Thwaites Glacier Discussion

[grixm]

25th - 19th

B22 further moves back toward land, however the crack in big melange field widens slightly.

[baking]

25th - 19th

B22 further moves back toward land, however the crack in big melange field widens slightly.

Actually, B22-A rotated clockwise between June 19 and 25.  Perhaps you got the order of the images reversed?  Also, the bulk of the melange moved slightly closer to shore although other new "cracks" did appear.

[baking]

Here is a GIF of B22-A and the nearby melange over the past month.  The final image has a longer pause to better see where the GIF loops.  Scaled down by a factor of 14.

Note:  You can see B22-A "walking" slightly to the West (downward) and the big shift of the melange is also to the West.

[grixm]

Perhaps you got the order of the images reversed?

Indeed. Very strange because I double checked, and still got it wrong....

[baking]

6-Day GIF of B22-A showing substantial shift to the North, although most of the motion was in the past 4 days.

[oren]

It appears the "top" part is at least temporarily dislodged.

[Tor Bejnar]

So, moving to the left or to the right? 

I found a little iceberg (part of larger iceberg near lower right-of-center of ice island) that is 'fully attached' in one image and 'floating away' in the other, so the ice island is moving to the left in this GIF.  [I found two of these, but one is more obvious.]]

Yup, Oren, the axis-of-rotation does not go through the ice island (and isn't close to it either) - no pinning point at this time.

(I like dated images in a GIF, but having the images start or end with a longer pause (and telling us which) makes it easier to know 'where it starts.'  This is easy to do with https://ezgif.com/)

[Stephan]

And I observed a micro-calving of one of the attached grounded icebergs at the westernmost point of B-22.

[baking]

I could not find the calvings in the West that you both mention, but I found this large one in the East.  I think it is significant because calving in this younger end of the iceberg is not common and the possibility that it was caused by the underwater peak and combined with such a rapid movement to the North may mean that it has in fact broken away from the peak.

[Stephan]

I could not find the calvings in the West that you both mention, but I found this large one in the East.  I think it is significant because calving in this younger end of the iceberg is not common and the possibility that it was caused by the underwater peak and combined with such a rapid movement to the North may mean that it has in fact broken away from the peak.

I didn't realise this important calving at the eastern end of B-22.
The micro calving I mentioned can be found left of the "4" in "74S". The calving did not occur at B-22 itself but from one of the grounded icebergs at its western flank which supposedly once have been part of B-22 because they show the same texture/structure.

[vox_mundi]

Thwaites Glacier: Significant Geothermal Heat Beneath the Ice Stream
https://phys.org/news/2021-08-thwaites-glacier-significant-geothermal-beneath.html

Ice losses from Thwaites Glacier in West Antarctica are currently responsible for roughly 4 percent of the global sea-level rise. This figure could increase, since virtually no another ice stream in the Antarctic is changing as dramatically as the massive Thwaites Glacier. Until recently, experts attributed these changes to climate change and the fact that the glacier rests on the seafloor in many places, and as such comes into contact with warm water masses. But there is also a third, and until now one of the most difficult to constrain, influencing factors.

In a new study, German and British researchers have shown that there is a conspicuously large amount of heat from Earth's interior beneath the ice, which has likely affected the sliding behavior of the ice masses for millions of years. This substantial geothermal heat flow, in turn, are due to the fact that the glacier lies in a tectonic trench, where the Earth's crust is significantly thinner than it is, e.g., in neighboring East Antarctica. The new study was published today in the Nature online journal Communications Earth & Environment.

Unlike East Antarctica, West Antarctica is a geologically young region. In addition, it doesn't consist of a large contiguous land mass, where the Earth's crust is up to 40 kilometers thick, but instead is made up of several small and for the most part relatively thin crustal blocks that are separated from each other by a so-called trench system or rift system. In many of the trenches in this system, the Earth's crust is only 17 to 25 kilometers thick, and as a result a large portion of the ground lies one to two kilometers below sea level. On the other hand, the existence of the trenches has long led researchers to assume that comparatively large amounts of heat from Earth's interior rose to the surface in this region. With their new map of this geothermal heat flow in the hinterland of the West Antarctic Amundsen Sea, experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the British Antarctic Survey (BAS) have now provided confirmation.

"Our measurements show that where the Earth's crust is only 17 to 25 kilometers thick, geothermal heat flow of up to 150 milliwatts per square meter can occur beneath Thwaites Glacier. This corresponds to values recorded in areas of the Rhine Graben and the East African Rift Valley," says AWI geophysicist and first author of the study, Dr. Ricarda Dziadek.


