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Global cooling linked to increased glacial carbon storage via changes in Antarctic sea ice

Abstract

Palaeo-oceanographic reconstructions indicate that the distribution of global ocean water masses has undergone major glacial–interglacial rearrangements over the past ~2.5 million years. Given that the ocean is the largest carbon reservoir, such circulation changes were probably key in driving the variations in atmospheric CO2 concentrations observed in the ice-core record. However, we still lack a mechanistic understanding of the ocean’s role in regulating CO2 on these timescales. Here, we show that glacial ocean–sea ice numerical simulations with a single-basin general circulation model, forced solely by atmospheric cooling, can predict ocean circulation patterns associated with increased atmospheric carbon sequestration in the deep ocean. Under such conditions, Antarctic bottom water becomes more isolated from the sea surface as a result of two connected factors: reduced air–sea gas exchange under sea ice around Antarctica and weaker mixing with North Atlantic Deep Water due to a shallower interface between southern- and northern-sourced water masses. These physical changes alone are sufficient to explain ~40 ppm atmospheric CO2 drawdown—about half of the glacial–interglacial variation. Our results highlight that atmospheric cooling could have directly caused the reorganization of deep ocean water masses and, thus, glacial CO2 drawdown. This provides an important step towards a consistent picture of glacial climates.

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Fig. 1: Changes in ocean circulation and sea ice cover between the PI and LGM.
Fig. 2: Deep ocean carbon storage and atmospheric CO2 concentrations in the PI and LGM reference simulations.

Data availability

The input files used to run the MITgcm simulations that support the findings of this study are available from the corresponding author upon request. The model output data for all simulations can be obtained from https://github.com/alicemarzocchi/MITgcmOutput.

Code availability

The MITgcm code is freely available for download at https://doi.org/10.5281/zenodo.1409237. Computer code used to process the model output and generate figures is available from the corresponding author on request.

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Acknowledgements

This work was funded by the National Science Foundation under awards nos. OCE-1536454 and OCE-1846821, and computational resources were provided by the Research Computing Center at the University of Chicago. A.M. received funding from NERC grant no. NE/P019293/1 (TICTOC).

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A.M. and M.F.J. designed the study. A.M. performed the numerical simulations and analysed the results. A.M. and M.F.J. interpreted the results and wrote the manuscript.

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Correspondence to Alice Marzocchi.

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Supplementary text, Figs. 1–4 and Table 1.

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Marzocchi, A., Jansen, M.F. Global cooling linked to increased glacial carbon storage via changes in Antarctic sea ice. Nat. Geosci. 12, 1001–1005 (2019). https://doi.org/10.1038/s41561-019-0466-8

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