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Significance We quantified a 36-y trend of advanced spring recovery of carbon uptake across the northern hemisphere boreal evergreen forest zone. From this trend, we estimated the corresponding change in global gross primary production (GPP) and further quantified the magnitude and spatiotemporal variability of spring GPP, that is, the cross-photosynthetic carbon uptake by forest. Our main findings are the following: ( i ) We developed a proxy indicator for spring recovery from in situ flux data on CO 2 exchange and recent satellite snowmelt products and ( ii ) we established a relation between spring recovery and carbon uptake to assess changes in springtime carbon exchange showing a major advance in the CO 2 sink.
Pulliainen et al. (Mon,) studied this question.