Would you care to support that statement. In every historical survey of the MWP food production was greatly enhanced and because of that it was a time of real prosperity. Wars were of shorter duration and more limited goals do to the simple fact that people had what they needed so the wars were mainly fought as a means of personal advancement on the part of the gentry and not because of some grand political strategy, I am of course not counting the Crusades as they were a Church thing, however they were more apt to occur because the production of food in Europe was greater so the excess could be used to support the war overseas.
Now lets have a global cooling period like what was witnessed after the Tambora eruption of 1816 and then you will begin to see some real problems. That resulted in over 65,000 deaths (from starvation) alone in Europe and does not take into account the deaths due to food riots etc.
Nope, sorry Old Fraud, the historical record doesn't back up your contentions.
A rise of a few degrees will almost certainly result in a dieoff of a significant portion of humanity. But it will not be detrimental to everyone. Drill, baby, Drill.