Consider this article from a professor at SUNY Suffolk, himself a believer in man-made global warming. (H/T ECM)

Excerpt:

During the years 800-1200, Iceland and Greenland were settled by the Vikings. These people, also known as the Norse, included Norwegians, Swedes, Danes, and Finns.

The warm climate during the MWP allowed this great migration to flourish. Drift ice posed the greatest hazard to sailors but reports of drift ice in old records do not appear until the thirteenth century (Bryson, 1977.)

[…]The Greenland Vikings lived mostly on dairy produce and meat, primarily from cows. The vegetable diet of Greenlanders included berries, edible grasses, and seaweed, but these were inadequate even during the best harvests. During the MWP, Greenland’s climate was so cold that cattle breeding and dairy farming could only be carried on in the sheltered fiords. The growing season in Greenland even then was very short. Frost typically occurred in August and the fiords froze in October. Before the year 1300, ships regularly sailed from Norway and other European countries to Greenland bringing with them timber, iron, corn, salt, and other needed items. Trade was by barter. Greenlanders offered butter, cheese, wool, and their frieze cloths, which were greatly sough after in Europe, as well as white and blue fox furs, polar bear skins, walrus and narwhal tusks, and walrus skins. In fact, two Greenland items in particular were prized by Europeans: white bears and the white falcon. These items were given as royal gifts. For instance, the King of Norway-Denmark sent a number of Greenland falcons as a gift to the King of Portugal, and received in return the gift of a cargo of wine (Stefansson, 1966.) Because of the shortage of adequate vegetables and cereal grains, and a shortage of timber to make ships, the trade link to Iceland and Europe was vital (Hermann, 1954.)

Here’s where the Vikings settled:

It’s all covered in ice today, though. But it was not covered in ice back then. And that’s because it was warmer in the Medieval Times than it is today.