It’s truly frightening that the selfish, excessive consumption and especially the burning of fossil fuels by Americans is so damaging that it can go back and cause global warming even a millennium ago.
We are...that evil.
(I hope I don’t need a /sarc tag.)
Personally I don’t think the Medieval Warming Period was so bad. Back then you could grow grapes in England; it’s too cold to grow grapes there now. And the resulting boom in farm production pulled Europe out of the Dark Ages. On the other hand, it also caused overpopulation in Scandinavia, leading to the Vikings going forth to find more land for their surplus sons.