GiventhatmuchofWestAntarcticaisbelow
sea level, establishing a large ice sheet required
substantial cooling. When the global climate
began to cool again ~14 million years ago, ice
started to cover West Antarctica. Again, tectonics
mayhavecontributedtotheencasingofWest
Antarctica in ice as uplift of Marie Byrd Land ( 12 )
established elevated terrain on which an ice sheet
could grow. By 10 million years ago, the West An-
tarctic Ice Sheet filled the low-lying rift system.
Once fully established, two insolation cycles
paced the ice growth and decay: the obliquity
(or wobble) of the orbit, with a ~40,000-year
period, and the eccentricity, with a 100,000-year
period. The background temperature appears
to control which of these frequencies is domi-
nant. Currently, the ice core records indicate a
100,000-year frequency to the ice sheet expan-
sion and contraction ( 14 ). Before 800,000 years
ago, the 40,000-year period dominated ice ad-
vance and retreat. During cool periods, the ice
in both East and West Antarctica expanded,
reaching the edge of the continental shelf
margin (Fig. 1). At the ice sheet’s maximum
extent, sediments transported by the ice were
deposited where the ice went afloat, expanding
the shallow reaches of the continental shelf
each time the ice sheets grew. These low-lying,
marine portions of the continent were slower
to become ice-covered and are the ice sheet
regions most susceptible to collapse when the
climate warms. During some past warm periods,
the West Antarctic Ice Sheet retreated beyond
its current extent and in some instances com-
pletely collapsed ( 15 ). Low-lying portions of
East Antarctica are likely to have collapsed as
well during past warm periods ( 16 ). Evidence
for the collapse comes from sediments in the
Ross Sea and global sea level records where the
past sea level cannot be replicated without loss of
ice from both East and West Antarctica (Fig. 1).Structure
There are three major components of an ice
sheet system: grounded slow-moving ice, fast-
flowingicestreamsoroutletglaciersandfloating
ice shelves (Fig. 2). The grounded slow-moving
ice contains the vast majority of the ice (Fig. 2)
and is in contact with the underlying rocks.
This ice moves very slowly by means of inter-
nal deformation at rates on the order of 1 m/year
( 4 ) and is up to 4775 m thick ( 17 ). The fast
flowing ice streams and outlet glaciers are
conveyor belts that move the ice toward the
ocean, are up to 100 km across, and slide rapidly
over the underlying topography at rates of up to
4 km/year ( 4 ). Although the surface of glaciers
and ice streams are fractured by crevasses,
water and till (water-saturated sediments) at
their base reduce the basal friction and enable
their fast flow ( 18 ). The grounded Antarctic holds
enough ice to raise sea level rise by 58 m ( 19 ).
Ice shelves are expansive, flat, floating bodies
of ice attached to the fast flowing ice streams
that cover an area of >1.5 million km^2 all
around the continent ( 20 ). They do not affect
sea level rise directly but slow the flow of the
glaciers and the ice streams that feed them
by providing some backward stress. The loca-
tionwheretheiceinglaciersandicestreamsstarts floating over the ocean is called the
grounding line. If more ice crosses the ground-
ing line, global sea level will rise. Inland retreat
of the grounding line is indicative of a shrinking
ice sheet. The large ice shelves Ross, Filchner-
Ronne, and Amery provide substantial back-
stress and are on average 300 m thick. Snow
accumulation on these large ice shelves’sur-
faces and ice discharge from the grounded ice
balance ice loss from ocean melting at their bases
and large icebergs calving off their fronts ( 20 ).
East Antarctica is the largest ice sheet on the
planet, with thicknesses greater than 4600 m
( 19 ). In some areas, the bedrock underlying
the ice is above sea level, but extensive portions
are below sea level. The top of the ice sheet,
Dome A, is at 4200 m over the Gamburtsev
Mountains(Figs.2and3),whereasthedeepest
point, carved by erosion during successive ad-
vance and retreat of an ice stream, is located more
than 3500 m below sea level under Denman
Glacier ( 19 ). Beneath the thick ice are large lakes—
Vostok ( 21 ), 90°E, and Sovietskaya ( 22 )—with up
to 1000 m of water (Figs. 2 and 3). These systems
have been sealed from the atmosphere for ~34
million years since the onset of Antarctic glacia-
tion. The iconic ice core records of temperature
and CO 2 come from the deep cores at Vostok
[400,000 years ( 14 )], Dome C [800,000 years
( 23 )], and Dome F [720,000 years ( 24 )].
West Antarctica is classified as a marine
ice sheet because the topography beneath the
ice sheet is largely below sea level. The ice is
underlain by marine sediments deposited by
glacial processes, rifted sedimentary basins,1322 20 MARCH 2020•VOL 367 ISSUE 6484 SCIENCE
Unglaciated Antarctica Marine
Ice Sheet
Collapse56 50 40 30 20 10 051015202530Surface temperature (°C)02004006008001000120014001600C
O
2(ppm)Pliocene
PleistoceneEocene Oligocene MioceneMillions of years before the presentGlacial
MaximumEast Antarctica
West AntarcticaAlkenones Boron Isotopes Paleosols Stomata LOESS Probability maximumFig. 1. Development of Antarctic ice together with global CO 2 and ocean
surface temperature.(Top) Global CO 2. (Bottom) Ocean surface temperature.
Onset of East Antarctic ice occurred 34 million years ago as ocean temperatures
and CO 2 dropped ( 53 , 54 ). Development of West Antarctica marine ice sheet
at 14 million years ago began with the next major drop in global temperature.
Two extreme modes of Antarctic ice have occurred since the onset of glaciation
in West Antarctica was first covered with ice: ice extending all the way to
the continental shelf during cold periods, such as the Last Glacial Maximum
25,000 years ago, and retreat beyond the present extent, with partial collapse of
marine portions of Antarctica during some past warm periods ( 16 ).ANTARCTICA