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Sudden emergence of a shallow aragonite saturation horizon in the Southern Ocean

Abstract

Models project that with current CO2 emission rates, the Southern Ocean surface will be undersaturated with respect to aragonite by the end of this century1,2,3,4. This will result in widespread impacts on biogeochemistry and ocean ecosystems5,6,7, particularly the health of aragonitic organisms, such as pteropods7, which can dominate polar surface water communities6. Here, we quantify the depth of the present-day Southern Ocean aragonite saturation horizon using hydrographic and ocean carbon chemistry observations, and use a large ensemble of simulations from the Community Earth System Model (CESM)8,9 to track its evolution. A new, shallow aragonite saturation horizon emerges in many Southern Ocean locations between now and the end of the century. While all ensemble members capture the emergence, internal climate variability may affect the year of emergence; thus, its detection may have been overlooked by ensemble average analysis in the past. The emergence of the new horizon is driven by the slow accumulation of anthropogenic CO2 in the Southern Ocean thermocline, where the carbonate ion concentration exhibits a local minimum and approaches undersaturation. The new horizon is also apparent under an emission-stabilizing scenario indicating an inevitable, sudden decrease in the volume of suitable habitat for aragonitic organisms.

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Fig. 1: Depth of aragonite saturation horizon.
Fig. 2: Emergence of shallow aragonite saturation horizon.
Fig. 3: Drivers of the sudden emergence of the shallow horizon.
Fig. 4: Year of emergence of shallow saturation horizon.

Data availability

CESM ensemble output is available from the Earth System Grid (http://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html). GLODAPv2 data can be accessed from the project webpage (https://www.glodap.info/). WOA data are provided by the National Oceanographic Data Center (https://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html).

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Acknowledgements

N.S.L., G.N.-G. and K.M.K. are grateful for support from the National Science Foundation (grant nos. OCE-1558225 and PLR-1543457). G.N.-G. was supported, in part, by the Significant Opportunities in Atmospheric Research and Science (SOARS) programme, NSF grant no. AGS-1641177. C.H. acknowledges support from the National Science Foundation (grant no. OCE-1459834). CESM computing resources were provided by CISL at NCAR. We thank B. Medeiros for providing model re-gridding scripts and N. Freeman for helpful comments on an earlier version of the manuscript.

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N.S.L. and K.M.K. re-gridded the CESM-LE and CESM-ME DIC output to the GLODAP/WOA grid, corrected the model DIC bias and calculated the aragonite saturation state from the bias-corrected model DIC projections. S.K.L. provided the modified GLODAPv2 mapped climatologies and expertise. G.N.-G. analysed the bias-corrected projections and wrote the manuscript. All authors were involved in the study design, discussed the results and helped write the manuscript.

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Correspondence to Nicole S. Lovenduski.

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Negrete-García, G., Lovenduski, N.S., Hauri, C. et al. Sudden emergence of a shallow aragonite saturation horizon in the Southern Ocean. Nat. Clim. Chang. 9, 313–317 (2019). https://doi.org/10.1038/s41558-019-0418-8

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