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Anne

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Sea Level Rise: New Iceberg Theory
« on: July 23, 2013, 06:02:30 PM »
From Science Daily:
Quote
In events that could exacerbate sea level rise over the coming decades, stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean, according to new iceberg calving simulations from the University of Michigan.
(...)
The researchers have found the physics at the heart of iceberg calving, and their model is the first that can simulate the different processes that occur on both ends of Earth. It can show why in northern latitudes -- where glaciers rest on solid ground -- icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge. It can also illustrate why in the southernmost places -- where vast ice shelves float in the Antarctic Ocean -- icebergs form in larger, more horizontal plank shapes.
The model treats ice sheets -- both floating shelves and grounded glaciers -- like loosely cemented collections of boulders. Such a description reflects how scientists in the field have described what iceberg calving actually looks like. The model allows those loose bonds to break when the boulders are pulled apart or rub against one another.
(...)
Areas that border deep, unobstructed ocean rather than fjords or other waterways are at greater risk of rapid ice loss. The researchers point to the Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland, which is already retreating rapidly, as places vulnerable to "catastrophic disintegration".
"The ice in those places gets thicker as you go back. If our threshold is right, then if these places start to retreat as you expose the thicker calving font, they're susceptible to catastrophic breakup."

The paper is behind a paywall here.

Quote
Abstract
Iceberg calving has been implicated in the retreat and acceleration of glaciers and ice shelves along the margins of the Greenland and Antarctic ice sheets. Accurate projections of sea-level rise therefore require an understanding of how and why calving occurs. Unfortunately, calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature. Here we present a numerical model that simulates the disparate calving regimes observed, including the detachment of large tabular bergs from floating ice tongues, the disintegration of ice shelves and the capsizing of smaller bergs from grounded glaciers that terminate in deep water. Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two-stage process that requires both ice fracture and transport of detached icebergs away from the calving front. We suggest that, as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.
J. N. Bassis, S. Jacobs. Diverse calving patterns linked to glacier geometry. Nature Geoscience, 2013, doi:10.1038/ngeo1887

Csnavywx

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Re: Sea Level Rise: New Iceberg Theory
« Reply #1 on: July 24, 2013, 03:38:02 PM »
Hansen and Sato (2012) covers the effects of such voluminous iceberg discharge into regional oceans:

http://www.columbia.edu/~jeh1/mailings/2012/20121226_GreenlandIceSheetUpdate.pdf

In particular, the freshening and cooling effect exerted by these armadas of icebergs is modeled by GISS modelE in the paper, with fairly drastic effects. Note that they assume a 10 year doubling period for ice melt contribution to sea level (starting with 1mm/yr in 2010). Even a cooling effect half of what is stated in the paper would lead to drastic climactic changes eerily similar to that of Heinrich events of the past.

sidd

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Re: Sea Level Rise: New Iceberg Theory
« Reply #2 on: November 05, 2016, 04:12:33 AM »
Nice model from Feldmann and Levermann giving ice surges with a realistic (PISM) ice model. Open access

doi:10.5194/tc-2016-235

"During the surge phase mainly the process of hydraulic runaway (positive feedback between basal melt water production and flow acceleration; Fowler and Johnson, 1995) is in effect. It is complemented by creep instability (positive feedback between strain heating and ice deformation; Clarke et al., 1977), which additionally promotes rapid ice streaming [Ref. 20] (Figs. 4 and 5). The modeled cyclic alternation of ice streaming and stagnation provides a simple example of ice-stream shutdown and re-activation, a phenomenon which is characteristic for the dynamics of some of the Siple Coast outlets in West Antarctica.

The period duration of a full surge cycle in our model of about 1.8 kyr is very close to results from other recent studies (Bougamont et al., 2011; Robel et al., 2016) which is surprising considering the differences in degree of physical approximations, [Ref 25] parameterizations, and setup complexity between the three studies"

sidd

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Re: Sea Level Rise: New Iceberg Theory
« Reply #3 on: November 05, 2016, 10:13:13 PM »
I should add that there is an interesting graf (Fig 3e) in the Feldmann-Levermann paper of ice volume discharge against time. We see (approximately) pulses of 1e4 GT over timescales of 1e2 yr or so. Thats around 30mm sea level rise if evenly distributed over the oceans, but a la Mitrovica we know it will be about twice that in NHemis and a ccorresponding fall in SHemis. The glacier dimension in the study is a few hundred Km long and 80 km wide, say Thwaites, but without retrograde bed leading into Byrd deep.


I hope they put in retrograde bed and deep holes and see what happens ...


sidd