Heinrich events, the massive episodic disgorgement of sediment-laden ice from the Laurentide Ice Sheet to the North Atlantic Ocean, are a puzzling instability of the Ice-Age climate system. Although there is broad agreement on the defining characteristics of Heinrich events, the glaciological mechanisms remain controversial. Paleoceanographic records show that Heinrich events tend to occur at the culmination of a cooling cycle, termed the Bond cycle, and this has invited the interpretation that the events are a fast response of the Laurentide Ice Sheet to external atmospheric changes. A vexing issue for glaciologists is how a fast and timely response to an external forcing can possibly be reconciled with the known physics of glaciers and ice sheets. Fast changes in glacier behavior can only occur if some flow instability is excited. Thus glaciologists tend to favor the idea that the climate change occurring at the culmination of a Bond cycle is an atmospheric response to ice sheet instability. However, a free-running cyclic flow instability, such as that exhibited by surging glaciers, could not satisfy the timing requirements. Using computer modeling we explore ways to resolve these conflicts.