When the Vikings settled there prior to the last little ice age it was a green and fertile land along the coasts. The weather patterns shifted a few hundred years later and the populations had to shift with many going back to Norway. Weather by its nature is NOT static; with or without man.
And this was directly attributable to the vikings driving around in their gas-guzzling viking-mobiles, their decadent incandescent lights, bloated meat heavy diets, low taxes, and somehow, their owning of fire arms.