Years ago cores from New England pond bottoms suggested that the last ice age came on fast, with in a year or two. If I remember aright this was based on analysis of pollen grains in the pond bottom cores. This was called "snowblitz" at the time. People pointed out that snow is positive feedback. The white snowcover reflects mucho solar heat back into space, so that once the northern hemisphere bcomes snowcovered, it won't melt out. This was a topic of discussion back in the '60s or '70s when people worried about the ice ages coming back.
Right here is another pond bottom mud core study, this time in Ireland, where the investigators show another rapid onset of an ice age.
The global warming people have backed off a little since the world started cooling down ten years ago. This report suggests that it could get a lot colder, real fast.