If it was just Greenland, sure. There's lots of room for regional variation, and at the time of the first settlements in Greenland, the region was atypically habitable to pastoralists.

"This place is warmer than it was" is not, in itself, proof of AGW, any more than "We had a cold snap here" is disproof.

Focusing on the changes going on in Greenland isn't about proving AGW, or even just GW. Its about highlighting the costs and ramifications. Most of the world scientific community has moved past "proving" AGW, and started talking about predicting effects and planing amelioration.