National Geographic - Douglas Fox*
The massive iceberg that broke off the Larsen C Ice Shelf may be a harbinger of a continent-wide collapse that would swamp coastal cities around the world.
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A startling sunset reddens the Lemaire Channel, off the west coast of the Antarctic Peninsula. The continent's coastal ice is crumbling as the sea and air around it warm. Photograph Camille Seaman
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Seen from above, the Pine Island Ice Shelf is a slow-motion train
wreck. Its buckled surface is scarred by thousands of large crevasses.
Its edges are shredded by rifts a quarter mile across. In 2015 and 2016 a
225-square-mile chunk of it broke off the end and drifted away on the
Amundsen Sea. The water there has warmed by more than a degree
Fahrenheit over the past few decades, and the rate at which ice is
melting and calving has quadrupled.
On the Antarctic Peninsula, the warming has been far greater—nearly
five degrees on average. That's why a Delaware-size iceberg just broke
off the Larsen C Ice Shelf and why smaller ice shelves on the peninsula
have long since disintegrated entirely into the waters of the Weddell
Sea. But around the Amundsen Sea, a thousand miles to the southwest on
the Pacific coast of Antarctica, the glaciers are far larger and the
stakes far higher. They affect the entire planet.
See the Giant Crack in Larsen C Ice Shelf That Yielded Antarctica Iceberg
The Pine Island Ice Shelf is the floating terminus of the Pine Island
Glacier, one of several large glaciers that empty into the Amundsen
Sea. Together they drain a much larger dome of ice called the West
Antarctic Ice Sheet, which is up to two and a half miles thick and
covers an area twice the size of Texas. The ice sheet is draped over a
series of islands, but most of it rests on the floor of a basin that
dips more than 5,000 feet below sea level. That makes it especially
vulnerable to the warming ocean. If all that vulnerable ice were to
become unmoored, break into pieces, and float away, as researchers
increasingly believe it might, it would raise sea level by roughly 10
feet, drowning coasts around the world.
The ice sheet is held back only by its fringing ice shelves—and those
floating dams, braced against isolated mountains and ridges of rock
around the edges of the basin, are starting to fail. They themselves
don't add much to sea level, because they're already floating in the
water. But as they weaken, the glaciers behind them flow faster to the
sea, and their edges retreat. That's happening now
all around the
Amundsen Sea. The Pine Island Ice Shelf, about 1,300 feet thick over
most of its area, is a dramatic case: It thinned by an average of 150
feet from 1994 to 2012. But even more worrisome is the neighboring
Thwaites Glacier, which could destabilize most of the West Antarctic Ice
Sheet if it collapsed.
"These are the fastest retreating glaciers on the face of the Earth,"
says Eric Rignot, a glaciologist at the NASA Jet Propulsion Laboratory
in Pasadena, California. Rignot has studied the region for more than two
decades, using radar from aircraft and satellites, and he believes the
collapse of the West Antarctic Ice Sheet is only a matter of time. The
question is whether it will take 500 years or fewer than a hundred—and
whether humanity will have time to prepare.
"We have to get these numbers right," Rignot says. "But we have to be careful not to waste too much time doing that."
Getting the predictions right requires measurements that can be made
only by going to the ice. In December 2012 a red-and-white Twin Otter
plane skimmed low over the Pine Island Ice Shelf. The pilot dragged the
plane's skis through the snow, then lifted off and circled back to make
sure he hadn't uncovered any crevasses. After the plane landed, a single
person disembarked. Tethered to the plane by a rope and harness, he
probed the snow with an eight-foot rod.
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A diver watches an emperor penguin as it swims nearby. The brown patches above are microalgae, which cling to sea ice and photosynthesize in the spring. Photograph Laurent Ballesta
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Finally the scout was satisfied: There were no buried crevasses that
might swallow a landing party. More scientists got out of the plane. The
team, led by glaciologist Martin Truffer of the University of Alaska,
proceeded to set up camp. Their plan was to spend two months on the ice
shelf; they would be the first humans to spend even a single night. The
ice had long been considered too dangerous to visit. But Truffer's team
wanted to bore holes all the way through the ice shelf, so they could
measure the heat eating at it from the seawater below.
