Jens Terhaar

We assess the Southern Ocean CO 2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO 2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr −1 ) and p CO 2 ‐observation‐based products (0.73 ± 0.07 PgC yr −1 ). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present‐day net uptake is to first order a response to rising atmospheric CO 2 , driving large amounts of anthropogenic CO 2 (C ant ) into the ocean, thereby overcompensating the loss of natural CO 2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with p CO 2 ‐products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr −1 decade −1 , respectively). This is despite nearly identical p CO 2 trends in GOBMs and p CO 2 ‐products when both products are compared only at the locations where p CO 2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non‐thermal processes of biology and mixing/circulation. Ocean interior accumulation of C ant points to an underestimate of C ant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle.