May 18, 2016 at 1:01 PM
Scientists ringing alarm bells about the melting of Antarctica have focused most of their attention, so far, on the smaller West Antarctic ice sheet, which is grounded deep below sea level and highly exposed to the influence of warming seas. But new research published in the journal Nature Wednesday reaffirms that there's a possibly even bigger — if slower moving — threat in the much larger ice mass of East Antarctica.
The Totten Glacier holds back more ice than any other in East Antarctica, which is itself the biggest ice mass in the world by far. Totten, which lies due south of Western Australia, currently reaches the ocean in the form of a floating shelf of ice that's 90 miles by 22 miles in area. But the entire region, or what scientists call a "catchment," that could someday flow into the sea in this area is over 200,000 square miles in size — bigger than California.
Moreover, in some areas that ice is close to 2.5 miles thick, with over a mile of that vertical extent reaching below the surface of the ocean. It's the very definition of vast.
Warmer waters in this area could, therefore, ultimately be even more damaging than what's happening in West Antarctica — and the total amount of ice that could someday be lost would raise sea levels by as much as 13 feet.
"This is not the first part of East Antarctica that's likely to show a multi-meter response to climate change," said Alan Aitken, the new study's lead author and a researcher with the University of Western Australia in Perth. "But it might be the biggest in the end, because it's continually unstable as you go towards the interior of the continent."
The research — which found that Totten Glacier, and the ice system of which it is part, has retreated many times in the past and contains several key zones of instability — was conducted in collaboration with a team of international scientists from the United States, Australia, New Zealand and the United Kingdom. A press statement about the study from the U.S. group, based at the University of Texas at Austin, described the study as showing that "vast regions of the Totten Glacier in East Antarctica are fundamentally unstable."
Indeed, the Totten Glacier watch has been ramping up lately: Scientists have already documented that warm ocean waters can reach the glacier's base and that the enormous ice shelf that currently stabilizes it, extending over the top of the ocean, is melting from below. The glacier is thinning quickly, and its grounding line, where the ice shelf descends and meets the seafloor, has retreated inland three kilometers between 1996 and 2013 in some areas.
Finally, recent research has suggested that Totten can only lose a tiny 4.2 percent of its remaining ice shelf before the structure starts losing the ability to brace the larger glacier, holding it in place. It all points to a region of enormous vulnerability, and one that is already undergoing change.
"In a warming world, West Antarctica and regions of East Antarctica that are below sea level will be the most likely to change," says Robin Bell, an Antarctic expert with the Lamont-Doherty Earth Observatory at Columbia University, who reviewed the new study for The Post.
In the new research, researchers took aircraft-based measurements across the vastness of Totten Glacier, and the extremely deep and thick ice canyons behind it — which scientists call "subglacial basins" — in order to understand a critical yet invisible feature: precisely what the layers of rock beneath the ice are really like.
This, in turn, provides a clue to the behavior of this region in past warm eras. When marine-based glaciers move back and forth across a seascape repeatedly, they grind against the seafloor and dig up piles of looser sediment, such as sandstone, depositing them in a new location. But when glaciers move more quickly, sediment beneath them remains more undisturbed.
The radar, magnetic and gravity measurements conducted in the study found key regions where Totten Glacier and the connected systems of ice behind it lie atop plenty of sediment — suggesting the glacier retreats rapidly in these areas. But it also detected areas where there isn't much sediment at all, suggesting that it grinds away in these locations a great deal, or in the words of UT-Austin's Jamin Greenbaum, one of the researchers behind the study, is able to "ping-pong back and forth" over long time periods.
The gist is that while Totten may be in a relatively stable configuration now, if it retreats far enough, then it can start an unstable backslide into deep undersea basins and unload a great deal of ice, raising seas first by close to a meter and then considerably more than that.
"There's multiple stages. But at each stage, we see a bigger contribution to sea level rise and a bigger proportion of contribution to sea level rise from this system. This system keeps going, and its role keeps increasing, as we get to bigger and bigger amounts of sea level rise," Aitken says.
