Sack Lunch Seminar (SLS)

Abstract:

Eighty thousand years of ice sheet build-up came to a rapid end ~19-10 thousand years before present (ka BP), when ice sheets receded quickly, and our planet warmed by about 4C. It remains unresolved whether insolation changes due to changes of earth's tilt and orbit around the sun were sufficient to drive this deglaciation, or whether changes in atmospheric CO2 played an essential role, too. Using transient deglaciation simulations with a new coupled three-dimensional climate-ice sheet model, it is shown show that orbital forcing around 19 ka BP tipped the delicate balance between ice sheet accumulation, ablation, basal melting, and calving, thereby initiating the deglaciation. However, the transient model experiments also reveal that the deglacial CO2 rise not only accelerated the ice sheet retreat after 17 ka BP, but was necessary for a full termination. Without the deglacial atmospheric CO2 rise, most of Canada would remain ice-covered - even today. The talk will discuss potential causes of the glacial/interglacial CO2 variability and its orbital pacing.

Millennial-scale variability associated with Dansgaard Oeschger (DO) and Heinrich events (HE) is one of the most puzzling glacial climate phenomena ever discovered in paleo-climate archives. The presentation will describe the results of the first transient global climate hindcast simulation covering the period 50 ka B.P. to 30 ka B.P. The climate model is forced by time-varying external boundary conditions (greenhouse gasses, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing Northern Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. The analysis presented is consistent with the idea that ice-sheet instabilities and subsequent changes of the Atlantic Meridional Overturning Circulation were the main driver for the continuum of DO and HE variability seen in paleo-records across the globe.