Southern Ocean anthropogenic carbon sink constrained by sea surface salinity

Abstract

The ocean attenuates global warming by taking up about one quarter of global anthropogenic carbon emissions. Around 40% of this carbon sink is located in the Southern Ocean. However, Earth system models struggle to reproduce the Southern Ocean circulation and carbon fluxes. We identify a tight relationship across two multimodel ensembles between present-day sea surface salinity in the subtropical-polar frontal zone and the anthropogenic carbon sink in the Southern Ocean. Observations and model results constrain the cumulative Southern Ocean sink over 1850-2100 to 158 ± 6 petagrams of carbon under the low-emissions scenario Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6) and to 279 ± 14 petagrams of carbon under the high-emissions scenario SSP5-8.5. The constrained anthropogenic carbon sink is 14 to 18% larger and 46 to 54% less uncertain than estimated by the unconstrained estimates. The identified constraint demonstrates the importance of the freshwater cycle for the Southern Ocean circulation and carbon cycle.

Fig. 1. Projections of cumulative Southern Ocean C_ant uptake in CMIP6.

ESM projections of the 21st century Southern Ocean (>30°S) cumulative C_ant uptake since 1850 from (A) 11 CMIP6 models following the SSP5-8.5 scenario (21) with the data-based estimate for cumulative Southern Ocean uptake from 1850 to 2005 (black vertical line) (5, 7, 8). (B) Time series of the multimodel mean C_ant uptake under the SSP1-2.6 (blue) and SSP5-8.5 (red) scenarios with ±1 SD for the CMIP6 model ensembles before (transparent) and after the emergent constraint is applied (opaque). The bars indicate the range of ±1 SD of the cumulative C_ant uptake in 2100 under SSP1-2.6 (blue), SSP2-4.5 (yellow), and SSP5-8.5 (red) before (transparent) and after (opaque) the constraint is applied.

Fig. 2. Sea surface salinity and cumulative C_ant uptake in the Southern Ocean.

(A) Present-day sea surface salinity in August from World Ocean Atlas 2018 (80) and the sea surface salinity in August averaged over 1986–2005 simulated by the (B) CESM2 and (C) GFDL-CM4 models. Both models are part of CMIP6. Black and white contour lines delineate the PF and the STF in August. Simulated cumulative Southern Ocean (>30°S) C_ant uptake over 1850–2100 for the (D) CESM2 and (E) GFDL-CM4 models. CESM2 is the minimum of the CMIP6 ensemble for both present-day (1986–2005) mean sea surface salinity between the PF and STF (33.67) and projected cumulative C_ant uptake in 2100 (204 Pg of C), while GFDL-CM4 is the ensemble maximum (34.16 and 309 Pg of C). The observation-based mean sea surface salinity between the PF and STF is 34.07 [World Ocean Atlas 2018 (80)].

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).