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Climate warming during Antarctic ice sheet expansion at the Middle Miocene transition


During the Middle Miocene climate transition about 14 million years ago, the Antarctic ice sheet expanded to near-modern volume. Surprisingly, this ice sheet growth was accompanied by a warming in the surface waters of the Southern Ocean, whereas a slight deep-water temperature increase was delayed by more than 200 thousand years. Here we use a coupled atmosphere–ocean model to assess the relative effects of changes in atmospheric CO2 concentration and ice sheet growth on regional and global temperatures. In the simulations, changes in the wind field associated with the growth of the ice sheet induce changes in ocean circulation, deep-water formation and sea-ice cover that result in sea surface warming and deep-water cooling in large swaths of the Atlantic and Indian ocean sectors of the Southern Ocean. We interpret these changes as the dominant ocean surface response to a 100-thousand-year phase of massive ice growth in Antarctica. A rise in global annual mean temperatures is also seen in response to increased Antarctic ice surface elevation. In contrast, the longer-term surface and deep-water temperature trends are dominated by changes in atmospheric CO2 concentration. We therefore conclude that the climatic and oceanographic impacts of the Miocene expansion of the Antarctic ice sheet are governed by a complex interplay between wind field, ocean circulation and the sea-ice system.

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Figure 1: Palaeo-oceanographic records and temperature changes from the South Tasman Rise during the MMCT.
Figure 2: Climatic responses to an Antarctic ice sheet expansion in scenario ΔICE.
Figure 3: Climatic responses to a CO2 decline in scenario ΔCO2.


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We thank S. Barker, I. Hall and P. Köhler for comments on this study and the colleagues in the Paleoclimate Dynamics group at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven for general support. This study is promoted by the PACES programme of the AWI and by the Helmholtz Climate Initiative REKLIM (Regional Climate Change), a joint research project of the Helmholtz Association of German research centres (HGF).

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G.K. and G.L. designed the research. G.K. carried out the experiments. G.K. and G.L. carried out the analysis and wrote the paper.

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Correspondence to Gregor Knorr.

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The authors declare no competing financial interests.

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Knorr, G., Lohmann, G. Climate warming during Antarctic ice sheet expansion at the Middle Miocene transition. Nature Geosci 7, 376–381 (2014).

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