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nukefix

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Re: Greenland ice sheet retreat
« Reply #50 on: August 04, 2017, 11:56:42 AM »
Ice sheets that are grounded on an inward-sloping submarine bed are fundamentally unstable, for example WAIS. In Greenland this is not expected to happen as the topography of the bedrock is much more benign:

https://www.the-cryosphere.net/7/499/2013/tc-7-499-2013.pdf

Jakobshaven goes deep inland under present day sea level but it's narrow. Mean-image of S-1 over 2017 brings the movement out nicely..

ps. I'm not familiar with the acronym GDF?

« Last Edit: August 04, 2017, 12:09:22 PM by nukefix »

oren

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Re: Greenland ice sheet retreat
« Reply #51 on: August 04, 2017, 02:48:01 PM »
ps. I'm not familiar with the acronym GDF?
I believe is means Glaciogenic Debris-Flow, but now sure how it's related to the discussion.

gerontocrat

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Re: Greenland ice sheet retreat
« Reply #52 on: August 04, 2017, 03:25:13 PM »
NASA GRACE analysis from 2002 shows almost linear Greenland Ice Sheet mass loss trend. When will AGW accelerate the loss ?
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nukefix

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Re: Greenland ice sheet retreat
« Reply #53 on: August 04, 2017, 03:46:16 PM »
NASA GRACE analysis from 2002 shows almost linear Greenland Ice Sheet mass loss trend. When will AGW accelerate the loss ?
Arguably this already happened in the 1990's...but yes melting can potentially increase by a lot and we could have more summers with 2012-like extreme melt...

Daniel B.

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Re: Greenland ice sheet retreat
« Reply #54 on: August 04, 2017, 04:35:20 PM »
NASA GRACE analysis from 2002 shows almost linear Greenland Ice Sheet mass loss trend. When will AGW accelerate the loss ?

Certainly not this year.

johnm33

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Re: Greenland ice sheet retreat
« Reply #55 on: August 05, 2017, 12:23:36 PM »
"When will AGW accelerate the loss ?"
When the meltwater of the interior gets access to the ocean?

oren

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Re: Greenland ice sheet retreat
« Reply #56 on: August 05, 2017, 10:57:23 PM »
"When will AGW accelerate the loss ?"
When the meltwater of the interior gets access to the ocean?
Indeed. Or when Zachariae Isstrom decides to let all hell break loose.

johnm33

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Re: Greenland ice sheet retreat
« Reply #57 on: August 06, 2017, 05:39:11 PM »
It has to break out from the three rows of islands which are presently holding back it's melange and calving front, I begin to wonder if it'll have to wait to be weakened by ongoing melting to the north or south, although perhaps one almighty storm could clear it's path. That said it remains my favourite, there are so many ice streams falling into Jakobshavn that the fjord hardly ever clears, and Petermann tapers so much for so long, but maybe VAK is right.

nukefix

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Re: Greenland ice sheet retreat
« Reply #58 on: August 07, 2017, 07:19:00 AM »
"When will AGW accelerate the loss ?"
When the meltwater of the interior gets access to the ocean?
Indeed. Or when Zachariae Isstrom decides to let all hell break loose.
According to Bedmap2 there's only limited scope for retreat at NEGIS.

AbruptSLR

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Re: Greenland ice sheet retreat
« Reply #59 on: May 14, 2018, 10:45:40 PM »
It seems to me that the behavior of the NEGIS during the Holocene Optimum ~7.8 – 1.2 ka, provides a point of concern as to how much ice mass loss may occur for this key marine terminating ice stream in the coming decades:

