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Messages - steve s

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51
Antarctica / Re: PIG has calved
« on: March 02, 2016, 09:28:43 PM »
ASLR: "... as it is probable that the keel of the iceberg is deep enough to also be impacted by the relatively warm CDW discharge shown in the first of my previous two images."

I'm confused by the first part of this comment. The berg's keel should be little deeper than the PIG's front. The second part may offer a clue. The berg's pinned, so return advective water might be a factor as it flows around the pin. There may be also orographic lifting of fluids over the eastern side of the berg, dependent on the local terrain, and about that I am ignorant. However the advective inflows must be west of the berg where the depths are greater for any of this to work out.

52
Antarctica / Re: PIG has calved
« on: March 02, 2016, 09:54:52 AM »
" ... the specific gravity of warm water is higher than of cold ... "

wait, what ?

Sorry I was imprecise Sidd. Fresh water's density is greatest at about 4 degrees C, so ice forms at the water's surface, where the water is at 0 degrees. Dissolved salt depresses the freezing point and increases the density, but leaves the general relationship unchanged.

Advective outflows, having lost heat to the ice, are less dense and above the inflows. The density change is due to both cooling and freshening.

The motion of the berg indicates that a large volume water is traveling at the top of the water column -- along the bottom of glacier. To my amateur eye, the strength of the outflow at the top of the water column seems either (1)  too great to be explained by melt-water alone (as ASLR's figure suggests is the case); or (2) the volume of melting is higher than seems to be indicated by apparent volume changes in the glacier.

53
Antarctica / Re: PIG has calved
« on: March 01, 2016, 10:09:33 PM »
Thanks ASLR.

I was originally thinking of tides being too weak. Now I realize that the specific gravity of warm water is higher than of cold, so the warm basal waters become an outflow river at the lower surface of the glacier. The strength of the flow (surprises me) is made visible by the ongoing separation between the berg and the glacier. 



54
Antarctica / Re: PIG has calved
« on: March 01, 2016, 09:13:04 PM »
Thanks Wipneus; the sequences are always interesting. In this case, the ongoing forces on the separated berg seem impossibly strong.

Probably something I am missing in the way of force transmission, but the glacier's movement seems to remain correlated with the bergs rotation although the two are physically separated with the berg pinned. The forces on the berg are sufficient, whatever their sources, to cause rapid growth of peripheral crevasses and so on. If a river were flowing beneath the glacier generating the forces, it would have to be a very vigorous river. I think that can be ruled out.

Any ideas?
 

55
Antarctica / Re: NASA: Antarctic Ice Sheet gaining mass?
« on: November 07, 2015, 09:05:07 PM »
What would the impact on the estimates be if Zwally failed to properly factor in the effect of the past rise in mean sea level on the change in the mass of ice above sea level? The mass decrease implied, correcting for the mass grounded on rock above sea level, seems roughly similar to the increase Zwally claimed. 

56
I was looking at a spot of obvious retreat. I don't know the scale of the image or the area covered by a pixel to approximate the distance, but I do wonder.

Given the rather obvious increase in floating ice chunks, if the calving face did not retreat, the glacier must have surged. Big icequake; near the grounding line as I have read the various posts.

57
ST, would you please estimate from the image pixels the distance the Thwaites calving face retreated during August? Since the retreat distance varies along the face, the maximum point of retreat would do.

I think of the maximum as a relatively easy qualitative measure of the release of stored strain, suitable for quick comparison with new nearby events. In the case of the Thwaites in the next few years, time series on retreat maximums and on the intervals between events may prove both interesting and useful.









58
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: May 19, 2015, 09:44:45 AM »
Enjoy your well-earned break. 

59
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: April 24, 2015, 11:29:10 PM »
Any mechanism that is not gradual is hard to model. Ice dam formation and release is at its base a catastrophic process capable of moving far greater masses of solids quickly down riverine channels than a gradual process using the same amount of fluids. 

We know the Bretz Floods happened; any such process in Antarctica would increase the centennial slr over the quantity without it given the same rainfall and heat flux. The pressurized ponds under the Thwaites might provide surprises when combined with cliff fracturing. Gradualism is not a safe religion, if mathematically convenient. The highest Pollard estimates would prove low.

60
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: April 24, 2015, 01:44:11 PM »
The question that this hypothesis depends upon is the floating ice flow rate prior to becoming oceanic. Perhaps heavy enough rain could keep the bergs separated and provide the needed volume of water for flushing the ice. An Antarctic monsoon hypothesis, more or less.