New magnetic anomaly grid and Curie-depth estimates in the Amundsen Sea sector of West Antarctica.


Geothermal heat flow, lithospheric elastic thickness and magnetic anomaly distribution of the Amundsen Sea sector.

The heat flow could be a crucial factor that needs to be considered when it comes to the future of Thwaites Glacier. According to Gohl: "Large amounts of geothermal heat can, for example, lead to the bottom of the glacier bed no longer freezing completely or to a constant film of water forming on its surface. Both of which would result in the ice masses sliding more easily over the ground. If, in addition, the braking effect of the ice shelf is lost, as can currently be observed in West Antarctica, the glaciers' flow could accelerate considerably due to the increased geothermal heat."

Ricarda Dziadek, Fausto Ferraccioli, Karsten Gohl (2021): High geothermal heat flow beneath Thwaites Glacier in West Antarctica inferred from aeromagnetic data. Communications Earth & Environment,
https://www.nature.com/articles/s43247-021-00242-3

[baking]

I was pulling together some images from the past few months and noticed a couple of new developments.  The first is a calving just to the East of the Thwaites Eastern Ice Shelf (TEIS) and the second is a new calving line forming just to the West of the Tongue.

[icefisher]

Thwaites stress cracks are beginning to manifest as pinning point support weakens. 
https://doi.org/10.5194/tc-2021-288, in review, 2021

Rapid fragmentation of Thwaites Eastern Ice Shelf, West Antarctica
Douglas I. Benn1, Adrian Luckman2, Jan A. Åström3, Anna Crawford1, Stephen L. Cornford2, Suzanne L. Bevan2, Rupert Gladstone4, Thomas Zwinger3, Karen Alley5, Erin Pettit6, and Jeremy Bassis7

    1School of Geography and Sustainable Development, University of St Andrews, St Andrews, KY16 9AL, UK
    2Department of Geography, Swansea University, Swansea, SA2 8PP, UK
    3CSC-IT Center for Science, FI-02101 Espoo, Finland
    4The Arctic Centre, University of Lapland, 96101 Rovaniemi, Finland
    5Department of Environment and Geography, University of Manitoba, Winnipeg, MB R3T 2M6, Canada
    6College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA
    7Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA

Received: 08 Sep 2021 – Accepted for review: 15 Sep 2021 – Discussion started: 20 Sep 2021

Abstract. Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last five years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and release of tabular bergs on both eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear upstream of the main pinning point developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedbacks between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished through time as the pinning point has shrunk, accumulation of damage has ensured that the ice in the shear zone has remained the weakest link in the system. Experiments with the BISICLES ice sheet model indicate that additional damage to or unpinning of TEIS are unlikely to trigger significantly increased ice loss from WAIS, but the calving response to loss of TEIS remains highly uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice shelf margins or grounding points. Our results indicate a third mechanism, backstress-triggered failure, that can occur when ice ffractures in response to stresses associated with pinning points. In most circumstances, pinning points are essential for ice shelf stability, but as ice shelves thin and weaken the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.
How to cite. Benn, D. I., Luckman, A., Åström, J. A., Crawford, A., Cornford, S. L., Bevan, S. L., Gladstone, R., Zwinger, T., Alley, K., Pettit, E., and Bassis, J.: Rapid fragmentation of Thwaites Eastern Ice Shelf, West Antarctica, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-288, in review, 2021.

[baking]

Rapid fragmentation of Thwaites Eastern Ice Shelf, West Antarctica

"the  observed  fracture pattern requires backstress from the pinning point sufficient to initiate and sustain shear failure in the shelf. That is, backstress must consistently exceed the evolving effective strength of TEIS."

"The transformation of TEIS from a largely intact ice shelf into its currently fragmented state
suggests the pinning point has shifted from being a stabilising to a destabilising influence."

"Concomitant with the changes to grounded margins, there have been large changes in the relationship between TEIS and TWIT. First, acceleration of TWIT after 2002 (itself likely a response to weakening of a sub-shelf pinning point) was transmitted across shear margin to TEIS, causing the observed speed-up between 2002 and 2006 (Fig. 6). . . .  Subsequently,  fragmentation  and  opening  of  the  shear  margin  exposed  the western flank of TEIS, reducing lateral confining stress on that side."

