The "Medieval Warm Period" is one of those scientific/political footballs that is constantly being passed around. As a medievalist, I'm familiar with the historical evidence (or interpretation) that suggests the weather must have been nicer c. 1000 A.D. because why else would the Vikings believe that Iceland and Greenland would have viable farming land. Of course, it was only viable, not necessarily very productive. Then there is the research of NOAA and the findings of the Intergovernmental Panel on Climate Change, which both indicate that the Medieval Warm Period wasn't really that much warmer and that, contrary to being a global event, was much more regionalized.
The reason that the MWP is important is because those who are skeptical of human-created global warming believe that the MWP and the Little Ice Age indicate that changes in global temperatures occurred well before 20th century industrialization. While NOAA admits that attempts pin down temperature patterns in periods as early (and earlier) than 1000 A.D. are tough because of the lack of reliable data, they also point out that the warming that has ocurred in the 20th century easily eclipses any previous rise. Of course, that is, any measure rise.
What does this all mean? Science as politics ain't going away and, frankly, I'm not too interested in that debate. Sometimes it seems that the role played by contingency is forgotten in history and the MWP debated offers an important reminder that weather can affect history in multiple ways. What if the MWP continued and even got hotter? Would we all be talking about our Viking forefathers here in the U.S.? Or would more--and better--wine be coming out of Britain?