snowfall over the continent and mass loss, the
latter approximately equally split between ice-
berg calving and ice shelf basal melting ( 37 ).
Under the current situation of a warming global
atmosphere, AIS precipitation is likely to in-
crease, as a warmer atmosphere can hold more
moisture and therefore produce greater precip-
itation ( 38 ). At the same time, a warming atmo-
sphere would increase surface melt of the ice
sheet, leading to mass loss. Additionally, a warm-
ing atmosphere will lead to changes in wind
pattens, with unknown outcomes with respect
to increasingly warm ocean waters reaching
ice shelf cavities and accelerating ice loss. The
warm-water cavities are sensitive ( 39 ), as they
currently have a largely unconstrained flow of
warm water onto the continental shelf and into
their sub–ice-shelf cavities. The cold-water cavi-
ties are currently protected by dense and saline
continental shelf waters that block transport of
CDW to the ice shelf cavities (Fig. 2A), although
this situation could change under a changing
climate ( 40 ). It should also be noted that the
anticipated mass loss processes of surface
and basal melt have an inherently faster time
scale than that of the atmospheric-driven gain
through precipitation. This implies that the mass
loss effects may occur faster than they can be
offset by precipitation.
Reductions in the extent and thickness of
ice shelves and the retreat of their grounding
zones associated with the ice-ocean processes
occurring in cavities would have major conse-
quences for the inland ice, causing it to flow
faster to the grounding zone as the buttressing
force of the ice shelves is weakened or lost ( 41 ).
There is some evidence that the resulting reduc-
tion in buttressing produces a near-instantaneous
response in the speedup of the inland ice ( 42 ).
As the inland ice flows faster toward the ocean
and is melted, the ice feeding the ice shelf be-
comes thinner, leading to overall decreased mass
oftheAIS.Ofalltheprocessesgoverningthis
mass and implied sea-level change, perhaps the
most important ones are those in the ground-
ing zone, which are also arguably the least well
understood at present. This is a key motiva-
tion for the ongoing integrated field, remote-
sensing, and modeling campaign called the
International Thwaites Glacier Collaboration
( 43 ). Studies such as this into the retreat of
grounding zones are needed to feed into fully
coupled global climate models, so that climate
models can accurately represent such processes
to project future sea level with confidence [see
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ACKNOWLEDGMENTS
Funding:D.M.H. and A.B. were supported by NSF grant PLR-
1739003 and NYU Abu Dhabi grant no. G1204. K.W.N. is supported by
NERC grant NE/S006656/1.Competing interests:The authors
declare no competing interests. This is ITGC contribution no. 012.10.1126/science.aaz54911330 20 MARCH 2020•VOL 367 ISSUE 6484 SCIENCE
BLSAM
indexPolar cellsWesterliesEasterliesACTTropical
planetary
wave trains1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 202010Westerly winds (m/s)-1AFig. 4. Atmospheric context wind variability.(A)The approximately east-west
oriented Antarctic Circumpolar Trough (ACT) supports a band of westerly
winds to the north of the trough, and easterlies to the south. Coriolis force
causes the westerlies and easterlies to drive the upper ocean waters to the north
and south, respectively, creating a divergence of surface waters all along the
ACT. This creates an upwelling of the subsurface CDW, which can cause it to
rise above the depth of the continental shelf break in places and allow the
possibility of it crossing onto the shelf and interacting with the AIS. Whether
or not the CDW gets onto the continental shelf is determined by processes
sketched in Fig. 2, A and B. (B) A time series of westerly winds in the Amundsen
Sea adjacent to Thwaites Glacier ( 36 ). Positive values indicating westerly
winds favor a greater transport of warm CDW into the cavity below Thwaites Ice
Shelf, thereby increasing melting and consequently grounding-zone retreat.
Negative values indicating easterly winds represent the opposite behavior.
The winds were created using a global climate model that simulates large,
natural, decadal variability in the climate system, principally originating in the
tropics. Superimposed on this variability is a centennial-scale trend in
positive wind values that is likely being forced by anthropogenic sources.
The study suggests that this upward trend will lead to an increase in warm
waters at Thwaites Ice Shelf, and hence increased melting and retreat.ILLUSTRATION: (PANEL A) N. CARY/SCIENCEANTARCTICA