The continent is fairly dry now, but it seems to me that would require a huge volume of water, given the size of bergs to be carried. Lake Missoula-type releases as formed the Columbia Gorge might be sufficient, moving ice instead of basalt. So, if rainwater could form large ponds under or on the ice, to be followed by periodic releases as ice dams collapse, it seems to me that sufficient flows might be generated, repeatedly clearing channels that refill after each discharge.

61
The effort is so hopeful, and if it could be pulled off it would very useful. Seems like a difficult or even impossible model to debug, though. If debugged, an incredibly difficult model to fit with accurate parameters. Many statistical difficulties would have to be ignored, any of which could throw off results.

I hope that it points the way to better back-of-the-napkin thinking by showing critical bottlenecks.   

(I'm not picking on this effort, for it is impossible to test the computational validity/accuracy of large complex computer solutions under the theoretically simplest scenarios -- and this is not simple.)

62
Antarctica / Re: What's new in Antarctica ?
« on: July 08, 2014, 08:37:35 PM »
Two meters by 2100 is roughly an inch or 2.5 cm per year on average. If, as most of us on this thread seem to think, this proves low estimate, policy makers will soon be forced to accept SLR as important. Actually, much earlier as Rignot has set the stage for the rate of change of the rate of change entering the public awareness as a justification.

("Let the cry ring out to all who will listen: To arms, to arms, for the Thwaites is coming.")

A Pyrrhic victory, no matter how soon the recognition comes, but sooner is better.


63
Antarctica / Re: Surge of WAIS Ice Mass Loss
« on: June 30, 2014, 06:37:57 AM »
" In this study, we provide a statistically rigorous, error-bounded trend estimate of ice mass loss over the WAIS from 2003–2009 which is almost entirely data-driven."

These results are necessarily biased downward by the estimation procedure. Or, more precisely, are determined entirely by the trends during the estimation period and the choice of observations used. You can judge whether you feel the observation period is likely to be indicative of future melting.

64
Antarctica / Re: Will Antarctica sea ice set a new record in 2014?
« on: June 29, 2014, 09:44:32 PM »
Am I accurate if I interpret these numbers as indicating a massive increase in fresh water draining from the continent this year as compared with previous ones? Else I suspect there would have to be a major increase in an available heat sink, and that seems far less likely.

65
From the abstract of Rignot, et.al., 27 May 2014:

"Upstream of the 2011 grounding line positions, we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin."

66
nukefix,

ASLR did not mention one monster issue associated with the Thwaites -- the shape of the bed and the thickness of the overlying ice. Much more unstable than the PIG. Now that the Thwaites has retreated onto the bed downslope, it is unstable in a self-reinforcing pattern. Also, because of the shape of the shelf and overlying ice dome, unless there is an ice jam limiting the outflow rate, the outflows may accelerate for an appreciable period.

As I understand the situation, no energy inputs are needed for collapse, although energy inputs can influence the rate. Also, because the retreat to the downslope is new at the Thwaites, the downslope's contribution to the rate of collapse is not fully understood or factored in to the calculations. As Rignot has stated, his team's date estimates are upper bounds.

So, no. The currents are probably not important any longer. Also the next few years may be illuminating.

If I am off base in this assessment, I am sure ASLR will correct me.

Steve

67
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: May 19, 2014, 10:33:09 PM »
Talking of growth rates, anyone want to estimate what year the Thwaites Glacier is going to be recognized as contributing more to SLR than the Pine Island Glacier?

Steve

68
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: May 18, 2014, 08:47:54 PM »
ASLR, it's all in the phrasing. Or maybe the timing. Having facts is not the issue; using them is.

This new article was written for the popular press. Almost all comments I have seen in the popular press have downplayed the term "collapse". NASA has made no statements to the contrary. With this new Rignot put into the popular mind that collapse means collapse. The comment will probably shift the discussion in the popular press.

The article had to have been written by Rignot in the last week. If it was cleared by NASA, this signals a policy shift at high levels in the government. Or maybe last week's press conference indicated a shift. If not, Rignot put his career at risk.

Good deal either way for the ASLR problem's recognition. Doesn't matter that the technical report said the same thing in different words. Previously the time frame could be overlooked by pundits -- not so readily now.

69
Antarctica / Re: Potential Collapse Scenario for the WAIS
« on: May 18, 2014, 07:37:43 PM »
Thanks idunno,

One line struck me.

