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Author Topic: Thwaites Glacier Discussion  (Read 1969 times)

prokaryotes

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solartim27

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Re: Thwaites Glacier Discussion
« Reply #1 on: October 21, 2016, 08:40:48 AM »
Since there is no dedicated topic yet, here it is now.

Try the 5 pages of discussions here
http://forum.arctic-sea-ice.net/index.php/topic,72.0.html
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prokaryotes

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Re: Thwaites Glacier Discussion
« Reply #2 on: October 21, 2016, 10:02:23 AM »
Since there is no dedicated topic yet, here it is now.

Try the 5 pages of discussions here
http://forum.arctic-sea-ice.net/index.php/topic,72.0.html

Oh well, but do we want a dedicated discussion (without PIG), and here in this forum? Mods, go ahead delete/move as you see fit.
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oren

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Re: Thwaites Glacier Discussion
« Reply #3 on: October 21, 2016, 12:21:22 PM »
That discussion is mostly about Thwaites, including following its movements and calvings. Just the thread name is misleading.

AbruptSLR

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Re: Thwaites Glacier Discussion
« Reply #4 on: October 21, 2016, 05:08:01 PM »
That discussion is mostly about Thwaites, including following its movements and calvings. Just the thread name is misleading.

When looking at the bigger picture (when considering cliff failures and hydrofracturing as GMSTA approaches 2.7C), the collapse mode for Thwaites is closely linked to the behavior of PIG.
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DrTskoul

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Re: Thwaites Glacier Discussion
« Reply #5 on: February 16, 2017, 03:21:29 AM »
"There is currently large uncertainty about how much sea level will rise and much of this uncertainty is related to whether models incorporate the fact that ice sheets break," Bassis said. "What we are showing is that the models we have of this process seem to work for Greenland, as well as in the past so we should be able to more confidently predict sea level rise."

He added that portions of Antarctica have similar geography to Laurentide: Pine Island, Thwaites glacier, for example.

"We're seeing ocean warming in those region and we're seeing these regions start to change. In that area, they're seeing ocean temperature changes of about 2.7 degrees Fahrenheit," Bassis said. "That's pretty similar magnitude as we believe occurred in the Laurentide events, and what we saw in our simulations is that just a small amount of ocean warming can destabilize a region if it's in the right configuration, and even in the absence of atmospheric warming."

The study is called "Heinrich events triggered by ocean forcing and modulated by isostatic adjustment."


How an Ice Age paradox could inform sea level rise predictions

https://m.phys.org/news/2017-02-ice-age-paradox-sea.html
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FredBear

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Re: Thwaites Glacier Discussion
« Reply #6 on: February 16, 2017, 11:47:37 AM »
Heinrich events triggered by ocean forcing and modulated by isostatic adjustment.
They are saying that isostatic rebound reduced the contact of warm water with the ice sheet - but surely that reaction is much too slow? The relatively rapid rise in sea level would also increase water contact - although loss of mass could contribute by reducing local gravity? Recent calvings of ice shelves have led to the glaciers behind them speeding up and thinning, creating concerns about runaway reactions   .    .    .

RoxTheGeologist

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Re: Thwaites Glacier Discussion
« Reply #7 on: February 16, 2017, 06:39:56 PM »
Heinrich events triggered by ocean forcing and modulated by isostatic adjustment.
They are saying that isostatic rebound reduced the contact of warm water with the ice sheet - but surely that reaction is much too slow? The relatively rapid rise in sea level would also increase water contact - although loss of mass could contribute by reducing local gravity? Recent calvings of ice shelves have led to the glaciers behind them speeding up and thinning, creating concerns about runaway reactions   .    .    .

"On millennial timescales, isostatic adjustment causes the bed to uplift, isolating the terminus from subsurface warming and allowing the ice sheet to advance again"

It depends on the magnitude of the isostatic rebound and sea level rise. An ice sheet 1 km thick causes 1/2.6 *1000m of uplift. Basically you replace the ice mass with rock mass. However, I would tend to agree with you. I can't see how this can become a feedback cycle. The sea level change is on a decadal timescale, whereas the isostatic rebound takes much longer. I'd expect ice sheets to continually adjust to the new physical regime as the rebound progresses. Sedimentologists with muddy thoughts?


