RECENT geochemical data(1,2) have challenged the view that rapid climate fluctuations in the North Atlantic at the end of the last glacial mere caused by the thermohaline circulation of the ocean being switched ’on’ or ’off’. Instead, these data suggest that the circulation pattern must have switched between a warm, deep mode and a cold, shallow mode, probably associated with different sites of deep convection(3). Here I present simulations with a three-dimensional ocean model, coupled to an idealized atmosphere, which show this kind of transition. The mechanism for the transition is a rearrangement of convection in the North Atlantic, triggered by a brief freshwater pulse. This results in a drop in sea surface temperature in the North Atlantic by up to 5 degrees C within less than 10 years. The rate of North Atlantic Deep Water (NADW) formation is the same in the cold climate as in the warm climate, but NADW sinks to intermediate depths only, while Antarctic Bottom Water pushes north to fill the entire abyssal Atlantic.