The massive Greenland ice sheet has melted away at least once during the last 1.4 million years, according to a study published on Wednesday, raising fears that manmade climate change could provoke dangerous sea levels.

Bedrock samples retrieved through more than three kilometres (two miles) of ice reveal for the first time that the island's surface was exposed directly to the atmosphere in the not-so-distant past.

It may have been a single period of up to 280,000 years, or several shorter ones, researchers reported in the journal Nature. 

But either way the evidence shows that the island was largely ice-free.

"Unfortunately, this makes the Greenland ice sheet look highly unstable," said lead author Joerg Schaefer, a palaeoclimatologist at Columbia University in New York.

Covering an area larger than France, Spain and Germany combined, the northern hemisphere's largest ice block on land is kilometres thick and holds enough frozen water to lift the world's oceans by more than seven metres (24 feet).

Even a couple of metres would swamp cities that are home to hundreds of millions of people and planted with many of the crops that feed them. 

Hence the sense of urgency among climate scientists trying to figure out just how sensitive the ice sheet is to global warming, which has already pushed temperatures in the Arctic region two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial era levels -- twice the global average.

The rate of Greenland's ice loss has doubled since the 1990s. 

In the last four years alone, the ice sheet has shed more than a trillion tonnes of mass, according to earlier research.

Two questions loom large: What is the temperature "tipping point" for irretrievable melting? And how long would it take for the ice sheet to disintegrate once that threshold is crossed?

Knowing how Greenland's ice cover has behaved during past warming is key to the answer, but the historical picture has remained sketchy beyond 125,000 years ago.

- Obvious consequences -

Quite simply, the ebb and flow of glaciers wipes away the kind of physical evidence that neatly accumulates elsewhere in layers.

Taking advantage of new lab techniques, Schaefer and his team detected rare, radioactive chemicals -- produced by interactions with cosmic rays from space -- in the Greenland bedrock samples, which were collected more than two decades ago. 

The two isotopes identified have well-known decay rates which mark their age, and could only have come into being if the ground was ice-free, the researchers said.

It means that the entire, gargantuan ice sheet must have melted down to less than 10 percent of its current size if moonlight or sunlight could shine on the soil from which the sample was extracted.

"This study really nails it down," said Thomas Stocker, a professor at the University of Bern in Switzerland and co-chair of the UN's Intergovernmental Panel on Climate Change (IPCC).

"It is direct evidence that the Greenland ice sheet has seen phases where that central area was exposed to the atmosphere," he told AFP.

The findings have "evident and obvious consequences" for sea level rise, he added. 

"We should be concerned."

A second study, also in Nature, used the same technique to analyse sediment accumulated over millions of years in the ocean off Greenland's east coast.

The analysis appears to tell a different story: the ice sheet's eastern flank, the researchers concluded, has not completely melted for the last 7.5 million years.

But a closer look shows that the two studies are not in conflict.

"It is quite possible that both of these records are right for different places," said Paul Bierman, lead author of the second study and a researcher at the University of Vermont in Burlington.

Indeed, some climate models have calculated that ice caps 1,000 metres thick could persist atop Greenland's eastern highlands -- source of the marine sediments -- even if 95 percent of the ice sheet has disappeared.

For Stocker, who was not involved in either study, the evidence based on samples taken from central Greenland bedrock "is certainly stronger".

Both studies, he added, have the virtue of providing rare physical evidence for climate modellers, who will no doubt adjust their projections based on the new findings.