If I remember my history correctly, "Leif the Lucky" called it "Greenland" to attract additional settlers to what was largely a glacier covered Island (even at that time). It was disinformation, just as the name "Iceland" was designed to keep additional settlers away from a relatively green, smaller island.
Thats what we have always assumed. And there is probably some truth in it. The fact remains that they farmed it, and did well enough for a couple hundred years. Then, as the "little ice age" began to kick in, their crops began to fail, and by the fourth century or so they had disappeared. Starved out, returned to Iceland, no one knows.
Archaeologists are digging up a viking farm that was covered over as the glaciers advanced, pushing sand ahead of them. The farm is almost still as they left it, supposedly, they said you could still smell the manure. As the ice is receding now, it is being exposed.
Saw an article recently, a guy in Greenland is buying up farmland and is running livestock and growing crops; its the first time anyone has done it since the vikings. The difference in temperature is about a degree and a half, but thats enough to make all the difference between crops and no crops.
My opinion is that a little warming is a blessing. A little less Maldive Islands, which is a bummer, mainly if you're Maldivian, but longer growing seasons in Saskatchewan and Siberia are a rich compensation.