As the researchers lay in their tents at night, in the middle of a
4,000-mile arc of coastline that lacked a single permanent outpost, they
heard loud pops and bangs coming from the ice. Each morning they saw
new cracks, an inch wide and seemingly bottomless, cutting across its
surface. During their five weeks of studying it, the ice under their
boots thinned by another seven feet.
It took scientists a long time to realize just how quickly
West Antarctica's ice could melt. In part that's because the most
vulnerable glaciers are so well guarded. In front of the Pine Island Ice
Shelf—the floating end of the glacier—the sea surface itself freezes
each winter. In summer this fractured sea ice joins icebergs calved from
the ice shelves to form a shifting palisade that historically kept
ships at least a hundred miles from the ice shelf.
In March 1994 the U.S. icebreaker
Nathaniel B. Palmer became
perhaps only the second vessel ever to reach it. For a few days powerful
winds parted the ice floes, creating a narrow, ephemeral passage for
the
Palmer to thread. With no accurate maps to guide them, the
crew on the ship's bridge eyed the sonar monitor nervously. It showed a
chaotic seafloor of canyons and sharp ridges, including one that rose
within 20 feet of the ship's keel.
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The west side of
the Antarctic Peninsula is warming several times faster than the rest of
the planet. Ninety percent of its 674 glaciers are now in retreat and
are calving more icebergs into the sea, like this one in Andvord Bay.
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The
Palmer would spend just 12 hours at the front of the ice
shelf before encroaching sea ice forced it to retreat north. But that
gave the crew enough time to lower scientific instruments through the
water column. They made a disturbing discovery. Near the surface, a
current was streaming out from under the ice shelf that was slightly
less salty than the sea around it, because it was freshened by melted
ice. (The ice is fresh because it originated as snow falling on West
Antarctica.) And at depths of 2,000 to 3,000 feet, along a seafloor
canyon that ran straight under the ice, warmer seawater was streaming
in.
Stan Jacobs, an oceanographer from the Lamont-Doherty Earth
Observatory in New York, quickly understood what was going on. The warm
water was coming from the South Pacific, more than 200 miles north. It
was so heavy with salt that it was following the floor of a submarine
canyon, which sloped down toward the glacier. The glacier itself had
carved that canyon, thousands of years ago during the Ice Age, when it
and the other glaciers in West Antarctica advanced hundreds of miles out
from their present-day positions.
Now that same canyon was channeling warm ocean water under the Pine
Island Ice Shelf. Somewhere tens of miles inland, the warm water was
finding the "grounding line": the place where the glacier lifts off the
seafloor and becomes a floating ice shelf. Hitting that wall of ice, the
warm water was eroding it, producing a steady stream of melt-laden
seawater. Because it was cooler and fresher, it was less dense, and so
it was rising above the warmer, incoming water and flowing back out to
sea just under the shelf.
By measuring the amount of this freshwater, the researchers could
estimate how much ice was being lost. The melt rates "were just crazy,"
says Adrian Jenkins, a glaciologist from the British Antarctic Survey in
Cambridge. According to his calculations, the ice shelf was losing 13
cubic miles of ice per year from its underside; back near the grounding
line, the ice was probably thinning up to 300 feet per year.
"It was just beyond our concept that a glacier would melt that fast," Jenkins says.
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In East Antarctica, Australian researchers probe for crevasses on Totten Glacier—another one that has begun to look vulnerable—before deploying instruments to measure how fast it's moving and thinning. Photograph by Camille Seaman
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Over the next 13 years he and Jacobs tried three times to return to
Pine Island. Sea ice blocked them each time. When they finally got back
there on the
Palmer in January 2009, they found that the melt
rate had increased by about 50 percent. This time they came equipped
with a new tool: a yellow robotic submarine called
Autosub3.
