute an important carbon sink (4, 6, 7). The annual total boreal forest net carbon sink is estimated to be 0.5 ± 0.1 Pg·C·y −1 for the 2000-2007 period (4). Based on coupled carbon cycle-climate modeling, this sink is estimated to be increasing by 0.014 Pg·C·y −1 for boreal North America including all land areas and by 0.018 Pg·C·y −1 for boreal Asia, respectively (7). However, considerable uncertainty remains regarding the magnitude of this terrestrial sink, particularly how it changes with time due to external climate drivers including the timing of spring snowmelt. It is vital for future climate scenarios to reduce the forest sink uncertainty and obtain information on the spatiotemporal variability and trends. High-latitude warming over the land surface is associated with observed reductions in spring snow extent through earlier snowmelt (2, 8-10). Advanced snowmelt across the boreal zone has a potentially major impact on the carbon balance (11-14). Here, we combine spatially continuous time series of satellite-derived snowmelt data (clearance from the landscape, that is, the time when fractional snow cover reaches zero) with point-wise carbon flux observations to address this open question. This is performed by quantifying the relationship between the observed declines in spring snow cover extent (due to earlier snowmelt) and the carbon balance in springtime. ResultsWe define the dynamics of boreal forest carbon uptake by using the change of snow clearance day (SCD) as a proxy indicator for changes in evergreen boreal forest spring recovery (SR) of photosynthesis, def...

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