Earth's climate has shifted drastically many times in its history, but barring massive asteroid impacts, climate change tends to play out over thousands or tens of thousands of years. Plenty of time to pack away the summer clothes and buy a nice warm coat when you spot an ice age coming. But researchers at the University of Saskatchewan recently discovered that some ice ages come on quite rapidly.
By taking some very thin slices of a mud core sampled from a really old lake (Ireland's Lough Monreach), the Saskatchewan team got a high-resolution look at varying oxygen and carbon isotope levels in the lake's history. The analysis revealed events happening month by month in the lake's ancient past. They found that things got very cold very quickly during the "Big Freeze" (more scientifically referred to as the Younger Dryas), a small ice age that occurred about 13,000 years ago. How quickly? The lake basically froze solid within a few years, and it might even have happened within a few months.
Source: http://io9.com/5416491/there-could-be-an-ice-age-two-months-from-now 
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pretty!