Objection: Newfoundland was so warm in the Medieval Warm Period that when the Vikings landed they called it Vineland and brought boatloads of grapes back to Europe.
Answer: Once again: you can’t draw conclusions about global climate from an anecdote about a single region, or even a few regions. You need detailed analysis of proxy climate indicators from around the world. These proxy reconstructions have shown that the Medieval Warm Period (around the time the Vikings are said to have discovered North America) was not as pronounced or as warm as today’s warmth. From NOAA’s paleoclimate website comes these quotes:
What records that do exist show that there was no multi-century periods when global or hemispheric temperatures were the same or warmer than in the 20th century ….
… In summary, it appears that the 20th century, and in particular the late 20th century, is likely the warmest the Earth has seen in at least 1200 years.
As for the specific anecdote that Vineland was a warm land where grapes grew wild: as with the Greenland story, Vineland’s name was most likely a kind of marketing ploy. This 1988 article by Robert McGhee for Canadian Geographic had this to say:
There has been much argument over the location of Vinland, with scholars and local enthusiasts placing it anywhere between Labrador and Florida, and even in the Great Lakes or the Mississippi Valley. The geographical descriptions in the Norse sagas are too vague to allow certain placement on a modern map, but there is growing consensus that they best fit Newfoundland and Labrador (formerly Newfoundland). The main problem with a Newfoundland and Labrador (formerly Newfoundland) site is the absence of wild grapes. Still, there is a strong suspicion that what Leif found were only berries, and that he followed the practice of his father in “giving a land a good name so that men would want to go there”.
(That comes from this page, which has lots of good resources on the Viking expansion.)
So, grapes growing in Newfoundland: paleoclimate’s smoking gun, or medieval marketing ploy? You decide.