Possible cause of the little ice age

According to a report from Richard Black from the BBC, An international research team has suggested that a series of volcanic eruptions starting in 1250 through 1300 caused the little ice age that significantly lowered temperatures in Europe from the 1300's through 1500's.  Global temperatures also dropped but to a lesser degree than Europe.   The little ice age was one of the contributing factors to the destruction of the Norse culture on greenland.   Looking at the chart below it would be interesting to research what caused the Medieval Warm Period. 

http://en.wikipedia.org/wiki/File:2000_Year_Temperature_Comparison.png