February 2019, ScientificAmerican.com 45sea, they drag sediment with them. The ice stabilizes when it
reaches a local high in the seafloor and then builds the seafloor
higher there by piling the sediment into raised moraine shoals—
long, stony walls that grow where the ice ends. Ice can sit in such
a position for hundreds or thousands of years, rebuffing weak ef-
forts to dislodge it. But if enough warming occurs, the ice retreats
back down the sloping bed into the valley behind the shoal. The
ice rarely stabilizes again until it reaches the next high ridge, often
far behind it. Meanwhile icebergs float over the abandoned mo-
raine shoal, which is still below sea level, and out into the ocean.
This is now happening in many places around Antarctica and
Greenland. Jakobshavn Glacier has “jumped the bump” of a for-
mer moraine shoal and is retreating back through its valley-
shaped fjord, “unzipping” a path into the greater ice sheet. When
the first European explorers visited the area that is now Glacier
Bay in Alaska, it was filled with a vast glacier ending on a large
moraine shoal. Since then, the ice has retreated from that ridge, or
bump, more than 60 miles inland to get to the next high ground,
which today is the current shoreline of the beautiful bay.
Fortunately, most such retreats have only a limited effect on
global sea level. Even a big Glacier Bay–sized glacier is small com-
pared with the world ocean. Jakobshavn is just one of dozens of
major drainages around Greenland’s ice sheet, but they do not
quickly destabilize their neighbors in adjacent fjords, and they
end not too far inland where the bed rises again. Similarly, Ant-
arctica is drained by a great number of glaciers flowing down into
their own waffle-iron valleys. With enough warming, many of
them might retreat in unison, but each by itself is not a huge influ-
ence on the global sea.
The Bentley trench in West Antarctica and a few other deep re-
gions in East Antarctica, including the Wilkes and Aurora basins,
present a different story. Retreat through one of these to the next
high ground would have global importance. Models point to
Thwaites Glacier as the most likely path into the Bentley trench
and connecting basins. If it started unzipping into the interior as
Jakobshavn has, the melting could potentially raise sea level 11
feet before it stabilizes on high ground on the other side of the
trench. The East Antarctic basins by themselves could raise sea
level more than Thwaites would, but they require more warming
to cause those glaciers to jump their bumps.
Note that there is nothing bizarre about this scenario. With
sufficient warming, ice retreats, usually to the next high ground.
This has been observed over and over in the past and present. If
Thwaites becomes warm enough to start acting like ice in Green-
land and Alaska, then it should retreat.
A FRACTURED FUTURE?
HOW FAST COULD THWAITES GO? How much warming can we cause
before it goes there?
My colleagues David Pollard of Pennsylvania State University
and Robert M. DeConto of the University of Massachusetts Am-
herst programmed an ice-flow model that uses the relevant phys-
ics and can be run fast enough on advanced computers to study
big changes in ice sheets over long times. I helped them a little
with the physics of calving from high cliffs after ice shelves break
off, especially if surface meltwater wedges open crevasses.
Pollard and DeConto optimized this model to match data from
the geologic past and to assess the impacts of different amounts of
human-caused warming. They determined that we probably have
a few decades even under fast warming before the collapse of
Thwaites is triggered by loss of its shelf and meltwater widening
crevasses. Thwaites then would take a century or so to collapse
completely. They did not know how fast the ice could break,
though, so they set a top rate equal to what Jakobshavn had done
in Greenland. (It has already exceeded that rate briefly.) And be-
cause Thwaites is thicker, it could make much higher cliffs than
Jakobshavn. Higher cliffs tend to break faster (one reason high-
way engineers leave slopes rather than cliffs). So we could be un-
derestimating the worst-case scenario, but we really do not know.
This is a good model, but it surely is not the last word from
Pollard and DeConto or others. Some hope remains that Thwaites
could stabilize on a deeper ridge on the downslope of the trench,
behind its current position, before retreating still more, for ex-
ample. Or icebergs could break off and pile up for a while behind
the current ridge where the ice now starts to float, helping to re-
form a shelf that could lessen the ice loss.
To address these and other questions, the National Science
Foundation and the British Antarctic Survey, together with other
international collaborators, have launched a major effort to learn
even more about Thwaites’s history, how the glacier is flowing,
and what the seafloor surface is that it is flowing over, which will
help all of us involved to better predict its future. The data are al-
most guaranteed to reduce uncertainties and to be fascinating.
Some questions may remain difficult to answer. Think of all
the ceramic coffee cups you have seen dropped on a hard floor.
Some bounce, some crack, some chip, some break into a million
pieces. The physical processes of these fractures are well known
and readily calculated, and the behavior averaged across many
dropped cups is predictable. But you would not want to bet your
career, or anything else important, predicting the fate of the next
cup that hits the floor.
The future of Thwaites depends a lot on fractures. Will the ice
shelf fracture from the ice that now feeds it, causing the ice sheet
to jump the bump and retreat into the deep basins? Will huge ice-
bergs break off rapidly if ice-shelf loss produces a cliff along the
sheet’s face that is higher than any now on Earth, driving retreat
faster than any we have seen? Meltwater is important, but how
much of the water will run off in rivers to the sea, and how much
will percolate into snow and refreeze? How fast will the air warm?
I suspect that coffee cups are easy to predict in comparison.
If the world can muster the effort, slowing and stopping warm-
ing from greenhouse gas emissions will slow sea-level rise, easing
the mounting costs of coastal damage. But if Thwaites is poised to
retreat briskly, preventing warming by limiting the damage in-
curred by human activity could be vastly more valuable.MORE TO EXPLORE
Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More. 2`Dàm
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Contribution of Antarctica to Past and Future Sea-Level Rise. 2¹Uyàï$Îy ¹ ́ï¹D ́m
Dÿm0¹ ̈ ̈Dàm ́ Nature, <¹ ̈ÎñÀjÈDyåμÀμéè$Dà`ñÀj÷ĈÀêÎ
How Much, How Fast?: A Science Review and Outlook for Research on the
Instability of Antarctica’s Thwaites Glacier in the 21st Century. 5Î Î3`D®U¹åyïD ̈Î
́ Global and Planetary Change, <¹ ̈ÎÀñjÈDyåÀêñè ù ́y÷ĈÀéÎ
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