44 Scientific American, February 2019KONRAD STEFFEN, CIRES AND UNIVERSITY OF COLORADOTERRIBLE BEAUTY
AN ICE SHEET’S FLOW depends on how strong the mound is, how
well lubricated it is underneath on land, and whether or not it is
held back by a spatula—an attached, floating ice shelf. General at-
mospheric warming can soften ice and thaw the places where the
ice bottom is frozen to the rock below, allowing the ice to slide
faster toward the sea. But the heat takes a long time to be con-
ducted through two-mile piles. The big ice sheets have not fin-
ished warming from the rising air temperatures that ended the
most recent ice age more than 10,000 years ago!
A speedier way to warm the ice and its bed is for water melting
on top to pour down into crevasses. In some places on the flanks
of Greenland’s ice, meltwater in summer collects in large hollows
on the surface, forming big, beautiful blue lakes. The water, being
denser than ice, tends to wedge open crevasses that can reach the
bed at the bottom and drain the lake. An
expanding lake can break through half
a mile of ice or more, creating a flow of
water greater than Niagara Falls. In an
hour, that can warm the bed as much as
would have occurred over 10,000 years.
This process is important, and we are
studying it eagerly. But it is not the great-
est worry for people on Earth’s coasts,
because the bumpy bed can also keep the
ice from speeding toward the sea.
The same mechanism presents a
stronger threat if it happens on an ice
shelf. In very cold places, the ice flowing
into the ocean remains attached but
floating. These ice shelves almost always
occur in protected bays or fjords. The
motion of ice shelves is slowed by friction
along the shorelines around them and
perhaps with upward protrusions from
the seafloor, where the ice locally runs
aground. The shelf slows the flow of the
nonfloating ice on land toward the sea.
Warming air can create lakes on top
of the ice shelves. When the lakes break through crevasses, a shelf
can fall apart. For example, the Larsen B Ice Shelf in the Antarctic
Peninsula, north of Thwaites, disintegrated almost completely in
a mere five weeks in 2002, with icebergs breaking off and top-
pling like dominoes. That did not immediately raise sea level—the
shelf was floating already—but the loss of the shelf allowed the ice
sheet on land behind it to flow faster into the ocean—like pulling
a spatula away, allowing the batter to run. The ice flowed as much
as six to eight times quicker than it had been moving earlier. For-
tunately, there was not a lot of ice behind the Larsen B Ice Shelf in
the narrow Antarctic Peninsula, so it has raised sea level only a lit-
tle. But the event put society on notice that ice shelves can disinte-
grate quickly, releasing the glaciers they had been holding back.
Ice shelves can also be melted from below by warming seawater,
as happened to Jakobshavn.
When shelves are lost, icebergs calve directly from ice-sheet
cliffs that face the sea. Although this delights passengers on cruise
ships in Alaska and elsewhere, it speeds up the ice sheet’s demise.
At Jakobshavn today, the icebergs calve from a cliff that towers
more than 300 feet above the ocean’s edge—a 30-story building—
and extends about nine times that much below the water. As
these icebergs roll over, they make splashes 50 stories high and
earthquakes that can be monitored from the U.S.
So far ice-shelf loss and ice-cliff calving are contributing mod-
erately to sea-level rise. But at Thwaites, this process could make
the rise much more dramatic because a geologic accident has
placed the glacier near a “tipping point” into the great Bentley
Subglacial Trench.JUMP THE BUMP
ON AN AUTUMN MORNING in 1956, Charles Bentley (who years later
would be my Ph.D. adviser) defended his thesis at Columbia Uni-
versity. The next day he hopped a train to Panama, then caught a
ship heading south, to be part of the International Geophysical
Year research project that would analyze planet Earth. He spent
two years in West Antarctica before re-
turning to find that he had not graduat-
ed yet, because his thesis fee had not
been paid. In the meantime, he and his
team traversed more than 3,000 miles
of ice, to and from the Byrd Station re-
search base and across vast reaches of
West Antarctica. (Bentley died at age 87
in 2017.)
Of the many measurements and dis-
coveries they made, the most important
for our story involved the ice thickness.
They set off small explosions on the sur-
face and used seismometers to listen to
sound traveling through the ice sheet
and bouncing back off the bed. These
data showed that West Antarctica was
not a thin drape of ice overlying a high
continent, as some had expected. Instead
Bentley and his team found very thick ice,
and they discovered the Bentley Subgla-
cial Trench. There the bed plunges more
than a mile and a half below sea level—
Earth’s deepest place not under an ocean.
And the ice filling it extends more than a mile above sea level.
Bentley and glaciologists who followed him had found a tip-
ping point. The great trench and adjacent basins underlie the vast
center of the West Antarctic Ice Sheet. If the front edge of
Thwaites retreated from the coast back into the trench, it could
make an ice face thousands of feet high, extending from far above
the trench to deep down into it. Such a cliff—much bigger than at
Jakobshavn or anywhere else on Earth—could break fast, making
incredibly tall icebergs that would roll over and float away
through the trench outlet to the ocean, raising sea level a lot.
Decades of additional research have established just how im-
portant this mechanism is. John Anderson, who recently retired
after 43 years at Rice University, and many of his graduate stu-
dents tirelessly mapped the continental shelf under the ocean
around Antarctica, using side-scan sonar and other tools. During
ice ages, Antarctic ice spread many miles farther in all directions
and withdrew as ice ages ended. The seafloor around Antarctica
today was the bed under the ice sheet in the past. Telltale imprints
left in seafloor sediments give us accurate stories about ice sheets.
One story is that as expanding ice sheets push forward into theMELTWATER pours into the Greenland
ice sheet, hastening its slump toward the
sea—a sign of things to come in Antarctica.© 2019 Scientific American