
Huge Cavity in Antarctic Glacier Signals Rapid Decay - howard941
https://www.nasa.gov/feature/jpl/huge-cavity-in-antarctic-glacier-signals-rapid-decay
======
lenticular
West Antarctica is something that glaciologists are really worried about in
the long term. The West Antarctic Ice Sheet's bed slopes downward as you go
inland, especially around Amundsen Sea where Thwaites Glacier is. This reverse
slope means that if the grounding line (where the ice begins to float)
retreats back, it is in deeper water. This causes the ice flux to increase,
making it retreat backwards into deeper water still, etc.

This is called the marine ice sheet instability. There's no mechanism known
thus far that would countervail it, so it is something that should be taken
seriously. Antarctica is too cold to melt much from surface warming anytime
soon. Ocean warming and changing currents are a different story, though.

~~~
Simon_says
I'm sorry, but could you break this down more for stupid people? What exactly
are glaciologists afraid will happen and why is it bad?

~~~
lenticular
Ice sheets, of which the West Antartic, East Antarctic, and Greenland ice
sheets are the only current examples, are basically like very large domes of
ice, on the order of 1000 kilometers wide and 1-3 kilometers high. Unlike
mountain glaciers, they completely cover the underlying topography.

They flow outwards under their own weight into the ocean. In Antarctica, it is
cold enough that melting at the surface is negligible. This means that almost
all snow that falls ultimately must flow into the ocean. At some point at the
edge, the ice loses contact with the ground and floats. This is the grounding
line.

For a constant ice flow velocity, the amount of ice that gets spit out into
the ocean is proportional to how thick the ice is at the grounding line.

Now, suppose that the slope of the bed gets deeper as you go from the
grounding line into the interior of the ice sheet. Then, if the grounding line
retreats, it's in deeper water. So, the ice flux increases. The increasing ice
flux means that the ice sheet thins near the edge. Since it's thinner, it
can't stay grounded in as deep of water. Imagine a big chunk of ice in a
puddle of water that sits at the bottom of a puddle, versus a little piece
that floats.

Now, since it can't float in as deep of water, the grounding line retreats
because bouyancy is pulling up on the edge. This makes it go into deeper
water, ice flux increases more, etc.

This could cause a collapse of the ice sheet, resulting in a few meters of
rise. The timescale that this could happen is very poorly constrained
(decades? centuries? a couple millenia?). It may have already irreversibly
begun, or maybe not.

~~~
function_seven
I still had trouble visualizing what you're describing.

This is a typical cross-section[1]. To make it match what you're describing,
I'd draw a slope from the grounding line and downward to the left, right?
(Image depicts a flat bed underneath the ice, instead of a slope downward as
you go inward)

[1] [http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/09/...](http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/09/Fig-2.-Grounding-line.png)

EDIT: I think this image is a better representation:

[http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/05/...](http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/05/PIG.ai_.jpg)

~~~
dsfyu404ed
Flip the terrain illustration in that picture left to right and make it
steeper.

The terrain that this particular glacier is a bowl that is well below sea
level. The more the glacier retreats the deeper the water gets (because the
terrain is deeper), the more of the glacier can be floated, the faster the
glacier can melt.

Imagine a pool that slopes from 3ft to 8ft full of ice melting. The thin part
melts first, lifts up, water gets a little further underneath lifting the next
bit of ice and melting it faster but because the pool gets deeper as you go
the more ice can be lifted up and broken off so it melts faster as it goes.

Edit: yes, your second image is spot on.

~~~
Gibbon1
First image alludes to something. Near freezing slightly warmer water is
denser. So convection is reversed.

Unless I'm mistaken.

~~~
lenticular
That's right. Water is most dense at about 4 degrees, and lower in sea water
(around 1 C I believe). Below that, and water molecules start forming
ephemeral crystal lattices. Since, weirdly, the hexagonal structure of ice
lattices takes up more room than in liquid form, this makes the water less
dense. Just to be clear, it isn't actually crystallizing at these
temperatures, but due to fluctuations, crystal lattices can be briefly formed.

------
makerofspoons
Previous modeling predicted this glacier would collapse in the next 200-1000
years, producing a 3M rise in sea level:
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4655561/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4655561/)

~~~
robodale
Here's what a ~3 meter rise in the ocean would look like for the Miami-area of
Florida.

[https://ss2.climatecentral.org/#8/25.738/-80.398?show=satell...](https://ss2.climatecentral.org/#8/25.738/-80.398?show=satellite&projections=0-K14_RCP85-SLR&level=10&unit=feet&pois=hide)

~~~
WhompingWindows
Yikes, not to mention the storm surge during a hurricane, which is predicted
to be a much larger hurricane and much more frequent in future.

