Southern Ocean Carbon Sink Projected to Reverse at Sub-400 ppm CO2 Threshold

The New Scientist report draws on CMIP6-era models that couple ocean circulation, biology, and carbonate chemistry. These runs impose a declining CO2 trajectory after 2050 and track air–sea fluxes south of 30° S. Results indicate that reduced surface pCO2 from lower atmospheric CO2 outpaces the biological pump’s capacity to maintain a downward gradient, flipping the sign of the flux.

Earlier observational syntheses (e.g., Landschützer et al., 2015) established the Southern Ocean as the dominant anthropogenic carbon sink, yet those products assumed constant or rising atmospheric CO2. The new experiments reveal a non-linear response once the atmospheric driver reverses, an effect missed by linear extrapolations used in most net-zero policy scenarios.

Model spread remains large because of unresolved mesoscale eddies and iron-limitation parameterizations; ensemble members that include stronger eddy compensation show a weaker reversal. Independent constraints from Argo float pCO2 and atmospheric inversions will be required to narrow the uncertainty before the finding can inform emission budgets.

If confirmed, carbon-accounting frameworks will need dynamic sink terms rather than fixed removal credits, altering the timing and scale of negative-emission requirements.