The Medieval Warm Period, between the ninth and fourteenth centuries, made their voyages possible. Benevolent weather allowed them first to settle Greenland and later to reach and explore unknown portions of North America. The weather was considerably warmer during this period than it is today in North America and Greenland.
The Northmen, under Eirik Thorvaldsson (Eirik the Red) colonized Greenland. Their sheep, goats, cattle, and horses grazed the lush green pastures while they traded in walrus hides and ivory with their European homeland. However, the fragile environment soon became overgrazed and could not support their domestic animals in viable numbers, forcing a gradual shift from an agrarian to a hunter-gatherer society, as the contents of their middens indicate. Wild game was plentiful during the early years, but after a time, the hunting moved farther and farther afield as yak and caribou herds were depleted. Finally no game remained except a few ring seals.
It is particularly important for the reader to be aware that not a single document originating in Greenland exists. The Norse Greenlanders may have been illiterate for the most part. Everything about their personal history is conjecture because none of it comes to us from the source, they themselves. The runic alphabet they employed did not lend itself to lengthy dissertation.
Everything about the five-hundred-year history of the two main Greenland settlements comes to us from sources with no vested interest in telling the true story of these hardy people. In all cases, the information was compiled as long as two hundred years after the fact by saga writers who had never been to Greenland.
For the rest of the story, read my Axe of Iron series of historical fiction books, for therein you will find plausible answers to the most enduring mystery of the western hemisphere:
For the complete text, with endnotes, see Historical Perspective in the first novel, Axe of Iron: The Settlers.