A glacier in north-east Greenland with enough ice to raise world ocean levels by half a metre has begun to slide more quickly towards the sea, extending ice losses to all corners of the vast remote island, a US study shows.

Because of warmer water temperatures, the end of the Zachariae Isstrom glacier floated free from a ridge of bedrock below sea level on which it had rested until 2012, the study, which was reported in the journal Science on Thursday, said.

Without that natural brake, the glacier in the cold north was now sliding more quickly and more icebergs were snapping off, adding a net five billion tonnes of ice a year to the oceans, according to the study based on satellite and aerial surveys.

"Similar changes - even larger - are under way in the south," Jeremie Mouginot of the University of California-Irvine and his colleagues said.

Greenland contains enough ice to raise world sea levels by about six metres if it ever all melted in a slow-motion collapse that could take thousands of years.

Its ice losses, along with thawing ice from the Alps to Antarctica, have raised sea levels by about 20 centimetres since 1900.

That aggravates storm surges for cities from New York to Shanghai and threatens low-lying tropical island nations.

A nearby north Greenland glacier, Nioghalvfjerdsfjorden - often called "79" for ease of pronunciation - was also melting quickly, the study said.

But its slide was restricted by an uphill section of seabed under its icy base.

The two glaciers together contain a potential 1.1 metres of sea level rise.

"The changes are staggering and are now affecting the four corners of Greenland," Eric Rignot, a co-author at UCI, said.

Almost 200 nations will meet in Paris for a summit starting on November 30 to try to limit climate change.

The UN panel of climate scientists estimates that sea levels may rise by 26 to 82 centimetres this century.

Ruth Mottram of the Danish Meteorological Institute, who was not involved in the study, said Greenland's glaciers sometimes accelerate after small changes in the environment.

"A small retreat can become a much bigger retreat because these glaciers are only stable when they are resting on bedrock," she said.

Another study has estimated Greenland lost a net 211 billion tonnes of ice annually from 2000 to 2011 - roughly 0.6 millimetres of global sea level rise a year - with icebergs and melting ice exceeding snowfall that replenishes the ice sheet. 

The glacier has lost fully 95 per cent of the ice shelf that used to help stabilise it, they said, and now sports a 75-metre high ice cliff extending above the water (the glacier also extends hundreds of metres below it).

"This is sort of the second major floodgate from Greenland that has opened up," Eric Rignot of UC-Irvine and NASA's Jet Propulsion Laboratory, one of the authors of the study, said.

The first was the Jakobshavn glacier, Greenland's "fastest" moving, according to a recent study, which is based 1300 metres below sea level and also retreating into a deep basin, he said.

Now, at Zachariae, that seems to be happening again.

"If you see Greenland as a boat, it's like we're taking water from every side now," Mouginot said.

Furthermore, Mouginot, Rignot and their colleagues note that Zachariae and Nioghalvfjerdsfjorden together form the terminus of the north-east Greenland ice stream, "the only large, dynamic feature that extends continuously deep to the ice sheet interior near Greenland's summit".

Therefore, 12 per cent of the entire Greenland ice sheet drains through this region, they said.

The entire ice sheet of Greenland contains enough ice to raise sea levels about six metres. It has been losing ice rapidly in recent years, through a combination of meltwater run-off on the ice sheet's surface - which reaches the sea through complex channels and fissures - and the calving of large icebergs from its glaciers.

Both of these processes have elements that can be pretty spectacular. Atop the ice sheet, vast meltwater lakes can form and then suddenly vanish in a matter of hours, draining rapidly as crevasses open beneath them and they spill into the ice sheet's depths.

Meanwhile, at the front ends of marine glaciers, detaching icebergs can tumble and slam back with such force that they knock the glacier itself backwards, and trigger magnitude 5 level earthquakes.

NASA estimates that, through the combination of these mechanisms, the Greenland ice sheet is losing several hundred billion tonnes (or gigatonnes) a year and raising sea levels by three-quarters of a millimetre annually. If so, that would be a little under a third of the total global sea level rise, which is 3.24 millimetres a year.

And now, the Zachariae glacier may be poised to add to that total.

