Environment
Antarctica's
Totten Glacier has become 'dangerously unstable'
Research
suggests we are slowly awakening a process that, in the past, has
utterly transformed one of the biggest ice sources on Earth
Chris Mooney,
Washington Post
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 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.
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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.
totten-graphic.jpg
(Nature
Geoscience/Washington 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.
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“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.
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"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.
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