Science - USA (2020-03-20)

REVIEW

History, mass loss, structure, and dynamic

behavior of the Antarctic Ice Sheet

Robin E. Bell^1 *and Helene Seroussi^2

Antarctica contains most of Earth’s fresh water stored in two large ice sheets. The more stable East Antarctic

Ice Sheet is larger and older, rests on higher topography, and hides entire mountain ranges and ancient

lakes. The less stable West Antarctic Ice Sheet is smaller and younger and was formed on what was once a

shallow sea. Recent observations made with several independent satellite measurements demonstrate that

several regions of Antarctica are losing mass, flowing faster, and retreating where ice is exposed to warm ocean

waters. The Antarctic contribution to sea level rise has reached ~8 millimeters since 1992. In the future, if

warming ocean waters and increased surface meltwater trigger faster ice flow, sea level rise will accelerate.

T

wo hundred years ago, an expedition led

by Fabian Gottlieb von Bellingshausen

and Mikhail Lazarev discovered main-

land Antarctica, the most remote and

inhospitable continent. Today, Antarctic

is an ice-covered continent where change is

emblematic of the impacts humans have on

the global climate. Hidden beneath the ice

sheets are a rich diversity of terrains and hy-

drologic systems of mountains, lakes, and dy-

namic subglacial water networks ( 1 , 2 ). The

changes we are now witnessing ( 3 – 5 ) are con-

centrated in the low-elevation regions as well

as the Antarctic Peninsula, the furthest north

part of the continent. Evidence of change comes

from satellite measurements of ice mass, velocity

( 4 ), and elevations ( 3 ). Large floating ice shelves

have disintegrated ( 6 ), and the location where

the ice goes afloat is moving inland ( 7 ). Future

vulnerabilities arise from interactions with the

warming ocean, melting of the ice surface, and

the disappearance of the ice shelves. Looking

forward, coastal communities around the globe

need to know how much sea level will rise, and

how much Antarctica will change is one of the

greatest unknowns. Both improved models

and observations are essential to improve the

scientific community’s response to the question

of how much sea level will rise over the coming

decades and centuries.

Origin and history

Over the past 100 million years or more, Ant-

arctica shifted from a green tree-covered con-

tinent ( 8 ) to a continent encased in ice as it

became tectonically isolated while the global

climate cooled ( 9 ). The tectonic isolation of the

continent began more than 200 million years

ago, when it was at the center of the Gondwana

supercontinent with a climate similar to that

of modern New Zealand. The supercontinent

breakup occurred slowly, first with Africa

(170 million years ago), then India (145 million

years ago), and last, Australia (90 million years

ago), shifting away from what today is Antarctica

as the Southern Ocean began to form. The fi-

nal step occurred 34 million years ago as the

Drake Passage ( 10 ), between South America and

the Antarctic Peninsula, and the Tasmanian

Gateway, south of Australia, opened. The glob-

al oceans thereby were effectively linked. The

Antarctic Circumpolar Current, the strongest

ocean current on the planet, began to circulate

around the continent, and Antarctica was iso-

lated. As this tectonic isolation occurred, global

temperatures and the concentration of atmo-

spheric carbon dioxide (CO 2 ) began dropping

steadily (Fig. 1) ( 11 ). A period of gradual climate

cooling was marked by a sudden cooling of the

ocean temperatures together with a decline

in CO 2 34 million years ago (at the Eocene-

Oligocene boundary). The dropping temper-

atures and CO 2 , together with the tectonic

isolation, induced the first ice formation on

the continent’s high elevations.

The morphology of the Antarctic continent

determined how it became glaciated. The con-

tinent consists of the cratonic shield of East

Antarctica, the thin low-lying West Antarctic

rift system, and the Antarctic Peninsula, an

elongated ridge produced by convergent tec-

tonics ( 12 ). East Antarctica is dominated by two

major mountain ranges. In the center of the

craton at 80°S, the Gamburtsev Mountains rest

at an elevation of 3000 m. The Transantarctic

Mountains, with elevations reaching 4500 m

above sea level, separate the elevated shield of

East Antarctica from the thinned crust of the

West Antarctic rift system ( 13 ). The early ice

in East Antarctica formed on the Gamburtsev

and Transantarctic Mountains and in the high

elevations of Dronning Maud Land. This early

ice cover quickly grew from mountain glaciers

into an ice cap ( 2 ), slowly eroding the Gamburtsev

Mountains. For the next 20 million years, East

Antarctic remained glaciated, but little ice per-

sisted in West Antarctica.

SCIENCE 20 MARCH 2020•VOL 367 ISSUE 6484^1321

(^1) Lamont-Doherty Earth Observatory of Columbia University,
Palisades, NY 10964-8000, USA.^2 Jet Propulsion Laboratory,
California Institute of Technology, 4800 Oak Grove Drive MS
300-323, Pasadena, CA 91109-8099, USA.
*Corresponding author. Email: [email protected]
Transantarctic Mountains along the edge of the
Ross IceShelf from a ROSETTA-Ice imaging flight
PHOTO:SUSANHOWARD
ANTARCTICA