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Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 21 Nov 2017, 14:07:51
by dohboi
More on why sea level is likely to rise above standard projections within the century:

grist.org/article/antarctica-doomsday-glaciers-could-flood-coastal-cities/

Doomsday on Ice

Rapid collapse of Antarctic glaciers could flood coastal cities by the end of this century.


By Eric Holthaus on Nov 21, 2017

"The only place in the world where you can see ice-cliff instability in action today is at Jakobshavn glacier in Greenland, one of the fastest-collapsing glaciers in the world. DeConto says that to construct their model, they took the collapse rate of Jakobshavn, cut it in half to be extra conservative, then applied it to Thwaites and Pine Island."

Re: Sea Level Rise Pt. 2

Unread postPosted: Fri 01 Dec 2017, 19:42:13
by dohboi

Re: Sea Level Rise Pt. 2

Unread postPosted: Fri 01 Dec 2017, 20:15:25
by Plantagenet
dohboi wrote:
Doomsday on Ice

Rapid collapse of Antarctic glaciers could flood coastal cities by the end of this century.


By Eric Holthaus on Nov 21, 2017

"The only place in the world where you can see ice-cliff instability in action today is at Jakobshavn glacier in Greenland, one of the fastest-collapsing glaciers in the world. DeConto says that to construct their model, they took the collapse rate of Jakobshavn, cut it in half to be extra conservative, then applied it to Thwaites and Pine Island."


Pine Island Glacier is already shifting to a new regime. A huge iceberg just calved off it..... Once the ice calves behind the grounding line the whole glacier will collapse and rapidly retreat just like Jacobhavn Glacier in Greenland....and then collapse will eat its way into the interior of the West Antarctic Sheet, causing ice drawdown from the entire ice sheet.

Image
Recent Pine Island Glacier iceberg calving event

Cheers!

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 09 Jan 2018, 11:14:55
by onlooker
https://www.yahoo.com/news/ocean-floor- ... 02572.html
The Ocean Floor Is Sinking Under The Water Weight From Melting Glaciers, And It’s As Bad As It Sounds

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 09 Jan 2018, 11:37:34
by Tanada
onlooker wrote:https://www.yahoo.com/news/ocean-floor-sinking-under-water-130002572.html
The Ocean Floor Is Sinking Under The Water Weight From Melting Glaciers, And It’s As Bad As It Sounds


Everything in balance, the continental crust where the glaciers are receding is rising as well. All that mass makes a big difference no matter if it is on land or in the sea.

The interesting thing to me is, how will this change the rate of sea quakes and tsunami's as the forces balance out in a different direction? There are geological remnants of per-historic tsunamis in many places like Hawaii and the Bahamas and the Canary Islands. You would think with sea level rising nearly 400 feet from 20,000 ybp to 6,000 ybp that most such events would have been triggered. Unfortunately some of them are cyclical like the Southeast Asia Tsunami of Christmas 2006 and the Seattle Tsunami that last happened in 1700 that seems to take place every 290-350 years as the subduction zone slips.

Re: Sea Level Rise Pt. 2

Unread postPosted: Mon 22 Jan 2018, 21:35:13
by dohboi
This is bad:

M. S. Waibel, C. L. Hulbe, C. S. Jackson & D. F. Martin (16 January 2018), "Rate of mass loss across the instability threshold for Thwaites Glacier determines rate of mass loss for entire basin",

Geophysical Research Letters, DOI: 10.1002/2017GL076470

http://onlinelibrary.wiley.com/doi/10.1 ... ign=buffer

Abstract:

"Rapid change now underway on Thwaites Glacier (TG) raises concern that a threshold for unstoppable grounding line retreat has been or is about to be crossed.

We use a high-resolution ice sheet model to examine the mechanics of TG self-sustained retreat by nudging the grounding line just past the point of instability. We find that by modifying surface slope in the region of the grounding line, the rate of the forcing dictates the rate of retreat, even after the external forcing is removed. Grounding line retreats that begin faster proceed more rapidly because the shorter time interval for the grounding line to erode into the grounded ice sheet means relatively thicker ice and larger driving stress upstream of the boundary. Retreat is sensitive to short-duration re-advances associated with reduced external forcing where the bathymetry allows re-grounding, even when an instability is invoked."

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 23 Jan 2018, 12:20:39
by Tanada
It has been pointed out nearly a decade ago by Dr. Richard Alley and his colleagues that once the ice backs up off the grounding sill on the sea floor the rate of retreat will be limited only by how fast the ice can float away through the narrow gap in the fjord. Ice lacks the mechanical strength to maintain a cliff height over about 100 meters or 300 feet. Right now the ice is resting on the sill so it can extend a full 300 feet above the sea surface, but once it retreats back even a few hundred yards/meters the warping stress will cause it to shed icebergs off the face to reduce its height to match its material strength. In effect by resting on the sill the ice is prevented from warping forward under its own weight and only sheds icebergs as it is forced forward past the sill where the support no longer prevents the warping effect. IOW on either side of the sill the ice floats and water does not support it and prevent the bending action of its own weight from warping the front. This is why even the massive tabular icebergs floating around Antarctica are all the same height and have relatively flat tops, the laws of physics and mechanical properties of ice are at their limits at that height.

