It may come as a surprise to you, but the normal state for Earth is glaciated. We are in an "interglacial" that's called the "Holocene." We started into it around 12,000 years ago. The estimates are that it hasn't been this warm in 2,000 years. When the other interglacials ended, humans just adapted to colder temps and/or migrated south (or north, in the southern hemisphere). The wild card is that, in the earlier interglacials, the campfires of humans, after we appeared, were not enough to modify the natural cycle. That's no longer true, with the massive amounts of greenhouse gasses pumped into the atmosphere, it could go either way. Either we have an extraordinarily hot interglacial OR we get instant glaciation. There is some evidence that ice ages come on rapidly - mammoths frozen with fresh flowers in their mouths, etc. The North Atlantic circulation is slowing down, probably as a result of massive inflows of fresh water from Greenland ice-melt. There is some evidence that's caused ice ages before. Best case - Northern Europe becomes uninhabitable. Worst case - it's buried under ice, and north Alabama becomes beachfront. At 85 this year, I won't be around to see it. Probably...