Climate scientists now understand that the Medieval Warm Period was caused by an increase in solar radiation and a decrease in volcanic activity, which both promote warming. Other evidence suggests ocean circulation patterns shifted to bring warmer seawater into the North Atlantic.
I’ve always heard that in medieval times, Greenland was a green land, which is why that island was given that name.
Climate has changed throughout earth’s history. During the last ice age, Chicago was buried under a mile of ice. How did all that ice melt, if not for global warming due to natural cycles?
I really recommend that people read a book on natural history to gain some perspective on climate change and geological change. I have a book which focuses on the history of my area (the Puget Sound area), and its fascinating. As you say, during the last ice age (ending about 10,000 years ago) the ice where I am sitting was hundreds of feet thick. The north/south valleys here were carved out by that ice. Even more recently, just a few hundred years ago, a massive earthquake changed the elevation of Puget sound by about 20 feet.