"On the balance of available evidence, we propose the following sequence of events as the most likely  cause  of  the  recent  fragmentation.  The  TEIS  shear  zone  was  initiated,  or  was significantly  modified,  in  the  mid-2000s,  when  TEIS  accelerated  in  response  to  stresses transferred  across  the  strong  shear  margin  with  TWIT  (Alley  et  al.  2021).  During  the acceleration event, high longitudinal and shear strain rates developed on TEIS in response to elevated  longitudinal  compression  supported  by  backstress  from  the  pinning  point.  Large fractures within the TEIS shear zone first appear in satellite imagery after the acceleration event, suggesting that damage was initiated or increased in response to elevated stresses (Fig.  7).  Fragmentation,  opening  and  significant  weakening  of  the  shear  margin  between TEIS and TWIT removed confining pressure from the western flank of TEIS, encouraging transverse extension in response to ongoing longitudinal compression. Concurrently, damage on the shelf continued to accumulate due to positive feedbacks between damage and strain, and basal melting reduced ice-shelf thickness and possibly contributed to weakening the TEIS shear  zone  (Wåhlin  et  al.,  2021).  Sentinel-1  data  show  increasing  development  of  tensile fractures  since  2017  in  response  to  transverse  extension  (Fig.  3),  and  increasing concentration of strain and extensive rifting within the TEIS shear zone since 2020 (Fig. 5). Thus, although backstress from the pinning point has likely diminished through time (Wild et al., 2021), accumulation of damage on TEIS has ensured that the ice in the shear zone has remained the weakest link in the system. Fragmentation of TEIS was not a consequence of unpinning; on  the  contrary,  fragmentation  reflects  stresses  originating  at  the pinning point acting on progressively weakening ice.

Backstress from the pinning point, once a crucial source of support for TEIS, is now hastening its destruction. Complete fragmentation of TEIS appears to be imminent and disintegration could occur sooner than the ~10 year timescale implied by rates of thinning and unpinning (cf. Wild et al., 2021). Complete loss of the shelf may follow, although the former pinning point and fast ice may retard the evacuation of icebergs in much the same way as currently observed on TWIT."

I can only add that I've been watching this for a while and am very concerned about some new fractures developing towards the Northeastern end of the TEIS shear zone.  Of course this is a glacier and it moves slowly and fractures can refreeze, so there is that, but it doesn't look good overall.

[baking]

I noticed a difference in the latest Sentinel-2 optical satellite images and the usual Sentinel-1 radar images on the Eastern flank of the Thwaites Eastern Ice Shelf (TEIS.)  The time period is roughly the same, about 7 months, and I've used the "Highlight Optimized Natural Color Script" for the Sentinel-2 images from Sentinel Playground.  Sentinel-1 images are high-resolution via PolarView as usual.

There seems to be a new large rift forming that is not visible on radar.  The explanation is not clear, but I assume Sentinel-2 is seeing a shadow on the surface of the snow pack.  Something of this size cannot be wind related so I assume it is reflecting changes to the ice below.  All I can think is that the radar image, which usually picks up surface features in the snow due to backscattering, is not sensitive enough to a gradual slope.

[paolo]

Baking,

You will find attached the two images:
"TEIS_2021-10-13_Sentinel-2_L1C_Custom_script" (1458 x 1838 px)
and
"TEIS_2021-10-15_Sentinel-1_IW_GRDH_1SSH" (1370 x 1812 px)
These are two 1px=10m images

These two images are consistent with each other and do not show the structure you display in the animation.This structure seems to be a mirage due to the image processing you used.

Very large images, click twice to enlarge

[baking]

This is the custom script I use:

//Highlight Optimized Natural Color Script
//Author: Marko Repše

//For S2L1C:
return [Math.cbrt(0.6*B04 - 0.035),
        Math.cbrt(0.6*B03 - 0.035),
        Math.cbrt(0.6*B02 - 0.035)]

Try it yourself.

It's also there on September 26, 2021.

[baking]

I did some more research.  It is in the L2A data set, and not the L1C.  It is just supposed to be an atmospheric correction, but it doesn't really explain why it is there on October 13 and September 26, but not there on March 17.  Possibly some change to the L2A algorithm.

[paolo]

I confirm that the problem has nothing to do with the script, but with the L2A image
I did the test using :
> both L1C and L2A images
> the two scripts :
>> the one indicated by you (I noticed that I used the same formula for both images, the formula used for the L1C and L2A images being quite different, but this does not invalidate the test, only the contrast is increased for the L2A image)
>> the one I usually use (return [1 * sample.B04,1 * sample.B03,1 * sample.B02, sample.dataMask ]
Below are the four results:
2021-10-13_Sentinel-2_L1C_Custom_script Baking
2021-10-13_Sentinel-2_L1C_Custom_script Paolo
2021-10-13_Sentinel-2_L2A_Custom_script Baking
2021-10-13_Sentinel-2_L2A_Custom_script Paolo

Conclusions:
   Should we be wary of L2A images in our analyses?

