Hope y'all don't mind me putting about 11/2 cents in.
There was a mini-ice age around 1100 AD. Not sure of the dates, but it was probably caused by an astroid crash or volcano. There have been several TV shows on it. The effects were noted in civilizations all over the world and the rings of trees from different locales bear the theory out.
There's been a variety of interesting possibilities discussed here. I'll be looking into more sources on my next trip to the library.