LAUSR

The increasing strength of land CO2 uptake in the 2000s has been attributed to a stimulating effect of rising atmospheric CO2 on photosynthesis (CO2 fertilization). Using terrestrial biosphere models, we show that enhanced CO2 uptake is induced not only by CO2 fertilization but also an increasing uptake by plant regrowth (accounting for 0.33 ± 0.10 Pg C/year increase of CO2 uptake in the 2000s compared with the 1960s–1990s) with its effect most pronounced in eastern North America, southern-eastern Europe, and southeastern temperate Eurasia. Our analysis indicates that ecosystems in North America and Europe have established the current productive state through regrowth since the 1960s, and those in temperate Eurasia are still in a stage from regrowth following active afforestation in the 1980s–1990s. As the strength of model representation of CO2 fertilization is still in debate, plant regrowth might have a greater potential to sequester carbon than indicated by this study. Plain Language Summary The recent enhancement of CO2 uptake by the terrestrial biosphere is slowing down an acceleration of the atmospheric CO2 increase. A stimulating effect of rising atmospheric CO2 on plant photosynthesis (CO2 fertilization) provides the most pronounced impact on the enhanced CO2 uptake. However, the question remains on how much of the enhanced uptake CO2 fertilization accounts for and a possible contribution from past land use change. Here using multiple terrestrial biosphere models, we demonstrate that despite a large contribution from CO2 fertilization, the enhanced CO2 uptake in the 2000s cannot be fully explained without an increasing uptake by land use change, in particular, plant regrowth. The regrowth effect is most pronounced in North America, Europe, and temperate Eurasia, and they account for 94% of the global total CO2 uptake enhancement by plant regrowth. The strengthening trends in both CO2 fertilization and plant regrowth suggest that the deceleration of the atmospheric CO2 increase continues in the future.