A huge flotilla of icebergs launched into the world's oceans almost 15,000 years ago from melting Antarctic glaciers pushed sea levels up by some six feet, researchers say, and that kind of dramatic melting has been forecast for the icy continent's future as well.

Cycles of global warming have seen huge collapses of the Antarctic ice sheet at least eight times in the last 20 centuries, and current measurements taken on some of its biggest glaciers suggest a similar massive retreat and collapse may be imminent.

"The Antarctic Ice Sheet had been considered to be fairly stable and kind of boring in how it retreated," Oregon State University climate scientist Peter Clark says. "This shows the ice sheet is much more dynamic and episodic, and contributes to rapid sea-level rise."

The last large release of icebergs occurred around 9,000 years ago, scientists say, after which the frequency of glacial retreat and iceberg creation slowed -- until it picked up again in the last century as greenhouse gases from human activity accelerated climate change.

Computer models of current climate trends suggest global warming may be tipping the Antarctic Ice Sheet into another rapid shrinking, the researchers say in a report of their study published in the journal Nature.

Looking for evidence of past iceberg surges, the researchers drilled into the sea floor off Antarctica and found debris left behind from as many as eight such surges occurring over 20,000 years.

Sand, trapped inside the icebergs, has been deposited on the seafloor as the icebergs slowly melted in recurring episodes of glacier retreat.

The largest amount of debris suggests the biggest surge was around 14,500 years ago, a time when sea levels rose at a rate 20 times greater than is happening today as ice sheets in the both the Northern Hemisphere and in Antarctica melted.

"The question has been, 'Where did this ice come from?" Clark says. "This is the first clear evidence Antarctica did contribute to this sea-level rise."

Scientists said they now believe Antarctica's melting accounted for about half of that ancient jump in sea level.

The researchers say the study's findings confirm a warming climate can cause Antarctica's huge glaciers to undergo pulses of speedy withdrawal instead of melting and retreating in a slower, steadier fashion.

Such dramatic melting in the past -- and the resulting onslaught of icebergs and rises in sea level -- suggests future events may be less predictable than once believed, they say.

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