31 March 2002

Antarctica Key To Sudden Sea Level Rise

by Kate Melville

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, a new study has shown, in research that solves 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 event caused the Earth's sea level to rise about 70 feet. That's about four times faster than sea levels were rising most of the time during this period, and at least 20 times faster than the sea level is currently rising.

The findings were reported today in the journal Science by researchers from Oregon State University, the University of Toronto and the University of Durham in the United Kingdom.

The cause of this event, called the "global meltwater pulse 1A" since it was first identified in 1989, has until now been unknown. This study not only pinpoints the source of the meltwater pulse, but it also makes clear that significant climatic events can occur very rapidly and unpredictably.

This type of melting event thousands of years ago is different from the more recent events in Antarctica, researchers say, such as the breakup of a large percentage of the Larsen ice shelf earlier this month. 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," said Peter Clark, a professor of geosciences at OSU and 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, however, is the importance of Antarctica's huge ice sheets remaining stable. The West Antarctic ice sheet 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 Earth's sea levels another 200 feet.

And during this comparatively short period thousands of years ago, it is now known that these two huge ice sheets were anything but stable. One or the other, or some combination of the two, melted at a surprisingly 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 de-glaciation that lasted from about 21,000 years ago to 12,000 years ago," Clark said. "The average sea level rise during that period was about eight millimeters per year. But during this meltwater pulse there was an extremely rapid disintegration of an ice sheet and sea levels rose much faster than average."

The amount of sea level rise that occurred during a single year of that period, Clark said, is more than the total sea level rise that has occurred in the past 100 years.

For some time, researchers had speculated that the cause might have been the partial melting of a major ice sheet in North America. But the OSU and other university scientists were able to develop a method that "fingerprinted" each of the possible melting scenarios from known ice sheets in the world at that time, and found that a source from Antarctica most closely matched data about sea level rise available from fossil shoreline deposits.

Using this approach, it became clear that the melting of the North American ice sheet could not have been the sole source for the meltwater pulse, and some combination of ice sheet melting in Antarctica was the more likely culprit for the sudden spurt in sea level rise.

This period thousands of years ago, Clark said, was also a time of increasing temperature, sea level and atmospheric carbon dioxide that is conceptually similar to the present day.

Prior to the partial collapse of the Antarctic ice sheets 14,200 years ago, carbon dioxide levels had risen about 50 parts-per-million in the atmosphere. In the past 150 years, since the beginning of the Industrial Revolution, carbon dioxide levels in Earth's atmosphere have risen 85 parts-per-million.

But in comparing these two eras there were differences in Earth's overall temperature, atmosphere and location of ice sheets, so it's not possible to use the events that happened then as any certain predictor of what might happen to Earth today, Clark said. What is clear is that large ice sheets of the past were vulnerable to global warming.

This meltwater event thousands of years ago not only caused the sea level to rise, the researchers said in their report, but also may have affected the atmosphere, ocean circulation and global climate.