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Abrupt North Atlantic circulation changes in response to gradual CO2 forcing in a glacial climate state

Abstract

Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard–Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere–ocean model, gradual changes in atmospheric CO2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO2 levels of about 15 ppmv—comparable to variations during Dansgaard–Oeschger cycles containing Heinrich events—is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO2 levels, we infer that atmospheric CO2 may serve as a negative feedback to transitions between strong and weak circulation modes.

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Figure 1: Transient simulations of the experiment CO2_Hys (left) and LGM_0.15_CO2 (right).
Figure 2: AMOC hysteresis and trend analysis in the increasing CO2 scenario of the experiment CO2_Hys.
Figure 3: Summary cartoon of the proposed mechanism in this study.
Figure 4: Synthesis of AMOC stability diagrams.

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Acknowledgements

X.Z. thanks G. Leduc for helpful discussion about marine sediment core MD02-2529. We thank colleagues in the Paleoclimate Dynamics group at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven for general support and the AWI Computer Centre for keeping the supercomputer running. This study is supported by Helmholtz Postdoc Programme (PD-301), as well as the PACES program of the AWI and the BMBF funded project PalMod. The opening foundations of the Key Laboratory of Marine Sedimentology & Environmental Geology, SOA, (grant No. MASEG201701) and State Key Laboratory of Marine Geology, Tongji University (grant No. MGK1611) as well as the national Natural Science Foundation of China (grant No. 41575067) are grateful acknowledged (X.Z.). Furthermore, G.K. acknowledges funding by ‘Helmholtz Climate Initiative REKLIM’ (Regional Climate Change), a joint research project of the Helmholtz Association of German research centres (HGF). We also acknowledge financial support from the UK NERC (grants NE/J008133/1 and NE/L006405/1) to S.B.

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All authors conceived the study. X.Z. designed and performed the model simulations, analysed the results, and led the write up of the manuscript with G.K. All authors interpreted the results and contributed to the final version of the manuscript.

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Correspondence to Xu Zhang.

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Zhang, X., Knorr, G., Lohmann, G. et al. Abrupt North Atlantic circulation changes in response to gradual CO2 forcing in a glacial climate state. Nature Geosci 10, 518–523 (2017). https://doi.org/10.1038/ngeo2974

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