And thanks for the URL "https://custom-scripts.sentinel-hub.com/"

Large Images, click to enlarge

[FredBear]

Looking at Worldview recently there does seem to be some new sort of mark in the tongue just outside the marked standard extent of the ice shelf on the eastern side? 11th is a bit clearer than the 16th October.

(The image is reversed compared with the ones above)

[kassy]

Ice shelf holding back keystone Antarctic glacier within years of failure

An alarming crackup has begun at the foot of Antarctica’s vulnerable Thwaites Glacier, whose meltwater is already responsible for about 4% of global sea-level rise. An ice sheet the size of Florida, Thwaites ends its slide into the ocean as a floating ledge of ice 45 kilometers wide. But now this ice shelf, riven by newly detected fissures on its surface and underside, is likely to break apart in the next 5 years or so, scientists reported today at a meeting of the American Geophysical Union.

The most dramatic sign of impending failure is a set of diagonal fractures that nearly span the entire shelf. Last month, satellites spotted accelerating movement of ice along the fractures, says Erin Pettit, a glaciologist at Oregon State University, Corvallis, who is part of a multiyear expedition studying the glacier. The shelf is a bit like a windshield with a series of slowly opening cracks, she says. “You’re like, I should get a new windshield. And one day, bang—there are a million other cracks there.”

Once the ice shelf shatters, large sections of the glacier now restrained by it are likely to speed up, says Ted Scambos, a glaciologist at the University of Colorado, Boulder, and a leader of the Thwaites expedition. In a worst case, this part of Thwaites could triple in speed, increasing the glacier’s contribution to global sea level in the short term to 5%, Pettit says.

...

On top of the 300-meter-thick shelf, the researchers used ground-penetrating radar to image the underside of the ice. They were surprised to find it was not flat and smooth, but carved into a series of upside-down valleys, some 50 meters deep. These undulations stress the ice shelf, and the team saw signs of that stress: fractures had formed at the apex of each valley, Pettit says. “They’re just waiting to be activated in a new way.”

Meanwhile, at the upstream camp, researchers led by Britney Schmidt, a planetary scientist at Cornell University, dug a borehole and sent an instrument-studded robot called Icefin plunging through it to the ocean hidden below. Schmidt then navigated Icefin to the point where the ice and rock met. Nearly everywhere—even at the grounding line itself—the water was 1° or 2° above freezing. Although not unexpected, given the 10 kilometer retreat of the grounding line in the past decade, it was a sure sign of the extended reach of warm waters ushered in by climate change.

During its surveys, Icefin also scanned the underside of the ice with a laser and found valleys similar to those seen downstream. Local variations in water temperature suggested the valleys create turbulence that draws in warmer waters, which deepen them, says Peter Washam, an oceanographer at Cornell. “They’re really hot spots of melting.”

The researchers have plugged many of these observations into computer models of the ice shelf, as detailed in one of three papers about Thwaites at the journal Cryosphere. The models suggest the extensive surface cracks seen in the past 5 years have opened as ice thinned by melting grinds into an offshore, undersea mountain, which had long helped to hold the ice shelf back. Several of these cracks, including one nicknamed “the dagger,” are now extending toward the middle of the shelf. Once there, they may trigger the incipient cracks in the valleys underneath to grow and weaken the shelf further, Pettit says.

...

https://www.science.org/content/article/ice-shelf-holding-back-keystone-antarctic-glacier-within-years-failure

https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/978762

[oren]

Wow. This is extremely bad.

[Ajpope85]

The article feels kind of optimistically worded as well. I'm getting the creeping feeling that my fears of a modern day meltwater pulse in our lifetimes isn't so absurd.

[baking]

Video from the press conference is here:
Edit:  There are literally dozens of news stories today about Thwaites based on this press conference.  I recommend watching the press conference and hearing from the scientists directly.

[Jim Hunt]

My Arctic alter ego's thoughts on all the excitement:

https://twitter.com/GreatWhiteCon/status/1470750638405566470

Retweeted by the AGU!

Thanks for the video baking. Now added to my article.

[kassy]

The article feels kind of optimistically worded as well. I'm getting the creeping feeling that my fears of a modern day meltwater pulse in our lifetimes isn't so absurd.

Finding all that they did is a confirmation of the bleaker projections we had. Of course Thwaites is the most famous and biggest with these problems but this must be happening in other areas too to some degree if the local conditions allow for that.

It will be really interesting to see the new SLR projections after this starts happening.

[gerontocrat]

BBC Article on the same subject
"Things fall apart, the centre cannot hold"

https://www.bbc.co.uk/news/science-environment-59644494
Thwaites: Antarctic glacier heading for dramatic change

Scientists are warning of dramatic changes at one of the biggest glaciers in Antarctica, potentially within the next five to 10 years.

They say a 1,000-sq-km floating section at the front of Thwaites Glacier, which until now has been relatively stable, could "shatter like a car windscreen".

US and UK researchers are currently engaged in an intense study programme at Thwaites because of its melt rate.

Already it is dumping 50 billion tonnes of ice into the ocean each year.

This is having limited impact on global sea-levels today, but there is sufficient ice held upstream in the glacier's drainage basin to raise the height of the oceans by 65cm - were it all to melt.

Such a "doomsday" scenario is unlikely to come about for many centuries, but the study team says Thwaites is now responding to a warming world in really quite rapid ways.

"There is going to be dramatic change in the front of the glacier, probably in less than a decade. Both published and unpublished studies point in that direction," said glaciologist Prof Ted Scambos, a senior investigator with the International Thwaites Glacier Collaboration (ITGC).

"This will accelerate the pace (of Thwaites) and widen, effectively, the dangerous part of the glacier," he told BBC News.



The ITGC has established how this is happening. It is the result of warm ocean water getting under - and melting - Thwaites's floating front, or ice shelf as it's known.

The warm water is thinning and weakening this ice, making it run faster and pushing back the zone where the main glacier body becomes buoyant.

At the moment, the leading edge of the eastern ice shelf is pinned in place by an offshore underwater ridge, which means its flow speed is a third of that seen in the ice shelf's western sector which has no such constraint.

But the ITGC team says the eastern shelf is likely to become uncoupled from the ridge in the next few years which will destabilise it. And even if the pinning persists, the ongoing development of fractures in the shelf ice will almost certainly break up the area anyway.

At present, the eastern shelf, which has a width of about 40km, moves forward at about 600m per year. The coming change in status will probably see the following ice jump in speed to about 2km per year - the same as the current velocity recorded in the 60km-wide western sector.

Jointly funded by the US National Science Foundation and the UK's Natural Environment Research Council, the five-year ITGC project is putting Thwaites under unprecedented scrutiny.

Each Antarctic summer season, teams of scientists are investigating the glacier's behaviour in every way possible. From satellite, on the ice, and from ships in front of Thwaites.

Those teams are en route for the new season right now, some in Covid quarantine ahead of their deployment to the field.

One of the projects for the New Year will see the tubby yellow submarine known as "Boaty McBoatface" dive under Thwaites' floating ice to gather data on water temperature, current direction and turbulence - all factors that influence melting.



The autonomous vehicle will go on missions lasting one to four days, navigating its own path through the cavity beneath the shelf. This is high risk as the seafloor terrain is extremely rugged.

"It's scary. We might not get Boaty back," conceded Dr Alex Phillips from the UK's National Oceanography Centre.

"We've put a lot of effort this past year into developing collision avoidance for the vehicle, to make sure it doesn't crash into the seabed. We also have contingencies whereby if it does get into trouble, it can retrace its steps and retreat to safety."

[FredBear]

Looking on Worldview Thwaites has been changing in the 2000's, the land/ice margins appear to have been marked at about 2010 positions.
In 2000 there was a largely smooth thick Eastern Ice Shelf with calvings off the edge, which has not changed much(?).
To the west there was a strip of ice breaking up as a result of stresses between the EIS and the main glacier tongue, which has been breaking further back towards the continent edge with time. This strip was a little west of the ice margin indent on the maps until about 2007 when it moved eastwards into the marked indentation.
The main glacier tongue was smoother towards the east (with some longitudinal streaks), 2/3 of the width on the western side had many more transverse breaks across it.
By 2002 a major part of the tongue had detached and the residue started to lengthen over the following years in similar form.
By December 2012 there had been another major calving (the iceberg formed in the calving traveled east of B22A before drifting west) which had removed the western side of the tongue and transverse breaks subsequently seemed to have formed around the continental edge over the whole width of the tongue.
Tongue regrowth seems to have been centered along the eastern edge of the tongue marking, with a "spray" of icebergs forming towards the western (lower) marked edge.

It is my impression that the direction of flow of what remains of the Thwaites ice tongue has changed significantly   .    .     .

I have attached Worldview pictures  from 2000, 2002, 2012 and 2021 (various dates in December).

[baking]

It is my impression that the direction of flow of what remains of the Thwaites ice tongue has changed significantly   .    .     .

