A favorite argument of AGW "sceptics" has to do with Greenland and what is known as the medieval warm period (MWP). The idea is that if the Earth was warmer some time in the past, this would undermine AGW theory. I don't find this line of argumentation to be robust either way, but let's examine it, shall we?
The Vikings colonized Greenland from 986 AD. They were able to farm, fish and raise cattle. The settlements disappeared by the 15th century, presumably because of the little ice age (LIA).
The first thing that needs to be pointed out is that Greenland was still a very cold place during the MWP. It was not a green paradise of any sort. Skeptical Science and A Few Things Ill Considered have the details on that.
Another counter-argument I've encountered is that Greenland might have been considerably warmer than it is now during the MWP, but this was most likely a local or North-Atlantic phenomenon, not a global one. Nearly all climate reconstructions that cover the MWP (and there are many of them, by many different authors, using several different methods) do not show it to be warmer than today.
Is there a way to corroborate that Greenland was indeed unusually warm during the MWP, unlike the rest of the northern hemisphere? Absolutely.
Alley (2000) provides a 50,000 year temperature reconstruction from Central Greenland whose resolution is not bad at all. The reconstruction is ice-core based, and it provides temperatures as absolute values. In order to calculate a "temperature anomaly" for purposes of graphical comparison, I added 31.29 to the temperature values provided by Alley (2000).
Let's start by looking at data from 200 AD to 1850 AD. I will use the CPS Northern-Hemisphere reconstruction from Mann et al. (2008) for comparison.
The difference between the MWP and the LIA in Greenland was around 1.6°C. Interestingly, the MWP temperature peak in Greenland occurs almost exactly at the time of the Viking colonization. Presumably, that's not a coincidence. Additionally, there are already some indications in this graph that the climate of Greenland experiences abrupt changes from time to time.
Not convinced? Let's look at 50,000 years of data. For comparison, I will use the temperature reconstruction from Vostok station, Antarctica made available by Petit et al. (1999). This is also an ice-core based reconstruction.
Here we see that the temperature of Greenland fluctuates in a manner that is not matched by the temperature of Antarctica. In particular, notice the effect of the Younger Dryas stadial on each of the series.
In synthesis, the climate of Greenland is quite peculiar, and as a result, it should not be thought of as a proxy of global or even hemispheric climate.