Little Ice Age
Another possibility is that there was a shutdown or slowing of Thermohaline circulation, also known as the "great ocean conveyor" or "meridional overturning circulation". The Gulf Stream could have been interrupted by the introduction of a large amount of fresh water to the North Atlantic, possibly caused by a period of warming before the little ice age. There is some concern that shutdown of thermohaline circulation could happen again as a result of global warming.