CO₂ 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 () but overestimated the response in grasslands () since the 1850s. Combining the constrained (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.