The research, which matches new ice core data with a simulation of past and future changes in the West Antarctica Ice Sheet (WAIS), reveals for the first time regular cycles of "catastrophic collapse" and reformation reaching back five million years.

Cycles lasted 40,000 years during the first three-fifths of this period, but have since more than doubled in length, explained David Pollard, a scientist at Pennsylvania State University and lead author of one of the studies.

"But with global warming we are cutting short a natural cycle," he told AFP by phone.

"The two studies combined show it is really likely that the WAIS will collapse in the next few thousand years. In the absence of human influence, it would probably happen only 100,000 years from now," he said.

Rising sea levels is arguably the most serious long-term threat from climate change.

The global ocean water mark is likely to go up by at least a metre before the end of the century, recent research has shown.

That is enough to wipe out several small island nations, and to disrupt or displace tens of millions of people living in heavily-populated and low-lying delta areas in East Asia, African and the Indian subcontinent.

Part of that rise will come from thermal expansion as ocean temperatures rise, a process scientists understand well and are able to forecast.

But the world's two great ice sheets sitting atop Greenland and Antarctica remain climate change wild cards, with great uncertainty as to whether -- or how quickly -- they might shed their mass.

A team of more than 50 scientists led by Tim Naish of Victoria University in Wellington, New Zealand extracted sediment samples reaching 600 metres below the surface of the WAIS.

The findings showed a geological metronome of massive change across five million years, and provided the first direct evidence of total collapse.

"Before there were hints of it collapsing like that, but we really didn't know until now," said Pollard, a co-author of the study, published in Nature.

The new data also confirmed that the cycles of ice destruction and formation are closely linked to shifts in the tilt of Earth's axis as it rotates around the Sun, a process called obliquity.

The period covered by the sediment samples -- the early Pliocene -- is of special interest to climate scientists because it so closely resembles the conditions forecast for Earth over the next 100 years.

With global temperatures set to rise about 3.0 degrees Celsius (5.4 degrees Fahrenheit) by 2100, "more significance is being placed on the early Pliocene as an analogue for understanding the future behaviour of the WAIS and its contribution to global sea levels," the study says.

The concentration of carbon dioxide in the atmosphere -- some 400 parts per million (ppm) -- was also in line with projected 21st century levels, which have already hit 385 ppm and are still rising.

In the second study, Pollard and Robert DeConto of the University of Massachusetts in Amherst simulate Antarctic ice sheet variations in a mathematical model over the past five million years in order to track the "grounding line", the shifting border between land and sea ice.

"We found that the dominant mechanism attacking the West Antarctic ice has been variations in ocean melting under its large floating ice shelves," rather than changes in temperature or surface melt, Pollard said.

"One of the next steps is to determine if human activity will make it warm enough to start the collapse," said Pollard.

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