He's an assistant professor of climate communication at George Mason University, and has coauthored several textbooks on climate change. https://communication.gmu.edu/people/jcook20
I'm not able to find any source describing the extent of Greenland's ice sheet during the medieval warm period. If you have one, you should post it. All I can find are temperature records which indicate greenland has been hotter than in the medieval warm period since the 1930s.