There was a period of time (edit: 2002-2006) when the tongue and the eastern ice shelf (TEIS) were strongly linked and the tongue flowed more to the east and TEIS had many calvings and was flattened out.  But then a rift between the tongue and TEIS formed and TEIS pretty much froze in place.  Not sure if that is what you are seeing.

Edit: Image from December 2002 below shows what I meant.  Flow of ice sheet was along red arrow until rift occurred at blue line and the ice to the East slowed down significantly because it was no longer being pushed by the main ice stream.

Also, ice is being pushed from upstream.  Downstream ice will follow the path of least resistance.  It doesn't imply changes in the direction of the upstream flow.

See: https://tc.copernicus.org/articles/15/5187/2021/tc-15-5187-2021.html

[FredBear]

It is my impression that the direction of flow of what remains of the Thwaites ice tongue has changed significantly   .    .     .

There was a period of time (edit: 2002-2006) when the tongue and the eastern ice shelf (TEIS) were strongly linked and the tongue flowed more to the east and TEIS had many calvings and was flattened out.  But then a rift between the tongue and TEIS formed and TEIS pretty much froze in place.  Not sure if that is what you are seeing.

Edit: Image from December 2002 below shows what I meant.  Flow of ice sheet was along red arrow until rift occurred at blue line and the ice to the East slowed down significantly because it was no longer being pushed by the main ice stream.

Also, ice is being pushed from upstream.  Downstream ice will follow the path of least resistance.  It doesn't imply changes in the direction of the upstream flow.

See: https://tc.copernicus.org/articles/15/5187/2021/tc-15-5187-2021.html

Thwaites Glacier has two floating ice areas: the Thwaites Western Ice Tongue (TWIT) and the Thwaites Eastern Ice Shelf (TEIS).
What I am seeing is the seaward end of the 2010 TWIT not having much ice from the Thwaites Glacier now. I do not recognise the movement represented by your red arrow but would see movement in the direction of your blue line stressing the flow and forming rifts because of the faster movement of the TWIT.
Originally (2000) the rifts between TEIS and TWIT formed in the eastern margins of the 2010 marked TWIT, later moving east to the gulf between the two.
The main rifted flow which used to form the tongue has now joined the rift with the TEIS and is flowing along the top edge of the old tongue location (about horizontally/north in the image of 2021). The western portion of the glacier has formed a curved rift pattern towards the west and icebergs appear to be breaking off and not reforming the tongue seen traveling towards the lower left corner in the year 2000 image. To my mind this represents a change in flows from the land-based glacier.
This has resulted in the marked "2010 tongue end" having no thick glacier ice in a large triangle by 2021, along with increased rifting of the glacier flow as it crosses the grounding line.
On closer examination there has also been more rifting in the TEIS as the rift between the TEIS and the TWIT has moved east ("up" in the images), indicating that it may be becoming more fragile.

[baking]

The western portion of the glacier has formed a curved rift pattern towards the west and icebergs appear to be breaking off and not reforming the tongue seen traveling towards the lower left corner in the year 2000 image. To my mind this represents a change in flows from the land-based glacier.

If recently calved icebergs are held close together in tight formation by the surrounding ice, they can refreeze together and stay in formation until broken up by other forces.  If, on the other hand, they calve into a loose formation, they will just form a melange and most likely be swept away when the sea ice clears.  I think that is all you are seeing.

There is also an underwater mount somewhere near the middle of that 2010 outline that used to help hold the tongue together by providing some resistance to the advancing ice, keeping it packed more closely, but it is no longer having much of an effect for whatever reason.

I'm still not sure you can draw any conclusions about any "change in flows from the land-based glacier" from this.

[iwantatr8]

So a couple of interesting points following the AGU meeting:

Firstly Dr Anna Crawford has done a more in depth presentation of her work on the Dominos projects and how the model currently sees the calving of the TEIS.  I suggest watching the whole thing but the model animation is at this time:

And to see the difference in speed between the east and west side  have a look at the CPOM page here:
http://www.cpom.ucl.ac.uk/csopr/iv/index.php?glacier_number=4&image_date=211204_211210#output

[baking]

And to see the difference in speed between the east and west side  have a look at the CPOM page here:
http://www.cpom.ucl.ac.uk/csopr/iv/index.php?glacier_number=4&image_date=211204_211210#output

Just to clarify, I have marked up the latest version.  At the upstream shear margin, the west side is moving faster than the east, but at the downstream shear margin near the pinning point, the east side is moving faster than the west.  This is because Thwaites Western Ice Tongue (TWIT) has become decoupled from Thwaites Eastern Ice Shelf (TEIS.)  If the downstream shear margin gives way as shown in Crawford's video, it will free up the eastern side of the upstream shear margin.

