what's the latitude of northern Scotland? I mean, bring it over to the United States and you're are sitting in the middle of Hudson's Bay. NARRATOR: What worries some scientists is that in the past, this circulation appears to have stopped. A close examination of polar ice cores reveals about 10,000 years ago, while the Earth warmed up from the last ice age, the climate suddenly switched back into a cold state. Temperatures fell nearly 10 degrees in a decade. Some scientists believe it was caused when fresh water from melting glaciers flowed into the north Atlantic and disrupted the ocean circulation. RON PRINN: First, the north Atlantic would get much colder than it is today. That would mean Europe, northern Europe, would cool down very rapidly. If that kept up over time, one would get more and more snowfall over time. And one of the hypotheses for going into an ice age is a shutdown of just that circulation, leading to more and more snowfall. Then it accumulates. It lasts over the summer. And steadily, over hundreds, thousands of years, you can build up glaciers on land that was previously, you know, vegetation-covered. Paradoxically, a warming, in that case, would be leading to a catastrophic cooling of some parts of the world. NARRATOR: The mighty oceans, some scientists argue, are the biggest uncertainty in the climate system, capable of dramatic climate surprises. After decades of research, climate scientists have presented many possible scenarios for the future of the planet.