Glacial greenhouse-gas fluctuations controlled by ocean circulation changes

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Abstract

Earth’s climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard–Oeschger climate oscillations1,2,3, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear4,5,6,7,8. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard–Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales.

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Figure 1: Model simulations of glacial climate and greenhouse-gas fluctuations.
Figure 2: Example of millennial changes in climate and greenhouse gases as recorded in polar ice cores from 45 to 50 kyr bp in comparison with model simulation.
Figure 3: Simulated ocean production of N 2 O.

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Acknowledgements

This study was funded by the NSF Marine Geology and Geophysics programme grant 0728315-OCE, and the NSF Paleoclimate programme grant 0602395-ATM. We thank J. Ahn, E. Brook, R. Toggweiler and V. Brovkin for discussions and comments on the manuscript.

Author Contributions A.S. designed the study and performed the numerical experiments. E.D.G. performed the comparison with the proxy record. A.S. and E.D.G. discussed the results and contributed equally to the analysis of the data and writing of the paper.

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Correspondence to Andreas Schmittner.

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Schmittner, A., Galbraith, E. Glacial greenhouse-gas fluctuations controlled by ocean circulation changes. Nature 456, 373–376 (2008) doi:10.1038/nature07531

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