Nature - Jane Qiu
The massive East Antarctic Ice Sheet looks stable from above — but it's a dangerously different story below.
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| A research team probes for crevasses on Totten glacier, the largest in East Antarctica. Hayden Henderson/Helires/AAD |
On a glorious January morning in 2015, the Australian icebreaker RSV
Aurora Australis
was losing a battle off the coast of East Antarctica. For days, the
ship had been trying to push through heavy sea ice. It rammed into the
pack, backed up and crashed forward again. But the ice, several metres
thick, hardly budged. Stephen Rintoul, an oceanographer at the
University of Tasmania in Hobart, Australia, nearly gave up his goal —
to reach a part of the continent that had thwarted all previous
expeditions. "I really thought that would be it," he says. "It'd be
another failed attempt."
Then the weather came
to the rescue, with a wind change that blew the ice away from the
shore, opening a path through the pack. The ship managed to break free
and wove its way through 100 kilometres of ice, reaching the edge of the
frozen continent shortly after midnight. Rintoul and his team were the
first scientists to reach the Totten Ice Shelf — a vast floating ice
ledge that fronts the largest glacier in East Antarctica. "It was a
really exhilarating experience," says Rintoul, chief scientist of the
expedition.
The team had to work fast before
the ice closed again and blocked any escape. For more than 12 hours,
Rintoul and his colleagues carried on non-stop, probing the temperature
and salinity of the water, the speed and direction of ocean currents as
well as the shape and depth of the ocean floor. They also deployed
instruments that would continue taking measurements after the ship had
departed.
These first direct observations confirmed a fear that researchers had long harboured: that
warm waters from the surrounding ocean can sneak underneath the floating glacier tongue and eat the ice away from below
1. "This could explain why Totten has been thinning in the past few decades," says Rintoul.
Findings such as these are revealing
some scary truths about East Antarctica — the vast, remote landmass to
the east of the Transantarctic Mountains (see
'Ice king').
This region is about as big as the entire United States and the
majority of it stands on a high plateau up to 4,093 metres above sea
level, where temperatures can plunge to −95 °C. Because the East
Antarctic Ice Sheet seems so cold and isolated, researchers thought that
it had been stable in the past and was unlikely to change in the future
— a stark contrast to the much smaller West Antarctic Ice Sheet, which
has raised alarms because
many of its glaciers are rapidly retreating.
In
the past few years, however, "almost everything we thought we knew
about East Antarctica has turned out to be wrong", says Tas van Ommen, a
glaciologist at the Australian Antarctic Division in Kingston, near
Hobart. By flying across the continent on planes with instruments that
probe beneath the ice, his team found that a large fraction of East
Antarctica is well below sea level, which makes it more vulnerable to
the warming ocean than previously thought. The researchers also
uncovered clues that the massive Totten glacier, which holds about as
much ice as West Antarctica, has repeatedly shrunk and grown in the past
2 — another sign that it could retreat in the future.
Although
East Antarctica doesn't seem to be losing much ice today,
there are indications that it is feeling the heat of climate change and
is responding in measurable ways. This is disconcerting because its ice
sheet is more than ten times bigger than the one in the west. If all
the ice below sea level in East Antarctica were to disappear, the height
of the world's oceans would swell by nearly 20 metres.
Researchers
are now trying to gather as much information as possible about East
Antarctica to better predict what's to come. Their concern is that over
the next few centuries, the ice sheet there might reach a tipping point.
"Once glaciers retreat beyond a certain point, things may go downhill
very quickly and cause rapid sea level rise," says Eric Rignot, a
glaciologist at the University of California, Irvine. "We don't want to
sleepwalk into a calamity like this."
Deep danger
Rignot was one of the first
scientists to warn about possible trouble in East Antarctica — a region
long neglected by climate researchers. In 2013, his team detailed the
behaviour of ice around the margin of Antarctica by combining satellite
imagery, airborne surveys and climate models. The researchers found
evidence that six East Antarctic ice shelves, including Totten, were
melting from below at rates much higher than expected — with some even
rivalling those of fast-retreating glaciers in West Antarctica
3.
More
surprises emerged when the researchers took a closer look at some of
those East Antarctic glaciers. Satellite imagery and airborne surveys
between 1996 and 2013 showed that the surface of the Totten glacier
dropped by 12 metres and that its grounding line — the point at which
the ice flowing off the continent begins to float on the ocean —
retreated inland by a shocking amount of up to 3 kilometres
4.
