Increasing global net land CO 2 sink since the 1980s is mainly driven by the fertilization effect from rising atmospheric CO 2 concentrations (Schimel et al., 2015; Sitch et al., 2015; Fernández-Martínez et al., 2019; O’Sullivan et al., 2019; Tagesson et al., 2020; Walker et al., 2021)