New Scientist has this:
Around 12,800 years ago the northern hemisphere was hit by the Younger Dryas mini ice age, or “Big Freeze”. It was triggered by the slowdown of the Gulf Stream, led to the decline of the Clovis culture in North America, and lasted around 1300 years.
Some climate scientists have suggested that the Greenland ice sheet could have the same effect if it suddenly melts through climate change, but the 2007 report of the Intergovernmental Panel on Climate Change concluded this was unlikely to happen this century.
Patterson’s team have now set their sights on even more precise records of historical climate. They have built a robot able to shave 0.05 micrometre slivers along the growth lines of fossilised clam shells, giving a resolution of less than a day. “We can get you mid-July temperatures from 400 million years ago,” he says.<\blockquote>