"This
is not an isolated incidence," says Chris Stokes, a glaciologist at
Durham University, UK. His team analysed satellite imagery obtained
between 1974 and 2012 that covers all the coastal regions in East
Antarctica. Most areas had no net ice gain or loss. The only exception
is the Wilkes Land region — an area larger than Greenland that includes
Totten glacier
5.
Three-quarters of the glaciers there retreated between 2000 and 2012.
"Wilkes Land may be East Antarctica's weak underbelly," says Stokes.
As researchers were pondering the surprising retreat of East
Antarctic glaciers, van Ommen and his colleagues were flying over Totten
to probe its underside. "The landscape underneath the ice is
fundamentally important for how glaciers flow and how they respond to
climate change," he says. When the team launched an international
initiative called ICECAP (International Collaboration for Exploration of
the Cryosphere through Aerogeophysical Profiling) a decade ago to
systematically survey the hidden landscape of East Antarctica, "we
almost knew nothing about what's going on down there", he says.
Every
Antarctic summer since then, ICECAP's aircraft have been criss-crossing
the vast continent to peer through the ice using radar as well as
gravitational and magnetic sensors. "They are the best flights in the
world," says Martin Siegert, a glaciologist at Imperial College London
and a principal investigator of the project. The seemingly featureless
ice sheet is ever-changing — with wind-sculpted snow dunes and ice
shimmering in thousands of shades under the unearthly Antarctic light.
"It's just like another planet," he says.
The
flights have revealed an astoundingly dramatic landscape hidden beneath
the relatively flat ice sheet. Preliminary results from airborne surveys
this January, led by glaciologist Sun Bo at the Polar Research
Institute of China in Shanghai, confirmed the existence of a
1,100-kilometre-long canyon — the longest in the world, and almost as
deep as the Grand Canyon in the United States. In previous flights over
Wilkes Land, van Ommen's team discovered that 21% of the Totten glacier
catchment is more than 1 kilometre below sea level — an area 100 times
larger than previous estimates. "We really didn't expect it to be as
extensive as it has turned out to be," says Donald Blankenship, a
geophysicist at the University of Texas at Austin and another ICECAP
principal investigator.
The team also found
underwater troughs that extend all the way from the edge of the Totten
Ice Shelf to the grounding line 125 kilometres inland, and as deep as
2.7 kilometres below sea level
6. This deeply contoured landscape could allow warming waters from offshore to quickly reach and erode the ice.
"Almost everything we thought we knew about east antarctica has turned out to be wrong."
The first chance to study the fate of that water came when the RSV
Aurora Australis
reached Totten in 2015. Near the glacier tongue, Rintoul and his
colleagues detected waters as warm as 0.3 °C — much warmer than the −2
°C freezing point of sea water
1.
"They are driving rapid rates of melt," says Rintoul. The instruments
he and his team left behind show that warm waters are present all year
round. If these waters follow the recently discovered channel beneath
Totten to the grounding line, they will be at least 3.2 °C warmer than
the freezing point at that depth. "That would be really bad news," he
says.
Threats to ice shelves could also come
from the Antarctic interior — from lakes under the ice sheet that
periodically send flood waters towards the coast. A decade ago, Lake
Cook beneath the ice sheet in Wilkes Land suddenly drained, gushing 5.2
billion cubic metres of flood water — the largest event of this type
ever reported in Antarctica. Such floods could be another destabilizing
factor, causing faster ice flow and more iceberg calving, says Leigh
Stearns, a glaciologist at the University of Kansas in Lawrence.
Troubled past
These
scenarios are not just hypothetical, say researchers. Studies in the
past few years have revealed that East Antarctica has lost a lot of ice
in the past, and could do so again in the near future.
Some
of the evidence for that comes from a 2010 expedition supported by the
Integrated Ocean Drilling Program, which retrieved sediments from the
sea floor off the coast of East Antarctica. Getting those sediments was a
dangerous endeavour. The ship had to repeatedly stop drilling and dodge
massive icebergs. "The waters around Antarctica present some of the
most challenging environments for ocean drilling," says Tina van de
Flierdt, a geochemist at Imperial College London and a principal
investigator of the expedition.
The efforts
paid off, however, by revealing surprising changes in the ice sheet's
history. "We had long thought when the East Antarctic Ice Sheet grew to
the current size about 14 million years ago, it's the end of the story,"
says van de Flierdt. "It's this big stable block of ice that isn't
really doing anything in the face of climate change."
