Ice sheet melting key to sea level rise.

• Position: Antarctic - Brief Article

A massive and unusually abrupt rise in sea level about 14,200 years ago was caused by the partial collapse of ice sheets in Antarctica, researchers have found, solving a mystery scientists have been heatedly debating for more than a decade. In less than 500 years at the end of the last ice age, this collapse caused the Earth's sea level to rise approximately 70 feet--about four times faster than sea levels were rising most of the time during this period and at least 20 times more rapidly than they are today. The cause of this event, called the "global meltwater pulse 1A" since it was first identified in 1989, has been unknown until now.

This type of melting thousands of years ago is different from the more recent events in Antarctica, such as the breakup of a large percentage of the Larsen ice shelf in March, 2002, but the dramatic melting illustrates the pressing need for a better understanding of Antarctica's huge ice sheets and their stability. "We can't say at this point whether the recent breakup of part of an ice shelf in Antarctica has any relevance to this type of huge meltwater event that originated from Antarctica thousands of years ago," notes Peter Clark, a professor of geosciences at Oregon State University, Corvallis, one of the world's leading experts on glaciers. "We don't know yet how important these ice shelves are to stabilizing the larger ice sheets of the continent." What is very clear is the importance of Antarctica's huge ice sheets remaining stable. The West Antarctic one is thought to be potentially unstable, and, if it collapsed, sea levels around the world would rise almost 20 feet. The melting of the larger and more-stable East Antarctic ice sheet would raise sea levels another 200 feet.

During the comparatively short period thousands of years ago, these two huge ice sheets were anything but stable. One or the other, or some combination of the two, melted at a rapid rate and caused a 70-foot surge in sea levels in just a few hundred years. "This event happened near the end of the last ice age, a period of deglaciation that lasted from about 21,000 years ago to 12,000 years ago," Clark says. "The average sea level rise [prior to] that period was...