The uncertainty of ocean carbon uptake in ESMs is projected to grow 2-fold by the end of the 21st century. We found that models that take up anomalously low (high) CO2
in the Southern Ocean (SO) today project low (high) cumulative CO2
uptake in the 21st century; thus the SO can be used to constrain future global uptake uncertainty. Inter-model spread in the SO carbon sink arises from variations in the p
seasonality, specifically bias in the simulated timing and amplitude of NPP and SST.