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| In 2015, the icebreaker RSV Aurora Australis transported the first scientists to the Totten Ice Shelf. Paul Brown/AAD |
Instead, the sea-floor sediments revealed that the ice sheet waxed
and waned many times between 5.3 million and 3.3 million years ago
7
— an epoch called the Pliocene, when air temperatures were up to 2 °C
higher than today. "We got a clear signal every time it was warm,
suggesting that the ice sheet was sensitive to climate warming," says
van de Flierdt.
The researchers say that they
have some intriguing preliminary results from the most recent
interglacial period, between 129,000 and 116,000 years ago — when the
globe was as warm as it is today. The ice sheet retreated just slightly
less at that time than it did during the much warmer Pliocene. "That's a
big surprise," says van de Flierdt.
"If the
results prove to be robust, I'd say it's really interesting," says
Maureen Raymo, a geochemist at the Lamont-Doherty Earth Observatory in
Palisades, New York. "This may mean that you can lose a certain amount
of ice quite easily with a little bit of warming," she says.
Fast forward
As
East Antarctica's vulnerability comes into focus, researchers are
increasingly concerned about the future. The only way to forecast
decades or centuries ahead is to use computer models that simulate how
ice sheets respond to a changing climate. But the models are relatively
simplistic, and until recently they couldn't accurately reproduce some
past events, such as the significant glacial retreats that scientists
have been discovering in East Antarctica's history.
Climate
researchers Robert DeConto of the University of Massachusetts in
Amherst and David Pollard of Pennsylvania State University in University
Park have tried to make the simulations more realistic by factoring in
some processes that were left out of earlier studies. Their model allows
meltwater on the ice surface to deepen crevasses and splinter the ice
shelves, and it simulates how ice cliffs collapse once they lose the ice
shelves that buttress them.
When DeConto and
Pollard included these processes, their model showed East Antarctica's
glaciers retreating substantially during the last interglacial period
and in the Pliocene
8.
"It's really the first successful attempt to roughly match ice-sheet
simulations with our best understanding of past glacier retreat and
sea-level rise," says van Ommen.
After looking
back in time, the researchers turned their model to the future. There,
they saw a mix of good and bad news. In their simulations, the entire
Antarctic Ice Sheet does not change much in the next 500 years if global
warming is limited to less than about 1.6 °C above pre-industrial
levels by the end of the century — roughly in line with what the Paris
climate agreement aims to achieve.
But if
temperatures rise more than about 2.5°C above pre-industrial levels by
2100 and continue climbing, Antarctic ice melt will raise ocean levels
by 5 metres by 2500 (ref.
8),
with nearly half of that coming from East Antarctica. With Greenland
ice also melting, the global sea level would rise by at least 7 metres —
enough to inundate large parts of major coastal cities such as Mumbai,
Shanghai, Vancouver and New York. "This would drastically reshape the
world's coastline and affect millions," says DeConto.
He
cautions that the model is still rather crude — mainly because
observations of East Antarctica are so limited. "Most of the coastline
is simply unmapped," he says.
The lack of data
has also resulted in extremely poor ocean models that grossly
underestimate the amount of warm water reaching the ice shelves, says
DeConto. "This really calls for long-term monitoring of ocean
conditions."
In East Antarctica now,
temperatures are dropping rapidly as the austral winter sets in;
researchers are cosy at home reviewing the latest haul of data from the
field season. A priority for the future is to map the bedrock beneath
all major ice shelves. That will help researchers to identify other
glaciers that might be eaten away by warm ocean waters, and to predict
how the interior might respond once the ice on the coastal margins
disappears.
One of the scariest finds would be
large valleys in the continental interior that get deeper as they slope
towards the ocean. These could destabilize large sections of East
Antarctica's glacial cap when its margins start to disintegrate over the
coming decades and centuries. "Then the entire ice sheet could slide
off easily," says Blankenship. "There would be nothing to hold it back."
References
-
Rintoul, S. R. et al. Sci. Adv. 2, e1601610 (2016).
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Aitken, A. R. A. et al. Nature 533, 385–389 (2016).
-
Rignot, E., Jacobs, S., Mouginot, J. & Scheuchl, B. Science 341, 266–270 (2013).
-
Li, X., Rignot, E., Morlighem, M., Mouginot, J. & Scheuchl, B. Geophys. Res. Lett. 42, 8049–8056 (2015).
-
Miles, B. W. J., Stokes, C. R. & Jamieson, S. S. R. Sci. Adv. 2, e1501350 (2016).
-
Greenbaum, J. S. et al. Nature Geosci. 8, 294–298 (2015).
-
Cook, S. P. et al. Nature Geosci. 6, 765–769 (2013).
-
DeConto, R. M. & Pollard, D. Nature 531, 591–597 (2016).
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