Natural air-sea flux of CO2 in simulations of the NASA-GISS climate model: Sensitivity to the physical ocean model formulation

► Uptake uncertainty due to mainly kinematic and chemistry model differences. ► Layer model underestimates the biological pump magnitude due to reduced mixing at the base of thermocline. ► The Southern Ocean is a key region where both models diverge most from observations. ► Indirect ocean-atmosphere feedbacks are less important in determining the CO2 sink sensitivity.