Enhanced ocean CO uptake due to near-surface temperature gradients.

The ocean annually absorbs about a quarter of all anthropogenic carbon dioxide (CO) emissions. Global estimates of air-sea CO fluxes are typically based on bulk measurements of CO 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 CO fluxes, because the air-sea CO 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 CO 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 d to 0.08 mmol m d ( = 148). This implies an increase in the Atlantic CO sink of 0.03 PgC yr (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 CO uptake. Accounting for this increased ocean uptake will probably require some revision to how global carbon budgets are quantified.