Climate cycles and sea level change also have been linked to shifts in Earth's orbit that occur every 21,000 years or so, but work by William Thompson, an ocean and climate-change scholar at Woods Hole Oceanographic Institution, and Steven Goldstein of the Lamont-Doherty Earth Observatory at Columbia University, found sea levels change too often to be explained solely by the planet's orbital cycles.

"The only reasonable thought is that all of it has to be driven by climate change," Thompson told UPI's Climate.

"Sea level was 120 meters (395 feet) lower 20,000 years ago. As climate warmed between then and now, most of that glacial ice melted and sea level rose to the current level."

In a paper published in the April 15 issue of the journal Science, Thompson and Goldstein described how they studied fossil corals from Barbados, using a new method to calculate the colonies' ages.

Their research not only offers a warning about the chance of unexpectedly rapid changes in sea level, but also proposes a clever way to conduct future studies using radiometric dating of fossilized coral.

"One of the important messages in this paper is that this is a fresh approach," Thompson said.

"If you put some isotopes in a box, then seal it and come back later to open the box, then count the parent isotopes and the daughters, by looking at the ratio, you can see how long the box has been sealed."

This offers a theoretical way to date the corals: counting their slow radioactive decay products.

Up to now, the trouble has been that corals do not sit in a closed box - they live in an open system, with other sources of decay products.

The two researchers were able to rewrite the equations to account for these external factors and determine the corals' ages more precisely.

This can be an important tool in climate research, because coral forms close to the sea surface and therefore the age of the coral gives a good approximation of sea level at that point.

Sea level changes because the oceans either gain or lose water, and most of that water originates from or comprises glaciers and polar ice caps.

If, for instance, the entire Greenland ice sheet melted, it would raise the level of the world's oceans about 23 feet (7 meters).

At present, most of the observed change in sea level is originating not from ice melting, but from thermal expansion of the oceans - as the waters warm, they expand and take up more space.

The glaciers and ice caps constitute enormous systems so scientists generally thought they would be slow to respond to climate change. Recent research such as Thompson's and Goldstein's, however, indicates warming may have a more rapid impact.

"Our records show that sea level change happens much more frequently," Thompson said, and added the accumulations he has measured are still slow by most human measurements: perhaps a centimeter a year.

"If you have a house on the beach and someone tells you that the sea level is going to rise 10 meters (33 feet), you would be worried," Thompson said, "but if it only occurs at the rate of about a centimeter a year for 1,000 years, that's the same thing.

"In the perspective of a human lifetime, you may not see a change. A thousand years is a long time, but it's within our historical memory."

Prasad Gogineni, a professor of electrical engineering and computer science at the University of Kansas, has been studying the Antarctic ice caps from satellite data.

The loss of the west Antarctic ice sheet has the potential to raise sea level by about 5 meters (16 feet), and both east and west Antarctic ice sheets by 70 meters (230 feet)."

No one is yet predicting the loss of these ice caps, but they are behaving differently than predicted in the most recent reports by the Intergovernmental Panel on Climate Change.

"There is concern that sea level will rise faster than it has been rising," Gogineni told Climate.

"The current thinking is that the ice sheets are contributing more than the IPCC is measuring in their earlier models, but the errors are so large, that there is a lot of work to be done to get better models."

He said current evidence is that outflow is exceeding the accumulation. The IPCC models in 2001 predicted the Antarctic ice sheet actually would expand under the global warming scenario, "but the evidence right now is that it is thinning."

A paper in the March 5 issue of Science Express by Hans Oerlemans, profes sor of meteorology at Utrecht University in the Netherlands, has found a dramatic decrease in glacier extent worldwide since about 1850.

"The worldwide retreat of many glaciers during the last few decades is frequently mentioned as a clear and unambiguous sign of global warming," Oerlemans wrote.

The changing climate seems to be having a profound effect on the dynamics of ice. The Larsen B ice shelf in Antarctica collapsed dramatically in 2002, for instance.

The sheet was floating already, so it did not itself affect sea level, but because it had acted like a stopper in a bottle for the glaciers on the Antarctic Peninsula, they soon began to speed up their flow into the ocean.

"Glaciers that were tributaries to the ice shelf accelerated - not just slowly, but almost immediately by glacier standards," said Ted Scambos, a glaciologist at the Snow and Ice Data Center in Denver.

"Within one year of having this ice shelf br eak away, the rates in the lower parts of the glaciers were five to eight times as fast as they had been. There was a major reorganization of the stresses that control glacier flow."

A warming climate in Antarctica, Scambos said, "is going to initiate a large, rapid change in the mass balance of the ice sheet."

Climate is a weekly series examining the latest science and issues related to global climate change, by veteran environmental reporter Dan Whipple. E-mail: [email protected]

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