Scientists who have been raising alarms about the endangered ice sheet of West Antarctica say they've identified a key glacier that could pose the single most immediate threat to the world's coastlines - and are pushing for an urgent new effort to study it.

The glacier is not one that most people will have even heard of - Thwaites Glacier along the Amundsen Sea. It's a monstrous body that is bigger than the South Island and has discharged over 100 billion tons of ice each year in recent years.

The glacier is both vast and vulnerable, because its ocean base is exposed to warm water and because of an unusual set of geographic circumstances that mean that if it starts collapsing, there may be no end to the process. But it's also difficult to study because of its location - not near any US research base, and in an area known for treacherous weather. As a result, the researchers are also calling for more support from the US government to make studying West Antarctica's glaciers, and Thwaites in particular, a top priority.

"In some scenarios, the next 50 years or 100 years could see, you could begin to see very rapid ice loss from central West Antarctica. It's the wild card," said Ted Scambos, the lead scientist at the National Snow and Ice Data Center who chaired a meeting earlier this month of West Antarctic scientists outside Loveland, Colorado. Thwaites, says Scambos, has "much more upward potential than we realised".

"It is hard to find tipping points in physical systems likely to be activated in the near future, with the possible exception of Thwaites," Richard Alley, a glaciologist at Penn State University, said at the meeting.

The scientists are backed by a just-released report by the US National Academy of Sciences, which concluded that the National Science Foundation - which runs US Antarctic programs - should make research on Antarctica's sea level implications its top priority, with a particular emphasis on West Antarctica. That's because much of its ice is below sea level and thus "vulnerable to a runaway collapse process known as marine ice sheet instability".

"There is an urgent need to understand this process in order to better assess how future sea level rise from ice sheets might proceed," the report stated.

"This was our No. 1 priority, across all disciplines," said Robin Bell, a senior scientist at Columbia University's Lamont-Doherty Earth Observatory who co-chaired the National Academy of Sciences panel, at the Colorado meeting. Bell said that the consensus on this point emerged from the community of Antarctic researchers themselves, whose views were extensively solicited in forming the document.

The National Science Foundation has not yet said how it will respond to this unusual show of unanimity among Antarctic scientists.

"NSF would like to see a wide distribution of the report's recommendations in order to prompt a discussion in the research community and among other agencies with a stake in Antarctic research," agency spokesman Peter West said in a statement. "Such an approach would help the NSF shape its response to the recommendations for the future of the US Antarctic Program, which it manages."

It's not just US scientific authorities worrying about Thwaites. In Colorado, representatives of the British Antarctic Survey, which along with the United States is a leader of Antarctic research, sounded a similar note.

David Vaughan, the survey's director of science, said he believed a central research goal should be "looking at the longer term commitment to sea level rise that is inherent in the West Antarctic ice sheet, with a geographic focus on Thwaites Glacier, what we already now agree is potentially the most vulnerable part".

Concerns about West Antarctica aren't new. In a landmark paper published in 1978, the Ohio State University glaciologist John Mercer singled out the region as posing a "threat of disaster" because the majority of the vast ice sheet is rooted far below sea level. Were it to melt due to global warming, "major dislocations in coastal cities, and submergence of low-lying areas such as much of Florida and the Netherlands, lies ahead," Mercer wrote.

But it took much longer for scientists to convince themselves that Mercer might be right - by studying West Antarctica to identify its most vulnerable regions, and then by using satellites and other means of remote observation to actually document their changes.

"There has been a pendulum in this community in the last 20 years, from, 'we're sure the West Antarctic Ice Sheet is going to collapse,' to 'actually we're not,' to 'oh yes we are,' to 'oh, it's happening now,'" says Eric Steig, a University of Washington glaciologist who spoke at the meeting on how to find cores of ancient ice that might prove whether West Antarctica collapsed during a past warm period, some 120,000 years ago.

"I think that many of the ideas that people came up with for why it won't collapse have been disproven," Steig says - although he emphasises that there is still a great deal of scientific uncertainty about the matter.

