(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: It was just as warm in the Medieval Warm Period (MWP) as it is today. In fact, Greenland was green and they were growing grapes in England!
Answer: There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth — Europe, for example — but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two thousand years.
Anecdotal evidence of wineries in England and Norse farmers in Greenland do not amount to a global assessment.
On its website, NOAA has a wide selection of proxy studies, accompanied by the data on which they are based. Specifically, they have this to say on the MWP:
The idea of a global or hemispheric “Medieval Warm Period” that was warmer than today, however, has turned out to be incorrect.
With regard to the “grapes used to grow in England” bit, here is some fairly solid evidence that grapes are in fact growing there now, denialist talking points aside. If that is not enough, RealClimate has a remarkably in-depth review of the history of wine in Great Britain, and how reliable it is as a proxy for global temperatures. (Hint: not very.)