Model Behavior The initial pulse of warming during the last deglaciation, which defined the start of an interval called the Bølling-Allerød, occurred abruptly about 14,500 years ago. To date, the most detailed simulations used models of intermediate complexity, not with more sophisticated Coupled Global Climate Models (CGCMs) that can synchronously couple both oceanic and the atmospheric components. Overcoming practical and technical challenges, Liu et al. (p. 310 ; see the Perspective by Timmermann and Menviel ) performed such a simulation using CCSM3, a state-of-the-art ocean-atmosphere CGCM. In contrast to previous studies, which indicated that the Bølling-Allerød was triggered by a nonlinear bifurcation between modes of deep ocean circulation in the Atlantic, the results suggest that the event was a transient response caused by the cessation of meltwater input into the surface ocean in the North Atlantic region.