From 1996 to 2010, the new research found, the Zachariae glacier's grounding line - where the glacier simultaneously meets both the sea floor and the ocean - retreated inland 3.5 kilometres. But then, from 2011 to 2015, it retreated another 3.5 kilometres, a sure sign of acceleration. A key event in letting the glacier speed up seems to have been the collapse of its ice shelf, a buttressing tongue that used to extend out over the fjord in front of the glacier, creating an underwater cavity beneath it. Now, in contrast, Zachariae is basically a steep cliff.

Other ways of measuring the rate of change of the glacier - its rate of thinning, for instance, or its flow speed into the sea - were also observed in the new study to be increasing.

"This study does a nice job of putting together data from multiple sensors to document the ongoing speed-up of this glacier. At present its contribution to sea level is relatively small, but there is certainly the potential for it to increase more over time," says Ian Joughin, a Greenland and polar science expert at the University of Washington, in Seattle, in a comment on the new study.

Joughin notes that, so far, despite the fast retreat, Zachariae is not losing as much ice each year as the Jakobshavn glacier in central western Greenland. The new study reports a loss of five gigatonnes annually for the Zachariae glacier, versus 25 to 35 gigatonnes for Jakobshavn, Joughin said.

Joughin thinks Zachariae may not catch up to the latter, since the basin into which it is retreating is not as deep.

But Rignot notes that with 1.1 metres of potential sea level rise between them, the Zachariae and Nioghalvfjerdsfjorden glaciers together contain about as much ice as the very worrisome glaciers along the Amundsen Sea coast of West Antarctica. Granted, in the latter case the situation is worse because losing those coastal glaciers would then unleash a bigger destabilisation of West Antarctica as a whole.

Greenland's glaciers face a kind of double threat - not only changes in the ocean, but also rapidly rising Arctic temperatures, meaning that they can melt both from above and below.

"A disaster is unfolding in slow motion with important sea level rise implications," says Jason Box, a professor with the Geological Survey of Denmark and Greenland, after reading the paper at The Washington Post's request.

The researchers forecast that for the next 20 to 30 years, Zachariae glacier will retreat quickly, as it is perched on a seabed that slopes downward in elevation, exposing more and more of the glacier to warm water. At the same time, the ice front exposed to the ocean will widen from 19 kilometres across to 50 kilometres across, increasing the amount of ice that will be able to flow out at once.

And most troubling of all in some ways, the glacier's "calving cliff" will grow taller, increasing from the current height of 75 metres above the water level. That's worrying because recent research has suggested that once major glaciers lose their ice shelves, as Zachariae has, they can then form cliffs extending up to 100 metres above sea level which will fail continually, because ice just isn't strong enough to sustain a cliff so high. Zachariae could be moving into that realm.

Finally, the study also notes that, while Nioghalvfjerdsfjorden glacier has not been retreating or losing ice nearly as quickly, it too could lose its stabilising ice shelf, and after that, start to move more quickly. For it, too, appears to be exposed to warmer ocean water.

"One can foresee [the retreat of Zachariae glacier] eventually inducing a race for biggest loser [of ice] between Zachariae and the other glacier, Nioghalfjerdsfjorden, which has a dense chain of islands (Norsk Oer Islands Barrier) blocking [its] free access to the sea,"  Box said.

"Nioghalfjerdsfjorden is thinning so, if climate warming remains unchecked, it is just a matter of time until Nioghalfjerdsfjorden disintegrates and the upstream underwater embayment becomes a traffic jam of megabergs shuffling to get out to sea."

However, the key question - as with all studies of glacial retreats in regions with the potential to cause major sea level rise - is how much and how fast. And that's also, of course, the hardest one to answer.

"The retreat of these marine-based sectors is likely to increase sea-level rise from Greenland for decades to come," the paper concludes, but it does not specify how rapidly a full loss of the Zachariae glacier could occur.

"Collapse of the entire basin is going to take a long time. It's not going to happen tomorrow," Mouginot said. "But it's a process, when you start, it's like Jakobshavn - [you don't] see the glacier recovering from that."

Washington Post, Reuters