The medieval warm period wasn't as warm as you're suggesting (I can't find any citations for more than 2 degrees, and the delta may well be less), and it wasn't world-wide: northern Europe (and some other parts of the northern hemisphere) was warmer, and as it turns out, europe ended up writing a disproportionate part of modern history, so that was remembered.
Globally, temperatures were lower than they are now.
This isn't a secret, nor is the information hard to find; e.g. https://en.wikipedia.org/wiki/...
There may be some truth to the inevitability of global warming, but make no mistake: our generation sure is screwing over the future thoroughly. Even in optimistic assumptions, it seems likely that greenland will lose most of its ice; which sounds to me like the world is likely to experience sea level rises of at least 10 meters (since greenland isn't the only glacier on the planet, and because warm water expands).
The question is whether that takes thousands of years - so cultures and populations get to adapt relatively calmly - or something scarier than that.
People aren't great at dealing with rapid change.