Scientists believe that Totten Glacier has collapsed, and ice has retreated deep into the inland Sabrina and Aurora subglacial basins, numerous times since the original formation of the Antarctic ice sheet over 30 million years ago. In particular, they believe one of these retreats could have happened during the middle Pliocene epoch, some 3 million years ago, when seas are believed to have been 10 or more meters higher (over 30 feet) than they are now.
"This paper presents solid evidence that there has been rapid retreat here in the past, in fact, throughout the history of the ice sheet," Greenbaum says. "And because of that, we can say it's likely to happen again in the future, and there will be substantial sea level implications if it happens again."
That said, the research suggests that the Totten system presents a very complex landscape and that the ice will have to surmount numerous different hurdles before all of it is able to empty into the ocean.
First, there's the current ice shelf and a marine-based area that extends back about 150 kilometers inland. Much of this area is very deep — with ice grounded over a kilometer below sea level — but it also shallows somewhat as you move inland, rising into a ridge whose peak is often just 200 meters below sea level, though it also contains much deeper channels.
The good news is that retreating up a ridge is harder for ice to do. The bad news is that the retreat has already begun in this area. If the ice sheet manages to retreat back over this region — which it could do if it loses its ice shelf entirely, Aitken says — then seas could increase 90 centimeters, or close to a meter.
Assuming Totten clears this region, though, 90 centimeters would be just the beginning. After that, there is a large area where it seems that the ice sheet can retreat very rapidly. That's because there is a plunge downhill in this area — dubbed the Sabrina Subglacial Basin — and the ice is not stable. Traversing it would raise the total to more than 2 meters of sea level rise, as the ice would retreat backward several hundred more kilometers.
"Once you're over that hill, you get this big runaway retreat into the interior of the continent, which gives you a very substantial amount of sea level rise," Aitken says.
Then, at the back of this area, the ice sheet would take on a new and more stable configuration again, says Martin Siegert, a glaciologist with Imperial College London and one of the study's authors. It would feature relatively shallow and stable areas cut into by deep fjords, or subsea valleys, that are as much as 1,000 meters deep. In these vulnerable fjords is where ice loss would occur, just as it does today at Greenland glaciers like Jakobshavn and Helheim.
"When the ice sheet starts to retreat, it will go back quickly toward those fjords there," Siegert says. "So it will go back to a Greenland style type ice sheet."
Once again, this area would prove harder for the ice to move across. But if it does, then all bets are off — the ice front would plunge down into the extremely deep Aurora Subglacial Basin, and total sea level rise could reach 4 meters, or over 13 feet (on top of major contributions from other parts of Antarctica, which would also surely have retreated at this point).
In light of this research, the key questions become: How rapidly Totten can pull off these various retreats? And how much warming of the atmosphere, and the ocean, would be required to push it into a motion even greater than what it has seen so far?
That's not easy to answer — different models retreat the Totten system at different speeds, in response to more and less warming. The current study uses an ice sheet model that the authors admit is conservative, and it takes thousands of years for the most extensive changes to happen — and warming well over the 2 degree threshold that the world has set for itself in international climate negotiations.
However, recent research has sped up estimates for ice loss from Antarctica by adding new melt and collapse processes to ice sheet models and is capable of producing a faster retreat of the Totten region.
In a study published two months ago in Nature by Rob DeConto of the University of Massachusetts at Amherst and his colleague David Pollard of Penn State University, the Totten region does retreat under moderate and especially high emissions scenarios, and it does so in the next 500 years. In a high emissions scenario, ice flows out of the deep Aurora basin in this study, and contributes to catastrophic global sea level rise.
"In recent years we have been very focused on the vulnerability of the marine-based West Antarctic Ice Sheet, but this is a good reminder that marine-based sectors of the much larger East Antarctic Ice Sheet could also be highly sensitive to a warming ocean and atmosphere," DeConto says of the new study.
So the big question about Antarctica remains just how fast it can change — but the new research suggests we are slowly awakening a process that, in the past, has utterly transformed one of the biggest ice sources on Earth.