Artist rendering of a supernova in the night sky
After analyzing layers of deposits in both Europe and North America, Scientists are speculating that the last ice age occurred in the blink of an eye: less than one year.
According to a recent study, the catalyst may have been a comet impact in the Northeast of America which caused massive forest fires, melting of northern ice sheets, and an influx of fresh water into the Artic seas which caused the moderating effect of the North Atlantic current to breakdown. The massive heating, curiously, caused Northern hemisphere cooling to occur due to the huge quantities of smoke and dust emanated into the atmosphere.
In addition to a comet impact, other causes include potential supernovae (cosmic rays), the catastrophic collapse of a distant galaxy (cosmic radiation/microwaves), or heavy remnants of the Taurid meteor shower impacting on Earth.
Once the cooling trend started, there was no moderating factor that could reverse it, so a warming trend took several millennia to develop.
It is speculated that this event may be responsible for the elimination of large mammal species in North America and Europe (mammoth, large cats, horses, sloths, camels) in the geological record, and the end of the human "Clovis" culture.
Sudden cooling was demonstrated in the film The Day After Tomorrow.