James Burke talks about this in "Connections" -- The 13th was THE GREATEST OF CENTURIES, because the weather had never been so good ...
The way the "North Atlantic Conveyor Belt" works is that if you pump enough warm water up to the ice caps, they melt, releasing fresh water into the North Atlantic ... Since fresh water is lighter than salt water, the polar water stays at the surface while the warming tropical water sinks ... This creates what you can think of as a wall of fresh water that blocks the Gulf Stream from getting to Europe ... Thus cooling the climate and tripping off the next ice age ...
That is the real danger of global warming : That it will cause an ice age ...
In that "paradigm," more evidence of a warm middle ages doesn't discredit global warming ... It is also troubling in highlighting how the global temperature changes quickly ...
The evidence also shows that the period of the Roman Empire was very warm and things cooled down after 450, bringing the "Dark Ages" ...
Since the earth itself pours out more gases than we do, I'm not very worried about most "greenhouse" gases, though the extremely odd chemicals we produce are best regulated, because they will last longer than plutonium ... For carbon, methane, water vapor, etc., cows and termites are the main sources, not man ...
Something Steven Gould said is right ... to focus on man instead of bacteria is to focus on the tail, not the dog ...