People are just not mentally prepared for this. In the late 21st century, we
are going to start seeing massive refugee crises all over the world every
year. We are in this "golden age" of wealth and prosperity now, even if it
isn't equitably distributed, however that whole system may come crashing down
if 15% or 25% of the world's vulnerable to massive ocean flooding and storms.

~~~
anigbrowl
I feel like people aren't able to get mentally prepared while we have to deal
with denialist propaganda on a daily basis.

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75dvtwin
This sentence helped me to visualize what might be happening.

"Just as a grounded boat can float again when the weight of its cargo is
removed, a glacier that loses ice weight can float over land where it used to
stick. When this happens, the grounding line retreats inland. That exposes
more of a glacier's underside to sea water, increasing the likelihood its melt
rate will accelerate. "

not clear from the article though, what do they think is causing the retreat?

Is it warming of ocean water, or we do not know yet?

~~~
pas
[https://en.wikipedia.org/wiki/Circumpolar_deep_water](https://en.wikipedia.org/wiki/Circumpolar_deep_water)
typically 1-2°C warm and salty.

[http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/05/...](http://cdn.antarcticglaciers.org/wp-
content/uploads/2014/05/PIG.ai_.jpg)

[http://www.antarcticglaciers.org/glaciers-and-climate/ice-
oc...](http://www.antarcticglaciers.org/glaciers-and-climate/ice-ocean-
interactions/changes-circumpolar-deep-water/)

------
kaybe
If you need some good news today, check page 34 on the new Ozone report:

[http://ozone.unep.org/sites/default/files/Assessment_Panel/S...](http://ozone.unep.org/sites/default/files/Assessment_Panel/SAP-2018-Assessment-
report.pdf)

The finding that the ozone layer is behaving as predicted - probably healing -
is not unexpected. It was depleting due to the gases now controlled under the
Montreal protocol, the emission of which is decreasing very rapidly.

This plot however shows the contribution of these gases to global warming. As
you can see there, _it is going down over time_! Of course the contribution
pales in comparison to CO2, Methane and N2O, but it is still significant. And,
if nobody messes up, _it will go down_.

It can be done! It's the easiest part, but it is a start.

edit: It also says that the main reason for 20th century climate changes in
the southern hemisphere summer is actually the ozone hole over Antarctica.
When it closes (projected to 2060) the changes will be undone. (p. 38)
(pending too much influence from other greenhouse gases) I haven't seen the
details, but maybe there's hope for Australia.

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infradig
Why no mention that this is a known area of undersea volcanoes? They are
already causing considerable melting of glaciers from below.

See: [https://www.livescience.com/46194-volcanoes-melt-
antarctic-g...](https://www.livescience.com/46194-volcanoes-melt-antarctic-
glaciers.html)

Literally the first paragraph at that link I supplied addresses this situation
at Thwaites Glacier.

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madengr
Did a little work on these ice penetrating radars when I was at KU.

------
fdggdfsvscvsd
"Thanks to a new generation of satellites, we can finally see the detail"

So at least it is not clear if this compares unfavorably to situations in the
past, as there is no data about it.

~~~
kaybe
There is a lot of proxy data for the amount of sea water missing though. It
only goes missing for so long when it is in ice form. The only question is
where exactly all that ice was.

~~~
fdggdfsvscvsd
What do you mean - they measured rising sea levels, and wanted to find the
missing ice it came from?

~~~
kaybe
No, basically the isotope composition of the sea changes when a lot of water
evaporates and doesn't come back over the rivers (because it fell as snow and
just stayed on land). The heavier isotopes need a little bit more energy to
evaporate, so the remaining sea water has more and more heavy isotopes left.
The most commonly used tracer is oxygen 16 vs oxygen 18 [0].

The isotopes are also used by sea organisms when they grow [1]. When they die
and their shells become part of the sea floor (this is optional, there are
also old corals that can be used), the signal can remain for a very long time
(essentially as long as the sea floor remains, a few 100,000 years).

So we know quite well how much water was missing from the ocean at these
points in time. The method can also be used to deduce temperatures from back
then.

[0]
[https://en.wikipedia.org/wiki/%CE%9418O](https://en.wikipedia.org/wiki/%CE%9418O)

[1]
[https://en.wikipedia.org/wiki/Foraminifera](https://en.wikipedia.org/wiki/Foraminifera)

~~~
fdggdfsvscvsd
Thanks for the explanation.

