Washington Post - Chris Mooney
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| A Conductivity-Temperature-Depth profiler being
deployed near the front of the Totten Glacier. (Credit: Steve Rintoul
CSIRO and ACE CRC) |
Scientists at institutions in the United States and Australia on Friday
published
a set of unprecedented ocean observations near the largest glacier of
the largest ice sheet in the world: Totten glacier, East Antarctica. And
the result was a troubling confirmation of what scientists already
feared — Totten is melting from below.
The measurements, sampling
ocean temperatures in seas over a kilometer (0.62 miles) deep in some
places right at the edge of Totten glacier’s floating ice shelf,
affirmed that warm ocean water is flowing in towards the glacier at the
rate of 220,000 cubic meters per second.
These waters, the paper
asserts, are causing the ice shelf to lose between 63 and 80 billion
tons of its mass to the ocean per year, and to lose about 10 meters (32
feet) of thickness annually, a reduction that has been previously noted
based on satellite measurements.
This matters because more of
East Antarctica flows out towards the sea through the Totten
glacier region than for any other glacier in the entirety of the East
Antarctic ice sheet. Its entire “catchment,” or the region of ice that
slowly flows outward through Totten glacier and its ice shelf, is
larger than California. If all of this ice were to end up in the ocean somehow, seas would raise by about 11.5 feet.
“This
ice shelf is thinning, and it’s thinning because the ocean is
delivering warm water to the ice shelf, just like in West Antarctica,”
said Don Blankenship, a glaciologist at the University of Texas at
Austin and one of the study’s co-authors. Blankenship was not on the
research vessel, but he and his colleagues helped the
Australia-based researchers with understanding the contours of the
seafloor so they could plan their field investigations into where warm
and deep waters could penetrate.
The lead author of the research,
published Friday in Science Advances, was Stephen Rintoul, a researcher
with the University of Tasmania in Hobart and Australia’s Commonwealth
Scientific and Industrial Research Organization, or CSIRO. Totten
glacier is, more or less, due south of Australia and relatively close to
one of Australia’s bases of operations on the ice continent, Casey
Station.
Rintoul and his colleagues, on board the government vessel
Aurora Australis,
were able to navigate extremely close to the Totten ice shelf edge in
January of 2015, when an opening in the sea ice allowed the ship to get
in closer than one ever has before. This is how they were able to gather
the required ocean observations — and to detect the warm water.
A view of the Totten Glacier from RSV Aurora Australis in January 2015.
(Australian Antarctic Division (Photo: Paul Brown, Australian Maritime College))
The researchers took ocean measurements at 10 separate points along
the floating Totten ice shelf. And at two of the stations, they found
that the ocean underneath was extremely deep. There was a six-mile-wide
canyon at a depth of 600 meters (nearly 2000 feet) that then branched
into two narrower canyons, each reaching greater depths. One of them was
over 800 meters deep (more than 2,500 feet) the other was 1,097 meters
deep (3,600 feet). Each was about one to two miles wide.
It was
in these deep undersea canyons, and a few shallower areas as well, that
warm ocean water, called modified circumpolar deep water, was flowing
inward powerfully towards Totten glacier. And the previously measured
loss of ice from the ice shelf matched closely with the amount of heat
that the ocean was delivering, the paper found.
Granted, calling
the waters reaching Totten at great depths “warm” is a bit of a misnomer
—they are slightly below the freezing point. However, at the extreme
pressures and depths involved, the freezing point of ice itself lowers,
making these waters more than warm enough to melt ice.
Measuring
the warm water reaching Totten was, until now, a missing puzzle piece in
determining what’s happening with the glacier. Prior research, for
instance, had
shown the presence of cavities that warm water
could enter,
and scientists believed this was occurring because they had observed
Totten thinning and lowering in the water. But as NASA glaciologist Eric
Rignot put it to the Post at the time, “it is one thing to find
potential pathways for warm water to intrude the cavity, it is another
to show that this is actually happening.”
Now, scientists are showing that it’s actually happening.
The
researchers are interested in Totten not only because of the massive
global consequences were it to be destabilized, but also because it
could help solve a riddle from the Earth’s past. Researchers have
calculated that
during previous warm eras,
such as during the Pliocene, about 3 million years ago, global
temperatures not too much higher than those that exist today led to
radical amounts of sea level rise. It’s too much of an ocean surge for
the loss of West Antarctica, alone, to explain — so they’ve been going
looking to East Antarctica to close the sea-level budget from those
eras.
And
it turns out that like West Antarctica, East Antarctica features
several regions — including Totten — where massive amounts of ice rise
above the ocean level, but are grounded deep below it. In the case of
Totten glacier, its so-called “grounding line,” which is where the
glacier begins to lift off the seafloor and to float, forming an ice
shelf with an ocean cavity beneath it, is nearly a mile and a half deep.
Granted,
none of this means that Totten is contributing much to sea-level rise —
yet. The large loss of ice from the ice shelf doesn’t raise seas
because that ice is already afloat. But the weakening of the ice shelf
is troubling because the shelf holds back Totten’s more dangerous ice,
and when it goes it will allow that ice to flow more easily into the
ocean.
For Blankenship, the new study, combined with past
aircraft and satellite research on Totten, puts the remaining piece in
place and suggests an increasingly clear picture of ocean-driven melt
that could lead to growing instability.
“The whole process is
here and going on,” he says. “This is the biggest potential contributor
in East Antarctica. It needs to be understood.”
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