Shaped like a torpedo and as long as a delivery truck, it could navigate
autonomously under the ice shelf, out of contact with the ship, for up
to 30 hours at a time.
On its first three dives,
Autosub3 discovered that the ice
shelf had thinned enough to lift off a submarine ridge that, running
across its width, had once supported and stabilized the ice shelf. That
had opened a gap that was allowing warm water to flow in and melt the
underside of the ice even faster. On its fourth dive the yellow robot
nearly died. When the crew winched it out of the water, they found its
nose cone smashed and some of its delicate internal equipment damaged.
Technicians reconstructed what had happened from the sub's navigation data. Thirty miles back, under the ice shelf,
Autosub3
had strayed into a chasm on the underside of the ice. Searching for a
way forward, it had smashed and scraped against the walls of the
chasm—ultimately rising 500 feet up into the labyrinthine bowels of the
ice shelf. Finally it had dropped back out and escaped into open water.
The ice shelves, Fricker says, 'are the canary in the coal mine.'
The sub's sonar data, meanwhile, revealed the breathtaking landscape
it had navigated. The bottom of the ice shelf was corrugated with not
just one but many channels, which cut as far as 600 feet up into it. The
walls of these inverted ice canyons were sculpted into terraces,
ledges, and sharp corners, and along the ceiling of each ran a gaping
crack that penetrated even farther into the ice.
"What the hell is going on?" Jenkins recalls thinking when he first saw the sonar maps.
What he and Jacobs came to realize was that the upside-down canyons
had been carved, like rock canyons on land, by flowing water. Apparently
the meltwater rising off the grounding line was still warm enough to
melt more ice. And as it flowed for tens of miles along the underside of
the ice shelf, back out to the open sea, it was melting a lot of it.
Antarctica is melting at a dangerous pace — here's why
Large swaths of West Antarctica are hemorrhaging ice these
days. The warming has been the most dramatic on the Antarctic Peninsula,
a spine of ice-cloaked mountains that reaches 700 miles up toward the
tip of South America. Catching the powerful winds and ocean currents
that swirl endlessly around Antarctica, the peninsula gets slammed with
warm air and water from farther north. Average annual temperatures on
its west side have risen nearly 5 degrees Fahrenheit since 1950—several
times faster than the rest of the planet—and the winters have warmed an
astonishing 9 degrees. Sea ice now forms only four months a year instead
of seven.
Since 1988, four ice shelves on the east side of the peninsula have
disintegrated into armadas of icebergs. (The Larsen C Ice Shelf may one
day do the same, judging from that Delaware-size ice chunk that's about
to break off it.) Warmer air helped trigger these collapses by forming
meltwater ponds on the surface of ice shelves; the ponds drained into
crevasses, wedging them deeper into the ice. As the shelves have
vanished, the glaciers they once stabilized have stampeded into the
ocean, accelerating to two, five, even nine times their original speed.
They're relatively small glaciers and won't raise sea level much—but
their acceleration has reinforced concerns that the same thing might
happen to the much larger glaciers along the Amundsen Sea.
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| Lauren
C. Tierney, NG Staff. Sources: Project MIDAS; Jeremie Mouginot, Bernd
Scheuchl, and Eric Rignot, UC Irvine and Nasa Cryospheric Sciences
Program and MEaSURes; British Antarctic Survey; NASA LIMA |
The Amundsen Sea is farther south than the peninsula, and the air
there is not as warm. The biggest threat to its glaciers is the
mechanism Jacobs and Jenkins helped uncover: deep submarine canyons that
channel warm water from the north under the ice shelves, and deep
inverted canyons that focus the warmth on the underside of the ice.
A satellite survey last year of many Antarctic ice shelves—led by
glaciologists Ted Scambos of the National Snow and Ice Data Center in
Boulder, Colorado, and Helen Fricker of the Scripps Institution of
Oceanography in San Diego—revealed that such melt canyons are common.
