Wonderful link Baitman, very interesting. In a nutshell:
"Such a huge series of climatic extremes would have been enough to kill off an civilization – even a modern one."
Now it didn't kill them, it just screwed them up so they ended up killing themselves.
My Roman point was a non-weather one, but connected to resources, which is a global issue today.
The elite crossed all boundaries in leisurous life, and slowly the pillars of Rome went crumbling, for when strength was needed (the Huns), Rome was chopping at it resources....farming. Rome would sent the peasants to war in the late days, and carry on with the parties. Result: lack of production...
The French revolution was also related to famine first and foremost, - the people wanted bread and were fed up with the unrealistic lifestyle of the aristocracy. A little weather issue sent the country (and some others) into a hunger period, and all hell broke loose.
This is my "doomsday theory" regarding GW, - if so-and-so much happens, the western societies are going to have a bad time, and the civilization there may collapse.
You just see what starts happening in the U.S. in a power-out or some floodings, - imagine it would last for years....
