And barbarian hordes will bring down empires. Just like every time before.
The world will survive this, but politically, it's going to be a very different place when it warms up again.
All they need to do is read history. The last one resulted in the collapse of the French Monarchy because the peasants wouldnt change their diet. All cereal based societies had to adapt to more than just wheat and other based grains.
Potato, Turnips, Carrots, Oats, Barley, for example. The Irish failed to continue to grow more rugged types of potatoes and relied on the one variety that was most liable to get the blight. They refused to grow other types of root crops and they ended up starving.
Winters will be extreme, like then, and summers will be cooler with extreme nasty thunderstorms.
BTW. This summer the swallows left northern Wisconsin the first week of August. They usually leave the last week.