CO 2 fertilization contributed more than half of the observed forest biomass increase in northern extra-tropical land.

The existence of a large-biomass carbon (C) sink in Northern Hemisphere extra-tropical ecosystems (NHee) is well-established, but the relative contribution of different potential drivers remains highly uncertain. Here we isolated the historical role of carbon dioxide (CO 2 ) fertilization by integrating estimates from 24 CO 2 -enrichment experiments, an ensemble of 10 dynamic global vegetation models (DGVMs) and two observation-based biomass datasets. Application of the emergent constraint technique revealed that DGVMs underestimated the historical response of plant biomass to increasing [CO 2 ] in forests ( β Forest Mod $$ {\beta}_{\mathrm{Forest}}^{\mathrm{Mod}} $$ ) but overestimated the response in grasslands ( β Grass Mod $$ {\beta}_{\mathrm{Grass}}^{\mathrm{Mod}} $$ ) since the 1850s. Combining the constrained β Forest Mod $$ {\beta}_{\mathrm{Forest}}^{\mathrm{Mod}} $$ (0.86 ± 0.28 kg C m -2  [100 ppm] -1 ) with observed forest biomass changes derived from inventories and satellites, we identified that CO 2 fertilization alone accounted for more than half (54 ± 18% and 64 ± 21%, respectively) of the increase in biomass C storage since the 1990s. Our results indicate that CO 2 fertilization dominated the forest biomass C sink over the past decades, and provide an essential step toward better understanding the key role of forests in land-based policies for mitigating climate change.