Mrs. BD and I have been taking the William and Mary course in Medieval History (with the delightfully Asperger's-ish Prof. Daileader via The Teaching Company) and we are enjoying it immensely. I do not like to sit unless I am at work, but this course gets me into a chair after work. (We live and thrive on the Teaching Company courses at my cottage, as readers know.)
The Prof says that the wealth of the Middle Ages came from a combination of trade and the renewal of currency in the form of the Italian Florin, the introduction of the heavy plow, the replacement of slavery with serfdom, a doubling of Europe's population - and the Medieval Warm Period which made it possible to grow better crops much further north than in the Dark Ages - and further north than today. Greenland was farmland.
The Warm Period was far warmer than the world today. People benefited. That's why we pray for Global Warming (but also doubt that humankind will be so lucky. With our luck, we'll get the next Ice Age and all be screwed except for Dr. Merc).