Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

The Younger Dryas (YD, dated between 12.7}11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat #ux as triggered by glacial meltwater. Recently, a CH 4 -based synchronization of GRIP d18O and Byrd CO 2 records (West Antarctica) indicated that the concentration of atmospheric CO 2 (CO!5. 2 ) rose steadily during the YD, suggesting a minor in#uence of the THC on CO!5. 2 at that time. Here we show that the CO!5. 2 change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater #ux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small e!ect on CO!5. 2 in this model, because it is spatially limited and compensated by far-"eld changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. d13C depletion and PO 4 enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO 2 record in terms of THC changes are discussed. O. Marchal ( ) 'T. F. Stocker 'F. Joos 'A. IndermuK hle ' T. Blunier ' J. Tschumi Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland E-mail: [email protected]