Global Warming - Greenland is what I say to that

Saying that you don't believe in Global warming these days is akin to denying the Holocaust. Actually, the real difference is that Holocaust denial probably attracts less grief than trying to argue that something other than Man was to blame for Global Warming.
Well, I am about to invite a shed load of odium. I don't believe in Global Warming. There I've said it. Although to be more accurate, I don't believe in Global warming as it is being promoted. And I believe even less in the eco-Nazis who have made it their life's work to proselytise on behalf of the new religion of GloWar. It seems to me that there is already far too much money, power and academic credibility riding on the whole topic of GW for it ever again to be the subject of rational debate.
I recently heard one estimate that GW is currently a £2 billion a year industry for various academics, pressure groups and other clingers-on. No wonder the GW promoters get so pissed off with anyone who might raise even the tiniest doubst that their case was unproven. Next year's scholarship or grant might be riding on their ability to keep the theory afloat.
Leaving aside the fact that the Romans were growing grapes as far north as Cumbria when they occupied England in the first century AD, what really makes me doubt GW is the island of Greenland.
When Eric the Red was promoting his latest real estate venture to some, probably sceptical, fellow Norwegians he called the island he had discovered, Greenland to emphasis its suitability for farming. Now, he couldn't have done that unless the island actually was, for the most part, green. When he and his fellow settlers put their roots down in the late 10th Century (984) they lived in two settlements on the west coast on the fjords near the very southwestern tip of the island. The Norse settlements thrived there for the next few centuries, and then disappeared sometime in the 15th century after nearly 500 years of habitation.
What drove them out?
According to data obtained from ice cores, between AD 800 and 1300 the regions around the fjords of the southern part of the island experienced a mild climate . Trees and herbaceous plants grew in the south of the island and they were able to farmland grow plants similar to those they had been used to in Norway.
By the time that the Danish church sent missionaries to Greenland in the early 18th Century to recover the souls of any Norse settlers suspected of relapsing into Paganism, there no Europeans left, just some late-arriving Innuit who were quite happy frolicing in the icy wastes. The Norwegians had been driven away by the onset of the mini ice age that extended glacial ice to virtually the whole of the island.
Now parts of that same ice field are showing signs of melting and that fact is used as proof of Global Warming caused by Man's use of fossil fuels.
So, my question is this. If the climate of Greenland was balmy enough between the 10th and 15th Centuries for people to thrive there, what caused the sudden change in climate that drove them away?
There were no cars, no internal combustion engines of any kind. Just farm animals and arable crops.
Until anyone can explain this to me by way of their pet theory on mn-made GW, I am afraid I will go on being a Global Warming Denier.