Nicolaj K. Larsen et al. (14 May 2018), "Instability of the Northeast Greenland Ice Stream over the last 45,000 years", Nature Communications, Volume 9, Article number: 1872, doi:10.1038/s41467-018-04312-7

http://www.nature.com/articles/s41467-018-04312-7

Abstract: "The sensitivity of the Northeast Greenland Ice Stream (NEGIS) to prolonged warm periods is largely unknown and geological records documenting such long-term changes are needed to place current observations in perspective. Here we use cosmogenic surface exposure and radiocarbon ages to determine the magnitude of NEGIS margin fluctuations over the last 45 kyr (thousand years). We find that the NEGIS experienced slow early Holocene ice-margin retreat of 30–40 m a−1, likely as a result of the buttressing effect of sea-ice or shelf-ice. The NEGIS was ~20–70 km behind its present ice-extent ~41–26 ka and ~7.8–1.2 ka; both periods of high orbital precession index and/or summer temperatures within the projected warming for the end of this century. We show that the NEGIS was smaller than present for approximately half of the last ~45 kyr and is susceptible to subtle changes in climate, which has implications for future stability of this ice stream."
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vox_mundi

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Re: Greenland ice sheet retreat
« Reply #60 on: September 19, 2018, 01:25:18 AM »
Study links natural climate oscillations in north Atlantic to Greenland ice sheet melt 

Quote
  Scientists have known for years that warming global climate is melting the Greenland Ice Sheet, the second largest ice sheet in the world. A new study from the Woods Hole Oceanographic Institution (WHOI), however, shows that the rate of melting might be temporarily increased or decreased by two existing climate patterns: the North Atlantic Oscillation (NAO), and the Atlantic Multidecadal Oscillation (AMO).

 Both patterns can have a major impact on regional climate. The NAO, which is measured as the atmospheric pressure difference between the Azores and Iceland, can affect the position and strength of the westerly storm track. The study found that when the NAO stays in its negative phase (meaning that air pressure is high over Greenland) it can trigger extreme ice melt in Greenland during the summer season. Likewise, the AMO, which alters sea surface temperatures in the North Atlantic, can cause major melting events when it is in its warm phase, raising the temperature of the region as a whole.

Depending on how the AMO and NAO interact, excess melting could happen two decades earlier than expected, or two decades later this century.

"We know the Greenland ice sheet is melting in part because of warming climate, but that's not a linear process," Ummenhofer said. "There are periods where it will accelerate, and periods where it won't."   
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vox_mundi

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Re: Greenland ice sheet retreat
« Reply #61 on: September 19, 2018, 10:36:38 PM »
More evidence that Ice Caps like Greenland may exhibit fundamental instability once they pass a tipping point ...

Unprecedented Ice Loss In Russian Ice Cap
https://m.phys.org/news/2018-09-unprecedented-ice-loss-russian-cap.html



In the last few years, the Vavilov Ice Cap in the Russian High Arctic has dramatically accelerated, sliding as much as 82 feet a day in 2015, according to a new multi-national, multi-institute study led by CIRES Fellow Mike Willis, an assistant professor of Geology at CU Boulder. That dwarfs the ice's previous average speed of about 2 inches per day and has challenged scientists' assumptions about the stability of the cold ice caps dotting Earth's high latitudes.

... Researchers suspect the ice cap began to dramatically advance when the bottom of the ice cap became wetter and the front of the glacier advanced onto very slippery marine sediments.

By 2015, the sediments and rock at the bed beneath the ice had become so slippery that the material couldn't stop the ice from flowing. It took just two years for the ice cap base to reach that tipping point, transforming into a near frictionless zone, which is well-lubricated and highly mobile. The glacier continues to slide today at accelerated speeds of 5-10 meters per day.

The Vavilov Ice Cap thinned by a total of a few meters, advanced about 2 km, and lost about 1.2 km3 in total volume into the ocean in the 30 years before the speedup. In the one year between 2015 and 2016, the ice advanced about 4 kilometers and thinned by about 100 meters (~0.3 m per day). The ice cap lost about 4.5 km3 of ice, enough to cover Manhattan with about 250 feet of water, or the entire state of Washington with an inch. And it's unlikely the ice cap will ever be able to recover ice mass in today's warming climate, the paper states.
Quote
Many scientists have assumed that polar ice caps that sit above sea level will only respond slowly to a warming climate—but the authors of this study urge that this assumption be questioned. The rapid collapse of the Vavilov Ice Cap has significant ramifications for glaciers in other polar regions, especially those fringing Antarctica and Greenland.   

