Years ago there was a show on PBS about climate.
Some egghead types had gone into Canada somewhere to take core sediment samples from a lake, I forget which one.
By studying the size of the sediment particles, the pollen in the samples, etc. They were able to map out timewise exactly how long it had taken from this particular area to go from a moderately warm, temperate climate to full-scale, deep freeze ice age.
We’re talking glaciers and ice caps hundreds of feet thick.
Ninety years.
Being a bit geezerly I recall some speculations from decades ago. One was that ice ages do or can occur quickly.
Someone claimed there were fossils showing animals frozen while still eating. The reason for that was speculated to be the Earth shifting suddenly on its axis. That would of course mean a shifting of the frozen poles and not a general cooling of the planet.
Oh, but don’t forget, these things happened *gradually*. [guffaw]