Edit: The labels on the image should read "faster" and "slower" since they are relative speeds.

[grixm]

Has anyone else noticed these cracks? They started popping up within the last year and they are still growing week by week. They look very ominous because they are so far upstream, they are not part of routine calvings.
If they spread enough to cause a giant calving, it looks like it would be catastrophic for the ice shelf.
Attached gif is a comparison between two days ago, and November last year.

[Stephan]

Thank you for sharing your observation with us.
Do you know whether these cracks are on the ice shelf (I think so) or are they positioned above the grounding line?

[grixm]

Thank you for sharing your observation with us.
Do you know whether these cracks are on the ice shelf (I think so) or are they positioned above the grounding line?

I was not sure, so I checked by overlaying the 2019 grounding line from Wild et al's 2021 paper. The red line is the grounding line, and the yellow line is the new cracks.

They seem to be downstream of the grounding line, but just barely. The biggest one actually seems to pass over a small underwater knoll that was still grounded in 2019, though maybe not anymore.

[Stephan]

Thank you for checking that! 

[jai mitchell]

does anyone have an idea of current shelf thickness at the pin points?

[Reginald]

Rolling Stone, By Jeff Goodell, December 29

New data suggests a massive collapse of the ice shelf in as little as five years. “We are dealing with an event that no human has ever witnessed,” says one scientist. “We have no analog for this”

https://www.rollingstone.com/politics/politics-features/doomsday-glacier-thwaites-antarctica-climate-crisis-1273841/

But in the long run, it is not clear that the dynamics of ice sheet collapse that are underway at Thwaites can be stopped. As glaciologist Eric Rignot put it in 2015, in Antarctica, “the fuse has been blown.” Even if we cut carbon emissions to zero tomorrow, warm water will continue to flow beneath the ice sheet for decades, destabilizing the ice and further pushing the glacier toward eventual collapse. This doesn’t means that cutting carbon pollution to zero isn’t an important goal — nothing, in fact, is more important or more urgent. “We may have a small safety margin in Antarctica, but not a large one,” says Alley. Even if the fuse is blown, cutting emissions fast could slow it all down to a millennium-long crack-up that will give us more time to adapt. One way or another, our future is written in ice.

[grixm]

Activity in the fast ice west of the tongue. Cracks along the edge of the ice shelf and B22 iceberg, looks like breakup is close. The iceberg moved about 200m during a short period sometime this week.

[Alexander555]

Just a question. The ice shelf is breaking off near land. Is that correct ? And how thick is the glacier at that point ?

[baking]

Just a question. The ice shelf is breaking off near land. Is that correct ? And how thick is the glacier at that point ?

I assume you are talking about the rifts near the grounding line mentioned above.  I think what is happening is that some areas are moving faster than others and the slow moving sections get fractured by being pushed by faster moving neighbors.  It does create a local weakness in the structure of the shelf, but it is not really directly due to melting or thinning.

[Alexander555]

I never payed much attention to this part of the forum. So i try to pick up a little. If it breaks near the grounding line it will be a pretty high wall that will face gravity. I have seen a couple glaciers in Alaska, they are just a couple dozens of meters above sea level at the point where they enter the sea.  This could be a couple hundreds of meters.

[Alexander555]

There is a youtube movie in the link. Most of the land below thwaites is below sea level. That makes it very risky. https://www.antarcticglaciers.org/2020/07/thwaites-glacier/

[Alexander555]

There was a lot about it in the news the last week. They were talking about the fact that the ice shelf is acting like a cork. So they are probably talking about the slow moving parts, or the parts that are not moving. Are there ice shelfs in the sea  left and right of the moving part ? If you look at the colour pic in the youtube movie, that shows the elevation. It's basically a giant valley.

[baking]

There was a lot about it in the news the last week. They were talking about the fact that the ice shelf is acting like a cork. So they are probably talking about the slow moving parts, or the parts that are not moving. Are there ice shelfs in the sea  left and right of the moving part ? If you look at the colour pic in the youtube movie, that shows the elevation. It's basically a giant valley.

There has been a lot of material posted here over the past few years that is probably worth looking at and we can probably answer any questions you have to get you up to speed.

The short answer is that the part of Thwaites that has been in the news recently is the Thwaites Eastern Ice Shelf (TEIS) that actually in the center, the trapezoid shaped shelf that sticks out and doesn't move very much.  It is holding back the central third of Thwaites and when it goes it should substantially widen the part of Thwaites that is fast moving and could be very disruptive over time.

