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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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).

Additional information

Journal Peer Review information: Nature Climate Change thanks Nancy Williams and other anonymous reviewer(s) for their contribution to the peer review of this work.

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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.

Author information

Author notes

    • Gabriela Negrete-García

    Present address: Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, CA, USA

    • Kristen M. Krumhardt

    Present address: Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA


  1. Department of Atmospheric and Oceanic Sciences and Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, USA

    • Gabriela Negrete-García
    •  & Nicole S. Lovenduski
  2. International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, USA

    • Claudine Hauri
  3. Environmental Studies Program and Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, USA

    • Kristen M. Krumhardt
  4. NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway

    • Siv K. Lauvset
  5. Geophysical Institute, University of Bergen and Bjerknes Centre for Climate Research, Bergen, Norway

    • Siv K. Lauvset


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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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The authors declare no competing interests.

Corresponding author

Correspondence to Nicole S. Lovenduski.

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