Rignot: "Two centuries – if that is what it takes –"

Finally the hedged comments have been put into perspective for the public. But one has to wonder, was this article cleared with NASA's bureaucracy before publication?

70
Antarctica / Re: PIG has calved
« on: February 15, 2014, 09:09:48 PM »
Thank you ASLR for posting these detailed images.

I've been watching that notch form and increase in size over the last few days, but using the less detailed MODIS images. How the notch could open to the extent it did without spilling visible ice into the ocean was difficult to understand. Apparently there are a pair of actions involved. The ice (which apparently was thin) crumbled, but remained along the other ice, apparently pinned by a current. The other action is a surface current flowing west under the ice which has shifted ice west opening that notch while closing the obvious crack in the first photo.

If one examines the movements of separating ice blocks along the coast between the Thwaites tongue and the PIG over series of images, the same current seems to be active.

Has to be a lot of water moving, possibly augmented by high flows through the Thwaites gateway. This area may prove interesting in the next week or so.


71
Antarctica / Re: PIG has calved
« on: February 07, 2014, 05:01:39 AM »
Aslr, thanks for your reply. However, I think further discussion is in order.

You mentioned and embedded Google Earth images. Google Earth and map images are updated infrequently. For example, T]the zoomed crevasse image you posted is dated 1/26/2010. The lovely image you report as being from early December 2013 is dated 12/31/1998, long before several calving events. The images' dates are printed along their bottom edges.

My preference is to use the MODIS Rapidfire daily image subsets for convenient comparisons. Because of the loss of historic images due to the failure of a disk that was not backed up, I cannot tell when the hole opened through. However photos dated December 2012, after the last major tongue retreat, do not show the hole. Thus the phenomenon is new and may be a coal-mine canary.

Whether it is a traditional polyna is in question. A polyna is usually associated with seasonal sea ice, not ice shelves. The area where this hole is occurring was reported to be an ice shelf until recently -- as was the area I mentioned in a previous post -- and, to my knowledge, has not had seasonal ice for a very long time. Clearly the shelf has broken up. but into bergs of fairly uniform size; apparently crevasse interval-determined width.

Polynas can be maintained by winds or currents, but winds work on bergs poorly because of their depth. In this case the open space is among tightly packed bergs.  More likely in my opinion is that an upwelling is keeping the hole open. You suggest from the PIG, but I suspect from under the Thwaites. In either case, it would seem to require a tremendous sustained flow, not the burst-emptying of a pond, to force bergs so far apart and then keep them away. Several mechanisms come to mind, one of which is geothermal melting somewhere under the Thwaites.

I don't have an answer to the cause of the open water, but I do wonder whether the TG's contribution to SLR is now higher than reported.


72
Antarctica / Re: PIG has calved
« on: February 06, 2014, 08:37:06 AM »
Anyone have an explanation for the ongoing open water at that one spot? It has been visible in images for at least the last two months despite being surrounded by what seems to be bergs. The bergs seem neither to shrink in size nearby (indicating melting) or to enter the open water.

If I read D in http://glacierchange.files.wordpress.com/2012/10/thwaites-bedrock-2.jpg correctly, the open water is behind the 2009 oceanic grounding line and likely where a deep water location with ungrounded ice is indicated. Have suitable currents or geothermal heat sources been reported in that area?

73
Antarctica / Re: PIG has calved
« on: January 25, 2014, 08:16:01 PM »
The Thwaite's Tongue area is worth following closely; or rather the area between the Thwaite's Tongue and the PIG outlet. On December 26th a set of crevasses appeared, they were not visible on the previous day's image:


For a sense of scale, (1) the grounded PIG iceberg is visible offshore; and (2) each pixel is 500 meters, so the cracks are both sudden and wide. The area covered in this huge event is far larger than the iceberg.

The following day the new crevasses were indistinct, their walls having calved, filling them in. The shelf was grounded, so this single event added more to SLR than the PIG's iceberg separating had. The area has been breaking up bit-by-bit since, only recently with large areas of ice rafting away. The last clear image was January 18, but the clouds did not hide much on January 21:


Through the cloud cover one can see hints that in the four days since the continued loss of ice has been extensive, but no details. I was hoping to report a month to month comparison.


74
NH=?


New glossary entry needed?

75
Antarctica / Re: PIG has calved
« on: December 30, 2013, 09:12:30 PM »
I suspect the image is of a cloud and shadow cast on a snow and water background..

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