the Larsen B collapse, the surface of the ice
shelf surface was covered by lakes, indicating
that warming air temperatures and surface
meltwater can destabilize ice shelves, leading
to faster flow of Antarctic ice into the global
oceans and highlighting the protecting role of
ice shelves ( 32 ).
Vulnerabilities
These remote sensing observations allow scien-
tists to observe ice sheet changes and decipher
thecausesofsuchchanges.Boththeocean
surrounding Antarctica and the atmosphere,
especially in the Peninsula region, have warmed
over the 25-year observational record of ice
change ( 33 , 34 ). Antarctica is losing most of its
mass through increased ice flow of the outlet
glaciers and ice streams. This contrasts with
theGreenlandIceSheet,wherehalfoftheloss
is due to faster ice flow and half is due to in-
creased melting of the ice sheet surface ( 35 ).
Surface melt is not yet a major contributor to
ice loss in Antarctica, and global climate models
suggest that an increase in snowfall in East
Antarctica could partially offset the dynamic
mass loss ( 36 ). Although these changes have
been ongoing for the past three decades, more
rapid and dramatic mass loss cannot be ex-
cluded. The marine portions of the ice sheet
with subglacial topography that deepens inland
and glaciers with thick marine terminating
fronts are prone to instabilities ( 37 , 38 ).
Although the surface waters surrounding
Antarctica are cold, the underlying waters of
the Circumpolar Deep Water are warmer and
can influence the ice sheet when they reach
the ice shelves and grounding lines, where the
ice becomes afloat. The concentration of changes
in West Antarctica points to the dominant role
the warming ocean plays in recently observed
change ( 39 , 40 ).Atthebaseoftheoutletglaciers
in the Amundsen Sea, the topography beneath
the ice either rises inland or drops. A bed topo-
graphy dropping inland with ice getting thicker
is referred to as a reverse slope. This reverse
slope for a marine ice sheet has long been at the
core of a concept called the marine ice sheet
instability ( 37 ). As a glacier retreats across a
reverse slope, glacier retreat means thicker ice
at the grounding line, and therefore, more iceis leaving the ice sheet, while the region that
accumulates snow is reduced. The ice sheet is
out of balance. The greater flux of ice results in
thinning and additional retreat until a region
with an inland rising slope is encountered to
stabilize the grounding line ( 38 ). In addition
to this, ice dynamics imbalance on a reverse
slope; the thicker ice at the grounding line
means more ice is exposed to warming ocean
waters ( 40 ). A perturbation can nudge an outlet
glacier off a stable point into a region with a
reverse slope and have consequences for de-
cades ( 39 , 41 ). An extreme El Nino event in the
1940s appears to have triggered the ground-
ing line retreat still ongoing in the Pine Island
catchment in West Antarctica ( 42 ).
Anothertriggerforrapidandsustainedin-
creased ice flux is the collapse of buttressing
ice shelves. This concept was widely debated
inthesciencecommunityuntiltheaccelera-
tion of the ice flow in the glaciers feeding the
Larsen B ice shelf after its collapse in 2003 was
observed ( 6 , 31 ). Shortly before the collapse, this
ice shelf surface was covered with lakes, leading
to the hypothesis that hydrofracture and load-
ing from lakes can damage an ice shelf suffi-
ciently to induce a catastrophic collapse ( 32 ).
This mechanism has been incorporated into
some ice sheet models ( 41 ) but assumes that
meltwater is stationary and that little water is
transported across an ice shelf. It is now clear
that surface water can flow from the grounded
ice onto ice shelves ( 43 ) and coalesce into
rivers atop the ice surface that end as water-
falls at their front ( 44 ). Hydrology could there-
fore have a stabilizing impact on ice sheet
mass balance as the distribution of meltwater
increases.
How large and thick marine ice sheets be-
have after the complete removal of ice shelf
buttressing has not yet been witnessed. The
question of how quickly ice shelves can collapse
and how the glaciers feeding them respond
remains open ( 45 ).Unknowns and future directions
Increasingly, communities around the globe
are asking how much the sea level will rise in
the coming decades. Although we now know
that the answer for each community must
incorporate knowledge of local processes, such
as isostatic uplift from unloading of ice at the
end of the last glacial period and subsidence
owing to sediment compaction, changing ocean
volume from Antarctic mass loss remains one
of the largest contributors to communities’un-
known future. Gaps in our fundamental knowl-
edge of the bathymetry close to the ice sheet
andinregionscoveredbyseaiceandiceshelves,
the temperature of the deep water masses, the
fate of surface meltwater, and the basal condi-
tions beneath the ice sheets introduce limits
into our ability to project the future. It is es-
sentialthatwerefineourprojectionsthrough1324 20 MARCH 2020•VOL 367 ISSUE 6484 SCIENCE
03000400010002000030004000100020000300040001000
2000(^0100)
Distance (km)
Distance (km)
Distance (km)
Depth (m)
Depth (m)
Depth (m)
200 250
0 100 200 250
0 40 70
Gamburtsev Mountains
Lake Vostok
WAIS Divide
< 68kyr
68 – 120kyr
120 – 220kyr
220 – 330kyr
330 – 420kyr
Age of ice
Dome A
Lake Vostok
Vostok Ice Core
Subglacial
Gamburtsev Mountains
water
Subglacial
topography
Subglacial
topography
Subglacial
Ice surface
Ice surface
Ice surface Freeze-On
topography
~12 0 ,000
~120,000
~220,000 ~420,000
~330,000
~68,000
~68,000
~68,000
Fig. 3. Radar cross sections over the Gamburtsev Mountains, Lake Vostok, and West Antarctica.
(To p)Gamburtsev Mountains. (Middle) Lake Vostok. (Bottom) West Antarctica. Location of profiles are approximately
along the profiles shown in Fig. 2. Radar layers indicate ice stratigraphy. Ice is deformed as it flows over mountains,
but layers remain flat as the ice flows over Lake Vostok, the location of the first deep ice core. The color shading
highlights the age of the ice sheets. Basal freeze-on is observed in the Gamburtsev Mountain profile.
ANTARCTICA