There are a number of scenarios that have been proposed in the scientific literature in the past few years:
1)  The oldest theory is that TEIS will eventually "unpin" from an underwater ridge at the front of the shelf as the ice melts and thins, but this should take 10-20 years.
2)  A more recent worry is that the shelf will shear away from the ridge through a series of fractures developing in a line at an angle to the ridge.  This could happen in the next 5-10 years, although that is probably a conservative estimate.
3)  The big news back in December was that researchers have found a weak area in the center TEIS, and that some transverse cracks that have been steadily growing could hit that weakness by as soon as early 2024.

This video from the press conference in December does a pretty good job at summarizing the latest research:

[Stephan]

"Boaty McBoatface", a torpedo-shaped submarine instrument will go beneath Thwaites Ice Shelf in Jan/Feb 2022 with the British Antarctic Survey. It is presented in the following youtube clip:

Worth while watching.

[Stephan]

A detailed report from Dr. Julia Wellner from the UK-US Thwaites Glacier Collaboration about their work on bathymetry and sediment sampling on the sea floor around Thwaites and Pine Island Glaciers that allow a look into the past where ice once had been grounded which now is open waters / ice shelf.
Worth while watching.

[VeliAlbertKallio]

There are other processes that might become relevant in future in the demise of Thwaites Glacier:

Meltwater Shearing of Ice Shelves has been a risk on the Antarctic Peninsula beginning from Larsen A in 1995 that lies furthest away from the South Pole, followed by Larsen B, and most recently, the beginning of Larsen C disintegration. In this process either meltwater and/or flash floods on ice build water that drains into the crevasses within the ice shelf. As the water is heavier it causes a shearing force from within the crevasse which then deepens it and widens it and gathers more sharing force to it. On the Weddell Sea, from Larsen C the next ice shelf to the south is in turn Ronne which would bring much more drama than the Larsens A, B, C.

Glacier Debris Flows (GDFs) are yet another process where melt water or rain from the flash floods penetrate underneath the ice. Even on relatively flat surfaces with perhaps even just a couple degrees angle, tiny layers of water under glacier detaches it from the moorings of the bed rocks leading to sudden ice collapses - much like the above-said ice shelf shearing by water accumulates in the crevasses. You can read more about this dangerous but little observed process here:

Meyer, Robinson: "When Glaciers Transform Into Deadly 150-mph Avalanches - After happening only once in the 100-year record, catastrophic glacial collapse occurred twice in Tibet this summer", The Atlantic | Science, 18 October 2016.
https://www.theatlantic.com/science/archive/2016/10/glaciers-can-collapse-in-seconds-not-years/504458/\

A growing isostatic disequilibrium between the Ice Sheet mass balances between the Eastern and the Western Antarctica drive fluid displacement in lithosphere - where ice accumulates the fluids gain higher pressures and move towards the shield areas of the ice sheet with lesser fluid pressures within magma reservoirs under the rocks. The inter-regional variations among the various parts of the Antarctic Ice Sheets also do it in lesser extent. Besides the pressure directing fluids towards the shield of the Antarctic ice sheets, the fluid motion is also pointed towards centre at the Gamburtsev Anomaly which receives the flows akin to shield area but sits just in the middle of Antarctic ice due to its sheer size. There are lesser flows of fluids towards the Gamburtsev Point at the centre of ice - with the greatest ice pressures between it and the shield that almost encircles the entire Antarctic ice as its periphery. Magma and hot water incursions are likely to occur there as the Antarctic ice load adjusts to anthropogenic effects over the east Antarctic. (The reason for its existence is the span of the Antarctic ice cover, as the earth is increasingly rigid at depth, entire Antarctic-wide span of a deepening subglacial bowl is just not possible; hence, there is this "oasis" sitting under the ice.

A change in dissolved gases in magmas. The increasing ice pressure enhances the liquid rocks to dissolve gases. The opposite of this is the nucleation of gases out of rocks when the ice sheet mass balance reduces. Both these and the above said begin to manifest only significantly on advanced stages of changes in the Antarctic Cryosphere and are known as the ice-volcano interactions on which I presented at CMPCC 1 Summit, an intermediate climate summit between COP15 and COP16 by the Southern Nations, Small Island States, and the Non-Sovereign Nations (indigenous peoples): https://www.academia.edu/50061469/The_Link_Between_Receding_Glaciers_and_Natural_Disasters_CMPCC_1_Summit_Interview_by_Veli_Albert_Kallio_Frozen_Isthmuses_Protection_Campaign_of_the_Arctic_and_North_Atlantic_Oceans

[baking]

Google says:  No results found for "Gamburtsev Anomaly".