We simulate three-dimensional ice temperature fields to examine spatial-temporal history of the subglacial thermal environment during the last glacial cycle. Model results suggest that 60--80% of the Laurentide Ice Sheet was cold-based (frozen to the bed) at the LGM, and therefore unable to undergo large-scale basal flow. The fraction of warm-based ice increases significantly through the ensuing deglaciation, with only 10--20% of the Laurentide Ice Sheet frozen to the bed by 8 kyr BP. This basal thermal evolution, a function of both climatic and ice sheet history, could enable a dynamical switch to widespread basal flow through the deglacial period. Because basal flow has the capacity to evacuate large amounts of ice from the interior of continental ice sheets, creating thin and climatically-vulnerable ice masses, this switch in flow regime may have played a significant role in glacial terminations and the 100-kyr glacial cycle.