They tend to fan out and steer warm water toward the edges of the
shelves. The ice there is crucial: It rubs against the stationary banks
and slows the flow of the shelf and the glacier behind it. But that edge
ice is also thinner than the rest. This "is something that bears
watching," Scambos said in early 2016.
Ian Howat, of the Byrd Polar and Climate Research Center in Columbus,
Ohio, is another glaciologist who's watching Pine Island closely. Last
November he reported two ominous new rifts spreading across the ice
shelf that threaten to prune it to its shortest length in recorded
history. As Howat looked back through monthly satellite photos, he
realized that the rifts had been triggered by a singular event that had
happened, unnoticed, three years before. The strip of torn-up ice
anchoring the ice shelf to its northern bank had suddenly fallen apart,
suggesting it had been undermined by melting from below. It blew out
"just in a matter of days," Howat says, "like a zipper, unzipping the
side of the glacier."
It's unclear when the entire ice shelf might disintegrate. The "warm"
water flowing underneath it from offshore is only 4 to 6 degrees
Fahrenheit above freezing. But roughly 3,000 cubic miles of it arrives
every year, which means the ice shelf is receiving an amount of heat
that exceeds the output of a hundred nuclear power plants, operating
24/7.
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An iceberg's
graceful curves bear witness to the rapid melt it has experienced since
being dumped by a glacier into the Lemaire Channel. Winters
on the west side of the Antarctic Peninsula have warmed by 9 degrees Fahrenheit since 1950.
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When Truffer and his team camped on the shelf in December 2012, they
could sense how it had already weakened. As the meltwater cuts deep into
the ice from below, the unsupported ice sags, causing the entire shelf
to bend and warp. Crevasses erupt along the lines of stress, on both the
top and the bottom of the ice. The pops and bangs the researchers heard
and the daily opening of new cracks bore witness to the ice's gradual
failure as it thinned and broke down beneath them.
As the Pine Island Ice Shelf has weakened and the glacier behind it
has accelerated, the ice has stretched and thinned for 150 miles inland
from the coast. The destabilizing effects spread farther into West
Antarctica every year. "A little nudge can get you to several decades of
retreating behavior that's hard to reverse," Truffer says.
In fact, research by Rignot and others over the past few years
indicates that the collapse of several major glaciers flowing into the
Amundsen Sea is now unstoppable. Between 2002 and 2009 alone, the ice
shelf in front of the Smith Glacier thinned by 1,500 feet in some
places, the one in front of the Pope Glacier by up to 800 feet. The
grounding lines of the Amundsen glaciers have retreated so far—tens of
miles in some cases—that they now rest on seafloor that slopes down
toward the center of the ice sheet. Each increment of retreat exposes a
greater ice surface to warm ocean water. It's a runaway process—and
scientists are urgently trying to figure out how fast it will run.
The ice shelves, Fricker says, "are the canary in the coal mine."
Because they're already floating, they don't raise sea level themselves
when they melt—but they signal that a rise is imminent, as the glaciers
behind them accelerate. Fricker and her team have found that from 1994
to 2012, the amount of ice disappearing from all Antarctic ice shelves,
not just the ones in the Amundsen Sea, increased 12-fold, from six cubic
miles to 74 cubic miles per year. "I think it's time for us scientists
to stop being so cautious" about communicating the risks, she says.
The retreat and hemorrhage of these glaciers "will accelerate over
time," agrees Rignot. "Maybe you don't care much about that for the next
30 to 40 years, but from 2050 to 2100 things could get really bad, and
at that point listening to scientists is irrelevant." Yet after things
get really bad, they could still get worse.
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Iceberg A56,
photographed through clouds from the International Space Station, is
several times the size of Manhattan. It has drifted over 1,000 miles
since breaking off the Filchner-Ronne Ice Shelf around 2000.
TIM PEAKE, ESA/NASA
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Most of the heat trapped by our fossil fuel emissions since
the industrial revolution began in the 19th century has gone into the
ocean. Most of the heat now hitting the Antarctic ice shelves, however,
comes from another effect of climate change: Intensified circumpolar
winds and currents have driven warmer water from offshore onto the
continental shelf and under the floating ice. Much more ocean warming is
yet to come, even if we begin to cut emissions. A lot more heat is on
the way to Antarctica.