AbruptSLR

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Re: Thwaites Glacier Discussion
« Reply #8 on: February 16, 2017, 07:41:00 PM »
Many people think that when scientist look at the paleo-record, that as the climate oscillates that the paleo-record can be used to directly answer what is going to happen in the Anthropocene in the new few decades.  Such thinking is ludicrous, as the purpose of looking at the paleo-record is to better calibrate state-of-the-art Earth System Models (with state-of-the-art modules for marine glaciers including isostatic adjustments) so that when Anthropogenic forcing is applied we can better estimate what is likely to happen in the near future.

Perhaps the most important issue to consider about isostatic rebound for Thwaites is that the satellite record of ice mass loss (GRACE etc.) need to be corrected for this rebound; which means that recent ice mass losses from Thwaites are likely higher than previously reported.
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RoxTheGeologist

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Re: Thwaites Glacier Discussion
« Reply #9 on: February 17, 2017, 01:59:04 AM »
Perhaps the most important issue to consider about isostatic rebound for Thwaites is that the satellite record of ice mass loss (GRACE etc.) need to be corrected for this rebound; which means that recent ice mass losses from Thwaites are likely higher than previously reported.

It would be interesting if there were exposed sections of rock close to to glacier, and undergoing substantially the same rebound. Height changes of the exposed rock would allow calibration of any rebound and perhaps allow for an isostatic adjustment to the mass loss.

sidd

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Re: Thwaites Glacier Discussion
« Reply #10 on: February 17, 2017, 04:37:53 AM »
Very pretty paper from a few years ago on seasonal uplift signal in Greenland.

www.pnas.org/cgi/doi/10.1073/pnas.1204664109

but thwaites is much harder logistically and otherwise


AbruptSLR

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Re: Thwaites Glacier Discussion
« Reply #11 on: February 17, 2017, 03:56:47 PM »
It would be interesting if there were exposed sections of rock close to to glacier, and undergoing substantially the same rebound. Height changes of the exposed rock would allow calibration of any rebound and perhaps allow for an isostatic adjustment to the mass loss.


I am re-posting the following from the "Antarctic Tectonics" thread.  The first paper indicates that the GRACE satellite SLR contributions previously reported by NASA are probably 40% too low for at least the ASE area and probably for all of the WAIS due to treating the GIA correction for the WAIS like any other part of the earth when, as I have indicated in my prior posts in this thread, West Antarctica has a relatively unique tectonic history and current condition:


An investigation of Glacial Isostatic Adjustment over the Amundsen Sea sector, West Antarctica
by: A. Groh; H. Ewert, M. Scheinert, M. Fritsche, A. Rülke, A. Richter, R. Rosenau, R. Dietrich
http://dx.doi.org/10.1016/j.gloplacha.2012.08.001

"Abstract
The present study focuses on the Amundsen Sea sector which is the most dynamical region of the Antarctic Ice Sheet (AIS). Based on basin estimates of mass changes observed by the Gravity Recovery and Climate Experiment (GRACE) and volume changes observed by the Ice, Cloud and Land Elevation Satellite (ICESat), the mean mass change induced by Glacial Isostatic Adjustment (GIA) is derived. This mean GIA-induced mass change is found to be 34.1 ± 11.9 Gt/yr, which is significantly larger than the predictions of current GIA models. We show that the corresponding mean elevation change of 23.3 ± 7.7 mm/yr in the Amundsen Sea sector is in good agreement with the uplift rates obtained from observations at three GPS sites. Utilising ICESat observations, the observed uplift rates were corrected for elastic deformations due to present-day ice-mass changes. Based on the GRACE-derived mass change estimate and the inferred GIA correction, we inferred a present-day ice-mass loss of − 98.9 ± 13.7 Gt/yr for the Amundsen Sea sector. This is equivalent to a global eustatic sea-level rise of 0.27 ± 0.04 mm/yr. Compared to the results relying on GIA model predictions, this corresponds to an increase of the ice-mass loss or sea-level rise, respectively, of about 40%."

The first accompanying figure shows an overview of the Amundsen Sea sector, West Antarctica. The red line defines the generalised drainage basins of Pine Island Glacier, Thwaites Glacier and Smith Glacier (PITS). Locations of three GPS campaign sites are marked by red triangles.

The second figures shows the GRACE data from 2003 to 2009 which the papers says needs to be corrected to indicate about 40% more ice mass loss than previously reported

The second paper finds that ice mass loss estimates for GRACE observations for the Antarctic are highly dependent upon the GIA correction used (which the authors state to be uncertain).  That said the latest GIA data makes me believe the −147 ± 80 Gt/yr average ice mass loss for AIS from 2003 thru 2012, cited below:

Time-variable gravity observations of ice sheet mass balance: Precision and limitations of the GRACE satellite data
by: I. Velicogna, and J. Wahr; Article first published online: 27 JUN 2013; Geophysical Research Letters, DOI: 10.1002/grl.50527

Abstract:
"Time-variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) mission have been available since 2002 to estimate the mass balance of the Greenland and Antarctic Ice Sheets. We analyze current progress and uncertainties in GRACE estimates of ice sheet mass balance. We discuss the impacts of errors associated with spherical harmonic truncation, spatial averaging, temporal sampling, and leakage from other time-dependent signals (e.g., glacial isostatic adjustment (GIA)). The largest sources of error for Antarctica are the GIA correction, the omission of l=1 terms, nontidal changes in ocean mass, and measurement errors. For Greenland, the errors come mostly from the uncertainty in the scaling factor. Using Release 5.0 (RL05) GRACE fields for January 2003 through November 2012, we find a mass change of −258 ± 41 Gt/yr for Greenland, with an acceleration of −31 ± 6 Gt/yr2, and a loss that migrated clockwise around the ice sheet margin to progressively affect the entire periphery. For Antarctica, we report changes of −83 ± 49 and −147 ± 80 Gt/yr for two GIA models, with an acceleration of −12 ± 9 Gt/yr2and a dominance from the southeast pacific sector of West Antarctica and the Antarctic Peninsula."

The third paper indicates up to 4.5 meters of bed uplift due to GIA for the Pine Island Bay in the next 100-years.  However, I believe that it is likely too conservative scientifically, and that basal melting rates, and earthquakes, will increase ice mass loss faster than the negative feedbacks mentioned in the article:

S. Adhikari, E. Ivins, E. Larour, H. Seroussi, M. Morlighem, and S. Nowicki, (2014), "Future Antarctic bed topography and its implications for ice sheet dynamics", Solid Earth Discuss., 6, 191–228, 2014, www.solid-earth-discuss.net/6/191/2014/; doi:10.5194/sed-6-191-2014

http://www.solid-earth-discuss.net/6/191/2014/sed-6-191-2014-print.pdf

Abstract: "The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) is generally losing its mass since the last glacial maximum (LGM). In a sustained warming climate, the AIS is predicted to retreat at a greater pace primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that the past loading is relatively less important than future loading on the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years 2100 and 2500AD, respectively, and that the East Antarctic Ice Sheet (EAIS) is likely to remain unchanged or subside minimally except around the Amery Ice Shelf.  The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay approaches roughly 45mmyr−1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is associated with the flattening of reverse bed, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote the stability to marine portions of the ice sheet in future."
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steve s

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Re: Thwaites Glacier Discussion
« Reply #12 on: March 24, 2017, 08:22:52 PM »
Looks like the Thwaites tongue is calving.



AbruptSLR

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Re: Thwaites Glacier Discussion
« Reply #13 on: March 24, 2017, 09:30:40 PM »
Looks like the Thwaites tongue is calving.

steve s,

Great catch, & the first attached image is a Sentinel image from March 22 2017, showing the same event.  That said, I think that rather than calling it a calving event it might be more accurate to say that the iceberg that calved in 2012 (see the second image) has finally become ungrounded (see the third image of the grounding point #2). 
« Last Edit: March 24, 2017, 09:52:30 PM by AbruptSLR »
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maga

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Re: Thwaites Glacier Discussion
« Reply #14 on: March 25, 2017, 11:00:03 AM »
I would call it a calving event. The berg that calved in 2012 which is labeled as new iceberg in the picture has moved away long ago and is currently floating in the Ross sea, roughly along a line from the Sulzberger Ice Shelf to Cape Adare. What is breaking off now is a part of the remaining Thwaites Ice Tongue. It may have been grounded or possibly still is, but it's breaking apart rapidly.  The sea is getting deeper towards the land and we have to expect a rapid breakup of the complete remaining ice tongue. The Eastern Thwaites Ice Shelf will follow soon. It is still grounded at the tip but cracks are developing all over the place. Probably less than three more years to go...

AbruptSLR

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Re: Thwaites Glacier Discussion
« Reply #15 on: March 26, 2017, 04:06:46 AM »
The first image shows the Thwaites Ice Tongue in December 2012, and the second shows the Ice Tongue in March 2015.  It looks to me like the large grounded at the seaward end of the degraded Ice Tongue is the same feature but somewhat degraded with time.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson