'Surely the Arctic was warmer a thousand years ago when Scandinavians settled in Greenland to farm?'
The Norse didn't settle the Arctic areas of Greenland, only the very margins in the far south.
But to answer your question - the settlement was from about 980CE into an environment slightly cooler than modern Greenland, and cooling from its maximum. Even at the best it was a lousy climate and the Norse only really survived because they could import materials from Norway, Iceland and (briefly) North America. Their agricultural economy was taken from their experience of Norway and Iceland - marginal pasture and hay to see their animals through the winters. As the climate cooled the growing season collapsed and they followed it into extinction.