However something to keep in mind, those tabular bergs are like every other floating piece of ice, 7/8th of their total height is submerged in the water. The key distance behind or in front of the sill is where the water depth is greater than the total height of a tabular iceberg including the submerged portion. While the ice is in the portion of the channel where the bottom is less than that height they remain grounded on the sea floor below. In the case of tabular icebergs this is typically a total height of around 800 feet with 710 feet below sea level. This means once the ice retreats back from the sill to water with a depth of 720 or more foot depth it will start retreating very rapidly as it will shed ice off its seaward side until it retreats back to water of less than 710 foot depth.

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 23 Jan 2018, 14:13:41
by vox_mundi
Interacting Antarctic Glaciers May Cause Faster Melt and Sea Level Rise

A new study shows that a large and potentially unstable Antarctic glacier may be melting farther inland than previously thought and that this melting could affect the stability of another large glacier nearby – an important finding for understanding and projecting ice sheet contributions to sea-level rise.

The findings, by a Stanford-led team of radar engineers and geophysical glaciologists, came from radar data collected at the same locations in 2004, 2012 and 2014, each revealing details of the glaciers miles below the surface. The surveys show that ocean water is reaching beneath the edge of the Pine Island Glacier about 7.5 miles further inland than indicated by previous observations from space.

The team also found that the Southwest Tributary of Pine Island Glacier, a deep ice channel between the two glaciers, could trigger or accelerate ice loss in Thwaites Glacier if the observed melting of Pine Island Glacier by warm ocean water continues down the ice channel. The results were published online in the Annals of Glaciology.
"This is a potentially really dynamic place between these two glaciers, and this is somewhere where further study is really warranted," ... "If this tributary were to retreat and get melted by warm ocean water, it could cause the melt beneath Pine Island to spread to Thwaites"

"These results show that the ocean is really starting to work on the edge of this glacier, which means that we're likely at the onset of it having an impact,"


- Dustin Schroeder, - assistant professor of geophysics - School of Earth, Energy & Environmental Sciences



Image

Dustin M. Schroeder et al. Ocean access beneath the southwest tributary of Pine Island Glacier, West Antarctica, Annals of Glaciology (2017).

Will Our Cities Survive the 21st Century?

Unread postPosted: Mon 29 Jan 2018, 22:07:02
by AdamB

Goodbye, Miami, American Atlantis. The city is sinking. Yes, in the literal sense, swaths of the metropolis—built on flat, permeable limestone—will be swallowed by seas expected to rise 4–8 feet globally by the century’s end. That includes the $50 million waterfront condos still being developed on Miami Beach. The working-class stucco homes miles from the coast in Hialeah. Beaches, hotels, office buildings, malls, hospitals, schools, highways, wastewater treatment plants, and airports in between. But even before the Atlantic submerges much of Miami, a metro area currently filled with 2.6 million inhabitants, it will be sunk by its leaders’ failures to reckon with its future. As of now, most of them refuse to warn the public about the threats posed by climate change. They balk at investing in most long-term adaptations. Some are in denial; others fear of spooking high-rolling condo buyers or vacationers.


Will Our Cities Survive the 21st Century?

Re: Sea Level Rise Pt. 2

Unread postPosted: Mon 29 Jan 2018, 23:20:20
by dohboi
So a (probably stupid) question occurred to me:

Sea levels will continue to rise, mostly from glaciers (well above sea level) and ice sheets (at, near or a good bit above sea level) melting.

When the mass of a spinning object moves closer to the center of that object (think of a twirling ice skater pulling in her/his arms to spin faster), that object spins faster.

So...is the earth starting to spin faster, and will it spin ever faster as more and more of this land ice finds its way to the sea? Or is it offset by other factors? Presumably we are still talking about relatively tiny changes, but even as an academic question...I am wondering...

Any light that could be shed would be welcome. Thanks ahead of time. :-D 8O :|

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 30 Jan 2018, 01:23:57
by Keith_McClary
dohboi wrote:When the mass of a spinning object moves closer to the center of that object (think of a twirling ice skater pulling in her/his arms to spin faster), that object spins faster.

Actually, it's "closer to the axis of rotation". Since the ice is near the poles the effect will be the opposite.

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 30 Jan 2018, 05:59:51
by vtsnowedin
The weight of all the ice compared to the weight (mass) of the earth and water in the oceans is minuscule. Move it anywhere you want and the difference in rotation speed will be measured in one or two seconds per year.

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 30 Jan 2018, 07:16:18
by dohboi
Thanks, Keith. And yes, vt, I said it would be minuscule. Thanks for repeating that point again for me :-D

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 30 Jan 2018, 09:35:33
by vtsnowedin
dohboi wrote:Thanks, Keith. And yes, vt, I said it would be minuscule. Thanks for repeating that point again for me :-D
Are we touchy this morning? I did not just parrot your point but added the logical reason why you were right.

Re: Sea Level Rise Pt. 2

Unread postPosted: Tue 30 Jan 2018, 10:47:49
by Tanada
The rotation of the earth has been calculated to slow slightly if all the ice melts but the biggest implication is the equatorial bulge will distribute much of the melt water to the tropics. This will cause mean sea level rise in for example, Brazil or Indonesia to be about double what it is in Scotland, or Alaska. Now we are talking about 270 feet or so maximum average rise so an extra 20 feet in Brazil will not seem all that important and a 250 foot rise in Scotland will still be devastating.