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.

"Wild Berries" Translation More Likely

The actual word in the sagas is "vinber" which means a berry used to make wine. It doesn't necessarily mean "grape". People made wine out of elderberries, gooseberries, currants, and many other berries that grow as far north as Newfoundland.

Helge Ingstad points this out in his book, The Viking Discovery of America: The Excavation of a Norse Settlement in L'Anse Aux Meadows, Newfoundland about his and Anne Stine's discovery of the L'Anse Aux Meadows Norse settlement site. Gwyn Jones also talks about it at length in his book, A History of the Vikings.

Samuel Eliot Morison says that this was first pointed out by Merritt L. Fernald, a botany professor at Harvard, in 1910. 

Even before the Vinland Sagas were written down, Adam of Bremen, in History of the Archbishops of Hamberg (1075), referred to Vinland as the land of wild vines that yield excellent wine:

"King Sven related that there was another island in that ocean which had been discovered by many and was called Vinland because vines grow wild there and yield excellent wine, and moreover, self-sown grain grows there in abundance. "

But even if, in 1075 or so, rumor had it that there were vines with berries on them in Vinland, it could easily have been just misinformation based on people's extrapolation of that fatal word "vinber".

"Most of what is guessed about the Norse colony in North America is derived from two Icelandic epics called The Vinland Sagas. There are three locations — Stoneland, probably the barren coast of Labrador, Woodland, possibly Maine; and Viniand-which the Norse visited. While Leif the Lucky gets the credit in history and the roads and festivals named after him, it was another Norseman, Bjarni Herjolfcson, who was the first European to sight North America, in 985 or 986. But it was Leif who supposedly built some huts and spent one winter in this land where wild grapes—more likely berries since there are no grapes in any of these places—grew before returning to Greenland."

Don't Know Much About History by Kenneth C. Davis  Pg. 9

Some Speculate a Medieval "Marketing Ploy"

A 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".

(This article can be found on this page.)

Myth Used as Evidence Against Global Warming

Sources:

• [1]  Ingstad, Helge; Ingstad, Anne Stine (2001). The Viking Discovery of America: The Excavation of a Norse Settlement in L'Anse Aux Meadows, Newfoundland. Checkmark Books. ISBN 0-8160-4716-2.
• Ingstad, Helge (1965). Vesterveg til Vinland; oppdagelsen av norrøne boplasser i Nord-Amerika. Gyldendal (Oslo).
• [2]  A History of the Vikings by Gwyn Jones Oxford University Press, 2 edition (July 19, 2001) ISBN-10: 0192801341
• [3]  Don't Know Much About History by Kenneth C. Davis  Pg. 9
• The "Medieval Warm Period"  NOAA's paleoclimate website [United States Department of Commerce]
• 'Recurring climactic phenomenon' nothing to fret over
  EMIL L IANNELLI. Bucks County Courier Times. Aug 21, 2007. pg. A.4
• Leif Ericson Day really has it all
  
  Ed Quillen   Denver Post. Oct 10, 2006. pg. B.07
• "The Vikings: They Got Here First, But Why Didn't They Stay?" Robert McGhee 1998Canadian Geographic