Read here. In peer-reviewed research published by the influential Proceedings of the National Academy of Sciences (PNAS), scientists reconstructed 5,600 years of climate conditions in Greenland. This new reconstruction confirmed the warm and optimum climate conditions of the Medieval Warming and the rapid onset of the frigid Little Ice Age.
In the case of Greenland's flourishing Viking settlements, the climate change to a cold phase meant total destruction of their existing society and settlements.
This new PNAS study totally refutes Michael Mann's entirely bogus 'hockey stick' portrayal of climate being benign and unchanging prior to the 20th century.
"What climate scientists have been able to ascertain is that an extended cold snap, called the Little Ice Age, gripped Greenland beginning in the 1400s. This has been cited as a major cause of the Norse’s disappearance. Now researchers led by Brown University show the climate turned colder in an earlier span of several decades, setting in motion the end of the Greenland Norse...The Vikings arrived in Greenland in the 980s...The arrival coincided with a time of relatively mild weather, similar to that in Greenland today. However, beginning around 1100, the climate began an 80-year period in which temperatures dropped 4 degrees Celsius (7 degrees Fahrenheit), the Brown scientists concluded...“You have an interval when the summers are long and balmy [Editor: yes, Greenland summers "long and balmy" during the Medieval period] and you build up the size of your farm, and then suddenly year after year, you go into this cooling trend, and the summers are getting shorter and colder and you can’t make as much hay. You can imagine how that particular lifestyle may not be able to make it,”...[William J. D’Andreaa, Yongsong Huanga, Sherilyn C. Fritz, N. John Anderson 2011: Proceedings of the National Academy of Sciences]