Actually Vikings invaded much of western Europe as well as Ukraine earlier starting in 800AD in response to Carolus Magnus proselytism, and it is contested that they were "murdering and pillaging".
Most probably this vision was propagated by their enemies in the Christian world.
We do not know for the "mass famines", but this time was the Medieval Warm Period (800–1300). It is thought that between c. 950 and c. 1100 was the Northern Hemisphere's warmest period since the Roman Warm Period.