Researchers in Britain and the United States devised computer models to test three scenarios that could account for rapid -- by the standards applied to glaciers -- melting of the Helheim Glacier, one of Greenland's largest.

Two were based on changes caused directly by global warming: an increase in the flow of water that greases the underbelly of the glacier as it slides toward the sea, and a general thinning due to melting.

If confirmed, either of these explanations would point to a sustained increase in runoff over the coming decades, fueling speculation that sea level could rise faster and higher than once thought.

The stakes are enormous: the rate at which the global ocean water mark rises could have a devastating impact on hundreds of millions of people living in low-lying areas around the world.

But a team led by Faezeh Nick of Durham University in Britain found that neither of these scenarios matched the data.

"They simply don't fit what we have observed," said colleague and co-author Andreas Vieli in an interview.

By contrast, the third computer model -- which hypothesised that melt-off was triggered by changing conditions in the confined area where the glacier meets the sea -- fit like a glove, he said.

"Whatever happens at the terminus provokes a strong and rapid reaction in the rest of the glacier. The result has been a significant loss of mass" as huge chunks of ice drop into the ocean, a process known as calving, Vieli explained.

These changes are also set in motion by global warming, but are not likely to last, he said.

"You cannot maintain these very high rates of peak mass loss for very long. The glaciers start to retreat and settle into a new an relatively stable state," he said.

The Helheim Glacier, along with several others in Greenland, started to slow down in 2007.

Vieli also noted that the data alarming the scientific community only covers a span of a few years. It may be ill-advised, he suggested, to project a trend on the basis of what may turn out to be a short-term phenomenon.

The UN Intergovernmental Panel on Climate Change (IPCC) predicted in 2007 that sea levels could creep up by 18 to 59 centimetres (7.2 to 23.2 inches) by 2100 due to thermal expansion driven by global warming.

Such an increase would be enough to wipe out several small island nations and seriously disrupt mega-deltas home in Asia and Africa.

But IPCC failed to take into account recent studies on the observed and potential impact of the melting ice sheets in Greenland and Antarctica, prompting the Nobel-winning body to later remove the upward bracket from its end-of-century forecast.

A new consensus has formed among experts that levels could rise by a metre or more by 2100, according to Mark Serreze of the National Now and Ice Data Center in Boulder, Colorodo.

"What has puzzled us is that the changes are even faster than we would have though possible," he said in a recent interview.

Vieli cautioned that his findings, published in Nature Geoscience, are narrowly focused on one glacier, and that sea levels could still rise higher than the IPCC's original projections.

Other Greenland glaciers behave differently, and the dynamics of the Antarctic ice sheet are still poorly understood, he noted.

Nor should the new study "be taken out of context to suggest that climate change is not a serious threat -- it is," he added.

The ice sitting atop Greenland could lift oceans by seven metres, though even the gloomiest of climate change projections do not include such a scenario.

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