The world was a warmer place back then, we know that now. we also know that the vikings were well travelled and made an attempt to sack Constantinople, and made it well into the Ukraine.
These guys loved their boats and were excellent seamen. None of this is far-fetched.
Yah, these political correct environmentalists want to close us down, let us all freeze in the dark, they never read Leif Ericsson's description of how mild America was a thousand years ago, or of King Woden-lithi's description that 2700 years ago, Petersborough, ONT had a climate like Atlanta more or less, etc.
It would take centuries more of recent warming to get us back to where the North was back when Scandinavians were in their salad days.
Read Barry Fell's classic Bronze Age America, or any of his works...