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It is this current, argue Broecker and Denton, that keeps the Arctic relatively warm and glacier free. When it stops running, an ice age -- or a cold spike -- begins. What causes a turnoff? An influx of fresh water might do it, by diluting the saltiness and density of the current, preventing it from sinking and heading back to the tropics. There is evidence that at just the time the Younger Dryas began, a huge North American lake (which no longer exists) began dumping Amazonian quantities of fresh water into the North Atlantic. The discharge stopped about 1,000 years later, as did the Younger Dryas. Broecker and Denton's model, says Penn State's Richard Alley, an expert on Greenland ice cores, "is probably the trigger for these abrupt changes."
Nobody knows what other factors might help trigger climate shifts, and how sensitive they are. "It scares us," says Alley. "We know that there are times when climate is very delicately poised. We know that for the past 8,000 or 10,000 years, it hasn't flipped over. But we don't really understand it well enough to say whether it's really stable or whether we are on thin ice."
In short, while there is no reason to think the next full-fledged Ice Age is upon us, a shorter episode of frigid conditions could happen at any time. The last interglacial period was warmer than this one and also, arguably, more unstable. It is conceivable that the greenhouse effect could heat up the planet for a while but then trigger changes that could plunge the earth into a sudden chill. And for an idea of what a mini-Ice Age might be like, just imagine last week's cold wave lasting all winter, every winter -- for the next thousand years.