According to a news report in the current issue of Science Magazine, four massive volcanic eruptions in the tropics caused a sudden cooling of the earth between 1275 and 1300. The report is titled:
A Volcanic Trigger for Europe's Little Ice AgeFull Story at http://www.sciencemag.org/cgi/content/full/335/6068/511-b?sa_campaign=Email/sntw/3-February-2012/10.1126/science.335.6068.511-b
The abrupt chill and worldwide disturbance of weather and crops may also be linked to the famines throughout Europe and Asia and Black Death of the fourteenth century.
The scientists' findings are reported in Geophysical Research Letters from January 31, 2012.