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More and more, the vulnerable point appears to be Thwaites Glacier on the coast of the Amundsen Sea - a remote and difficult to reach part of West Antarctica that is not particularly close to any of the US' three Antarctic research stations. The Amundsen Sea's glaciers are roughly midway along the Antarctic coast between the US's Palmer Station, near the tip of the Antarctic peninsula, and McMurdo Station, near the Ross Ice Shelf - representing a considerable trip from either point. The total distance between the peninsula and McMurdo is a staggering 4000 kilometres.

The Amundsen region was once considered to have "such terrible weather conditions that field operations would be near to impossible," as the British Antarctic Survey's Vaughan put it in a 2006 paper.

"The number of people who have stood on Thwaites Glacier, you can count on the fingers of a couple of hands," says Sridhar Anandakrishnan, a glaciologist at Penn State University who has actually been to Thwaites on several research expeditions, and says the glacier has "the highest probability of having a huge change on sea level."

But the location is not a problem for satellites, which have recently led to the conclusion that glaciers throughout the Amundsen coast - including Thwaites' partners Pine Island, Haynes, Smith and Kohler - have been retreating rapidly at their subsea bases and losing ice. The cause is not warming air temperatures, as Mercer had suspected, but rather warmer ocean waters reaching the base of the coastal glaciers - which are rooted hundreds of meters below sea level - and melting them from below.

This does appear to be caused by climate change, albeit in a complex way, explains Robert Bindschadler, a NASA researcher who headed the annual meeting of West Antarctic scientists for 20 years and has an Antarctic glacier named after him. Because the tropics and subtropics have been warming faster than the Antarctic, Bindschadler explains, the greater temperature differential from equator to pole has driven fiercer winds around the Antarctic continent. And those, in turn, have pushed so-called circumpolar deep water, which is the warm middle ocean layer in this region, onto continental shelves where it can reach the base of glaciers.

"There's so much heat in that layer, it could wipe out the entire ice sheet," says Bindschadler - even though the water is just a few degrees above freezing.

The water is also quite salty and dense, so it travels to the lowest accessible point of the submerged glacier - the so-called "grounding line," where the glacier front touches the seafloor, hundreds of meters below the sea surface. And then, it starts a process of rapid melting.

Last year, a pair of blockbuster research papers found not only that the Amundsen Sea's glaciers were experiencing rapid grounding line retreat, but suggested that there may be no apparent end to the process. That's because the glaciers lie on a "retrograde" bed, where the ground beneath the glacier slopes downhill rapidly as you move further inland - for Thwaites, over a kilometre in some regions lies below sea level.

"It's just a soup bowl. It just drops off," says Anandakrishnan. Thus, the thinking goes, the warm water can continually chase the retreating grounding line downhill.

That's true in much of the region, but what sets Thwaites apart from the other Amundsen Sea glaciers - including its fast-moving neighbour, Pine Island glacier - is its gigantic size. Its oceanfront expanse is 120 kilometres wide, where Pine Island's is only 30 kilometres. Also unlike Pine Island, Thwaites is "not confined in a valley, hence with the potential for a much larger increase in ice flux if the flow acceleration continues at the same pace," as a recent study put it.

Thwaites is also well over 2 kilometres thick in places. And while it is not synonymous with West Antarctica as a whole - which has been estimated to contain 3.3 metres of total potential sea level rise - it could blaze the path toward a larger retreat.

"It has the biggest basin, it would kind of eat out the heart of the West Antarctic Ice Sheet if it started to retreat rapidly," says Bindschadler.

The crucial question of timing. But how soon could that happen? Last year, Ian Joughin, a respected glaciologist at the University of Washington, published a paper in Science on Thwaites with two colleagues, in which he predicted that "a full-scale collapse of this sector may be inevitable," but added that his model "leaves large uncertainty in the timing". He put the range between 200 and 900 years for the "onset" of rapid collapse.

But some scientists now wonder if an important part of the change could happen still faster.

