Some key questions answered on the news West Antarctic glaciers are “collapsing"
- 14 May 2014, 10:00
- Roz Pidcock
Collapse, irreversible, unstoppable, catastrophic: Eye-catching
words like these pepper recent media coverage of
papers charting the demise of West Antarctic glaciers.
Both papers come to much the same conclusion - the region's
glaciers are shrinking under the pressure of rising temperatures
and will ultimately add several metres to sea levels worldwide.
The research prompted some dramatic headlines. The
Daily Mail's headline reads 'Nasa data reveals Antarctic ice
sheet is melting at an 'unstoppable' speed' while The
Guardian says, 'West Antarctic ice sheet collapse has begun
Are glaciers retreating or collapsing entirely? When does
retreat become a collapse? Do scientists think we're heading for a
collapse too? We've tried to answer some questions the coverage
What's the West Antarctic ice sheet? Why is it
Map showing West Antarctica and the location of the two
biggest glaciers, Thwaites and Pine Island. Source:
The West Antarctic Ice Sheet (WAIS) is one of three regions
making up Antarctica. The other two are East Antarctica and the
Antarctic Peninsula, with the Transantarctic Mountain range
dividing East from West. The two major glaciers in West Antarctica
- Pine Island Glacier and Thwaites Glacier - flow into the Amundsen
Sea, carrying ice from the mountains to the ocean.
long thought the WAIS to be the most vulnerable part of
Antarctica to rising temperatures. It's what's called a marine
based ice sheet, which means a lot of it sits on land that is below
Changes in wind and ocean circulations are bringing warm water
in contact with the Amundsen shelf and vast warm pools are forming
under the ice sheet, melting it from the bottom up.
Unlike the East Antarctic ice sheet, a lot of the West
Antarctic ice sheet is below sea level, making it vulnerable to
bottom-up melting by warm water. Source:
As the front edge of the ice sheet retracts inwards, it meets
with deeper water, because of the bowl shape of the WAIS basin.
This steepens the ice face, making it unstable and causing ice to
slide more quickly into the ocean. This point marks a critical
threshold, beyond which ice melt accelerates over time until the
glacier has disappeared completely.
The Antarctic Glaciers
website by ice sheet expert at Aberystwyth University Bethan
Davies features a good explanation of the ice sheet's
Is it the Amundsen glaciers that are at risk, or the
whole ice sheet, or both?
At the moment, it's the glaciers along the Amundsen coast that
are losing ice. Scientists have done
extensive satellite, air and ground surveys and found not only
are the glaciers shrinking but there's nothing stopping the ice
from melting away completely, like a ridge or other obstacle.
That's what scientists mean when they talk about the glaciers'
"collapse" - the chain of events leading to their disappearance
looks to be already underway.
Here's NASA glaciologist and lead author on one of the new
papers, Eric Rignot, explaining the processes leading to the
decline of six Amundsen glaciers in West Antarctica.
This isn't the same as saying the whole WAIS is doomed to
disappear - but it is all connected, Hamish Pritchard from the
British Antarctic Survey (BAS) tells us. As these glaciers lose ice
and thin out, they can draw down ice from neighbouring
The destabilising effect is amplified by the melting of ice
shelves at the edge of the ice sheet, which help keep the interior
of the ice sheet stable - an effect known as "buttressing". Losing
them helps unlock the rest of the WAIS to faster ice loss.
On their own, the six main Amundsen glaciers could add about 1.2
metres to global sea level, scientists say. But they are most
concerned about what's likely to happen to the WAIS without these
huge glaciers to prop it up.
What does it mean if the WAIS collapses? Will it lose
all of its ice, or just most of it?
Scientists generally don't think the WAIS will end up losing
absolutely all its ice, Pritchard explains.
"Ultimately, it's likely to mean most
rather than all [the ice] because even with a whole-ice-sheet
collapse, we'd expect some sizeable remnants to persist around the
Even so, scientists estimate the West Antarctic contribution to
sea level would still about 3.3
metres. That's on top of the sea level rise coming from
Greenland ice sheet, glaciers and other ice caps on land, as well
as sea level rise caused by seawater expanding as it warms up.
Do scientists think the collapse of the WAIS glaciers is
If the ice streams are truly inherently unstable and if
they have been forced beyond a critical threshold, then the
collapse is unstoppable, explains Pritchard. The question is
have we passed the critical point? The new research would suggest
we have, he says:
"There's no doubt that [the glaciers]
are being forced (by the ocean) to retreat and both of these papers
say that they do think that this unstable, unstoppable retreat has
begun. They both have some caveats though … Both studies fall
frustratingly just short of being definitive."
For example, scientists can't be absolutely sure the critical
threshold has been crossed for the Thwaites Glacier, Pritchard
explains. The authors consider a range of scenarios and it's
reached in all "except in the lowest-melt scenario" - which
observations suggest is far too optimistic.
We asked author on that paper, Ian Joughin from the University
of Washington, the same question. He told us:
"Definitively unstoppable no; likely
unstoppable yes. There is always some uncertainty as our model like
any model has to make some assumptions."
Does "unstoppable" mean the same as
Effectively yes. Rignot described the Amundsen glaciers to a
NASA press conference as having "passed the point of no return." In
other words, scientists see no scenario in which collapse of the
glaciers could now be avoided. Pritchard explains:
"An unstable (/unstoppable/irreversible)
retreat in this context means that once the threshold is crossed,
the retreat will occur regardless of what happens to the
Ice loss will continue until a new 'steady state' is reached
where ice loss equals ice gained through snowfall, Pritchard adds.
But this steady state would not be reached until much of the ice
was gone and the remnants had retreated onto higher ground, he
None of this will happen quickly. The scientists put a timeline
anywhere between 200 to 1000 years, depending on how quickly the
water warms. Observations would suggest it's more likely to be the
lower end of that range, scientists say.
The New York Times
quotes from NASA's Thomas P. Wagner, who was involved in the
research, as saying:
"This is really happening. There's
nothing to stop it now. But you are still limited by the physics of
how fast the ice can flow."
Could the changes in West Antarctica be described as
Though scientists don't tend to use those words lightly, the
consequences could well be described this way. Pritchard
"Collapse of the WAIS would (ultimately)
redraw every coastline, drive retreat of coastal cities and
farmland as damaging storm surges became more frequent, would
change the Earth's appearance (and albedo) from space and would
considerably alter regional and global ocean circulation with
probably important consequences for global weather patterns. On the
centuries timescale, it's probably fair to call it
It depends how you define catastrophic, Joughin tells us:
"[N]obody is going to wake up with water
all of a sudden in their living room ... [but] ... Over decades to
centuries though, populations will be displaced and billions of
dollars of infrastructure lost."
Do any these findings affect East Antarctic or the
Melting glaciers in West Antarctica shouldn't directly affect
the Antarctic Peninsula or East Antarctica, because of the
separation of the Transantarctic Mountains. However, there is some
separate evidence of smaller-scale retreat on some glaciers in East
Antarctica. Scientists warn glaciers are vulnerable if ocean
warming causes ice melt as it has in the WAIS.