What caused the Younger Dryas? The conventional wisdom is that, as the planet gradually warmed and the glaciers retreated, proglacial lakes were formed by the water melting from them. The largest of these was Lake Agassiz, which is theorised to have been at the very center of the continent (around present day Lake Winnipeg). Another was Lake Algonquin, which comprised most of the current great lakes, Georgian Bay, and parts of Northern Michigan. And a third was Lake Iroquois, which was an enlarged version of Lake Ontario.
For most of its existence, drainage from Lake Agazziz was Southward into the proto-Mississippi. Lake Algonquin flowed South towards the Gulf of Mexico, and Lake Iroquois drained South into the Hudson River. However, as the glaciers retreated (so the story goes), the ice-dam which prevented water from these lakes from flowing Eastward collapsed abruptly. An immense pulse of cold, fresh water flowed down the St. Lawrence Valley and out into the Atlantic, where it caused a shut-down of the North Atlantic thermocline (sometimes called the great ocean conveyor) which carries warm water Northward via the Gulf Stream. This in turn caused Northern Hemisphere temperatures to plunge, glaciers to advance again, and so on and so forth.
If you live in the Greater Toronto Area you can actually find evidence of the aftermath of this ice dam collapse in the form of Scarborough Bluffs. These bluffs are made of packed clay soil and in fact constitute the shore-line of old Lake Iroquois; previous to the great Eastward outflow which drained all of these pro-glacial lakes, the waters stood about 100 feet higher than today.
However, a new theory has come onto the scene. A recent paper has suggested (backed by some pretty good evidence) that an "impact event" occurred somewhere near the Great Lakes at the beginning of the Younger Dryas:
Iridium peaks, and spikes of ammonium and nitrate which could have been produced by extensive burning of biomass, are also found in the Greenland ice cores. The authors therefore propose that the black horizon is the result of massive forest fires raging across the whole North American continent, triggered by the fiery impact of a meteorite or comet - or possibly a low altitude explosion like the Tunguska event. Enough soot from the fires, and dust from the impact, would have been thrown into the atmosphere to significantly cool the climate, causing enough environmental stress to wipe out both the Clovis people and the mammoths.
So did the cooling associated with this impact trigger the Younger Dryas? Probably not on its own. Though enough soot from the fires, and dust from the impact, would have been thrown into the atmosphere to significantly cool the climate, these effects would only have lasted several years. It is more likely that the impact caused a "destabilization/melting" of North America's interior ice sheets--including the bust up of that ice dam--which triggered that massive Eastward outflow from our pro-glacial lakes, and so on and so forth.
Now, why did I just write all this? Well, if you encounter a skeptic who tells you that its all bullshit because how can warming produce colder temperatures? you can tell them about the Younger Dryas. Of course, when scientists talk about the possibility of a Gulf Stream shut-down caused by modern day warming, they are looking at an entirely different mechanism: cold fresh water melting from the Greenland Ice Sheet, for the most part, bringing about the same effect.
Furthermore, and luckily enough, this outcome does NOT seem imminent.