The Younger Dryas was probably caused by a massive flooding of the North Atlantic by freshwater outflow from the melting ice sheets. 
Studies with numerical models of the climate system indicate that a large meltwater pulse could cause a shutdown of the thermohaline circulation and cooling of the magnitude observed.
As we have noted in earlier lectures, the thermohaline circulation transports warm water from the tropics to the northern North Atlantic, maintaining warm temperatures in this region.
North Atlantic thermohaline circulation
Current evidence indicates the Younger Dryas was probably confined to the North Atlantic, consistent with the thermohaline shutdown mechanism.