"Today, because of the circulation of the Atlantic Ocean, we can grow roses in Norway and we have been able to do that for a long time," says Dr. Richard Alley, Evan Pugh Professor of Geosciences. "But there is no such thing as a stable climate; if the warm current were not there, we would see more polar bears and fewer roses."

While ice ages that come on over tens of thousands of years have periodically covered large areas of the globe with glaciers, Alley is more concerned with rapid climate changes -- within a decade -- that effect the northern and southern hemispheres differently.

"The secret of why the whole world rides a roller coaster in the ice age and freezes and thaws is probably greenhouse gases, especially carbon dioxide," Alley told attendees today (Feb. 17) at the annual meeting of the American Association for the Advancement of Science. "The seesaw effect of rapid climate change is probably caused by ocean circulation and the keys to this change are locked in the polar ice."

While during an ice age the whole Earth becomes cold together and warms up together, episodes of rapid climate change have chilled only the northern Atlantic and warmed the area around Antarctica or warmed the Atlantic and cooled Antarctica.

Normally, warm surface water off the coast of Brazil moves northward toward Scandinavia. When the current reaches the areas of cold air, the water cools, becoming more dense, and sinks. This sinking current then travels down to the tip of South America.

If something interrupts this current, the north becomes cold and can rapidly freeze while the heat south of the equator remains there. The north becomes cold and the south becomes warm.

Despite the frozen north, the trade winds continue to blow westward over the Atlantic and the Isthmus of Panama. The moisture picked up by the wind rains out in the Pacific, leaving the Atlantic saltier.

Pacific rain never makes it to the Atlantic because of the large continents in between, and the salty water that used to leave the Atlantic around the tip of South America cannot because the current no longer flows. Eventually, the Atlantic becomes salty enough that the sinking resumes and turns on the ocean circulation.

"We don't know how this cycle begins, nor do we know geographically where the salty water begins to sinks to return circulation. However, this pattern of cold north with warm far south has occurred repeatedly," says Alley.

With global warming, more precipitation in the far north and melting of glaciers there may freshen the north Atlantic and slow or stop the ocean circulation.

"If this happens in the near future, then the north may become colder even though global warming is affecting the rest of the world," says the Penn State researcher. "However, ultimately, the warming will take over and not just the mountain glaciers, but the Greenland glacier could melt as well."

What will melt under various conditions is difficult to predict. The mountain glaciers, which are currently melting, will raise sea level only a little. If Greenland melts, it will raise sea levels much higher. While these sea-level increases will be noticeable and perhaps inconvenient, people will adapt to them, according to Alley.

"The question always asked is if the West Antarctic Ice sheet will fall into the ocean in the next 100 years," says Alley. "This event would have high impact on continental shores and terrestrial lowlands. It is a low probability event, but not a zero possibility."

The mountain glaciers only hold enough frozen water to raise sea level a foot or two if they all melt. Greenland could melt over many millennia, raising sea level 20 feet or more.

However, changes in West Antarctica could raise sea level 20 feet in centuries or faster. To monitor the potential for the unlikely event of West Antarctica melting and to try to predict and plan for rapid climate change, researchers must continue to study the past climate record locked in the frozen poles, according to Alley.

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