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 (CO2) fertilization by integrating estimates from 24 CO2‐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 [CO2] in forests (βForestMod) but overestimated the response in grasslands (βGrassMod) since the 1850s. Combining the constrained βForestMod (0.86 ± 0.28 kg C m−2 [100 ppm]−1) with observed forest biomass changes derived from inventories and satellites, we identified that CO2 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 CO2 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. [ABSTRACT FROM AUTHOR]

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