Conveyors
The Younger Dryas event is now believed to have resulted when a massive glacial lake busted loose from ice dams after the Ice Age and flooded a massive load of fresh water into the Atlantic in a very short time.
It's unlikely that a slower melt of fresh water will result in that kind of drastic temperature change, but it could certainly have some effect. I've seen enough different work on the effects of deep ocean conveyors on climate to credit it.
The chart on this page shows that the drastic effect seems to have been mostly on the northern hemisphere.
Even now, for some reason, modern temperature changes and warming seem to be mostly confined to the northern hemisphere.