Sorry for the thread drift here. I am offering an alternative to the cooling period in the medieval period. Climate change is a very complex process that involves far to many variables to be pinned down to a single or even a few causes.

I read an article about the little ice age on the BBC site not long ago. In the article some researchers from Utrecht University, Netherlands were trying to tie the black death into the cooling period known as the little ice age. They said with the depleted human population that a lot of cleared farm land reverted back to forests. With the sudden increase in forested land the carbon dioxide levels dropped causing the cooling period.

Full article is here http://news.bbc.co.uk/1/hi/sci/tech/4755328.stm