Fig. 3. Emergent constraints on the cumulative Southern Ocean C_ant uptake in CMIP6.

The projected cumulative C_ant uptake in the Southern Ocean south of 30°S across the CMIP6 model ensemble for (A) the historical period from 1850 to 2005, and the historical and future period from 1850 to 2100 under (C) SSP1-2.6, (E) SSP2-4.5, and (G) SSP5-8.5 against the present-day (1986–2005) mean sea surface salinity between the PF and STF. Linear regression fits (red dashed lines) and the associated 68% prediction intervals are shown in (A), (C), (E), and (G), as are observation-based estimates of present-day annual sea surface density between the PF and STF (black dashed lines) with the associated uncertainty (black shaded area). Probability density functions for the cumulative Southern Ocean C_ant uptake from (B) 1850 to 2005 and from 1850 to 2100 under (D) SSP1-2.6, (F) SSP2-4.5, and (H) SSP5-8.5, before (“CMIP6 prior,” transparent) and after (“after constraint,” opaque) the emergent constraint is applied. Data-based estimates are shown in (B) as dash-dotted (5, 7) and dotted (8) lines and are scaled to the same definition of C_ant as the models (see Materials and Methods).