I found this source to be more well-researched and accurate than Wikipedia about the lifestyle of the Vikings there:
http://www.archaeology.org/online/features/greenland/
In short, they could not have developed such a social structure without the benefit of better environmental conditions than exist today.
To prove that it wasn’t all in a name: wasn’t it Iceland where they used to grow grapes?
From ac’s link:
“An ice core drilled from the island’s massive icecap between 1992 and 1993 shows a decided cooling off in the Western Settlement during the mid-fourteenth century.”
Greenland had warmer times recently, without AGW.
Of course, though, that doesn’t negate AGW theory.