Research ArticleOCEANOGRAPHY

Southern Ocean anthropogenic carbon sink constrained by sea surface salinity

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Science Advances  28 Apr 2021:
Vol. 7, no. 18, eabd5964
DOI: 10.1126/sciadv.abd5964
  • Fig. 1 Projections of cumulative Southern Ocean Cant uptake in CMIP6.

    ESM projections of the 21st century Southern Ocean (>30°S) cumulative Cant 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 Cant 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 Cant 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 Cant 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) Cant 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 Cant 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 Cant uptake in CMIP6.

    The projected cumulative Cant 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 Cant 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 Cant as the models (see Materials and Methods).

  • Table 1 Unconstrained (prior) and constrained (after constraint) cumulative Southern Ocean Cant uptake (petagrams of C) in CMIP5 and CMIP6*.

    SimulationsPriorAfter constraint
    CMIP6
      Historical
        1850–200549 ± 555 ± 3
        1850–201458 ± 665 ± 4
      SSP1–2.6
        1850–2100134 ± 13158 ± 6
        2015–210078 ± 1096 ± 4
      SSP2-4.5
        1850–2100173 ± 16200 ± 8
        2015–2100117 ± 12138 ± 6
      SSP5-8.5
        1850–2100245 ± 28278 ± 13
        2015–2100187 ± 22214 ± 11
    CMIP5
      Historical
        1850-200551 ± 553 ± 4
      RCP2.6
        1850–2100138 ± 10142 ± 6
        2006–210086 ± 888 ± 7
      RCP4.5
        1850–2100174 ± 15183 ± 7
        2006–2100122 ± 12129 ± 7
      RCP8.5
        1850–2100241 ± 24253 ± 9
        2006–2100190 ± 21200 ± 10

    *Not all models were available for all future scenarios so that historical and future estimates do not always add up exactly to the estimates of the full period.

    †GFDL-ESM2M and GFDL-ESM2G simulations started in 1861.

    Supplementary Materials

    • Supplementary Materials

      Southern Ocean anthropogenic carbon sink constrained by sea surface salinity

      Jens Terhaar, Thomas L. Frölicher, Fortunat Joos

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