Revised budget for the oceanic uptake of anthropogenic carbon dioxide

Tracer-calibrated models of the total uptake of anthropogenic CO₂ by the world's oceans give estimates of about 2 gigatonnes carbon per year¹, significantly larger than a recent estimate² of 0.3-0.8 Gt C yr^-1 for the synoptic air-to-sea CO₂ influx. Although both estimates require that the global CO₂ budget must be balanced by a large unknown terrestrial sink, the latter estimate implies a much larger terrestrial sink, and challenges the ocean model calculations on which previous CO₂ budgets were based. The discrepancy is due in part to the net flux of carbon to the ocean by rivers and rain, which must be added to the synoptic air-to-sea CO₂ flux to obtain the total oceanic uptake of anthropogenic CO₂. Here we estimate the magnitude of this correction and of several other recently proposed adjustments to the synoptic air-sea CO₂ exchange. These combined adjustments minimize the apparent inconsistency, and restore estimates of the terrestrial sink to values implied by the modelled oceanic uptake.