Climate variability recorded in Greenland ice cores over the past 100,000 years is interspersed by a number of rapid warming events. Each warming event is approximately 10 to 15 degrees C in annual average temperature at summit Greenland. Each of these events in the climate record is also estimated to have occurred in less than a couple of decades. A modelling simulation of the glacial climate with a fully coupled climate model reveals that the initial warming event is triggered by thermohaline instability under a North Atlantic covered by sea ice. The initial instability is characterized by the formation of a wintertime glacial super polynya at the beginning of each warming event. Afterwards, the resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a reduced state occurs within a couple of decades. I will describe some of the interesting physics underlying the instability.