Enhanced ocean CO₂ uptake due to near-surface temperature gradients

The ocean annually absorbs about a quarter of all anthropogenic carbon dioxide (CO₂) emissions. Global estimates of air-sea CO₂ fluxes are typically based on bulk measurements of CO₂ in air and seawater and neglect the effects of vertical temperature gradients near the ocean surface. Theoretical and laboratory observations indicate that these gradients alter air-sea CO₂ fluxes, because the air-sea CO₂ concentration difference is highly temperature sensitive. However, in situ field evidence supporting their effect is so far lacking. Here we present independent direct air-sea CO₂ fluxes alongside indirect bulk fluxes collected along repeat transects in the Atlantic Ocean (50° N to 50° S) in 2018 and 2019. We find that accounting for vertical temperature gradients reduces the difference between direct and indirect fluxes from 0.19 mmol m^-2 d^-1 to 0.08 mmol m^-2 d^-1 (N = 148). This implies an increase in the Atlantic CO₂ sink of ~0.03 PgC yr^-1 (~7% of the Atlantic Ocean sink). These field results validate theoretical, modelling and observational-based efforts, all of which predicted that accounting for near-surface temperature gradients would increase estimates of global ocean CO₂ uptake. Accounting for this increased ocean uptake will probably require some revision to how global carbon budgets are quantified.