Scientists are especially concerned about the Thwaites Glacier, which
by itself could raise global sea level four feet; last fall the British
and American science foundations announced a coordinated $20 million to
$25 million field campaign that will deploy ships, planes, satellites,
and underwater robots to assess the glacier's status starting in 2018.
For now, the best estimates suggest that Antarctica will sweat off
enough ice to raise global sea levels by 1.5 to 3.5 feet by 2100,
depending on how quickly humans continue to pump out greenhouse gases.
Throw in Greenland and other rapidly melting glaciers around the world,
and sea level could plausibly rise three to seven feet by 2100.
But that's not the worst case: Sea level won't stop rising in 2100.
Earth's past offers worrisome clues to what the more distant future
might bring. Geologists studying ancient shorelines have concluded that
125,000 years ago, when the Earth was only slightly warmer than today,
sea levels were 20 to 30 feet higher. Some three million years ago, the
last time atmospheric carbon dioxide was as high as it is today, and the
temperature was about what it's expected to be in 2050, sea levels were
up to 70 feet higher than today. Yet a collapse of the Greenland and
West Antarctic Ice Sheets would raise sea level only about 35 feet.
To consider the worst case, then, scientists must turn their eyes
toward East Antarctica, home to more than three-fourths of all the ice
on Earth.
This past January a twin-propeller DC-3 made a series of flights from
Australia's Casey Station along the East Antarctic coast. Built in
1944, the plane was packed with modern scientific equipment. As it flew
over the Totten Glacier, a radar recorded the thickness of the ice.
Another instrument recorded tiny changes in Earth's gravitational
field—clues to the topography of the seafloor under the glacier's
floating ice shelf. Now and then a crew member opened the plane's rear
door, knelt in the windy opening, and tossed out a torpedo-shaped
object. As the device splashed into the water, it split in two: One part
floated, sending radio signals back to the plane, while the other part
reeled down 2,600 feet of wire, measuring the water temperature all the
way down.
Until recently the East Antarctic Ice Sheet was considered secure;
unlike West Antarctica, it sits on high ground. But mapping with
ice-penetrating radar has revealed a low-lying region cut by glacially
carved channels that drop as far as 8,500 feet below sea level—perfect
for guiding warm ocean water deep into the heart of the ice sheet. The
Totten Glacier is the largest coastal outlet in this region. If it
collapsed, global sea level could rise 13 feet—"roughly as much as all
of West Antarctica," Rignot points out. "One glacier alone."
In January 2015, the Australian icebreaker
Aurora Australis became the first ship to reach the front of Totten. Like the
Palmer
at Pine Island in 1994, it found deep, warm water flowing under the ice
shelf, at a rate of 4.5 cubic miles a day. The glacier is already
losing a couple of cubic miles of ice per year—small potatoes, in
Antarctic terms. But Donald Blankenship, a University of Texas
glaciologist who oversees the aerial survey, fears it could blow up.
In 2016 his team reported evidence from the bedrock that Totten
repeatedly has retreated 100 to 200 miles inland from its current
position—meaning it might help explain why sea level was so much higher
three million years ago. Blankenship's surveys have also identified two
seafloor grooves deep enough to let warm water under Totten's ice shelf.
Last January the team was refining those seafloor maps.
Totten will lose its ice more slowly than West Antarctica. The worst
case coming out of Antarctica still seems to be centuries away. But it
would mean abandoning many of the world's largest cities, including New
York, Los Angeles, Copenhagen, Shanghai, and dozens of others—and it's
looking less crazy all the time. "The fuse is lit," says Blankenship.
"We're just running around mapping where all the bombs are."
Writer
Douglas Fox has traveled to Antarctica five times and has spent months on the ice there. This is his first feature for
National Geographic magazine.
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