One problem is that in addition to last year's research documenting rapid grounding line retreat in the Thwaites region, modelling work published earlier this year highlighted two more processes involved in glacial collapse, both of which could accelerate its speed. One of them is the concept of "ice cliff failure," which suggests that a sheer wall of ice more than 100 metres high (above water) can't hold. Ice, after all, is not an extremely strong or durable material.

"The maximum ice cliff is 100 metres, beyond that the yield stress of ice will just disintegrate," says David Pollard, a glaciologist at Penn State University who led the new research. The fear is that as Thwaites (or potentially some other glaciers) become destabilised, it may produce ice cliffs higher than 100 metres which will then collapse.

Granted, scientists are not saying yet that this will happen on a time scale that will immediately affect people alive today, or their children and grandchildren - just that they fear it could. Contending with a very hard phenomenon to study, they're still trying to understand what's truly possible.

The suggestion of mechanisms like cliff collapse "create a need to understand the ice-ocean processes in that area of Antarctica now, and also to understand the bedrock topography, in more detail - now," says Scambos.

That means a great deal more research and direct measurements in this extremely remote environment. It isn't research on the moon or at the ocean's greatest depths, but in terms of work on or near the surface of Earth, it's about as tough as it gets.

To understand the difficulty of the scientific task, consider this - one key problem will be figuring out exactly what is going on at the ground level beneath over 1.6km of ice. A key unknown involves precisely what kind of terrain the base of Thwaites glacier rests upon, and what it is composed of - which will affect just how much resistance there is to the glacier's movement.

"Whether that base is slippery, or not slippery, makes all the difference in whether Thwaites glacier disappears in 100 years or 1000 years," says Anandakrishnan.

Such research can be conducted through seismic measurements atop the glacier. Even more complicated, though, is determining what's happening at the Thwaites glacier's dynamic oceanfront and ice shelf, where there is continual ice flow into the sea and large crevasses.

The entire situation is further complicated by the limited resources available for Antarctic science. The National Academies report called for greater investment in logistical support for scientists, including all-weather aircraft and heavy icebreakers, noting that the research must be conducted in "a notoriously challenging area to work in, with difficult weather conditions often limiting flight operations".

"This really is the logistics we need in order to be able to answer those questions about the West Antarctic ice sheet," said Columbia's Bell, who co-chaired the report.

Meanwhile, at the Colorado meeting, the British Antarctic Survey's David Vaughan suggested the need for a similar British research investment in studying the Amundsen Sea and Thwaites - a follow up to extensive work the survey just helped to conduct on nearby Pine Island glacier.

"We would like a very high-resolution subglacial topography, to see if there's any major points where this grounding line retreat would hang up for a couple of decades, enough to make a difference," Vaughn said.

Yet another aspect of the required research, meanwhile, will be trying to discover whether Thwaites and other parts of West Antarctica vanished in past warm periods of Earth's history - as researchers suspect - which would give a further hint about their vulnerability. That requires more "paleo-climate" research, in which scientists drill huge cores of ancient ice and collect samples of sediments or date the ages of rocks to try to determine what happened thousands or even hundreds of thousands of years ago.

"There is a clear signal that could be found in ice cores, but we haven't drilled in the right place," says the University of Washington's Steig.

So in sum - scientists are not yet convinced that a disaster is emerging at Thwaites glacier. Further inquiries could still demonstrate new reasons that the glacier might be more stable than some fear, particularly if there are aspects of the topography beneath Thwaites that would in some way slow its retreat.

The researchers aren't stating outright that the glacier poses a threat this century - rather, they want to find out if it does.

"I've been working on this for 38 years," says Penn State's Richard Alley, one of the most outspoken of the WAIS scientists. "And I always believed that we would learn, and we would take useful information and we would give it to policymakers and they would use it. And that we'd do this in time to be really useful."

Now, Alley says he just hopes that's still the case. "It may be that West Antarctica is still going to hang around a while, but the impetus to understand better I think has gotten way stronger."