Another possibility is as follows. The fall in temperature 12,800 years BP occurred in the middle of a cyclical temperature rise that had started to melt and de-stabilize the ice sheets. This temperature rise was brought to an abrupt end by a large (? chondritic) meteorite impact or impacts, somewhere on land but not necessarily on the Laurentide ice sheet. Especially if this reactivated extensive volcanism, it would have caused a return to the ice age (the Younger Dryas), but would not in itself have caused a human macro-catastrophe. It would certainly have been a notable social and climatic event, and would account for the evidence of global fires and the nano-diamonds and other impact ejecta in the ice sheets and in the black sedimentary bands found all over the world.
Let’s assume the skies start to clear in a few years but the volcanic activity continues, and the temperature stays down for a millennium. The civilization starts or continues to thrive, the population explodes due to extensive agriculture on low land, and mankind starts or continues to built ever grander megalithic monuments. As suggested by Graham, these are meant to warn future generations of an impact event, (anger of the gods) linked to the 26,000 year precessional cycle which they have already documented. It has 1000 plus years in which to develop and refine the necessary megalifting technology.
Human activity, agriculture and the greenhouse effect consequent on anaerobic methane release, is finally combined with a return to the warming conditions driven primarily by solar activity. (I am struck by the exceptionally rapid temperature rise Graham shows at the end of the Younger Dryas). These lead together to the sudden temperature rise, which hits thickened ice sheets that are hard on top, but already weakened underneath by the previous partial melting that was arrested in 12,800 BP by the above-mentioned impact.
We arrive at the onset of the YD global warming event, that hits an advanced maritime culture with megalithic astronomical monuments, some at least on higher ground. The populations and cities are concentrated for obvious reasons near coastlines, and bingo! On YD-day the Ice sheets melt catastrophically, causing the biblical flood, tsunamis and a 100 metre global rise in sea levels, massive large animal extinctions and destruction of civilisation. Luckily for us Noah and friends saw it coming and took the necessary precautions to escape with his family, friends, farm animals, and his seed stock!