Scientific American - USA (2020-03)

March 2020, ScientificAmerican.com 23

SOURCE: “DEEP GLACIAL TROUGHS AND STABILIZING RIDGES UNVEILED BENEATH THE MARGINS
OF THE ANTARCTIC ICE SHEET,” BY MATHIEU MORLIGHEM ET AL., IN

NATURE GEOSCIENCE;

DECEMBER 12, 2019

Below

Ross Ice Shelf

Transantarctic

Mountains

EAST

ANTARCTICA

WEST

ANTARCTICA

Below Denman Glacier

Below Recovery

Glacier

Below

Thwaites Glacier

GEOGRAPHY

Mapping the Frozen Continent

A new view of Antarctica’s bedrock could improve predictions about sea-level rise

Where and how fast will Antarctica lose

ice as the climate warms, and how much

will the sea level rise as a result? To answer

these questions, scientists must learn as

much as possible about the vast conti-

nent—despite the challenge of accurately

surveying its topography underneath all

that ice. The contours of Antarctica’s bed-

rock help determine the behavior of glacial

grounding lines, the zones where glacier

ice transitions from resting on ground to

floating on ocean water; if the line moves

inland, a glacier loses more ice. Last

December researchers published an

upgraded bedrock map in Nature

Geoscience, combining mea-

surements from sources that

included airborne radar, satellite, seismic

and snow accumulation data. The team

estimated the topography for gaps

between radar measurements using a

more accurate, physics-based method, and

they found striking differences from older

maps—for instance, prior bedrock eleva-

tion estimates in some areas were up to

2,000 meters off.

The new map reveals good and bad

news about potential ice loss in different

parts of the continent. “There is not much

hope for West Antarctica,” says Mathieu

Morlighem, the study leader and an earth

system scientist at the University of Califor-

nia, Irvine. “But East Antarctica is a mixed

picture.” The study fills in crucial knowl-

edge gaps about the continent’s bedrock

topography, says Jonathan Bamber, a glaci-

ologist at the University of Bristol in Eng-

land, who was not involved in the map

project: “This is going to make our projec-

tions and simulations that much better for

predicting sea-level rise.” — Annie Sneed

The researchers made an astounding discovery under-

neath Denman Glacier: a vast canyon more than

3,500 meters below sea level, marking Earth’s deepest

point on land. Unfortunately, they also found

that the glacier’s underlying

bedrock slopes downward

going inland, which makes

this region “very vulnerable,”

the study says. This kind of

slope means the ice over

the grounding line will be

increasingly thick when

the line recedes inland,

leading to greater ice loss—

and even more ground-

ing-line recession.

This region alone

could contribute

about 1.5 meters

to sea-level rise.

Glaciers carry a significant portion of East Antarctica’s

ice through valleys in the Transantarctic Mountains

and out onto the Ross Ice Shelf, so the bedrock

topography here matters for that region. Thankfully,

the researchers found a large ridge that runs below

each glacier, which will help stabilize the area if the

Ross Ice Shelf collapses.

Previous Antarctica maps indicated that Recovery

Glacier’s underlying bedrock sloped upward going

inland, which would make the glacier less vulnerable

to grounding-line recession and increased ice loss. But

the new map shows it actually slopes in the opposite

direction, which means the glacier is more vulnerable

and may foreshadow a faster recession of the

grounding line. So far the glacier has been steady,

but “this region is a major point of vulnerability

in East Antarctica,” the study authors write.

Thwaites is the fastest-changing

Antarctic glacier, Morlighem says.

Instead of discovering that the

underlying bedrock has many

ridges—which would help slow the

glacier’s loss of mass—researchers found

only two, and they look unlikely to stop the recession

of its grounding line and resulting ice loss.

Elevation

–3,000 meters 4,200 m

Land below

sea level

( blue )

Land above

sea level

( brown )

Current

ice extent

( edge )

Current

grounding line (black)

Boundary where ice goes

from resting on bedrock

to floating on water.

50 kilometers

Map by Katie Peek

Under the Ice

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