Michael J. Willis et al, Massive destabilization of an Arctic ice cap, Earth and Planetary Science Letters (2018)

Quote
Abstract:

  Ice caps that are mostly frozen at the bedrock-ice interface are thought to be stable and respond slowly to changes in climate. We use remote sensing to measure velocity and thickness changes that occur when the margin of the largely cold-based Vavilov Ice Cap in the Russian High Arctic advances over weak marine sediments. We show that cold-based to polythermal glacier systems with no previous history of surging may evolve with unexpected and unprecedented speed when their basal boundary conditions change, resulting in very large dynamic ice mass losses (an increase in annual mass loss by a factor of ∼100) over a few years. We question the future long-term stability of cold and polythermal polar ice caps, many of which terminate in marine waters as the climate becomes warmer and wetter in the polar regions.
“There are three classes of people: those who see. Those who see when they are shown. Those who do not see.” ― Leonardo da Vinci

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sidd

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Re: Greenland ice sheet retreat
« Reply #62 on: September 19, 2018, 10:41:28 PM »
For those interested, I posted some quotes from the paper on the Vavilov icecap at

https://forum.arctic-sea-ice.net/index.php/topic,1350.msg173421.html#msg173421

in another thread.

sidd

AbruptSLR

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Re: Greenland ice sheet retreat
« Reply #63 on: October 03, 2018, 11:49:52 PM »
As continued global warming should increase the frequency with which atmospheric rivers reach Greenland, we may be in for some rude surprises in the coming decades (w.r.t. increasing rates of ice mass loss from the Greenland Ice Sheet):

William Neff (2018), "Atmospheric rivers melt Greenland", Nature Climate Change 8, 857-858, DOI: https://doi.org/10.1038/s41558-018-0297-4

http://www.nature.com/articles/s41558-018-0297-4

Abstract: "Recent years have seen increased melting of the Greenland Ice Sheet, contributing to accelerated rates of sea-level rise.  New research suggests that this melting due to an increased frequency of atmospheric rivers, narrow filaments of moist air moving polewards."
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sidd

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Re: Greenland ice sheet retreat
« Reply #64 on: October 04, 2018, 05:34:51 AM »
Thanks for the Neff reference. Neff has been looking at the effect of atmospheric rivers on greenland forawhile, eg

doi: 10.1002/2014JD021470

which i believe i have discussed before.

The latest Neff article is a precis of the results of Mattingly et al. (Fettweis is an author)

doi:10.1029/2018JD028714

on the impact of atmospheric rivers on greenland ice over the period 1980-2016. The latter, among other things, use a self organizing map classifier to identify these phenomena. They find, unsurprisingly that the effect is spatially and seasonally diverse, but that surface mass balance is adversely affected.

"Furthermore, our investigation of the short- and long-term relationships between moisture transport events and modeled GrIS surface properties proves that this correspondence between the years of enhanced AR activity and anomalous GrIS mass loss is not a coincidence. Strong AR impacts cause increased melt in all areas of the GrIS and decreased SMB in the ablation zone during summer, and warm seasons with above-average GrIS melt extent are characterized by anomalously strong moisture transport by ARs over Greenland. ARs typically result in SMB gains in the GrIS ablation zone during nonsummer seasons and in the accumulation zone during all seasons. However, the intense summer SMB losses in the ablation zone during years of enhanced moisture transport outweigh the positive AR contributions to SMB in other regions and seasons. The scaling of melt versus snowfall in Figures 9 and 11 shows that the magnitude of mass loss from summer melt in the ablation zone has a much greater upper limit than mass gain from snowfall."

I attach figure 9.

sidd

RoxTheGeologist

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Re: Greenland ice sheet retreat
« Reply #65 on: October 05, 2018, 05:47:58 PM »

Less just hope that the large parts of the ice sheet doesn't get warm enough and well lubricated enough to collapse en-mass. The images of glaciers collapsing catastrophically in the Himalayas may set precedent for that to happen.

https://www.thethirdpole.net/en/2017/08/17/experts-explore-reasons-for-glacier-collapses-in-tibet/