I saw something interesting on Discovery or whatever a few years back.
Scientists went to a lake in Canada (not sure which one) and took core samples from the sediments being deposited in the lake by the feeder streams.
By measuring the thickness of the sediments and the size of the particles and the pollens in the layers, they were able to map out the time it took for one of the ice age events to happen.
Time span from normal temperate climate to full ice age glaciation:
Ninety years
So buy lots of warm socks and will them to your great grandchildren!
That's interesting, I think I'll read up on that